Module Objectives
- Distinguish prokaryotic and eukaryotic cell structure and function
- Describe the role of key organelles (nucleus, mitochondria, ribosomes, ER, Golgi)
- Explain Mendel's laws and apply them to monohybrid and dihybrid crosses
- Outline the central dogma: DNA replication → transcription → translation
- Describe glycolysis, the Krebs cycle and oxidative phosphorylation
- Identify the major human organ systems and their primary functions
1. Cell Biology
The cell is the fundamental unit of life. All living organisms are composed of one or more cells, and all cells arise from pre-existing cells (Cell Theory).
Prokaryotes vs Eukaryotes
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Examples | Bacteria, Archaea | Animals, plants, fungi, protists |
| Nucleus | Absent (nucleoid region) | Present (membrane-bound) |
| Size | 1–10 µm | 10–100 µm |
| Membrane-bound organelles | Absent | Present |
| DNA | Circular, in cytoplasm | Linear chromosomes, in nucleus |
| Ribosomes | 70S (50S + 30S) | 80S (60S + 40S) |
| Cell wall | Usually present (peptidoglycan in bacteria) | Present in plants (cellulose), fungi (chitin); absent in animals |
Cell Membrane — Fluid Mosaic Model
The plasma membrane is a phospholipid bilayer with embedded proteins. Key properties:
- Phospholipids — hydrophilic head (faces water) + hydrophobic tails (form bilayer interior)
- Integral proteins — span the membrane; include channels, transporters, receptors
- Peripheral proteins — attached to inner/outer surface; involved in signalling
- Cholesterol — modulates membrane fluidity
Membrane Transport
- Passive transport (no ATP): simple diffusion, facilitated diffusion, osmosis
- Active transport (requires ATP): Na⁺/K⁺-ATPase pump (3 Na⁺ out, 2 K⁺ in)
- Bulk transport: endocytosis (phagocytosis, pinocytosis) and exocytosis
2. Cell Organelles
| Organelle | Function | Membrane |
|---|---|---|
| Nucleus | Contains DNA; site of transcription; controls cell activities | Double membrane (nuclear envelope) |
| Mitochondria | ATP production via oxidative phosphorylation; site of Krebs cycle | Double membrane; cristae increase surface area |
| Rough ER | Protein synthesis (ribosomes) and folding; lipid synthesis | Single membrane, studded with ribosomes |
| Smooth ER | Lipid/steroid synthesis; detoxification; Ca²⁺ storage | Single membrane, no ribosomes |
| Golgi Apparatus | Modifies, packages and sorts proteins for secretion or organelle delivery | Flattened cisternae (single membrane) |
| Ribosome | Protein synthesis (translation) | No membrane (RNA + protein) |
| Lysosome | Intracellular digestion; autophagy; contains hydrolytic enzymes (pH 5) | Single membrane |
| Peroxisome | Oxidation of fatty acids; detoxification (H₂O₂ → H₂O) | Single membrane |
| Chloroplast | Photosynthesis (plants/algae only) | Double membrane + thylakoid |
| Vacuole | Storage of water, nutrients, waste (large central vacuole in plants) | Single membrane (tonoplast) |
3. Genetics
Key Terminology
- Gene — a specific DNA sequence that codes for a functional product (protein or RNA)
- Allele — alternative versions of a gene (e.g., B = brown eyes, b = blue eyes)
- Genotype — the genetic constitution (e.g., Bb)
- Phenotype — the observable characteristics (e.g., brown eyes)
- Homozygous — two identical alleles (BB or bb)
- Heterozygous — two different alleles (Bb)
- Dominant — allele expressed in heterozygous state
- Recessive — allele expressed only in homozygous state
Mendel's Laws
- Law of Segregation — Each parent has two alleles per gene; these separate during gamete formation so each gamete receives only one allele
- Law of Independent Assortment — Alleles of different genes assort independently during gamete formation (applies to genes on different chromosomes)
Cell Division
Mitosis — PMAT (Prophase, Metaphase, Anaphase, Telophase) → 2 identical diploid cells. For growth, repair, asexual reproduction.
Meiosis — Two rounds of division (Meiosis I + II) → 4 genetically unique haploid cells (gametes). Includes crossing over in Prophase I for genetic diversity.
