A human body is made of many different types of cell. A nerve cell looks nothing like a muscle cell, because they do very different jobs. A cell that is built to carry out one particular job is called a specialised cell.
The key idea in this lesson is simple: structure matches function. The shape of a cell and the parts inside it help it to do its job well. In the exam you may be given a picture or some information about an unfamiliar cell. You will need to explain how its structure helps its function.
Key terms:
Job: to carry the male's genetic information to the egg cell.
It has a long tail to swim to the egg. Its streamlined head helps it to move through fluid. It has lots of mitochondria to release the energy needed for swimming. The head also holds enzymes that break down the outside of the egg cell so that the sperm can get in.
Job: to carry electrical impulses from one part of the body to another.
It is very long, so one cell can carry a message over a large distance. It has branched connections at its ends, which let it link up with many other nerve cells and with muscles.
Job: to contract (get shorter) and so move parts of the body.
It contains special protein fibres that slide over each other to make the cell shorter. It has lots of mitochondria to release the energy needed for contraction.
Notice that two of these cells, sperm and muscle, have lots of mitochondria. Both do energetic work, so both need a large supply of energy. Remember, mitochondria are where aerobic respiration takes place.
A good answer has two parts: the feature, then why it helps. For example: "A nerve cell is long, so it can carry an impulse over a large distance." Never just list the features without saying what they do.
Job: to take up water and mineral ions from the soil.
It has a long, thin extension that sticks out into the soil. This gives the cell a very large surface area, so more water and mineral ions can be absorbed.
Job: to carry water and mineral ions up the plant.
The cells are joined end to end to make long, hollow tubes, so water can flow through easily. The walls are strong, which holds the tubes open. Xylem cells are dead when they do their job.
Job: to carry dissolved sugars around the plant.
The cells are joined end to end to make tubes. Small holes in the end walls let the dissolved sugars flow through from cell to cell.
You will see how these cells work in the whole plant in the lesson on transpiration and translocation. For now, just link each cell's structure to its job.
Question: Root hair cells have no chloroplasts. Why do they not need them?
Answer: Root hair cells are underground, where there is no light, so they cannot photosynthesise. Their job is to take up water and mineral ions, not to make food.
An organism begins life as just one cell. As the organism develops, its cells differentiate to form different types of cell. As a cell differentiates, it gains different sub-cellular structures. These let it carry out a certain function. Once it has changed, it has become a specialised cell.
For example, a muscle cell gains the protein fibres it needs to contract, and it gains lots of mitochondria. A cell that has differentiated like this has the structures for one job.
Most types of animal cell differentiate at an early stage. In a mature animal, cell division is mainly restricted to repair and replacement. For example, new cells are made to replace dead cells or to heal a cut.
Many types of plant cell keep the ability to differentiate throughout life. This means a plant can keep making new types of cell as it grows, not just when it is young.
Differentiation is important because it lets an organism have cells with different jobs. Each type of specialised cell does its job better than a general cell could. Working together, the cells form tissues, organs and organ systems, and the whole organism can do many more things than a single cell can.
Writing that a cell "needs" a feature, such as "the sperm needs a tail". Instead, say what the feature does: "the tail lets the sperm swim to the egg".
Saying that animal cells never divide once the animal is mature. They do divide, but mainly for repair and replacement.
Mixing up the plant tubes. Xylem carries water and mineral ions. Phloem carries dissolved sugars.
(a) Name the process by which a cell becomes specialised. [1 mark]
(b) Explain how the structure of a sperm cell helps it to carry out its function. [3 marks]
(c) Describe when cells differentiate in most animals and in many plants. [2 marks]
(a) Differentiation. (1)
(b) It has a tail, which lets it swim to the egg cell (1). It has lots of mitochondria, which release the energy needed for swimming (1). It has enzymes in its head, which break down the outside of the egg so it can get in (1). (Other correct answer: a streamlined head helps it move through fluid.)
(c) Most animal cells differentiate at an early stage (1). Many plant cells keep the ability to differentiate throughout life (1).
In "explain how the structure helps" questions, link every feature to its job with a word such as "so" or "which". Aim for one feature plus one reason for each mark.
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