Learning objectives
- Compare the structure and function of xylem and phloem
- Explain the transpiration stream using cohesion-tension
- Describe translocation and the source-to-sink idea
1. Xylem — water and minerals, one direction only
Xylem vessels are dead, hollow, lignified tubes joined end to end with no cross walls. Water enters the root hairs by osmosis, crosses the cortex, and is pulled up the stem to the leaves where it evaporates through the stomata.
Two forces drive it: root pressure pushes weakly from below, while transpiration pull — created as water evaporates from mesophyll cell walls — pulls strongly from above. Because water molecules stick to each other (cohesion) and to the vessel wall (adhesion), the column does not break. This is the cohesion-tension theory.
- Xylem — dead, lignified, no cross walls, upward flow only
- Carries water + dissolved mineral ions
- Also gives mechanical support to the stem
2. Phloem — sugars, both directions
Phloem is living tissue made of sieve tubes with perforated sieve plates, each supported by a companion cell packed with mitochondria. Sucrose and amino acids are loaded actively at a source (a photosynthesising leaf or a sprouting tuber) and unloaded at a sink (a root, fruit, seed or growing bud).
Because loading uses ATP, translocation stops if the phloem is poisoned or starved of oxygen — the standard exam evidence that it is an active process, unlike the passive transpiration stream.
3. Why this matters in the field
Vascular-wilt pathogens such as Fusarium oxysporum and bacterial wilt block the xylem, so leaves wilt even when the soil is wet. Phloem-limited pathogens and sap-sucking insects (aphids, whiteflies) drain assimilates and spread viruses. When you diagnose a whole-plant wilt with no leaf spot, always suspect the vascular system first.
Key definitions
- Transpiration
- Loss of water vapour from the aerial parts of a plant, mainly through the stomata.
- Translocation
- Movement of manufactured food (mainly sucrose) in the phloem from source to sink.
- Cohesion-tension
- Theory that water is pulled up the xylem as an unbroken column held together by cohesion between water molecules.
Exam tips
- Answer xylem/phloem comparisons in a two-column table — structure, contents, direction, living or dead.
- Never say phloem 'carries food upwards'; say it moves assimilates from source to sink, in either direction.
Practice exam questions & model answers
Q1.Give four differences between xylem and phloem.
Xylem is made of dead lignified vessels while phloem is made of living sieve tubes; xylem carries water and mineral salts while phloem carries dissolved organic food; xylem transport is unidirectional (upward) while phloem transport is bidirectional (source to sink); xylem transport is passive, driven by transpiration pull, while phloem loading is active and requires ATP.
Q2.Explain why a plant may wilt although the soil is moist.
Water uptake or conduction is blocked — for example a vascular-wilt fungus or bacterium plugs the xylem vessels, or the roots are rotted or waterlogged so they cannot absorb. Transpiration continues, cells lose turgor and the plant wilts despite available soil water.
Q3.State three factors that increase the rate of transpiration.
High temperature, low humidity, moving air (wind) and bright light, which opens the stomata wider, all increase transpiration rate.
Continue revising
Plant Cell Structure and Function
Plant BiologyPhotosynthesis: Process, Equation and Limiting Factors
BotanyLeaf Structure, Stomata and Transpiration
Agricultural SciencePlant Nutrition, NPK and Soil Fertility
Put the theory to work — diagnose a real leaf photo or browse the disease library.