BIOCHEMISTRY OF CARBON

ASSIMILATION

BIOCHEMISTRY OF CARBON ASSIMILATION

CIMMYT Maize In-Service Training Course,

Mexico

CONTENTS

PAGE

INTRODUCTION ........................................................

PHOTOSYNTHESIS ...................................................... 1)

2)

3)

4)

5)

6)

Site of Action .................................... • ............ . Water .. • ....................................................... . Carbon Dioxide ................................................. Light ..........................................................

................................. Photosynthetic Light Reactions

Fixation of Carbon Dioxide .....................................

4

4

5

6

10

14

20

7) The C4 Dicarboxylic Acid Pathway ••••••••••••••••••••••••••••••• 22

PHOTORESPIRATION .............................................. • ..... . SYNTHESIS OF SUCROSE AND POLYSACCHARIDES

RESPIRATION ......................................................... 1) Formation of Hexose Sugars f.rom Polysaccharide Reserves

30

35

41

42

2) Glycolysis ••••••••••••••••••••••••••••••••••••••••••••••••••••• 44

3)

4)

Mitochondria ................................................... The Tricarboxylic Acid Cycle (TCA or Krebs Cycle)

47

48

5) The Electron Transport System•••••••••••••••••••••••••••••••••• 50

INTRODUCTION

Consider a maize plant at maturity. Most of the total dry weight is

made up of compounds containing carbon, hydrogen and oxygen

(carbohydrates) e.g. sugars, starch, cellulose, hemicellulose. The '

mature grain contains 85% carbohydrate. Without carbohydrates we are not

going to have much yield. It is the same for other plants. Where do

these compounds ·:come from? The answer is that they are formed from the

products of the process we know as photosynthesis.

Photosynthesis is the basis of life since it is essentially the

only mechanism of energy input into the living world. The process of

photosynthesis involves the oxidation of water by light (energy source)

and the reduction of co2 to form sugars.

co2

+ 2n H2o + Light chloroplasts)(CH

20)n + n o

2 + n H

20

s~. for photosynthesis to occur ~e first need the correct site cf action

(the chloroplasts), energy from light, and the building blocks of the

products. These building blocks are co2 and n2o. To stay alive the plant

needs chemical energy. Photosynthesis does not provide this energy in a

readily useable form. Instead, photosynthesis fixes energy in the form

of carbohydrates, some of which are available for future use as an

energy source.

Another process, respiration (essentially the reverse of

photosynthesis) uses the products of photosynthesis to form compounds

containing available energy. Energy in this form can be used in the

thousands of other reactions necessary for the plant to survive, grow,

develop and mature. Respiration involves the removal of electrons from

carbohydrates and the passage of these electrons to a lower energy level

+ where they combine with H and the willing electron acceptor, o2 , to

form stable water.

®

In this unit we aim to cover the basic aspects of carbon

assimilation in higher plants. First, let us look at the site of

photosynthesis in the plant and how water, co2 and light reach the site

of action in the chloroplast. Then we will look at the biophysics and

biochemistry of photosynthesis and the production of carbohydrates.

Finally, we will take a look at losses from the system firstly by .•

photorespiration; the amount of co2

that is lost and the differences

between c3

and c4

plants; and secondly ~y respiration, the amount of co2

lost and the uses of respiration.

PHOTOSYNTHESIS

1) Site of Action

Photosynthesis occurs in green plant parts, especially the leaf. It

occurs in green organelles present in the cytoplasm of mesophyll cells

in the leaf. These organelles are called chloroplasts (Fig. 1.1.).

r-)-------

©

thylakoids

stroma

·- ->- -- - ,._ ___ FIG.

~a.rt of o.. ceU.. Oeta."1l. o~ c.hlo rop\.~ - ·,~~~no.\. st.ru.ct.urc. 1.1. Location of chloroplasts in the leaf and the internal

structure of the chloroplast.

The structure of the chloroplast is very important because

different parts of photosynthes14 occur in different parts of the

chloroplast. The chloroplast is surrounded by two membranes with the

inner one controlling molecular traffic into and out of the chloroplast.

However, there are other specialized membranes within the chloroplast.

These internal membranes act as the wall of a flat tube, the lamella,

and contain the ·;photosynthetic pigments. In some areas the lamellae are

stacked to form grana (one stack is a granum). An individual lamella is

often called a thylakoid. This whole system of l~mellae is surrounded by

the chloroplast matrix or stroma. Note that there is a channel between

the lamellae of each thylakoid. This channel contains water and solutes

and, as we will see later, is very important in the light reactions of

photosynthesis. The thylakoids contain the following pigments:

chlcrcphyll ~ ~nd chlorophyll b, ~hich &re green, and sm&ller

of the yellow/orange carotenes and xanthophylls. Chloroplasts either

develop from non-green pro-plastids or from other chloroplasts by

division. These processes depend on light, as does the synthesis of

chlorophyll itself.

2) Water

The water required in photosynthesis is taken into the chloroplasts

from the cytoplasm and into the cytoplasm from other cells. These

processes occur because the use of water in photosynthesis and the

production of solutes by photosynthesis lowers the osmotic potential and

thus the water potential inside photosynthesizing cel.1s (see the unit on

Water in the Plant). The amount of water needed directly by

photosynthesis is very small compared to the amount of water lost by

transpiration. These large transpiration losses· (see the unit on Water

Use by the Cereal Crop) are necessary to obtain sufficient co2 from the

5

atmosphere for photosynthesis. When water is in short supply,

photosynthesis is impaired because <1>2 uptake is reduced. This occurs

well before any possibility of a direct water shortage for use in

photosynthesis.

3) Carbon Dioxide

The atmosphere is the source of carbon dioxide for photosynthesis.

Carbon dioxide moves from the atmosphere to the chloroplasts by the

process of diffusion. Almost all of this carbon dioxide enters the plant

via stomates that are located mainly on the leaf. Since the

concentration of carbon dioxide in the atmosphere is very low (0.035% by

volume, 350 ppm), the diffusion gradient to the chloroplast is shallow.

In addition, the diffusion path is long and restricted by various

resistances to the flow of co2. All these factors make ,it difficult for

the plant to get co2. The stomates have to remain wide open throughout

the light period if co2 is not to limit photosynthesis. While the

stomates are open to allow assimilation of ~o2 , a lot of water is lost

to the atmosphere by transpiration.

For co2 to diffuse into the leaf, the co2 concentration in the

photosynthetic reaction center in the chloroplast must be less than that

in ambient air. If so, then co2 can move in the gaseous phase through.

stomates and intercellular spaces to the mesophyll cells, where it then

diffuses in the aqueous phase through cell walls into the cytoplasm and

to the chloroplasts.

Trans£ er by molecular diffusion is explained by Fick' s Law which

states that the rate of flow is related to the difference in

concentration between the point of supply and the point ~f consumption,

and is inversely related to the distance separating these two points.

The quantity S, flowing per unit time t through an area A, when the

6

dif f~rence in concentration is de and the distance from the source and

the sink is dx, is expressed by:

dS • _ KA de dt · dx

K • co§fficient of diffusion (with dimensions of cm2/s if S is in cm )

2 A is in cm

x is in cm

3 ·:3 de is cm /cm

The value obtained depends on the substance diffusing and the

medium through which it diffuses.

We can now examine the path from the atmosphere to the chloroplast

reaction center. (Fig. 1.2.). It is more than just a certain distance

with equal resistance properties throughout its length. Certain parts

offer more resistance to molecular diffusion than others. To understand

these ideas better it is useful to imagine the system as analogous to

the flow of electricity through an electrical conductor.

• "D j( 0 ;; c 0

A

PHOTOSYNTHESIS CARBON OUTSIDE DIOXIDE AIR

Shearing layer ____________ !._ _________ _

Stomata . r, - ----------------------- -·

~ SOIL ::. Micro-organisms 0

Sub-stomata! cavity

------- - ----- __ !:! _______ ----· • ~ :a 0 .. t-..

"O c 0

" • . .,,

Mesophyll cell wall

r.,,

Diffusion within cell to reaction centre

In chloroplast ,, FIG. 1.2. A resistance analog model of co2 diffusion from the at~osphere

to the photosynthetic reaction center in the chloroplast.

1

We can, by analogy to Ohm's Law of electrical resistance write:

FC02 •

fJ. c •

I R •

FC02 • A C I R

flux of co2 into the leaf (analogous to current)

co2 concentration gradient (analogous to potential difference)

Total resistance of leaf to co2 diffusion (analogous to •lectrical resistance)

So, the pathway of co2 diffusion between the atmosphere and the

point of carboxylation in the chloroplast can be considered as a group

of resistances in series.

What level should AC have and what are these resistances that make

up IR? We know the co2

concentration in the atmosphere, but what is it

inside the 1£af at the reaction center in the chloroplast? We tlon't knuw

the answer to this second question, but as we will see later most plants

(those with the c3

photosynthetic system) cannot fix co2 if it is

present in concentrations below about 60 ppm. For those plants with the

c4 photosynthetic system this concentration is about 10 ppm. This level

is called the compensation point (r). The concentration of co2 at r is

the concentration of co2 normally assumed for the reaction center.

