Module 1: Cell Biology - Ultrastructure, Membranes and Cell Division
Cell biology links organelle structure with function, membrane exchange and cell division. This chapter reviews shared foundations and interprets supplied water-potential data. It does not assert a target paper structure or marking scheme.
Organelles: structure fits function
Prokaryotic cells lack a membrane-bound nucleus. Bacteria commonly have a nucleoid, 70S ribosomes and a peptidoglycan wall, but wall-less bacteria exist; archaeal walls do not contain peptidoglycan. Plasmids and flagella occur in some cells, not all. Eukaryotes compartmentalise functions: nucleus with chromatin and nucleolus; mitochondria with inner membrane and matrix; chloroplasts with thylakoids and stroma in photosynthetic cells; rough and smooth ER; Golgi apparatus and ribosomes. A typical plant cell additionally has a cellulose wall and a large vacuole. A structure should be identified by its actual features rather than assuming every member has every listed organelle.
Membranes and transport
The fluid mosaic model describes a lipid bilayer with proteins and, in suitable cells, sterols, glycoproteins and glycolipids. Cholesterol is characteristic of many animal membranes; sterol composition differs among organisms. Simple diffusion moves suitable molecules down a chemical gradient. Osmosis is net water movement down water potential across a selectively permeable membrane. Facilitated diffusion uses proteins without direct ATP hydrolysis; carrier-mediated flux can saturate. Primary active transport can use ATP directly, while secondary transport uses an electrochemical gradient maintained by another energy-dependent process. The mammalian sodium-potassium pump moves three sodium ions out and two potassium ions in per ATP in its ordinary cycle. Sodium-coupled glucose uptake illustrates secondary transport. Vesicular transport uses energy. Surface area, diffusion distance, gradients and permeability together influence exchange.
Mitosis and the cell cycle
Interphase: G1 growth, S DNA replication, G2 organelle replication. Prophase - chromosomes condense, spindle forms, nuclear envelope breaks down. Metaphase: spindle microtubules attach through kinetochores at centromeric regions 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, giving water potential -800 kPa. In an external solution at -900 kPa, initial net water movement is out because -800 is higher than -900. As water leaves, pressure and solute concentration can change. The initial comparison does not prove complete plasmolysis: a new equilibrium may retain positive pressure. Distinguish a direction predicted by initial potentials from a final cellular condition that requires further information.
Traps that cost marks
Common errors include adding potentials with inconsistent signs, describing osmosis without comparing water potentials, and confusing centromeres with centrioles. Mitosis normally preserves the starting chromosome complement; it does not automatically turn every cell into a haploid or diploid product. DNA replication precedes chromosome separation. Specify whether an active transport step uses ATP directly or a pre-existing electrochemical gradient.