Photosynthesis and Cellular Energy Conversion

Autotrophic and Heterotrophic Organisms

Organisms are classified based on how they obtain energy and organic molecules. This distinction is fundamental in understanding ecological roles and energy flow in the biosphere.

• Autotrophic organisms: These organisms produce their own organic molecules from inorganic sources, typically using CO2 as a carbon source. They are known as producers in the biosphere.

• Heterotrophic organisms: These organisms obtain organic material by consuming other organisms. They are consumers and depend on autotrophs for food and oxygen.

Example: Plants are autotrophs, while animals and fungi are heterotrophs.

Chloroplasts and Photosynthesis

Photosynthesis occurs in chloroplasts, specialized organelles found in plant cells and algae. The process converts light energy into chemical energy stored in glucose.

• CO2 and O2 exchange: CO2 enters and O2 exits via stomata.

• Chloroplast structure:

  • Double membrane surrounds the organelle.

  • Thylakoids are membrane-bound sacs, often stacked into grana.

  • Grana are connected by stroma thylakoids.

• Overall photosynthesis equation:

Endergonic process: Requires energy input from sunlight.

Light Reactions of Photosynthesis

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

• Photosystems:

  • Photosystem II (PSII): Initiates the light reactions, absorbs light at P680.

  • Photosystem I (PSI): Absorbs light at P700, acts later in the electron transport chain.

• Light harvesting complexes: Pigments bound to proteins that transfer energy to the reaction center.

• Reaction center: Contains special chlorophyll a molecules that donate excited electrons to the primary electron acceptor.

• Electron flow: Linear electron flow involves eight steps, including photon absorption, water splitting, electron transport, and ATP/NADPH formation.

Linear Electron Flow Steps

1. Photon excites pigment LH→P680.

2. Electron moves from P680 to primary electron acceptor.

3. Water is split, providing electrons and releasing O2.

4. Electrons are transported via cytochromes, creating a proton gradient.

5. ATP is synthesized via chemiosmosis.

6. Electrons reach PSI, are re-excited, and transferred to NADP+ to form NADPH.

7. ATP and NADPH are used in the Calvin cycle.

Chemiosmosis and ATP Synthesis

Chemiosmosis is the process by which ATP is produced using the energy of a proton gradient across a membrane.

• Photosystem II, cytochrome complex, and NADP+ reductase contribute to the proton gradient.

• ATP synthase uses the gradient to synthesize ATP from ADP and inorganic phosphate.

Calvin Cycle (Light-Independent Reactions)

The Calvin cycle uses ATP and NADPH to convert CO2 into glucose. It is an anabolic process that regenerates its starting material.

• Phase 1: Carbon Fixation

  • Enzyme rubisco incorporates CO2 into a 5-carbon sugar (RuBP).

  • Forms two 3-carbon molecules.

• Phase 2: Reduction

  • 3-phosphoglycerate is phosphorylated and reduced by NADPH to form G3P.

  • For every three CO2 molecules, six G3P are produced, but only one exits the cycle.

• Phase 3: Regeneration

  • Five G3P are rearranged to regenerate three RuBP.

  • Requires ATP input.

For one G3P molecule: 9 ATP and 6 NADPH are consumed.

Calvin Cycle Summary Table

Photosynthetic Pigments

Pigments absorb light energy for photosynthesis. Different pigments absorb different wavelengths.

• Chlorophyll a: Main pigment, absorbs violet-blue and red light.

• Chlorophyll b: Accessory pigment, broadens absorption spectrum.

• Carotenoids: Accessory pigments, protect against excess light.

Visible light range: 380–750 nm.

Comparison: Photosynthesis vs. Cellular Respiration

Key Equations

• Photosynthesis:

• Calvin Cycle (for one G3P):

Additional info: The notes also reference mitochondria and chemiosmosis, which are central to cellular respiration, but the main focus is on photosynthesis and the Calvin cycle.