Photosynthesis: Metabolic Pathways and Adaptations (Chapter 10.1–10.5)

Topic 9: Metabolic Pathways – Photosynthesis

Overview

This section covers the fundamental processes of photosynthesis, focusing on the light reactions, the Calvin cycle, and adaptations in C4 and CAM plants. It is based on Chapter 10, Concepts 10.1–10.5, and is essential for understanding how plants convert light energy into chemical energy.

Photosynthesis: Key Concepts

Organisms Capable of Photosynthesis

• Photosynthetic organisms include plants, algae, and certain bacteria.

• These organisms use chloroplasts to capture light energy and convert it into chemical energy.

• Example: Green plants, cyanobacteria, and algae.

Structure of Chloroplasts

• Chloroplasts are double-membraned organelles found in plant cells.

• Key structures: Thylakoid membranes (site of light reactions), stroma (site of Calvin cycle).

• Thylakoids are stacked into grana.

Nature of Light

• Light exhibits both wave and particle properties.

• As a wave: characterized by wavelength and frequency.

• As a particle: consists of photons, discrete packets of energy.

• Equation: $E = h\nu$ (Energy of a photon = Planck's constant × frequency)

Absorption of Light by Pigments

• Pigments such as chlorophyll a, chlorophyll b, and carotenoids absorb specific wavelengths of light.

• Absorbed photons excite electrons to higher energy states.

• Different pigments absorb different wavelengths, broadening the spectrum of usable light.

10.3 Light Reactions

Linear Electron Flow

• Occurs in the thylakoid membrane, involving Photosystem II (PSII) and Photosystem I (PSI).

• Water is split in PSII, releasing electrons, protons (H+), and O2.

• Electrons move through an electron transport chain, generating NADPH and ATP.

• Summary Equation: $2H_2O + 2NADP^+ + 3ADP + 3P_i + 2H^+ + light \rightarrow 2NADPH + 2H^+ + 3ATP + O_2$

Cyclic Electron Flow

• Electrons cycle back from ferredoxin to the cytochrome complex and then to PSI.

• Produces ATP but not NADPH or O2.

• Used to supplement ATP synthesis when the Calvin cycle requires more ATP than NADPH.

• Example: Purple sulfur bacteria use only one photosystem and cyclic flow.

Chemiosmosis and ATP Production

• Chemiosmosis is the process by which ATP is generated as protons (H+) flow back across the thylakoid membrane through ATP synthase.

• Both mitochondria and chloroplasts use chemiosmosis for ATP production.

• Equation: $ADP + P_i \xrightarrow{ATP\ synthase} ATP$

Light Reactions Summary

• Water is split in PSII, providing electrons and releasing O2.

• Electrons are transferred via the electron transport chain, generating NADPH and ATP.

• Proton gradient drives ATP synthesis via chemiosmosis.

10.4 The Calvin Cycle

Overview

• Anabolic pathway that builds the 3-carbon sugar glyceraldehyde 3-phosphate (G3P).

• Uses ribulose bisphosphate (RuBP) as the initial 5-carbon sugar.

• Each turn fixes one molecule of CO2.

Phases of the Calvin Cycle

• Phase 1: Carbon Fixation

  • CO2 is attached to RuBP by Rubisco, forming a 6-carbon intermediate.

  • Intermediate splits into two 3-carbon molecules: 3-phosphoglycerate.

• Phase 2: Reduction

  • 3-phosphoglycerate is phosphorylated (using ATP) and reduced (using NADPH) to form G3P.

  • One G3P is removed per cycle to build glucose and other organic molecules.

• Phase 3: Regeneration of RuBP

  • Remaining G3P molecules are rearranged to regenerate RuBP, using ATP.

10.5 Rubisco and Photorespiration

Rubisco: Imperfect Enzyme

• Rubisco can bind both CO2 and O2 due to their similar structures.

• When CO2 is bound, the Calvin cycle proceeds efficiently (C3 plants).

• When O2 is bound, photorespiration occurs, producing 2-phosphoglycolate, which must be recycled and consumes ATP.

• Photorespiration is wasteful and reduces photosynthetic efficiency.

Factors Affecting Rubisco Activity

• CO2/O2 ratio in the chloroplast (higher CO2 favors carboxylation).

• Temperature (higher temperatures favor oxygenation and photorespiration).

Photorespiration

• Complex process involving chloroplasts, peroxisomes, and mitochondria.

• Consumes ATP and releases CO2, reducing net carbon fixation.

10.5 Adaptations: C4 and CAM Plants

C4 Plants

• Spatially separate carbon fixation and the Calvin cycle.

• Mesophyll cells fix CO2 into a 4-carbon compound (using PEP carboxylase).

• 4-carbon compound is transported to bundle sheath cells, where CO2 is released for the Calvin cycle.

• Reduces photorespiration by concentrating CO2 near Rubisco.

CAM Plants

• Temporally separate carbon fixation and the Calvin cycle.

• Stomata open at night, fixing CO2 into organic acids.

• During the day, CO2 is released from acids for the Calvin cycle while stomata are closed.

• Adapted to arid environments to minimize water loss.

10.5 Climate Change and Plant Productivity

Impact of Rising CO2 Levels

• CO2 levels have increased from 300 to 400 ppm over the last 200 years.

• C4 plants show little change in productivity with increased CO2, while C3 plants may increase productivity.

• However, higher temperatures may increase photorespiration in C3 plants, potentially offsetting gains and leading to crop decreases.

Table: Plant Productivity at Different CO2 Levels

Plant Type	350 ppm CO2	600 ppm CO2	1000 ppm CO2
C4 plant	91	89	90
C3 plant	35	54	60

Values represent average dry mass of one well-watered plant per day (g).

Summary

• Photosynthesis involves light reactions and the Calvin cycle, with adaptations in C4 and CAM plants to improve efficiency under different environmental conditions.

• Rubisco's dual affinity for CO2 and O2 leads to photorespiration, which is minimized in C4 and CAM plants.

• Climate change may affect plant productivity, especially in C3 crops, due to increased photorespiration at higher temperatures.