Biology - Cell Biology | DSE - Wyatt's Notes

• Uses visible light; magnification up to $\sim 1500\times$
• Resolution: $\sim 200\ \mathrm{nm}$ — limited by wavelength of light
• Can observe live specimens
• Staining (e.g. iodine for starch, methylene blue for nuclei) increases contrast

• Uses electron beam (shorter wavelength than light)
• Transmission EM (TEM): thin sections; internal detail; up to $\sim 500\,000\times$
• Scanning EM (SEM): 3D surface images
• Resolution: $\sim 0.2\ \mathrm{nm}$ — can see organelles, viruses, large molecules
• Specimens must be fixed, dehydrated, and placed in a vacuum (dead specimens only)

$\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}$

• Phospholipid bilayer: hydrophilic heads face outward, hydrophobic tails face inward
• Proteins: embedded (intrinsic/integral) or attached to surface (extrinsic/peripheral)
• Cholesterol: between phospholipid tails, regulates fluidity
• Glycoproteins/glycolipids: carbohydrate chains on the outer surface for cell recognition

Fluid: phospholipids and proteins can move laterally within the bilayer Mosaic: pattern of different proteins scattered like tiles

• Partially permeable barrier
• Cell-cell recognition (glycoproteins)
• Receptor sites for hormones and neurotransmitters
• Enzyme surfaces (e.g. ATP synthase on inner mitochondrial membrane)

Water potential ($\Psi$):

• Pure water: $\Psi = 0$
• Solutions: $\Psi < 0$ (more negative = more concentrated)
• Water moves from less negative to more negative water potential

Turgor pressure: pressure exerted by cell contents pressing against the cell wall in plant cells.

• Turgid: cell fully swollen with water (normal, healthy state for plants)
• Plasmolysed: cell membrane pulls away from cell wall (in concentrated solution)

• Movement against the concentration gradient (low → high)
• Requires carrier proteins and energy from ATP hydrolysis
• Example: absorption of mineral ions by root hair cells; sodium-potassium pump

• Endocytosis: membrane engulfs material to bring it in (phagocytosis for solids, pinocytosis for liquids)
• Exocytosis: vesicles fuse with membrane to release contents (e.g. secretion of hormones, enzymes)

Purpose: growth, repair, asexual reproduction; produces 2 genetically identical diploid cells

Result: $2n \to 2n$ (diploid → diploid)

Purpose: production of gametes (sex cells); produces 4 genetically unique haploid cells

Result: $2n \to n$ (diploid → haploid)

1. Genetic variation through:

   • Crossing over (recombination) during prophase I — new combinations of alleles
   • Independent assortment — random alignment of homologous chromosomes at metaphase I
   • Random fertilisation — fusion of any sperm with any egg

2. Halving chromosome number so that fertilisation restores the diploid number

Problem: A cell in a light micrograph measures 45 mm across. The magnification is $\times 400$. Calculate the actual size of the cell in $\mu$m. Solution: $\text{Actual size} = \frac{\text{Image size}}{\text{Magnification}} = \frac{45\ \text{mm}}{400} = 0.1125\ \text{mm} = 112.5\ \mu\text{m}$

Problem: A cell has 46 chromosomes and undergoes division. After division, each daughter cell has 23 chromosomes. Was this mitosis or meiosis? Explain. Solution: This was meiosis. Mitosis produces daughter cells with the same chromosome number as the parent (diploid to diploid). The halving from 46 to 23 (diploid to haploid) is characteristic of meiosis, which produces gametes.

flowchart TD
    A[Cell Biology] --> B[Key Concepts]
    A --> C[Core Principles]
    A --> D[Practical Applications]
    B --> E[Fundamental definitions]
    C --> F[Design patterns]
    D --> G[Real-world usage]

Life’s building blocks: Cells are like factories — the nucleus is the management office, mitochondria are power plants, and ribosomes are assembly lines. Biochemical tests identify which molecules are present.

Why it matters: From diagnosing diseases to understanding genetics, cell biology is the foundation of modern medicine and biotechnology.

The key insight: Structure determines function — the shape of a protein determines what it does, and mutations that change shape can cause disease.

• Confusing resolution and magnification: Magnification enlarges the image; resolution determines the clarity of detail. Electron microscopes have higher resolution (not just higher magnification) than light microscopes.
• Mixing up osmosis direction: Water moves from less negative water potential to more negative water potential (high water concentration to low), not the other way around.
• Stating mitosis produces “identical cells” without qualification: Mitosis produces genetically identical cells only in ideal conditions. DNA mutations can introduce variation.

• Structure and function are linked: The folded cristae of mitochondria increase surface area for oxidative phosphorylation, directly enhancing ATP production capacity.
• Membrane fluidity depends on composition: More cholesterol and unsaturated fatty acid tails increase fluidity; saturated tails and cholesterol at low temperatures decrease it.
• Osmosis and water potential drive plant cell health: Turgor pressure from osmosis keeps plant cells rigid; loss of turgor causes wilting.
• Mitosis and meiosis maintain the chromosome cycle: Meiosis halves the chromosome number so that fertilisation restores it; mitosis maintains it during growth.
• Microscopy resolution limits what can be observed: The diffraction limit of light (~200 nm) means organelles like ribosomes (20-30 nm) require electron microscopy.
• Active transport maintains electrochemical gradients: The sodium-potassium pump uses ATP to maintain concentration gradients essential for nerve impulse transmission.

• Medical diagnostics: Understanding cell structure helps identify diseased cells (e.g. cancer cells show abnormal nuclei and uncontrolled division).
• Antibiotic design: Many antibiotics target prokaryotic-specific structures (70S ribosomes, peptidoglycan cell walls) without harming eukaryotic host cells.
• Forensic science: DNA analysis relies on understanding chromosome structure and the difference between mitosis and meiosis.
• Agriculture: Knowledge of osmosis and water potential informs irrigation strategies and salt-tolerant crop development.
• Biotechnology: Understanding of the endomembrane system (ER, Golgi) enables engineering of cells to produce and secrete therapeutic proteins.

• When asked to compare prokaryotic and eukaryotic cells, always mention the nucleus, membrane-bound organelles, ribosome size, and cell wall composition.
• For microscopy questions, state both magnification and resolution differences between light and electron microscopes.
• In osmosis problems, always draw a water potential gradient and indicate the direction of water movement.
• When describing mitosis or meiosis phases, include the key event of each phase and the final chromosome number.

• Cell Biology: Cell biology is foundational
• Genetics: Genetics studies heredity
• Ecology: Ecology studies environments