Introduction
Every living thing on Earth (from the tiniest bacterium to the largest whale) is made of cells. Cells are the basic unit of life. They carry out all the processes that keep living things alive: obtaining energy, growing, reproducing, responding to the environment, and removing waste.
You cannot see most cells with the naked eye. A typical human cell is about 0.01mm across — so small that thousands of them could fit on the full stop at the end of this sentence. Yet inside each one is an astonishing level of organisation, with different structures carrying out different jobs with remarkable precision.
Understanding cells is the starting point for all of biology. Everything else (tissues, organs, organ systems, reproduction, genetics) builds on what happens at the cellular level.
What Is a Cell?
A cell is the smallest unit of life that can carry out all the basic functions of a living organism. Most cells are microscopic and can only be seen using a microscope.
There are two broad categories of cells:
- Prokaryotic cells — simple cells with no nucleus. Bacteria are prokaryotes.
- Eukaryotic cells — complex cells with a nucleus. Animal and plant cells are eukaryotes.
At Year 7, the focus is on eukaryotic cells — specifically animal cells and plant cells.
Animal Cell Structure
An animal cell is a eukaryotic cell that makes up the tissues and organs of animals, including humans. Animal cells do not have a fixed shape — they vary depending on their function.
Parts of an Animal Cell and Their Functions
1. Cell Membrane: A thin, flexible layer that surrounds the entire cell. It controls what enters and leaves the cell — allowing useful substances like glucose and oxygen in, and waste products like carbon dioxide out. It is selectively permeable, meaning it allows some substances through but not others.
2. Nucleus: The control centre of the cell. It contains DNA — the genetic information that controls all cell activities and is passed on during reproduction. The nucleus directs the cell to make proteins and tells the cell when to divide.
3. Cytoplasm: A jelly-like fluid that fills the cell and surrounds all the other structures. Most chemical reactions that keep the cell alive take place in the cytoplasm.
4. Mitochondria (singular: mitochondrion): The powerhouse of the cell. Mitochondria are where aerobic respiration takes place — the process that releases energy from glucose. Cells that need a lot of energy (such as muscle cells) have large numbers of mitochondria.
5. Ribosomes: Tiny structures found in the cytoplasm where protein synthesis takes place. Every protein the cell needs (including enzymes and hormones) is made at a ribosome. They are too small to see with a light microscope.
Animal Cell — Summary Table
Plant Cell Structure
A plant cell is a eukaryotic cell found in plants. Plant cells contain all the structures found in animal cells, plus three additional structures that animal cells do not have.
Parts of a Plant Cell and Their Functions
1. Cell Membrane: It controls what enters and leaves the cell.
2. Nucleus: It contains DNA and controls cell activities.
3. Cytoplasm: The is the site of chemical reactions.
4. Mitochondria: This is the site of aerobic respiration and energy release.
5. Ribosomes: This is the site of protein synthesis.
6. Cell Wall (plant cells only): A rigid outer layer made of cellulose that surrounds the cell membrane. It provides strength and support to the cell and helps the plant stay upright. Unlike the cell membrane, the cell wall is fully permeable — it allows all substances to pass through freely.
7. Chloroplasts (plant cells only): Green structures found in the cytoplasm of plant cells that are exposed to light. They contain a green pigment called chlorophyll which absorbs light energy. Chloroplasts are where photosynthesis takes place — the process by which plants make their own food using light, water and carbon dioxide.
8. Permanent Vacuole (plant cells only): A large, fluid-filled sac in the centre of the cell filled with cell sap — a mixture of water, sugars, and salts. The vacuole helps maintain the shape and firmness of the cell by keeping it turgid (firm). Animal cells may have small, temporary vacuoles but never a large permanent one.
Plant Cell — Summary Table
Comparison Table: Animal Cell vs Plant Cell
What Is a Specialised Cell?
Most cells in your body are not the generic animal cell you see in textbooks. They are specialised cells — cells that have developed a specific structure perfectly adapted to carry out a particular function. The process by which cells become specialised is called differentiation.
Specialised cells have:
- A particular shape suited to their job
- An unusual number of certain organelles (e.g. more mitochondria where more energy is needed)
- Adaptations — specific features that make them better at their function
Key Specialised Animal Cells
1. Red Blood Cells (Erythrocytes)
Function: Carry oxygen from the lungs to every cell in the body.
Adaptations:
- Biconcave disc shape — flattened with a dip on both sides. This increases the surface area for absorbing and releasing oxygen.
- No nucleus — the nucleus has been removed to make more room for haemoglobin, maximising the amount of oxygen the cell can carry.
- Packed with haemoglobin — the red protein that binds to oxygen to form oxyhaemoglobin.
