Module 1: Cell Biology - Ultrastructure, Membranes and Cell Division
Module 1 treats the cell as the basic unit of life. Questions ask you to link organelle structure to function, explain how membranes control exchange, and describe how cells divide. Marks are lost on terminology and direction of movement rather than on ideas.
Organelles: structure fits function
Prokaryotes lack membrane-bound organelles: circular DNA in a nucleoid, 70S ribosomes, a murein wall, plasmids and often flagella. Eukaryotes compartmentalise - nucleus (chromatin, pores, nucleolus making rRNA), mitochondrion (double membrane, cristae, matrix; aerobic respiration), chloroplast (thylakoids in grana, stroma; photosynthesis), rough ER (protein synthesis and transport), smooth ER (lipid synthesis), Golgi (modification, sorting, vesicles), lysosome (hydrolytic enzymes), 80S ribosomes; plants add a cellulose wall and a large permanent vacuole.
Membranes and transport
The fluid mosaic model: a phospholipid bilayer with tails inward, plus cholesterol, glycoproteins and glycolipids for recognition, and integral or peripheral proteins. Simple diffusion carries small non-polar molecules down a gradient. Osmosis is water moving down a water potential gradient, through the bilayer or aquaporins. Facilitated diffusion uses channel or carrier proteins - passive but saturable. Active transport uses ATP-driven carriers against the gradient (the Na⁺/K⁺ pump moves 3 Na⁺ out for 2 K⁺ in). Co-transport brings glucose into ileum cells with Na⁺ down its gradient; glucose then leaves by facilitated diffusion. Endocytosis and exocytosis move bulk material using ATP. Fick's law links rate to surface area, gradient, distance and permeability, so small cells exchange efficiently.
Mitosis and the cell cycle
Interphase: G1 growth, S DNA replication, G2 organelle replication. Prophase - chromosomes condense, spindle forms, nuclear envelope breaks down. Metaphase - centromeres attach to spindle fibres and chromosomes align at the equator. Anaphase - centromeres divide and sister chromatids move to opposite poles. Telophase - nuclei reform and chromosomes decondense. Cytokinesis is by cleavage furrow in animals, cell plate in plants. Mitosis maintains genetic constancy for growth, repair and asexual reproduction.
Worked example
A plant cell has solute potential −1200 kPa and pressure potential +400 kPa, so its water potential is −1200 + 400 = −800 kPa. In a solution of water potential −900 kPa, water moves from higher to lower water potential - out of the cell, since −800 > −900. The protoplast shrinks, pressure potential falls, the cell becomes flaccid, and if pressure potential reaches zero with the membrane pulling away from the wall it is plasmolysed. The chain is: combine potentials, compare, then state direction and consequence.
Traps that cost marks
Writing solute potential as positive, or omitting pressure potential. Saying water moves 'towards the sugar' without comparing water potentials. Claiming mitosis produces haploid cells or omitting S-phase replication. Confusing centromere with centriole. Saying active transport happens 'because concentration is high' - it needs ATP hydrolysis.