CHAPTER 10

PHOTOSYNTHESIS

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I. OVERVIEWA. Photosynthesis

1. Photosynthesis is the process that converts lightenergy (sunlight) into chemical energy stored in sugars and other organic molecules

2. Directly or indirectly, photosynthesis nourishes almost the entire living world

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B. Two Types of Nutrition1. Autotrophic

•Producers•Synthesize organic molecules from inorganic raw materials (CO2)•Require an energy source•Ex: plants, algae, some protists, and some prokaryotes•Photoautotrophs—use light energy source

-Ex: plants•Chemoautotrophs—use oxidation of inorganic subs (sulfur or ammonia) as energy source

-rare form of autotrophic nutrition found in some bacteria

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Examples of

Autotrophs

2. Heterotrophic• Consumers• Acquire organic molecules from compounds

produced by other organisms• Includes decomposers (fungi and bacteria) and

animals that eat plants or other animals• Depend on photoautotrophs for food and oxygen

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II. CONCEPT 10.1: CHLOROPLASTS: SITES OF PHOTOSYNTHESISA. Introduction

1. Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria 2. The structural organization of these cells allows for the

chemical reactions of photosynthesisB. Chloroplasts

1. All green plant parts have chloroplasts, but leaves are the major organs of PS

2. Their green color is from chlorophyll, the green pigment within chloroplasts3. Are surrounded by 2 membranes

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4. Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast5. CO2 enters and O2 exits the leaf through microscopic pores called stomata 6. Chloroplasts are found mainly in cells of the mesophyll,

the interior tissue of the leaf7. A typical mesophyll cell has 30–40 chloroplasts8. The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana9. Chloroplasts also contain stroma, a dense fluid

10. Photosynthetic prokaryotes lack chloroplast but have chlorophyll built into plasma membrane or into the membranes of vesicles.

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5 µm

Mesophyll cell

StomataCO2 O2

Chloroplast

Mesophyll

Vein

Leaf cross section

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C. Photosynthesis: An Overview1. Equation of PS

•Reverse of CR•6 CO2 + 12 H2O* + light energy C6H12O6 + 6 O2* + 6H2O•Simplest photosynthetic equation:

CO2 + H2O [CH2O] + O2

2. Splitting of Water•C. B. van Niel hypothesized that plants split H2O as a source of H+ and release O2 as a by-product.

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

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Fate of Atoms During Photosynthesis

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D. A Preview of Photosynthesis

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

2. The light reactions (in the thylakoids):•Split H2O

•Release O2

•Reduce NADP+ to NADPH•Generate ATP from ADP by photophosphorylation

3. The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH

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

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III. Concept 10.2: The light reactions

Chloroplasts are solar-powered chemical factoriesTheir thylakoids transform light energy into the chemical energy of ATP and NADPH

A. The Nature of Sunlight1. Light is a form of electromagnetic energy, also called

electromagnetic radiation2. Like other electromagnetic energy, light travels in

rhythmic waves3. Wavelength is the distance between crests of waves4. Wavelength determines the type of electromagnetic

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5. The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation

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

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

B. Photosynthetic Pigments: Light Receptors1. Pigments are substances that absorb visible light2. Different pigments absorb different wavelengths3. Wavelengths that are not absorbed are reflected or

transmitted4. Leaves appear green because chlorophyll reflects

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

6. Chlorophyll a is the main photosynthetic pigment and absorbs best in the red and blue wavelengths, and least in the green.

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

8. Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

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C. Excitation of Chlorophyll by Light1. When a pigment absorbs light, it goes from a ground

state (lowest-energy state) to an excited state (orbit of higher potential energy), which is unstable

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

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

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D. Photosystem1. A photosystem consists of a reaction-center complex

(a type of protein complex) surrounded by light-harvesting complexes

2. The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center

3. A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a

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

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5. There are two types of photosystems in the thylakoid membrane

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

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

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

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

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E. Linear Electron Flow 1. During the light reactions, there are two possible

routes for electron flow: cyclic and linear2. Linear electron flow, the primary pathway, involves

both photosystems and produces ATP and NADPH using light energy

3. A photon hits a pigment and its energy is passed among pigment molecules until it excites P680

4. An excited electron from P680 is transferred to the primary electron acceptor

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5. P680+ (P680 that is missing an electron) is a very strong oxidizing agent

6. H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680

7. O2 is released as a by-product of this reaction

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

9. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

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

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11. In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor

12. P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain

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13. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)

14. The electrons are then transferred to NADP+ and reduce it to NADPH

15. The electrons of NADPH are available for the reactions of the Calvin cycle

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Linear Electron Flow

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F. Cyclic Electron Flow1. Cyclic electron flow uses only photosystem I and

produces ATP, but not NADPH2. Cyclic electron flow generates surplus ATP, satisfying

the higher demand in the Calvin cycle3. Cyclic electron flow is thought to have evolved before

linear electron flow and may protect cells from light-induced damage

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Cyclic Electron Flow

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G. Comparison of Chemiosmosis in Chloroplast and Mitochondria

