Cell Biology Advanced

1. Microscopy: Visualising the Cell

To study cells, we must magnify them. The two key parameters of microscopy are magnification and resolution.

* Magnification is how many times larger an image is compared to the actual object. The formula is crucial for calculations:

Magnification (M) = Image Size (I) ÷ Actual Size (A)

* Application: When using this formula, ensure both 'I' and 'A' are in the same units, typically micrometres (µm) or nanometres (nm). For example, to find the actual size of a mitochondrion that is 30 mm long in a micrograph with a magnification of ×20,000:

1. Convert image size to µm: 30 mm = 30,000 µm
2. Rearrange formula: A = I / M
3. Calculate: A = 30,000 µm / 20,000 = 1.5 µm

* Resolution is the minimum distance between two points at which they can still be distinguished as separate entities. It determines the level of detail visible. A lower value means better resolution.

Types of Microscopes:

* Light Microscope: Uses light and lenses. Maximum magnification is about ×1500 with a resolution of ~200 nm. Its main advantage is the ability to view living specimens.

* Electron Microscope: Uses a beam of electrons, which have a much shorter wavelength than light, providing far greater resolution (~0.1 nm). Specimens must be dead, fixed in a vacuum, and often stained with heavy metals.

* Transmission Electron Microscope (TEM): Passes electrons through an extremely thin specimen to produce a 2D image, revealing internal ultrastructure (e.g., cristae in mitochondria).

* Scanning Electron Microscope (SEM): Scans the surface of a specimen with electrons to produce a detailed 3D image.

2. The Fluid Mosaic Model of Membranes

The cell surface membrane is not a static structure but a dynamic, fluid one, described by the fluid mosaic model.

* Phospholipid Bilayer: The fundamental structure. Hydrophilic (water-attracting) phosphate heads face the aqueous exterior and cytoplasm, while hydrophobic (water-repelling) fatty acid tails form a non-polar core. This barrier is selectively permeable.

* Cholesterol: A lipid molecule found only in animal cell membranes. It fits between the phospholipids, regulating fluidity. At higher temperatures, it reduces fluidity; at lower temperatures, it prevents packing and maintains fluidity.

* Proteins: Scattered throughout the bilayer like tiles in a mosaic.

* Intrinsic (Integral) Proteins: Span the entire membrane. They act as channel proteins (pores for ion transport) and carrier proteins (change shape to move molecules across).

* Extrinsic (Peripheral) Proteins: Located on the surface.

* Glycocalyx: A carbohydrate layer on the outer surface, formed by glycoproteins (proteins with attached carbohydrate chains) and glycolipids (lipids with attached carbohydrate chains). It is vital for cell-cell recognition, cell adhesion, and acting as receptors for hormones.

3. Transport Across Membranes

Passive Processes (No ATP required):

* Diffusion: The net movement of particles from an area of higher concentration to an area of lower concentration. Affects small, non-polar molecules like O₂ and CO₂.

* Facilitated Diffusion: Diffusion across a membrane through channel or carrier proteins, for ions and larger polar molecules like glucose. It is still passive, moving down a concentration gradient.

* Osmosis: The net movement of free water molecules from a region of higher water potential (Ψ) to a region of lower water potential through a partially permeable membrane. Water potential is a measure of the tendency of water to move, measured in kilopascals (kPa). Pure water has a Ψ of 0 kPa; adding solutes makes it more negative.

Active Processes (ATP required):

* Active Transport: Moves substances against their concentration gradient using energy from ATP. It involves specific carrier proteins that act as 'pumps'.

* Bulk Transport: For moving large quantities.

* Endocytosis: Engulfing material by infolding the membrane to form a vesicle.

* Exocytosis: Fusing a vesicle (e.g., from the Golgi apparatus) with the cell membrane to release its contents.

4. The Cell Cycle and Division

Mitosis and meiosis are two forms of nuclear division.

The Cell Cycle:

1. Interphase: The longest phase. The cell grows and prepares for division.

* G1: Cell growth and protein synthesis.

* S (Synthesis): DNA replication occurs. Each chromosome is now two sister chromatids.

* G2: Further growth and organelle replication.

1. M (Mitotic) Phase: Nuclear division (mitosis) and cytoplasmic division (cytokinesis).

Mitosis: Produces two genetically **identical** diploid (2n) daughter cells. Its purpose is growth, tissue repair, and asexual reproduction.

* Prophase: Chromosomes condense and become visible. Centrioles move to opposite poles, forming the spindle. The nuclear envelope breaks down.

* Metaphase: Chromosomes align along the cell's equator (metaphase plate).

* Anaphase: Spindle fibres contract, pulling the sister chromatids apart to opposite poles. Each chromatid is now considered a chromosome.

* Telophase: Chromosomes decondense. Nuclear envelopes reform around the two new sets of chromosomes.

Exam Trap: A common error is stating that mitosis only happens for repair. It is fundamental to the growth of all multicellular organisms from a single zygote.

Meiosis: Produces four genetically **different** haploid (n) gametes. It involves two divisions (Meiosis I and Meiosis II) and is the basis of sexual reproduction.

* Genetic Variation in Meiosis:

1. Crossing Over: During Prophase I, homologous chromosomes pair up. Non-sister chromatids exchange segments of DNA at points called chiasmata. This creates new combinations of alleles on the chromosomes.
2. Independent Assortment: During Metaphase I, homologous pairs align randomly at the equator. The orientation of one pair does not affect the orientation of any other pair, leading to many different combinations of maternal and paternal chromosomes in the gametes.