PhotosynthesisChapter 10

Chemical Energy & ATP The activities of all cells

are powered by chemical fuels The principal compound

that living things use to store energy is adenosine triphosphate (ATP)

The characteristics of ATP make it an exceptionally useful molecule that is used by all types of cells as their basic source of energy

What is Photosynthesis? Photosynthesis is the process whereby light energy is

converted to chemical energy and carbon is fixed into organic compounds.

In the presence of light, plants transform carbon dioxide and water into carbohydrates and release oxygen Photosynthesis uses the energy of sunlight to convert water and CO2

into O2 and high energy sugars 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2

carbon dioxide + water + light → sugar + oxygen

Plants then use the sugars to produce complex carbohydrates such as starches Plants obtain carbon dioxide from the air or water in which they grow

Figure 7.18 The chloroplast, site of photosynthesis

Inside a Chloroplast

Key structures INSIDE Chloroplasts

Thylakoid saclike structure in chloroplasts made of photosynthetic

membranes – these sacs are made up of lipid bilayers

Granum a stack of thylakoids

Stroma region outside of the thylakoid membranes

Chlorophyll molecules are embedded in the thylakoid membrane

Stages of Photosynthesis The reaction that occurs during photosynthesis can be broken

into 2 stages:

1. Light Dependent Reactions Take place within the thylakoid membranes inside a chloroplast “PHOTO” phase – make ATP & NADPH

USE LIGHT ENERGY TO PRODUCE ATP & NADPH

2. Light Independent Reactions (Calvin Cycle) Take place in the stroma of the chloroplast “SYNTHESIS” phase – coverts CO2 to sugar

PRODUCE SUGAR

Photosynthetic Pigments Photosynthetic pigments absorb light energy and use it to

provide energy to carry out photosynthesis. Chlorophylls (absorb light in the red, blue, and violet range):

Chlorophyll a - directly involved in transformation of photons to chemical energy

Chlorophyll b - helps trap other wavelengths and transfers it to chlorophyll a Carotenoids (absorb light in the blue, green, and violet range):

xanthophyll - Yellow beta carotene - Orange Phycobilins – Red

Chlorophyll b, the carotenoids, and the phycobilins are known as ANTENNA PIGMENTS – they capture light in other wavelengths and pass the energy along to chlorphyll a.

Chlorophyll a is the pigment that participates directly in the light reactions of photosynthesis!

The Structure of Chlorophyll a Chlorophyll a is a large molecule

with a single magnesium atom in the head surrounded by alternating double and single bonds. The head is called the porphyrin

ring and is attached to a long hydrocarbon tail The double bonds are the source of the

electrons that flow through the electron transport chains during photosynthesis.

Figure 10.9 Location and structure of chlorophyll molecules in plants

The pigment molecules have a large head section that is

exposed to light in the surface of the membrane; the hydrocarbon

tail anchors the pigment molecules into the lipid bilayer.

Photosystems Photosystems are light-harvesting complexes

in the thylakoid membranes of chloroplasts. Each photosystem consists of a reaction center

containing chlorophyll a and a region of many atenna pigment molecules that funnel energy into chlorophyll a.

Two types of photosystems cooperate during photosynthesis:

1. Photosystem I2. Photosystem II

PS I and PS II Named in the order they were discovered –

however, PS II occurs first, followed by PS I. PS I absorbs light best in the 700nm range (so

called P700). PS II absorbs light best in the 680nm range (so

called P680).

Stages of Photosynthesis There are 2 stages in Photosynthesis:

1. Light dependent reactions

2. Light independent reactions (Calvin Cycle)

BOTH REQUIRE LIGHT (SOMEWHAT): Even the dark reactions in most plants occurs during daylight

– because that is the only time the light reactions can operate AND the dark reactions depend on the light reactions!!!