1. Ways Similar:•ETC located in membranes•ATP synthase in same membrane as ETC•ATP synthase complexes and some electron carriers and cytochromes are very similar

2. Ways Different:•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

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•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

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

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

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H. Summary of the Light Reaction

1. Linear Electron Flow produces:•NADPH—electron donor used to reduce CO2 to sugar in the Calvin Cycle•ATP—energy source for the Calvin Cycle•O2—released as by-product

2. Cyclic Electron Flow produces:•ATP

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IV. Concept 10.3 Calvin Cycle A. The Calvin cycle, like the citric acid cycle, regenerates

its starting material after molecules enter and leave the cycle

B. The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH

C. Requires ATP and NADPH from the light reactionD. Carbon enters the cycle as CO2 and leaves as a sugar

named glyceraldehyde-3-phospate (G3P)E. For synthesis of 1 G3P, the cycle must take place three

times, fixing 3 molecules of CO2

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F. Three Phases of Calvin Cycle for each CO2 1. Carbon Fixation•Each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP)•This reaction is catalyzed by RuBP carboxylase, or rubisco which is probably the most abundant protein on Earth and is the most abundant protein in chloroplasts•The six-carbon intermediate is unstable and splits in half to form two molecules of 3-phosphoglycerate for each CO2

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2. Reduction •Couples ATP hydrolysis with reduction (pair of electrons donated by NADPH) of 3-phosphoglycerate to glyceraldehyde 3-phosphate (G3P)•Requires 6 ATP and 6 NADPH•1 G3P exits the cycle to be used by the plant cell

3. Regeneration of CO2 Acceptor (RuBP)•5 G3P are used to regenerate 3 RuBP•Requires 3 ATP

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G. Summary of the Calvin Cycle1. To produce one G3P molecule requires:

•9 ATP•6 NADPH•3 CO2

2. G3P is the raw material used to synthesize glucose and other carbohydrates. 3. Two G3P molecules are needed to produce 1 glucose

molecule. 4. It takes 18 ATP, 12 NADPH and 6 CO2 to produce 1 glucose molecule.

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V. Concept 10.4 Alternative Mechanisms of Carbon Fixation

Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesisOn 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 a seemingly wasteful process called photorespiration

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A. Photorespiration: An Evolutionary Relic?1.In most plants (C3 plants), initial fixation of CO2, via

rubisco, forms a three-carbon compound2.In photorespiration, rubisco adds O2 instead of CO2 in

the Calvin cycle3.Photorespiration consumes O2 and organic fuel and

releases CO2 without producing ATP or sugar

4.Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2

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

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6.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

7.The two most important photosynthetic adaptations are C4 photosynthesis and CAM

B. C4 Plants 8.C4 plants minimize the cost of photorespiration by

incorporating CO2 into four-carbon compounds in mesophyll cells

9.This step requires the enzyme PEP carboxylase10.PEP carboxylase has a higher affinity for CO2 than

rubisco does; it can fix CO2 even when CO2 concentrations are low

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4.These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

5.Adaptive process which enhances carbon fixation under conditions that favor photorespiration (hot, arid)

6.Occurs in corn, sugarcane, agricultural grasses

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C4 Leaf Anatomy

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C. CAM Plants1.Some plants, including succulents, use crassulacean

acid metabolism (CAM) to fix carbon2.CAM plants open their stomata at night, incorporating

CO2 into organic acids

3.Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

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D. The Importance of Photosynthesis: A Review1.The energy entering chloroplasts as sunlight gets

stored as chemical energy in organic compounds2.Sugar made in the chloroplasts supplies chemical

energy and carbon skeletons to synthesize the organic molecules of cells

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

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

5.50% for cellular respiration6.Sucrose—transport form of carbohydrate

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Overview of Photosynthesis

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E. Comparison of CR and PS

Cellular Respiration Photosynthesis

Exergonic redox process Endergonic redox process

Energy released from oxidation of sugar

Energy required to reduce CO2

Electrons from sugar’s H+ lose potential energy in ETC as transported to O2 forming water

Light is energy source that boost potential energy of e- as they move from water to sugar

Produces ATP Produces sugar

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You should now be able to:1. Describe the structure of a chloroplast2. Describe the relationship between an action spectrum

and an absorption spectrum3. Trace the movement of electrons in linear electron flow4. Trace the movement of electrons in cyclic electron flow5. Describe the similarities and differences between

oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts

5. Describe the role of ATP and NADPH in the Calvin cycle6. Describe the major consequences of photorespiration7. Describe two important photosynthetic adaptations that

minimize photorespiration

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