1. CHAPTER 8 PHOTOSYNTHESIS Brenda Leady, University of Toledo
Prepared by
Brenda Leady, University of Toledo
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Trophic organization Heterotroph Autotroph
Must eat food, organic molecules from their environment, to sustain life
Autotroph
Make organic molecules from inorganic sources
Photoautotroph
Use light as a source of energy
Green plants, algae, cyanobacteria

3. CO2 + H2O + light energy → C6H12O6 + O2 + H2O
Photosynthesis
Energy within light is captured and used to synthesize carbohydrates
CO2 + H2O + light energy → C6H12O6 + O2 + H2O
CO2 is reduced
H2O is oxidized
Energy from light drives this endergonic reaction

4. Biosphere
Regions on the surface of the Earth and in the atmosphere where living organisms exist
Largely driven by the photosynthetic power of green plants
Cycle where cells use organic molecules for energy and plants replenish those molecules using photosynthesis
Plants also produce oxygen

5. Chloroplast
Organelles in plants and algae that carry out photosynthesis
Chlorophyll- green pigment
Majority of photosynthesis occurs in leaves in central mesophyll
Stomata- carbon dioxide enters and oxygen exits leaf

6. Chloroplast anatomy Outer and inner membrane Intermembrane space
Thylakoid membrane contains pigment molecules
Thylakoid membrane forms thylakoids
Enclose thylakoid lumen
Granum- stack of thylakoids
Stroma- fluid filled region between thylakoid membrane and inner membrane

8. 2 stages of photosynthesis
Light reactions
Take place in thylakoid membranes
Produce ATP, NADPH and O2
Calvin cycle
Occurs in stroma
Uses ATP and NADPH to incorporate CO2 into organic molecules

10. Light energy Type of electromagnetic radiation Travels as waves
Short to long wavelengths
Also behaves as particles- photons
Shorter wavelengths have more energy

12. Photosynthetic pigments absorb some light energy and reflect others
Leaves are green because they 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

14. After an electron absorbs energy, it is an excited state and usually unstable
Releases energy as
Heat
Light
Excited electrons in pigments can be transferred to another molecule or “captured”
Captured light energy can be transferred to other molecules to ultimately produce energy intermediates for cellular work

15. Pigments
Chlorophyll a
Chlorophyll b
Carotenoids

16. Absorption vs. action spectrum
Absorption spectrum
Wavelengths that are absorbed by different pigments in the plant
Action spectrum
Rate of photosynthesis by whole plant at specific wavelengths

17. Photosystems Thylakoid membrane Photosystem I (PSI)
Photosystem II (PSII)

18. Photosytem II (PSII) 2 main components
Light-harvesting complex or antenna complex
Directly absorbs photons
Energy transferred via resonance energy transfer
Reaction center
P680 →P680*
Relatively unstable
Transferred to primary electron acceptor
Removes electrons from water to replace oxidized P680
Oxidation of water yields oxygen gas

20. Photosystem II (PSII) Redox machine
Recent research in biochemical composition of protein complex and role of components
3 dimensional structure determined in 2004 using x-ray crystallography

22. Electron releases some of its energy along the way
Electrons accepted by primary electron acceptor is PSII to a pigment molecule in the reaction center of PSI
Electron releases some of its energy along the way
Establishes H+ electrochemical gradient
ATP synthesis uses chemiosmotic mechanism similar to mitochondria

23. Photosystem I (PSI) Key role to make NADPH
Light striking light-harvesting complex of PSI transfers energy to a reaction center
High energy electron removed from P700 and transferred to a primary electron acceptor
NADP+ reductase
NADP+ + 2 electrons + H + → NADPH
P700+ replaces its electrons from plastocyanin
No splitting water, no oxygen gas formed

24. Summary O2 produced in thylakoid lumen by oxidation of H2O by PSII
2 electrons transferred to P680+
ATP produced in stroma by H+ electrochemical gradient
Splitting of water places H+ in the lumen
High-energy electrons move from PSII to PSI, pumping H+ into the lumen
Formation of NADPH consumes H+ in the stroma
NADPH produced in the stroma from high-energy electrons that start in PSII and boosted in PSI
NADP+ + 2 electrons + H + → NADPH

26. Cyclic and noncyclic electron flow
Electrons begin at PSII and eventually transfer to NADPH
Linear process produces ATP and NADPH in equal amounts
Cyclic photophosphorylation
Electron cycling releases energy to transport H+ into lumen driving synthesis of ATP

28. The cytochrome complexes of mitochondria and chloroplasts have evolutionarily related proteins in common
Homologous genes are similar because they are derived from a common ancestor
Comparing the electron transport chains of mitochondria and chloroplasts reveals homologous genes
Family of cytochrome b-type proteins plays similar but specialized roles

30. Calvin cycle ATP and NADPH used to make carbohydrates
Somewhat similar to citric acid cycle
CO2 incorporated into carbohydrates
Precursors to all organic molecules
Energy storage

31. CO2 incorporation Also called Calvin-Benson cycle
Requires massive input of energy
For every 6 CO2 incorporated, 18 ATP and 12 NADPH used
Glucose is not directly made

32. 3 phases Carbon fixation Reduction and carbohydrate production
CO2 incorporated in RuBP using rubisco
6 carbon intermediate splits into 2 3PG
Reduction and carbohydrate production
ATP is used to convert 3PG into 1,3-bisphosphoglycerate
NADPH electrons reduce it to G3P
6 CO2 → 12 G3P
2 for carbohydrates
10 for regeneration
Regeneration of RuBP
10 G3P converted into 6 RuBP using 6 ATP

34. The Calvin cycle was determined by isotope labeling methods
14C-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 14C-labeled molecules makes dark spots on the film
Identified 14C-labeled spots and the order they appeared
Calvin awarded Nobel Prize in 1961

36. Variations in photosynthesis
Certain environmental conditions can influence both the efficiency and way the Calvin cycle works
Light intensity
Temperature
Water availability

37. Photorespiration RuBP + CO2 → 2 3PG Rubisco can also be an oxygenase
Rubisco functions as a carboxylase
C3 plants make 3PG
Rubisco can also be an oxygenase
Adds O2 to RuBP eventually releasing CO2
Photorespiration
Using O2 and liberating CO2 is wasteful
More likely in hot and dry environment
Favored when CO2 low and O2 high

39. C4 plants
C4 plants make a 4 carbon compound in the first step of carbon fixation
Hatch-Slack pathway
Leaves have 2-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 compound transferred that releases steady supply CO2

40. In cooler climates, C3 plants use less energy to fix CO2
In warm dry climates C4 plants have the advantage in conserving water and preventing photorespiration
In cooler climates, C3 plants use less energy to fix CO2
90% of plants are C3

42. CAM plants Some C4 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
Malate broken down into CO2 to drive Calvin cycle