The first resistance layer encountered by a co2 molecule on its

journey to the reaction center in the chloroplast is the boundary layer

resistance (rb). When a gas passes over a flat surface such as a leaf,

there is a small non-turbulent layer of air molecules associated with

the. surface. This is the boundary layer. Its depth depends on leaf

geometry (it is deeper for leaves with a large surface area) and the

velocity of the gas flowing over its surface (the lower the wind speed,

the deeper it is). When the boundary layer is deep the diffusion of co2

into . or out of the leaf is slower. This resistance (10-30 s/m) is

usually only a small part of the total resistance to co2

uptake. The

next resistance is that offered by the stomata (r ) • This resistance s

depends on stomata! opening and closing activity and represents the

variable resistance in the non-metabolic part of the co2

assimilation

pathway. The r .,s in the range 250-1000 s/m. Even when fully open, the s

stomates represent the largest resistance to co2 in the gaseous

diffusion part of the pathway. The r often represents the major s

limitation to co2 assimilation for leaves of plants with the c4

type of

photosynthesis. In many tropical environments stomata may present the

largest single limitation to the conversion of solar energy into

chemical energy stored in carbohydrates by higher plants.

Once inside the piant leaf, the cu2 molecule faces a further series

of resistances, collectively known as the mesophyll resistance (r ) • 1!I.

This includes the physical gaseous diffusion resistance from the stoma

to the mesophyll cell wall, through the sub-stomata! cavity (re); the

movement in the aqueous phase through the mesophyll cell wall (rw), and

aqueous movement through the mesophyll cell cytoplasm into the

chloroplast to the sites of carboxylation, together with. biochemical

rate limitations (rr). Between 60-80% of the total resistance to C02

diffusion from the atmosphere to its point of use in photosynthesis is

r + r , 30% for r itself. This is mainly because the coefficient of w r w

1 diffusion for co2 in H20 is about lO,OOOth of that in air. So, the rm

can be very large, especially in c3 plants (250-4000 s/m) (25-500 s/m in

c4 plants).

Adding all these components together we can write:

rb + r8

+ r + r + r c w r

Ca • concentration of co2 in the atmosphere

r • compensation point

Note, resistanc~ to water loss by transpiration are about equal to rb +

r + r + r , but in the other direction. So the total resistance to 8 c w

water loss is less than that for co2 use in photosynthesis - see the

unit on Water Use by the Cereal Crop for a fuller treatment.

In conclusion, there is no simple relationship between the

concentration of co2 outside the boundary layer in the atmosphere and at

the carboxylation reaction center, nor in the flux of co2 along the

~athway. It is greatly altered by leaf structure: in~ernAl RnA~omy ~~rl

physiological/biochemical aspects of photosynthesis. The largest

difference in these attributes occurs between the c3

and c4 groups of

plants and so it ~s not surprising that important differences in

photosynthetic rates are known between these two groups.

Before we consider. the biochemical aspects of carbohydrate

production we need to consider how the plant captures the last major

input into the photosynthesis summary equation. That input is the energy

itself, in the form of light from the sun.

4) Light

How is the energy carried by light from the sun converted into

chemical energy stored in carbohydrates? Let's look at the first stages

of the answer here.

Light can be considered in two forms, as a wave or as a particle.

When considered as particles, light comes in quanta or photons, packets

10

of energy, each with a specific wavelength. The energy of each photon is

inversely proportional to its wavelength. So photons of the violet and

blue wavelengths are more energetic than the longer wavelength ·Orange 23 .

and red ones. One mole (6.02 X 10 ) of photons is called an einstein.

A pigment (colored molecule) can absorb only one photon at a time

and this photon causes the excitation of one electron. Bonding electrons . . in the stable ground state orbitals of pigment molecules are the ones

usually exited. Each electron is driven away from.the positive nucleus

for a distance that depends on the amount of energy carried by the

photon absorbed. The pigment molecule is then in the excited state and

it is this excitation energy that is used in photosynthesis (Fig. 1.3.).

The photosynthetic pigments can remain in the excited state for

. -9 only very short periods of time (less than one billionth, 10 , of a

second) before the electron moves back to tiu::: ground stat~ and the

energy is lost as heat or as red light (fluorescence). When the full

photosynthetic system is intact as it is in vivo in the plant, the

excitation energy is channeled into photosynthesis.

For photosynthesis to occur, the energy in excited electrons of

various pigments must be transferred to an energy collecting pigment, or

reaction center. There are two kinds of reaction centers in thylakoids,

both with special types of chlorophyll a molecules. Fig. 1.3. shows that

energy in an excited pigment is transferred through a series of pigment

molecules until the energy arrives at the reaction center. So,

excitation of any one of numerous pigment molecules allows the momentary

collection of the light energy in a chlorophyll a reaction center.

Leaves absorb more than 90% of the violet and blue wavelengths that

hit them and almost as much of the orange and red. Almost all this

absorption is by the chloroplasts. Chlorophylls are green and so absorb

green wavelengths relatively ineffectively. If we measure the relative

absorption by a purified pigment as a function of wavelength and plot it

on a graph we get an absorption spectrum. (Fig. 1.4.).

··-----,.

chlorophyll chlorophyll chlorophyll

etc .• to '98ction canter

FIG. 1.3. Model to explain how light energy striking a chlorophyll molecule is given up. Upon excitation the energy can be lo~t by decay back to the ground state (producing heat or fluorescence of red lightr or can be transferred to an adjacent pigment by the process known as inductive resonance.

Fig. 1.4. shows the absorption spectra for chlorophylls a and b.

Note the strong absorption of both ehlorophylls in the violet/blue and

orange/red wavelengths, with little in the green and yellow-green. Some

carotenoids can also absorb light and pass it to the photosynthetic

reaction center, but they only absorb blue and violet wavelengths in

vitro (i.e. outside of the living plant). If we compare the effects of

different wavelengths on the rate of photosynthesis we obtain an action

spectrum. Fig. 1.5. plots the relative rates of photosynthesis for 22

12 •

species of crop plants as a function of the number of photons of each

wavelength striking the leaf.

chlorophyll a

FIG. 1.4. Absorption spectra of chlorophyll a and b dissolved in diethyl ether. Note - the absorptivity coefficient is equal to the absorbance (optical density) given by a solution at a concentration of lg/l with a thickness (light path length) of 1 cm.

I !

i o ......... .____.~__..~__.~___....____....____......__........_ __ ..........

400 e\.\IC. 500 G.reea( 6001 Rea 700 wavelengtn nm

FIG. 1.5. The action spectra of 22 species of crop plants.

Note how the peak in the red region is higher than in the blue.

Also note how high the action of green and yellow light is in

photosynthesis and also that a high amount is absorbed .!.!!. vivo even

though our absorption spectrum (Fig. 1.4.) showed very poor absorption

in these wavelengths. The main reason for this absorption is said to be

that unabsorbed yellow and green photons are repeatedly reflected from

chloroplast to chloroplast with a little absorbed at each reflection,

giving a total absorption of a half or more. Also absorption by

carotenoids in vivo is less in the blue and more in the green, and it is

this absorption that results in much of the photosynthesis from green

light.

If we prov:Cde light of long red wavelengths together with shorter

wavelengths, the rate of photosynthesis is greater than we would expect

from adding the rates obtained if either color was provided alone. This

effect is known as the Emerson enhancement effect (after its discoverer,

Robert Emerson). So the long red wavelengths help the shorter

wavelengths, or vice versa. Two groups of cooperating pigments called

photosystems cause this enhancement~ Photosystem I is rich in

cbloro}lli.yll a com.pared to photosystem II, which contains more

chlorophyll b. Photosystem I is the only photosystem to absorb

wavelengths above 680 nm but both absorb shorter wavelengths. Each

photosystem has some special chlorophyll a molecules that collect

excitation energy from other pigments. These special molecules are

the reaction centers. The reaction center for photosystem II absorbs at

about 680 nm and is called P680 while that for photosystem I absorbs at

about 700 nm and is called, P700. So although P680 and P700 represent

only about 1% of the total chlorophyll molecules present, these special

chlorophylls act as temporary traps for excitation energy which can be '

used in photosynthesis. We will now look at how this energy drives

photosynthesis.

5) Photosynthetic Light Reactions. (Transport of Electrons from Water to

NADP+).

There are two essential functions of light in photosynthesis.·

1) Light provides electrons for the reduction of NADP + to NADPH

(reducing power) (NADP is nicotinamide adenine dinucleotide

phosphate). NADPH reduces co2 in the chloroplasts.

2) Light provides energy to form ATP {adenosine triphosphate)

from ADP and H2Po4- in chloroplasts i.e. the process of

photophosphorylation.

a<h 2+ -a.uP + H2Po4 + light chloroplast)ATP3+ + H 0 2

Once NADPH and ATP have been formed, their energy is used up in the

reduction of co2 and in carbohydrate synthesis.

What is the function of P680 and P700? In their excited states P680

and P700 can, if a suitable primary electron acceptor molecule is

available, transfer an electron to that acceptor. P680 and P700 can

reduce primary electron acceptors that are energetically difficult to

reduce (those that have negative reduction potentials), and each reduced

acceptor can transfer an electron to another molecule with a more

positive reduction potential.

In thylakoids, several kinds of proteins and a few other kinds of

molecules are arranged in the two photosystems to form an efficient

electron transport system that moves electrons froa P680 and P700 to

+ NADP • Once oxidized, P680 is a very strong electron acceptor and can

attract electrons from water resulting in the splitting of water and the

release of oxygen. The function of the electron transport system is to

+ quickly move electrons from water to NADP • The reduction potentials of

these components and their arrangement in the thylakoids ensures that

electron transport occurs from water to P680 in photosystem II and then

on photocxcitation of P700, to the redox components of photosystem I,

+ + -and finally to NADP • During the electron transport process H and OH

are separated across the thylakoid lamellae. This charge separation

lli

provides the potential energy used in photosynthetic phosphorylation.

Remember the channel between lamellae of thylakoids?. This channel is

very important in both electron transport and photosynthetic

phosphorylation. Fig. 1.6. (taken from Salisbury and Ross (1978] will

let us look at the electron transport system in more detail •

. • ;':

2NAOPH+2H+

lhyfakoid c:hlnnel

Photolystllm I

.......... , .......

