Photosynthesis AP Biology

Mrs. Stahl

The Process That Feeds the Biosphere

• Photosynthesis is the process that converts solar energy into chemical energy and occurs in the chloroplast.

• Directly or indirectly, photosynthesis nourishes almost the entire living world –All organisms use organic compounds for

energy and for carbon skeletons –Organisms obtain organic compounds either

through autotrophic nutrition or heterotrophic nutrition

• Autotrophs sustain themselves without

eating anything derived from other organisms, they are the main primary producers of the biosphere

• Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules

• Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules

Figure 10.1

Figure 10.1a

Other organisms also benefit from photosynthesis.

• Photosynthesis occurs in plants,

algae, certain other unicellular eukaryotes, and some prokaryotes

• These organisms feed not only themselves but also most of the living world

(a) Plants

(b) Multicellular alga

(c) Unicellular eukaryotes

(d) Cyanobacteria

(e) Purple sulfur bacteria

40 μm

1 μ

m

10

μm

• Heterotrophs obtain their organic material from other organisms

• Heterotrophs are the consumers of the biosphere

• Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2.

• Heterotrophs can also decompose and feed on dead organisms or on organic litter, like feces and fallen leaves

–Fungi and many prokaryotes feed via decomposition, also known as detritivores.

• Earth’s supply of fossil fuels was formed from the remains of organisms that died hundreds of millions of years ago

• In a sense, fossil fuels represent stores of solar energy from the distant past

Chemoautotrophs

• Their energy comes from chemicals instead of sunlight

• Ex- Deep Sea Vents and the existing food webs that thrive without sunlight and photosynthesis, also known as chemosythnesis

Definition from: http://link.springer.com/referenceworkentry/10.1007

%2F978-3-642-11274-4_271 Chemoautotrophs are organisms that obtain their energy from a

chemical reaction (chemotrophs) but their source of carbon is the most oxidized form of carbon, carbon dioxide (CO2). The best known chemoautotrophs are the chemolithoautotrophs that use inorganic energy sources, such as ferrous iron, hydrogen, hydrogen sulfide, elemental sulfur or ammonia, and CO2 as their carbon source. All known chemoautotrophs are prokaryotes, belonging to the Archaea or Bacteria domains. They have been isolated in different extreme habitats, associated to deep-sea vents, the deep biosphere or acidic environments. This form of energy conservation is considered one of the oldest on Earth. These microorganisms are of astrobiological interest because they could develop in the extreme conditions existing in different extraterrestrial planetary bodies, like Mars or Europa.

Photosynthesis converts light energy to the chemical energy of food

• Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria

• The structural organization of these organelles allows for the chemical reactions of photosynthesis

Tracking Atoms Through Photosynthesis: Scientific Inquiry

• Photosynthesis is a complex series of reactions that can be summarized as the following equation: 6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O

• The overall chemical change during

photosynthesis is the reverse of the one

that occurs during cellular respiration

Equation for Photosynthesis

6CO2 + 6H2O ------> C6H12O6 + 6O2

Sunlight energy

Where: CO2 = carbon dioxide

H2O = water Light energy is required

C6H12O6 = glucose O2 = oxygen

https://www.msu.edu/user/morleyti/sun/Biology/photochem.html

The Splitting of Water • Chloroplasts split H2O into hydrogen and oxygen,

incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product

Figure 10.5

Reactants:

Products:

6 CO2 12 H2O

C6H12O6 6 H2O 6 O2

Chloroplasts: The Sites of Photosynthesis

in Plants

• Leaves are the major locations of photosynthesis

• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

• Each mesophyll cell contains 30–40 chloroplasts

• CO2 enters and O2 exits the leaf through microscopic pores called stomata

• Veins deliver water from the roots and carry off sugar from mesophyll cells to nonphotosynthetic areas of the plant

• A chloroplast has an envelope of two

membranes surrounding a dense fluid called the stroma, surrounds the thylakoid

• Thylakoids are connected sacs in the chloroplast which compose a third membrane system

• Thylakoids may be stacked in columns called grana

• Chlorophyll, the pigment which gives leaves their green color, resides in the thylakoid membranes, plays an important role in the absorption of light

Leaf cross section

Stomata

Chloroplast

Mesophyll

Chloroplasts Vein

Mesophyll cell

20 μm

CO2 O2

Figure 10.4b

Chloroplast

Stroma Granum

Thylakoid Thylakoid space

Outer membrane

Intermembrane space

Inner membrane

1 μm

Three Stages

• 1. Capture energy from sunlight

• 2. Using the energy to make ATP and to reduce the compound NADP+, an electron carrier, to NADPH

