A specialised cell is built for one job, and a full answer states each feature together with the advantage it gives. The same thinking extends upward to tissues, organs and systems.
This lesson is part of cells and organisation. It uses the structure-function sentences from relating organelles to functions.
How do I explain why a cell suits its job?
Name the feature, then join it to the advantage with “so”. This turns a description into an explanation, which is what “explain” asks for.
Here is a worked example with a cell you may not have in your notes. A guard cell in a leaf has unevenly thickened walls and many chloroplasts.
- Feature: the inner wall is thicker than the outer wall. Advantage: when water enters the cell, the thin outer wall stretches more, so the pair bends apart and opens the stoma.
- Feature: chloroplasts are present. Advantage: the cell can photosynthesise, so it can produce solutes that draw in water.
What is the ladder from cell to system?
Cells of one type form a tissue, tissues form an organ, and organs working towards one function form a system. Each level does something the level below cannot.
| Level | Example in the leaf | Example in a human |
|---|---|---|
| Cell | Palisade cell | Red blood cell |
| Tissue | Palisade mesophyll | Cardiac muscle |
| Organ | Leaf | Heart |
| System | Shoot system | Circulatory system |
Worked example: a small intestine villus
Question. A villus in the small intestine has a wall one cell thick and a network of blood capillaries. Explain how the villus is suited to absorbing digested food. (3 marks)
The wall is one cell thick, so digested food has a short distance to diffuse into the blood. The capillary network carries the absorbed food away, so the concentration gradient is maintained. The surface is covered with projections, so the surface area for absorption is large.
Each sentence pairs a feature with its effect, so three features earn three marks.
The mistake that costs marks
A frequent slip is to write a feature only, such as “the villus has capillaries”. That is a description, and the marker will not give the mark for “explain”.
Another slip is to confuse tissue and organ. If your example contains more than one kind of tissue, it is an organ. Muscle alone is a tissue, while the stomach is an organ.
Check yourself
A red blood cell has no nucleus and a biconcave shape. Explain how each feature helps the cell carry oxygen. (2 marks)
Answer
With no nucleus, there is more space inside the cell, so more haemoglobin can be carried and more oxygen transported.
The biconcave shape gives a larger surface area compared with its volume, so oxygen can diffuse in and out faster.
What to study next
Turn to numbers in calculating image magnification with consistent units, then try the chapter practice set. If you would like a teacher to check your feature-and-advantage writing, see online one-to-one Biology tuition.