. + FIG. 1.6. A model of light-stimulated transport of electrons and H in chloroplasts. Two water molecules (at the lower right) are oxidized by the cooperation of both photosystems. The many small circles represent chlorophyll and carotenoid molecules that extend across the lamellae. Each dot within a small circle represents the absorption of one photon. Connecting arrows indicate energy transfer between pigments to reaction center P680 or P700. Transfer of the four available electrons in two water ~olecules requires the absorption of eight photons, four by each photosystem. The two upward arrows from P680 and P700 represent electron excitation and transfer to the corresponding primary electron acceptor, Q or Fd (see text). ATPase proteins are loosely bound to the lamella near the stroma side, with the other proteins buried more deeply.

16

The diagram tries to show the relative energy changes and the

positional aspects of the electron transport chain. It assumes two H2

0

molecules give one oxygen molecule, with the release of four electrons •

. Note, electrons move from water {at the lower right) to NADP + at the

upper left. Energetically this net movement is uphill and is driven by

light, although most of the individual steps themselves. are downhill

{the light driven steps from P680 and P700 are uphill). Also note that

+ water is oxidized inside the thylakoid channel, releasing H , decreasing

the pH, causing a more positive electropotential inside the channel

+ relative to the outer, stroma side of the lamella. The H ions produced

in the channel attract OH into the channel while the excess OH ions on

+ the stroma side attract H • These forces remove water from where ATP and

water are being formed from ADP and H2Po4-, favoring ATP formation. At

the membrane, where it catalyzes the formation of ATP. The H+ released

during the oxidation of water is one of two primary 'sites' of

non-cyclic photophosphorylation. Electrons draw-n from water to

photo-oxidized P680 in photosystem II are accepted by a protein

containing at least four manganese ions.

The primary acceptor to which P680 transfers an electron is of ten

called Q. Its complete structure is unclear, but it may be a quinone.

Various quinones exist in chloroplasts, including the plastoquinones

{PQ). One of the commonest plastoquinones in the chloroplast is

plastoquinone A (PQA). It accepts electrons from Q. Two electrons plus

2H+ reduce one molecule of PQA to PQAH2• Note, a number of herbicides

are known to act by blocking the electron transport system between Q and

PQA. Both monuron and diuron do this. The triazines, atrazine and

simazine, and some substituted uracil herbicides such as bromacil and

17 •

isocil also block electron transport at or near this site.

· ~o iron containing proteins, cytochrome ·f and cytochrome b6 exist

in photosystem I and are known to be part of the electron transport

system. In cytochromes, the iron atom is responsible for accepting and

donating one electron. As PQAH2 transfers one electron to cytochrome f,

+ one B is released to the thylakoid channel and since the model uses

+ four electrons, -~it is repeated four times. This uptake of B from the

stroma side of the lamella and release into the channel is probably the

second site of non-cyclic photophosphorylation. Cytochrome f hands the

electron on to plastocyanin (PC),, a copper-containing protein. Again

repeated four times in the figure. Reduced PC is the electron donor to

cytochrome f (Fe2+) + PC (cu2+) -------) cytochrome f (Fe 3+) + PC (Cu+)

P700 in photosystem I. But remember that photosystem I is also

collecting photons. P700 can only accept an electron if it has just lost

one of its own, and this happens when the energy of a photon collected

in photosystem I is passed to P700. In order for four electrons to fall

down the ene·rgy gradient from PQH2 to P700 between the two photosystems,

the energy of four photons collected in photosystem I has to be

transmitted to P700.

The first acceptor of electrons from excited P700 is ferredo>:in

(Fd). From f erredoxin an electron is transferred ~o a protein called

+ ferredoxin-NADP reductase. This enzyme contains FAD (flavin adenine

dinucleotide), as a prosthetic group (a smaller organic, but non-protein

part of the enzyme). It is the FAD that is the electron acceptor. Two

+ . electrons and two H are needed to convert FAD to its reduced form,

FADH2• So to pass the four electrons driven from P7l», the FAD has to be

18

·reduced twice and re-oxidized twice. This re-oxidation occurs by

reducing NADP + and is the final step in electron transport. NADP +

accepts two electrons as it is converted to NADPH (reducing power); so

two NADPH molecules are formed with the four electrons.

What happens if co2 is in short supply because stomates are closed,

but light is available to drive the light reaction~? This situation may

occur with water stress. Does NADPH accumulate under these conditions

because little co2 is available to accept its electrons and convert it + .

back to NADP ? Two control methods generally prevent this happening.

First there is a pathway for electrons to cycle from f erredoxin back to

P700 via cytochrome b6 • This light-driven pathway of electron transport

from P700 via ferredoxin and then back to P700 is called cyclic electron

transport in contrast to non-cyclic electron transport which is the

normal rout·e of eJ ectrori fl ow to form NADPH. Cyclic electron tr~m:pcrt

+ also requires H uptake at the PQ step~ creating a pH gradient across

the lamella and so again forming ATP by the process known as cyclic

photophosphorylation.

Another mechanism, not shown in the model, for controlling the

amounts of ATP and NADPH is called pseudocyclic photophosphorylation

which is similar to cyclic photophosphorylation but the electrons are

transferred to oxygen, eventually forming water. ATP is formed by this

process, but how important it is in ATP formation is not known.

The following equation is a summary of the light reactions in

photosynthesis:

2H20 + 2NADP+ + 3ADP2- + 3H2Po4- + 8-12 photons -------~

+ 3-02 + 2NADPH + 2H + 3 ATP + 3Hi0

19

So, eight photons are required to oxidize two water molecules and

+ reduce two NADP molecules to two NADPH. Four photons are used in

photosystem II and four in photosystem I, each photon being needed to

push one electron up part of the energy gradient. Since two water

molecules are required to release one o2 and reduce one co2, the model

predicts eight photons are required to fix one co2

molecule. Most

measurements indicate between 8 and 12 photons are needed to fix one co2

molecule, so the model is close to measured values.

Now that we have seen how ATP and NADPH are formed, how do they

enter into the next part of photosynthesis; the fixation of carbon

dioxide into carbohydrates?

6) Fixation of Carbon Dioxide

14 We had to wait until the radioactive isotope of carbon, C , became

available, ~bo~t 194!}, before the sequence of rsac~iona incorporating

co2 into the plant and the production of complex molecules from water,

carbon and oxygen, became known. Paper chromatography allowed the

separation of products, while X-ray film, liquid scintillation counters

and direct chemical analysis were used to determine the amounts of

particular products. Calvin and his group at the University of

California, Berkeley were the first people to use cl4 and paper

chromatography in photosynthesis. They were the first to identify most

of the carbon products of photosynthesis.

Working with the green alga, Chlorella, they found that with an

exposure to c14o2 of just two seconds, most of 14 the C was in a

phosphorylated three-carbon acid called 3-phosphoglyceric acid (3-PGA).

What 2-carbon substance could react with co2 to form 3-PGA? The

researchers never found the 2-carbon substance. Instead they found a 5-C

sugar, phosphorylated at both ends of the molecule, ribulose-1,5-

20

,; .

. .

bisphospha te (RuBP) • Note RuBP was previously known as ribulose-1, 5-

diphosphate (RuDP), and is shown as RuDP in the figures. An enzyme,

ribulose bisphosphate carboxylase, was found to catalyze the combination

of co2 with RuBP, to form two molecules of 3-PGA. This reaction occurs

via an unstable intermediate that splits into two 3-PGA molecules when

water is added.

.. co2 + RuBP 2 3-PGA

Further work showed that other sugar phosphates were formed from

3-PGA in a cycle that is now known as the Calvin cycle. The Calvin cycle

can be summarized as:

3C02 + 9ATP + 6NADPH + 6H+ + 5H20 ---~ 1 triose P + 9ADP + 8Pi + 6NADP+

For each molecule of co2 converted to sugar, two molecules of NADPH

and three molecules of ATP are required.

There are four main parts to the Calvin· cycle:

1. co2

and H2o are added to RuBP to give two molecules of 3-PGA.

2. 3-PGA is reduced to 3-phosphoglyceraldehyde using electrons

provided by NADPH (reducing power) and energy from ATP (both

from the light reactions of photosynthesis) •

3. Some 3-phosphoglyceraldehyde is converted to fructose

diphosphate and part of the fructose diphosphate is converted

to xylulose-5-phosphate. Other 3-phosphoglyceraldehyde

molecules unite with sedoheptulose-7-phosphate to give

ribose-5-phosphate and xylulose-5-phosphate.

Ribulose-5-phosphate is produced from either of these two

pentose phosphates.

2l

4. Ribulose-5-phosphate is phosphorylated by ATP giving RuBP

which can accept co2 and continue the cycle. These reactions

are summarized in Figs. 1.7. and 1.8.

So the Calvin cycle shows how regeneration of RuBP occurs to accept

more co2 and how at the same time some 3-phosphoglyceraldehyde is

channeled for use in the• synthesis of sucrose, starch, cellulose,

pectins and oth4r polysaccharides. Before we go to look at how the

synthesis of these major components of a plant occurs, in some plants

the Calvin cycle is supplemented by another mechanism which first fixes

co2 into organic acids, malic and aspartic acids, not into 3-PGA. This

mechanism turns out to be very important for plants adapted to the

tropics, amongst them crop plants such as maize. This mechanism is

called the ft. dicarboxylic acid pathway.

7) 1he c4 Dicarboxylic Acid Pathway.

