Cellular Respiration, Cell Cycle, and Meiosis: Core Concepts in College Biology

Cellular Respiration and Fermentation

Overview of Catabolic Processes

Catabolic processes are metabolic pathways that break down complex molecules into simpler ones, releasing energy that can be used by the cell. The main types include aerobic respiration, fermentation, and anaerobic respiration.

• Aerobic respiration: Requires oxygen and produces a large amount of ATP.

• Fermentation and anaerobic respiration: Do not require oxygen and yield less ATP.

Cellular Respiration: Definition and Function

Cellular respiration is the process by which cells convert nutrients, primarily glucose, into ATP, the main energy currency of the cell. This process can occur with or without oxygen, but aerobic respiration is the most efficient.

• The main function is to extract usable energy from glucose.

• Occurs in several stages, mainly within the mitochondria.

• Oxygen acts as the final electron acceptor in aerobic respiration.

Chemical Equation and Redox Nature

The overall chemical equation for aerobic cellular respiration is a redox reaction:

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP\ (energy)}$

• Glucose is oxidized (loses electrons), and oxygen is reduced (gains electrons).

• By-products: 6 molecules of carbon dioxide and 6 molecules of water.

Stages of Aerobic Cellular Respiration

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

2. Pyruvate Oxidation: Converts pyruvate into acetyl-CoA in the mitochondria.

3. Krebs Cycle (Citric Acid Cycle): Processes acetyl-CoA, generating NADH, FADH2, and a small amount of ATP.

4. Electron Transport Chain & Chemiosmosis: Electrons from NADH and FADH2 are transferred through proteins in the inner mitochondrial membrane, producing a large amount of ATP via oxidative phosphorylation. Oxygen is the final electron acceptor, forming water.

ATP Production Mechanisms

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP, occurs in glycolysis and Krebs cycle.

• Oxidative phosphorylation: ATP is produced using energy derived from the electron transport chain.

Fermentation and Anaerobic Respiration

When oxygen is unavailable, cells switch to fermentation or anaerobic respiration to regenerate NAD+ and produce ATP.

• Fermentation: Uses organic molecules as final electron acceptors; yields little ATP.

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid; occurs in muscle cells and some bacteria.

• Alcohol Fermentation: Pyruvate is reduced to ethanol and CO2; occurs in yeast and some bacteria.

• Anaerobic Respiration: Uses inorganic molecules (e.g., nitrate, sulfate) as final electron acceptors; more ATP than fermentation but less than aerobic respiration.

Types of Anaerobic Organisms

• Obligate anaerobes: Only survive without oxygen; oxygen is toxic.

• Facultative anaerobes: Can switch between aerobic respiration and fermentation (e.g., yeast, some bacteria, muscle cells).

The Cell Cycle

Introduction to Cell Division

Cell division is the process by which a parent cell divides to produce two or more daughter cells. It is essential for reproduction, growth, and tissue repair.

• Binary Fission: Prokaryotic cell division (e.g., bacteria, archaea).

• Mitosis: Eukaryotic cell division producing somatic (body) cells; results in genetically identical diploid cells.

• Meiosis: Eukaryotic cell division producing gametes (sperm, eggs); results in genetically diverse haploid cells.

Importance of Cell Division

• Essential for reproduction, development, and tissue repair.

• DNA replication must occur before division to ensure each daughter cell receives a complete genome.

The Cell Cycle: Phases and Regulation

The cell cycle describes the sequence of events from cell formation to division. It consists of two main phases:

• Interphase: Non-dividing phase for cell growth, DNA replication, and organelle production. Subphases: G0, G1, S, G2.

• M (Mitotic) Phase: Dividing phase, includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Cell Cycle Regulation and Checkpoints

• Regulated by growth factors and internal checkpoints (G1, S, G2, M).

• p53 protein can trigger DNA repair or apoptosis if errors are detected.

• Loss of checkpoint control can lead to cancer.

Phases of Mitosis

Mitosis is the process of dividing the nucleus and genetic material of a somatic cell. It consists of five stages:

1. Prophase: Chromosomes condense, nucleolus disappears, mitotic spindle forms.

2. Prometaphase: Nuclear envelope breaks down, spindle attaches to kinetochores.

3. Metaphase: Chromosomes align at the metaphase plate.

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

5. Telophase: Nuclear envelopes reform, chromosomes decondense.

Cancer and Cell Cycle Control

• Cancer: Uncontrolled cell division leading to malignant tumors.

• Malignant tumors: Cancerous, invasive, can metastasize.

• Benign tumors: Non-cancerous, localized, slow-growing.

• Proto-oncogenes: Stimulate normal cell division; mutations can create oncogenes (cancer-promoting).

• Tumor-suppressor genes: Inhibit cell division; mutations can lead to cancer (e.g., p53).

Meiosis and Sexual Life Cycle

Genes, Chromosomes, and Inheritance

• Gene: Unit of heredity, located at a specific locus on a chromosome.

• Genes are passed to offspring via gametes (sperm and eggs).

Asexual vs. Sexual Reproduction

• Asexual reproduction: Single parent, offspring genetically identical to parent.

• Sexual reproduction: Two parents, offspring genetically unique due to meiosis and fertilization.

Life Cycle of Sexual Reproducers

• Meiosis: Produces haploid gametes from diploid germ cells.

• Fertilization: Fusion of gametes to form a diploid zygote.

• Zygote: First diploid cell of a new organism; undergoes mitosis for growth.

Meiosis: Process and Phases

Meiosis consists of two sequential divisions:

1. Meiosis I (Reductional Division): Homologous chromosomes separate, reducing chromosome number by half.

2. Meiosis II (Equational Division): Sister chromatids separate, similar to mitosis.

• Results in four genetically diverse haploid gametes.

Genetic Variation in Meiosis

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I, creating non-identical sister chromatids.

• Synapsis: Pairing of homologous chromosomes.

• Chiasma: Site of crossing over.

• Independent Assortment: Random alignment of homologous pairs during Metaphase I, resulting in genetic diversity. Number of combinations: $2^n$ (n = haploid number).

Nondisjunction and Aneuploidy

• Nondisjunction: Failure of chromosomes to separate properly during meiosis, leading to aneuploid cells (extra or missing chromosomes).

• Aneuploidy: Can cause genetic disorders such as Down syndrome (Trisomy 21).