Cells are the basic building blocks of all living organisms. In this topic you compare the two main cell types — prokaryotic (such as bacteria) and eukaryotic (animal and plant cells) — and learn what each sub-cellular structure does. You also study how cells become specialised, how scientists use microscopes to see them, how cells divide by mitosis as part of the cell cycle, the role of stem cells, and the three ways substances enter and leave cells: diffusion, osmosis and active transport. A solid grasp of cell structure underpins almost every other topic in AQA GCSE Biology (8461).
Eukaryotic and prokaryotic cells
Cells fall into two broad groups. Eukaryotic cells have a true nucleus enclosed in a membrane and contain membrane-bound organelles such as mitochondria. Animals, plants, fungi and protists are all made of eukaryotic cells. Prokaryotic cells, such as bacteria, are much smaller and do not have a nucleus.
- In a prokaryote the genetic material is a single loop of DNA floating free in the cytoplasm (it is not enclosed in a nucleus).
- Prokaryotes may also carry small extra rings of DNA called plasmids.
- Prokaryotic cells are typically about 1–5 micrometres across, while eukaryotic cells are usually 10–100 micrometres — much larger.
- Both cell types have a cell membrane, cytoplasm and ribosomes; only prokaryotes have plasmids and a single loop of DNA instead of a nucleus.
Animal and plant cells: sub-cellular structures
Most animal cells contain a nucleus (controls the cell and holds the DNA), cytoplasm (where most chemical reactions happen), a cell membrane (controls what enters and leaves), mitochondria (site of aerobic respiration, releasing energy) and ribosomes (where proteins are made).
- Plant cells contain all of those structures and, in addition, a cell wall made of cellulose for strength and support.
- Many plant cells also contain a permanent vacuole filled with cell sap, which helps keep the cell firm (turgid).
- Green plant and algal cells contain chloroplasts, which hold the green pigment chlorophyll and absorb light for photosynthesis.
- Bacterial cells also have a cell wall, but it is not made of cellulose.
Specialised cells and differentiation
As an organism develops, cells become specialised to carry out a particular function. This process is called differentiation. As a cell differentiates it gains different sub-cellular structures that suit its job.
- Sperm cells have a tail (flagellum) for swimming and many mitochondria to release energy for movement.
- Nerve cells are long with branched connections to carry electrical impulses around the body quickly.
- Muscle cells contain many mitochondria and protein fibres that contract.
- Root hair cells have a large surface area to absorb water and mineral ions from the soil.
- Xylem cells form hollow tubes to transport water, while phloem cells form tubes to transport dissolved sugars.
- In animals most differentiation happens at an early stage; in plants many cells keep the ability to differentiate throughout life.
Microscopy and resolution
Microscopes let us see cells and sub-cellular structures. Light microscopes use light and lenses; they are cheap and easy to use but have limited magnification and resolution (the ability to distinguish two close points as separate).
- Electron microscopes use a beam of electrons instead of light. They have much higher magnification and resolution, so we can see sub-cellular structures (such as the internal detail of mitochondria) in far more detail.
- Greater resolution means scientists can study cells in ways that were impossible with light microscopes alone, deepening our understanding of how cells work.
- When drawing or labelling, always include a scale or magnification so the true size can be worked out.
Magnification calculations
Magnification tells you how many times bigger an image is than the real object. The key equation is: magnification = size of image ÷ size of real object. You can rearrange it to find any of the three values.
- Always convert measurements to the same units first. Remember 1 mm = 1000 micrometres (µm).
- Worked example: if a cell measures 50 mm in an image and the real cell is 0.05 mm wide, magnification = 50 ÷ 0.05 = ×1000.
- To find real size: real object = size of image ÷ magnification.
- You should be able to express answers in standard form, for example writing 1000 as 1 × 103.
The cell cycle and mitosis
Body cells divide in a series of stages called the cell cycle. During the cycle the genetic material is copied and then the cell divides by mitosis to produce two genetically identical daughter cells. This is needed for growth, development and the repair of damaged tissue.
- Stage 1 (interphase): the cell grows, increases the number of sub-cellular structures such as ribosomes and mitochondria, and the DNA replicates to form two copies of each chromosome.
- Stage 2 (mitosis): one set of chromosomes is pulled to each end of the cell and the nucleus divides.
- Stage 3 (cytokinesis): the cytoplasm and cell membrane divide to form two identical daughter cells.
- The two daughter cells are genetically identical to each other and to the parent cell.
Stem cells
A stem cell is an undifferentiated cell that can divide to produce more cells of the same type, and which can differentiate into other, specialised cell types.
- Embryonic stem cells can differentiate into almost any kind of cell.
- Adult stem cells are found in places such as bone marrow; they can form many types of cell but are more limited than embryonic ones.
- In plants, stem cells in meristem tissue (in shoot and root tips) can differentiate throughout the plant's life, allowing cloning of plants quickly and cheaply.
- Therapeutic cloning produces an embryo with the same genes as the patient, so stem cells from it are not rejected; they could treat conditions such as diabetes or paralysis.
- Risks and issues include the possible transfer of viral infection and ethical or religious objections to using embryos. Students should be able to evaluate these arguments.
Diffusion and osmosis
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process — it does not need energy from respiration. Oxygen and carbon dioxide move in and out of cells by diffusion, as does urea.
- Diffusion is faster when the concentration gradient is steeper, the temperature is higher, or the surface area of the membrane is larger.
- Osmosis is the diffusion of water across a partially permeable membrane, from a dilute solution (high water concentration) to a more concentrated solution (lower water concentration).
- Single-celled organisms have a large surface area to volume ratio, so simple diffusion meets their needs. Larger organisms need specialised exchange surfaces and transport systems.
- Higher tier: surface area to volume ratio decreases as an organism gets larger, which is why bigger organisms need exchange surfaces with adaptations such as a large surface area, thin membranes, and a good blood supply.
Active transport
Active transport moves substances from a more dilute solution to a more concentrated solution — that is, against a concentration gradient. Because this is the opposite direction to diffusion, it requires energy released by respiration.
- Active transport allows mineral ions to be absorbed into plant root hair cells from very dilute solutions in the soil, where the concentration of ions is higher inside the cell.
- It also lets the gut absorb sugar molecules (such as glucose) from low concentrations in the gut into the blood, which has a higher sugar concentration.
- Cells carrying out a lot of active transport, such as root hair cells, contain many mitochondria to supply the energy needed.