Photosynthesis and Cellular Respiration

Overview of Photosynthesis and Respiration

Photosynthesis and cellular respiration are fundamental biological processes that cycle energy and carbon through the biosphere. Photosynthesis captures solar energy to produce organic molecules, while respiration breaks down these molecules to release energy.

• Respiration: An overall exergonic sequence of chemical reactions that oxidize glucose to CO2 and result in the production of ATP.

• Photosynthesis: An endergonic sequence of chemical reactions that reduces CO2 to glucose (and other sugars).

Photosynthesis is the pathway by which energy enters the biosphere and is the basis for carbon fixation, synthesizing organic compounds from inorganic carbon sources.

Photosynthesis and Cellular Respiration Cycle

Photosynthesis creates the starting materials for cellular respiration, forming a cycle that sustains life in the biosphere.

• Photosynthesis generates O2 and organic molecules.

• Cellular respiration uses these molecules to produce ATP, releasing CO2 and H2O.

Organisms capable of photosynthesis:

• Plants

• Unicellular protists

• Multicellular algae

• Cyanobacteria

• Purple sulfur bacteria

Photosynthesis: The Redox Reaction

Overall Chemical Equation

Photosynthesis is a redox reaction, where carbon dioxide is reduced and water is oxidized. The general formula is:

• CO2 is reduced: Carbon gains electrons.

• H2O is oxidized: Oxygen loses electrons.

Stages of Photosynthesis

Main Stages

Photosynthesis occurs in two main stages within the chloroplast:

1. Light reactions: Capture solar energy and convert it to chemical energy (ATP and NADPH).

2. Calvin cycle: Uses ATP and NADPH to fix carbon dioxide and synthesize sugars.

Chloroplast Structure and Function

Chloroplasts are the site of photosynthesis in plant cells. Gas exchange occurs through stomata, allowing CO2 in and O2 out.

• Thylakoid membranes: Location of light reactions.

• Stroma: Location of the Calvin cycle.

Pigments and Light Absorption

Pigments in Photosynthesis

Pigments are substances that absorb visible light, enabling the capture of solar energy for photosynthesis.

• Different pigments absorb different wavelengths of light.

• Wavelengths not absorbed are reflected or transmitted, giving the pigment its color.

• Leaves appear green because chlorophyll reflects and transmits green light while absorbing most other colors.

Light Spectrum and Photosynthesis

Not all wavelengths of light are equally useful for photosynthesis. The absorption spectrum of pigments determines which wavelengths are most effective.

• George Engelmann's experiment (1883) demonstrated that red and blue light are most effective for photosynthesis, as shown by oxygen-seeking bacteria clustering in these regions.

• Red light is especially important for oxygen production in photosynthetic bacteria.

Absorption by Pigments

• Absorption spectrum: Plots a pigment's light absorption vs. wavelength.

• Chlorophyll a: Main photosynthetic pigment; absorbs violet and red light best.

• Accessory pigments: Chlorophyll b and carotenoids broaden the spectrum used for photosynthesis (action spectrum).

Nature of Light

• Light exists as particles called photons, which excite electrons.

• Photons are discrete packets of light energy that transfer energy to electrons in pigment molecules.

Photosystems and Light Harvesting

Structure of Photosystems

Photosystems are complexes of proteins and pigments in the thylakoid membrane that capture light energy.

• Two types: Photosystem II (PSII) and Photosystem I (PSI).

• Each photosystem has:

  • Light-harvesting complex: Captures light energy as excited electrons and funnels it to the reaction center.

  • Reaction-center complex: Site of the redox reaction; contains the primary electron donor and acceptor.

Key Components

• P680: Primary electron donor in PSII.

• P700: Primary electron donor in PSI.

• Primary electron acceptors are special chlorophyll molecules.

Electron Transport and ATP/NADPH Production

Electron Transport Chains (ETC)

Light reactions involve two electron transport chains:

• First ETC: Transfers electrons from PSII to PSI, creating a proton gradient for ATP synthesis.

• Second ETC: Transfers electrons from PSI to NADP+ reductase, forming NADPH.

Summary of Light Reactions

• ATP and NADPH are produced on the thylakoid membrane facing the stroma.

• ATP synthesis is driven by a proton gradient across the thylakoid membrane.

• NADPH and ATP are used in the Calvin cycle.

The Calvin Cycle

Overview

The Calvin cycle uses ATP and NADPH to convert CO2 to sugar. It occurs in the stroma of the chloroplast and is also known as the light-independent reactions.

• Initial reactant: RuBP (ribulose-1,5-bisphosphate), a 5-carbon molecule.

• Key enzyme: Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), which fixes CO2.

• Produces 3-carbon molecules: 3-phosphoglycerate (3C).

Phases of the Calvin Cycle

1. Carbon fixation: CO2 is added to RuBP by Rubisco.

2. Reduction: 3-phosphoglycerate is reduced to glyceraldehyde-3-phosphate (G3P).

3. Regeneration: RuBP is regenerated from G3P.

For synthesis of one G3P, the cycle must occur three times, fixing three molecules of CO2.

Isotope Labeling and the Calvin Cycle

Isotope labeling methods (using 14C-labeled CO2) helped determine the steps of the Calvin cycle by tracking the incorporation of carbon into intermediates.

• Allowed researchers to identify the order of intermediates and the timing of carbon fixation.

Carbon Cycle in the Food Chain

Heterotrophs vs. Autotrophs

• Heterotrophs: Consumers in the food chain; break down organic compounds for energy but cannot make them from inorganic sources.

• Autotrophs: Producers in the food chain; make organic compounds from atmospheric carbon via photosynthesis and can also consume organic compounds for energy.

Table: Comparison of Photosynthetic Pigments