How are cells adapted to their jobs, and how do they build up into a whole organism?
Describe how cells are specialised for their functions and explain how cells are organised into tissues, organs, organ systems and organisms
A scaffolded answer to the N(A)-Level Biology outcome on specialised cells and levels of organisation. Examples such as the red blood cell, root hair cell and nerve cell, and the ladder from cell to tissue to organ to organism.
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What this dot point is asking
This outcome wants two things. First, you should be able to describe how a cell is specialised, which means how its shape and parts are adapted to suit one particular job. Second, you should be able to put the levels of organisation in order, from a single cell up to a whole organism, and give an example of each. The key skill is linking a feature of a cell to the job it does, always saying how the feature helps.
The answer
What specialised cells are
A specialised cell is one whose structure is suited to a particular job. Most cells in a multicellular organism are specialised, which makes the organism more efficient because each cell does its task well. The exam loves three examples.
Three examples to know
- Red blood cell. Its job is to carry oxygen. It has no nucleus, leaving more space for the red pigment haemoglobin that carries oxygen. It has a biconcave (flattened disc) shape, giving a large surface area for oxygen to diffuse in and out quickly.
- Root hair cell. Its job is to absorb water and mineral salts from the soil. It has a long thin extension (the root hair) that gives a large surface area for absorption. It has no chloroplasts, because it is underground.
- Nerve cell (neurone). Its job is to carry electrical signals quickly. It is very long, so it can carry a signal a long way, and it has thin branches at its ends to connect to other cells.
Levels of organisation
Cells do not work alone. They are organised into larger and larger structures:
- Cell. The basic unit of life, for example a muscle cell.
- Tissue. A group of similar cells working together to do one job, for example muscle tissue.
- Organ. Several different tissues working together, for example the heart.
- Organ system. Several organs working together, for example the circulatory system (heart and blood vessels).
- Organism. All the organ systems together make up one whole living thing, for example a human.
Examples in context
Example 1. The leaf as an organ. A leaf is made of several tissues: the upper layers for photosynthesis, the veins (xylem and phloem) for transport, and the lower layer with pores for gas exchange. Because it contains several tissues working together for one overall job, the leaf is an organ, not a tissue.
Example 2. Muscle from cell to system. A single muscle cell can contract a little; muscle tissue is many such cells; a muscle like the biceps is an organ; and the muscles working with the bones make up the muscular and skeletal systems that let you move. The same cell type scales up through every level.
Try this
Q1. State what is meant by a specialised cell. [1 mark]
- Cue. A cell whose structure is adapted (suited) to carry out one particular job.
Q2. Explain one way a nerve cell is adapted to its function. [2 marks]
- Cue. It is very long, so it can carry an electrical signal over a long distance from one part of the body to another.
Q3. Name the level of organisation that is made of several different tissues working together, and give one example. [2 marks]
- Cue. An organ; for example the heart (or a leaf), which is made of muscle, blood and nerve tissues working together.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksA red blood cell carries oxygen around the body. Describe two ways the red blood cell is adapted to this job, and explain how each adaptation helps.Show worked answer →
Adaptation 1: A red blood cell has no nucleus. This leaves more room inside the cell to hold haemoglobin, so it can carry more oxygen.
Adaptation 2: A red blood cell has a biconcave shape (a flattened disc, thinner in the middle). This gives it a large surface area for oxygen to diffuse in and out quickly.
What markers reward: a named adaptation plus an explanation of how it helps the cell do its job. The word because (or so that) links the two. Naming an adaptation with no reason scores only half the marks. Haemoglobin and large surface area are the key terms.
Original4 marksPut these four levels of organisation in order from smallest to largest, then give one example of each: organ, cell, organism, tissue.Show worked answer →
The correct order from smallest to largest is: cell, tissue, organ, organism.
Examples (any correct one each): a cell could be a muscle cell; a tissue could be muscle tissue; an organ could be the heart; an organism could be a human (or any whole living thing).
What markers reward: the four levels in the right order, and one valid example at each level. A common error is to give the heart as a tissue; the heart is an organ because it is made of several tissues working together.
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