A good structure-function answer has three parts: the feature, what it does, and the effect on the process. Questions on this appear as “suggest how this cell is adapted” or “explain why this tissue is suited to its job”, often with an unfamiliar diagram to look at.
It pulls together the tissues from distinguishing xylem and phloem and the route from water movement from roots to leaves. It belongs to plant transport.
Which features link to which roles?
| Structure | Feature | What it does | Effect on transport |
|---|---|---|---|
| Root hair cell | Long, thin extension | Increases surface area | More water and ions absorbed |
| Root hair cell | Thin cell wall | Shortens the distance | Faster uptake of water |
| Xylem vessel | Hollow, no end walls | Forms a continuous tube | Water flows with little resistance |
| Xylem vessel | Lignin in the walls | Strengthens and waterproofs | Does not collapse under the pull |
| Phloem sieve tube | Sieve plates with pores | Let contents pass | Sucrose moves from cell to cell |
| Phloem companion cell | Many mitochondria | Release energy | Sucrose is loaded into the sieve tube |
| Leaf | Stomata with guard cells | Open and close the pores | Controls water vapour loss and gas entry |
| Leaf mesophyll | Air spaces, wet cell walls | Give a moist surface | Evaporation and diffusion of vapour |
How do you build the answer?
Ask three questions about any feature you are shown.
- What is the feature? Name it precisely, such as “long extension” rather than “big”.
- What does it change physically? Surface area, distance, resistance, strength or energy supply.
- So what for the process? Say whether more water is absorbed, vapour leaves faster or sucrose moves more easily.
Worked example
The data are invented for teaching. A 10 mm length of root has a surface area of about 30 mm² without root hairs and about 180 mm² with root hairs. Explain how root hair cells are adapted for absorbing water, and compare the surface areas.
Step 1, the feature: each root hair cell has a long, thin extension into the soil, and the wall is thin.
Step 2, what it changes: the extensions increase the surface area in contact with the soil water. The thin wall and membrane give a short distance for water to cross.
Step 3, the effect: more water moves in by osmosis in a given time.
Step 4, the comparison: 180 ÷ 30 = 6, so root hairs give a surface area six times as large.
Check: 30 × 6 = 180, so the multiple is right.
The mistake to watch for
Students often stop at the feature or use a vague word such as “helps”.
Mistaken answer: Xylem vessels have lignin, which helps them carry water.
There is no link between the feature and the job. “Helps” does not tell the examiner what the lignin actually does.
The correction is to name the physical change and the effect. Lignin makes the walls strong and waterproof, so the vessels do not collapse under the pull. Water can then keep moving up the stem.
Check yourself
Try these first, then open each answer.
1. Explain why xylem vessels have no end walls between cells.
Show answer
Without end walls, the vessel forms a continuous hollow tube. There is little resistance, so water can flow up easily in a continuous column.
2. Suggest why phloem companion cells contain many mitochondria.
Show answer
Mitochondria release energy in respiration. This energy is needed to load sucrose into the sieve tubes, so the companion cells support the transport of sucrose.
3. Stomata have guard cells on each side. State the function of this arrangement and the effect on water loss.
Show answer
The guard cells open and close the stomatal pore. When they are open, water vapour can diffuse out, and when they are closed, less water is lost.
Where this leads next
Try the plant transport practice set, which mixes pathway, conditions, data, tissues and structure in one paper-style set. Return to the module overview if a lesson needs a second read.
A structure-function answer is a short reasoning chain, and the gaps are hard to see in your own writing. Online one-to-one Biology tuition gives you a teacher who can point out exactly which link is missing.