Visual Overview of Photosynthesis

Light Dependent Reactions - Overview

require presence of light occur in thylakoids of chloroplasts use energy from light to produce ATP and

NADPH (a temporary, mobile energy source that helps store even more energy)

oxygen gas is produced as a by-product

Light Independent Reactions - Overview

do not require light directly – so also known as the Dark Reactions or the Calvin Cycle

take place in the stroma of chloroplasts ATP and NADPH produced during light

dependent reactions are used to make glucose

LIGHT DEPENDENT REACTIONS

Stage 1 of Photosynthesishttp://www.sumanasinc.com/webcontent/animations/content/harvestinglight.html

The Light Reactions

Light is absorbed by PS I and PS II in the thylakoid membranes and electrons flow through electron transport chains.

There are 2 possible routes for electron flow:1. Noncylic photophosphorylation

2. Cyclic photophosphorylation

Photophosphorylation is a method of generating ATP by using light to add P to ADP

Occurs in Light Reactions

Figure 10.11 How a photosystem harvests light

Chlorophyll a

Noncyclic Photophosphorylaton

During noncyclic photophosphorylation, electrons enter two electron transport chains, and ATP and NADPH are formed.

The process begins in PS II and proceeds to PS I.

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 1)

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 2)

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 3)

Section 8-3

Figure 8-10 Light-Dependent Reactions

Go to Section:

A. Photosystem II -- Light is absorbed by pigment. Energy is transferred to e-, which go into ETC (electron transport chain.) Hydrolysis breaks water up into e-, H+, and O2

B. ETC moves H+ ions from stroma into inner thylakoid.

C. Photosystem I -- light is absorbed by pigments, energy goes to e-, NADPH is formed

D. Hydrogen movement makes inside positively charged.

E. As H+ diffuses through ATP synthase, ADP is made into ATP.

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 4)

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 5)

Cyclic Photophosphorylaton Under certain conditions…photoexcited electrons take an short-

circuit path called cyclic electron flow. Involves Photosystem I only…no production of NADPH and no release of

oxygen…but ATP is produced. During Cyclic Electron Flow, photo-excited electrons travel

from PS II electron transport chain to PS I, to a primary electron acceptor, and then back to the cytochrome complex in PS II. THE SOLE PURPOSE OF CYCLIC PHOTOPHOSPHORYLATION

IS TO PRODUCE ATP!!! Why Cyclic?

Because noncyclic electron flow produces ATP and NADPH in roughly equal quantities…but Calvin cycle consumes more ATP than NADPH. Cyclic flow makes up the difference for more ATP needed.

Figure 10.14 Cyclic electron flow

Cyclic vs. Noncyclic Electron Flowhttp://highered.mcgraw-hill.com/olc/dl/120072/bio12.swf

Noncyclic – pg 186 uses Photosystem II, and ETC (with the electron

carrier plastoquinone, Pq) , Photosystem I, and another ETC using an iron-containing protein called ferredoxin.

produces ATP and NADPH Cyclic – pg. 187

uses only Photosystem I and the second ETC – no production of NADPH and no release of Oxygen

DOES produce ATP to be used to make up the difference needed due to Calvin cycle demands.

Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplastshttp://bcs.whfreeman.com/thelifewire/content/chp08/0802002.html

LIGHT INDEPENDENT REACTIONS (Calvin Cycle)

Stage 2 of Photosynthesishttp://highered.mcgraw-hill.com/sites/0070960526/student_view0/chapter5/animation_quiz_1.html

The Dark Reactions (Calvin cycle) Calvin cycle can be divided into 3 phases:

Phase 1: Carbon Fixation Carbon fixation. CO2 is incorporated and attached to RuBP (catalyzed by

enzyme rubisco). Product of reaction is 6-carbon intermediate so unstable that it splits in

half to form two molecules of 3-phosphoglycerate. Phase 2: Reduction

Each molecule of 3-phosphoglycerate receives additional phosphate group from ATP to become 1,3 biphosphoglycerate.

