Organisation of the Organism
Everything else in biology sits on this topic. Cells are the building blocks; the organelles inside them do jobs that reappear in respiration, photosynthesis, transport and enzymes.
1. Cell structure
Structures in animal and plant cells
| Structure | Function | Animal | Plant |
|---|---|---|---|
| Cell membrane | Controls what enters and leaves — partially permeable | ✔ | ✔ |
| Cytoplasm | Jelly-like; site of most chemical reactions | ✔ | ✔ |
| Nucleus | Contains genetic material (DNA) and controls the cell’s activities | ✔ | ✔ |
| Mitochondria | Site of aerobic respiration, where energy is released | ✔ | ✔ |
| Ribosomes | Site of protein synthesis | ✔ | ✔ |
| Cell wall | Made of cellulose; provides support and stops the cell bursting | ✘ | ✔ |
| Chloroplasts | Contain chlorophyll; site of photosynthesis | ✘ | ✔ |
| Permanent vacuole | Contains cell sap; helps keep the cell turgid | ✘ | ✔ |
The three plant-only structures — cell wall, chloroplasts, permanent vacuole — are the answer to almost every “differences between plant and animal cells” question.
The cell wall and the cell membrane are different things. The wall is made of cellulose, is fully permeable, and provides support. The membrane is partially permeable and controls what enters and leaves. Plant cells have both.
Not every plant cell has chloroplasts. Root cells are underground and have none. Saying “all plant cells contain chloroplasts” is wrong.
Bacterial cells
Bacteria are prokaryotes — the key point is that they have no nucleus.
| Present in bacteria | Absent in bacteria |
|---|---|
| Cell wall (not made of cellulose) | Nucleus |
| Cell membrane | Mitochondria |
| Cytoplasm | Chloroplasts |
| Ribosomes | Permanent vacuole |
| Circular DNA, loose in the cytoplasm | |
| Plasmids — small rings of extra DNA |
Plasmids matter beyond this topic: they are what makes bacteria so useful in genetic modification.
A bacterial cell wall is not made of cellulose. Both plants and bacteria have cell walls, but of different materials — that distinction is often the whole question.
2. Specialised cells
Each of these appears repeatedly across the syllabus. Learn the adaptation and the reason together, because “state and explain” questions want both.
| Cell | Adaptation | Function |
|---|---|---|
| Ciliated cell | Cilia that beat | Move mucus (with trapped dust and bacteria) away from the lungs |
| Root hair cell | Long, thin extension; many mitochondria | Large surface area for absorbing water and mineral ions |
| Palisade mesophyll cell | Column-shaped, packed with chloroplasts, near the leaf surface | Maximum absorption of light for photosynthesis |
| Neurone | Very long; myelin sheath | Conduct electrical impulses over long distances |
| Red blood cell | Biconcave; no nucleus; contains haemoglobin | Transport oxygen |
| Sperm cell | Flagellum; acrosome; many mitochondria | Swim to and penetrate the egg cell |
| Egg cell | Energy store in cytoplasm; jelly coat | Nourish the embryo; prevent further sperm entering |
The recurring pattern: where a cell absorbs or exchanges, the adaptation is large surface area and a short diffusion distance. Where a cell needs energy, it has many mitochondria. Spotting which pattern applies usually gets you most of the marks.
3. Levels of organisation
cell → tissue → organ → organ system → organism
| Level | Definition | Example |
|---|---|---|
| Cell | The basic unit of all living organisms | Muscle cell |
| Tissue | A group of cells with similar structures, working together to perform a shared function | Muscle tissue, xylem |
| Organ | A structure made up of a group of tissues, working together to perform specific functions | Heart, stomach, leaf |
| Organ system | A group of organs with related functions, working together | Digestive system, circulatory system |
| Organism | A complete living thing | A human, an oak tree |
“A group of similar cells” is not enough for a tissue. The definition needs them working together to perform a shared function — that clause is usually a mark.
4. Size of specimens
magnification = image size ÷ actual size
Rearranged: actual size = image size ÷ magnification.
A memory triangle many students find useful: I over A × M — cover the one you want.
Getting the calculation right
Units are where the marks are lost. Both measurements must be in the same unit before you divide.
| Conversion | |
|---|---|
| 1 cm | = 10 mm |
| 1 mm | = 1000 μm (micrometres) |
| 1 cm | = 10 000 μm |
Method:
- Measure the image with a ruler, usually in mm
- Convert both values to the same unit
- Divide
- Check the answer against the unit the question asks for, and any instruction about significant figures
The commonest error in this whole topic is forgetting to convert. Answer options in multiple-choice questions are usually designed so that the un-converted value appears as one of the wrong answers — so getting a number that matches an option is not proof you were right.
Magnification has no units. It is a ratio, written as “×400”.
5. Mistakes that cost marks
Confusing the cell wall with the cell membrane.
Saying all plant cells have chloroplasts.
Saying bacterial cell walls are made of cellulose.
Saying bacteria have a nucleus. They have circular DNA free in the cytoplasm.
Saying mitochondria “produce energy.” They are the site of aerobic respiration, which releases energy.
Defining a tissue as “a group of similar cells” without the shared function.
Forgetting to convert units in a magnification calculation.
Giving magnification a unit.
Stating a cell adaptation without its reason.
Frequently asked questions
What are the three structures found in plant cells but not animal cells? Cell wall, chloroplasts and permanent vacuole.
What is the difference between the cell wall and the cell membrane? The wall is cellulose, fully permeable, and gives support. The membrane is partially permeable and controls what enters and leaves.
How is a bacterial cell different from a plant cell? No nucleus (circular DNA instead), no mitochondria, no chloroplasts, and the cell wall is not cellulose. Bacteria also have plasmids.
What is the function of the mitochondria? Site of aerobic respiration, releasing energy.
What is the difference between a tissue and an organ? A tissue is a group of similar cells working together; an organ is made of several tissues working together.
How do I calculate actual size? Actual size = image size ÷ magnification — with both lengths in the same unit.
Why do root hair cells have no chloroplasts? They are underground, so there is no light for photosynthesis.
Quick revision checklist
- I can label an animal and a plant cell and give every function
- I know the three plant-only structures
- I can distinguish the cell wall from the cell membrane
- I can describe a bacterial cell, including plasmids and circular DNA
- I know bacterial cell walls are not cellulose
- I can give the adaptation and reason for all seven specialised cells
- I can list the levels of organisation in order
- I can define tissue, organ and organ system precisely
- I can rearrange the magnification formula
- I can convert between cm, mm and μm without error
- I know magnification has no units
These notes cover topic 2 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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