Cell Structure and Function
Scope
Cell theory, prokaryotic and eukaryotic organisation, organelles, biomolecules, enzymes, membrane transport and the cell cycle are tested. Questions often ask for structure–function links.
Core rules
- Prokaryotes lack a nucleus and membrane-bound organelles; their ribosomes are 70S, eukaryotic cytoplasmic ribosomes are 80S.
- Mitochondria and chloroplasts have double membranes and their own DNA; the endosymbiotic theory explains this.
- The plasma membrane is a fluid mosaic: phospholipid bilayer with proteins, cholesterol and glycoproteins.
- Enzymes lower activation energy, are not consumed, and have an optimum pH and temperature.
- Cell cycle: G1, S (DNA replication), G2, M. Chromosome number doubles at anaphase, not at S phase; DNA doubles at S phase.
- Water moves from higher to lower water potential; plasmolysis occurs in hypertonic solutions.
Worked example
A diploid cell has 2n = 8. How many chromatids are present at metaphase of mitosis, and how many chromosomes at anaphase? Reasoning: before mitosis, each chromosome has replicated into two sister chromatids, so 8 chromosomes give 16 chromatids at metaphase. At anaphase the centromeres split, so sister chromatids become separate chromosomes: 16 chromosomes. The cell is still diploid in content until cytokinesis.
Traps
- S phase doubles DNA but chromosome number stays the same.
- Ribosome sizes are Svedberg units, not linear sizes.
- Cellulose is a structural polysaccharide; starch is storage.
- A competitive inhibitor raises the apparent Km but does not change Vmax.