CHAPTER 8

Chapter 8: Photosynthesis: Energy from the Sun

Photosynthesis: Energy from the Sun

CHAPTER 8


Photosynthesis Biochemical process in which light

energy is converted to chemical energy

Photos = lightsynthesis = to put together

In plants, photosynthesis takes place in chloroplasts.

It involves many enzyme controlled steps


Photosynthetic Reactants and Products

6 CO2 + 12 H2O + light C6H12O6 + 6 O2 + 6 H2O


Figure 8.1

Figure 8.1

figure 08-01.jpg


Photosynthesis

Photosynthesis can be divided into two pathways:

– The light reaction - driven by light energy captured by chlorophyll. Consists of Photosystem I and Photosystem II. It produces ATP and NADPH + H+.

– The Calvin–Benson cycle - does not use light directly. It uses ATP, NADPH + H+, and CO2 to produce sugars.


Properties of Light and Pigments

Light is the source of energy that drives photosynthesis

Molecules that absorb light energy in the visible range are called pigments.


Figure 8.5

Figure 8.5

figure 08-05.jpg



When light and a pigment molecule meet, one of 3 things happen

Reflection – the light bounces off the molecule

Transmission – the light passes through the molecule

Excitation – the light is absorbed by the molecule. If absorbed, the molecule goes from its ground state to and excited state of higher energy

An electron is boosted to another orbital


Pigments When a beam of white light shines on an

object, and the object appears to be red in color, it is because it has absorbed all other colors from the white light except for the color red.

In the case of chlorophyll, plants look green because they absorb green light less effectively than the other colors found in sunlight and reflect the green light not absorb



Different pigment molecules absorb different wavelengths of light

The particular set of wavelengths that a pigment absorbs is called its absorption spectrum

Review Figures 8.7


Figure 8.7

Figure 8.7

figure 08-07.jpg



Chlorophylls are the most important pigments in photosynthesis

Chlorophyll a is the primary pigment in photosynthesis.

Chlorophylls and accessory pigments trap light and transfer energy to a reaction center


Chlorophyll


An excited pigment molecule may lose its energy by emitting light of

longer wavelength or transfer the absorbed energy to

another pigment molecule as a redox reaction.

Chapter 8: Photosynthesis: Energy from the SunFigure 8.8 Energy Transfer and Electron Transport


There are two different systems for transport of electrons in photosynthesis.

1. Noncyclic electron transport produces NADPH + H+ and ATP and O2.

2. Cyclic electron transport produces only ATP.


Noncyclic In noncyclic electron transport, two

photosystems are required. Photosystems consist of many

chlorophyll molecules and accessory pigments bound to proteins.


Photosystem I Photosystem I uses light energy to

reduce NADP+ to NADPH + H+. The reaction center contains a

chlorophyll a molecule called P700 because it best absorbs light at a wavelength of 700 nm.


Photosystem II Photosystem II uses light energy to split

water, producing electrons, protons, and O2. The reaction center contains a chlorophyll a

molecule called P680 because it best absorbs light at a wavelength of 680 nm.

To keep noncyclic electron transport going, both photosystems must constantly be absorbing light.


After absorbing light energy: an energized electron leaves the Chl*

in the reaction center and participates in a series of redox reactions.

the electron flows through a series of carriers in the thylakoid membrane.

producing ATP

Chapter 8: Photosynthesis: Energy from the SunFigure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 1)

Chapter 8: Photosynthesis: Energy from the SunFigure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 2)


Cyclic electron transport produces only ATP.

The electron passes from an excited P700 molecule and cycles back to the same P700 molecule.

No O2 is released.In cyclic electron flow, photosystem I

acts on its own.

Cyclic Electron Transfer

Chapter 8: Photosynthesis: Energy from the SunFigure 8.10 Cyclic Electron Transport Traps Light Energy as ATP


“Z” Scheme

Photosystem I & II (P680 & P700) work together to generate ATP and NADPH.

