Organisation of the Organism
Cell structure and organelle functions in plant, animal and bacterial cells; the six specialised cell types; the cell–tissue–organ–organism hierarchy; and magnification calculations including mm–µm conversion for Extended candidates.
Cell Structure
COREAll living organisms are made of cells. The syllabus requires you to compare plant, animal and bacterial cells, describe organelle functions, and understand how specialised cells are adapted to their roles. Examiner reports consistently flag photomicrograph identification as a weak area — misidentifying organelles in images is one of the most common reasons for lost marks.
Three Cell Types — The Big Comparison
The key to this table is knowing which structures are present, absent, or present but different across the three cell types.
| Structure | Plant cell | Animal cell | Bacterial cell |
|---|---|---|---|
| Cell wall | ✓ (cellulose) | ✗ | ✓ (not cellulose) |
| Cell membrane | ✓ | ✓ | ✓ |
| Nucleus | ✓ | ✓ | ✗ (no membrane-bound nucleus) |
| Cytoplasm | ✓ | ✓ | ✓ |
| Mitochondria | ✓ | ✓ | ✗ |
| Ribosomes | ✓ | ✓ | ✓ (smaller than eukaryotic) |
| Chloroplasts | ✓ (in green parts) | ✗ | ✗ |
| Vacuoles | ✓ (large, central, permanent) | ✗ (none, or only small temporary) | ✗ |
| Circular DNA | ✗ | ✗ | ✓ (in cytoplasm) |
| Plasmids | ✗ | ✗ | ✓ |
The three bacterial-only features are the most frequently tested in MCQs. Bacteria are prokaryotes — their DNA floats freely in the cytoplasm as a single circular molecule. They may also carry small extra rings of DNA called plasmids (important later in Topic 21 — genetic modification). Bacteria do have ribosomes for protein synthesis, but these are smaller than the ribosomes in eukaryotic cells.
Organelle Functions
Provides structural support and prevents the cell from bursting when water enters. Made of cellulose in plants; different material in bacteria. Fully permeable — water and solutes pass through freely.
Controls which substances enter and leave the cell (selective permeability). Present in all three cell types. Site of active transport and regulated diffusion.
Contains the cell's DNA (genetic material, organised into chromosomes). Controls the cell's activities by directing protein synthesis. Surrounded by a nuclear membrane in eukaryotic cells.
Jelly-like fluid filling the cell. Site of many metabolic reactions. Holds the organelles in suspension. Contains enzymes for chemical processes.
Site of aerobic respiration — the reactions that release energy from glucose. Cells with high energy demand (e.g. muscle cells, sperm cells) have more mitochondria.
Site of protein synthesis. Very small — visible only with electron microscopes. Found in all three cell types (but smaller in bacteria). Essential for producing all enzymes and structural proteins.
Site of photosynthesis. Contain chlorophyll, the green pigment that absorbs light energy. Found only in the green parts of plant cells.
Large central vacuole filled with cell sap. Provides turgor pressure — pushes outward against the cell wall to keep the plant firm and upright. Also stores dissolved substances.
The single chromosome of bacteria, arranged in a loop and lying free in the cytoplasm. Carries the genes that control all bacterial functions. Not enclosed in a nucleus.
Small, extra, circular DNA molecules in bacteria. May carry genes for antibiotic resistance. Can be transferred between bacteria — key to how resistance spreads, and key in genetic engineering (Topic 21).
Which row correctly describes features found in a bacterial cell?
| Option | Cell wall | Membrane-bound nucleus | Plasmids | Mitochondria |
|---|---|---|---|---|
| A | ✓ | ✓ | ✓ | ✗ |
| B | ✓ | ✗ | ✓ | ✗ |
| C | ✗ | ✗ | ✓ | ✓ |
| D | ✓ | ✓ | ✗ | ✓ |
Specialised Cells — Structure Linked to Function
The syllabus requires you to know six specialised cell types. For each one, be ready to explain how the structural features adapt the cell for its function — not just state the function.