4. Molecular Biology — Central Dogma
The central dogma describes the flow of genetic information:
DNA Replication
- Helicase — unwinds and separates the DNA double helix
- Primase — synthesises short RNA primers to start replication
- DNA Polymerase III — adds nucleotides in 5'→3' direction; proofreads
- Ligase — joins Okazaki fragments on the lagging strand
- Topoisomerase — relieves supercoiling ahead of the replication fork
Transcription (DNA → mRNA)
- Occurs in the nucleus (eukaryotes)
- RNA Polymerase binds to the promoter region and synthesises pre-mRNA
- Pre-mRNA is processed: 5' cap + poly-A tail added; introns removed (spliced); exons joined
Translation (mRNA → Protein)
- Occurs at ribosomes (cytoplasm or rough ER)
- mRNA codons (triplets) are read; tRNA brings matching amino acids
- Ribosome has 3 sites: A (aminoacyl), P (peptidyl), E (exit)
- Start codon: AUG (Methionine) | Stop codons: UAA, UAG, UGA
5. Cellular Metabolism
Cellular Respiration — ATP Production
| Stage | Location | Input | Net ATP Output | Other Products |
|---|---|---|---|---|
| Glycolysis | Cytoplasm | 1 Glucose | 2 ATP | 2 Pyruvate, 2 NADH |
| Pyruvate Oxidation | Mitochondrial matrix | 2 Pyruvate | 0 ATP | 2 Acetyl-CoA, 2 NADH, 2 CO₂ |
| Krebs Cycle (×2) | Mitochondrial matrix | 2 Acetyl-CoA | 2 ATP | 6 NADH, 2 FADH₂, 4 CO₂ |
| Oxidative Phosphorylation | Inner mitochondrial membrane | 10 NADH, 2 FADH₂ | ~32 ATP | H₂O |
Total yield: ~36–38 ATP per glucose molecule under aerobic conditions.
Anaerobic Respiration
When O₂ is unavailable, pyruvate is converted to:
- Lactic acid — in animal muscle cells (causes muscle fatigue)
- Ethanol + CO₂ — in yeast (fermentation used in brewing, baking)
Anaerobic respiration yields only 2 ATP per glucose (glycolysis only).
6. Human Physiology — Major Organ Systems
| System | Key Organs | Primary Function |
|---|---|---|
| Nervous | Brain, spinal cord, nerves | Rapid electrical signalling; sensory processing; motor control |
| Endocrine | Pituitary, thyroid, pancreas, adrenals, gonads | Hormonal regulation of metabolism, growth, reproduction, stress |
| Cardiovascular | Heart, arteries, veins, capillaries | Transport of O₂, nutrients, hormones, waste removal |
| Respiratory | Lungs, trachea, bronchi, diaphragm | Gas exchange (O₂ in, CO₂ out); pH regulation |
| Digestive | Stomach, small intestine, large intestine, liver, pancreas | Digestion and absorption of nutrients |
| Immune | Lymph nodes, spleen, thymus, bone marrow, WBCs | Pathogen defence; innate and adaptive immunity |
| Musculoskeletal | Muscles, bones, tendons, ligaments | Movement, support, mineral storage, blood cell production |
| Urinary/Renal | Kidneys, ureters, bladder, urethra | Filtration of blood; excretion of waste; fluid/electrolyte balance |
| Reproductive | Testes/ovaries, accessory organs | Gamete production; hormone production; reproduction |
Homeostasis
All organ systems work together to maintain homeostasis — the stable internal environment necessary for optimal cellular function. Regulation occurs via negative feedback loops (most common) and occasionally positive feedback.
Example: Body temperature rises → hypothalamus detects change → triggers sweating and vasodilation → body cools → temperature returns to set point → sweating stops.
Knowledge Check
1. What is the role of helicase in DNA replication?
2. A plant cell (BB) is crossed with a plant cell (bb). What fraction of offspring will be heterozygous?
3. Why is the inner mitochondrial membrane folded into cristae?
4. What distinguishes introns from exons in pre-mRNA processing?
Ready for Biochemistry?
Now that you understand how cells work biologically, dive into the molecular chemistry that powers them — enzymes, metabolism, and signal transduction.