Concept 8.1 – Photosynthesis Feeds the Biosphere

Structure and Processes in Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, sustaining life on Earth. It is essential for the flow of matter and energy between organisms.

• Autotrophs: Organisms that produce their own food from inorganic substances (e.g., plants via photosynthesis).

• Heterotrophs: Organisms that obtain energy by consuming other organisms.

• Carbon fixation: The process of converting inorganic CO2 into organic compounds.

• Energy flow: Photosynthesis captures solar energy, which is then transferred through food webs.

• Example: Plants use sunlight to synthesize glucose, which animals consume for energy.

Concept 8.2 – Photosynthesis Converts Light to Chemical Energy

Anatomy of the Chloroplast and Photosynthetic Processes

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

• Chloroplast Structure: Contains thylakoids (site of light reactions), stroma (site of Calvin cycle), and grana (stacks of thylakoids).

• Reactants: CO2, H2O, and light energy.

• Products: Glucose (C6H12O6) and O2.

• General Equation for Photosynthesis:

• Example: In green plants, chlorophyll absorbs sunlight, initiating the conversion of CO2 and H2O into glucose.

Concept 8.3 – Light Reactions Produce ATP and NADPH

Mechanisms and Outcomes of the Light Reactions

The light reactions are the first stage of photosynthesis, occurring in the thylakoid membranes. They convert solar energy into chemical energy in the form of ATP and NADPH.

• Light Energy and Pigments: Light's energy relates to its wavelength; pigments like chlorophyll absorb specific wavelengths, determining their color.

• Absorption Spectrum: Shows which wavelengths a pigment absorbs; action spectrum indicates the effectiveness of different wavelengths in driving photosynthesis.

• Photosystems: Complexes (PSII and PSI) containing chlorophyll that capture light energy and initiate electron transport.

• Electron Transport Chain: Transfers electrons from water to NADP+, forming NADPH and generating ATP via chemiosmosis.

• ATP Formation: Occurs as protons flow through ATP synthase, converting ADP to ATP.

• Key Equation:

• Example: The splitting of water in PSII releases O2 and supplies electrons for the electron transport chain.

Concept 8.4 – The Calvin Cycle Reduces CO2 to Sugar

Phases and Components of the Calvin Cycle

The Calvin cycle is the set of light-independent reactions in photosynthesis that convert CO2 into glucose. It occurs in the stroma of the chloroplast.

• Major Reactants: CO2, ATP, NADPH.

• Major Products: Glucose, ADP, NADP+.

• Three Phases:

  1. Carbon Fixation: CO2 is attached to RuBP by the enzyme Rubisco.

  2. Reduction: ATP and NADPH are used to convert 3-PGA into G3P (a sugar).

  3. Regeneration: RuBP is regenerated from G3P, allowing the cycle to continue.

• Key Equation:

• Example: The Calvin cycle enables plants to synthesize glucose from atmospheric CO2.

Concept 8.5 – Life Depends on Photosynthesis

Photosynthesis as the Foundation of Life

Photosynthesis is the primary source of organic matter and oxygen in ecosystems, supporting both plants and animals. It is fundamental to the energy flow and carbon cycling in the biosphere.

• Importance: Provides food and oxygen for heterotrophic organisms.

• Application: All aerobic life depends directly or indirectly on photosynthetic organisms.