Cell Biology: Structure, Membranes, Transport and Division
The cell biology domain of the Basque PAU Biology exam asks you to identify cell types and organelles, explain membrane transport and describe the cell cycle and division. Expect comparisons between prokaryotic and eukaryotic cells, interpretations of osmosis experiments and questions on mitosis versus meiosis.
Cell structure
- Prokaryotes have 70S ribosomes, a peptidoglycan wall, a nucleoid and plasmids, but no nucleus or membrane-bound organelles.
- Eukaryotes have 80S ribosomes and compartmentalised organelles: nucleus, mitochondria with cristae, chloroplasts with thylakoids and stroma, endoplasmic reticulum, Golgi apparatus, lysosomes and peroxisomes.
- Mitochondria and chloroplasts are double-membraned, contain their own DNA and 70S ribosomes, and are explained by endosymbiotic theory.
Membranes and transport
- The fluid mosaic model describes a phospholipid bilayer with proteins, cholesterol and glycocalyx.
- Simple diffusion and osmosis are passive; facilitated diffusion uses channels or carriers; active transport uses ATP and moves solutes against a gradient; endocytosis and exocytosis move material in vesicles.
Cell cycle and division
- Interphase has G1, S and G2; DNA replicates in S phase.
- Mitosis produces two genetically identical diploid cells; meiosis halves the chromosome number and generates variation by crossing over and independent assortment.
- Cytokinesis differs: plant cells form a cell plate; animal cells form a cleavage furrow.
Worked example
A plant cell has solute potential Ψs = −0.6 MPa and pressure potential Ψp = 0.2 MPa, so its water potential is Ψ = Ψs + Ψp = −0.4 MPa. It is placed in a solution with Ψ = −0.3 MPa. Water moves from higher water potential to lower water potential, so water enters the cell. Entry continues until the cell’s Ψ equals −0.3 MPa; at that point Ψp must be 0.3 MPa.
Traps
- Water potential is usually negative; more negative means lower water potential.
- Active transport is not the same as facilitated diffusion; only active transport requires energy.
- Mitochondria and chloroplasts have 70S ribosomes, like bacteria.
- Mitosis maintains ploidy; meiosis reduces it.
- Osmosis is water movement across a semipermeable membrane, not solute movement.