Levels of organisation — each built from the level below
Cell: the basic structural and functional unit of a living organism. Tissue: cells of similar structure sharing one function. Organ: several different tissues working together. Organ system: organs with related functions — the digestive system holds stomach, small intestine, liver and pancreas. Organism: one or more organ systems working together; a unicellular organism such as a bacterium is a single cell carrying out every life process itself.
Cell structures — shared, plant-only, bacterial
Animal and plant cells share: cell membrane (controls entry and exit), cytoplasm (most chemical reactions), nucleus (DNA that controls the cell, inside a nuclear membrane), mitochondria (aerobic respiration) and ribosomes (protein synthesis). Plant cells add: a cellulose cell wall (support, stops bursting), chloroplasts (photosynthesis) and a permanent vacuole of cell sap (keeps the cell turgid). Bacterial cells have wall, membrane, cytoplasm and ribosomes only.
Specialised cells — six named, each feature tied to its job
Ciliated cell: cilia beat to move mucus. Root hair cell: long extension, big surface area, absorbs water and mineral ions. Palisade mesophyll cell: tall, chloroplast-packed, photosynthesis. Neurone: long, axon and dendrites, carries impulses fast. Red blood cell: biconcave disc, no nucleus, full of haemoglobin for oxygen. Gametes: sperm — tail plus many mitochondria; egg — large, with a food store. New cells come from cell division — growth and repair.
Drawn from real examiner reports.
"Root cell" and "hair cell" score zero
The examinable name is root hair cell — the shortened forms "root cell" and "hair cell" are not credited. The cell's long, thin extension gives a large surface area for absorbing water and mineral ions from the soil, so the name and the adaptation are usually marked together in the same part.
Flagged Jun 2022 P41 Q1ai
Cilia move mucus, they do not absorb
A ciliated cell's beating cilia sweep mucus — with the dust and pathogens trapped in it — along a surface, for example up the trachea and away from the lungs. Candidates mistake it for a villus and answer "absorption". Cilia move material along a surface; they do not absorb it, and they do not produce the mucus either.
Flagged Jun 2022 P41 Q1aii · Nov 2023 P41 Q1bi
The female gamete is the ovum, not the ovary
The female gamete is the egg cell (ovum). The ovary is the organ that produces it and the ovule is a plant reproductive structure — neither one IS the gamete. The male gamete is the sperm cell, made in the testes. Name the cell itself, never the structure that makes it.
Flagged Nov 2023 P31 Q1b
Bacteria have circular DNA, not a nucleus
A bacterial cell has no nucleus: its genetic material is one loop of circular DNA lying free in the cytoplasm, plus one or more smaller plasmids (which can carry extra genes, such as antibiotic resistance). Labelling that loop a "nucleus", or merging the plasmid with the main loop, loses the mark. Bacteria also have no mitochondria and no chloroplasts.
Chloroplast ≠ mitochondrion
The chloroplast contains chlorophyll and is the site of photosynthesis — plant cells only. The mitochondrion is the site of aerobic respiration, which releases energy, and is present in plant AND animal cells but not bacterial cells. Swapping the two processes, or putting chloroplasts in an animal cell, is a standard labelling trap.
A tissue is not an organ
A tissue is a group of cells of similar structure sharing one function, such as muscle tissue. An organ is built from several different tissues working together — a leaf contains palisade mesophyll, xylem, phloem and epidermal tissue. Answers like "an organ is a body part" or "a tissue is a type of cell" score nothing.
Cell wall ≠ cell membrane
The cell membrane controls what moves into and out of the cell — it is partially permeable, letting only some substances through. The cell wall (plant and bacterial cells only) is freely permeable and does a different job: strength, support, stopping the cell bursting. If a question says "controls entry/exit", the answer is membrane — even on a diagram showing both.
Flagged Nov 2022 P32 Q1ai, P43 Q4ci · Jun 2023 P32 Q10aii
Match the command word
"Identify" on a diagram = the correct name only. "State the function of…" = the mark scheme's exact wording (the cell membrane "controls entry and exit of substances", not "protects the cell"). "Describe/explain how it is adapted" = a full reasoning chain, not one feature.
Use the feature → enables → function chain
Three steps every time: name the structural feature ("many mitochondria") → what it enables ("releases more energy by respiration") → state the function ("powers the tail to reach the egg"). Stopping at "large surface area" without saying what it is for costs the last mark.
"Compare" wants matched pairs
A compare question (plant vs animal, or animal vs bacterial) wants paired statements: for each structure say which cell has it AND which does not — "a plant cell has a cell wall but an animal cell does not". Two separate lists of structures do not earn the comparison marks.
Palisade adaptation needs all three points
Adapting the palisade mesophyll cell needs three points: many chloroplasts, a tall column shape so cells pack tightly, and a position near the upper leaf surface where light intensity is greatest. One feature alone rarely carries a 3-mark adaptation question.
Cambridge 0654 spec reference: Section B2 "Cells", sub-topic B2.1 (Core content). This leaf covers the structure and functions of plant, animal and bacterial cells; identifying these structures in diagrams/images; six named specialised cells; new cells being produced by cell division; and the definitions of cell, tissue, organ, organ system and organism.
Out of scope for B2.1 (do not use): the stages/mechanism of mitosis and meiosis (B16 Inheritance); magnification calculations (B2.2); surface-area:volume ratio calculations (B3 Movement into and out of cells).
| Term | Mark-scheme-precise definition |
|---|---|
| Cell | The basic structural and functional unit of a living organism |
| Tissue | A group of cells with similar structure, working together to perform a shared function |
| Organ | A structure made of a group of different tissues, working together to perform specific functions |
| Organ system | A group of organs with related functions, working together to perform body functions |
| Organism | An individual living thing, built from one or more organ systems working together |
Structures in both animal and plant cells:
| Structure | Function |
|---|---|
| Cell membrane | Controls the movement of substances into and out of the cell |
| Cytoplasm | Site of most of the cell's chemical reactions |
| Nucleus | Contains the genetic material (DNA) that controls the cell's activities |
| Mitochondria | Site of (aerobic) respiration, releasing energy |
| Ribosomes | Site of protein synthesis |
Structures ONLY in plant cells:
| Structure | Function |
|---|---|
| Cell wall (made of cellulose) | Provides strength/support; prevents the cell bursting |
| Chloroplasts (contain chlorophyll) | Site of photosynthesis |
| Permanent vacuole (contains cell sap) | Keeps the cell turgid (rigid) |
Bacterial cell (prokaryotic): cell wall, cell membrane, cytoplasm and ribosomes are present, but there is no nucleus, mitochondria or chloroplasts. Genetic material is a single loop of circular DNA, free in the cytoplasm, plus one or more small circular plasmids.
Define: cell, tissue, organ, organ system, organism.
Define each of the following terms:
(a) tissue
(b) organ
(c) organ system
(3 marks)