| Cell type | Function | Structural adaptations |
|---|---|---|
| Ciliated cells trachea & bronchi |
Move mucus (which traps dust and bacteria) up and out of the airways | Densely packed cilia on the surface — hair-like projections that beat rhythmically. Many mitochondria provide the energy for ciliary movement. |
| Root hair cells plant roots |
Absorb water and mineral ions from the soil | Long hair-like extension dramatically increases the surface area for absorption. Thin cell wall and membrane allows rapid diffusion/osmosis. No chloroplasts (underground, no light). |
| Palisade mesophyll cells leaf upper layer |
Carry out photosynthesis | Packed with many chloroplasts (up to ~50 per cell) positioned near the upper leaf surface for maximum light capture. Tall and column-like — more cells can fit per unit area. |
| Neurones nervous system |
Conduct electrical impulses from one part of the body to another | Extremely long axon allows impulses to travel great distances without interruption. Some have a myelin sheath (insulation) for faster conduction. Dendrites increase surface area for receiving signals. |
| Red blood cells blood |
Transport oxygen around the body | No nucleus — maximises space for haemoglobin. Biconcave disc shape increases surface area for oxygen diffusion. Flexible — squeezes through narrow capillaries. Packed with haemoglobin. |
| Sperm cells male gametes |
Fertilise the egg cell (carry genetic information to the egg) | Flagellum (tail) for swimming to the egg. Many mitochondria in the midpiece to power the flagellum. Acrosome (enzyme cap) on the head to penetrate the egg. Streamlined head carries compact nucleus. |
You may be asked to describe or draw cells from photomicrographs or microscope slides. Key skills: identify the cell type from its shape and features; label visible structures; calculate the actual size using magnification (see 2.2). The most commonly tested photomicrograph identifications: plant vs animal vs bacterial cells, and red blood cells vs white blood cells.
A student examines a cell under a microscope and observes: a large central vacuole, a cell wall, chloroplasts, and a nucleus. State the type of cell and explain why the large central vacuole is important for this cell. [3 marks]
- Cell type: plant cell [1 mark] — correctly identifying that all four features together are characteristic of plant cells (large central vacuole is not found in animal or bacterial cells)
- The vacuole is filled with cell sap [1 mark]
- It pushes outwards against the cell wall, providing turgor pressure / keeping the cell firm/turgid / supporting the plant [1 mark]
Cell Organisation Hierarchy
In multicellular organisms, cells are organised into increasingly complex structures. You must be able to describe each level and give examples from the human body or a plant.
| Level | Definition | Human example | Plant example |
|---|---|---|---|
| Cell | The basic unit of life; the smallest unit capable of carrying out life processes | Muscle cell, red blood cell, neurone | Palisade mesophyll cell, root hair cell |
| Tissue | A group of similar cells working together to perform the same function | Muscle tissue, nervous tissue, epithelial tissue | Palisade mesophyll tissue, xylem tissue |
| Organ | A group of different tissues working together to perform a specific function | Heart (muscle + connective + nerve tissue), lung, liver | Leaf (epidermis + mesophyll + vascular tissue) |
| Organ system | A group of organs that work together to carry out a major body function | Circulatory system (heart + blood vessels + blood), digestive system | Shoot system (stem + leaves) |
| Organism | All organ systems working together as a complete, functioning individual | A human being | An oak tree |
❌ "Bacteria have a nucleus" → Wrong. Bacteria are prokaryotes: no membrane-bound nucleus. Their circular DNA lies free in the cytoplasm. This is the single most tested difference between bacterial and eukaryotic cells.
❌ "Plant cells always have chloroplasts" → Wrong. Only cells in the green parts of plants (leaves, green stems). Root cells, for example, have no chloroplasts because they receive no light.
❌ "Animal cells have no vacuoles" → Slightly wrong. Animal cells have no large permanent central vacuole, but they may have small temporary vacuoles. The distinction is size and permanence.
❌ Confusing cell membrane and cell wall → Cell wall = structural support; fully permeable; made of cellulose in plants. Cell membrane = selective permeability; controls entry/exit; present in ALL cells including bacteria.
❌ Red blood cells "have no organelles" → They have no nucleus (the nucleus is lost during development), but they retain haemoglobin, enzymes, and their cell membrane. Saying "no organelles" is too broad.
❌ Confusing ribosomes and mitochondria in photomicrographs → Ribosomes are tiny dots; mitochondria are larger oval/rod-shaped bodies. This is flagged in examiner reports every year. Check the scale bar in the image.
Size of Specimens
CORE EXTENDEDMicroscopes produce magnified images. The formula connecting image size, actual size, and magnification must be known and applied. Extended candidates must also be able to convert between millimetres and micrometres.
The Magnification Formula
Magnification = Image size ÷ Actual size
Actual size = Image size ÷ Magnification
Image size = Actual size × Magnification
Units must be consistent when using this formula. For Core, use millimetres (mm). For Extended, you may need to convert between mm and µm first.
