Photosynthesis in Higher Plants Notes: Class 11 Biology Chapter 11
Class 11 Biology Chapter 11 Photosynthesis in Higher Plants covers the site of photosynthesis, light and dark reactions, the C3 and C4 pathways, and factors affecting the rate of photosynthesis.
Overview of Topics & Subtopics (Click to Jump)
1. Site and Pigments of Photosynthesis
Short Note:
• Photosynthesis occurs mainly in the chloroplast; the light reactions occur in the thylakoid membranes (containing chlorophyll a, chlorophyll b, and carotenoids), and the dark reactions occur in the stroma.
2. Light Reaction
Short Note:
• Involves absorption of light, splitting of water (photolysis, releasing O2), and generation of ATP and NADPH through electron transport in Photosystem I and Photosystem II.
3. Dark Reaction (Calvin Cycle / C3 Pathway)
Short Note:
• Uses ATP and NADPH from the light reaction to fix CO2 and synthesise sugar; the first stable product is a 3-carbon compound (3-PGA), hence called the C3 pathway, occurring in the stroma.
4. C4 Pathway and Factors Affecting Photosynthesis
Short Note:
• The C4 pathway (Hatch-Slack pathway) is found in plants like maize and sugarcane, where the first stable product is a 4-carbon compound (oxaloacetic acid); more efficient in hot, dry conditions due to reduced photorespiration, using Kranz anatomy.
• Rate of photosynthesis is affected by factors including light intensity, CO2 concentration, temperature, and water availability (Blackman's Law of Limiting Factors).
NCERT Textbook Exercise Questions
Explain the light reaction of photosynthesis.
Answer: In the light reaction, light energy is absorbed by pigments in the thylakoid membrane, exciting electrons in Photosystem II, which are passed through an electron transport chain, generating ATP (via chemiosmosis) and eventually reducing NADP+ to NADPH via Photosystem I; water is split (photolysis) to replace lost electrons in PS II, releasing oxygen as a by-product.
Explain the Calvin cycle (C3 pathway).
Answer: The Calvin cycle occurs in three stages: carboxylation (CO2 fixed by RuBisCO onto RuBP, forming an unstable 6-C compound that splits into two 3-PGA molecules), reduction (3-PGA is reduced to G3P using ATP and NADPH from the light reaction), and regeneration (some G3P is used to regenerate RuBP, while the rest is used to synthesise glucose).
What are the differences between C3 and C4 pathways?
Answer: In the C3 pathway, the first stable product is a 3-carbon compound (3-PGA), CO2 fixation occurs directly via RuBisCO in mesophyll cells, and it is less efficient in hot/dry conditions due to photorespiration. In the C4 pathway, the first stable product is a 4-carbon compound (OAA), CO2 fixation occurs via PEP carboxylase in mesophyll cells with the actual Calvin cycle occurring in bundle sheath cells (Kranz anatomy), and it is more efficient in hot, dry climates as it minimises photorespiration.
Why is the Z scheme of light reaction called so?
Answer: The Z scheme is named for the zig-zag shape traced when the redox potentials of the various carriers involved in the electron transport chain (from Photosystem II to Photosystem I) are plotted against the sequence of electron carriers, showing the flow of electrons from a lower to higher energy level and back down, forming a 'Z' shaped pattern.
