1. The cell as the unit of life; cell theory; protoplasm and cytoplasm
A cell is the smallest unit of structure and function of a living organism. Organisms may be unicellular (Amoeba, bacteria) or multicellular (a banyan tree, a human). In a multicellular body cells form tissues, tissues form organs, and organs form organ systems.
Cell theory (Schleiden and Schwann, extended by Virchow):
- All living organisms are made of one or more cells.
- The cell is the basic unit of structure and function.
- New cells arise only from pre-existing cells by division.
Protoplasm is the entire living content of a cell: the cytoplasm and the nucleus (nucleoplasm) together. The cytoplasm is the jelly-like part between the cell membrane and the nucleus; it holds the organelles. (Some books use "protoplasm" and "cytoplasm" loosely; here the ICSE distinction is used.)
| Term | Meaning |
|---|---|
| Protoplasm | Cytoplasm + nucleus |
| Cytoplasm | Between membrane and nucleus |
Key terms
- Cell
The smallest unit of structure and function of a living organism.
- Protoplasm
All the living matter of a cell: cytoplasm and nucleus.
- Cytoplasm
The protoplasm outside the nucleus, inside the cell membrane.
2. Cell boundaries: cell wall and cell membrane
| Point | Cell wall | Cell membrane |
|---|---|---|
| Present in | Plant cells (also fungi, bacteria; composition differs) | All cells |
| Composition | Cellulose in plants | Lipids and proteins |
| Nature | Non-living, rigid | Living, flexible |
| Permeability | Freely permeable | Selectively (semi-) permeable |
| Function | Gives shape, support and protection | Controls what enters and leaves the cell; gives shape to animal cells |
The cell wall lies outside the cell membrane. Animal cells have only the membrane.
Key terms
- Cell wall
Non-living, freely permeable outer layer of plant cells, made of cellulose.
- Cell membrane
Living, selectively permeable boundary of every cell, made of lipids and proteins.
3. Nucleus, nucleolus, mitochondria, endoplasmic reticulum, ribosomes and Golgi bodies
- Nucleus: usually one, bounded by a double nuclear membrane with pores; contains nucleoplasm, chromatin (DNA with proteins, which forms chromosomes) and the nucleolus. It controls all cell activities and carries heredity.
- Nucleolus: dense, round body in the nucleus; makes ribosomes (and stores RNA).
- Mitochondria: rod- or oval-shaped, with an outer membrane and an inner membrane folded into cristae. They carry out aerobic respiration and release energy as ATP, so they are called the powerhouses of the cell.
- Endoplasmic reticulum (ER): a network of membrane-bound channels. Rough ER carries ribosomes and helps make and transport proteins; smooth ER has none and helps make lipids and detoxify. It gives mechanical support and joins the nuclear envelope to the cell surface.
- Ribosomes: tiny granules without a membrane, free in the cytoplasm or on rough ER; the site of protein synthesis.
- Golgi bodies: stacks of flat, curved sacs with vesicles; they modify, pack and secrete substances and help form lysosomes.
Key terms
- Nucleolus
Dense body in the nucleus that makes ribosomes.
- Mitochondrion
Organelle with folded inner membrane that releases energy in respiration.
- Ribosome
Granule that is the site of protein synthesis.
- Golgi body
Stack of sacs that packs and secretes substances.
4. Plastids, lysosomes, centrosome and vacuoles
Plastids occur in plant cells (and some protists):
- Chloroplasts: green, contain chlorophyll in stacks called grana; carry out photosynthesis.
- Chromoplasts: contain red, orange or yellow pigments; colour flowers and fruits.
- Leucoplasts: colourless; store starch (amyloplasts), oils or proteins.
Lysosomes: small single-membrane bags of digestive enzymes; digest worn-out organelles and foreign matter; called suicide bags. Centrosome: near the nucleus in animal cells, with two centrioles; helps form the spindle in cell division. Absent in higher plants. Vacuoles: sacs of cell sap bounded by a membrane (tonoplast in plants). Plants have one large central vacuole that stores water, salts and food and keeps the cell turgid; animal cells have small, temporary vacuoles.
Key terms
- Chloroplast
Green plastid where photosynthesis occurs.
- Chromoplast
Coloured plastid giving colour to flowers and fruits.
- Leucoplast
Colourless plastid that stores food.
- Lysosome
Bag of digestive enzymes; suicide bag.
- Vacuole
Sac of cell sap.
5. Prokaryotic and eukaryotic cells
| Point | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No true nucleus; DNA lies free as a nucleoid (no nuclear membrane) | True nucleus with nuclear membrane |
| Organelles | No membrane-bound organelles | Mitochondria, ER, Golgi, etc. |
| Ribosomes | Present (smaller) | Present (larger) |
| Size | Usually small (about 1 to 10 micrometres) | Usually larger (10 to 100 micrometres) |
| Examples | Bacteria, blue-green algae (cyanobacteria) | Amoeba, yeast and other fungi, plants, animals |
Sizes are approximate and differ between books.
Key terms
- Prokaryotic cell
Cell without a true nucleus or membrane-bound organelles.
- Eukaryotic cell
Cell with a true nucleus and membrane-bound organelles.
- Nucleoid
The region of a prokaryotic cell holding its DNA.
6. Plant cell versus animal cell
| Point | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Shape | Usually fixed, regular | Usually irregular |
| Plastids | Present | Absent |
| Vacuoles | One large central vacuole | Small and few |
| Centrosome | Absent in higher plants | Present |
| Nucleus | Often at the side | Usually central |
Drawing: draw a generalised cell in outline, then label with ruled lines that touch the part: plant cell (cell wall, membrane, cytoplasm, nucleus with nucleolus, large vacuole, chloroplast, mitochondrion) and animal cell (membrane, cytoplasm, nucleus with nucleolus, centrosome, Golgi body, mitochondrion, lysosome).
Key terms
- Turgid
Firm because the vacuole is full of water.
Exam tips
Common mistakes
- Calling the cell wall living or selectively permeable.
- Saying protoplasm and cytoplasm are the same thing without the nucleus distinction.
- Drawing a nuclear membrane in a bacterium, or a cell wall in an animal cell.
- Giving "makes energy" for mitochondria: they release energy from food (respiration).
- Writing only "powerhouse" without the reason.
How marks are usually awarded
- 1 mark: a term, an odd one out with its category, a function in one line.
- 2 marks: two differences, each matched point for point; two functions.
- 3 marks: a neat, labelled diagram (one mark for the outline, one for key features, one for correct labels).
- 10-mark structured question: a short situation or diagram with parts of 3, 3 and 4 marks; one part is usually a labelled diagram.