Worked Examples — Core
A cell measures 0.05 mm in real life. Its image in a drawing is 30 mm wide.
Magnification = 30 ÷ 0.05 = ×600
A drawing shows a cell 45 mm wide. The microscope magnification is ×900.
Actual size = 45 ÷ 900 = 0.05 mm
A bacterium is 0.002 mm long. It is drawn at ×5000 magnification.
Image size = 0.002 × 5000 = 10 mm
Both image size and actual size must be in the same unit before you divide. If the question gives actual size in mm and asks you to measure the image in mm — use mm throughout. If you mix mm and cm (or mm and µm), the answer will be wrong by a factor of 10 or 1000.
Practical tip: When measuring an image on paper with a ruler, always measure in mm and state your measurement. Examiners award a mark for showing your measurement even if the final answer is slightly off.
Converting mm and µm — Extended
1 mm = 1000 µm
mm → µm: multiply by 1000 | µm → mm: divide by 1000
0.04 × 1000 = 40 µm
250 ÷ 1000 = 0.25 mm
Image = 32 mm. Magnification = ×400.
Actual = 32 ÷ 400 = 0.08 mm
= 0.08 × 1000 = 80 µm
Under a microscope at ×1000 magnification, the image of a cell is 28 mm wide. What is the actual width of the cell in micrometres (µm)?
- A. 0.028 µm
- B. 2.8 µm
- C. 28 µm
- D. 28 000 µm
Step 1: Actual size (mm) = image size ÷ magnification = 28 ÷ 1000 = 0.028 mm
Step 2: Convert to µm: 0.028 × 1000 = 28 µm
Trap: Option D (28 000) comes from multiplying instead of dividing in step 1. Option B (2.8) comes from incorrect decimal placement.
A student draws a cell 54 mm long. The actual cell is 0.06 mm long. What is the magnification of the drawing?
- A. ×90
- B. ×108
- C. ×900
- D. ×3240
Magnification = image size ÷ actual size = 54 ÷ 0.06 = 900
Trap: Option A (×90) comes from 54 × 0.06 (multiplied instead of divided) then incorrectly placed. Always double-check: image size is always larger than actual size when magnification > 1, so your answer should be > 1.
A student uses a microscope to observe a plant cell. The image of the cell on a screen appears to be 63 mm long. The actual length of the cell is 0.07 mm.
(a) Calculate the magnification. Show your working. [2 marks]
(b) The student draws the cell at ×1200 magnification. Calculate the length of the cell in the drawing. [2 marks]
- Magnification = image size ÷ actual size [1 mark for correct formula or rearrangement]
- = 63 ÷ 0.07 = ×900 [1 mark for correct answer]
- Image size = actual size × magnification [1 mark for correct rearrangement]
- = 0.07 × 1200 = 84 mm [1 mark for correct answer]
❌ Dividing the wrong way → Magnification = image ÷ actual (not actual ÷ image). Quick check: magnification must be greater than 1 for any standard microscope image.
❌ Inconsistent units → If image size is measured in mm and actual size is given in cm, convert first. Otherwise the number is wrong by a factor of 10.
❌ Ext: Multiplying instead of dividing when converting µm to mm → Remember: µm → mm means the number gets smaller, so divide by 1000. A cell that is 50 µm is 0.05 mm, not 50 000 mm.
❌ Not showing working in calculation questions → Even if your final answer is slightly wrong due to a measurement error, a method mark is given for showing the correct formula with values substituted. Always write out the formula first.
Comprehensive Practice Questions
Mixed questions in the style of Cambridge IGCSE 0610 Papers 1–4. Attempt each before revealing the answer.
Which structure is present in a plant cell but NOT in an animal cell?
- A. Mitochondria
- B. Ribosomes
- C. Nucleus
- D. Cell wall
A red blood cell has no nucleus. Which statement best explains why this adaptation is beneficial?
- A. It allows the cell to carry out aerobic respiration more efficiently
- B. It enables the cell to divide more rapidly
- C. It provides more space for haemoglobin, increasing oxygen-carrying capacity
- D. It prevents the cell from being destroyed by the immune system
A student studies the human heart.
(a) State the level of organisation that the heart represents. [1 mark]
(b) Name the level of organisation that contains the heart. [1 mark]
(c) Describe the difference between a tissue and an organ. [2 marks]
- (a) Organ [1 mark]
- (b) Organ system (the circulatory system) [1 mark]
- (c) A tissue is a group of similar cells performing the same function [1 mark]; an organ is a group of different tissues working together to perform a specific function [1 mark]
Root hair cells and palisade mesophyll cells are both specialised plant cells.
