1. Chapter 7: Photosynthesis (Outline)
Photosynthetic Organisms
Flowering Plants as Photosynthesizers
Photosynthesis
Light Reactions
Noncyclic Pathway
Calvin Cycle Reactions
Carbon Dioxide Fixation
Reduction of Carbon Dioxide
Regeneration of RuBP C4
CAM

2. Photosynthetic Organisms
Organisms cannot live without energy
Almost all the energy that organisms used comes from the solar energy
Photosynthesis:
A process that captures solar energy and transforms it into chemical energy (carbohydrate)
Occurrs in higher plants, algae, mosses, photosynthetic protists and some bacteria (cyanobacteria)

3. Photosynthetic Organisms
Autotrophs – organisms having the ability to synthesize carbohydrates
Heterotrophs – are consumers that use carbohydrates produced by photosynthesis

4. Flowering Plants & Photosynthesis
Photosynthesis takes place in the green portions of plants
Leaf of flowering plant contains mesophyll tissue with chloroplasts
Specialized to carry on photosynthesis
CO2 enters leaf through stomata
Diffuses into chloroplasts in mesophyll cells
In the stroma, CO2 is combined with H2O to form C6H12O6 (sugar)
Chlorophyll is capable of absorbing solar energy, which drives photosynthesis

6. Photosynthesis
The process of photosynthesis is an example of a redox reaction (i.e. movement of electrons from one molecule to another)
In living things, electrons are accompanied by hydrogen ions (H+ + e)
Oxidation is the loss of hydrogen atoms; Reduction is the gain of hydrogen atoms

7. Photosynthesis
Carbon dioxide reduction requires energy and hydrogen atoms
Solar energy will be used to generate ATP needed to reduce CO2 to sugar
Electrons needed to reduce CO2 will be carried by a coenzyme (NADP+)
NADP+ is the active redox coenzyme of photosynthesis
When NADP+ is reduced, it accepts 2 e- and H+
When NADP+ is oxidized, it gives up its electrons

8. Photosynthetic Reactions
In the 1930’s, C. B. van Niel studying photosynthesis in bacteria discovered that CO2 was not split in the process of carbohydrate synthesis
Researchers later confirmed using the isotope oxygen (18O) that O2 given off from photosynthesis comes from water not CO2
When water splits, O2 is released and the hydrogen atoms (2 e- + H+) are taken up by NADPH

9. Photosynthetic Reactions
Light Reactions:
Chlorophyll present in thylakoid membranes absorbs solar energy
This energizes electrons
Electrons move down electron transport chain
Pumps H+ into thylakoids
Used to make ATP out of ADP and NADPH out of NADP+
Calvin Cycle Reactions:
In the stroma, CO2 is reduced to a carbohydrate
Reduction requires the ATP and NADPH produced in the light reactions

10. Photosynthesis Overview

11. Photosynthetic Pigments
Molecules that absorb some colors in rainbow (visible light) more than others, why?
Colors least absorbed reflected/transmitted most
Absorption Spectra
Graph showing relative absorption of the various colors of the rainbow
Chlorophylls a and b (main pigments) are green because they absorb much of the reds and blues of white light
Carotenoids are accessory pigments

12. Phosynthetic Pigments and Photosynthesis

13. Spectrophotometer
An instrument which measures the amount of light of a specified wavelength which passes through a medium
Amount of light absorbed at each wavelength is plotted on a graph, resulting in an absorption spectra

14. Light Reactions The light reactions utilize two photosystems:
Photosystem I (PSI)
Photosystem II (PSII)
A photosystem consists of
Pigment complex (molecules of chlorophylls a & b the carotenoids) and electron acceptor molecules that help collect solar energy like an antenna
Occur in the thylakoid membranes

15. The Noncyclic Electron Pathway
Uses two photosystems, PS I and PS II
Energy enters the system when PS II becomes excited by light
Causes an electron to be ejected from the reaction center (chlorophyll a)
Electron travels down electron transport chain to PS I
PS II takes replacement electron from H2O, which splits, releasing O2 and H+ ions
The H+ ions concentrate in thylakoid chambers
Which causes ATP production, how?

