Discussion - Chapters 5-8

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Chapter 8

Lecture Outline

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Chapter 8

Photosynthesis

Key Concepts:

• Overview of Photosynthesis

• Reactions That Harness Light Energy

• Molecular Features of Photosystems

• Synthesizing Carbohydrates via the Calvin Cycle

• Variations in Photosynthesis

Photosynthesis

• Energy within light is captured and used to synthesize carbohydrates

• CO2 is reduced

• H2O is oxidized

• Energy from light drives this endergonic reaction

Photosynthesis powers the biosphere

Biosphere – regions on the surface of the Earth and atmosphere where living organisms exist

Largely driven by the photosynthetic power of green plants

Energy cycle – cells use organic molecules for energy and plants replenish those molecules using photosynthesis

• In the process plants also produce oxygen

Trophic levels

Heterotroph

• Must eat food (organic molecules from their environment) to sustain life

Autotroph

• Makes organic molecules from inorganic sources

Photoautotroph

• Use light as a source of energy

• Green plants, algae, cyanobacteria

Figure 8.1

Chloroplast

• Organelle in plants and algae that carries out photosynthesis

• Green pigment is chlorophyll

• Majority of photosynthesis occurs internally in leaves, in the mesophyll

• Carbon dioxide enters and oxygen exits leaf through pores called stomata

Figure 8.2 1

(1): ©McGraw-Hill Education/Mark Dierker, photographer;

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Chloroplast anatomy

Outer and inner membrane separated by intermembrane space

A third membrane, the thylakoid membrane contains pigment molecules

• Membrane forms thylakoids

• Enclose thylakoid lumen

Granum – stack of thylakoids

Fluid filled region between thylakoid membrane and inner membrane is the stroma

Figure 8.2 2

CO2 enters the leaf via stomata, and O2, a product of photosynthesis, exits via stomata.

(2): ©Biophoto Associates/SPL/Science Source; (3): ©Omikron/Science Source

Two stages of photosynthesis

Light reactions

• Use light energy

• Take place in thylakoid membranes

• Produce ATP, NADPH and O2

Calvin cycle

• Occurs in stroma

• Uses ATP and NADPH to incorporate CO2 into carbohydrate

Figure 8.3

The light reactions at the thylakoid membrane produce ATP, NADPH, and O2.

The Calvin cycle in the stroma uses CO2, ATP, and NADPH to make carbohydrates.

Reactions That Harness Light Energy

Light is a type of electromagnetic radiation

Travels as waves

• Short to long wavelengths

Also behaves as particles called photons

• Shorter wavelengths have more energy

Figure 8.4

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Photosynthetic pigments 1

Pigments absorb some light energy and reflect others

• Leaves are green because they absorb red and violet, and reflect green wavelengths

Absorption boosts electrons to higher energy levels

Wavelength of light that a pigment absorbs depends on the amount of energy needed to boost an electron to a higher orbital

Having different pigments allows plants to absorb light at many different wavelengths

Photosynthetic Pigments 2

After an electron absorbs energy, it is an excited state and usually unstable

Releases energy as heat or light

Excited electrons in pigments can be transferred to another molecule or “captured”

Structure of pigment molecules

• Chlorophylls a and b contain a porphyrin ring

• A magnesium ion is bound to the porphyrin ring

• β-carotenecontains two smaller rings

a) Chlorophylls a and b

b) β-Carotene

(a carotenoid)

Absorption versus action spectrum

Absorption spectrum • Wavelengths that are absorbed by different pigments

Action spectrum • Rate of photosynthesis by whole plant at specific wavelengths

a) Absorption spectra

b) Action spectrum

Photosystems I and II 1

Captured light energy can be transferred to other molecules to produce energy intermediate molecules for cellular work

Thylakoid membranes of chloroplast contain two distinct complexes of molecules

• Photosystem I (PSI) – discovered first

• Photosystem II (PSII) – first step in photosynthesis

Light excites pigment molecules in both PSII and PSI

Photosystems I and II 2

Photosystem II • The initial step in photosynthesis

• Excited electrons travel from PSII to PSI

• Oxidizes water, generating O2 and H 

• Releases energy in electron transport chain (ETC)

