From the smallest bacterium to the tallest tree, all living things share a common cellular foundation — yet display astonishing diversity. This topic covers cells, the classification of organisms and the ecological relationships between them.
| Letter | Process | What it means |
|---|---|---|
| M | Movement | Living things can move part or all of themselves (plants move more slowly — e.g. turning towards light) |
| R | Respiration | Releasing energy from food using chemical reactions in cells (not the same as breathing!) |
| S | Sensitivity | Responding to changes in the environment (stimuli) |
| G | Growth | Increasing in size, mass and complexity permanently |
| R | Reproduction | Producing offspring of the same kind |
| E | Excretion | Removing metabolic waste products from the body |
| N | Nutrition | Taking in and using food for energy and growth |
Exam trap: "Breathing" is NOT the same as "respiration". Breathing is the physical process of moving air in and out of the lungs. Respiration is the chemical process of releasing energy from glucose in cells — it happens in every living cell.
All living things are made of cells. Cells are the smallest unit of life that can carry out all the functions needed to be alive. Most cells are so small they can only be seen with a microscope.
Cell → Tissue → Organ → Organ System → Organism. Each level has more complex organisation than the one below it.
Controls what enters and leaves the cell. Present in all cells.
Rigid outer layer in plant cells (made of cellulose). Gives support and shape. Not in animal cells.
Controls cell activities. Contains DNA (genetic information). Present in plant and animal cells.
Jelly-like fluid inside the cell membrane. Where most chemical reactions occur.
Site of aerobic respiration — releases energy from glucose for the cell's activities.
Site of photosynthesis. Contains chlorophyll (green pigment). Only in plant cells and algae.
Storage space. Plants have a large central vacuole (stores water, helps maintain shape). Animal cells have small temporary vacuoles.
Site of protein synthesis. Present in all cells — very small.
| Feature | Animal cell | Plant cell |
|---|---|---|
| Cell wall | ✗ Absent | ✓ Present (cellulose) |
| Chloroplasts | ✗ Absent | ✓ Present (in green parts) |
| Large central vacuole | ✗ Absent (may have small vacuoles) | ✓ Present |
| Cell membrane | ✓ Present | ✓ Present |
| Nucleus | ✓ Present | ✓ Present |
| Mitochondria | ✓ Present | ✓ Present |
| Cytoplasm | ✓ Present | ✓ Present |
| Shape | Irregular, flexible | Regular, fixed (due to cell wall) |
Memory tip: Plant cells have 3 extras not found in animal cells — Cell wall, Chloroplasts, and a large Central vacuole. The "3 Cs".
| Cell type | Adaptations | Function |
|---|---|---|
| Red blood cell | No nucleus (more space for haemoglobin); biconcave shape (large surface area) | Carry oxygen around the body |
| Sperm cell | Long tail (flagellum); many mitochondria; streamlined | Swim to and fertilise the egg |
| Root hair cell | Long thin extension (root hair); large surface area | Absorb water and minerals from soil |
| Palisade mesophyll cell | Many chloroplasts; tall and closely packed | Photosynthesis in leaves |
| Nerve cell (neuron) | Very long; many connections (dendrites) | Transmit electrical signals rapidly |
With millions of species on Earth, scientists use classification to organise living things into groups based on shared characteristics. This makes it easier to study them, find patterns, and predict properties.
The hierarchy (from largest group to smallest): Kingdom → Phylum → Class → Order → Family → Genus → Species
Mnemonic: King Philip Came Over For Good Soup
Multicellular, no cell wall, heterotrophic (consume food)
Multicellular, cell wall, autotrophic (make own food via photosynthesis)
Multicellular/unicellular, cell wall (chitin), decompose organic matter
Mostly unicellular, some autotrophic, some heterotrophic (e.g. Amoeba, Paramecium, algae)
Prokaryotes — no nucleus; single-celled; can be autotrophic or heterotrophic
| Group | Key features | Examples |
|---|---|---|
| Fish | Scales, fins, gills, ectothermic | Salmon, shark, clownfish |
| Amphibians | Moist skin, lay eggs in water, larvae have gills, adults have lungs, ectothermic | Frog, toad, salamander |
| Reptiles | Dry scaly skin, lay leathery eggs on land, ectothermic | Lizard, snake, crocodile, turtle |
| Birds | Feathers, wings, beak, lay hard-shelled eggs, endothermic | Sparrow, eagle, penguin |
| Mammals | Hair/fur, endothermic, give birth to live young (mostly), nurse with milk | Human, dog, whale, bat |
Exam trap: A bat is a mammal, not a bird. A whale is a mammal, not a fish. Use the defining characteristics, not superficial appearance (shape, habitat, etc.).
A dichotomous key is used to identify an unknown organism. At each step, you choose between two statements (yes/no or either/or) until you reach the organism's name.
When writing your own key: each question must have exactly two options; use observable features (not size, colour alone); each option leads to another question or a final identification.
Trap: Arrows in a food chain show energy flow, not "eats". So "grass → rabbit → fox" means energy flows from grass to rabbit to fox. The arrow does NOT mean grass eats rabbit.
A student looks at two cells under a microscope. Cell A has a large central vacuole, chloroplasts and a cell wall. Cell B has an irregular shape and no cell wall. (a) Identify which cell is a plant cell and which is an animal cell. (b) Name one organelle present in both cells that is responsible for releasing energy. (c) Why does Cell B not have a fixed shape?
A student finds an organism with the following features: it has scales, breathes through gills, lays eggs in water, and is ectothermic. (a) Identify the group this organism belongs to. (b) Explain why a dolphin, despite living in the sea, does NOT belong to the same group.
Grass → Grasshopper → Frog → Snake → Eagle. (a) Identify the producer. (b) Name a secondary consumer. (c) If a disease kills most of the frogs, predict what would happen to the grasshopper population. Explain your answer.