• 3. Using the ATP and NADPH to power the synthesis of organic molecules from CO2 in the air

F.F. Blackman’s Contribution

• Found out that different light intensities, CO2 concentrations, and temperatures alter photosynthetic rates

• Concluded that photosynthesis is a multi-step process with one portion requiring light, while the other occurs with and without light

C.B. van Niels, 1930’s

• Studied purple sulfur bacteria

• They do not release oxygen, instead of using water they converted hydrogen sulfide into pure sulfur globules (released as waste)

• He hypothesized that plants split water as a source of electrons from hydrogen atoms, releasing oxygen as a byproduct

• He proposed the equation for photosynthesis

Photosynthesis as a Redox Process

• Photosynthesis reverses the direction of electron flow compared to respiration

• Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

• Photosynthesis is an endergonic process; the energy boost is provided by light

Figure

10.UN01

becomes reduced

becomes oxidized

The Nature of Sunlight

• Light is a form of electromagnetic energy, also called electromagnetic radiation

• Like other electromagnetic energy, light travels in rhythmic waves

• Wavelength is the distance between crests of waves

• Wavelength determines the type of electromagnetic energy

Pigments- molecules that absorb light energy in the visible range

• The color we see is the color that is not absorbed, but reflected

• The electromagnetic spectrum is the

entire range of electromagnetic energy, or radiation. Wavelengths of 400-740 nm

• Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see

• Light also behaves as though it consists of discrete particles, called photons

Figure 10.7

Visible light

Gamma rays

X-rays UV Infrared Micro- waves

Radio waves

380 450 500 550 600 650 700 750 nm

Shorter wavelength

Higher energy Lower energy

Longer wavelength

10− 5 10− 3 nm nm 1 nm 3 nm 10 6 nm 10 9 (10 nm) 1 m

10 3 m

Photosynthetic Pigments: The Light Receptors

• Pigments are substances that absorb visible light

• Different pigments absorb different wavelengths

• Wavelengths that are not absorbed are reflected or transmitted

• Leaves appear green because chlorophyll reflects and transmits green light

Figure 10.8 Light

Chloroplast

Reflected light

Granum

Transmitted light

Absorbed light

Animation: Light and Pigments

• A spectrophotometer measures a pigment’s ability to absorb various wavelengths

• This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

White light

Refracting prism

Chlorophyll solution

Photoelectric tube

Galvanometer

The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light.

Slit moves to pass light of selected wavelength.

Green light

Blue light

The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light.

1

2 3

4

• An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

• The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis

• An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

Figure

10.10

Chloro- phyll a Chlorophyll b

Carotenoids

400 500 600 700 Wavelength of light (nm)

(a) Absorption spectra A

bso

rpti

on

o

f lig

ht

by

chlo

rop

last

p

igm

en

ts

Rat

e o

f p

ho

tosy

nth

esi

s (m

eas

ure

d b

y O

2 re

leas

e)

400 500 600 700

400 500 600 700

(b) Action spectrum

(c) Engelmann’s experiment

Aerobic bacteria

Filament of alga

• The action spectrum of photosynthesis was

first demonstrated in 1883 by Theodor W. Engelmann

• In his experiment, he exposed different segments of a filamentous alga to different wavelengths

• Areas receiving wavelengths favorable to photosynthesis produced excess O2

• He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production

• Chlorophyll a is the main photosynthetic

pigment

• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis

• The difference in the absorption spectrum between chlorophyll a and b is due to a slight structural difference between the pigment molecules

• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

Figure 10.11

Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center

Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown

CH in chlorophyll a

in chlorophyll b 3

CHO CH 3

Video: Space-Filling Model of Chlorophyll a

• Accessory pigments called carotenoids function in photoprotection; they absorb excessive light that would damage chlorophyll

Excitation of Chlorophyll by Light

• When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable

• When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence

• If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

Figure 10.12

Excited state

Heat

(a) Excitation of isolated chlorophyll molecule (b) Fluorescence

Ground state

Photon (fluorescence)

Photon

Chlorophyll molecule

Ene

rgy

of

ele

ctro

n

e−

A Photosystem: A Reaction-Center Complex Associated with Light-

Harvesting Complexes

• Photosystem- located in the thylakoid membrane where chlorophyll is organized along with proteins and smaller organic molecules.