In some plants photosynthesis can occur at a higher rate and is

more efficient than in others. These plants fix co2 into carbon-4 of a

four carbon acid, malic or aspartic acid. After about one second of

14 photosynthesis most C is found to be in these acids in this class of

plant and not in 3-PGA, indicating that these acids are the first

products of photosynthesis. So in these species the primary

carboxylation reaction does not involve ribulose bisphosphate. Species

that show four carbon acids as the first co2 fixation products are

called ft. species, while those fixing co2 into 3-PGA first, are c3

species. Most c4 species are monocots and they are generally considered

well adapted to the sub-tropic~ and tropics. c4 species are sub-divided

into those that fix co2 into malic acid and those that fix it into

aspartic acid. Table 1.1 shows some of the more important monocot crops

and weeds, broken down into c4 and c3 types.

22

Firstly C02 (as HC03-) combines with phosphoenolpyruvic acid (PEP)

to form oxaloacetic acid and H2Po4 - • The enzyme, phosphoenolpyruvate

carboxylase (PEP carboxylase) is responsible for this conversion.

Oxaloacetic acid is then converted to L-malic acid by the enzyme malic

acid dehydrogenase, with electrons provided by NADPH. Note, in those c4

species that form L-aspartic acid, the amino acid alanine is required to

transfer an aminb group to form L-aspartic acid plus pyruvic acid.

12 3-PGA

6 ribulose diP

• 6AOP

6 ribulose-5-P

)

/

light react\ions ~chloroplast envelope

,..-12ATP

12 NADPH + 12 H•

12 3-PGaldehycle- -

4Hz0 'or or 2 3-PGalclehyde

·-~ +ATPt--H2 PO:t

ltllrch

-2]fl ~H2PO:t+AOP

+Ao/+ ATP'+ A~ fructosans sucrose cell wall

polysaccharides

FIG. 1. 7. Summary of the Calvin cycle. Six co2 molecules are shown entering the cycle (upper left) with a net output of two 3-C molecules (3-phosphoglyceraldehyde, abbreviated as 3-PGaldehyde), or of one hexose phosphate. Starch is formed inside the chloroplast, but sucrose and polysaccharides other than starch are formed outside. Probably 3-PGaldehyde is the building block for all these molecules. 3-PGaldehyde can be readily transported across the chloroplast envelope. The model shows two 3-PGaldehyde molecules being used in carbohydrate synthesis with 10 others needed to regenerate ribulose-5-phosphate. (Note ribulose diP • ribulose bisphosphate).

23

FIG 1.8.

~/ ' tr1nsport trom ATP ADP ... + NAOPH NAOp+ H po- / chloroplast

COa + H:aO 3 ..,.A \. L 1.3-diPGA \... / L ~ ;~Id /c4> '7

1) -ru 121 131

H:aCOP03 tr ~ I

AOP'{2) Hz10P03tr Hz~~~ ATP_.;/11 C•O OHAP

Hz~OH H~H SOP C•O I I HCOH

HCOH I I HCOH

> I Hf"·· HCOH HzCOP03tr I

Ru-6P ~~l-o 1e1j--;.~o;~,tr HCOH 11. .. po- HC•O

I S7P z • I ~H ~OH HCOH HCOH

I I HzCOP03tr HzCOP03tr

Ri-5P- E4P

HzCOP03tr I C•O I

HOCH ----I HCOH

I H:aCOPO,tr XYL-6P

(5)

HzCOP03tr I C•O I

HOCH I

HCOH I

HCOH I

HzCOP03tr

FOP HzO

{81

HzP04 F6P

-~ The Calvin cycle. ~bbreviations: 3-PGA. 3-phosphoglyceric aeid; 1,3-diPGA, 1,3-diphosphoglyceric acid: OHAP. dihydroxyacetonephosphate; E4P, erythrose-4-phosphate; FOP. fructose-1. 6-diphosphate; ASP, fructose-6-phosphate: SOP. sedoheptulose-1, 7-diphosphate; S7P, sedoheptulose-7-phosphate; Ri-SP. ribose-5-phosphate; xyl-SP, xylulose-5-phosphate: Ru-SP, ribulose-5-phosphate; RuOP, ribulose-1,5-diphosphate. Structures of RuOP, 3-PGA, 1,3-diPGA. and 3-PGald are given in text reactions R10-1 and R10-2. Names of enzymes catalyzing individual reactions are: (1) ribulose diphosphate carbOxylase, (2) 3-phosphOglyceric acid kinase. (3) 3-phosphOglyceraldehyde dehydrogenase. (4) triose phosphate isomerase. (5) aldolase. (6) fructose-1.6 di phosphatase, (7) aldolase. (8) sedoheptulose-1, 7-diphosphatase, (9) transketolase. (10) ribulose phosphate epimerase. (11) ribose phosphate isomerase. (12) ribulose phosphate kinase. Note that aldolase and transketolase have dual reaction specificities. Transketolase transfers a two,.carbOn fragment (attached to a coenzyme, thiamine pyrophosphate. not shown) only from sugar phosphates having a keto group ar C-2 and the specific arrangement of OH and H about C-3 that occurs in S7P, xyl-SP, and F6P. All reactions are pnys101ogically reversible except (1) and the phosphatase reactions (6) and (8).

. There is a division of labor between two types of photosynthetic

cells in c4 species (Fig. 1.9.). We will just consider the grasses. c3

grasses have two bundle sheaths, an inner 'mesotome' sheath without

chloroplasts, surrounded by a second sheath, the parenchyma sheath, with

24

FIG. 1.9. Section of A) c3 species leaf, and B) c4 species leaf.

a few chloroplasts. The bundle-she~th cells of c3 species do not play a

significant role in photosynthetic co2 assimilation and metabolism. The

co2 is fixed in the chloroplasts of mesophyll cells by RuBP carboxylase,

and starch is formed in these same chloroplasts. In contrast, the c4

grasses have one, or sometimes two bundle sheaths. However, the bundle

sheath cells (bundle sheath chlorenchyma) are large compared to the

mesophyll cells and contain specialized chloroplasts. Mesophyll cells

are arranged radially around the bundle-sheath chlorenchyma so that each

mesophyll cell is in direct contact with a bundle sheath cell, or at

most one cell removed. This arrangement of the chlorenchyma is called

25

Kranz (german for halo or wreath) anatomy and is essential to the

functioning of the c4 photosynthetic system.

Species with only the mesotome bundle sheath (this includes maize)

are malic acid formers. They are called NADP-ME type after the

NADP-dependent malic decarboxylase enzyme (malic enzyme) which

decarboxylates malic acid giving co2• NADPH2 (reducing power) and

.• ::

TABLE 1.1. Some c3 and c4 Monocots

Weeds:

Crops:

Weeds:

Weeds:

Weeds:

Crops:

Gramineae (grass family) Agropyron repens (quackgrass) Agrostis alba (redtop) Festuca arundinacea (tall fescue) Phalaris canariensis (canary grass)

Avena sativa (oat) Hordeum vulgare (barley) Lolium multiflorum (Italian ryegrass) Oryza sativa (rice) Triticum aestivum (wheat)

Cyperaceae (sedge family) Cyperus rotundus (purple nutsedge) Cyperus esculentus (yellow nutsedge)

Gramineae Chloridoideae (aspartate formers}

Cynodon dactyion (bermuda grass) Eragrostis spp. (love grass) Panicoideae (malate formers)

Echinochloa crusgalli (barnyard grass) Paspalum spp. Pennisetum purpureum (elephant grass)

Saccharum of ficinarum (sugar cade) Setaria italica (foxtail millet) Sorghum bicolor (sorghum) !!!_ mays (maize) [Euchlaena mexicana ( teosint·e) J

26

pyruvate. Other c4 grasses, which have two bundle sheaths (includes the

chloridoideae) form aspartic acid. Since maize and other c4

cereals are

of the NAPD-ME type let us just consider this.

<al C3 L19ht

(bl C4 -"NAOP-ME type"

Mesophyll

Light u9ht

PS I photosys'tem I PS II photosystem II

NAOPH n1cot1nom1de adenine

ATP GAP RuP Ru IP Rt.BPC PEP PEPC

d1nuc1eot1de phosphate (reduced l

adenosone tropnospnate 9lycera1denyde onospnate robu lose phosphate r1bulose brspnosphate ribulose b1spnosphate carboxylase phosphoenolpyruv1c acid phosphoenolpyruvate corbaxylase

Bundle sheath chloroplast (without 9ronal

Ass1m1lation products

FIG. 1.10. A summary of the carbon reduction pathways found in a) c3 and b) c4 (NADP-ME type) plants, showing the 'division of labor' between·the mesophyll and the bundle sheath chloroplast in the c4 system.

FIG. 1.10 shows the division of labor between the mesophyll and

bundle sheath for a c4 plant of the NADP-ME type (e.g. maize), in

comparison with the carbon reduction pathway in a c3 plant such as

wheat. The malic acid is formed in the mesophyll cells and 3-PGA,

sucrose and starch are formed mainly in the bundle sheath cells. Most

enzymes of the Calvin cycle are present only in the bundle sheath cells

and the complete Calvin cycle is known to occur in these cells. PEP

27

carboxylase is, in contrast, almost entirely restricted to mesophyll

cefl~. So c4 plants really have two types of co2-fixing mechanisms, both

of which operate in the light. co2 first appears in malic acid because

co2

enters mesophyll cells before it could enter bundle sheath cells.

Remember the route that co2 takes into the leaf and how the mesophyll

and the bundle sheath cells are arranged. Table 1.2. summarizes some of

the important p~ysiological differences between c3 and c4 grasses.

