Cell biology, cell division and organisation of the human body
What this domain tests
Cell questions in the Human Biology paper ask you to name organelles and link each to a function, compare plant and animal cells, and explain why cells are small. Cell division questions ask for the sequence and outcome of mitosis and meiosis and why each matters. Organisation questions test the ladder from cell to tissue to organ to organ system to organism, and the four tissue types.
Core rules
- The cell membrane is a phospholipid bilayer that is selectively permeable; it controls entry and exit of substances. The cell wall is fully permeable and is not the barrier.
- Mitochondria carry out aerobic respiration and release energy as ATP. The more active a cell (muscle, sperm, liver), the more mitochondria it holds.
- Ribosomes build proteins; the rough endoplasmic reticulum transports them; the Golgi body modifies and packages them; lysosomes hold digestive enzymes.
- Mitosis gives two genetically identical diploid cells and serves growth, repair and asexual reproduction. Interphase is not resting: DNA replicates there.
- Meiosis is two divisions giving four haploid cells. Crossing over in prophase I and independent assortment in metaphase I create variation.
- Surface area to volume ratio falls as a cell enlarges, so large organisms need specialised exchange surfaces and transport systems.
Worked example
Human body cells have 2n = 46. How many chromosomes are in a cell at metaphase of mitosis, at anaphase of mitosis, and in a sperm? At metaphase there are still 46 chromosomes, but each consists of two sister chromatids, giving 92 DNA molecules. At anaphase the centromeres split and chromatids move to opposite poles, so the cell briefly contains 92 chromosomes. After cytokinesis each daughter cell has 46. A sperm is haploid with 23 chromosomes, each single-stranded. Rule: when asked for chromosome number, count centromeres, not chromatids.
Traps that cost marks
Writing that meiosis produces two cells, or that mitosis produces variation. Confusing anaphase of mitosis (sister chromatids separate) with anaphase I of meiosis (homologous chromosomes separate). Describing the cell wall as selectively permeable. Naming the nucleus as the site of respiration or protein synthesis.