BIG IDEA II: Energy is stored, used, and transformed in living systems.
Unit 10: Photosynthesis
Main Concept: Light energy is harnessed into chemical bond energy of organic molecules through the process of photosynthesis.
Learning Objectives:
A. Leaf Anatomy, Chloroplast Structure, and Photosystem Components
Explain how these components effectively harvest light energy to produce ATP and NADPH.
B. Link Between Light Reactions and Calvin Cycle
Connect how products of the light reactions (ATP and NADPH) power carbon dioxide fixation and carbohydrate synthesis in the Calvin cycle.
C. Chloroplast Functionality
Summarize chloroplast locations, major inputs and outputs of light reactions and Calvin cycle, and describe how these reactions are interconnected.
Discuss how RuBisCO behavior can limit Calvin cycle rates in certain conditions.
D. Comparison of ATP Production
Compare and contrast the mitochondria and chloroplasts in their structural adaptations for ATP production during oxidative phosphorylation (in mitochondria) and photo-phosphorylation (in chloroplasts).
E. Understanding RuBisCO Behavior
Explain the changes in RuBisCO behavior under varying O2:CO2 concentrations and the adaptations of C4 and CAM pathways to improve efficiency in these conditions.
F. Carbon Cycle Context
Relate CO2 fixation processes (photosynthesis) and CO2 release (respiration) to the broader global carbon cycle.
OVERVIEW OF PHOTOSYNTHESIS
Analogy: Photosynthesis as a play in three acts:
Act I: Get that light energy!
Act II: Store that light energy!
Act III: Use that light energy!
Focus: Carbon, electrons, and energy transformation between potential to kinetic forms.
Questions and Answers:
Overall function of photosynthesis: Convert light energy to chemical energy for building blocks and energy for cellular work.
Photosynthesis reaction formula:
ext{Light energy} + 6 ext{ H}2 ext{O} + 6 ext{ CO}2 = C6H{12}O6 + 6 ext{ O}2
Exergonic or Endergonic?: Endergonic reaction; products have more free energy than reactants.
Two basic stages of photosynthesis: Light reactions and Calvin cycle.
Contribution of light reactions: Light energy excites electrons, strips them from H2O, moves through the electron transport chain creating ATP and NADPH for the Calvin cycle.
Organelle responsible for photosynthesis: Chloroplast.
Structure where photosynthesis occurs: Thylakoid.
Membrane Structure
Thylakoid membrane: The structure of the internal thylakoid where light reactions occur.
PROPERTIES OF LIGHT:
Nature of Light Energy:
Travels as particles (photons) or waves.
Wavelength: Determines energy; longer wavelengths carry less energy (e.g., red light) vs. shorter wavelengths carry more energy (e.g., blue light).
Energy ranking: Blue > Yellow > Red (
Blue: ~500 nm
Yellow: ~580 nm
Red: ~700 nm
).
Color of a ball reflecting 500 nm light: Blue.
Function of pigments: Absorb light; chlorophyll absorbs all wavelengths except between 565-520 nm.
Effect of sunlight on chlorophyll electrons: Sunlight energizes chlorophyll electrons to a higher state, releasing energy as they return to normal states.
THE LIGHT REACTIONS:
Products of the light reactions: ATP and NADPH.
Usage in the cell: Both are utilized in the Calvin cycle to convert CO2 into G3P, with ATP providing phosphate and NADPH supplying electrons.
Key diagram components:
Chloroplast (light receptor)
Photon (sun energy)
Photosystem
Primary electron acceptor
Light harvesting complex
Reaction complex.
Excitation of special chlorophyll a molecules: They lose electrons.
Definition of an oxidizing agent: Substance that accepts electrons during a chemical reaction.
Oxidizing agent of Photosystem II (PSII): P680+ is the strongest known biological oxidizing agent that steals electrons from water, releasing O2.
Symbol for a proton: H+; H+ moves against its gradient into the thylakoid space (lumen).
Source of protons: Splitting of water contributes protons, as do electrons moving through the ETC.
Final product of PSII activity: NADPH and ATP; comparison to cellular respiration shows NADPH correlates with NADH from cellular respiration.
Proton buildup source: Similar processes from water split and ETC pumping H+ into lumen.
