General Biology: Energy, Metabolism, Cellular Respiration, Photosynthesis, and Cell Division

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Energy and Thermodynamics

Kinetic vs. Potential Energy

Energy exists in different forms, primarily as kinetic and potential energy.

Kinetic energy: The energy of motion.
Potential energy: Stored energy due to position or structure.
Chemical potential energy: Energy stored in chemical bonds, such as in glucose.

Laws of Thermodynamics

The laws of thermodynamics govern energy transformations in biological systems.

First Law: Energy cannot be created or destroyed.
Second Law: Energy transfers increase entropy (disorder).

Free Energy (Gibbs Free Energy)

Free energy determines whether a reaction can occur spontaneously.

Symbol:
Negative ΔG: Exergonic (energy-releasing) reactions.
Positive ΔG: Endergonic (energy-requiring) reactions.

Exergonic vs. Endergonic Reactions

Exergonic: Release energy; ΔG is negative; e.g., cellular respiration.
Endergonic: Require energy input; ΔG is positive; e.g., photosynthesis.

Catabolic vs. Anabolic Pathways

Catabolic: Breakdown of molecules, releases energy (e.g., cellular respiration).
Anabolic: Build-up of molecules, requires energy (e.g., protein synthesis).

Energy Coupling

Cells use energy released from exergonic reactions (like ATP hydrolysis) to drive endergonic reactions.

ATP Hydrolysis

ATP hydrolysis: Exergonic; releases energy for cellular work.
Equation:

Enzymes and Metabolism

Enzyme Function

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

Active site: Region where substrate binds.
Induced fit: Enzyme changes shape to better bind substrate.
Catalyst: Substance that speeds up reactions without being consumed.

Enzyme Regulation

Competitive inhibitors: Bind active site, block substrate.
Noncompetitive inhibitors: Bind elsewhere, change enzyme shape.
Feedback inhibition: End product inhibits an early enzyme in the pathway.

Cofactors and Coenzymes

Cofactors: Inorganic ions (e.g., Zn2+).
Coenzymes: Organic molecules (e.g., vitamins).

Cellular Respiration and Fermentation

Overview of Cellular Respiration

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

Stages: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, Oxidative Phosphorylation.
Location: Glycolysis in cytoplasm; other stages in mitochondria.

Glycolysis

Inputs: Glucose, 2 ATP, 2 NAD+
Outputs: 4 ATP (net 2), 2 NADH, 2 pyruvate
Oxygen required? No (anaerobic process).

Pyruvate Oxidation and Citric Acid Cycle

Pyruvate oxidation: Produces acetyl-CoA, CO2, NADH.
Citric Acid Cycle: Produces 2 CO2, 2 NADH, 2 Acetyl-CoA per glucose.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

Function: Transfers electrons, builds proton gradient, powers ATP synthesis.
Final electron acceptor: Oxygen (forms H2O).
ATP yield: ~30–32 ATP per glucose (aerobic respiration).

Fermentation

Occurs when oxygen is absent.
Products: 2 ATP (from glycolysis), lactic acid or ethanol.
Electron acceptor: NAD+ (regenerated for glycolysis to continue).

Substrate-Level vs. Oxidative Phosphorylation

Substrate-level phosphorylation: Direct transfer of phosphate to ADP.
Oxidative phosphorylation: ATP synthesis via chemiosmosis and ETC.

Autotrophs vs. Heterotrophs

Autotrophs: Make their own food (producers).
Heterotrophs: Consume others (consumers).

Photosynthesis

Overview

Photosynthesis converts light energy into chemical energy in plants, algae, and some bacteria.

Light reactions: Occur in thylakoid membrane; produce ATP, NADPH, O2.
Calvin cycle: Occurs in stroma; uses ATP and NADPH to fix CO2 into carbohydrates.

Light Reactions

Water is split, O2 released, NADP+ reduced to NADPH.
ATP made via photophosphorylation.

Calvin Cycle

CO2 is fixed and reduced to carbohydrate using NADPH and ATP.
ATP and NADPH from light reactions provide energy and reducing power.

Wavelength and Energy

Shorter wavelength = higher energy.
Violet-blue and red light maximize photosynthesis.

Cell Cycle and Cell Division

Phases of the Cell Cycle

Interphase: G1, S, G2 (cell growth, DNA replication).
Mitosis: Division of the nucleus.
Cytokinesis: Division of the cytoplasm.

Chromosomes and Chromatin

Chromatin: DNA + proteins; uncondensed chromosomes.
Chromosome: Condensed, visible during mitosis.
Chromatid: One copy of a duplicated chromosome.

Human Chromosome Number

Somatic cells: 46 chromosomes (diploid, 2n).
Gametes: 23 chromosomes (haploid, n).

Mitosis

Produces two genetically identical daughter cells.
Occurs in somatic (body) cells.

Meiosis and Sexual Reproduction

Meiosis: Produces gametes with half the chromosome number (haploid).
Occurs in ovaries and testes.
Increases genetic diversity via independent assortment, crossing over, and random fertilization.

Key Terms and Definitions

Gene: Unit of heredity.
Locus: Location of a gene on a chromosome.
Allele: Different versions of a gene.
Homologous chromosomes: Same genes, possibly different alleles, one from each parent.
Haploid (n): One set of chromosomes.
Diploid (2n): Two sets of chromosomes.

Meiosis Details

Two cell divisions: Meiosis I and II.
Crossing over: Exchange of genetic material between homologous chromosomes (Prophase I).
Independent assortment: Random distribution of chromosomes to gametes.
Random fertilization: Increases genetic variation.
Reduction division: Chromosome number halved in Meiosis I.

Comparison Table: Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Genetic identity	Identical	Unique
Chromosome number	Diploid (2n)	Haploid (n)
Function	Growth, repair	Gamete production

Genetic Variation in Meiosis

Sources: Independent assortment, crossing over, random fertilization.
Number of combinations: = ~8.4 million (from independent assortment alone).
Random fertilization: ~70 trillion possible combinations (8.4 million × 8.4 million).

Advantages of Sexual Reproduction

Increases genetic diversity, beneficial in changing environments.
Alternation between haploid and diploid stages in life cycle.