Most of the co2 fixed in the c~rboxyl groups of malic acid is

rapidly transferred to the bundle sheath cells via plasmodesmata (pores

in cell wall allowing cytoplasmic connections between cells), and then

enters the bundle sheath chloroplasts. Here malic acid is decarboxylated

+ by the NADP -dependent malic enzyme producing NADPH, (which reduces

3-PGA in the Calvin cycle, as shown in Figs. 1. 7. and 1. 8.), pyruvate

:ine co2 • This source of ?:ADPII is very important 111 malatE: formers

because they cannot generate sufficient NADPH in the light reactions of

photosynthesis in the bundle sheath cells. This is because bundle sheath

chloroplasts have few grana. Photosystem II, upon which NADPH production

by the light reactions is dependent, is located mainly in the grana.

The pyruvate produced by the decarboxylation of malic acid is

transported back to the mesophyll cells where it is used to regenerate

PEP. A chloroplast enzyme, pyruvate, phosphate dikinase carries out this

conversion with ATP as an energy source and H2Po4- as a reactant. This

enzyme is found only in the mesophyll of c4 plants; it is not known in

c3 plants. The co2 generated in the bundle-sheath chloroplasts is then

used in the Calvin cycle.

For each molecule of co2 fixed by the c4 system, five ATP molecules

are required. This is two more than required in the Calvin cycle. There

is no increased NADPH requirement. Even so, at high light intensities

28

TABLE 1.2. Some important physiological differences between c3 and c4 grasses.

Characteristic

Distribution of chloroplasts

Carboxylating enzyme

.•

Theoretical energy requirement (C0

2: ATP:NADPH)

Transpiration ratio (g n2o/g

. dry weight increase

co2 compensation polnt (ppm co2)

Photosynthesis inhibited at ambient atmospheric o2 concentration (21% 02)

Photorespiration detectable

Almost all in mesophyll cells, very few in bundle sheaths

Ribulose bisphosphate carboxylase in mesophyll chloroplasts

1 : 3 2

450 - 950

30 - 70

Yes

Yes

Optimum temperature 15 - 25°C for photosynthesis

Dry matter production (tonnes/ha/year)

29

Specialized chloro­plasts in both the mesophyll cells and in the bundle sheath(s)

Pbosphoenol pyruvate carboxylase in the mesophyll cytoplasm,

·then ribulose bisphosphate carboxy­lase in the bundle sheath.chloroplasts

1 : 5 : 2

250 - 450

0 - 10

lllo

Yes, but only in bundle sheath and at low rates

lO - 40° c

20-25 35-45

and temperatures, c4 plants show higher rates of photosynthesis, on a

leaf area basis, and higher crop growth rates than c3 plants. Under high

light and temperature conditions the atmospheric concentration of co2 is

the usual limiting factor for c4 plants. At temperatures of 25°-35°C and

high light intensities, c4 plants are often twice as efficient as c3

plants in converting light into biomass. (These aspects are very

important for m)Jize and are considered in later units of this module).

Thus, they often have a higher transpiration water use efficiency. Most

of this difference occurs because high light levels cause the liberation

of co2

in c3

plants but to a lesser extent in c4 _plants. In c3 plants

this co2 is largely lost back into the atmosphere.

Now we can look at the biochemical/physiological aspects of this

liberation of co2 by the process known as photorespiration, and also

look ct why the losses of co2 arc minimal in c4 plants.

PHOTORESPIRATION

Respiration in the leaves of c3 species is of ten 2-3 times as rapid

in the light as in the dark and can reach one half of the photosynthetic

rate. This respiration in the light is really made up of two components,

the respiration that also occurs in all plant parts in the dark i.e.

dark respiration, and an additional, more rapid process called

photorespiration. Dark respiration occurs in the cytoplasm and in

mitochondria while photorespiration occurs cooperatively in chloroplasts

peroxisomes and mitochondria. Photorespiration is stimulated by high

light intensity, high 02 levels, low co2 levels aad high temperatures

(Fig. 1.11.). Photorespiration involves the oxygenation of ribulose

bisphosphate (RuBP) and metabolic pathways taken by the P-glycolate .

carbon produced by the oxygenation.

Oz~ ~C02 Photosynthesis l - ..... s

.• ) c::::) ( Epidermis >!c:::: ,. •

Effect pf jncrcut jn· 00 ycllng_

~ Down Up

c~ Up Ollwn

light Up Up

-Temperature Up Up

light off Stops lurst -

~lG. 1.11. A summary of how environmental facturs afft!ct L11e Lctl.: uf photosynthesis and the rate of photorespiration.

Photorespiration is an integral part· of photosynthesis in c3

plants. It is less important in c4

plants, firstly because less occurs

(the co2

concentrating mechanisms and high co2

concentrations in the

bundle sheath competitively inhibit the oxygenation of RuBP and so

inhibit photorespiration} and secondly, because any co2 liberated by

photorespiration can be re-fixed in the mesophyll by PEP carboxylase.

Because photorespiration is such a drain on C assioilation in c3 plants,

attempts to reduce it are the subject of much research, but have met

with little success.

Fig. 1.12. is a simplified view of the flow of carbon and of

nitrogen in photorespiration. Note that aspects of photorespiratory

carbon and nitrogen metabolism occur in three organelles, the

flmglprhen

fADH 2G1yox1late

~~-z;.,. :""'~ lGlrCine

COz

flpw pf njtrm-n

I

GIUta' lne ; ·--

1Glyoxytate

"'t~:it-., •. ~ l ~ 1Glyclne

Serine- ..- z.Gtrclne

NHJ{_

FIG. 1.12. The cycling of carbon and nitrogen in photorespiration.

chloroplast, the peroxisome and the mitochondrion. 02 can inhibit co2

fixation by ribulose bisphosphate carboxyla~e; both o2 and co2 compete

for RuBP. RuBP carboxylase can catalyse the oxidation of RuBP by o2• So

RuBP carboxylase can act as an oxygenase. The products are 3-PGA and

phosphoglycolate (P-Glycollate in Fig. 1.12.). P-glycolate is converted

to glycolate by P-glycolate phosphatase. Both of these reactions occur

in the chloroplast. Glycolate is transported to the peroxisome where it

is oxidized to glyoxylate, followed by transamination to glycine. We

will not name all enzymes here. Glycine then enters the mitochondrion

where it is converted to serine and photorespiratory co2• This reaction

is the major, if not only .• source of co2 released in photorespiration.

Serine returns to the peroxisome and is deaminated and reduced to

glycerate (glyceric acid~. The glycerate enters th~ chloroplast where it.

is phosphorylated to give PGA which can re-enter the c3 photosyntheti~

cycle.

i

The cycling of ammonia is an important, but often neglected, aspect

of photorespiration (Fig. 1.12). Nitrogen enters the cycle through the

amination of glyoxylate by glutamate in the peroxisome. Half of the N is

released as NH3 during glycine oxidation in the mitochondrion while the

other half is returned to the peroxisome in serine and donates the amino

group to glyoxylate. The NH3 is re-incorporated into glutamate (free NH3

is toxic to the ·plant) giving glutamine which is re-cycled. Although the

biochemical pathways of C and N in photorespiration are now well

understood we still know very little about transport between the

organelles.

We should note that most of the apparent photorespiratory loss of

co2 in c3 plants is not really a release of co2 from glycine but simply

the failure of co2 to be fixed in the Calvin cycle because of o2 comp.::tition. In these plant.o, since: o2 and co2 are competing for th.::

same substrate (RuBP) and enzyme (RuBP carboxylase), the lower the co2

concentration relative to o2 , the less likely the co2 will be fixed and

the more likely it is that phosphoglycolate will be formed. Each

molecule of o2 consumed by the RuBP oxygenase reaction prevents the

fixation of one co2

molecule, and leads to the photorespiratory

evolution of half of a co2 molecule. So direct inhibition of

photosynthesis by o2 accounts for two-thirds of the total o2 inhibition

of photosynthetic co2 fixation. What happens in c4 species?

c4

species can be distinguished from c3 species on the basis of

several photosynthetic co2 exchange responses •

. 1) Oxygen concentrations of up to 60% have little or no

inhibitory effect on the light-limited or light-saturated rate

of photosynthesis by c4 plants.

2) Leaves of c4 species do not envolve co2 in the light.

33

3) C species have a compensation point of less than 10 ppm (10µ 4

liter/liter).

They thus appear to lack photorespiration and the associated o2

inhibition of photosynthesis, characteristic of c3 species. The growth

rate of c3

plants is also essentially unaffected by increases in co2

concentration or by o2 levels of up to 40%. However, the RuBP

carboxylase/oxyg~nase extracted from c4 tissue has very similar kinetic

properties to that 11:' c3 tissue. Alth~ugh isolated c4 mesophyll cells

cannot oxidize glycolate or glycine to co2 , isolated bundle sheath cells

can, and are sensitive to o2 concentrations. So, c4 species can

metabolize P-glycolate to co2 in the bundle sheath via the

photorespiratory cycle. So, does this liberation of co2 occur, and if

so, why does this not result in the loss of co2 as in c3 plants?

The absence of detectable photorespiration in intact leaves of c4

plants has most frequently· been attributed to an efficient co2

re-fixation mechanism linked to the special_ leaf anatomy. co2 produced

by photorespiration in bundle sheath cells is considered to be refixed

by PEP carboxylase in the mesophyll before it escapes from the leaf.

Recent evidence, however, indicates that re-fixation is not the primary

mechanism by which c4 plants reduce photorespiratory losses. From c14

labeling studies the flow of C through glycine and serine in the

photorespiratory cycle is now known to be much less than in c3

plants.

So since the glycine to serine conversion is the major source of co2

lost by photorespiration this suggests that, even in the bundle sheath

cells, the rate of photorespiration in c4 plants is much lower than in

c3 plants.

