Cellular Respiration and Photosynthesis: Key Concepts and Pathways

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose and other organic molecules to produce ATP, the main energy currency of the cell. This process occurs in several stages and involves both aerobic and anaerobic pathways.

• Purpose: To convert biochemical energy from nutrients into ATP.

• Location: Mainly in the mitochondria of eukaryotic cells.

• General Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP (energy)}$

Stages of Cellular Respiration

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into two molecules of pyruvate, producing a small amount of ATP and NADH.

• Pyruvate Oxidation: Pyruvate is transported into the mitochondria and converted to acetyl-CoA, producing NADH and releasing CO2.

• Krebs Cycle (Citric Acid Cycle): Acetyl-CoA enters the cycle, generating NADH, FADH2, ATP, and CO2.

• Electron Transport Chain (ETC) and Oxidative Phosphorylation: NADH and FADH2 donate electrons to the ETC, which powers ATP synthesis via chemiosmosis. Oxygen is the final electron acceptor, forming water.

ATP Yield

• Total ATP per glucose: Approximately 30-32 ATP molecules.

• Key electron carriers: NAD+ and FAD are reduced to NADH and FADH2 during earlier stages and donate electrons to the ETC.

Anaerobic Respiration and Fermentation

• Anaerobic Respiration: Occurs when oxygen is not present; uses other molecules as final electron acceptors.

• Fermentation: Allows glycolysis to continue by regenerating NAD+. Two main types: lactic acid fermentation and alcoholic fermentation.

Fermentation Types

Type	Products	Organisms
Lactic Acid Fermentation	Lactic acid, ATP	Muscle cells, some bacteria
Alcoholic Fermentation	Ethanol, CO2, ATP	Yeast, some bacteria

Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It occurs in the chloroplasts of plant cells.

• General Equation:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Stages of Photosynthesis

• Light Reactions: Occur in the thylakoid membranes; convert light energy into chemical energy (ATP and NADPH), and produce O2 as a byproduct.

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

Chloroplast Structure

• Thylakoids: Flattened sacs where light reactions occur.

• Stroma: Fluid surrounding thylakoids; site of the Calvin cycle.

Electron Transport in Photosynthesis

• Light excites electrons in chlorophyll, which are transferred through an electron transport chain, generating ATP and NADPH.

• Water is split to provide electrons, releasing O2 as a byproduct.

Comparison of Cellular Respiration and Photosynthesis

Feature	Cellular Respiration	Photosynthesis
Location	Mitochondria	Chloroplasts
Reactants	Glucose, O2	CO2, H2O, light
Products	CO2, H2O, ATP	Glucose, O2
Energy Conversion	Chemical to ATP	Light to chemical

Key Terms and Concepts

• ATP (Adenosine Triphosphate): Main energy carrier in cells.

• NADH/FADH2: Electron carriers in cellular respiration.

• NADPH: Electron carrier in photosynthesis.

• Glycolysis: First step of cellular respiration, occurs in cytoplasm.

• Krebs Cycle: Series of reactions in mitochondria that generate electron carriers.

• Electron Transport Chain: Series of proteins in mitochondria/chloroplasts that produce ATP.

• Fermentation: Anaerobic process to regenerate NAD+ for glycolysis.

• Calvin Cycle: Light-independent reactions of photosynthesis that produce glucose.

Additional info: These notes synthesize and expand upon handwritten class notes, providing a structured overview of cellular respiration and photosynthesis, including their stages, key molecules, and comparative features.