Photosynthesis: Structure, Function, and Importance in the Biosphere

Concept 8.1 – Photosynthesis Feeds the Biosphere

Structure and Processes of Photosynthesis

Photosynthesis is a fundamental biological process that sustains life on Earth by converting light energy into chemical energy. It involves the flow of matter and energy between plants and animals, and distinguishes between autotrophic and heterotrophic modes of nutrition.

• Flow of Matter and Energy: Photosynthesis in plants captures solar energy and converts carbon dioxide and water into glucose and oxygen. Animals consume plants (or other animals) to obtain energy and matter.

• Autotrophs vs. Heterotrophs: Autotrophs (e.g., plants, algae) produce their own food via photosynthesis. Heterotrophs (e.g., animals, fungi) obtain energy by consuming other organisms.

• Examples: Green plants, cyanobacteria, and algae are autotrophs; humans and most animals are heterotrophs.

Concept 8.2 – Photosynthesis Converts Light to Chemical Energy

Anatomy of the Chloroplast and Photosynthetic Process

Photosynthesis occurs in the chloroplasts, specialized organelles found in plant cells. The process involves the absorption of light and the transformation of energy into chemical bonds.

• Chloroplast Structure: Major anatomical features include the thylakoid membranes (site of light reactions), stroma (site of the Calvin cycle), and grana (stacks of thylakoids).

• Supply of Reactants and Products: Chloroplasts absorb CO2 and H2O, producing glucose and O2 during photosynthesis.

• General Equation for Photosynthesis:

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

• Energy Change: Light energy is converted into chemical energy stored in glucose.

• Example: A leaf absorbs sunlight, takes in carbon dioxide from the air, and releases oxygen as a byproduct.

Concept 8.3 – Light Reactions Produce ATP and NADPH

Mechanisms and Outcomes of the Light Reactions

The light reactions of photosynthesis occur in the thylakoid membranes and are responsible for generating ATP and NADPH, which are used in the Calvin cycle.

• Light Energy and Pigments: Light's energy relates to its wavelength and color. Chlorophyll absorbs light most efficiently in the blue and red regions of the spectrum.

• Absorption Spectrum: Shows how different wavelengths are absorbed by pigments; action spectrum indicates the rate of photosynthesis at each wavelength.

• Chlorophyll Excitation: When chlorophyll absorbs light, electrons are excited to higher energy states.

• Photosystems: Photosystem II (PSII) and Photosystem I (PSI) are protein complexes that capture light energy and facilitate electron transport.

• Electron Transport Chain: Electrons move through a series of carriers, resulting in the production of ATP and NADPH.

• ATP and NADPH Formation: ATP is produced via photophosphorylation and NADPH via reduction of NADP+.

• Key Equations:

$ADP + P_i + light\ energy \rightarrow ATP$

$NADP^+ + 2e^- + 2H^+ + light\ energy \rightarrow NADPH + H^+$

• Example: In the light reactions, water is split to provide electrons, releasing oxygen as a byproduct.

Concept 8.4 – The Calvin Cycle Reduces CO2 to Sugar

Phases and Components of the Calvin Cycle

The Calvin cycle is the set of chemical reactions that take place in the stroma of chloroplasts during photosynthesis. It uses ATP and NADPH to convert CO2 into glucose.

• Major Reactants: CO2, ATP, NADPH

• Major Products: Glucose (C6H12O6), ADP, NADP+

• Three 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 is used to regenerate RuBP, enabling the cycle to continue.

• Example: Six turns of the Calvin cycle are required to produce one molecule of glucose.

Concept 8.5 – Life Depends on Photosynthesis

Importance of Photosynthesis in the Biosphere

Photosynthesis is essential for life on Earth, providing the primary source of energy and organic matter for nearly all living organisms. It also maintains atmospheric oxygen levels.

• Key Point: All food chains and webs depend on photosynthetic organisms as primary producers.

• Example: Forest ecosystems rely on photosynthetic plants to support herbivores, which in turn support carnivores.