• Energy used to make H  electrochemical gradient

Photosystem I

• Primary role to make NADPH

• Addition of H  to NADP

 contributes to H

 gradient

by depleting H from the stroma

Formation of ATP in chloroplasts

ATP synthesis in chloroplasts • Achieved by chemiosmotic mechanism called

photophosphorylation

• Driven by flow of H from thylakoid lumen into stroma via ATP synthase

H  gradient generated three ways: • H

  in thylakoid lumen by splitting of water

• H 

 by ETC pumping H into lumen • H

  in stroma from formation of NADPH

Figure 8.8

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Three chemical products

Oxygen, O2 • Produced in thylakoid lumen by oxidation of H2O

by PSII • Two electrons transferred to P680

 molecules

NADPH • Produced in the stroma from high-energy electrons

that start in PSII and are boosted in PSI • NADP 2electron H NADPH

    

ATP • Produced in stroma by ATP synthase using the

H 

electrochemical gradient

Noncyclic and cyclic electron flow

Noncyclic

• Electrons begin at PSII and eventually transfer to NADPH, a linear process

• Produces both ATP and NADPH in equal amounts

Cyclic photophosphorylation (cyclic electron flow)

• Electron cycling releases energy to transport H 

into lumen driving ATP synthesis

• Produces only ATP • PSI electrons excited, release energy and eventually

return to PSI

Figure 8.9

When light strikes photosystem I, electrons are excited and sent to ferredoxin (Fd). From Fd, the electrons are then transferred to QB, to the cytochrome complex, to plastocyanin (Pc), and back to photosystem I. This produces an H electrochemical gradient, which is used to make ATP via ATP synthase.

The Cytochrome Complexes of Mitochondria and Chloroplasts Contain

Evolutionarily Related Proteins • Descent with modification – related genes play

similar but specialized roles in cells

• Homologous genes are similar because they are derived from a common ancestral gene

• The electron transport chains of mitochondria and chloroplasts use homologous genes

• Family of cytochrome b proteins plays similar but specialized roles in both mitochondria and chloroplasts

Figure 8.10

a) Cytochrome 6

b - f in the chloroplast

b) Cytochrome 1

b c in the mitochondrion

Molecular Features of Photosystems

Photosystem II (PSII)

Two main components: • Light-harvesting complex (or antenna complex)

• Directly absorbs photons

• Energy transferred via resonance energy transfer

• Reaction center • P680 → P680*

• P680* is relatively unstable, so energy is transferred quickly

• Electron transfers to primary electron acceptor and captured

• Water is oxidized to replace the electron on P680 ,

producing oxygen gas in the process

Figure 8.11

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Z scheme

• Zigzag shape of energy curve

• Photosynthesis involves increases and decreases in the energy of an electron as it moves from PSII through PSI to NADPH

• Electron on a nonexcited pigment molecule in PSII starts with the lowest energy

• Light excites the electron in PSII

• Photosystem I boosts the electron to an even higher energy level

Figure 8.12

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Synthesizing Carbohydrates via the Calvin Cycle

Calvin Cycle (aka Calvin-Benson Cycle)

CO2 incorporated into carbohydrates

• Precursors to other organic molecules

• Energy storage

Requires massive input of energy

• For every 6 CO2 incorporated, 18 ATP and 12 NADPH must be used

Product is glyceraldehyde-3-phosphate (G3P)

• Glucose is later made from G3P in separate process

Calvin cycle

Phase 1 – Carbon fixation • CO2 incorporated into RuBP using rubisco • Reaction product is a six-carbon intermediate that splits into

two 3-phosphoglycerate molecules (3PG)

Phase 2 – Reduction and carbohydrate production • ATP is used to convert 3PG into

1,3-bisphosphoglycerate (1,3-BPG) • NADPH electrons reduce it to glyceraldehyde-3-phosphate (G3P) • 6 CO2 → 12 G3P

• Only 2 G3P molecules used for carbohydrates • 10 G3P molecules must be used for regeneration of RuBP

Phase 3 – Regeneration of RuBP • 10 G3P are converted into 6 RuBP using 6 ATP

Figure 8.13

1. Phase 1: Carbon fixation. CO2 is incorporated into an organic molecule via rubisco.

2. Phase 2: Reduction and carbohydrate production. ATP is used as a source of energy, and NADPH donates high-energy electrons.

