A guided tour of the human cell: membrane, nucleus, organelles, protein production, energy, transport, division and specialised cell types.
Your body is built from tens of trillions of cells, and every one of them is a tiny, busy, self-regulating unit. This guide takes you inside a human cell: its boundary, its control centre, its factories, its power stations and its transport system. You will also learn how cells make proteins, move materials, copy themselves and specialise for different jobs.
Written for students up to O-Level (grades 8 to 10), and useful for parents and teachers. This is general education, not medical advice.
A cell is the smallest unit of an organism that can carry out the processes of life: it takes in materials, releases energy, makes new substances, responds to signals and can reproduce. The adult human body contains tens of trillions of cells, and they come in hundreds of different types.
Cells are grouped into tissues (similar cells doing one job), tissues form organs (such as the heart), and organs work together in systems (such as the circulatory system). Understanding the cell is therefore the key to understanding every other topic in biology, from digestion to genetics.
Most human cells are between about 10 and 100 micrometres (µm) across. One micrometre is one-thousandth of a millimetre, so you need a microscope to see them. A red blood cell is only about 7 to 8 µm wide, while a mature egg cell is about 0.1 mm, just visible to the naked eye as a speck.
Materials such as oxygen and glucose enter a cell through its surface, but they are used throughout its volume. As a cell gets bigger, its volume grows much faster than its surface area, so the surface can no longer supply the inside quickly enough. Small cells therefore exchange materials efficiently.
Compare two cube-shaped 'cells'. Cell A has sides of 1 unit. Cell B has sides of 2 units.
Doubling the side made the ratio half as large. The bigger cell has less surface for each unit of volume, so it would struggle to get enough oxygen. That is one reason cells stay small, and why cells that need a lot of exchange (such as intestine lining cells) have folded surfaces to increase area.
The plasma membrane (also called the cell surface membrane) is a thin double layer of phospholipid molecules with proteins embedded in it. Each phospholipid has a water-loving (hydrophilic) head and two water-hating (hydrophobic) tails. In water the tails face inwards, away from water, and the heads face outwards, forming the bilayer.
The membrane is partially permeable (selectively permeable): small uncharged molecules such as oxygen and carbon dioxide pass through easily, while ions and large molecules need help from protein channels or carriers. Other membrane proteins are receptors that detect signals such as hormones, and some help cells recognise and stick to each other. Animal cell membranes also contain cholesterol, which helps keep the membrane flexible but stable.
| Way of crossing | Energy from ATP needed? | What moves and why |
|---|---|---|
| Diffusion | No | Small molecules (oxygen, carbon dioxide) move from high to low concentration. |
| Facilitated diffusion | No | Larger or charged substances move down a concentration gradient through protein channels or carriers. |
| Osmosis | No | Water moves across the membrane toward the more concentrated solution. |
| Active transport | Yes | Carrier proteins pump substances against their gradient, for example sodium and potassium ions. |
| Endocytosis / exocytosis | Yes | Large particles or packages are taken in or released using vesicles. |
A red blood cell is placed in a salt solution that is much more concentrated than the liquid inside the cell. What happens?
Reverse the situation (pure water outside) and water enters, so the cell swells and may burst. This is why medical fluids given into the blood are made to match the body's own salt level (about 0.9% salt solution is the usual match for blood).
The cytoplasm is everything inside the membrane except the nucleus. Its watery part, the cytosol, contains dissolved salts, sugars, amino acids and enzymes; many chemical reactions, including the first stage of respiration, happen here. Floating in it are the organelles.
Running through the cytoplasm is the cytoskeleton, a network of protein fibres. It holds the cell's shape, anchors organelles, acts like a set of tracks for moving vesicles, and helps pull chromosomes apart during cell division. It is also what lets some cells, such as white blood cells, crawl.
The nucleus is surrounded by a double membrane called the nuclear envelope, which has small holes (nuclear pores) that let messenger molecules pass. Inside, DNA is wound around proteins to form chromatin, which condenses into visible chromosomes when the cell divides. Human body cells contain 46 chromosomes (23 pairs); egg and sperm cells contain 23.
A gene is a section of DNA that carries instructions for making a particular protein or functional RNA. Almost every cell in your body has the same set of genes, but each cell type switches on only the genes it needs. This is why a muscle cell and a nerve cell look and behave so differently. The dense spot inside the nucleus, the nucleolus, makes parts of ribosomes.
Proteins do most of the work in a cell: enzymes, channels, receptors, hormones and structural fibres are all proteins. The pathway below is one of the most important in biology.
The smooth ER has no ribosomes. It makes lipids, stores calcium ions in muscle cells and, in liver cells, helps break down some drugs and poisons.
Insulin is a protein hormone made by beta cells in the pancreas. Follow one insulin molecule:
Pancreatic beta cells therefore have a lot of rough ER and Golgi, which is a good clue when you identify a cell under a microscope.
