A-Level Biology: Transport in Plants

Plants move two very different things over long distances: water and mineral ions upwards in the xylem, pulled by evaporation from the leaves, and sugars in the phloem from where they are made to where they are needed. Both are classic extended-answer topics in A level biology, and both come with experimental evidence that questions love to ask about.

The xylem part is built around the cohesion–tension theory. You explain how evaporation of water from the leaf and its diffusion out through the stomata lower the water potential, how hydrogen bonds between water molecules (cohesion) let the whole column be pulled up as a continuous stream, what adhesion to the vessel walls adds, and how the structure of xylem vessels — dead, lignified, without end walls, with pits — suits them to carrying water under tension. The daily narrowing of a tree trunk is used as evidence. Transpiration is then treated as a rate: how light intensity (through stomatal opening), temperature, humidity and air movement change it and why, and how a potometer is set up and used, including the calculation of water uptake from the movement of a bubble in a capillary with V = πr²l.

The phloem part follows the mass-flow hypothesis step by step: active loading of sucrose at the source through an H⁺ gradient and co-transport, water entering the sieve tubes by osmosis, the resulting high hydrostatic pressure, flow towards the sink, and unloading where sucrose is respired or stored. You then weigh the evidence: ringing experiments, radioactive carbon tracers, sap exuding from aphid stylets — and the observations that a simple bulk-flow model struggles with, such as different solutes moving at different speeds.

The material offers a multiple-choice quiz with explained answers, a flashcard deck of structures and mechanisms, a printable written-work sheet of eight open questions (mechanism explanations, a potometer calculation and evaluation of evidence) answered by hand and checked, and an oral exam in which an examiner asks one question at a time and gives feedback at the end.

The content is based on the mass transport in plants content of the DfE A level biology subject content, as taught in current exam-board specifications (for example AQA topic 3, mass transport in plants). It is practice material and does not replace your own board's specification.

• Relate the structure of xylem vessels and sieve tube elements to their functions
• Explain the cohesion–tension theory of water movement in the xylem
• Explain how light, temperature, humidity and air movement affect transpiration
• Set up a potometer and calculate the rate of water uptake
• Describe translocation of sucrose by the mass-flow hypothesis
• Evaluate evidence for and against mass flow

Practice material written by Zestly, based on the DfE A level biology subject content (mass transport in plants: xylem and phloem).