Thermodynamics and Energy in Biology

The Laws of Thermodynamics

The laws of thermodynamics govern the flow and transformation of energy in biological systems.

• First Law (Law of Conservation of Energy): Energy cannot be created or destroyed; it can only be transferred or transformed from one form to another (e.g., potential to kinetic energy).

• Second Law: Every energy transfer increases the entropy (disorder) of the universe. No energy transfer is 100% efficient; some energy is always lost as heat.

Endergonic and Exergonic Reactions

Chemical reactions in cells can either require or release energy.

• Endergonic Reactions: Require an input of energy (e.g., photosynthesis).

• Exergonic Reactions: Release energy during the reaction (e.g., cellular respiration). Exergonic reactions often drive endergonic ones in cells.

ATP as an Energy Shuttle

ATP (adenosine triphosphate) acts as the main energy currency in the cell.

• The phosphate bonds (especially the terminal phosphate) in ATP store potential energy.

• Hydrolysis of ATP (breaking a phosphate bond) releases energy that can be used to power cellular processes.

Enzyme-Substrate Interaction and Enzyme Catalysis

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Enzyme Specificity: Each enzyme binds to a specific substrate due to the shape of its active site.

• Enzyme Regulation: Enzyme activity can be regulated by:

  • Allosteric Regulation: Molecules bind to sites other than the active site, changing enzyme shape and activity.

  • Covalent Modification: Enzyme is chemically modified (e.g., phosphorylation).

  • Feedback Inhibition: The end product of a pathway inhibits an earlier step, preventing overproduction.

  • Denaturation: Loss of enzyme structure (and function) due to changes in pH or temperature.

• Enzyme Inhibition: Decreases enzyme activity (can be competitive or noncompetitive).

• Enzyme Pathways: Series of linked reactions where the product of one reaction becomes the substrate for the next.

Cellular Respiration

Overview and Stages

Cellular respiration is the process by which cells extract energy from glucose to produce ATP.

• Glycolysis: Occurs in the cytoplasm; glucose is split into two pyruvate molecules, producing ATP and NADH.

• Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondria; pyruvate is broken down, producing CO2, ATP, NADH, and FADH2.

• Electron Transport Chain (ETC): Occurs in the inner mitochondrial membrane; electrons from NADH and FADH2 are transferred through proteins, creating a proton gradient that powers ATP synthesis.

Overall Equation for Cellular Respiration:

Fermentation

When oxygen is absent, cells can generate ATP through fermentation.

• Alcoholic Fermentation: Produces ethanol and CO2 (e.g., yeast).

• Lactic Acid Fermentation: Produces lactic acid (e.g., muscle cells under anaerobic conditions).

Photosynthesis

Overview and Equation

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

Overall Equation for Photosynthesis:

• Location: Light-dependent reactions occur in the thylakoid membranes; light-independent reactions (Calvin cycle) occur in the stroma of the chloroplast.

Light Reactions and Calvin Cycle

• Light Reactions: Capture light energy to produce ATP and NADPH; split water to release O2.

• Calvin Cycle: Uses ATP and NADPH to fix CO2 and synthesize sugars.

Steps in the Calvin Cycle

1. Carbon fixation: CO2 is attached to RuBP by rubisco, forming 3-phosphoglycerate (3-PGA).

2. Reduction: 3-PGA is phosphorylated and reduced to form G3P (glyceraldehyde-3-phosphate).

3. Regeneration: Some G3P is used to regenerate RuBP, enabling the cycle to continue.

Types of Photosynthesis: C3, C4, and CAM

Cell Division: Mitosis and Meiosis

Phases of the Cell Cycle

• Interphase: Cell grows, DNA replicates, prepares for division (G1 → S → G2).

• Mitosis: Division of the nucleus into two identical diploid cells (growth, repair).

• Meiosis: Two divisions produce four unique haploid gametes, increasing genetic diversity.

Phases of Mitosis

• Prophase: Chromosomes condense, spindle fibers form, nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equatorial plate.

• Anaphase: Sister chromatids are pulled apart to opposite poles.

• Telophase: Chromatids decondense, nuclear envelope reforms, spindle fibers disassemble.

Cytokinesis: Plant vs. Animal Cells

• Animal Cells: Cytokinesis occurs by cleavage furrow (pinching in).

• Plant Cells: Cytokinesis occurs by formation of a cell plate (building a wall from the center outward).

Phases of Meiosis (with Comparison to Mitosis)

• Meiosis includes two divisions, resulting in four unique haploid cells (gametes).

• Increases genetic diversity through crossing over and independent assortment.

Genetics: Mendelian and Non-Mendelian Inheritance

Mendel's Experimental Framework

Mendel studied inheritance in pea plants, focusing on traits with clear, observable differences.

Key Genetic Terms

Mendel's Principles

• Principle of Segregation: Each individual has two alleles for every gene, which separate during gamete formation. Offspring receive one allele from each parent.

• Principle of Independent Assortment: Genes for different traits are inherited independently if they are on different chromosomes, creating new combinations in offspring.

Non-Mendelian Inheritance Patterns

Sex-Linked Inheritance

Traits inherited on the sex chromosomes (usually the X chromosome) show unique patterns, often affecting males more frequently due to having only one X chromosome.

Additional info: These notes provide a comprehensive overview of key General Biology concepts, including energy transformations, cellular processes, photosynthesis, cell division, and genetics, suitable for exam preparation.