• A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes

• The light-harvesting complexes / antenna complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center chlorophylls

Reaction Center in Photosystems

• Transmembrane protein- pigment complex.

• This is where the light energy is converted into chemical energy.

• Primary electron acceptor

• An excited electron is passed to an acceptor, transferring energy away from the chlorophylls.

• Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

• Each photosystem reaction center chlorophyll and primary electron acceptor surrounded by an antenna complex- functions in the chloroplast as a light harvesting unit

Figure 10.13a

(a) How a photosystem harvests light

Pigment molecules

Primary electron acceptor

Reaction- center complex

STROMA

Photosystem

Light- harvesting complexes

Photon

Transfer of energy

Special pair of chlorophyll a molecules

THYLAKOID SPACE (INTERIOR OF THYLAKOID)

Thyl

ako

id m

em

bra

ne

e−

Figure 10.13b

(b) Structure of a photosystem

Chlorophyll STROMA

THYLA- KOID SPACE

Protein subunits

Thyl

ako

id m

em

bra

ne

• There are two types of photosystems in

the thylakoid membrane

• Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

• The reaction-center chlorophyll a of PS II is called P680

• Photosystem I (PS I) is best at absorbing a wavelength of 700 nm

• The reaction-center chlorophyll a of PS I is called P700

Differences Between PS II and PS I

• Differences are not in the chlorophyll molecules, but in the proteins associated with each reaction center

• These two photosystems work together to use light energy to generate ATP and NADPH

Noncyclic (Linear) Electron Flow

• During the light reactions, there are two possible routes for electron flow: cyclic and linear

• Linear (noncyclic) electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy

• There are 8 steps in linear electron

flow:

1. Photosystem II absorbs a photon of light and that photon hits a pigment and its energy is passed among pigment molecules until it excites P680

2. An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+), leaving the reaction center oxidized

Light

H2O CO2

O2

LIGHT REACTIONS

CALVIN CYCLE

ATP

NADPH

ADP

NADP+

[CH2O] (sugar) Light

Reaction Schematic

Figure 10.14-1

Pigment molecules

e−

1

2

P680

Light

Photosystem II

(PS II)

Primary acceptor

Step 1 and 2

3. An enzyme extracts electrons from H2O and

supplies them to the oxidized reaction center. This reaction splits water into two hydrogen ions and an oxygen atom that combines with another oxygen atom to form O2 . The electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680.

– P680+ is the strongest known biological oxidizing agent

–O2 is released as a by-product of this reaction

Figure 10.14-2

Pigment molecules

e−

1

2

P680

Light

Photosystem II

(PS II)

Primary acceptor

3

e− e−

2 H+

+ O2

H2O

½

Step 3

4. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I

5. Energy released by the “fall” drives the creation of a proton gradient across the thylakoid membrane

–Diffusion of H+ (protons) across the membrane drives ATP synthesis

Cytochrome Complex

• Connects the two photosystems via complex systems of electron carriers

• Uses the energy from the passage of electrons to move protons across the thylakoid membrane to generate the proton gradient used by an ATP synthase enzyme.

Figure

10.14-3

Pigment molecules

e−

1

2

P680

Light

Photosystem II

(PS II)

Primary acceptor

3

e− e−

2 H+

+ O2

H2O

ATP

4

5

Electron transport chain

Cytochrome complex

Pq

Pc ½

Step 4 & 5

6. In PS I (like PS II), transferred light energy excites P700 reaction center, which loses an electron to an electron acceptor, creating an electron “hole” in P700. This hole is filled by an electron that reaches the bottom of the electron transport chain from PS II

Figure 10.14-4

Pigment molecules

e−

1

2

P680

Light

Photosystem II

(PS II)

Primary acceptor

3

e− e−

2 H+

+ O2

H2O

ATP

4

5

Electron transport chain

Cytochrome complex

Pq

Pc P700

Light

Photosystem I

(PS I)

6

Primary acceptor

e−

½

Step 6

7. Photoexcited electrons are passed from PS I’s

primary electron acceptor down a second electron transport chain through the protein ferredoxin (Fd)

8. The enzyme NADP+ reductase transfers electrons from Fd to NADP+ . Two electrons are required for NADP+ ‘s reduction to NADPH. NADPH will carry the reducing power of these high energy electrons to the Calvin cycle.

