Building Blocks: Cells in Animals and Plants — AQA Combined Science: Synergy
This topic covers cell structure, how we view cells, how substances move in and out, and cell division by mitosis and meiosis.
Electron microscopy
Microscopes let us see cells. Electron microscopes have much higher magnification and resolution than light microscopes, so they reveal fine detail (e.g. inside mitochondria).
$$\text{magnification} = \frac{\text{size of image}}{\text{size of real object}}$$ Convert units so they match (1 mm = 1000 µm = 1 000 000 nm) and be able to use standard form.
Cell structures
Animal cells contain: nucleus, cytoplasm, cell membrane, mitochondria (respiration) and ribosomes (protein synthesis).
Plant cells have all of these plus a cellulose cell wall, a permanent vacuole and chloroplasts (photosynthesis).
Bacterial (prokaryotic) cells are much smaller, have no nucleus — their DNA is a single loop, sometimes with plasmids — and no mitochondria or chloroplasts.
Cells differentiate to become specialised (e.g. sperm, nerve, muscle, root hair cells), with structures suited to their function.
Transport into and out of cells
- Diffusion — net movement of particles from a high to a low concentration (passive). Faster with a bigger concentration gradient, higher temperature and larger surface area.
- Osmosis — diffusion of water across a partially permeable membrane from a dilute to a more concentrated solution.
- Active transport — movement against a concentration gradient, requiring energy from respiration (e.g. mineral ions into root hairs).
Required practical: investigating osmosis by measuring the change in mass of plant tissue (e.g. potato) in different concentrations of solution.
Small organisms have a large surface area to volume ratio and can exchange materials by diffusion; larger organisms need specialised exchange surfaces and transport systems.
Mitosis and the cell cycle
Body cells divide by mitosis for growth, repair and replacement, producing two genetically identical cells. The cell cycle: the cell grows and copies its DNA, then the chromosomes separate and the cell divides.
Meiosis
Gametes (sex cells) are produced by meiosis in the reproductive organs:
- produces four genetically different cells,
- each with half the chromosome number (haploid). At fertilisation two gametes fuse, restoring the full chromosome number, and the cell then divides by mitosis.
Cell differentiation and stem cells
Stem cells are undifferentiated cells that can divide and form other cell types. Embryonic stem cells can form any cell type; adult stem cells (e.g. bone marrow) form a limited range; plant stem cells in meristems can differentiate throughout life.
Exam tips
- Compare animal, plant and bacterial cells — know which structures each has.
- Define osmosis precisely (water, partially permeable membrane).
- Active transport needs energy from respiration; diffusion and osmosis do not.
- Distinguish mitosis (identical, 2 cells) from meiosis (different, 4 haploid cells).