Pair of electrons donated from NADPH reduces 1,3 to G3P (a sugar)….notice for every 3 molecules of CO2 there are six molecules of G3P

Phase 3: Regeneration of CO2 acceptor (RuBP) In a series of reactions, the carbon skeletons of 5 molecules of G3P are

rearranged by the last steps of the Calvin cycle into three molecules of RuBP….the RuBP is now prepared again to receive CO2…and the cycle continues.

Figure 10.17 The Calvin cycle (Layer 1)

Figure 10.17 The Calvin cycle (Layer 2)

Figure 10.17 The Calvin cycle (Layer 3)

A. 6 CO2’s combine with 6 5-C molecules – make 12 3-C molecules

C. 2 of the 12 3-C molecules are made into glucose

D. Other 10 3-C molecules are broken down into six 5-C molecules to start cycle over…

http://bcs.whfreeman.com/thelifewire/content/chp08/0802003.html

Figure 10.20 A review of photosynthesis

Factors Affecting Photosynthesis

Amount of water available too little, stop photosynthesis

Temperature best between O°C and 35° C (too high, damage

enzymes; too low, stop photosynthesis) Intensity of light

up to a point, increasing light intensity increases rate of photosynthesis – after this point, the rate of photosynthesis will NOT continue to increase

Metabolic Challenges – Water vs. CO2

How do we maintain photosynthesis while still preventing water loss? CO2 comes in via stomata, but water goes out at

same time (transpiration): So, plants close stomata during hot, dry days. BUT, closing stomata decreases photosynthetic yield

because if stomata are closed then CO2 can’t enter the plant leaf and O2 can’t exit!

Normal Pathway - C3 Plants In most plants, initial fixation of carbon occurs using rubisco – the

enzyme in Calvin cycle that adds CO2 to ribulose biphosphate These plants are called C3 plants because the first organic product of carbon

fixation is a three-carbon compound: 3-phosphoglycerate: Ex. rice, wheat, soybeans

THESE PLANTS PRODUCE LESS FOOD WHEN THEIR STOMATA CLOSE ON HOT, DRY DAYS. The declining level of CO2 in the leaf starves the Calvin cycle. Making matters worse, rubisco can accept O2 in place of CO2 – and as O2

concentrations overtake CO2 concentrations, rubisco adds O2 to the Calvin cycle instead of CO2. The product formed splits, leaves the chloroplast, and is broken down by

mitochondria and peroxisomes – KNOWN AS PHOTORESPIRATION (because it occurs in the light – photo AND because it consumes oxygen – respiration).

PHOTORESPIRATION GENERATES NO ATP AND NO SUGAR!!!!

Photorespiration The environmental conditions that foster

photorespiration are hot, dry, bright days (the conditions that cause stomata to close). Photorespiration occurs in the light and consumes O2!

When plants close stomata they get declining levels of CO2 – which starves the Calvin Cycle

If CO2 levels are low, rubisco can also accept O2 in place of CO2. When CO2 levels are high CO2 fixation dominates. When CO2 levels are low and O2 levels are high, respiration dominates.

Photorespiration DOES NOT PRODUCE ANY ATP – or SUGARS!!! Process actually takes organic materials AWAY from the Calvin Cycle – NOT

GOOD!

Alternative Methods of Carbon Fixation – Controlling Photorespiration

In certain plants, alternative methods of carbon fixation that minimize photorespiration have evolved! All plants do not use RuBP directly to fix their

carbon! C4 PLANTS CAM PLANTS

Modification for DRY ENVIRONMENTS (combats photorespiration) A unique leaf anatomy is required here - spatial

separation of processes (mesophyll cells v/s bundle-sheath cells)

C4 Plants

C4 Plants

In C4 plants, there are TWO distinct types of photosynthetic cells: bundle-sheath cells and mesophyll cells. The Calvin cycle is confined to the chloroplasts of bundle sheath

cells, HOWEVER, the cycle is preceded by incorporation of CO2 into organic compounds in the mesophyll cells.