This pathway is called the “Z” scheme. Noncyclic


Noncyclic Electron Flow or Z Scheme

In Photosystem II chlorophyll a absorbs light energy to become energized chloropyll a

2 electrons are released and caught by the primary electron acceptor.

H20 ½ O2 + 2 e- + 2H+


The electrons pass through a redox chain for chemiosmotic ATP production.

The electron transport chain pumps protons across the membrane into the thylakoid space.

The protons accumulate establishing a proton concentration gradient

ATP synthases open and the protons diffuse to generate ATP from ADP.


Z Scheme Cont’d

The electrons are passed to P700 chlorophyll P700 loses electrons to Ferredoxin (Fd) NADP combines with H to form NADPH. NADPH is the source of H used to make

C6H12O6

Chapter 8: Photosynthesis: Energy from the SunFigure 8.11 Chloroplasts Form ATP Chemiosmotically


The Calvin–Benson Cycle

The Calvin–Benson cycle makes sugar from CO2

ATP and NADPH provide the needed energy

This pathway was elucidated through use of radioactive tracers


The Calvin–Benson Cycle

Three phases: 1. Carbon Fixation – RuBP + CO2 carbon

sugarPG (first stable product) The reaction is catalyzed by rubisco (ribulose

bisphosphate carboxylase). 2. Series of reactions to produce G3P 3. Regeneration of RuBP (7 enzymatic steps) RuBP (ribulose biphosphate) is the initial CO2

acceptor


The Calvin–Benson CycleThe end product of the cycle is

glyceraldehyde 3-phosphate, G3P. There are two fates for the G3P:

–One-third ends up as starch, which is stored in the chloroplast and serves as a source of glucose.

–Two-thirds is converted to the disaccharide sucrose, which is transported to other organs.

Chapter 8: Photosynthesis: Energy from the SunFigure 8.13 The Calvin-Benson Cycle


RubiscoRubisco is a carboxylase, adding

CO2 to RuBP. It can also be an oxygenase, adding O2 to RuBP.

These two reactions compete with each other.

When RuBP reacts with O2, it cannot react with CO2, which reduces the rate of CO2 fixation.


Photorespiration A specialized metabolic pathway in

which rubisco reacts with O2 instead of CO2

Occurs under stress conditions of hot, dry, bright days when the internal leaf concentration of O2 is greater than CO2 concentration.

Glucose production is reduced thereby limiting plant growth


C3 Plants

Most common type of plants on earth. Grow best in temperate zones Includes rice, wheat, soybeans, bluegrass On hot days the stomata close, O2 builds up

and photorespiration occurs. The first product is the 3-C molecule of 3PG CO2 + RuBP 3 phosophoglycerate (3 C

compound)

Chapter 8: Photosynthesis: Energy from the SunFigure 8.16 Leaf Anatomy of C3 and C4 Plants


C4 Plants C4 plants have 2 enzymes (PEP

carboxylase & rubisco) for CO2 fixation in 2 different parts of the leaf.

PEP carboxylase does not have an affinity for O2 and fixes CO2 even at very low CO2 levels.

What is the significance of this fact? C4 plants include sugarcane, corn and other

plants that grow in hot, dry climates.


C4 Plants Cont’d CO2 + PEP carboxylase Oxaloacetate (4

C compound). Occurs in cells near top of leaf Oxaloacetate diffuses into bundle sheath

cells in the interior of the cells. Here oxaloacetate loses a C forming CO2

CO2 enters the Calvin-Benson Cycle


Crassulacean Acid Metabolism (CAM)

CAM plants are succulents or water storing plants.

Include cacti and pineapples CAM plants open their stomata only at night CO2 enters and forms malic acid which is

stored as an acid in the vacuoles until morning

In daylight, the CO2 is released from the acid and enters the Calvin Benson Cycle.


Stomates Stomates close when weather is hot & dry. O2 concentration increases, CO2

concentration decreases. Why? Ribulose requires high concentrations of

CO2

If sufficient CO2 is unavailable, photorespiration occurs.


Metabolic Pathways in Plants

Both photosynthesis and respiration occurs in plants.

Compare photosynthesis and respiration.

CHAPTER 8

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