But why does the bundle sheath of c4 plants photorespire less than ·

the mesophyll of c3

plants? The most commonly accepted answer at the

•

. - '

:','

moment stems from the fact that co2 competitively inhibits RuBP

carboxylase from acting as an oxygenase. At atmospheric levels of o2

,

photosynthetic co2 fixation by the c4 cycle in mesophyll cells proceeds

unhindered. leading to an increased co2 concentration in the bundle

sheath. The elevated co2/o2 ratio in the bundle sheath allows co2 to

compete more effectively with 02 for Ru BP carboxylase during bundle

sheath photosynthesis, reducing o2 inhibition of net co2 uptake in the

leaves of C4 plants. In addition, the increased level of co2 in the

bundle sheath will reduce the formation of P-glycolate by RuBP

oxygenase, thereby decreasing the amount of glycolate available for

oxidation to co2 in photorespiration. Should any photorespiration occur

then the co2 produced can be re-fixed by PEP carboxylase and/or RuBP

carboxylase before it can escape from the leaf.

In conclusion, c4 plants have th~ potential foe glycolate synthesis

and its metabolism to co2 in the bundle sheath.by the process known as

photorespiration. They are also capable of re-fixing photorespired co2

in mesophyll cells with the help of PEP carboxylase. However, the lack

of photorespiration and lack of o2 inhibition of photosynthesis in c4

plants is mainly due to the increased co2 concentration at the site of

RuBP carboxylase/oxygenase activity in the bundle sheath, resulting from

the co2-concentrating mechanism of the c4 cycle. The refixation of

photorespired co2

by PEP carboxylase is probably a contributing factor,

but is not.considered a major component in~·

SYNTHESIS OF SUCROSE AND POLYSACCHARIDES

The normal end products of co2 fixation are sucrose and starch.

Free hexose sugars such as glucose and fructose are less abundant. In

many grasses, especially temperate ones, starch (made. up of l,OOO's of

•

i' I,

glucose units) is less important and instead polysaccharides called

fructosans (made up of between three and 35 fructose units) predominate.

Sucrose is the most abundant sugar in most plants and it is the

sugar most commonly translocated. in the phloem of plants. The process of

sucrose synthesis begins with the transport of some of the

3-phosphoglyceraldehyde out of the chloroplast, where some molecules are

converted to g~ucose-1-phosphate and fructose-6-phosphate. These two

sugar phosphates are the two basic hexose units needed to form sucrose,

but they cannot combine directly to form sucrose. Instead, some ATP is

required to combine these two sugar phosphates; the reaction taking

place outside of the chloroplast. The glucose unit in

glucose-I-phosphate is activated by energy from another nucleotide

triphosphate, uridine triphosphate (UTP), not from ATP. Pyrophosphate

and a nucleotide suear cal] ed url<tine dipbm~phogJucose (tmPG) .ATe

formed. Another nucleotide sugar, adenosine diphosphoglucose (ADPG), can

also be used in sucrose synthesis. Sucrose phosphate synthetase is the

enzyme involved in the addition of UDPG to fructose-6-phosphate, forming

sucrose-6-phosphate. A glycosidic bond is formed between C-1 of glucose

and C-2 of fructose-6-phosphate as seen in Fig. 1.13. on the structure

of sucrose.

UDPG + fructose-6-phosphate ----~ sucrose-6-phosphate + UDP

Finally the phosphate is hydrolyzed from sucrose-6-phosphate by a

phosphatase enzyme, giving sucrose.

36

.. HQC;zOH O :0HP.

2C O H

HO OH H O H HQ CH2 QH

H HO OH H C·1 C-2

FIG. 1.13. Sucrose, a disaccharide.

The energy from ATP is used to regenerate UTP, allowing the process

tv conti~uc. ?!ot~, there io ~nether ~oute fer t~c p~cduct!cn cf s~crose

from UDPG and free fructose. This involves the enzyme sucrose

synthetase, but it is not very important in photosynthesizing cells.

The formation of starch is a very important event for us as cereal

production scientists because most of the dry weight of the economic

product, the cereal grain, is starch. Starch, in contrast to sucrose and

most other carbohydrates is formed inside the chloroplast. Two forms of

starch can be identified in the chloroplast, amylose and amylopectin.

Both are made up of D-glucose units connected by a 1,4-linkages, but

differences occur in their branching patterns. The a 1,4-linkages cause

the chains to coil into a helix as shown in the following figure, Fig.

1.14. Amylopectins are branched molecules, with the branches occurring

between C-6 of a glucose in the main chain and C-1 of the first glucose

in the branch chain. Amylopectins have between· 2,000 and over 200,000

glucose units while the almost entirely unbranched amylose molecules

37

FIG. 1.14. Organization of glucose molecules in amylose and amylopectin.

have a few thousand sugar units. The starch in the endospe?'lll of the

so-called "waxy" cereals (i.e. maize. sorghum, barley and rice) is

almost entirely amylopectin, although some high amylose genotypes are

known (e.g. high amylose maize has between 50 and 80% amylose).

The first step in the synthesis of starch in chloroplasts involves

the formation of glucose-1-phosphate, as in sucrose synthesis. There are

three enzymes known to form the a -1,4 bonds in both amylase and

amylopectin, but their relative importance is uncertain. Starch

phosphorylase is one of these enzymes, but this enzyme is thought to be

·more important in starch degradation, not starch synthesis. The other .

two enzymes use an activated form of glucose, rather than

glucose-1-phosphate directly. ADPG-starch transglucosylase transfers

glucose from ADPG while UDPG-starch transglucosylase does the same from

+ UDPG. Both enzymes are activated by K ions:

... ADPC (or UDPC) --------> starch + ADP (or UDP).

38

:

ADPG is considered much more important than UDPG for starch

synthesis in chloroplasts and in growing seeds. The UDPG (or ADPG) is

formed as follows:

UDP (or ADP) + sucrose ---~UDPG (or ADPG) + fructose

The fructose released in this reaction can also be converted, via

glucose-I-phosphate, to starch. So both hexose units of sucrose can be

used for starch synthesis. A summary of starch synthesis is given in

Fig. 1.15.

FIG. 1.15. A summary of the reactions involved in starch synthesis

Other important carbohydrates in plants are the cell wall

polysaccharides. Primary cell walls in plants consist of three major

fractions, cellulose, hemicelluloses and pectins. These fractions are

built up from five sugars and five sugar derivatives. There are many

ways in which these sugars can be connected because each hydroxyl group

represents a potential branch point, but the complexity of cell wall

polysaccharides is less than we might expect.

The five sugars involved are D-glucose, D-mannose, D-galactose (all

hexoses) and the pentoses, D-xylose and L-arabinose. The sugar

, i

derivatives include two uronic acids (D-glucuronic acid and

D-galacturonic ~cid) and two deoxy sugars (L-rhamnose and L-fucose). The

fifth sugar derivative, galacturonic acid methyl ester, is present only

in the pectin fraction. Primary cell walls also contain about 10%. by

weight of glycoprotein.

The least complex cell wall polysaccharide is cellulose. Cellulose

molecules are l~ng, unbranched and not coiled. The number of D-glucose

units in a cellulose molecule is of~en around 2 ,000 in primary cell

walls and at least 14,000 in some secondary walls. The linkage between

H OH

0

CH2 0H H OH

FIG. 1.16. How glucose residues are linked together to form cellulose.

The J3 linkage.

the D-glucose units differs from starch. In cellulose it is a B 1 ,4

linkage. About 40 cellulose chains, each containing two cellulose

molecules, are held together along their axes by hydrogen bonds to form

microfibrils. Together these chains of microfibrils give strength and

strongly resist stretching. Embedded between the microfibrils lie most

of the other polysaccharides (the pectins and hemicelluloses) and the

wall protein. Usually the cellulose makes up less than 20% of the cell

wall and sometimes less than 10% in grasses. The rest is usually made of

hemicellulose polymers such as xyloglucan and arabinogalactan and the

pectin polymer, rharnnogalacturonan. Xyloglucan is H-bonded to cellulose •

while all the other constituents are covalently bonded to each other,

' forming one giant 'molecule'.

Non-cellul&se polysaccharides are not synthesized in the wall.

Instead they are produced in the dictyosomes of the Golgi apparatus.

Vesicles from the dictyosomes move out to fuse with the plasma membrane,

allowing the secretion of the polysaccharides. This . method of

polysaccharide production and incorporation into the primary cell wall

does not include cellulose. Where cellulose is synthesized and how it is

incorporated into the cell wall are largely unsolved. The biochemistry

of the synthesis ot all cell wail polymers is largely unknown, although

the sugars must first be combined with a nucleotide such as UDP or

guanosine diphosphate (GDP) to form UDPG or GDPG, before they can be

polymerized.

RESPIRATION

Respiration (dark respiration) is essentially the reverse of

photosynthesis. The common respiration of glucose can be written as:

c6H12o6 + 602 --------)6co2 + 6H2o + energy

Compare this with the summary equation for photosynthesis at the

beginning of the section on photosynthesis in this unit. About 60% of

the energy released during respiration is in the form of heat and is

usually lost to the soil or the atmosphere. The other 40% is not lost as

heat but is captured in compounds that can be used in the many reactions

of growth and maintenance that can only occur if sufficient chemical

41

energy is available. We have already encountered these energy rich

compounds in photosynthesis.. The most important compound is ATP, with

NADH and NADPH playing major roles, especially in the transfer of

electrons.

