Cellular Respiration and Photosynthesis

Overview

Cellular respiration and photosynthesis are fundamental biological processes that enable organisms to convert energy for life functions. Cellular respiration occurs in both plants and animals, while photosynthesis is unique to plants, algae, and some bacteria.

Glycolysis

• Definition: Glycolysis is the first step in cellular respiration, where glucose is broken down into pyruvate.

• Products: 2 pyruvate, 2 ATP, and 2 NADH molecules.

• Location: Occurs in the cytoplasm of the cell.

• Anaerobic Steps: Glycolysis does not require oxygen.

Fermentation

• Definition: Fermentation is an anaerobic process that allows glycolysis to continue in the absence of oxygen.

• Types: Alcoholic fermentation (produces ethanol and CO2), lactic acid fermentation (produces lactic acid).

• Organisms: Yeast performs alcoholic fermentation; muscle cells perform lactic acid fermentation.

Electron Transport Chain (ETC) and Chemiosmosis

• Oxidative Phosphorylation: The process by which ATP is produced using energy from electrons transferred through the ETC.

• Location: Inner mitochondrial membrane.

• Products: Water and ATP.

• Substrate Level Phosphorylation: Direct formation of ATP in glycolysis and Krebs cycle.

• Endergonic/Exergonic Reactions: Exergonic reactions release energy; endergonic reactions require energy.

• Oxidation/Reduction: Oxidation is loss of electrons; reduction is gain of electrons. Both occur in ETC and photosynthesis.

Krebs Cycle (Citric Acid Cycle)

• Purpose: Completes the breakdown of glucose, producing NADH and FADH2 for the ETC.

• Location: Mitochondrial matrix.

• Pyruvate Entry: Pyruvate is converted to acetyl-CoA before entering the cycle.

• Intermediate Step: Conversion of pyruvate to acetyl-CoA.

ATP Production

• ATP: Adenosine triphosphate, the main energy currency of the cell.

• How Much ATP: Glycolysis (2 ATP), Krebs cycle (2 ATP), ETC (about 32 ATP).

• Where Produced: Cytoplasm (glycolysis), mitochondria (Krebs cycle and ETC).

Photosynthesis

• Definition: The process by which plants, algae, and some bacteria convert light energy into chemical energy (glucose).

• Location: Chloroplasts in plant cells.

• Equations:

• Light-Dependent Reactions: Occur in the thylakoid membranes; produce ATP and NADPH.

• Light-Independent Reactions (Calvin Cycle): Occur in the stroma; use ATP and NADPH to fix CO2 into glucose.

Electron Flow in Photosynthesis

• Linear Electron Flow: Electrons move from water to NADP+, producing NADPH and ATP.

• Cyclic Electron Flow: Electrons cycle back to photosystem I, producing ATP but not NADPH.

• Why Cyclic Flow is Necessary: Balances the ATP/NADPH ratio for the Calvin cycle.

Pigments and Light Absorption

• Pigments: Molecules that absorb specific wavelengths of light (e.g., chlorophyll, carotenoids).

• Chlorophyll: Main pigment in photosynthesis; absorbs red and blue light, reflects green.

• Accessory Pigments: Carotenoids and others expand the range of light absorption.

• Action Spectrum: Shows the effectiveness of different wavelengths in driving photosynthesis.

• Absorption Spectrum: Shows which wavelengths are absorbed by pigments.

Calvin Cycle

• Purpose: Fixes carbon dioxide into organic molecules (glucose).

• Phases: Carbon fixation, reduction, regeneration of RuBP.

• Key Enzyme: Rubisco catalyzes the first step.

Photorespiration

• Definition: A process where oxygen is consumed and carbon dioxide is released, reducing photosynthetic efficiency.

• Why Problematic: Wastes energy and reduces sugar production.

C3, C4, and CAM Pathways

• C3 Plants: Use the Calvin cycle directly; most common.

• C4 Plants: Minimize photorespiration by separating initial CO2 fixation and Calvin cycle in different cell types.

• CAM Plants: Fix CO2 at night, store it as malate, and use it during the day.

Relationship Between Cellular Respiration and Photosynthesis

• Interdependence: Photosynthesis produces glucose and oxygen, which are used in cellular respiration to produce ATP, CO2, and water.

• Energy Flow: Energy from sunlight is stored in glucose by photosynthesis and released by cellular respiration.

Additional Key Terms and Concepts

• Autotrophic Bacteria: Bacteria that produce their own food via photosynthesis or chemosynthesis.

• Redox Reactions: Chemical reactions involving the transfer of electrons.

• NADH and NADPH: Electron carriers in cellular respiration and photosynthesis, respectively.

• Source of Oxygen Released: Oxygen released during photosynthesis comes from water.

Table: Comparison of Cellular Respiration and Photosynthesis

Example: Photosynthesis in Plants

• Plants use sunlight to convert carbon dioxide and water into glucose and oxygen, providing energy for growth and metabolism.

Example: Cellular Respiration in Humans

• Humans consume glucose and oxygen, producing ATP for cellular activities and releasing carbon dioxide and water as waste.