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 is the process by which cells convert nutrients, primarily glucose, into ATP, the main energy currency of the cell. This process can be aerobic (with oxygen) or anaerobic (without oxygen).

The overall chemical equation for aerobic cellular respiration is:

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP} + \text{heat}}$

• 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, which enters the Krebs cycle.

3. Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondria; processes acetyl-CoA to produce 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, generating a proton gradient that drives ATP synthesis. Oxygen is the final electron acceptor, forming water.

ATP is produced by two mechanisms:

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP during glycolysis and the Krebs cycle.

• Oxidative phosphorylation: ATP synthesis powered by 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 the final electron acceptor. Produces ATP only via glycolysis.

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid. Occurs in muscle cells, fungi, and 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. Produces more ATP than fermentation but less than aerobic respiration.

Types of anaerobic organisms:

• Obligate anaerobes: Only survive without oxygen.

• 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 and eggs); results in genetically diverse haploid cells.

Phases of the Cell Cycle

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, including mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Cell Cycle Regulation

Cell division is tightly regulated by growth factors and checkpoints to ensure proper timing and accuracy.

• Growth factors: Proteins that stimulate cell division.

• Checkpoints: Control points where the cell verifies completion of key processes before proceeding. Major checkpoints: G1, S, G2, and M (metaphase).

• 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, resulting in two genetically identical diploid cells. The stages are:

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.

Microtubules play a key role in chromosome movement. Kinetochore microtubules attach to chromosomes, while non-kinetochore microtubules help elongate the cell.

Cancer and Cell Growth Regulation

Cancer is characterized by uncontrolled cell division, often resulting from mutations in genes that regulate the cell cycle.

• Malignant tumors: Cancerous, can invade other tissues (metastasize).

• Benign tumors: Non-cancerous, do not spread.

• Proto-oncogenes: Stimulate normal cell division; mutations can create oncogenes that drive cancer.

• Tumor-suppressor genes: Inhibit cell division; mutations can remove this inhibition (e.g., p53 gene).

Meiosis and Sexual Life Cycles

Inheritance and Chromosomes

Genes, made of DNA, are units of heredity located on chromosomes. Each gene's position is called its locus. Genes are passed to the next generation via gametes.

Asexual vs. Sexual Reproduction

• Asexual reproduction: Single parent, offspring are genetically identical (clones).

• Sexual reproduction: Two parents, offspring have unique gene combinations due to meiosis and fertilization.

Life Cycle of Sexual Reproducers

Sexual life cycles involve both mitosis and meiosis. Meiosis produces haploid gametes; fertilization restores diploidy in the zygote, which grows by mitosis.

Introduction to Meiosis

Meiosis is a two-division process that reduces chromosome number by half, producing four genetically diverse haploid gametes from a diploid germ cell.

• Meiosis I (reductional division): Homologous chromosomes separate, reducing chromosome number.

• Meiosis II (equational division): Sister chromatids separate, similar to mitosis.

Genetic Variation During Meiosis

Meiosis generates genetic diversity through two main mechanisms:

• Crossing Over: Homologous chromosomes exchange genetic material during Prophase I, creating non-identical sister chromatids.

• Independent Assortment: Homologous chromosome pairs align randomly during Metaphase I, resulting in many possible genetic combinations. The number of combinations is $2^n$, where n is the haploid number.

Nondisjunction and Aneuploidy

Nondisjunction is an error in meiosis where chromosomes fail to separate properly, leading to aneuploid cells (with extra or missing chromosomes). This can cause genetic disorders such as Down syndrome (Trisomy 21).

Summary Table: Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Genetic identity	Identical	Diverse
Chromosome number	Same as parent (diploid)	Half of parent (haploid)
Role	Growth, repair, asexual reproduction	Sexual reproduction