Transport in Plants IGCSE Biology: Xylem, Phloem, Transpiration and Translocation

Transport in Plants IGCSE Biology: Xylem, Phloem, Transpiration and Translocation.

What's the difference between xylem and phloem?

Xylem is made of dead, hollow cells joined end to end with their end walls broken down, forming one continuous tube that carries water and mineral ions upward from roots to leaves. Phloem is made of living cells, sieve tube elements supported by companion cells, that carry sucrose and amino acids in either direction, a process called translocation. Xylem transport is always one-way. Phloem transport goes wherever the plant currently needs sugar.

The two sit right next to each other in a vascular bundle, which is why students confuse them on a diagram. Cell structure gives away which is which every time, though.

Feature

Xylem

Phloem

Cell type

Dead, empty, hollow cells

Living cells (sieve tube elements + companion cells)

Structure

End walls disappeared: one continuous open tube

Sieve plates with pores between cells

Wall material

Strengthened with cellulose and lignin (hard, waterproof)

Thin cellulose walls, no lignin

What's transported

Water and mineral ions

Sucrose and amino acids

Direction

One-way: roots → leaves

Either direction: source → sink

Extra function

Lignified xylem gives structural support (it's what wood is)

Companion cells provide the energy the sieve tubes can't generate themselves

The pattern examiners reward: dead, lignified cells suit passive water transport under pressure. Living cells with a membrane and cytoplasm are needed because moving sucrose into and out of cells requires active processes a dead cell simply can't perform.

How do root hairs absorb water into the plant?

Root hair cells absorb water by osmosis because their cytoplasm and cell sap is more concentrated than the dilute water in the surrounding soil, so water moves in down a concentration gradient across the partially permeable cell membrane. Root hairs are elongated epidermal cells just behind the root tip, constantly replaced as soil particles damage them.

Their huge surface area is the whole point. A root hair is a single cell stretched into a thin projection, massively increasing contact with soil water compared to a normal epidermal cell. Water then crosses the root cortex, partly through the cell walls themselves, to reach the xylem, where its journey upward begins.

What causes transpiration pull?

Transpiration pull is the upward movement of water through the xylem, caused by continuous water loss from the leaves lowering the pressure at the top of the xylem relative to the bottom, so the whole column of water is pulled up, held together by cohesion between water molecules. Transpiration itself is defined as water loss as vapour, mainly from the leaves.

Here's the actual mechanism, step by step, in the order an exam wants it:

1. Water moves from the xylem into surrounding mesophyll cells by osmosis.

2. It evaporates from the moist cell walls of the spongy mesophyll into the air spaces between cells. The spongy mesophyll's large internal surface area is why most evaporation happens here, not in the palisade layer.

3. Water vapour diffuses out of the leaf, mostly through stomata on the lower epidermis.

4. This constant water loss lowers the pressure at the top of the xylem.

5. Because pressure at the bottom of the xylem, in the root, is now higher than at the top, water is drawn upward as one continuous, cohesive column. That's transpiration pull.

If water loss outpaces uptake, cells lose turgor and become flaccid, and the plant wilts. For "explain why a plant wilts" questions, name which cells lose turgor and why. Don't just say "it loses water."

What factors affect the rate of transpiration?

Transpiration speeds up with higher temperature, stronger wind, and lower humidity. All three increase the rate water vapour diffuses away from the leaf, either by adding energy to evaporation or by keeping the water vapour concentration outside the leaf lower than inside it. Humid air slows transpiration because that diffusion gradient shrinks.

How does a potometer measure transpiration rate?

A potometer measures transpiration rate indirectly by tracking how fast an air bubble, the meniscus in a water-air column, moves along a graduated capillary tube connected to a cut shoot. The further and faster the bubble moves in a given time, the faster the plant is taking up water, and that's used as a proxy for its transpiration rate.

Worked example, the way you'd actually run this in a practical:

• Cut a leafy shoot underwater (stops air entering the xylem and breaking the water column) and fit it into the potometer via rubber tubing, watertight at every joint.

• Let an air bubble enter the capillary tube at one end and record its starting position on the tube's scale.

• Start a stopwatch and measure how far the bubble moves in a fixed time, say 5 minutes. Distance moved, converted using the tube's known diameter, gives a rate of water uptake (e.g. mm³ per minute).

• Reset the bubble, then repeat after changing one variable at a time: a fan for wind, warmth and light for temperature, or a plastic bag over the shoot for humidity, keeping everything else constant.

• Compare distance travelled per minute across conditions. A shoot under a fan or in warm, dry air should push the bubble noticeably further per minute than the same shoot in still, cool, humid air.

The mark scheme isn't checking that you can describe a potometer. It's checking you understand you're measuring water uptake as a proxy for water loss, because you can't easily measure vapour leaving a live leaf directly.

What is translocation and how does it differ from transpiration?

Translocation is the movement of sucrose and amino acids through the phloem, from "source" tissues that produce or release them to "sink" tissues that need them, for example from a photosynthesising leaf, or a storage organ converting starch back to sucrose, to growing points, roots, or developing flowers and fruit. Transpiration, by contrast, is water loss as vapour from the leaves, driving passive upward water movement in the xylem.

They're easy to confuse because they sound similar, but they differ on every axis: transpiration is passive, one-way (up), happens in dead xylem cells, and moves water; translocation can go in any direction depending on where sugar is needed, happens in living phloem cells, and moves organic nutrients. If a question asks you to compare them, structure your answer around those three axes, direction, cell type, what's transported, rather than two separate paragraphs.

For how the water and sugars delivered by xylem and phloem actually get used and circulated around the rest of the organism in animals (a useful contrast question), see our guide to the double circulatory system.

FAQ

Is xylem alive or dead? Xylem is made of dead cells. The cells lose their cytoplasm and the end walls between them break down, leaving an empty, hollow tube strengthened with cellulose and lignin, ideal for carrying water under tension without collapsing.

Can water move down the phloem as well as up? Phloem carries sucrose and amino acids, not water, and it moves either direction, source to sink, which could mean downward from a leaf to a root, or upward from stored starch to a growing shoot tip. Xylem, by contrast, only ever moves water upward.

Why does a wilted plant recover after watering? Once water is available again, root hairs resume absorbing it by osmosis, restoring turgor pressure in the mesophyll and other cells, which pushes the cell membrane back against the cell wall and makes the plant rigid again.

What's the difference between transpiration and translocation, in one line? Transpiration is water loss as vapour from leaves, driving one-way water movement up the dead xylem. Translocation is the two-way movement of sucrose and amino acids through living phloem, from source to sink.

Do all IGCSE boards test transport in plants the same way? The core biology here, xylem, phloem, transpiration, translocation, root hair structure, and the potometer method, matches Cambridge/CIE-style IGCSE Biology conventions. On a different specification, the science is the same, but check your own syllabus wording and past papers for how questions are phrased.