What you'll learn
Cell biology is the foundation unit of AQA GCSE Combined Science: Trilogy, and everything else in the biology papers builds on it. A cell is the smallest unit of living material, and this unit explains what cells are made of, how they differ between organisms, how they divide, how they become specialised, and how substances move in and out of them. By the end you should be able to name the sub-cellular structures of animal, plant and bacterial cells and state what each one does, calculate magnification and convert between millimetres, micrometres and nanometres, describe the cell cycle and mitosis, explain what stem cells are and argue both sides of their use in medicine, and distinguish clearly between diffusion, osmosis and active transport. Cell biology is assessed on Biology Paper 1, and the required practicals on microscopy and osmosis appear regularly.
Key terms and definitions
Eukaryotic cell — a cell in which the genetic material is enclosed in a nucleus; animal and plant cells are eukaryotic
Prokaryotic cell — a cell with no nucleus, where the genetic material is a single loop of DNA free in the cytoplasm; bacteria are prokaryotic
Plasmid — a small extra ring of DNA found in bacterial cells, separate from the main chromosomal loop
Differentiation — the process by which a cell becomes specialised for a particular function
Stem cell — an undifferentiated cell that can divide to produce more cells of the same type, or differentiate into other cell types
Mitosis — the stage of the cell cycle in which the nucleus divides to produce two genetically identical nuclei
Magnification — how many times larger an image is than the real object; magnification equals size of image divided by size of real object
Resolution — the smallest distance between two points that can still be distinguished as separate
Diffusion — the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient
Osmosis — the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
Active transport — the movement of substances against a concentration gradient, from a dilute to a more concentrated solution, using energy from respiration
Core concepts
Eukaryotic and prokaryotic cells
All living things are made of cells, but they fall into two categories. Eukaryotic cells keep their DNA inside a nucleus and are found in animals, plants and fungi. Prokaryotic cells have no nucleus; bacteria are the example you need. Prokaryotic cells are much smaller, typically around 1 micrometre across compared with 10 to 100 micrometres for a eukaryotic cell.
A bacterial cell has cytoplasm, a cell membrane and a cell wall, but its genetic material is a single loop of chromosomal DNA sitting free in the cytoplasm. It may also carry one or more plasmids, which are small separate rings of DNA. Plasmids matter because they are how bacteria pass on useful genes, including genes for antibiotic resistance, and they are the tool used in genetic engineering.
Animal and plant cells
An animal cell contains a nucleus, which controls the cell's activities and holds the chromosomes; cytoplasm, where most chemical reactions take place; a cell membrane, which controls what enters and leaves; mitochondria, where aerobic respiration releases energy; and ribosomes, where proteins are made.
A plant cell has all of those plus three extra structures. The cell wall, made of cellulose, strengthens the cell and stops it bursting. The permanent vacuole holds cell sap and helps keep the cell firm. Chloroplasts contain chlorophyll and are where photosynthesis occurs. A common exam error is writing that plant cells have a cell wall instead of a cell membrane. They have both, with the membrane inside the wall.
Cell specialisation and differentiation
Most cells in a multicellular organism are specialised. A sperm cell has a tail for swimming, many mitochondria to release the energy for movement, and enzymes in its head to digest through the egg membrane. A nerve cell is long, to carry impulses over distance, with branched endings to connect to other cells. A muscle cell contains protein fibres that shorten, and many mitochondria. A root hair cell has a long extension that increases surface area for absorbing water and mineral ions. A xylem cell is a hollow dead tube with lignin-strengthened walls, carrying water. A phloem cell is a living column with sieve plates between cells, carrying dissolved sugars.
In animals, most cells differentiate early in development and then lose the ability to change. Many plant cells keep the ability to differentiate throughout life, which is why a whole plant can be grown from a cutting.
Microscopy and magnification
Light microscopes use light and lenses and can magnify up to about 2,000 times. Electron microscopes use a beam of electrons and have much higher magnification and much higher resolution, which is why they revealed sub-cellular structures such as ribosomes that light microscopes cannot show.
Magnification is calculated as image size divided by real size, and the same relationship rearranges to give real size equals image size divided by magnification. The arithmetic is straightforward; the marks are usually lost on units. Remember that 1 millimetre is 1,000 micrometres, and 1 micrometre is 1,000 nanometres. Always convert both measurements to the same unit before dividing.
The cell cycle and mitosis
Body cells divide by the cell cycle so that an organism can grow, repair damaged tissue and replace worn-out cells. The cycle has two main stages. First, the cell grows, increases the number of its sub-cellular structures such as mitochondria and ribosomes, and copies its DNA so each chromosome becomes two identical strands. Then mitosis occurs: one set of chromosomes is pulled to each end of the cell and the nucleus divides. Finally the cytoplasm and cell membrane divide, producing two daughter cells identical to the original and to each other.
Because the DNA is copied first and then shared out, both daughter cells receive a full set of chromosomes and are genetically identical. This is the basis of asexual reproduction as well as growth.
Stem cells
Stem cells are undifferentiated. Embryonic stem cells, taken from an early embryo, can differentiate into any type of cell. Adult stem cells, found for example in bone marrow, can form a more limited range, mainly blood cells. Plant stem cells are found in meristem tissue at the tips of roots and shoots and can produce any plant cell type throughout the plant's life.
Medically, stem cells offer possible treatments for conditions such as diabetes and paralysis, and therapeutic cloning could produce cells genetically identical to a patient so they are not rejected. Against that, obtaining embryonic stem cells destroys the embryo, which many people object to on ethical or religious grounds, and there is a risk that transferred cells carry viral infection. In plants, meristem cells are used to produce identical clones of rare species to protect them from extinction, or to mass-produce crop plants with useful characteristics.
