Energy Flow, Aerobic Respiration & Fermentation, and Photosynthesis

Ch. 6: Energy Flow

Introduction to Energy in Biological Systems

Energy flow is fundamental to all living organisms, enabling cellular processes and life itself. This section explores the sources, transformations, and regulation of energy in biological systems.

• Sources of Energy: Organisms obtain energy as autotrophs (self-feeders, e.g., plants via photosynthesis) or heterotrophs (consume other organisms).

• Thermodynamics in Biology: The first law of thermodynamics states that energy cannot be created or destroyed, only transformed. The second law states that entropy (disorder) increases in energy transformations, impacting biological order and life processes.

• Electron Flow and Redox Reactions: Movement of electrons (via oxidation-reduction reactions) is central to energy transfer in cells. Oxidation is the loss of electrons; reduction is the gain of electrons.

• Energy Diagrams: Energy diagrams illustrate the changes in free energy during biochemical reactions, helping to predict reaction spontaneity.

• Coupled Reactions: Cells often couple exergonic (energy-releasing) and endergonic (energy-consuming) reactions to drive essential processes.

• ATP as Energy Currency: Adenosine triphosphate (ATP) stores and transfers energy for cellular work. ATP hydrolysis releases energy to power endergonic reactions.

• Activation Energy: The minimum energy required to initiate a chemical reaction. Enzymes lower activation energy, increasing reaction rates.

• Enzyme Function: Enzymes are biological catalysts that speed up reactions without being consumed. Their activity can be regulated by inhibitors or activators.

Example: The breakdown of glucose during cellular respiration involves a series of redox reactions, transferring electrons and releasing energy stored in ATP.

Ch. 7: Aerobic Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP. It includes both aerobic (with oxygen) and anaerobic (without oxygen) pathways.

• Redox Reactions in Respiration: Glucose is oxidized, and oxygen is reduced, resulting in the production of ATP, water, and carbon dioxide.

• Electron Carriers: Molecules such as NAD+, FAD, and ATP play key roles in transferring electrons and energy during respiration.

• Stages of Aerobic Respiration:

  1. Glycolysis: Occurs in the cytoplasm; breaks glucose into pyruvate, producing ATP and NADH.

  2. Pyruvate Oxidation: Converts pyruvate to acetyl-CoA, generating NADH and releasing CO2.

  3. Citric Acid Cycle (Krebs Cycle): Completes the oxidation of acetyl-CoA, producing ATP, NADH, FADH2, and CO2.

  4. Electron Transport Chain (ETC): Located in the inner mitochondrial membrane; uses electrons from NADH and FADH2 to create a proton gradient, driving ATP synthesis via chemiosmosis.

• Fermentation: In the absence of oxygen, cells undergo fermentation to regenerate NAD+ and allow glycolysis to continue. Types include lactic acid fermentation and alcoholic fermentation.

• ATP Yield: Aerobic respiration yields up to 36-38 ATP per glucose, while fermentation yields only 2 ATP per glucose.

• Evolutionary Significance: The universality of glycolysis suggests it is an ancient metabolic pathway.

• Non-Carbohydrate Fuels: Fats and proteins can also be catabolized for energy, entering the respiration pathway at various points.

Example: Muscle cells switch to lactic acid fermentation during intense exercise when oxygen is scarce, producing lactate and regenerating NAD+.

Ch. 8: Photosynthesis

Introduction to Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing organic molecules and oxygen.

• Importance: Photosynthesis is the foundation of most ecosystems, providing energy and organic matter for heterotrophic organisms.

• Evolutionary Impact: The development of photosynthesis led to the accumulation of oxygen in Earth's atmosphere, enabling aerobic life.

• Energy Conversion: Light energy is captured by pigments (e.g., chlorophyll) and converted into chemical energy stored in carbohydrates.

• Stages of Photosynthesis:

  1. Light Reactions: Occur in the thylakoid membranes; convert light energy to ATP and NADPH, releasing O2 as a byproduct.

  2. Calvin Cycle (Dark Reactions): Occurs in the stroma; uses ATP and NADPH to fix CO2 into organic molecules (e.g., glucose).

• Electron Flow: Electrons move from water to NADP+ via photosystems II and I, generating a proton gradient for ATP synthesis.

• Carbon Fixation: The enzyme Rubisco catalyzes the incorporation of CO2 into organic molecules. Limitations include photorespiration and environmental factors.

• Storage of Products: Photosynthetic organisms store energy as complex carbohydrates (e.g., starch).

Example: In the Calvin cycle, three molecules of CO2 are fixed to produce one molecule of glyceraldehyde-3-phosphate (G3P), a precursor to glucose.

Comparison Table: Aerobic Respiration vs. Fermentation

Feature	Aerobic Respiration	Fermentation
Oxygen Requirement	Required	Not required
ATP Yield (per glucose)	36-38	2
End Products	CO2, H2O	Lactate or ethanol + CO2
Electron Carrier Regeneration	ETC regenerates NAD+	Organic molecules regenerate NAD+
Location	Mitochondria (mainly)	Cytoplasm

Key Equations

• Cellular Respiration:

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

• Photosynthesis:

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

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard General Biology curriculum.