Biology - Cell Biology

1. Cell Structure

Prokaryotic vs Eukaryotic Cells

Plant vs Animal Cells

2. Organelles

3. Microscopy

Light Microscopy

• 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

Electron Microscopy

• 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)

Magnification and Scale

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

4. Cell Membrane — The Fluid Mosaic Model

Structure

• 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

Functions of Cell Membrane

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

5. Transport Across Membranes

Passive Transport (no ATP required)

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)

Active Transport (ATP required)

• 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

Bulk Transport

• 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)

6. Cell Division

Mitosis

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

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

Meiosis

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

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

Significance of Meiosis

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

Mitosis vs Meiosis

Worked Examples

Example 1: Calculating Actual Size from a Micrograph

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: $\text{Actual size} = \frac{\text{Image size}}{\text{Magnification}} = \frac{45\ \text{mm}}{400} = 0.1125\ \text{mm} = 112.5\ \mu\text{m}$

Example 2: Distinguishing Mitosis and Meiosis

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.

Common Pitfalls

• 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.

Summary

Cell biology covers the structure and function of prokaryotic and eukaryotic cells, including organelle functions, the fluid mosaic model of cell membranes, transport mechanisms (passive, active, bulk), and cell division (mitosis and meiosis). Microscopy techniques (light and electron) and the ability to calculate actual size from magnification are essential practical skills for the DSE exam.