Mitochondria (singular: mitochondrion) are the main sites of aerobic respiration, the process that uses oxygen to release energy from glucose:
glucose + oxygen → carbon dioxide + water + energy (transferred to ATP)
The first stage, glycolysis, takes place in the cytosol. Most of the ATP is made in mitochondria. Each mitochondrion has a smooth outer membrane and a folded inner membrane (the folds are called cristae), which increases the area available for ATP production. Mitochondria also contain a small amount of their own DNA and can divide independently.
ATP (adenosine triphosphate) is the cell's immediate energy currency: when it is broken down, the energy released powers muscle contraction, active transport, building molecules and other work. Cells that use a lot of energy, such as heart muscle cells, contain very large numbers of mitochondria. Mature red blood cells contain none, so they cannot use oxygen for their own energy and rely on glucose breakdown without oxygen.
Lysosomes are small sacs filled with acidic digestive enzymes. They break down worn-out organelles, food particles and bacteria taken in by white blood cells. Peroxisomes are another type of small sac; they contain enzymes that break down some fats and neutralise hydrogen peroxide, a harmful by-product of some reactions. Breaking things down inside enclosed sacs keeps the digestive enzymes from damaging the rest of the cell.
Cells constantly communicate. A signalling molecule, such as a hormone or a neurotransmitter, binds to a matching receptor protein, like a key fitting a lock. The binding triggers a chain of changes inside the cell, which can change enzyme activity, open channels or switch genes on or off. Because each step can activate many molecules in the next, a tiny signal can cause a large response.
Cells in contact can also pass small molecules directly through protein tunnels called gap junctions, which is important in heart muscle where cells must contract together.
Cells that divide pass through a repeating series of stages called the cell cycle. In interphase (G1, S and G2) the cell grows and copies its DNA; then mitosis shares the copies equally between two new nuclei, and the cytoplasm divides (cytokinesis). The result is two genetically identical daughter cells. Mitosis is used for growth and repair, for example healing a cut.
At checkpoints, control proteins check that the DNA is undamaged and fully copied before allowing the cell to continue. Cancer happens when changes in the controlling genes allow cells to divide when they should not. Not all cells keep dividing: many nerve cells and most muscle cells divide very rarely in adults.
Specialised cells have features (adaptations) that suit their function. To answer exam questions, always link a feature to its job.
| Cell | Adaptation | Why it helps |
|---|---|---|
| Red blood cell | Biconcave disc, no nucleus, full of haemoglobin | More space for haemoglobin; large surface area for gas exchange; flexible enough to squeeze through capillaries |
| Neuron (nerve cell) | Long fibre (axon) and many branching endings | Carries electrical signals over long distances and connects with many other cells |
| Skeletal muscle cell | Contractile protein filaments, many mitochondria | Shortens to produce movement; mitochondria supply ATP |
| Sperm cell | Tail, many mitochondria, enzyme-filled cap | Swims to the egg; mitochondria give energy; enzymes help it enter the egg |
| Ciliated cell (airways) | Tiny hair-like cilia | Sweep mucus and trapped particles away from the lungs |
| White blood cell | Flexible shape, many lysosomes | Squeezes out of vessels and engulfs and digests microbes |
Blood tests count cells because their numbers and shapes carry information about health. For example, red blood cells that are too few carry less oxygen. Doctors, not websites, interpret such results; the point here is that cell structure links directly to body function.
Cells are studied with microscopes. Magnification tells you how many times bigger an image is than the real object:
magnification = image size ÷ actual size (use the same units for both)
A student draws a cheek cell. The drawing is 40 mm wide, and the magnification is ×800. What is the real width of the cell in micrometres?
Check: 50 µm is within the usual 10 to 100 µm range for human cells, so the answer is reasonable. Always check that your answer is sensible.
Cells are part of a hierarchy: cell → tissue → organ → organ system → organism. For example, cardiac muscle cells form cardiac muscle tissue; this tissue, with connective tissue, blood vessels and nerves, forms the heart; the heart, blood and vessels make up the circulatory system. The whole is able to do things (such as pumping blood around the body) that no single cell can do alone.
| Term | Meaning |
|---|---|
| Cell | Smallest unit of life; basic building block of the body |
| Organelle | A specialised structure inside a cell with a particular job |
| Partially permeable | Allowing some substances through but not others |
| Osmosis | Net movement of water across a partially permeable membrane toward the more concentrated solution |
| Gene | A section of DNA that codes for a protein or functional RNA |
| Ribosome | Tiny structure that builds proteins from amino acids |
| ATP | Molecule that carries usable energy for cell work |
| Mitosis | Division that gives two genetically identical cells |
| Specialised cell | A cell adapted in structure for a particular function |
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