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.
Key terms to connect with this topic
Use these definitions inside explanations and questions rather than memorising them as isolated sentences.
- Phloem: A plant transport tissue that carries dissolved sugars (mainly sucrose) and amino acids from sources (such as leaves where they are made) to sinks (such as roots, fruits and growing tips where they are used or stored). This transport is called translocation.
- Potometer: A piece of apparatus used to measure the rate of water uptake by a cut shoot, which is used as an estimate of the rate of transpiration. It works by tracking the movement of an air bubble along a graduated capillary tube as the shoot takes up water.
- Root hair cell: A specialised cell on the surface of a plant root that has a long, thin extension (the root hair) projecting into the soil. This greatly increases the surface area for absorption of water by osmosis and mineral ions by active transport.
- Translocation: The movement of sucrose and amino acids in the phloem, from regions of production (sources, e.g. Leaves) to regions of storage or use (sinks, e.g. Roots and fruits).
- Transpiration: The loss of water vapour from the leaves of a plant, mainly through the stomata, as water evaporates from the surfaces of mesophyll cells.
- Transpiration rate: The speed at which water is lost from the leaves of a plant by evaporation through the stomata. Transpiration rate is increased by high temperature, low humidity, wind and high light intensity, and is decreased by the opposite conditions.
- Wilting: The drooping of a plant's leaves and stems that occurs when water loss by transpiration exceeds water absorption by the roots. Cells lose turgor pressure and become flaccid, so the plant can no longer support itself.
- Xylem: A plant transport tissue that carries water and dissolved mineral ions from the roots to the leaves and stems. Xylem vessels are made of dead, hollow cells joined end to end with no end walls, and their walls are strengthened with lignin.
Browse the full IGCSE Biology glossary when a related term needs checking.
Apply the topic in practical work
These practical guides show how the Biology appears in methods, variables, measurements, graphs and evaluation.
- Investigating the Rate of Transpiration: The rate of transpiration can be measured using a potometer: a device that tracks how fast water is taken up by a cut shoot. Factors such as wind, humidity, light, and temperature can be tested.