Cell Structure and Function

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

The cell is the basic structural and functional unit of all living organisms. All life processes — metabolism, growth, reproduction, response to stimuli, and homeostasis — occur within cells or are co-ordinated by them. Rudolf Virchow’s principle “Omnis cellula e cellula” (all cells arise from cells) established that the cell is not just a structural unit but a functional one as well.

Two Fundamental Cell Types:

Feature	Prokaryotic Cell	Eukaryotic Cell
Genetic material	Circular DNA in nucleoid	Linear DNA in nucleus with histones
Membrane-bound organelles	Absent	Present
Nucleus	No (nucleoid region)	Yes (nuclear envelope)
Size	0.1–5 μm	10–100 μm
Ribosomes	70S (smaller)	80S (larger)
Cell division	Binary fission	Mitosis/Meiosis
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists
DNA replication	Single origin of replication	Multiple origins per chromosome

Animal Cell Organelles — Quick Reference:

• Nucleus: Contains hereditary information (DNA); surrounded by double nuclear envelope with pores; contains nucleolus (rRNA synthesis)
• Mitochondria: “Powerhouse of the cell” — aerobic respiration produces ATP; has own 70S ribosomes and circular DNA (endosymbiotic origin)
• Ribosomes: Site of protein synthesis; 80S in eukaryotic cytoplasm (60S + 40S); made of rRNA and proteins
• Endoplasmic reticulum (ER): Rough ER (ribosome-studded; protein synthesis for secretion) and Smooth ER (lipid synthesis, detoxification, carbohydrate metabolism)
• Golgi apparatus: Receives proteins from ER; modifies, sorts, and packages them for secretion or delivery to lysosomes
• Lysosomes: Contain hydrolytic enzymes (proteases, lipases, nucleases) at pH ~5; involved in intracellular digestion
• Centrosome/Centrioles: Organise microtubules; form spindle apparatus during cell division; form basal bodies of cilia and flagella
• Cytoskeleton: Microfilaments (actin — cell movement, muscle contraction), Microtubules (tubulin — intracellular transport, cilia, flagella), Intermediate filaments (mechanical strength — keratin, vimentin)

⚡ Exam Tip (MDCAT): The key distinguishing feature of animal cells vs plant cells: animal cells have centrioles (involved in spindle formation during mitosis), while plant cells do NOT. Plant cells have a cell wall (cellulose), large central vacuole, and chloroplasts. Both have mitochondria. A common MDCAT diagram question: identifying an organelle from its electron micrograph appearance, or matching organelles to their functions.

⚡ MDCAT Memory Trick: Animal cells have centrosomes (with centrioles), lysosomes, and no cell wall. Remember: “Animals are Loco” — Lysosomes, Centrosomes, Locomotion uses cilia/flagella.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of cellular processes.

The Cell Membrane — Fluid Mosaic Model:

The cell membrane (plasma membrane) is described by the Fluid Mosaic Model (Singer and Nicolson, 1972):

• Phospholipid bilayer: Amphipathic phospholipids arrange with hydrophilic heads outward and hydrophobic tails inward
• Cholesterol: Inserts between phospholipids; increases membrane rigidity at high temperatures; prevents close packing at low temperatures
• Proteins: Integral proteins span the membrane; peripheral proteins attach to one surface
• Glycocalyx: Carbohydrate chains on glycoproteins and glycolipids — functions in cell recognition, adhesion, and protection

Membrane Transport:

Transport Type	Direction	Energy	Carrier Protein	Example
Simple diffusion	High → Low conc.	None	No	O₂, CO₂, N₂
Facilitated diffusion	High → Low conc.	None	Yes (channel or carrier)	Glucose via GLUT, ions via channels
Osmosis	Water, across semipermeable membrane	None	No	Water in plant roots
Active transport	Low → High conc.	ATP required	Yes	Na⁺/K⁺ ATPase
Bulk transport (endo/exocytosis)	In/out of cell	ATP required	No (membrane fusion)	Phagocytosis, exocytosis

The Na⁺/K⁺ ATPase: An antiport pump in the cell membrane. For each ATP hydrolysed: 3 Na⁺ pumped OUT, 2 K⁺ pumped IN. This maintains:

• Resting membrane potential (~-70 mV in neurons)
• Volume regulation (prevents osmotic lysis)
• Drive for secondary active transport (Na⁺ gradient powers glucose and amino acid uptake)

Cell Nucleus:

• Nuclear envelope: Double membrane; outer connected to rough ER; nuclear pores (~3000 per nucleus) regulate transport (mRNA exits; DNA does not)
• Chromatin: DNA + histone proteins; nucleosomes (DNA wrapped around histone octamer) form the basic structure
• Histones: H1 (linker), H2A, H2B, H3, H4 (core histones) — positively charged proteins that bind negatively charged DNA
• Nucleolus: Dense region where rRNA genes are transcribed and ribosome subunits are assembled

Cell Communication — Junctions:

Junction Type	Function	Structure
Tight junction	Prevent passage of molecules between cells	Claudin and occludin proteins seal intercellular space
Adherens junction	Mechanical attachment between cells	Cadherin proteins connect to actin microfilaments
Desmosomes	Strong mechanical attachment	Cadherin proteins connect to intermediate filaments
Gap junction	Allow ions and small molecules to pass	Connexin proteins form channels (connexons)

⚡ Common MDCAT Error: Students confuse phagocytosis (“cell eating”) and pinocytosis (“cell drinking”). Phagocytosis is the engulfment of large solid particles (bacteria, dead cells) — produces a phagosome. Pinocytosis is the non-specific uptake of extracellular fluid and dissolved substances. Receptor-mediated endocytosis is specific — ligands bind to receptors on the cell surface, which cluster in clathrin-coated pits before being internalised.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

The Endomembrane System:

The endoplasmic reticulum, Golgi apparatus, lysosomes, and cell membrane form a continuous, interconnected system for protein and lipid trafficking:

1. Proteins synthesised on rough ER → enter ER lumen → fold and undergo initial modifications
2. Packaged into COPII vesicles (ER → Golgi) → travel to cis-Golgi network
3. Progress through medial Golgi (further modifications: glycosylation, phosphorylation)
4. Exit from trans-Golgi network → sorted to: lysosomes (via mannose-6-phosphate tagging), secretory vesicles, or cell membrane
5. Constitutive secretion: continuous release; regulated secretion: stored until signal received

Cell Cycle Control:

The cell cycle is regulated by:

• Cyclins: Regulatory subunits that oscillate in concentration throughout the cell cycle
• Cyclin-dependent kinases (CDKs): Catalytic subunits that phosphorylate target proteins; require binding to cyclins for activity
• Cell cycle checkpoints: G₁ checkpoint (Restriction point — commits cell to S phase), G₂ checkpoint (ensures DNA replication is complete and errors repaired), M checkpoint (ensures all chromosomes are properly attached to spindle)

Proto-oncogenes vs Tumour Suppressor Genes:

• Proto-oncogenes (e.g., RAS, MYC, EGFR): Normal genes that promote cell division. When mutated (become oncogenes), they drive excessive proliferation.
• Tumour suppressor genes (e.g., p53, RB, BRCA1/2): Normal genes that inhibit cell division. When mutated/lost, the “brakes” on cell division are removed.

p53 (“guardian of the genome”): responds to DNA damage by arresting the cell cycle at G₁/S checkpoint to allow repair, or triggering apoptosis if damage is irreparable. Mutated in ~50% of all human cancers.

Apoptosis (Programmed Cell Death) vs Necrosis:

Feature	Apoptosis	Necrosis
Nature	Regulated, programmed	Uncontrolled, accidental
Cell size	Shrinks (condensation)	Swells (lysis)
Membrane	Intact until late stages	Ruptures early
Chromatin	Condensed, fragmented	Degrades irregularly
Inflammatory response	None	Significant (releases intracellular contents)
Energy	Requires ATP	No ATP requirement
Morphology	Apoptotic bodies engulfed by phagocytes	Cell lysis, inflammation

Specialised Cell Types — Structure-Function Relationships:

Cell Type	Key Adaptations	Function
Red blood cell	Biconcave disc, no nucleus, haemoglobin	O₂ transport
Neuron	Long axon, dendrites, myelin sheath	Electrical signalling
Muscle cell (skeletal)	Multinucleated, striated, actin-myosin	Contraction
Pancreatic β-cell	Well-developed ER, many vesicles	Insulin secretion
Sperm cell	Flagellum, mitochondria (mid-piece), acrosome	Fertilisation
Epithelial cell (intestinal)	Microvilli (brush border)	Maximise absorption surface

Energy Metabolism Summary:

For one glucose molecule (C₆H₁₂O₆):

• Glycolysis (cytoplasm): glucose → 2 pyruvate + 2 ATP + 2 NADH
• Pyruvate oxidation (mitochondria): 2 pyruvate → 2 acetyl-CoA + 2 NADH + 2 CO₂
• Krebs cycle (mitochondrial matrix): 2 acetyl-CoA → 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂
• Electron transport chain (inner mitochondrial membrane): ~32-34 ATP from oxidative phosphorylation

Net: ~36-38 ATP per glucose (aerobic) vs 2 ATP (anaerobic glycolysis alone)

⚡ MDCAT Exam Pattern: Questions often test the fluid mosaic model, organelle functions, differences between prokaryotic and eukaryotic cells, and the stages of mitosis. In diagrams, be able to identify: mitochondria (cristae), rough ER (ribosomes visible), Golgi (cisternae stacks), centrioles (9+0 arrangement of microtubules), nucleus (nuclear envelope). A classic MDCAT question: if a cell lacks mitochondria, which processes cannot occur? Answer: aerobic respiration (requires mitochondria for Krebs cycle and electron transport chain).

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