3. Phase 3: Regeneration of RuBP. Two G3P are used to make glucose and other sugars; the remaining 10 G3P are needed to regenerate RuBP via several enzymes. ATP is required for RuBP regeneration.

The Calvin Cycle Was Determined by Isotope Labeling Methods

• 14

C labeled‐ CO2 injected into cultures of green algae

• Allowed to incubate different lengths of time

• Separated newly made radiolabeled molecules using two- dimensional paper chromatography

• Autoradiography – radiation from 14 C labeled‐ molecules makes dark spots on the film

• Identified 14 C labeled‐ spots and the order they appeared

• Melvin Calvin awarded Nobel Prize in 1961

Figure 8.14 1 GOAL The incorporation of CO2 into carbohydrate involves a biosynthetic pathway. The aim of this experiment was to identify the steps.

KEY MATERIALS The green alga Chlorella pyrenoidosa and 14 C labeled‐ CO2.

1. Grow Chlorella in an apparatus called a “lollipop.” Add 14 C labeled‐ CO2 and incubate

for various lengths of time (from fractions of a second to many minutes). Stop the Calvin cycle by placing a sample of cells into a solution of alcohol.

2. Take a sample of the internal cell contents and spot on the corner of chromatography paper. This spot is called the origin.

3. Place edge of paper in a solvent, such as phenol-water, and allow time for solvent to rise and separate the mixture of molecules that were spotted at the origin.

4. Dry paper, turn 90 , and then place the edge in a different solvent such as butanol-propionic acid-water. Allow time for solvent to rise.

5. Dry paper and place next to X-ray film. The developed film reveals dark spots where 14

C labeld– molecules were located. This procedure is called autoradiography.

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Figure 8.14 2

GOAL The incorporation of CO2 into carbohydrate involves a biosynthetic pathway. The aim of this experiment was to identify the steps.

KEY MATERIALS The green alga Chlorella pyrenoidosa and 14 C labeled– CO2 .

6. THE DATA* 7. CONCLUSION The identification of the molecules in each spot elucidated the steps of the Calvin cycle.

8. SOURCE Calvin, M. 1961. The path of carbon in photosynthesis, Nobel Lecture, 618 to 644.

*An autoradiograph from one of Calvin’s experiments.

Variations in Photosynthesis

Environmental conditions can influence both the efficiency and way the Calvin cycle works

• Light intensity

• Temperature

• Water availability

Photorespiration

Rubisco functions as a carboxylase

RuBP + CO2 → 2 3PG

C3 plants make 3PG

• Rubisco can also be an oxygenase • Adds O2 to RuBP eventually releasing CO2 • This is called photorespiration

• Using O2 and liberating CO2 is wasteful

• More likely in hot and dry environments

• Favored when CO2 low and O2 high

Figure 8.15

C4 plants

• Evolved a mechanism to minimize respiration

• 4C plants make oxaloacetate (4 carbon molecule) in the first step of carbon fixation

• Hatch-Slack pathway

• Leaves have two-cell layer organization • Mesophyll cells

• CO2 enters via stomata and 4 carbon compound formed (PEP carboxylase does not promote photorespiration)

• Bundle-sheath cells • 4 carbon molecule transferred that releases steady

supply of CO2, minimizing photorespiration

Which is better – C3 or C4?

• It depends on the environment

• In warm dry climates 4

C plants conserve water and prevent photorespiration

• In cooler climates, 3

C plants use less energy to fix CO2

• 90% of plants are 3

Figure 8.16

Mesophyll cells: Form a protective layer around bundle-sheath cells so they are not exposed to high O2.

High O2 and low CO2 diffuse around the mesophyll cells.

Bundle-sheath cells: Site of the Calvin cycle.

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CAM plants

• Some 4

C plants separate processes using time

• Crassulacean Acid Metabolism

• CAM plants open their stomata at night

• CO2 enters and is converted to malate

• Stomata close during the day to conserve water

• Oxaloacetate converted to malate

• Malate broken down into CO2 to drive Calvin cycle during the day

Figure 8.17

CO2 is initially incorporated into a 4-carbon molecule.

The 4-carbon molecule releases CO2, which is incorporated into the Calvin cycle.

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