- Small and flexible — can squeeze through the narrowest blood vessels (capillaries).
2. White Blood Cells (Leukocytes)
Function: Defend the body against infection and disease.
Adaptations:
- Large, irregular nucleus — allows the cell to engulf pathogens (bacteria and viruses) in a process called phagocytosis.
- Can change shape — to squeeze through the walls of blood vessels and move towards infection sites.
- Produce antibodies — proteins that target and destroy specific pathogens.
- No fixed shape — flexibility is essential for their defensive role.
3. Sperm Cells
Function: Carry the male's genetic information to the egg cell during fertilisation.
Adaptations:
- Streamlined head — contains the nucleus with the father's DNA and an acrosome (enzyme store) that helps the sperm penetrate the egg.
- Long tail (flagellum) — whips from side to side to propel the sperm towards the egg.
- Many mitochondria — packed into the middle section to provide energy for the long journey to the egg.
- Small and lightweight — produced in enormous numbers (millions per ejaculation) to increase the chance of fertilisation.
4. Egg Cells (Ova)
Function: Carry the female's genetic information and, after fertilisation, develop into a new organism.
Adaptations:
- Large size — the largest human cell. Contains a large store of nutrients (yolk) to feed the developing embryo in the early stages.
- Nucleus contains the mother's DNA — half the genetic information of the future organism.
- Zona pellucida — a protective jelly coat around the egg that hardens immediately after one sperm enters, preventing any other sperm from getting in.
- Cell membrane changes after fertilisation — becomes impermeable to prevent polyspermy (fertilisation by more than one sperm).
5. Ciliated Epithelial Cells
Function: Line the airways (trachea and bronchi) and move mucus and trapped particles away from the lungs.
Adaptations:
- Covered in cilia — tiny hair-like projections that beat rhythmically in coordinated waves to sweep mucus (containing trapped dust, bacteria, and other particles) upwards towards the throat where it can be swallowed.
- Many mitochondria — to provide energy for the constant beating of the cilia.
6. Muscle Cells
Function: Contract to produce movement in the body.
Adaptations:
- Long and cylindrical — can contract (shorten) and relax repeatedly.
- Contain contractile proteins — actin and myosin filaments that slide past each other to cause contraction.
- Very large numbers of mitochondria — muscle contraction requires enormous amounts of energy, particularly during exercise.
- Can store glycogen — an energy reserve that can be quickly converted to glucose when needed.
7. Nerve Cells (Neurones)
Function: Carry electrical impulses rapidly around the body.
Adaptations:
- Very long — some neurones stretch from the spinal cord to the tip of the toe, allowing signals to travel long distances without interruption.
- Myelin sheath — a fatty insulating layer around the axon that speeds up the transmission of electrical impulses.
- Dendrites — branching extensions at one end that receive signals from other neurones.
- Axon terminals — at the other end, release neurotransmitter chemicals to pass signals to the next neurone across a synapse.
Key Specialised Plant Cells
1. Root Hair Cells
Function: Absorb water and mineral ions from the soil.
Adaptations:
- Long, thin extension (root hair) — greatly increases the surface area in contact with the soil, maximising absorption.
- No chloroplasts — root hair cells are underground and receive no light, so photosynthesis does not occur here.
- Large permanent vacuole — helps maintain the concentration gradient that draws water into the cell by osmosis.
- Many mitochondria — to provide energy for the active uptake of mineral ions against the concentration gradient.
2. Palisade Mesophyll Cells
Function: The main site of photosynthesis in a leaf.
Adaptations:
- Packed with chloroplasts — more than any other cell type, maximising the amount of light absorbed for photosynthesis.
- Column-shaped and tightly packed — positioned at the top of the leaf to receive the maximum amount of sunlight.
- Chloroplasts can move — within the cell to position themselves where light is most intense.
- Large surface area to volume ratio — allows rapid absorption of carbon dioxide for photosynthesis.
3. Guard Cells
Function: Control the opening and closing of stomata (tiny pores in the leaf surface) to regulate gas exchange and water loss.
Adaptations:
- Kidney-shaped — two guard cells surround each stoma. When they absorb water and become turgid, they curve and the stoma opens. When they lose water and become flaccid, the stoma closes.
- Contain chloroplasts — unlike most epidermal cells, guard cells can photosynthesise to produce the glucose needed for osmotic changes.
- Unevenly thickened cell walls — the inner wall is thicker than the outer wall, causing the characteristic bending shape when turgid.
Microscopy — How We See Cells
A microscope is a scientific instrument used to magnify objects that are too small to be seen clearly with the naked eye.
Cells are too small to see with the naked eye. Scientists use microscopes to observe them.