– This process also removes an H+ from the stroma

Figure

10.14-5

Pigment molecules

e−

1

2

P680

Light

Photosystem II

(PS II)

Primary acceptor

3

e− e−

2 H+

+ O2

H2O

ATP

4

5

Electron transport chain

Cytochrome complex

Pq

Pc P700

Light

Photosystem I

(PS I)

6

Primary acceptor

e− e−

7

8

Fd

e−

Electron transport chain

NADP+

reductase NADPH

NADP+

+ H+

½

Step 7 & 8

• The light reactions use the solar power of photons absorbed by both photosystem I and photosystem II to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH

Cyclic Electron Flow

• Under certain conditions, photoexcited electrons from PS I, but not PS II, can take an alternative pathways = cyclic electron flow – Excited electrons cycle from their reaction center to a

primary acceptor, along an ETC, and return to the oxidized P700 chlorophyll

– As electrons flow along the ETC, they generate ATP by cyclic photophosphorylation.

– There is no production of NADPH and no release of oxygen

– The purpose of this flow is that it allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle.

Figure 10.16

Primary acceptor

Primary acceptor

Fd

Cytochrome complex

Pc

Pq

Photosystem II

Photosystem I

Fd

NADP+

+ H+ NADP+

reductase

NADPH

ATP Cyclic

Electron Flow

• Some organisms such as purple sulfur bacteria have PS I but not PS II

• Cyclic electron flow is thought to have evolved before linear electron flow

• Cyclic electron flow may protect cells from light-induced damage

Cyclic versus Noncyclic

Cyclic • Allows the chloroplast to

generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle.

Noncyclic • Produces ATP and NADPH in

roughly equal amounts

Chemiosmosis

• The process in which ATP is generated in the mitochondria and chloroplast

• Defined as: Energetic electrons excited by light (chloroplasts) or extracted by oxidation in the Krebs cycle (mitochondria) are used to drive proton pumps, creating a proton gradient. When protons subsequently flow back across the membrane, they pass through channels that couple their movement to make ATP.

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

• Chloroplasts and mitochondria generate ATP by

chemiosmosis, but use different sources of energy • Mitochondria transfer chemical energy from food to

ATP; chloroplasts transform light energy into the chemical energy of ATP

• ETC pumps protons across a membrane as electrons are passed along a series of increasingly electronegative carriers. This transforms redox energy to a proton-motive force in the form of H+ gradient across the membrane. ATP synthase molecules harness the proton-motive force to generate ATP as H+ diffuses back across the membrane.

• Spatial organization of chemiosmosis differs

between chloroplasts and mitochondria but also shows similarities

• In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix

• In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma

Figure 10.17

MITOCHONDRION STRUCTURE

CHLOROPLAST STRUCTURE

Thylakoid membrane

Stroma

ATP

Thylakoid space

Inter- membrane

space

Inner membrane

Matrix

Key

Diffusion

Electron transport

chain

ATP synthase

ADP +

H+

H+

Higher [H+]

Lower [H+]

P i

• ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

• In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH

Figure 10.18

Photosystem II Photosystem I Cytochrome

complex Light

Pq

Light 4 H+

+2 H+ 4 H+ O2

H2O

Pc

Fd

3

2

1

NADP+

To Calvin Cycle

NADP+ reductase

STROMA (low H+ concentration)

ATP synthase

THYLAKOID SPACE (high H+ concentration)

Thylakoid membrane

ADP +

H+ ATP

P i

e e

NADPH

½

+ H+

The Two Stages of Photosynthesis: A Preview

• Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

• The light reactions (in the thylakoids)

– Split H2O

– Release O2

– Reduce the electron acceptor NADP+ to NADPH

– Generate ATP from ADP by photophosphorylation

• The Calvin cycle (in the stroma) forms sugar from

CO2, using ATP and NADPH

• The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

Figure 10.6-1 Light

Thylakoid Stroma

Chloroplast

LIGHT REACTIONS

NADP+

ADP

P i

+

H2O

Figure 10.6-2 Light

Thylakoid Stroma

Chloroplast

LIGHT REACTIONS

NADP+

ADP

P i

+

H2O

NADPH

ATP

O2

Figure 10.6-3 Light

Thylakoid Stroma

Chloroplast

LIGHT REACTIONS

NADP+

ADP

P i

+

H2O

O2

CO2

NADPH

ATP

CALVIN CYCLE

Figure 10.6-4 Light

Thylakoid Stroma

Chloroplast

LIGHT REACTIONS

NADP+

ADP

P i

+

H2O

[CH2O] (sugar)