Respiration, like photosynthesis, is not just one reaction, it is a

series of reactions, each catalyzed by a different enzyme. The gradual,

stepwise breakdpwn of large molecules allows energy to be trapped in

ATP, NADH and NADPH and also provides carbon skeleton intermediates for

other essential compounds, such as amino acids for proteins, nucleotides

for nucleic acids, carbon precursors for .porphyrin pigments (such as

chlorophyll and cytochromes), fats, sterols etc. Thus not all of the

potential respiratory substrate is fully oxidized to co2 and H2o, some

is used in synthetic (anabolic) processes. Energy trapped during

oxidation can be used to synthesize large molecules required for growth.

So, when plants are growing rapidly, most of the sugars being used up

are diverted into the synthesis of large molecules. They are not

respired, with the production of co2.

1) Formation of Hexose Sugars From Polysaccharide Reserves.

Starch is a very important storage polysaccharide in plants. It is

stored in chloroplasts as water-insoluble granules or grains. Starch

stored in this form is a very important food reserve for leaves. Another

very important role for starch is as the primary food reserve in most

seeds. The starch stored in the endosperm of a maize seed is broken down

to sucrose and glucose once the seed starts to germinate. Some of this

is respired to produce energy for growth, while the rest is transpo~ted

to the root and shoot where it may be respired or used to synthesize

other molecules, such as cell wall polysaccharides.

Most steps in the degradation of starch to glucose can be catalyzed

by three enzymes, but others are necessary to complete the degradation.

42

•

!

'

The first three enzymes are alpha amylase, beta amylase and starch

phosphorylase. It seems that only a -amylase can start to break down

intact starch molecules, the other two enzymes attacking the first

products released by a-amylase. Alpha amylase randomly attacks the a 1,4

linkages throughout the amylopectin and amylose molecules, releasing at

first large, complex products; but later, products with about 10 glucose

units called d~xtrins, and eventually, a -maltose (a disaccharide of

two glucose units) and glucose from amylose. However, a-amylase cannot

attack the 1,6 bonds at the branch points in amylopectin so amylopectin

breakdown stops when branched dextrins of very short chain lengths still

remain.

Beta amylase can hydrolyze starch into B -maltose, starting from

one end of the starch molecule. However, as with a -amylase, the 1,6

bon.! in amylcpect:!.n cannot be atta.:ked. Fur e:c.ch bu11d eleaved, uue n2o

molecule has to be used by these enzymes. For this reason these enzymes

are called hydrolytic enzymes.

The third enzyme, starch phosphorylase, breaks down starch by

incorporating phosphate into the products, not water as the amylases do.

This produces glucose-I-phosphate. It is therefore a phosphorolytic

enzyme. Note, this reaction catalyzed by starch phosphorylase is

reversible, unlike the reaction catalyzed by the amylases. Again the 1,6

bond in amylopectin is not broken. All three enzymes appear functional

in leaves, with the initial attack being by a-amylases. However, in

germinating cereal grains, only the amylases are active •

. The 1,6 linkages in amylopectin not attacked by these enzymes are

broken by a debranching enzyme, sometimes called the R enzyme, and by

dextrinases, producing new end groups available for attack by the other

enzymes. This allows the complete digestion of starch into ·glucose,

maltose and glucose-i-phosphate.

43

The maltose is slowly hydrolyzed to glucose by a -amylase or more

rapidly by maltase. The glucose units are now available as building

blocks for other polysaccharides or are channeled into respiration.

In many grasses and in the temperate cereals. the principal stored

carbohydrate is not starch but fructosans. made up of between 10 and 35

fructose units. Most fructosans in grasses have fructose units connected

by S-2 9 6 linkages. These linkages are hydrqlyzed by the enzyme ::

-S -fructofuranosidase.

We have seen how glucose, glucose-I-phosphate and fructose, the

substrates of respiration, are produced. Now we can look at respiration

itself. ·There are three main parts to respiration. These are glycolysis

(Embden-Meyerhof-Parnas pathway), the tricarboxylic acid cycle (TCA

cycle, or Krebs cycle) and the electron transport system (linked with

oxidative phosphoryl Rt ion). WP. wi Jl Jook at each of these p!'oceeses in

turn.

2) Glycolysis

Glycolysis is a· group of reactions in which glucose, glucose-1-P or

fructose are converted to pyruvic acid in the cytoplasm, not in an

organelle. Really. the reactions of glycolysis are part of the process

of fermentation, by which sugars are converted to ethanol and co2 , or to

lactic acid or malic acid. Fig. 1.17. is a simplified version of

glycolysis showing the main compounds involved but omitting enzymes etc.

(compiete diagrams can be found in many biochemistry textbooks).

Note that glycolysis does not involve any reaction in which o2 is

absorbed or co2 released. These reactions occur later in respiration.

Glycolysis has three important functions in plants:

1) Molecules are formed that can be removed from the pathway to

synthesize other plant constituents (Fig. 1.18).

44

!

•

St8fh + (Phosphoryl->

Glucose-I-phosphate (He•ok.inase) J (Phospho1lucomutuc)

Glucose---• Glucose-6-phosphate . ATP I

.•

t (Phosphoheaoisomcra~)

Fructose-6-phosphate . ATP J (Phosphoheaoki11a1e)

Fruftose-1,6-diphosphate 't (Aldolase)

. Glyferaldehyde-3-phosphate - Dihydroxyacctone ph~sphate 2 NAD• + (TrioS<phosphatc dchydrogen-) .

2 I 3-Diphosptioglyceric acid 2 ADP l (Phospho1lycerate kinase)

2 3-Phosphoglyceric acid'+ 2 ATP J (Phosphoglyc:eromu1ase)

2 2-Phosphoglyceric acid J (Enolase) .

2 P~osphoenolpyruvic acid 2 ADP ~ (Pyruvate kinase)

/ 2 Pyruvic ac!d + 2 ATP :-

FIG. 1.17. A simplified diagram of the pathway of glycolysis in plants.

To form many large molecules such as lipids, proteins and nucleic

acids the high transfer potential of the tenrlnal phosphate bond

and ATP is required and in some reactions the e1ectrons present in

NADH and NADPH are also needed. So, respiratf.on is required to

provide energy for the production of these large molecules and the

first two parts of respiration, glycolysis and the TCA cycle,

provide the chemical building blocks (Fig. 1.18).

2) We have already mentioned the second major function of glycolysis.

This is the production of ATP. Two ATP molecule<s are required in

the utilization of glucose or fructose, but la;ter two ATP' s are

released for each 3-carbon unit participating in ~he pathway. Since

two 3-carbon units are produced per hexose sugar unit then four

45

l'

ATP's are produced per hexose unit; giving a net production of two

ATP' s per hexose. However, if glucose-1-P, glucose-6-P or

fructose-6-P is the substrate then one less ATP molecule is used to

start with, giving a net production of 3 ATP molecules per molecule

of hexose ~hosphate.

.• ,. ltfth

1 hex cm-phosphates

l tr lose-phosphate

' · phosphoeuolpyrwic ICid

- ::

cell well

glycerol of fits. oils. phospholipids

serine --- cvsteine - protein phenolic compounds (tyrosine. phenylalanine. anthocyanins. lignins. tryptophan, euxin hormone) I

pyrwic1eid

ethanol

/ lactic acid

k::'_ 1lanine ---- protein

j ecetyf CoA

fatty ICids, CUlicular compounds

~eno~ds lcarotenoids. phytol of chlorophyll, sterols. gibberelhns, misc. terpenes) _

111191'11 aromatic compou_nds le. g., ring A of flavonoidsl

.,,.rlgine - . citric ICid

L ./ .,,.,tlC --- ............ F . ICid "'- chlorophyll

proteiy \ oxaloacetic acid isoc:itric ICid /

pyrimidines ) ( \ /~tHmi;::ic9::~ine L ot!"' amino (

ICids -::aloids malic\- -~· -1- /J~~ IUll'llfic ICid IUCCinyl CoA

other amino ecids

' IUCClnic _/ I ICid

cvtoc:hromes. phytochrome>

FIG. 1.18 Diagram to show the roles of glycolysis and TCA cycle intermediates in the formation of other plant constituents.

46 -

•

.

3) The third essential function of glycolysis is the production of

another energy-rich molecule, NADH. NADH (or in leaves, both NADH

+ + and NADPH) is formed by the reduction of NAD (or NADP ) during the

oxidation of 3-phosphoglyceraldehyde to 1, 3-diPGA. This is the

reverse of what happens in photosynthesis. The NADH is used either

as a source of electrons in the many anabolic reactions in the

plant or -~t enters the mitochondria where electron transport

reactions oxidize it and convert the energy into two ATP molecules.

Whether NADU is oxidized in the mitochondria. or used to drive

other reductive reactions, is dependent on the internal o2 + concentration. Mitochondria cannot oxidize NADH back to NAD unless o2

+ is present. In the absence of o2, there is soon less NAD because it is

converted to NADH, and the pyruvic acid is reduced Co form ethanol and

cc2, or lactic scid, by the process vf fermentation.

In order to continue the process of respirati.on, the product of

glycolysis, i.e. pyruvic acid, has to pass to the -.1..Cochondrion. We can

now have a quick look at the structure of this organelle.

3) Mitochondria

Each plant cell contains many mitochondria, frequently about 200,

but values as high as 2,000 per cell have been reported. A typical

mitochondrion is about one micro meter long and like the chloroplast it

has a double membrane with an extensive inner me!l!lbrane system (Fig.

1.19.). The inner membrane of the mitochondrial envelope is highly

convoluted, protruding into the interior in sheet like patterns in many

places. Each convolution is called a crista (plural: cristae). In many

cases, one crista is fused to another in the interior of the

mitochondrion, forming a continuous sac-like compartment. The cristae

contain most of the enzymes that catalyze the steps of the electron

47

t~ansport system following the tricarboxylic acid cycle. So the

increased surface area provided by the cristae is very important. The

tricarboxylic acid cycle reactions occur in the protein rich region

between the ~ristae.

