IGCSE Biology: Organisation of the Organism — Revision Notes

Condensed for the final weeks. For the full explanation, use the Organisation of the Organism study guide.

Cell structure at a glance — all Core, no Supplement content

Structure	Plant cell	Animal cell	Bacterial cell
Cell wall	Yes	No	Yes
Cell membrane	Yes	Yes	Yes
Nucleus	Yes	Yes	No (circular DNA instead)
Cytoplasm	Yes	Yes	Yes
Chloroplasts	Yes	No	No
Ribosomes	Yes	Yes	Yes
Mitochondria	Yes	Yes	No
Vacuole (permanent)	Yes	No	No
Plasmids	No	No	Yes

Every outcome in 2.1 is examined identically for Core and Extended candidates — there is no Supplement-only cell-structure content. Learn this table as one grid, not three separate lists, so you can instantly answer “what does a plant cell have that an animal cell doesn’t” (chloroplasts, cell wall, permanent vacuole) and “what does a bacterial cell have that neither does” (circular DNA, plasmids, no true nucleus).

The six named specialised cells — learn the exact wording

Cell	Function (syllabus wording)
Ciliated cell	Movement of mucus in the trachea and bronchi
Root hair cell	Absorption
Palisade mesophyll cell	Photosynthesis
Neurone	Conduction of electrical impulses
Red blood cell	Transport of oxygen
Sperm and egg cell (gametes)	Reproduction

Markers check for the precise idea in each function, not a vaguer paraphrase — “conduction of electrical impulses” for a neurone, not “sending messages.” Learn these six exactly as listed.

The organisation hierarchy

Cell → basic structural and functional unit of a living organism. Tissue → group of cells with similar structures, working together to perform a shared function. Organ → made of several tissues working together to perform specific functions. Organ system → group of organs with related functions working together. Organism → an individual living thing capable of all the life processes. In multicellular animals and plants it is built from organ systems; a unicellular organism, such as a bacterium (a prokaryote) or many protoctists, carries out all the life processes within a single cell, with no tissues, organs or organ systems.

This five-term chain is a short-answer favourite precisely because it is easy to learn imprecisely. Practise applying it to examples the syllabus has not already given you (a leaf, a nephron, a red blood cell) rather than only reciting the five words in order.

Magnification — the formula and the units trap

Core: magnification = image size ÷ actual size. Practise rearranging this to find actual size or image size when magnification is given instead.

Supplement adds: converting between millimetres (mm) and micrometres (μm) — 1 mm = 1000 μm.

The single most common way marks are dropped on this calculation is a units mismatch: image size measured in mm against a scale bar given in μm, or vice versa, without converting first. Always check both measurements are in the same unit before applying the formula.

Worked example: magnification calculation

A drawing of a cell measures 45 mm across. The actual cell is
0.03 mm across. Calculate the magnification.

magnification = image size ÷ actual size
              = 45 ÷ 0.03
              = ×1500

If the actual size had instead been given in micrometres (30 μm), convert first: 30 μm ÷ 1000 = 0.03 mm, then apply the same formula.

Why this topic underpins the rest of the syllabus

Organisation of the organism is topic 2 of 21 in 0610, sitting right after Characteristics and classification of living organisms. Almost everything that follows assumes this content is secure: the transport topics depend on knowing what a cell membrane and cytoplasm are, and the nutrition, respiration and coordination topics all describe a specialised cell doing a job, which only makes sense once cell specialisation from this topic is fixed in memory. Treat fluency here — naming a structure from a diagram instantly, stating its function without hesitation — as a prerequisite for later topics, not content to revisit only in the final weeks before the exam.

Practical microscopy links

If your course includes microscopy practicals, connect them directly to this topic rather than treating them as a separate skill. Examiners can present a genuine micrograph, or a drawing based on one, and ask you to identify structures, estimate size using a scale bar, or calculate magnification from a stated actual size. Practise with real or simulated images where possible, since these are messier and harder to read than the clean labelled diagrams found in most textbooks — closer to what actually appears on the exam paper.

Exam traps

• Describing cell structures in three separate isolated lists instead of comparing plant, animal and bacterial cells directly.
• Paraphrasing a specialised cell’s function loosely instead of using the syllabus’s precise wording.
• Reciting the cell/tissue/organ/organ system/organism hierarchy without being able to apply it to an unfamiliar structure.
• Mixing millimetres and micrometres in a magnification calculation without converting first.

Self-test

1. Name three structures a plant cell has that an animal cell does not.
2. Name two structures a bacterial cell has that neither a plant nor an animal cell has.
3. State the function of a ciliated cell, using the syllabus’s exact wording.
4. State the magnification formula, and the conversion between mm and μm.
5. Put these in order from smallest to largest level of organisation: organ, cell, organ system, tissue, organism.

Answers: 1. Cell wall, chloroplasts, permanent vacuole (any three). 2. Circular DNA and plasmids (a bacterial cell also lacks a true nucleus). 3. Movement of mucus in the trachea and bronchi. 4. Magnification = image size ÷ actual size; 1 mm = 1000 μm. 5. Cell, tissue, organ, organ system, organism.