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Chapter 6Pathways of

Photosynthesis

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Photosynthesis Produces Food and Releases Oxygen

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6.1 Photosynthesizers are autotrophs that produce

their own food Photosynthesis converts solar energy

into the chemical energy of a carbohydrate Photosynthesizers are the base of almost

every food chain in the world Autotrophs and heterotrophs (consumers)

use organic molecules from photosynthesis

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Figure 6.1 Photosynthetic organisms

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6.2 In plants, chloroplasts carry out photosynthesis

Photosynthesis occurs in green plant parts Carbon dioxide enters leaves via stomata Water and carbon dioxide diffuse into

chloroplasts, the photosynthetic organelles Chloroplasts contain the chlorophyll and

other pigments that absorb solar energy

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Figure 6.2 Leaf structures specialized for photosynthesis

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6.3 Photosynthesis is a redox reaction that releases O2

Oxidation is the loss of electrons and reduction is the gain of electrons Because they always occur together, the entire

reaction is a redox reaction

Photosynthesis is a redox reaction in which hydrogen atoms are transferred from water to carbon dioxide

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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES

6.4 Experiments showed that the O2 released by photosynthesis

comes from water In 1930 C.B. van Niel used the oxygen isotope

O18 to track the path of oxygen production in photosynthesis

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6.5 Photosynthesis involves two sets of reactions

The light reactions Only occur when solar energy is available Chlorophyll molecules absorb solar energy to

energize electrons used in ATP production The Calvin cycle reactions

CO2 is taken up and reduced to a carbohydrate that can be converted to glucose

ATP and NADPH from light reactions are needed

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Figure 6.5A Overview of photosynthesis

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First, Solar Energy Is Captured

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6.6 Light reactions begin: Solar energy is absorbed by pigments

Solar energy can be described in terms of its wavelength and its energy content

Most common pigments chlorophylls a and b and carotenoids are capable of absorbing various portions of visible light

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Figure 6.6A The electromagnetic spectrum includes visible light

Figure 6.6B Absorption spectrum of photosynthetic pigments

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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES

6.7 Fall temperatures cause leaves to change color

Chlorophyll is not very stable and in the fall, sufficient energy to rebuild chlorophyll is not available

Chlorophyll in leaves disintegrates,

and we begin to see yellow and

orange pigments in the leaves

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6.8 Solar energy boosts electrons to a higher energy level

Photosystem I (PS I) & Photosystem II (PS II) consist of antenna molecules and a reaction center

Antenna molecules absorb light and pass energy to the reaction center

In reaction center excited electrons are passed to electron acceptors

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Figure 6.8 A general model of a photosystem

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6.9 Electrons release their energy as ATP forms

Electron transport chain - a series of carriers that pass electrons from one to another

Each electron transfer releases energy that is ultimately used to make ATP ATP synthase complexes - hydrogen ions flow

through, providing energy for ATP synthesis

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Figure 6.9 High-energy electrons (e−) release energy as they pass down an electron transport chain

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6.10 During the light reactions, electrons follow a noncyclic pathway

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6.11 The thylakoid membrane is organized to produce ATP and NADPH

Chemiosmosis - ATP production is tied to an H+ gradient across a membrane

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Figure 6.11 Organization of a thylakoid

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Second, Carbohydrate Is Synthesized

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Fig 6.12 Calvin cycle reactions

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6.12 The Calvin cycle uses ATP and NADPH from the light reactions to

produce a carbohydrate Calvin cycle reduces CO2 from the atmosphere to

produce carbohydrate CO2 Fixation – CO2 combines with RuBP with the help of RuBP

carboxylase CO2 Reduction – CO2 is reduced to a carbohydrate

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RuBP Regeneration – every three turns of the Calvin cycle, five molecules of G3P reform three molecules of RuBP

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6.13 In plants, carbohydrate is the starting point for other molecules

Figure 6.13 Fates of G3P

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C3, C4, and CAM Photosynthesis Thrive Under

Different Conditions

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6.14 C3 photosynthesis evolved when oxygen was in limited supply

In C3 plants the first detectable molecule after CO2 fixation is a C3 molecule 3PG Common in plants where temperature and rainfall are moderate

Under water stress, stomata close, limiting CO2 and reducing efficiency O2 competes with CO2 for the active site of RuBP carboxylase, and

less C3 is produced

Figure 6.14 Carbon dioxide fixation in C3 plants as exemplified by these wildflowers

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6.15 C4 photosynthesis boosts CO2 concentration for RuBP carboxylase

C4 plant - the first detectable molecule following CO2 fixation is a four carbon molecule

Avoid the uptake of O2 by RuBP carboxylase by increasing the amount of CO2 available to the enzyme

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Figure 6.15A Anatomy of a C3 plant compared to a C4 plant

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Figure 6.15B Carbon dioxide fixation in C4 plants as exemplified by corn

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6.16 CAM photosynthesis is another alternative to C3 photosynthesis

CAM – crassulacean-acid metabolism Partitioning based on time

CAM plants open their stomata only at night; during the day, the stomata are closed, conserving water

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Figure 6.16 Carbon dioxide fixation in CAM plants as exemplified by pineapple

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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES

6.17 Destroying tropical rain forests contributes to global warming

Tropical rain forests contribute greatly to uptake of CO2 because they are the most efficient of all terrestrial ecosystems CO2 in our atmosphere traps radiant heat from the

sun and warms the world Deforestation in tropical rain forests accounts for

20–30% of all carbon dioxide in the atmosphere Burning also removes trees that would ordinarily

absorb CO2

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Figure 6.17 Global warming: Past trends and future predictions

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Connecting the Concepts:Chapter 6

General equation for photosynthesis6CO2 + 6H2O → C6H12O6 + 6O2

Two separate sets of reactions Light reactions (in thylakoid membrane)

Absorb solar energy and produce NADH and ATP, which are provided to the Calvin cycle to reduce CO2

Calvin cycle reactions (in stroma) Reduce carbon dioxide to a carbohydrate

C4 and CAM are alternative means of supplying RuBP carboxylase with CO2 while limiting its exposure to oxygen.

Photosynthesis keeps the biosphere functioning by supplying energy to all organisms Organisms use cellular respiration, which occurs in the mitochondria, to

get energy out of carbohydrates