Use of H+ concentration gradient: Powers ATP synthase movement to generate ATP.
CYCLIC VS. NON-CYCLIC ELECTRON FLOW:
Linear electron flow: Electrons excite in PSII, enter ETC, excite in PSI, and combine with NADP+ to form NADPH.
Cyclic electron flow: Occurs when electrons from PSI return to the original electron transport chain, repowering ATP synthase while reducing NADP+ is not involved.
Work of first transport chain: Powers ATP synthase to contribute to ATP generation for Calvin cycle.
Second transport chain work (after PS-I): Donates electrons to NADPH for CO2 reduction to G3P.
ATP contribution: First transport chain contributes more towards ATP synthesis.
CHEMIOSMOSIS:
Similarities in chemiosmosis: High proton concentration drives movement through ATP synthase.
Protein channel name: ATP synthase.
Difference in proton movement: Mitochondria (Intermembrane Space to Mitochondrial Matrix) vs Chloroplasts (thylakoid space to stroma).
ATP production sites: Mitochondrial ATP from the matrix vs. chloroplast ATP from stroma.
Redox equation of photosynthesis:
ext{Light energy} + 6 ext{ H}2 ext{O} + 6 ext{ CO}2 = C6H{12}O6 + 6 ext{ O}2
Reduction products: CO2 to C6H12O6 (reduced) and H2O to O2 (oxidized).
THE CALVIN CYCLE:
Carbon source for G3P: CO2 from the atmosphere diffused through stomata.
Location of the Calvin cycle: Stroma of the chloroplast.
First action with CO2: Carbon fixation by RuBisCO, fixing CO2 to RuBP.
Enzyme catalyzing reaction: RuBisCO.
Unstable 6-C molecule: Immediate product is unstable due to phosphate positioning.
End-product of carbon fixation: Two 3-carbon molecules.
Determining ATP usage: ATP adds phosphate, leading to 1-3 bisphosphoglycerate.
NADPH's role in reduction stage: Supplies electrons for reduction.
NADPH's resulting state: Converts to NADP+ after donating electrons.
Carbon product of reduction stage: G3P.
Regeneration stage: Necessary for RuBP regeneration, enabling cycle continuation.
Cost for one G3P production: Uses 3 CO2 and 3 RuBP, costing 9 ATP and 6 NADPH.
G3P requirement for glucose: Two G3P molecules needed to form glucose.
PHOTORESPIRATION and ALTERNATIVE PHOTOSYNTHETIC PATHWAYS:
The trade-off for plants: Choice between food (CO2) and water (H2O); gas exchange occurs via stomata, which also causes water loss.
Dominant factor: Usually water takes precedence.
Increased internal O2: Occurs on hot, dry days when stomata are closed, building O2.
Problems of high internal O2:
1) Reactive oxygen species (ROS) can form and damage cells.
2) RuBisCO may fix O2 instead of CO2, diverting energy.
3) Non-fixation of carbon leads to inability to synthesize necessary sugars.
C3 plants definition: Normal plants with a stable first product of the Calvin cycle as a 3-carbon molecule (3-PGA).
Evolution of C4 plants: Adapted to higher oxygen environments and hotter, dryer climates necessitating stomata closure.
C4 nomenclature: Named for the 4-carbon organic molecule produced via PEP-carboxylase fixation.
Enzymatic differences:
A) C3 plants: RuBisCO.
B) C4 plants: PEP-C.
Functionality in climates: C4 enzyme is suited for high O2:CO2 ratios typical in hot, dry conditions.
Differences in the Calvin cycle between C3 and C4 plants: Anatomy differs—C4’s carbon is fixed in mesophyll cells using PEP-carboxylase, whereas Calvin cycle occurs in bundle sheath cells using RuBisCO.
CAM plant strategies:
Night: Open stomata, fix CO2 to 4-C acids.
Day: Close stomata to conserve water, utilize stored CO2 for Calvin cycle.
GENERAL UNDERSTANDING:
Diagrams:
O2 and CO2 cycle through producers, consumers, decomposers, and atmosphere, including energy sources.
General equations for cellular respiration (C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy) and photosynthesis.
Energy flow diagram: From the sun to biological movement, highlighting electron shuttle molecules like NAD+ and NADP+.