Cells, Organisation and Biodiversity

Cell Types: Animal, Plant and Bacterial

All living organisms are composed of cells. WJEC Biology distinguishes three main cell types you must compare:

Animal cells contain: nucleus (controls cell activity; contains DNA on chromosomes), cell membrane (controls entry/exit; selectively permeable), cytoplasm (site of reactions), mitochondria (aerobic respiration → ATP).

Plant cells contain all the above PLUS: cell wall (cellulose; provides support and shape), chloroplasts (contain chlorophyll; site of photosynthesis), large permanent vacuole (filled with cell sap; maintains turgor pressure). Not all plant cells have chloroplasts (e.g. root hair cells).

Bacterial cells (prokaryotic): no membrane-bound nucleus (DNA is a circular loop in cytoplasm), no chloroplasts or mitochondria, cell wall (not cellulose), may have a plasmid (small circular DNA strand), flagellum for movement, much smaller (~1–10 μm vs ~10–100 μm for eukaryotes).

Microscopy — Required Practical

WJEC requires you to prepare slides and use a light microscope. Key skills:

• Calculating magnification: Magnification = image size ÷ actual size (rearrange for actual size).
• Converting units: 1 mm = 1000 μm. A cell drawn 20 mm that is actually 0.02 mm → magnification = 20 ÷ 0.02 = ×1000.
• Stains used: iodine (starch and nuclei), methylene blue (nuclei in animal cells).
• Electron microscopes have greater magnification and resolution than light microscopes.

Cell Organisation

Cells → Tissues → Organs → Organ Systems → Organism.

Examples:

• Muscle cells → muscle tissue → heart → circulatory system → human body.
• Palisade cells → mesophyll tissue → leaf → shoot system → plant.

Specialised cells are adapted to their function (e.g. red blood cells — biconcave disc, no nucleus, haemoglobin for O₂ transport; root hair cells — large surface area, no chloroplasts).

Mitosis

Mitosis produces two genetically identical daughter cells. It is used for: growth, repair, replacement of worn cells, asexual reproduction.

Stages (WJEC examiners accept a simplified sequence):

1. Cell grows; DNA replicates → each chromosome becomes two joined chromatids.
2. Chromosomes line up at the cell equator.
3. Chromatids pulled to opposite poles.
4. Two nuclei form; cell divides → two identical cells.

Human body cells have 46 chromosomes (23 pairs). After mitosis, each daughter cell has 46.

Classification and Biodiversity

The five-kingdom classification: Animals, Plants, Fungi, Protists, Prokaryotes (Monera). Modern classification also uses the domain system: Bacteria, Archaea, Eukarya.

Binomial nomenclature: genus then species (e.g. Homo sapiens). Italicised; genus capitalised.

Biodiversity = variety of life in an area. Measured using species richness and abundance. High biodiversity = more stable ecosystem; loss of species can destabilise food webs.