IGCSE Biology Transport in plants notes

A labelled diagram to picture: a stem section with vascular bundles arranged in a ring — xylem on the inside of each bundle, phloem on the outside. In a root, the bundle sits in the centre with xylem in a star shape and phloem between the arms. In a leaf, vascular bundles form the veins, with xylem on the upper side of each vein and phloem below.

Supplement: xylem vessels are structured for their job: mature vessels are cells joined end to end with no cross walls, forming a long continuous tube like a water pipe; their walls are thickened with lignin for strength (support) and waterproofing; and the cells have no cell contents, leaving a clear bore for water to flow through unobstructed.

Root hair cells are specialised for absorption (Topic 2): their long extension gives a large surface area, speeding the uptake of water by osmosis and mineral ions by active transport from the thin film of soil water around each root.

1. 1Root hair cell

   Water enters from the soil film by osmosis (soil water is more dilute than the cell contents).

2. 2Root cortex cells

   Water passes cell to cell across the root cortex, again by osmosis.

3. 3Xylem

   Water enters the xylem vessels in the centre of the root and is drawn up the stem.

4. 4Mesophyll cells

   Water leaves the leaf xylem and spreads through the mesophyll, where most evaporates from cell surfaces.

Transpiration is the loss of water vapour from leaves. Water evaporates from the surfaces of mesophyll cells into the air spaces, then diffuses out through the stomata down a water-vapour concentration gradient — the air inside a leaf is nearly saturated, while the outside air usually holds less. Only about 1–2% of the water a plant takes up is used in photosynthesis; almost all the rest is lost this way, and the loss is the engine that pulls water up the xylem.

Rate is investigated with a potometer: a leafy shoot in a water-filled tube with a graduated scale; as the shoot transpires, it draws water along the scale. The distance moved per minute estimates the rate of water uptake — which closely tracks transpiration when the shoot is healthy.

Supplement: the mechanism pulling water up is the transpiration pull. Evaporation from mesophyll surfaces removes water molecules from the top of the xylem column; because water molecules attract each other, the whole column is pulled up as one unbroken chain, like a rope — drawing more water in at the roots. If the pull outstrips supply, the plant wilts: cells lose more water than they gain, turgor pressure falls (Topic 3), and soft tissues droop. Wilting is protective — closing the stomata and shading the leaf surface cut losses — but a wilted plant photosynthesises slowly because it cannot hold its leaves in the light.

Supplement: translocation is the movement of sucrose and amino acids in the phloem from sources to sinks. A source is a part of the plant that releases sucrose or amino acids — a mature photosynthesising leaf, or a bulb in spring. A sink is a part that uses or stores them — growing roots, developing fruits, new shoots.