Overview
This topic examines the core energy processes that sustain life: photosynthesis in plants and cellular respiration in both plants and animals. These biochemical pathways are critical to the flow of energy and cycling of matter within ecosystems.
Key Concepts and Structures
- Photosynthesis: A two-stage process in which light energy is converted into chemical energy. The light-dependent reactions convert solar energy into ATP and NADPH, while the light-independent reactions (Calvin Cycle) fix carbon dioxide into glucose.
- Chloroplast Structure: The site of photosynthesis, containing thylakoids (which form stacks called grana) and the surrounding fluid called stroma. Thylakoids house pigments like chlorophyll that absorb light.
- Light Reactions: Occur in the thylakoid membranes. Water is split (photolysis) to release oxygen, and the electrons released are used to generate ATP and NADPH through the electron transport chain and chemiosmosis.
- Calvin Cycle: Takes place in the stroma. It uses ATP and NADPH to convert CO2 into glucose through a series of enzyme-controlled steps, including carbon fixation, reduction, and regeneration of RuBP.
- Cellular Respiration: The process by which cells convert glucose into ATP. It consists of glycolysis, the Krebs cycle, and the electron transport chain. Oxygen is essential for the complete oxidation of glucose.
- Glycolysis: Occurs in the cytoplasm. Breaks down glucose into two molecules of pyruvate, producing a small amount of ATP and NADH. Does not require oxygen.
- Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix. Processes pyruvate into CO2 and transfers energy to NADH and FADH2, which carry electrons to the ETC.
- Electron Transport Chain (ETC): Takes place in the inner mitochondrial membrane. Electrons from NADH and FADH2 pass through protein complexes, releasing energy used to pump H+ ions and drive ATP synthesis via ATP synthase. Oxygen is the final electron acceptor, forming water.
- Anaerobic Respiration: In the absence of oxygen, cells undergo fermentation. In animals, this produces lactic acid; in yeast, alcohol and CO2. Yields much less ATP than aerobic respiration.
- Energy Balance: Aerobic respiration yields up to 36-38 ATP per glucose molecule, while anaerobic yields only 2 ATP. Photosynthesis stores energy in glucose; respiration releases that energy for cellular work.
Quick Tip
Don’t confuse where each process happens: Photosynthesis occurs in chloroplasts (plants only); cellular respiration occurs in mitochondria (both plants and animals). Know the stages and where ATP is produced.