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Honors Biology

Chapter 8Photosynthesis

John Regan

Wendy Vermillion

Columbus State Community College

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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8.1 Overview of photosynthesis

• Flowering plants as photosynthesizers– Photosynthesis occurs in the green parts of

plants– Water is taken up by roots and transported to

leaves by veins– Carbon dioxide enters through openings in

the leaves called stomata– Light energy is absorbed by photopigments in

thylakoids of chloroplasts

Chloroplast Anatomy• Double membrane on outside• Inner membrane-thylakoids

– Stacks of thylakoids- grana– Chlorophyll and other pigments in thylakoid membranes– Absorbs light

• Stroma- space between grana– Carbohydrates made here

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Plants as photosynthesizers

• Fig 8.2

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Overview of photosynthesis cont’d.• Photosynthetic reaction Solar energy + 6CO2 + 6H2O C6H12O6 + 6O2

– Carbon dioxide is reduced to form glucose, therefore oxygen in glucose comes from CO2

– Water is oxidized to form oxygen, therefore the oxygen given off by photosynthesis comes from water

• Two set of reactions– Photosynthesis consists of 2 sets of reactions– The light reactions occur first followed by the Calvin

cycle– NADP+ carries hydrogens from the light reactions to

the Calvin cycle– ATP formed in the light reactions is used in the Calvin

cycle

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Overview of photosynthesis

• Fig 8.3

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8.2 Solar energy capture• Visible light

– Visible light is a part of the electromagnetic spectrum

– Visible light includes the colors violet,blue green, yellow, orange, and red

– Blue-violets have the shortest wavelengths and the highest energy content

– Photopigments chlorophyll a and b and carotenoids absorb specific portions of the light spectrum

– Blue, violet, and red are best absorbed– Green is reflected

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The electromagnetic spectrum and visible light

• Fig 8.4

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Photosynthetic pigments and photosynthesis

• Fig 8.5

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Solar energy capture cont’d.

• Light reactions– 2 types of photosystems (PS I and PS II)

composed of photopigments– Composed of chlorophyll a and b and carotenoids– Reaction center of each photosystem is

chlorophyll a– Electrons of chlorophyll a absorb light energy and

move to an electron acceptor molecule– Electrons then pass down one of 2 paths-

noncyclic or cyclic

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Solar energy capture cont’d.• The noncylic pathway

– Produces ATP and NADPH– Light energy is absorbed by chlorophyll a in PSII– Excited electrons are removed– Those electrons are replaced by splitting a water

molecule

2H2O 4H+ + 4e- + O2

– High energy electrons removed from PSII pass down an ETC releasing energy to build a chemiosmotic gradient

– ATP is produced– Electrons are picked up by NADP+ to make NADPH– Electrons lost by PSI chlorophyll a can be replaced by

those from PSII

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The light reactions: the noncyclic electron pathway

• Fig 8.6

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Solar energy capture cont’d.• Cyclic pathway

– Generates only ATP– PSI absorbs solar energy and electrons from

chlorophyll a are removed– Electrons pass down an ETC – Energy released is stored in an

electrochemical gradient for chemiosmosis– ATP is produced

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The light reactions: the cyclic electron pathway

• Fig 8.7

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Solar energy capture cont’d.

• Organization of thylakoid– Thylakoid space acts as reservoir of

hydrogen ions– Hydrogen ions flow down gradient

through an ATP synthase complex in thylakoid membrane to produce ATP

– Thylakoid membranes contain the following complexes:

• PS I and II• ETC• ATP synthase complexes

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Organization of the thylakoid

• Fig 8.8

Light Reaction In/Out Table

In Out

H2O O2

NADP+ NADPH

ADP + P ATP

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8.3 Carbohydrate synthesis

• Stages of the Calvin cycle1. Carbon fixation

• carbon fixation occurs when a reaction occurs that attaches CO2 to an organic compound

• Ribulose biphosphate (RuBP) combines with CO2 to form a 6C molecule

• Enzyme which catalyzes this reaction is RuBP carboxylase

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The Calvin cycle reactions

• Fig 8.9

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Stages of the Calvin Cycle

2. Reduction of carbon dioxide– 6C molecule breaks into 2 3C 3PG molecules– Each undergoes reduction to G3P in a 2 step process– CO2 is reduced to a carbohydrate and NADPH is

oxidized to NADP+

– ATP provides the energy – Every 3 turns of the Calvin cycle produce 6 G3P; one

molecule leaves the pathway to synthesize carbohydrates

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Stages of Calvin Cycle

3. Regeneration of RuBp– Constant supply of RuBP needed to keep the

Calvin cycle turning– For every 3 turns of the Calvin cycle 5 G3P

molecules are used to reform RuBP– This reaction also requires energy from ATP

Calvin Cycle In/Out Table

In Out

NADPH NADP+

ATP ADP + P

CO2 Glucose

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Carbohydrate synthesis cont’d.

• Importance of the Calvin cycle– G3P is the Calvin cycle product that can be converted

to glucose phosphate – Glucose phosphate can then be converted into many

different organic molecules

Glucose phosphate

Glucose and starches Amino acids Fatty acids and glycerol

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8.5 Photosynthesis versus cellular respiration

• Photosynthesis vs cell respiration– Both plant and animal cells carry out cell

respiration– Only plant cells photosynthesize– Both processes utilize an electron transport

chain and chemiosmosis for ATP production– Photosynthesis reduces CO2 to carbohydrates

and releases O2

– Respiration utilizes O2 and gives off CO2

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Photosynthesis vs cell respiration

• Fig 8.12

Objectives Chapters 71. Know the equation for aerobic cellular respiration.

2. Know the 4 stages of aerobic respiration and where each happens.

3. Know the terms oxidation, reduction.

4. Know how many ATPs are made from NADH and FADH2 in ETC.

5. Know the 2 categories of metabolism and whether they are exergonic or endergonic.

6. Know the efficiencies of aerobic and anaerobic respiration.

7. Know the 2 types of fermentation.

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Objectives Chapter 81. Know the equation for photosynthesis.

2. Know what colors of light have the most energy.

3. Know what colors of light are absorbed the best by chlorophyll.

4. Know the structure of a chloroplast.

5. Know the source of oxygen released by photosynthesis.

6. Know the 2 stages of Photosynthesis.

7. Know the reactants and products of the light reaction and Calvin cycle.

8. Know the differences between the cyclic and noncyclic electron pathways.

9. Know how RuBP is used in the Calvin cycle.

10. Compare photosynthesis and cellular respiration.1-27