(a) State the function of each cell type. [2 marks]
(b) Explain how the structure of a root hair cell is adapted for its function. [3 marks]
(c) State one structural feature found in a palisade mesophyll cell that is not found in a root hair cell, and explain why. [2 marks]
- (a) Root hair cell: absorption of water and mineral ions from soil [1 mark]; palisade mesophyll cell: photosynthesis [1 mark]
- (b) Long hair-like extension increases surface area [1 mark] for absorption of water by osmosis and mineral ions by active transport [1 mark]; thin wall and membrane allow rapid uptake [1 mark]
- (c) Chloroplasts are present in palisade mesophyll cells but not in root hair cells [1 mark]; because root hair cells are underground and receive no light, so chloroplasts would be non-functional there — photosynthesis cannot occur without light [1 mark]
A student observes a bacterial cell under an electron microscope. The image shows the cell to be 15 mm long at a magnification of ×30 000.
(a) Calculate the actual length of the bacterial cell. Give your answer in µm. Show your working. [3 marks]
(b) State two ways the structure of a bacterial cell differs from that of an animal cell. [2 marks]
- Actual size = image size ÷ magnification [1 mark for formula]
- = 15 ÷ 30 000 = 0.0005 mm [1 mark for correct mm answer]
- = 0.0005 × 1000 = 0.5 µm [1 mark for correct conversion]
- Bacterial cell has no membrane-bound nucleus / DNA is free in cytoplasm
- Bacterial cell has circular DNA; animal cell has linear chromosomes in a nucleus
- Bacterial cell has plasmids; animal cell does not
- Bacterial cell has a cell wall; animal cell does not
- Bacterial cell has no mitochondria; animal cell has mitochondria
- Bacterial cell has no membrane-bound organelles; animal cell has membrane-bound organelles
High-Frequency Mistakes — Topic 2 Overall
- 🔮Bacteria have a nucleusNo. Bacteria are prokaryotes — their DNA (a single circular molecule) lies free in the cytoplasm. There is no membrane-bound nucleus. This single point is tested in nearly every exam session.
- 🌿All plant cells have chloroplastsOnly cells in green parts of the plant (leaves, green stems). Root cells, storage cells, and flower petals contain no chloroplasts.
- 🔄Confusing the cell membrane with the cell wallCell wall: structural support, fully permeable, made of cellulose (plants) or other materials (bacteria). Cell membrane: selective permeability, controls entry/exit, present in ALL cells. Both can be present in the same cell.
- 🔫Misidentifying organelles in photomicrographsRibosomes: tiny dots, only visible with electron microscope. Mitochondria: larger, oval/rod-shaped. Chloroplasts: larger, green, oval, in plant cells. Nucleus: largest organelle, round, with visible membrane. Flagged in examiner reports every year.
- 🍂"Red blood cells have no organelles"More precisely: red blood cells have no nucleus. They do contain haemoglobin, enzymes, and their cell membrane. "No organelles" is too broad and would lose marks.
- ✖Magnification calculation upside-downMagnification = image ÷ actual size (NOT actual ÷ image). Quick sanity check: the result should always be greater than 1 for any microscope image. If your answer is less than 1, you divided the wrong way.
- 📏Mixed units in magnification calculationsImage size and actual size must both be in the same unit before you divide. Convert everything to mm first, then calculate. Only convert to µm at the final step if the question asks for µm.
- 📈Ext: µm → mm conversion direction1 mm = 1000 µm. Going from µm to mm means the number gets smaller (divide by 1000). Going from mm to µm means the number gets larger (multiply by 1000). A cell that is 30 µm = 0.030 mm, not 30 000 mm.
- ✎Not writing the formula before calculatingExaminers award a method mark for showing magnification = image ÷ actual (with values substituted), even if a minor arithmetic error gives a wrong final answer. Always write the formula first.
Topic 2 accounts for cell structure questions that appear in almost every paper — it underpins Topics 3 (diffusion/osmosis), 5 (enzymes), 6 (photosynthesis), 9 (blood/heart), 12 (respiration), and 14 (nervous system). Highest-yield items: the three-way comparison table (plant/animal/bacterial cells), the six specialised cells with structural justifications, and magnification calculations with correct units. In photomicrograph questions, use the scale bar to estimate actual size before identifying structures.