Xylem vs phloem: the contrast table examiners reward
| Xylem | Phloem | |
|---|---|---|
| Carries | Water and mineral ions | Sucrose and amino acids |
| Direction | Upwards only (roots → leaves) | Both directions (source → sink) |
| Cells | Dead, hollow tubes; no cell contents; walls thickened with lignin | Living cells |
| Process name | Transpiration stream | Translocation |
The structural adaptations of xylem are a standing 2-3 marker: no cell contents / dead cells so water flows unimpeded, and lignified walls for strength and support. Say 'sucrose', never 'sugar' or 'glucose', for phloem. Sucrose is the transported form and the only word that scores. In a stem cross-section, xylem sits on the inside of the vascular bundle, phloem on the outside; in a root, xylem forms the central X.
The water pathway: root hair to leaf
The path is a fixed sequence worth learning as one line: root hair cell → root cortex cells → xylem → leaf mesophyll cells → evaporation into air spaces → diffusion out of stomata. Root hair cells appear with their own adaptation question: the long extension gives a large surface area for absorbing water (by osmosis) and mineral ions (by active transport, which is why root cells need many mitochondria). Water moves between cells by osmosis down a water potential gradient; the gradient exists because water is constantly being pulled up and lost at the leaves.
Transpiration: the five-step chain that earns full marks
Define transpiration as the loss of water vapour from leaves: water evaporates from the surfaces of mesophyll cells into air spaces, then water vapour diffuses out through the stomata. Any explanation question is answered with the same chain:
- Water evaporates from mesophyll cell surfaces into the leaf air spaces.
- Water vapour concentration inside the leaf is higher than outside.
- Water vapour diffuses out through open stomata, down the concentration gradient.
- This pulls water up the xylem (the transpiration stream).
- More water is absorbed by the roots to replace it.
Factor questions just bend one link: heat speeds evaporation and diffusion; wind blows vapour away, steepening the gradient; humidity raises the outside concentration, flattening the gradient and slowing transpiration. Always name the gradient. That is where the Extended mark lives.
Potometers and wilting: the practical edge
The potometer measures water uptake, which examiners accept as an estimate of transpiration rate. But the precise mark distinguishes the two (some absorbed water is used in photosynthesis or keeping cells turgid). Standard technique marks: cut the shoot underwater and assemble underwater (prevents air locks), seal joints with grease (prevents leaks), let the plant acclimatise, measure the distance the bubble moves per minute, and repeat. Wilting closes the loop: when water loss exceeds uptake, cells lose turgor pressure, the plant wilts, which closes stomata and reduces further loss. Wilting questions are turgor questions in disguise; link back to osmosis vocabulary (turgid, flaccid) for the marks.