Chapter 10: Photosynthesis

Concept 10.1: Photosynthesis Feeds the Biosphere

Photosynthesis is a fundamental biological process that sustains life on Earth by converting solar energy into chemical energy. It is the primary means by which energy enters the biosphere.

• Autotrophs vs. Heterotrophs: Autotrophs are organisms that produce their own food from inorganic substances (e.g., plants, algae, some bacteria), while heterotrophs obtain energy by consuming other organisms.

• Photoautotrophs: These are autotrophs that use light as an energy source to synthesize organic compounds. Examples include green plants, algae, and cyanobacteria.

• Importance: Photosynthesis provides the organic molecules and oxygen necessary for most life forms.

Concept 10.2: Photosynthesis Converts Light Energy to the Chemical Energy of Food

Photosynthesis occurs in chloroplasts, specialized organelles in plant and algal cells. The process involves capturing light energy and converting it into chemical energy stored in glucose and other organic molecules.

• Chloroplast Structure: Chloroplasts contain internal membranes called thylakoids, which are stacked into grana and surrounded by stroma. These compartments are essential for the light-dependent and light-independent reactions.

• General Equation for Photosynthesis:

• Redox Reactions: Photosynthesis involves the transfer of electrons from water (which is oxidized to O2) to CO2 (which is reduced to glucose).

• Stages of Photosynthesis:

  • Light Reactions: Occur in the thylakoid membranes; convert solar energy to chemical energy (ATP and NADPH).

  • Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to fix CO2 into sugars.

Concept 10.3: The Light Reactions Convert Solar Energy to the Chemical Energy of ATP and NADPH

The light reactions capture energy from sunlight and use it to produce ATP and NADPH, which are then used in the Calvin cycle.

• Sunlight: The ultimate energy source for photosynthesis; contains photons of varying wavelengths.

• Action Spectrum vs. Absorption Spectrum:

  • Action Spectrum: Shows the rate of photosynthesis at different wavelengths of light.

  • Absorption Spectrum: Shows the wavelengths of light absorbed by pigments (e.g., chlorophyll a, chlorophyll b, carotenoids).

• Photosystems: Complexes of pigments and proteins that capture light energy. There are two main types:

  • Photosystem II (PSII): Absorbs light, splits water, and generates O2, electrons, and protons.

  • Photosystem I (PSI): Absorbs light and facilitates the transfer of electrons to NADP+, forming NADPH.

• Electron Flow:

  • Linear Electron Flow: Electrons move from water through PSII and PSI to NADP+, producing ATP and NADPH.

  • Cyclic Electron Flow: Electrons cycle back from PSI to the electron transport chain, producing ATP but not NADPH or O2.

• Photophosphorylation: The process of generating ATP from ADP and inorganic phosphate using the energy of light-driven electron transport.

Concept 10.4: The Calvin Cycle Uses the Chemical Energy of ATP and NADPH to Reduce CO2 to Sugar

The Calvin cycle is a series of biochemical reactions that occur in the stroma of chloroplasts. It uses ATP and NADPH to convert CO2 into glucose.

• Phases of the Calvin Cycle:

  1. Carbon Fixation: CO2 is attached to ribulose bisphosphate (RuBP) by the enzyme rubisco.

  2. Reduction: ATP and NADPH are used to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P).

  3. Regeneration: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.

• Role of ATP and NADPH: ATP provides energy, and NADPH provides reducing power for the synthesis of G3P.

Concept 10.5: Alternative Mechanisms of Carbon Fixation Have Evolved in Hot, Arid Climates

Plants in hot, dry environments have evolved adaptations to minimize water loss and photorespiration, which can reduce photosynthetic efficiency.

• Photorespiration: A process where rubisco adds O2 instead of CO2 to RuBP, leading to the loss of fixed carbon and energy.

• Consequences: Photorespiration decreases photosynthetic output, especially under high O2 and low CO2 conditions.

• Adaptations:

  • C4 Plants: Use a spatial separation of steps to concentrate CO2 in bundle-sheath cells, reducing photorespiration (e.g., maize, sugarcane).

  • CAM Plants: Use temporal separation by fixing CO2 at night and storing it as malate, which is used during the day (e.g., cacti, succulents).

Concept 10.6: Photosynthesis is Essential for Life on Earth: A Review

Photosynthesis is crucial for maintaining atmospheric oxygen and providing organic molecules for all living organisms. The process involves complex energy transformations and the coordinated action of multiple biochemical pathways.

• Inputs and Outputs of Photosystems and Calvin Cycle:

• Role of Rubisco: The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) catalyzes the first step of carbon fixation in the Calvin cycle.

Example: In a typical leaf cell, the light reactions generate ATP and NADPH, which are then used by the Calvin cycle to synthesize glucose from CO2.