Chapter 10: Photosynthesis

Overview

Photosynthesis is a fundamental biological process by which plants, algae, and some bacteria convert light energy into chemical energy, producing organic compounds and oxygen. This chapter explores the mechanisms, stages, and evolutionary adaptations of photosynthesis, as well as its significance in the biosphere.

Photosynthesis and the Biosphere

Autotrophs and Heterotrophs

• Autotrophs are organisms that produce their own food from inorganic substances using light or chemical energy. Producers in ecosystems, such as plants and algae, are autotrophs.

• Heterotrophs obtain energy by consuming other organisms. Consumers include animals, fungi, and many bacteria.

• Example: Grass (autotroph) is eaten by a rabbit (heterotroph).

Where Photosynthesis Occurs and Conversion of Light Energy

Chloroplast Structure

• Photosynthesis occurs in chloroplasts, which contain:

  • Stroma: Fluid-filled space where the Calvin cycle occurs.

  • Thylakoids: Membranous sacs containing chlorophyll; site of light reactions.

  • Thylakoid space: Interior of thylakoids.

  • Inner and outer membranes: Enclose the chloroplast.

Overall Equation for Photosynthesis

• The general formula for photosynthesis is:

$6CO_2 + 6H_2O + light\ energy \rightarrow C_6H_{12}O_6 + 6O_2$

• This equation is essentially the reverse of cellular respiration.

Source of Oxygen in Photosynthesis

• The oxygen released during photosynthesis comes from water, not carbon dioxide.

• Experiments using isotopes (e.g., $^{18}O$) have shown that the O2 produced is derived from H2O.

Stages of Photosynthesis

• Light Reactions: Occur in the thylakoid membranes; convert solar energy to chemical energy (ATP and NADPH); release O2 as a byproduct.

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

The Light Reactions: Converting Solar Energy to Chemical Energy

Light and Pigments

• Photosynthesis uses visible light (wavelengths ~380–750 nm).

• Pigments such as chlorophyll absorb light energy; different pigments absorb different wavelengths.

• Relationship between wavelength and energy: Shorter wavelengths have higher energy.

Absorption and Action Spectra

• Absorption spectrum: Shows wavelengths absorbed by a pigment.

• Action spectrum: Shows effectiveness of different wavelengths in driving photosynthesis.

• Engelmann's experiment: Used algae and aerobic bacteria to map the action spectrum before spectrophotometers existed.

Photosystems and Electron Flow

• Photosystem: A complex of proteins and pigments that captures light energy.

• Composed of a reaction-center complex and light-harvesting complexes.

• Primary electron acceptor: Receives excited electrons from chlorophyll.

• Photosystem II (PSII): Contains a special pair of chlorophyll a molecules called P680.

Linear Electron Flow

• Electrons flow from water through PSII and PSI to NADP+, forming NADPH.

• Produces ATP via chemiosmosis and releases O2.

• Key steps:

  • Light excites electrons in PSII; water is split to replace lost electrons, releasing O2.

  • Electrons move through an electron transport chain, generating a proton gradient for ATP synthesis.

  • Electrons reach PSI, are re-excited, and reduce NADP+ to NADPH.

Chemiosmosis in Photosynthesis

• ATP is produced as protons flow through ATP synthase from the thylakoid space to the stroma.

• Similarities to cellular respiration: Both use electron transport chains and chemiosmosis to generate ATP.

• Differences: In photosynthesis, the electron source is water and the final electron acceptor is NADP+; in respiration, the electron source is organic molecules and the final acceptor is O2.

The Calvin Cycle: Using ATP and NADPH to Reduce CO2 to Sugar

Overview of the Calvin Cycle

• The Calvin cycle is a series of enzyme-catalyzed reactions that fix CO2 into organic molecules.

• Occurs in the stroma of the chloroplast.

• Uses ATP and NADPH from the light reactions.

Stages 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 to glyceraldehyde-3-phosphate (G3P).

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

Key Points

• For every three CO2 molecules fixed, one G3P is produced (after three turns of the cycle).

• Net production of one G3P requires 9 ATP and 6 NADPH.

• Rubisco is the most abundant protein on Earth and catalyzes the first step of carbon fixation.

Alternative Mechanisms of Carbon Fixation

Adaptations in Hot, Arid Climates

• Plants have evolved mechanisms to minimize water loss and photorespiration.

• C4 plants: Use a spatial separation of steps to concentrate CO2 in bundle-sheath cells, reducing photorespiration.

• CAM plants: Open stomata at night to fix CO2, storing it as organic acids for use during the day.

• Photorespiration: Occurs when rubisco fixes O2 instead of CO2, wasting energy and reducing photosynthetic output.

• Photorespiration can be problematic in agriculture, especially under hot, dry conditions.

Summary Table: Comparison of Photosynthetic Pathways

Key Equations

• Overall photosynthesis:

$6CO_2 + 6H_2O + light\ energy \rightarrow C_6H_{12}O_6 + 6O_2$

• ATP and NADPH production (light reactions):

$ADP + P_i + NADP^+ + H_2O + light \rightarrow ATP + NADPH + O_2$

• Calvin cycle (per 3 CO2):

$3CO_2 + 9ATP + 6NADPH + 6H^+ \rightarrow G3P + 9ADP + 8P_i + 6NADP^+ + 3H_2O$

Summary

• Photosynthesis is essential for life on Earth, providing food and oxygen.

• It involves complex processes that have evolved to maximize efficiency and adapt to environmental challenges.