Unit 3: Carbon Assimilation
1. Photosynthetic Pigments and Reaction Centres
Photosynthesis begins with the absorption of light by specialized pigments.
- Photosynthetic Pigments: These include chlorophylls, carotenoids, and phycobilins which harvest light energy.
- Reaction Centres: Specific chlorophyll a molecules where the actual conversion of light energy to chemical energy takes place.
2. Photosynthetic Electron Transport
Light absorption triggers a flow of electrons through a series of membrane-bound carriers.
- Process: Electrons are removed from water (photolysis) and passed through Photosystem II (PSII) and Photosystem I (PSI).
- Output: This transport generates ATP and NADPH, which are essential for the subsequent reduction of CO2.
3. CO2 Reduction (C3 Cycle) and Photorespiration
The chemical energy produced in the light reactions is used to fix carbon dioxide.
- C3 Cycle (Calvin Cycle): The primary pathway for CO2 reduction where the first stable product is a 3-carbon compound (3-PGA).
- Photorespiration: A process where the enzyme RuBisCO binds with oxygen instead of CO2, leading to a loss of fixed carbon. It is considered a wasteful process that reduces photosynthetic efficiency.
4. C4 Pathways and Crassulacean Acid Metabolism (CAM)
Some plants have evolved mechanisms to concentrate CO2 and minimize photorespiration.
- C4 Pathway: Carbon is first fixed into a 4-carbon compound in mesophyll cells before being released to the C3 cycle in bundle sheath cells. This separation allows plants to thrive in high light and temperature.
- CAM (Crassulacean Acid Metabolism): A mechanism used mainly by succulents where CO2 is fixed at night and stored as malic acid, then released during the day for the C3 cycle. This conserves water by keeping stomata closed during the day.
[Image comparing the pathways of C3, C4, and CAM photosynthesis]
5. Red Drop, Emerson Effect, and Limiting Factors
Several concepts describe the efficiency and regulation of photosynthesis.
- Red Drop: A sharp decline in photosynthetic efficiency at wavelengths longer than 680 nm.
- Emerson Enhancement Effect: The observation that the rate of photosynthesis is higher when both long-red and short-red light are provided simultaneously compared to the sum of the rates when provided separately. This indicates the existence of two photosystems working in tandem.
- Blackman's Law of Limiting Factors: States that when a biological process is influenced by several factors, its rate is limited by the factor that is at its lowest or "minimum" value.
6. Exam Focus: Tips and FAQs
Exam Tip: Be ready to explain the structural differences between C3 and C4 plants, especially Kranz Anatomy in C4 plants. Also, understand that CAM plants separate CO2 fixation by time (night vs. day), while C4 plants separate it by space (mesophyll vs. bundle sheath cells).
Common Pitfalls
- Mistake: Assuming RuBisCO only fixes CO2. Correction: RuBisCO stands for Ribulose-1,5-bisphosphate carboxylase/oxygenase; it can bind both CO2 and O2.
- Mistake: Thinking C4 plants don't use the C3 cycle. Correction: All photosynthetic plants eventually use the C3 cycle; C4 and CAM are just "add-on" mechanisms to concentrate CO2 for it.
Frequently Asked Questions
Q: What is the "Red Drop"?
A: It refers to the sudden decrease in the quantum yield of photosynthesis when light of wavelength greater than 680 nm is used alone.
Q: Why is photorespiration considered wasteful?
A: Because it consumes ATP and O2 and releases CO2 without producing any sugar or energy-rich molecules like ATP or NADPH.