CALVIN CYCLE

CO2

NADPH

ATP

O2

BioFlix: The Carbon Cycle

BioFlix: Photosynthesis

The light reactions convert solar energy to the chemical energy of ATP

and NADPH • Chloroplasts are solar-powered chemical

factories

• Their thylakoids transform light energy into the chemical energy of ATP and NADPH

The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2

to sugar • The Calvin cycle, like the citric acid cycle, regenerates its

starting material after molecules enter and leave the cycle • The cycle builds sugar from smaller molecules by using ATP

and the reducing power of electrons carried by NADPH • Anabolic-> uses energy to build sugars from smaller

molecules • Carbon enters the cycle as CO2 and leaves as sugar (three

carbon sugar called glyceraldehyde-3-phosphate (G3P) • Each turn in the cycle fixes one carbon • For the net synthesis of one G3P molecule, the cycle must

take place three times, fixing three molecules of CO2 • To make one glucose molecule requires six cycles and the

fixation of six CO2 molecules.

• The Calvin cycle has three phases

1. Carbon fixation (catalyzed by rubisco)

2. Reduction

3. Regeneration of the CO2 acceptor (RuBP)

Figure

10.UN03 Light

H2O CO2

O2

LIGHT

REACTIONS

CALVIN

CYCLE

ATP

NADPH

ADP

NADP+

[CH2O] (sugar)

Figure

10.19-1

Input 3 CO2, entering one per cycle

Phase 1: Carbon fixation Rubisco

3-Phosphoglycerate

Calvin Cycle

RuBP P P 3

P 3 P

P 6

Each CO2 molecule is attached to a five carbon sugar, ribulose biphosphate (RuBP), which is catalyzed by Rubisco (enzyme). Rubisco is the most abundant

protein in chloroplasts and possibly the most abundant on Earth.

The six carbon intermediate is unstable and splits in half to form two molecules of 3-phosphoglycerate for

each CO2

Figure 10.19-2 Input 3 CO2, entering one per cycle

Phase 1: Carbon fixation Rubisco

3-Phosphoglycerate

1,3-Bisphosphoglycerate

Phase 2: Reduction

G3P

Calvin Cycle

RuBP P P 3

P 3 P

P 6

6

6 ADP

P 6 P

ATP

P 6

6

6 NADP+

6 P i

G3P P 1

Output

Glucose and other organic compounds

NADPH

Each 3-phosphoglycerate receives another phosphate group from ATP to form 1,3 biphosphoglycerate. A pair of

electrons from NADPH reduces each 1,3 biphosphoglycerate to G3P.

Electrons reduce a carboxyl group to the aldehyde group of G3P (stores

more potential energy). If our goal was the net production of one G3P, we

would start with 3CO2 (3C) and 3 RuBP (15C) . After fixation and reduction we would have 6 molecules of G3P (18C). One of these three 6 G3P (3C) is a net gain of carbohydrate. This molecule

can exit the cycle and be used by the plant cell.

Input 3 CO2, entering one per cycle

Phase 1: Carbon fixation Rubisco

3-Phosphoglycerate

1,3-Bisphosphoglycerate

Phase 2: Reduction

G3P

Calvin Cycle

G3P

Phase 3: Regeneration of RuBP

ATP

3 ADP

3

5 P

RuBP P P 3

P 3 P

P 6

6

6 ADP

P 6 P

ATP

P 6

6

6 NADP+

6 P i

G3P P 1

Output

Glucose and other organic compounds

NADPH

Regeneration: The other five G3P (15C) remain in the cycle to regenerate RuBP. In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle to regenerate 3RuBP

For the net synthesis of one G3P molecule, the Calvin cycle consumes nine ATP and six NADPH. The light reactions regenerate ATP and NADPH. The G3P from the Calvin cycle is the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbs.

Alternative mechanisms of carbon fixation have evolved in hot, arid

climates • Dehydration is a problem for plants, sometimes

requiring trade-offs with other metabolic processes, especially photosynthesis

• On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis

• The closing of stomata reduces access to CO2 and causes O2 to build up

• These conditions favor an apparently wasteful process called photorespiration.

Photorespiration: An Evolutionary Relic?

• In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)

• In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound = photorespiration – The two carbon fragment is exported from the

chloroplast and degraded to CO2 by the mitochondria and peroxisomes

• Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar, but it actually consumes ATP

• Photorespiration may be an evolutionary relic

because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2

• Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle

• In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle. Certain plant species have evolved alternate modes of carbon fixation to minimize photorespiration.