FIG. 1.19.

,• :_.

An idealized drawing of a mitochondrion with cristae.

4) The Tricarboxylic Acid Cycle (TCA or Krebs Cycle)

The initial step leading to the TCA cycle involves the loss of co2

from pyruvic acid and the combination of the remaining two-carbon

acetate unit with a sulfur-containing compound, co-enzyme A (CoA) to

produce acetyl CoA. This reaction is not reversible. Besides the loss of

co2 , two hydrogen atoms are removed from pyruvic acid during the

formation of acetyl CoA. The hydrogen atoms are finally accepted by

+ NAD , yielding NADH. The complete TCA cycle is shown in Fig. 1.20;

r

The TCA cycle has the following main functions:

1) Production of the electron donors, NADH and FADH2• These donors are

oxidized in the electron transport part of respiration.

2) Synthesis of a limited amount of ATP (one ATP for each pyruvate

oxidized. In other words half of that formed in glycolysis).

o,,~ c I C•O I CH,

thiamine pyrophosphate. lipoamide: coenzvmes ·CoASH

H:zO

C02

o, C-SCoA I

CH3

ecetyl CoA

H20~ W +NAOH ~OOH

' r-n NAO•~ i-""' OKaloacetic CH2 acid I

COOH COOH I

HOCH malic acid I CH, dehydrofena•

1~00H

L-malic acid

· fumarntt

COOH I CH I

HC I

COOH

fumaric ~AOH2 acid FAD

iuccinic COOH dehydrogena1t1 I

CH2 I

CH2 I

COOH

ATP

) H•

CoASH

COOH I

CH2 I

I z COOH _,;,_, . ..

citric acid Fe

HO-~:C~COn HzO

~· ~OOH

thiamine pyro-P

HCOH I

HOOC-CH I

CH2 I COOH

isoci tric acid

~ac~·d ~,,..

Mn-

~ COOH NAO• I C-0 NAO+ I

?Cz fHz COOH

H•

11 laeloglutaric acid

FIG. 1.20. Reactions of the tricarboxylic acid cycle.

49

3) ·Formation of carbon skeletons that can be used to synthesize

certain amino acids which can be converted into larger molecules

(see Fig. 1.18.)

So the TCA cycle removes electrons from organic acid intermediates

and transfers them to NAD+ or FAD. Note that NADP+ is not used as an

electron acceptor, unlike in the chloroplast where NADP + is the most

common electro~1:.acceptor. The one molecule of ATP is formed from ADP and

B2Po

4 - during the conversion of succinyl coenzyme A to succinic acid.

Two more co2 molecules are released in the TCA cycle and a net loss of

both the carbon atoms from the incoming acetate of acetyl CoA. Note, no

co2

is absorbed during the TCA cycle.

An overall equation for the TCA cycle is:

+ ·-2 -2 pyruvate + 8NAD + 2FAD + 2ADP + 2H2Po4 + 4H20 ---..!)

6Cu2

+ 2ATP-l + B NADii + 8H+ + 2F'ADH2

S) The Electron Transport System

When NADH and FADH2, produced in glycolysis or in the TCA cycle,

are oxidized, ATP is produced. This oxidation involves the uptake of o2

and the production of water, but neither NADH nor FADH2 can combine

directly with o2 to form H2o. Instead, the electrons have to be

transferred via several intermediate electron carriers, collectively

called the electron transport system of the mitochondria. Note, in some

+ ways this is similar to the transport of electrons from H2o to NADP via

a 'bucket brigade' of acceptors in the photosynthetic light reactions in

the chloroplast •

. In the mitochondrion, the transfer of an electron proceeds from

carriers that are difficult to reduce (have low reduction potentials) to

those that have a greater tendency to accept electrons (higher reduction

potentials). Oxygen has the greatest tendency to accept electrons and

..

..

this occurs as the last step in electron transport. The acceptors are

thought to be arranged on the cristae in order of acceptance of

electrons, with several thousand electron transport systems per

mitochondrion.

As with the chloroplast electron transport system, the

mitochondrial system involves cytochromes. Also involved are several

flavoproteins, •ome iron-sulfur proteins similar to ferrodoxin, another

cytochrome-containing substance called cytochrome oxidase, and a few

other unidentified electron carriers. A simple model of the

mitochondrial electron transport system is presented in Fig. 1.21.

The most important feature of the electron transport system is the

formation of ATP from ADP and H2Po4-, driven by the flow of electrons to

Or This production of ATP in the mitochondrion is called oxidative

phc~;:hcry!ation. Compared to the production of ATF by glycolysis, AT?

production by oxidative phosphorylation is very efficient, with in most

·cases three ATP molecules produced per NADH (if it comes from the TCA

cycle). Clearly this is why fast growing organisms need a good supply of

as:

A summary equation for the electron transport system can be written

10 NADH + lOH+ + 2FADH2 + 32 ADP-2 + 32 B2Po4- + 602 ---~

10 NAD+ + 2 FAD + 32 ATP-3 + 42H20

This assumes both of the NADH molecules from glycolysis along with

eight NADH molecules, and two FADH2 molecules from the TCA cycle are all

oxidized.

The amount of ADP and H2Po4- available can liait tbe rate of ATP

produced and the amount of o2 used. It does seem that the processes of

oxidative phosphorylation and electron transport are very closely

51

related to each other. ATP synthesis in the electron transport system

depends on the formation of a pH gradient across the inner mitochondrial

+· membrane with the direction of H movement toward the outside of the

+ membrane. As o2 pulls electrons from NADH or FADH2 to form n2o, H ions

are removed from the inner matrix between the cristae and are released

into the space between the two membranes. During the formation of ATP

and a2o from ADP and H2Po

4-, the OH- ions in this n2o are attracted to

the high pH region in the matrix. This removal of n2o allows ATP

formation to occur.

Note - while aerobic respiration of animals and some plants is

strongly inhibited by certain negative ions such as cyanide (CN-) and

azide (N3-), and by carbon monoxide forming a complex wit·h the iron in

cytochrome oxidase, many other plants are little affected. This is

electron transport system (See Fig. 1.21.)

One final point in this section on the biochemistry of respiration.

There is another pathway, other than through glycolysis and the TCA

cycle, by which plants can obtain energy from the oxidation of sugars

into carbon dioxide and water. Because five-carbon sugar phosphates are

the intermediates, this other pathway is often called the pentose

phosphate pathway. We will not outline the pathway here; it can be found

in many modern plant biochemistry and physiology textbooks. The pentose

phosphate pathway is important in plants because it acts as a mechanism

for glucose breakdown, it provides reduced NADPH for synthetic

reactions, it provides ribose-5-phosphate, used in nucleotide and

nucleic acid synthesis, and because the erythrose-4-phospaate needed in

the synthesis of lignin and other aromatic compounds is produced in this

pathway.

52

• ~

~'. .·

•

• ...

~ ZW+1'0z -T ( exogenous NAOH r fllvoprotein ---

1 ~ H:zO

.~ cyeniclH9list1nt PlthMv ...._,otein

' \ . . cytOChroma • Z W i 1' Oz

FADH, y .;:::---~' - .. .,-... ., - ... :;:-• H,D

FAD fe.Sprotein ~ I . ,. ~

NH~)~ 2W

-7"' 2W NADH

nG. 1.21. A proposed pathway of electron transport in· the inner mitochondrial membrane. The main pathway (which is cyanide sensitive) can accept electrons from FADH2 or NADH, both of which usually obtain their elect~ons from the TCA cycle. NADH also comes from glycolysis. The FAD and NAD regenerated are cycled back for use in glycolysis and the TCA cycle. There are three sites of ATP formation (oxidative phosphorylation) for each NADH arising from the TCA cycle but only two sites if the NADH comes from outside of the aitochondrion, as in glycolysis. Cytochrome oxidase catalyses the absorption of o2 and production of H

2o. It is this last step that is sensitive to cyanide,

azide and carbon monoxide. The oxidase in the cyanide-insensitive pathway (top of figure) has not been identified.

So, by adding together each of the three summary reactions for

resp.iration we can produce the following net equation for the

respiration of glucose.

glucose -2 -(c6n12o6) + 602 + 36ADP + 36 H2Po4 ----~

6C02

+ 36 ATP-l + 42 n2o

53

Among the products in this net equation of respiration, only the

terminal phosphate bond of ATP is additional usable energy for the

plant. How much of the energy in glucose (not directly usable) can be

trapped as readily usable energy in the terminal phosphate bond of ATP?

The available energy contained in one mole of glucose or fructose is

-686,000 cal. The available energy in the terminal phosphate in each

mole of ATP is ~bout -7,600 cal/mole, or -273,500 cal in 36 moles of ATP

(36 molecules of ATP are produced per molecule of glucose). This is an

efficiency of about -273,600/-686,000, or about 40%. The other energy,

about 60%, is lost as heat.

• l ,.

• A

•

• '

REFERENCES

1) Govindjee (1982). Photosynthesis Vol. II. Development, Carbon

Metabolism, and Plant Productivity. Academic

Press.

2) _ Ray, P.M. (1972). The Living Plant, 2nd. Edition. Saunders

College Publishing. Ch. 3.

3) Salisbury, F.B. and Ross, C.W. (1978). Plant Physiology, 2nd.

Edition. Wadsworth. Chs. 9, 10, 12.

4) Stoskopf, N.C. (1981). Understanding Crop Production. Reston

Publishing Co. Chs. 1, 5, 9.

5) Zelitch, I. (1971). Photosynthesis, Photorespiration, and Plant

Productivity. Academic Press •

. . 55