Diffusion
Diffusion is the spreading out of the particles of a gas or of a dissolved substance, giving a net movement from a region of higher concentration to a region of lower concentration. It is a passive process: no energy from respiration is needed. Oxygen and carbon dioxide move in and out of cells by diffusion, and urea diffuses from cells into the blood plasma to be excreted.
Three factors increase the rate of diffusion: a steeper concentration gradient, a higher temperature, and a larger surface area of membrane. Exchange surfaces in organisms are adapted to all three, which is why the lungs have millions of alveoli and the small intestine has villi.
Surface area to volume ratio
A single-celled organism has a large surface area compared with its volume, so diffusion alone supplies everything it needs. As an organism gets larger, volume increases faster than surface area, so the ratio falls and diffusion becomes too slow. This is why large organisms need specialised exchange surfaces and a transport system. Effective exchange surfaces share the same features: a large surface area, a thin membrane giving a short diffusion path, and in animals a good blood supply and ventilation to maintain the concentration gradient.
Osmosis and active transport
Osmosis is a special case of diffusion that applies only to water, moving through a partially permeable membrane from a dilute solution to a concentrated one. Put a plant cell in pure water and it takes in water and becomes turgid; the cell wall stops it bursting. Put it in a concentrated sugar solution and it loses water, becoming flaccid and eventually plasmolysed as the membrane pulls away from the wall. An animal cell in pure water has no wall to protect it and will burst.
Active transport moves substances the other way, against the concentration gradient, and therefore requires energy released by respiration. Root hair cells use it to absorb mineral ions from very dilute soil water, and the small intestine uses it to absorb the last of the glucose when the concentration in the gut is already lower than in the blood.
Worked examples
Example 1: Magnification calculation (3 marks)
A cell measures 0.05 mm across. Under a microscope its image measures 30 mm. Calculate the magnification.
Both measurements are already in millimetres, so no conversion is needed. Magnification equals image size divided by real size, which is 30 divided by 0.05, giving 600. The answer is written as times 600, with no units, because magnification is a ratio. If the question had given the real size as 50 micrometres, you would first convert: 50 divided by 1,000 gives 0.05 mm.
Example 2: Explaining osmosis in potato tissue (4 marks)
A cylinder of potato is left in a concentrated sugar solution for an hour and loses mass. Explain why.
The sugar solution is more concentrated than the cell sap inside the potato cells. Water therefore moves out of the cells by osmosis, from the dilute solution inside to the concentrated solution outside, through the partially permeable cell membranes. Losing water decreases the mass of the tissue. Notice that the mark scheme wants the direction stated and the words partially permeable used; writing only that water left the potato earns very little.
Example 3: Comparing transport processes (3 marks)
Give one similarity and two differences between diffusion and active transport.
Both move dissolved substances across cell membranes. However, diffusion moves substances down a concentration gradient while active transport moves them against it; and diffusion is passive whereas active transport requires energy from respiration, so cells carrying it out contain many mitochondria.
Common mistakes and how to avoid them
Students often say a substance moves from high to low concentration without the word net. Particles move in both directions; it is the net movement that is down the gradient. Write net movement and the mark is secure.
Osmosis is frequently described as the movement of water from high concentration to low concentration, which confuses examiners because the water is moving from high water concentration to low. The safest phrasing is from a dilute solution to a concentrated solution.
Another recurring error is claiming that active transport uses energy without saying where the energy comes from. The answer must link it to respiration.
In magnification questions, marks are routinely lost by dividing without converting units first. Convert everything to micrometres or everything to millimetres before you touch the calculator.
Finally, many students describe mitosis as producing cells with half the chromosomes. That is meiosis, which is not part of this unit. Mitosis produces two genetically identical cells with a full set.
Exam technique for "Biology: Cell Biology"
Cell biology questions on Trilogy Biology Paper 1 mix straightforward recall with calculation and practical analysis. When a question asks you to describe a structure's function, name the structure and give the function in the same sentence: mitochondria are the site of aerobic respiration, which releases energy for the cell.
Calculation questions carry marks for method as well as answer, so write the relationship, show the substitution, then give the answer with the unit. If you are asked for real size and you have image size and magnification, rearrange before substituting rather than trying to do it in your head.
For the required practical on osmosis, be ready to describe how you would control variables — same length and diameter of potato cylinder, same volume of solution, same temperature, same time — and to calculate percentage change in mass, which is change in mass divided by starting mass, multiplied by 100. Using percentage change rather than raw change is what makes the cylinders comparable.
Questions on stem cells often ask for a balanced evaluation. Give at least one benefit and one objection, and finish with a short conclusion that refers back to the specific context in the question.
Quick revision summary
Eukaryotic cells have a nucleus; prokaryotic cells such as bacteria do not, and carry plasmids. Plant cells add a cellulose cell wall, permanent vacuole and chloroplasts to the animal cell structures. Cells differentiate to become specialised, and their structure always matches their function. Magnification is image size divided by real size, with units converted first. The cell cycle copies the DNA and then divides the nucleus by mitosis, giving two genetically identical cells. Stem cells are undifferentiated and offer medical possibilities alongside ethical objections. Diffusion is passive movement down a gradient, osmosis is diffusion of water through a partially permeable membrane, and active transport moves substances against the gradient using energy from respiration. As organisms get larger their surface area to volume ratio falls, so they need specialised exchange surfaces with a large area, a short diffusion path and a maintained concentration gradient.