Key Stage 4 science · Biology – Cell biology

GCSE biology: cells, stem cells and biological molecules

Cell biology is the foundation of GCSE biology. Students learn that cells are the basic building blocks of all living things, that animal, plant and bacterial cells have different structures, that specialised cells are adapted to do particular jobs, that stem cells and plant meristems can make new cells of many kinds, and that living things are built from four key kinds of biological molecule.

Cells, stem cells and biological molecules, explained

All living organisms are made of cells, and the cell is the smallest unit that can carry out the processes of life. Cells fall into two types. Eukaryotic cells, found in animals, plants, fungi and protists, have a nucleus that holds the genetic material. Prokaryotic cells, which at GCSE means bacteria, are much smaller and have no nucleus. Animal and plant cells share a nucleus, cytoplasm where chemical reactions happen, a cell membrane that controls what enters and leaves, mitochondria that release energy by respiration, and ribosomes that make proteins. Plant cells also have a cell wall made of cellulose, a permanent vacuole filled with cell sap, and, in the green parts, chloroplasts that carry out photosynthesis. Bacterial cells have cytoplasm, a cell membrane, a cell wall made of a different material from a plant’s, ribosomes, and DNA that floats freely in the cytoplasm as a single loop, often with small extra rings called plasmids.

Many cells are adapted, or specialised, for a particular function, and students must be able to link structure to function. A sperm cell has a tail to swim, many mitochondria for energy and a head carrying the DNA. A nerve cell is long and has branched endings so that it can carry electrical impulses to many other cells. A muscle cell contains protein fibres that contract and many mitochondria. A red blood cell has no nucleus and a biconcave shape, which leaves room for haemoglobin to carry oxygen and gives a large surface area. A root hair cell has a long thin extension to absorb water and mineral ions from the soil. Specialised cells in one body almost all contain the same genes: they are different because different genes are active.

Stem cells are unspecialised cells that can divide and then turn into different types of cell, in a process called differentiation. In animals, embryonic stem cells can become almost any type of cell, while adult stem cells, such as those in bone marrow, can form a more limited range, including blood cells. Stem cells are used in medicine, for instance in bone marrow transplants, and are being researched as a way to repair damaged tissues. They also raise ethical questions, especially when they come from embryos, and students are expected to put forward arguments on both sides. In plants, the same job is done by meristems, regions at the tips of shoots and roots where cells keep dividing and can become any type of plant cell throughout the plant’s life. Meristem tissue is used to grow many identical copies of a plant, for example to protect rare species or to grow crops with useful features.

The final strand is the four key biological molecules. Carbohydrates, such as sugars and starch, are made of carbon, hydrogen and oxygen and provide energy and storage. Proteins are long chains of amino acids that form structures such as muscle, and also enzymes and many hormones. Nucleic acids, DNA and RNA, carry and use the genetic code. Lipids, which are fats and oils, store energy, insulate and form the cell membrane. At GCSE students are expected to describe these four groups and their jobs, and many schools go on to explain the building blocks of each: sugars, amino acids, nucleotides, and fatty acids with glycerol.

Teachers use microscope work, labelled diagrams and comparison tables to teach this topic. Students practise drawing cells carefully, using a sharp pencil and clear label lines, and calculating magnification and the real size of a cell from an image. Expect questions that begin “explain how this cell is adapted to its function”, which are answered with a feature, a link and a function.

Words your child will hear

Eukaryotic cell: A cell with a nucleus, as in animals, plants and fungi.
Prokaryotic cell: A cell without a nucleus, such as a bacterium. Its DNA lies free in the cytoplasm.
Differentiation: A cell becoming specialised to do a particular job.
Stem cell: An unspecialised cell that can divide and turn into other types of cell.
Meristem: A region in a plant where cells keep dividing and can become any type of plant cell.
Mitochondria: The parts of a cell where energy is released by respiration.
Plasmid: A small extra ring of DNA found in many bacteria.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about cells as the basic structural unit of all organisms; adaptations of cells related to their functions; the main sub-cellular structures of eukaryotic and prokaryotic cells
Students should be taught about stem cells in animals and meristems in plants
Students should be taught about carbohydrates, proteins, nucleic acids and lipids as key biological molecules

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A model answer

A root hair cell absorbs water and mineral ions from the soil. Explain how it is adapted to its function.

State the function first. The root hair cell takes in water by osmosis and mineral ions from the soil, so every feature you describe should link back to absorbing water and minerals.
Feature one: a long, thin extension that sticks out into the soil. This gives the cell a large surface area, so more water and mineral ions can enter in a given time.
Feature two: a large permanent vacuole filled with cell sap. The sap has a lower concentration of water than the soil water, so water moves into the cell by osmosis.
Feature three: many mitochondria. They release energy by respiration, which is needed for the active transport of mineral ions into the cell against a concentration difference.
Mention what is missing. It has no chloroplasts, because roots are underground and there is no light for photosynthesis.

The root hair cell has a long thin extension to give a large surface area for absorption, a large vacuole whose sap helps water enter by osmosis, and many mitochondria that supply the energy for active transport of mineral ions. It has no chloroplasts because it is underground.

Common mistakes and how to help

Saying plant cells have chloroplasts instead of mitochondria

Photosynthesis and respiration are often taught as opposites, and the textbook picture of a plant cell highlights chloroplasts.

What helps: Remind students that plants respire all the time, including in the dark and in roots, so every plant cell has mitochondria. Green cells have chloroplasts as well.

Thinking bacteria have no DNA because they have no nucleus

The nucleus is often described as the place where DNA is kept, so cells without one seem to have none.

What helps: Explain that bacterial DNA is there, floating in the cytoplasm as a loop, with plasmids as well. The difference is that it is not enclosed in a nucleus.

Believing that stem cells only come from embryos and can become anything

News stories about stem cells tend to focus on embryos and on dramatic cures.

What helps: Describe three cases: embryonic cells (which can become almost any type), adult stem cells such as those in bone marrow (which form a more limited range) and plant meristems. Ask which kind fits each example.

Thinking specialised cells have different DNA from each other

The cells look and behave so differently that it seems they must have different instructions.

What helps: Explain that almost every cell in the body has the same full set of genes, and differentiation happens because different genes are switched on in different cells. The exceptions are few: sex cells carry half a set, and red blood cells lose their nucleus as they mature.

How to help at home

Look at cells. Once an adult has cut an onion, peel the thin, almost transparent skin from the inside of one layer and look at it under a simple microscope if you have one. You should see a brick-like pattern of cells. A strong magnifying glass may just show the outlines. Ask your teenager what an onion cell has that an animal cell does not (a cell wall), and why it has no chloroplasts when it grows underground.
Grow roots from a cutting. Put a stem cutting of a plant such as mint or busy lizzie in a glass of water on a windowsill, and watch roots appear over a couple of weeks. New roots form from cells that start dividing again, which links to meristems. An adult should cut the stem.
Cell as a factory. Ask your child to compare a cell with a factory or a town, matching the nucleus, mitochondria, ribosomes and membrane to a job. Then ask where the comparison breaks down. Both the matching and the limits are part of good model use.
Sort your food by molecule. Use a nutrition label to sort what is in the fridge into mainly carbohydrate, protein or fat. Ask what each is used for in the body, and which of them is made of amino acids.
Talk about the ethics. Discuss a question such as “should stem cells from embryos be used in medical research?” and ask for one scientific reason and one ethical reason on each side. The aim is balanced reasoning, not agreement.