Lecture #11 – Cellular Energetics, Part 3

12.8 Chloroplasts and Photosynthesis
12.9 Use of Light Energy to Generate Molecular Oxygen, NADPH, and ATP in Stages 1-3 of Photosynthesis
12.10 ATP and NADPH Drive Carbon Fixation in the Calvin Cycle and Carbohydrate Synthesis in Stage 4 of Photosynthesis

Discuss chloroplasts and the role of photosynthesis in the evolution of life.
Clarify the roles of the light-dependent and light-independent reactions.
Explain the operation of photosynthetic units and their reaction centers.
Describe the mechanism for ATP production by electron flow.
Compare and contrast cyclic and non-cyclic photophosphorylation.
Describe the C3 pathway and its role in carbon fixation.
Clarify the effects of the environment on C3 photosynthesis.
Describe the process of C4 photosynthesis and its benefits to plants.
Describe the mechanism by which CAM plants manage to survive in very hot, dry climates and compare it with C3 and C4 photosynthesis.
Describe the cooperation and interdependence among peroxisomes, chloroplasts, and mitochondria and their roles in plant physiology.

Heterotrophs: Organisms that depend on an external source of organic compounds.
Autotrophs: Organisms capable of surviving on CO₂ as their principal carbon source.
- Chemoautotrophs: Utilize chemical energy stored in inorganic molecules (e.g., NH₃, H₂S, NO₂) to convert CO₂ into organic compounds.
- Photoautotrophs: Use the radiant energy of the sun to convert CO₂ into organic compounds and carry out photosynthesis.

Photosynthesis: A process in which energy from sunlight is transformed into chemical energy stored in carbohydrates and other organic molecules. The principal end products of plant photosynthesis are O₂ and polymers of six-carbon sugars (starch and sucrose).

Photosynthesis Stages:
1. Light absorption, generation of high-energy electrons, and O₂ formation from H₂O.
2. Electron transport leading to the reduction of NADP⁺ to NADPH and pmf (proton motive force) generation.
3. Synthesis of ATP.
4. Conversion of CO₂ into carbohydrates (carbon fixation).

The simplified equation for photosynthesis is:
6 ext{CO}2 + 12 ext{H}2 ext{O}
ightarrow ext{C}6 ext{H}{12} ext{O}6 + 6 ext{H}2 ext{O} + 6 ext{O}_2

Photosynthesis involves a redox reaction where electrons are transferred from water to carbon dioxide:
- O₂ produced originates from H₂O, not CO₂.
Photorespiration, the process where RuBP (ribulose bisphosphate) is oxygenated instead of carboxylated, impairs photosynthesis efficiency.

Light-dependent reactions: Energy from sunlight is absorbed and stored as chemical energy in ATP and NADPH.
Light-independent reactions (Calvin Cycle): CO₂ fixation and synthesis of carbohydrates from CO₂ using energy from ATP and NADPH.
The Calvin cycle describes a series of reactions that fix CO₂ into organic molecules, occurring in the stroma of chloroplasts.

Chloroplast Structure:
- Outer Membrane: Contains porins permeable to low molecular weight metabolites.
- Intermembrane Space: Continuous with the lumen of each crista.
- Inner Membrane: Acts as a permeability barrier; contains transport proteins.
- Stroma: Contains enzymes that catalyze CO₂ fixation/starch synthesis and chloroplast DNA.
- Thylakoid Membrane: Comprised of flattened vesicles enclose a single interconnected luminal space, containing chlorophylls.

Chlorophyll: Main pigment with a porphyrin ring that absorbs light; chlorophyll a absorbs violet-blue and red light.
Accessory pigments (Carotenoids): Absorb light in the blue-green region, allowing for greater absorption of incoming photons.

Light-Harvesting Complexes (LHCs): Increase efficiency by transferring energy to reaction-center chlorophylls.
Photosystems: PSII and PSI have distinct roles. PSII initiates photolysis (splitting of water), generating O₂, while PSI reduces NADP⁺ to NADPH.
Cyclic vs. Non-Cyclic Photophosphorylation:
- Non-cyclic involves both photosystems and produces ATP and NADPH.
- Cyclic involves only PSI and produces ATP.

C3 Pathway (Calvin Cycle):
- Rubisco catalyzes the carboxylation of ribulose 1,5-bisphosphate (RuBP), producing 3-phosphoglycerate (PGA).
C4 Photosynthesis: CO₂ fixed into 4-carbon compounds that enhance CO₂ capture efficiency, reducing photorespiration losses.
CAM Pathway: Stomata open at night for CO₂ uptake, storing it as malate for use in the Calvin cycle during the day.

Overall Calvin Cycle Equation:
6 ext{CO}2 + 12 ext{NADPH} + 18 ext{ATP} ightarrow ext{C}6 ext{H}{12} ext{O}6 + 18 ext{ADP} + 18 ext{P}_i + 12 ext{NADP}^+
Light-dependent and independent reactions combined yield oxygen and glucose:
6 ext{CO}2 + 6 ext{H}2 ext{O}
ightarrow ext{C}6 ext{H}{12} ext{O}6 + 6 ext{O}2

The efficiency of photosynthesis is heavily influenced by environmental factors such as CO₂ concentration and light intensity, with significant impacts on plant growth and agricultural productivity.