60-second revision
- Robert Hooke named cells (cork, 1665); Leeuwenhoek saw living cells; Robert Brown found the nucleus (1831).
- Mitochondria = powerhouse (ATP); ribosomes = protein factories; lysosomes = suicide bags.
- Only plant cells have a cellulose cell wall, chloroplasts and a large central vacuole.
- Xylem carries water up; phloem carries food. Chlorophyll contains magnesium.
- Ethylene ripens fruit; auxin bends shoots towards light; abscisic acid is the stress hormone.
- Rhizobium fixes nitrogen in legume root nodules; Azotobacter is free-living.
- Potato = stem tuber; sweet potato = root; ginger = rhizome (stem); onion = bulb.
- Whittaker (1969) gave five kingdoms; Linnaeus gave binomial names like Homo sapiens.
The cell: discovery and theory
- Robert Hooke (1665) looked at a thin slice of cork under his microscope and named the box-like compartments cells. They were dead cells.
- Anton van Leeuwenhoek (1670s) was the first to see living cells — bacteria, protozoa and sperm — with simple lenses he ground himself.
- Robert Brown discovered the nucleus in 1831.
- Cell theory: Schleiden (plants, 1838) and Schwann (animals, 1839) said all living things are made of cells; Virchow (1855) added that every cell comes from a pre-existing cell.
- Prokaryotic cells (bacteria) have no nuclear membrane and no membrane-bound organelles; eukaryotic cells (plants, animals, fungi) do.
- The nerve cell is the longest cell in the human body; an ostrich egg is the largest single cell.
Organelles and their jobs
| Organelle | Function | Exam tag |
|---|---|---|
| Nucleus | Holds DNA (chromosomes), controls the cell | Control centre |
| Mitochondria | Cellular respiration, releases energy as ATP; has its own DNA | Powerhouse |
| Ribosomes | Make proteins; no membrane | Protein factory |
| Endoplasmic reticulum | Rough ER makes proteins, smooth ER makes lipids | Transport network |
| Golgi apparatus | Packages and dispatches materials | Post office |
| Lysosomes | Contain digestive enzymes; can digest a damaged cell | Suicide bags |
| Plastids (chloroplasts) | Photosynthesis; contain chlorophyll | Kitchen of the cell |
| Vacuole | Storage; large and central in plant cells | Storage sac |
| Cell membrane | Selectively permeable boundary of every cell | Gatekeeper |
| Cell wall | Rigid outer layer; cellulose in plants, chitin in fungi | Absent in animals |
Cell division
- Mitosis: one cell gives two identical cells with the same number of chromosomes. Used for growth and repair.
- Meiosis: forms gametes (sperm, egg, pollen, ovule) and halves the chromosome number, so fertilisation restores the full number.
Plant tissues, nutrition and transport
Green plants make food by photosynthesis: carbon dioxide and water, in the presence of sunlight and chlorophyll, give glucose and oxygen. The oxygen released comes from splitting water. Gases move in and out through tiny pores called stomata, mostly on the lower surface of leaves.
- Chlorophyll has a magnesium atom at its centre; haemoglobin has iron. Easy to swap in an exam — do not.
- Xylem: carries water and minerals upwards from the roots. Mostly dead cells at maturity (vessels, tracheids).
- Phloem: carries food (sugars) from leaves to other parts, in both directions — this is called translocation.
- Transpiration (loss of water vapour through stomata) creates the pull that draws water up the xylem.
- Meristematic tissue (growing tips, cambium) divides; permanent tissues do not.
Plant hormones and movements
| Hormone | Main effect |
|---|---|
| Auxin | Cell elongation; shoot bends towards light (phototropism); rooting of cuttings |
| Gibberellin | Stem elongation, seed germination, bigger fruit |
| Cytokinin | Cell division; delays ageing of leaves |
| Abscisic acid | Stress hormone: closes stomata, causes dormancy and leaf fall |
| Ethylene | A gas; ripens fruits |
Shoots grow towards light (positive phototropism) and roots grow towards gravity (positive geotropism). The touch-me-not plant (Mimosa) folds its leaves on touch — a movement that is not growth-related.
Nitrogen fixation
Plants cannot use the nitrogen gas in air directly. Rhizobium bacteria living in the root nodules of legumes (pulses such as gram, pea and moong) convert it into usable compounds; that is why pulses enrich the soil. Azotobacter does the same while living freely in soil, and blue-green algae (cyanobacteria such as Anabaena and Nostoc) do it in paddy fields. Lightning also fixes some nitrogen.
Reproduction
| Asexual method | Example |
|---|---|
| Binary fission | Amoeba, Paramecium, bacteria |
| Multiple fission | Plasmodium (malaria parasite) |
| Budding | Yeast, Hydra |
| Fragmentation | Spirogyra |
| Regeneration | Planaria |
| Spore formation | Rhizopus (bread mould), ferns |
| Vegetative propagation | Potato (stem tuber), ginger (rhizome), onion (bulb), Bryophyllum (leaf buds), sweet potato (root), sugarcane and rose (stem cuttings) |
In flowering plants the stamen is the male part (anther makes pollen) and the pistil or carpel is the female part (stigma, style, ovary). Pollination brings pollen to the stigma; after fertilisation the ovule becomes the seed and the ovary becomes the fruit. Flowering plants show double fertilisation: one male gamete fuses with the egg (zygote) and the other with the polar nuclei (endosperm, the food store).
Heredity and classification
- Gregor Mendel worked with garden peas and gave the laws of inheritance; a monohybrid cross gives a 3:1 ratio in the second generation.
- Carl Linnaeus introduced binomial nomenclature: genus + species, e.g. Homo sapiens, Mangifera indica (mango).
- R.H. Whittaker (1969) proposed five kingdoms: Monera (bacteria), Protista (single-celled eukaryotes like Amoeba), Fungi, Plantae and Animalia.
- Fungi have no chlorophyll and get food from dead or living matter; yeast ferments sugar and is used to make bread and alcohol.
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