16. The Noncyclic Electron Pathway
PS I captures light energy and ejects the energized electron (e-) from the reaction center
Transferred permanently by electron acceptors to a molecule of NADP+
Causes NADPH production
NADP+ + 2 e- + H+
NADPH and ATP produced are used by the Calvin cycle reaction (reduction of CO2) in the stroma

18. Organization of the Thylakoid Membrane
PS II:
Pigment complex and electron-acceptors
Adjacent to an enzyme that oxidizes water
Oxygen is released as a gas (by-product)
Electron transport chain:
Consists of Pq (plastoquinone) and cytochrome complexes
Carries electrons from PS II to PS I
Pumps H+ from the stroma into thylakoid space
PS I:
Pigment complex and electron acceptors
Adjacent to enzyme that reduces NADP+ to NADPH
ATP synthase complex:
Has a channel for H+ flow
Which drives ATP synthase to join ADP and Pi

19. Organization of a Thylakoid

20. ATP Production Thylakoid space acts as a reservoir for H+ ions
Each time water is oxidized, two H+ remain in the thylakoid space
Electrons yield energy
Used to pump H+ across thylakoid membrane
Move from stroma into the thylakoid space
Flow of H+ back across thylakoid membrane
Energizes ATP synthase
Enzymatically produces ATP from ADP + Pi
This method of producing ATP is called chemiosmosis

21. Calvin Cycle Reactions: Overview of C3 Photosynthesis
A cyclical series of reactions
Utilizes atmospheric carbon dioxide to produce carbohydrates
Known as C3 photosynthesis
Involves three phases:
Carbon dioxide fixation
Carbon dioxide reduction
RuBP Regeneration

22. Calvin Cycle Reactions: Phase 1: Carbon Dioxide Fixation
CO2 enters the stroma of the chloroplast via the stomata of the leaves
CO2 is attached to 5-carbon RuBP molecule
Result in an intermediate 6-carbon molecule
This splits into two 3-carbon molecules (3PG)
Reaction is accelerated by RuBP Carboxylase (Rubisco)
CO2 now “fixed” because it is part of a carbohydrate

23. Calvin Cycle Reactions: Phase 2: Carbon Dioxide Reduction
ATP phosphorylates each 3PG molecule and creates 1,3-bisphosphoglycerate (BPG)
BPG is then reduced by NADPH to glyceraldehyde-3-phosphate (G3P)
NADPH and some ATP used for the reduction reactions are produced in light reactions

24. Calvin Cycle Reactions: Phase 3: Regeneration of RuBP
RuBP used in CO2 fixation must be replaced
Every three turns of Calvin Cycle,
Five G3P molecules are used
To remake three RuBP molecules
5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)

25. The Calvin Cycle

26. C3 Plants
C3 plants include more than 95% percent of the plant species on earth (e.g. trees)
Called C3 because the CO2 is first incorporated into a 3-carbon compound
In hot dry conditions, the photosynthetic efficiency of C3 plants suffers because of photorespiration
Stomata must close to avoid wilting
CO2 decreases and O2 increases
O2 starts combining with RuBP instead of CO2
CO2 + RUBP Rubisco    PG

27. C4 Photosynthesis: C4 Plants
Use a supplementary method of CO2 uptake forming a 4-carbon molecule instead of the two 3-carbon molecules
In C4 plants
CO2 is inserted into a 3-carbon molecule called phosphoenolpyruvate (PEP) forming
Oxaloacetate, a C4 molecule in the mesophyll cells
Oxaloacetate is converted into malate, which is transported into the bundle sheath cells
Here the 4-carbon molecule is broken down into CO2, which enters the Calvin cycle to form sugars & starch

28. Chloroplast distribution in C4 vs. C3 Plants

29. CO2 Fixation in C4 vs. C3 Plants
In hot and dry climates
C4 plants avoid photorespiration as PEP in mesophyll cells does not bind to O2
Net productivity of C4 is about 2-3 times of C3 plants
In cool, moist conditions, CO2 fixation in C3 is three times more efficient than in C4

30. CAM Photosynthesis
Called CAM after the plant family in which it was first found (Crassulaceae)
CAM plants partition carbon fixation by time
Crassulacean-Acid Metabolism
During the night through their open stomata
CAM plants use PEPCase to fix CO2
Forms C4 molecules (malate)
Stored in large vacuoles in mesophyll cells
During daylight
NADPH and ATP are available
Stomata closed for water conservation
C4 molecules release CO2 to Calvin (C3) cycle

31. CO2 Fixation in a CAM Plant
Adaptive Value:
Better water use efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.)