C4 Plants

Preface the Calvin Cycle with carbon fixation that forms a 4-C compound as its first product – CO2 is added to PEP to form oxaloacetate. Requires unique leaf anatomy – see page 192

PEP has a very high affinity for CO2, so when it is hot and dry (and stomata close), PEP can fix CO2 when rubisco cannot.

Mesophyll cells of a C4 plant pump CO2 into the bundle sheath, keeping [CO2] high enough for rubisco to accept carbon dioxide

C4 photosynthesis minimizes photorespiration and enhances sugar production Ex. Sugarcane, corn, members of the grass family

Figure 10.18 C4 leaf anatomy and the C4 pathway

In mesophyll cells, the enzyme PEP carboxylase fixes carbon dioxide (instead of RuBP).

A 4-carbon compound conveys the atoms of the CO2 into a bundle-sheath cell via plasmodesmata.

In bundle-sheath cells, CO2 is released and enters the Calvin cycle.

C4 Plants

In effect, the mesophyll cells of a C4 plant pump CO2 into the bundle-sheath, keeping the CO2 concentration in the bundle-sheath cells high enough for rubisco to accept carbon dioxide rather than oxygen. In this way, C4 photosynthesis minimizes

photorespiration and enhances sugar production. This adaptation is especially advantageous in hot regions

with intense sunlight – and these environments are where C4 plants thrive today!

Adaptation for dry environments (combats photorespiration). Succulents (water-storing plants), cacti, pineapples are CAM plants.

Open stomata during night and close them during day: Temporal separation – night v/s day – closing stomata during the day

helps desert plants conserve water, but it also prevents CO2 from entering the leaves.

During the night when their stomata are open, CAM plants take up CO2 and incorporate it into a variety of organic acids.

This is a mode of carbon fixation called CRASSULACEAN ACID METABOLISM (CAM). The mesophyll cells of CAM plants store the organic acids they make

during the night in their vacuoles until day when light reactions can supply ATP and NADPH needed for the Calvin Cycle.

During day, the CO2 is released from these organic acids and becomes incorporated into sugar via the Calvin cycle.

CAM Plants

Figure 10.19 C4 and CAM Photosynthesis Compared

BOTH pathways are evolutionary solutions to the problem of maintaining photosynthesis with stomata partially or completely closed on hot, dry days.

Spatial Separation of Steps: In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells (mesophyll and bundle-sheath).

Temporal Separation of Steps: in CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times.

C3 (Normal) v/s C4 (Adaptation)

Light Reactions:

-carried out by molecules in thylakoid membranes

-convert light E to chemical E of ATP and NADPH

-split H2O and release O2 to the atmosphere

Calvin Cycle Reactions:

-take place in stroma

-use ATP and NADPH to convert CO2 into the sugar G3P

-return ADP, inorganic phosphate, and NADP+ to the light reactions

USEFUL ANIMATIONS1. http://www.sumanasinc.com/webcontent/animations/content

/harvestinglight.html2. http://highered.mcgraw-hill.com/sites/0070960526/student_

view0/chapter5/animation_quiz_1.html3. http://www.tvdsb.on.ca/westmin/science/Biology12/

Metabolic%20Processes/Metabolic%20Processes.htm

4. http://www.fw.vt.edu/dendro/forestbiology/photosynthesis.swf

5. http://www.cix.co.uk/~argus/Dreambio/photosynthesis/photosynthsis%20animation.htm

6. http://www.viten.no/?fotosyntese2_en

Photosynthesis Concept Map

Spectrophotometer Instrument used to measure what wavelengths

of light are being absorbed by pigments Directs beams of light through a solution of

pigment and measures the fraction of light transmitted at each wavelength

Figure 10.7 Determining an absorption spectrum