Cell structure and organelles: what every Biology student must know
The cell as the unit of life, each organelle and its function, plant versus animal cells, prokaryotes versus eukaryotes, and how cells are organised into tissues and organs — for WAEC, NECO, JAMB and BECE.
By the Levia editorial team · Updated 27 October 2026 · 5 min read
The cell is where Biology begins. It's usually the first topic in the senior secondary Biology syllabus, it appears in Basic Science at junior level, and questions on organelles and plant versus animal cells show up in nearly every WAEC, NECO and JAMB Biology paper. A clear understanding of the cell also makes later topics (respiration, photosynthesis, genetics and cell division) much easier.
The cell: the basic unit of life
A cell is the smallest structural and functional unit of a living organism. Everything an organism does (feeding, respiring, growing, reproducing) happens in its cells.
The cell theory states that:
- All living things are made up of one or more cells.
- The cell is the basic unit of structure and function in living things.
- All cells come from pre-existing cells.
Some organisms, such as Amoeba and Paramecium, are unicellular (one cell). Others, such as humans and mango trees, are multicellular (made of many cells).
The main organelles and their functions
An organelle is a specialised structure inside a cell that performs a particular function.
| Organelle | Function | Found in |
|---|---|---|
| Nucleus | Controls the cell's activities; contains genetic material (DNA) in chromosomes | Plant and animal cells |
| Cell membrane (plasma membrane) | Controls what enters and leaves the cell; selectively permeable | Plant and animal cells |
| Cytoplasm | Jelly-like fluid where most chemical reactions take place | Plant and animal cells |
| Mitochondria | Site of aerobic respiration; releases energy (the "powerhouse" of the cell) | Plant and animal cells |
| Ribosomes | Site of protein synthesis | Plant and animal cells |
| Endoplasmic reticulum | Transports materials within the cell; rough ER (with ribosomes) makes proteins, smooth ER makes lipids | Plant and animal cells |
| Golgi apparatus (Golgi body) | Packages and modifies proteins for secretion | Plant and animal cells |
| Lysosomes | Contain enzymes that digest worn-out parts and foreign materials | Mainly animal cells |
| Cell wall | Gives shape, support and protection; made of cellulose in plants | Plant cells (not animal cells) |
| Chloroplasts | Site of photosynthesis; contain the green pigment chlorophyll | Plant cells (green parts) |
| Large central vacuole | Stores cell sap; keeps the cell firm (turgid) | Plant cells |
| Centrioles | Help in cell division in animal cells | Animal cells |
The nucleus in more detail
The nucleus is surrounded by a nuclear membrane (nuclear envelope) and contains:
- Chromatin/chromosomes: made of DNA, carrying the genes that determine the organism's characteristics.
- Nucleolus: involved in making ribosomes.
Because it holds the genetic material, the nucleus is often described as the "control centre" of the cell.
Mitochondria in more detail
Mitochondria are where aerobic respiration releases energy from food (glucose), in the form of ATP. Cells that need a lot of energy have many mitochondria: muscle cells, sperm cells, and cells of the liver and kidney.
Chloroplasts in more detail
Chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis. They are found in the green parts of plants, especially the palisade cells of leaves. Root cells do not have chloroplasts, a fact examiners like to test, because roots grow underground away from light.
Plant cells versus animal cells
This comparison appears in exams constantly:
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Chloroplasts | Present (in green parts) | Absent |
| Vacuole | Large, central, permanent | Small, temporary (if present) |
| Shape | Regular, fixed (often rectangular) | Irregular, flexible (often rounded) |
| Centrioles | Absent (in most higher plants) | Present |
| Food storage | Starch | Glycogen |
| Position of nucleus | Pushed to the side by the vacuole | Usually central |
Both have a nucleus, cell membrane, cytoplasm, mitochondria, ribosomes, endoplasmic reticulum and Golgi apparatus.
Prokaryotic versus eukaryotic cells
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No true nucleus (DNA lies free in the cytoplasm) | True nucleus with a nuclear membrane |
| Membrane-bound organelles | Absent | Present (mitochondria, chloroplasts and others) |
| Size | Smaller | Larger |
| Examples | Bacteria | Plants, animals, fungi, protists |
Levels of organisation
In multicellular organisms, cells are organised into increasingly complex levels:
- Cell: for example, a muscle cell.
- Tissue: a group of similar cells performing the same function, such as muscle tissue.
- Organ: different tissues working together, such as the heart.
- Organ system: organs working together, such as the circulatory system.
- Organism: the complete living thing, such as a human being.
Example question: Arrange in order of increasing complexity: organ, cell, system, tissue. Answer: cell → tissue → organ → system.
Specialised cells
Cells are adapted to their functions. Common exam examples:
- Red blood cells: no nucleus (more room for haemoglobin), biconcave shape for a large surface area to carry oxygen.
- Nerve cells (neurones): long fibres to carry impulses over distances.
- Root hair cells: long, thin extensions to increase surface area for absorbing water and minerals.
- Sperm cells: a tail for swimming, and many mitochondria for energy.
- Palisade cells: packed with chloroplasts for photosynthesis.
How to answer cell questions well
- Use precise terms. Say "the mitochondrion is the site of aerobic respiration", not "the mitochondrion gives energy".
- For diagrams, draw clean outlines in pencil, label with straight lines that don't cross, add a title, and draw large enough to see details.
- For comparison questions, set your answer out as a table with matching points side by side.
Test yourself
- Which organelle is the site of protein synthesis?
- Name two structures found in plant cells but not in animal cells.
- Why do root hair cells lack chloroplasts?
- What is the main difference between prokaryotic and eukaryotic cells?
Answers: 1. Ribosome. 2. Any two of: cell wall, chloroplasts, large central vacuole. 3. Roots grow underground where there is no light, so they don't photosynthesise. 4. Prokaryotic cells have no true nucleus (no nuclear membrane); eukaryotic cells do.
Practise with explanations
Cell Biology is the foundation of Levia's Biology Level 1, and the junior Basic Science subject covers cells, tissues and organs for BECE. Every question comes with an explanation that uses correct biological terminology.
Try the real Level 1 question set free — no account needed — with an explanation after every question.
Levia is an independent learning platform, not affiliated with JAMB, WAEC, NECO or any examination body. Exam formats, dates and fees can change — always confirm current details on the official body's website.