C4 Plants • C4 plants minimize the cost of photorespiration by

incorporating CO2 into four-carbon compounds.

• Ex- sugarcane and corn

• There are two distinct types of cells in the leaves of C4 plants:

– Bundle-sheath cells are arranged in tightly packed sheaths around the veins of the leaf

– Mesophyll cells are loosely packed between the bundle sheath and the leaf surface

Sugar production in C4 plants

occurs in a three-step process:

1. The production of the four carbon precursors is catalyzed by the key enzyme PEP carboxylase in the mesophyll cells

1. PEP carboxylase has a higher affinity for CO2 than rubisco does (i.e., on hot, dry days when the stomata are closed); it can fix CO2 even when CO2 concentrations are low

2. These four-carbon compounds are exported to bundle-sheath cells

2. The bundle sheath cells strip a carbon from the four carbon compound as CO2, and return the three carbon remainder to the mesophyll cells.

3. The bundle sheath cells then use rubisco to start the Calvin cycle with an abundant supply of CO2

3. In effect, the mesophyll cells pump CO2 into the bundle sheath cells, keeping CO2 levels high enough for rubisco to accept CO2 and not O2

C4 Photosynthesis

• Minimizes photorespiration and enhances sugar production

• Thrive in hot regions with intense sunlight • Since the Industrial Revolution in the 1800s, CO2

levels have risen greatly • Increasing levels of CO2 may affect C3 and C4

plants differently, perhaps changing the relative abundance of these species

• The effects of such changes are unpredictable and a cause for concern

Figure 10.20

Mesophyll cell

Bundle- sheath cell

Photo- synthetic cells of C4 plant leaf

Vein (vascular tissue)

C4 leaf anatomy

Stoma

The C4 pathway

Mesophyll cell PEP carboxylase

Oxaloacetate (4C)

Malate (4C)

Pyruvate (3C)

CO2

ADP

PEP (3C)

ATP

CO2

Calvin Cycle

Bundle- sheath cell

Sugar

Vascular tissue

CAM Plants

• Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon

• Ex- cacti and pineapple • CAM plants open their stomata at night, incorporating CO2

into organic acids – Temperatures are typically lower at night and close them during

the day. – During the night, these plants fix CO2 into a variety of organic

acids in mesophyll cells

• Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle – Light reactions supply ATP and NADPH to the Calvin cycle, and CO2

is released from the organic acids

Sugarcane Pineapple

C4 CO2 CO2 CAM

Organic acid

Organic acid

Night

Day

CO2 CO2

Calvin Cycle

Calvin Cycle

Sugar Sugar

Bundle- sheath cell

(a) Spatial separation of steps

(b) Temporal separation of steps

Mesophyll cell

2

1 1

2

The Importance of Photosynthesis: A Review

• The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

• Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

• Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits

• In addition to food production, photosynthesis produces the O2 in our atmosphere

Figure 10.22a O2 CO2

H2O

Sucrose (export)

H2O

Light

LIGHT REACTIONS:

Photosystem II Electron transport

chain Photosystem I

Electron transport chain

Chloroplast

NADP+

ADP

+ P

i

NADPH

ATP

RuBP

G3P

CALVIN

CYCLE

Starch (storage)

3-Phosphoglycerate

Sucrose (export) O2 H2O

Mesophyll cell

CO2

Figure 10.22b

LIGHT REACTIONS CALVIN CYCLE REACTIONS

• Are carried out by molecules in the thylakoid membranes

• Convert light energy to the chemical energy of ATP and NADPH

• Split H2O and release O2 to the atmosphere

• Take place in the stroma

• Use ATP and NADPH to convert CO2 to the sugar G3P

• Return ADP, inorganic phosphate, and NADP+ to the light reactions

Figure

10.UN06

Regeneration of

CO2 acceptor

Calvin Cycle

Carbon fixation

Reduction

1 G3P (3C)

5 x 3C

3 x 5C 6 x 3C

3 CO2

CAMPBELL

BIOLOGY Reece • Urry • Cain • Wasserman • Minorsky • Jackson

© 2014 Pearson Education, Inc.

TENTH

EDITION

CAMPBELL

BIOLOGY Reece • Urry • Cain • Wasserman • Minorsky • Jackson

TENTH

EDITION

10 Photosynthesis

Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick