Cell Biology

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

The cell is the fundamental structural and functional unit of all living organisms. All cells arise from pre-existing cells through cell division (Rudolf Virchow’s principle: “Omnis cellula e cellula”). Two major categories exist: prokaryotes (bacteria, archaea — no membrane-bound nucleus) and eukaryotes (plants, animals, fungi, protists — have a true nucleus and membrane-bound organelles).

Cell Theory Principles:

1. All living organisms are composed of cells
2. The cell is the basic unit of life
3. All cells arise from pre-existing cells
4. Chemical reactions of life occur within cells

Prokaryotic Cell Structure:

Typical bacteria (e.g., Escherichia coli):

• Cell wall: peptidoglycan (murien) — not cellulose
• Cell membrane: phospholipid bilayer with proteins
• Nucleoid region: circular DNA chromosome (not membrane-bound)
• Ribosomes: 70S (30S + 50S subunits)
• Plasmids: small circular DNA (extra-chromosomal, used in gene cloning)
• Flagella: protein flagellin, rotate for movement
• Pili: hair-like structures for attachment to host cells

Eukaryotic Plant Cell Structure:

Organelle	Function	Key Features
Cell wall	Protection, structural support	Cellulose, hemicellulose, pectin; absent in animal cells
Cell membrane	Selective permeability	Fluid mosaic model; phospholipid bilayer with proteins
Nucleus	Stores genetic information	Nuclear envelope with pores; contains nucleolus
Nucleolus	rRNA synthesis	Dense region within nucleus
Mitochondria	Aerobic respiration (ATP production)	Double membrane; cristae; own DNA, 70S ribosomes
Chloroplasts	Photosynthesis	Absent in animal cells; own DNA, 70S ribosomes
Endoplasmic reticulum	Protein/ lipid synthesis	Rough ER (ribosomes attached); Smooth ER (lipid synthesis)
Golgi apparatus	Processing and packaging of proteins	Cisternae; receives from ER, sends to membrane or lysosomes
Lysosomes	Intracellular digestion	Contain hydrolytic enzymes; pH ~5
Vacuole	Storage, turgidity	Large central vacuole in plants (contains cell sap)
Ribosomes	Protein synthesis	80S in eukaryotes; 70S in prokaryotes
Cytoskeleton	Structural support, cell motility	Microfilaments (actin), microtubules (tubulin), intermediate filaments

⚡ Exam Tip (MDCAT): Plant cells have a cell wall (cellulose), large central vacuole, chloroplasts, and can have plastids. Animal cells do not have these. Both have mitochondria. The 70S ribosome is found in prokaryotes and in mitochondria/chloroplasts of eukaryotes — this is one piece of evidence supporting the endosymbiotic theory (mitochondria and chloroplasts evolved from ancient bacteria that were engulfed by ancestral eukaryotic cells).

⚡ MDCAT Memory Trick: A mnemonic for remembering membrane-bound organelles: “Rough ER, Golgi are the CELL Sorting PACKING shippingcent RESpiration Mitochondria Powerhouse” — ER-Golgi-cell membrane; Mitochondria-powerhouse; Nucleus; Lysosomes; Ribosomes.

---

🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of cell structure and function.

The Fluid Mosaic Model:

Proposed by Singer and Nicolson (1972). The cell membrane is described as a “fluid” because phospholipids and proteins can move laterally within the membrane (lateral diffusion). It is described as a “mosaic” because the membrane contains a mosaic of various types of molecules: phospholipids, cholesterol, glycoproteins (proteins with carbohydrate chains), glycolipids (lipids with carbohydrate chains), and integral and peripheral proteins.

Membrane Transport Mechanisms:

Mechanism	Description	Energy Required	Example
Simple diffusion	Movement of molecules from high to low concentration	No (passive)	O₂, CO₂ across membrane
Osmosis	Diffusion of water across semipermeable membrane	No	Water movement in plant roots
Facilitated diffusion	Transport via channel or carrier proteins	No	Glucose via GLUT transporters
Active transport	Movement against concentration gradient	Yes (ATP)	Na⁺/K⁺ ATPase pump
Bulk transport (endocytosis)	Large particles into cell	Yes	Phagocytosis, pinocytosis
Bulk transport (exocytosis)	Large particles out of cell	Yes	Neurotransmitter release

The Na⁺/K⁺ ATPase Pump: This antiport pump actively transports 3 Na⁺ out of the cell and 2 K⁺ into the cell per ATP hydrolysed. It maintains the resting membrane potential (~-70 mV in neurons) and is essential for nerve impulse transmission and muscle contraction. It is an electrogenic pump (net positive charge moves out).

Cell Nucleus:

The nucleus is the largest organelle, typically 5–10 μm in diameter. It contains:

• Nuclear envelope: Double membrane with nuclear pores (each ~125 MDa, allows mRNA export but not DNA)
• Chromatin: DNA + histone proteins (DNA wrapped around histone octamers to form nucleosomes)
• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly

Endoplasmic Reticulum and Golgi:

The ER-Golgi system is the cell’s protein processing and sorting hub: Rough ER → proteins synthesised by ribosomes on ER surface → enter ER lumen → processed and folded → transported to Golgi via vesicles → further processed in cis (receiving) face → sorted in trans (shipping) face → sent to: cell membrane (for secretion), lysosomes (for intracellular digestion), or outside the cell (exocytosis)

Mitochondria — The Powerhouse:

• Inner membrane highly folded into cristae (increasing surface area for electron transport chain)
• Matrix contains: Krebs cycle enzymes, mitochondrial DNA (circular, 16.6 kb in humans), 70S ribosomes
• Intermembrane space: contains protons pumped during oxidative phosphorylation
• Produces ~36-38 ATP per glucose (aerobic respiration)

⚡ Common MDCAT Error: Students confuse the function of the smooth ER (no ribosomes = no protein synthesis) vs rough ER. Smooth ER is involved in lipid and steroid hormone synthesis, carbohydrate metabolism, and detoxification (in liver cells). Rough ER synthesises proteins destined for secretion, the cell membrane, or other organelles.

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

The Cell Cycle and Mitosis:

The cell cycle has four phases:

1. G₁ phase: Cell growth; organelles replicate; protein synthesis active. Checkpoint at the G₁/S boundary (Restriction point in mammalian cells) — commits the cell to division
2. S phase: DNA replication — each chromosome duplicates to form sister chromatids joined at the centromere
3. G₂ phase: Further growth; preparation for mitosis; checkpoint ensures DNA replication is complete
4. M phase: Mitosis (nuclear division) + cytokinesis (cytoplasmic division)

Mitosis Stage	Key Events
Prophase	Chromatin condenses into visible chromosomes; each has two sister chromatids; mitotic spindle forms from centrioles
Metaphase	Chromosomes align at the metaphase plate (cell equator); spindle fibres attach to kinetochores at centromeres
Anaphase	Sister chromatids separate (each now called a chromosome) and are pulled to opposite poles by shortening spindle fibres
Telophase	Nuclear envelopes reform around each set of chromosomes; chromosomes decondense; cytokinesis begins
Cytokinesis	Animal cells: cleavage furrow (actin-myosin contractile ring); Plant cells: cell plate (vesicles from Golgi fuse at centre)

Meiosis — Reduction Division:

Meiosis reduces chromosome number by half (diploid 2n → haploid n) for gamete formation. It has:

• Meiosis I: Prophase I (leptotene, zygotene, pachytene, diplotene, diakinesis — crossing over in pachytene), Metaphase I (bivalents align at metaphase plate), Anaphase I (homologous chromosomes separate, NOT sister chromatids), Telophase I
• Meiosis II: Essentially a mitotic division of the haploid cells produced in Meiosis I

Crossing Over and Genetic Recombination:

During Prophase I (pachytene stage), homologous chromosomes form bivalents and exchange genetic material at chiasmata. This creates new combinations of alleles — genetic recombination — which contributes to genetic diversity. This is a major difference between meiosis and mitosis.

Endosymbiotic Theory:

Lynn Margulis (1967) proposed that mitochondria and chloroplasts originated as ancient prokaryotic cells that were engulfed by ancestral eukaryotic cells in a symbiotic relationship. Evidence:

• Both have their own circular DNA (like bacteria)
• Both have 70S ribosomes (prokaryotic type)
• Both replicate by binary fission
• Both have double membranes (the inner derived from the original bacterium, the outer from the host cell’s endocytic vesicle)

Cancer and Cell Division Control:

Cancer results from uncontrolled cell division. Normal cells have:

• Density-dependent inhibition (contact inhibition — cells stop dividing when crowded)
• Anchorage dependence (cells must be attached to a substrate to divide)

Cancer cells lose both these controls. Mutations in proto-oncogenes (e.g., RAS, MYC) convert them to oncogenes, driving uncontrolled growth. Mutations in tumour suppressor genes (e.g., p53, RB) remove brakes on the cell cycle. p53 is called the “guardian of the genome” — it arrests the cell cycle for DNA repair or triggers apoptosis if damage is irreparable.

⚡ MDCAT Exam Pattern: Questions on cell biology frequently involve comparisons (plant vs animal cell, prokaryote vs eukaryote, mitosis vs meiosis), organelle functions, and the fluid mosaic model. A common MDCAT question type: identifying stages of mitosis from diagrams, or distinguishing between the two division types based on number of divisions, chromosome number reduction, and presence/absence of crossing over.

---

Content adapted based on your selected roadmap duration. Switch tiers using the pill selector above.