Light microscope:
- Uses visible light and glass lenses to magnify objects
- Can magnify up to approximately ×1500
- Sufficient to see cell structures like the nucleus, cell wall, vacuole, and chloroplasts
- Used in school laboratories
Electron microscope:
- Uses beams of electrons instead of light
- Can magnify up to ×2,000,000
- Reveals fine detail of structures like mitochondria, ribosomes, and cell membranes
- Used in research laboratories — too large and expensive for schools
Magnification Formula
Magnification = Image size ÷ Actual size
Or rearranged:
Actual size = Image size ÷ Magnification
Example 1:
A cell appears 30mm long under a microscope with a magnification of ×100. What is the actual size of the cell?
Actual size = 30mm ÷ 100 = 0.3mm
Example 2:
A cell is viewed under a microscope with a magnification of ×400. The image of the cell measures 80mm. What is the actual size of the cell?
Actual size = Image size ÷ Magnification
Actual size = 80 ÷ 400
Actual size = 0.2mm
Example 3:
A student draws a cell that measures 60mm in her notebook. The actual size of the cell is 0.03mm. What magnification was the microscope set to?
Magnification = Image size ÷ Actual size
Magnification = 60 ÷ 0.03
Magnification = ×2000
Common Mistakes to Avoid
① Saying plant cells have a cell wall instead of a cell membrane — plant cells have both. The cell wall is outside the cell membrane, not a replacement for it.
② Confusing the cell wall and cell membrane — the cell wall is rigid and made of cellulose; the cell membrane is thin and flexible and controls what enters and leaves.
③ Saying all plant cells have chloroplasts — only cells in the green parts of the plant that are exposed to light contain chloroplasts. Root cells, for example, have no chloroplasts.
④ Forgetting that red blood cells have no nucleus — this is one of the most common exam questions. The nucleus was removed to make room for more haemoglobin.
⑤ Confusing mitochondria and chloroplasts — mitochondria release energy through respiration; chloroplasts capture energy through photosynthesis. They do opposite jobs.
⑥ Describing the nucleus as where energy is produced — energy is produced in the mitochondria, not the nucleus. The nucleus contains DNA and controls cell activities.
Frequently Asked Questions
Q1. What is the difference between a cell, a tissue, an organ and an organ system?
A1: These are the four levels of organisation in a multicellular organism. A cell is the basic unit of life. A group of similar cells working together forms a tissue (e.g. muscle tissue). A group of different tissues working together forms an organ (e.g. the heart). A group of organs working together to perform a major bodily function forms an organ system (e.g. the circulatory system). The organism is the complete living thing made up of all its organ systems.
Q2. Why do some cells have more mitochondria than others?
A2: Mitochondria produce energy through aerobic respiration. Cells that require large amounts of energy have more mitochondria to meet that demand. Muscle cells, sperm cells, root hair cells (for active transport), and ciliated epithelial cells all have high numbers of mitochondria because their functions are energy-intensive. A cell like a red blood cell, which has no nucleus and a simple function, has far fewer mitochondria.
Q3. What is differentiation and why does it matter?
A3: Differentiation is the process by which a cell becomes specialised for a particular function. All cells in your body contain exactly the same DNA — the same genetic instructions. Differentiation determines which genes are switched on in each cell, causing it to develop a specific shape and set of organelles suited to its job. Differentiation happens during the development of an embryo and is responsible for the fact that a muscle cell looks and behaves completely differently from a nerve cell, even though both contain the same DNA.
Q4. Do all plant cells contain chloroplasts?
A4: No. Only plant cells that are found in green parts of the plant and are exposed to light contain chloroplasts. Cells in roots, for example, receive no light and therefore have no chloroplasts — photosynthesis cannot occur there. Similarly, cells in the centre of a thick stem may have few or no chloroplasts. Chloroplasts are only found where there is sufficient light for photosynthesis to be worthwhile.
Q5. What is the difference between a light microscope and an electron microscope?
A5: A light microscope uses visible light and glass lenses to magnify specimens. It can magnify up to about ×1500 and is used in school laboratories. It allows you to see large cell structures such as the nucleus, cell wall, vacuole, and chloroplasts. An electron microscope uses beams of electrons instead of light and can magnify up to ×2,000,000. It reveals the fine detail of structures too small to see with a light microscope — such as ribosomes, the detailed structure of mitochondria, and the cell membrane. Electron microscopes are large, expensive, and found only in research settings.
Q6. Why are cells so small?
Cells need to exchange substances with their surroundings — taking in oxygen and glucose, and removing carbon dioxide and waste products. The smaller a cell is, the larger its surface area to volume ratio — meaning it can exchange substances more efficiently relative to its size. If cells were much larger, the centre of the cell would be too far from the surface for substances to diffuse in and out quickly enough to sustain life.
