Understanding Transport in plants: IGCSE Biology Explained

The transport problem every plant has to solve

A tall oak tree needs to move water from its roots to leaves that might be 20 metres above the soil. It has no heart, no blood, no muscles. So how does it do it? That question sits at the centre of this IGCSE Biology topic, and once you understand the answer, every exam question on plant transport becomes a variation of the same logic.

Plants solve their transport problem with two specialised tissue systems: xylem for moving water and mineral ions upward, and phloem for moving dissolved sugars and amino acids to wherever they are needed. Your job for the Cambridge IGCSE exam is to know the structure of each tissue, how substances enter the plant, what drives their movement, and which factors speed up or slow down the process.

Xylem and phloem: structure and function

Think of xylem and phloem as two different delivery systems running side by side through the plant. They carry different cargo, move in different directions, and are built differently at the cellular level.

Where xylem and phloem sit in the plant

The position of these tissues changes depending on which organ you are looking at. This is a favourite exam question: you get a cross-section diagram and have to label or identify the vascular tissues.

• In roots: xylem forms a central star shape (or cross shape) in the middle of the root. Phloem sits between the arms of the star. The vascular tissue is concentrated at the centre for strength against the pulling forces of water uptake.
• In stems: xylem and phloem are arranged in vascular bundles around the outer edge of the stem. Within each bundle, xylem is on the inside (closer to the centre) and phloem is on the outside. This ring arrangement provides support like the steel rods in a concrete column.
• In leaves: xylem and phloem run through the veins (midrib and branching network). Xylem is on the upper side of the vein, phloem on the lower side.

[Extended] Xylem vessel structure and function

For the Supplement syllabus, you need to connect three structural features of xylem vessels to their transport function:

1. Thick walls reinforced with lignin: lignin is a tough, waterproof substance. It prevents the vessel from collapsing under the negative pressure created during transpiration, and it stops water leaking out sideways. It also provides mechanical support to the plant.
2. No cell contents: xylem vessels are dead at maturity. The cytoplasm, nucleus and organelles break down, leaving a completely hollow tube. This means there is nothing to obstruct the flow of water.
3. No cross walls between cells: the end walls between adjacent xylem cells break down during development, creating one continuous tube from root to leaf. Water flows without having to cross any barriers.

The logic is straightforward, and IGCSE examiners reward answers that make these connections explicit: every structural feature removes resistance to water flow. Thick walls stop collapse. No contents remove obstruction. No end walls remove barriers. The result is an efficient, continuous pipeline.

Water uptake: how water enters the plant

Root hair cells

Water enters the plant through root hair cells. These are epidermal cells near the tip of each root that have a long, thin extension (the "hair") projecting into the soil. The hair massively increases the surface area of the cell, which increases the rate of water absorption.

Think of it this way: if you needed to soak up a puddle, you would spread out a towel rather than scrunch it into a ball. The root hair does the same thing. More surface in contact with soil water means faster uptake.

The osmosis mechanism

Water moves into root hair cells by osmosis: the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (more concentrated solution) through a partially permeable membrane.

In practical terms: the soil water around the root is a dilute solution of mineral ions. The cell sap inside the root hair cell is a more concentrated solution (it contains sugars, amino acids and other dissolved substances). The cell membrane acts as a partially permeable membrane. So water moves from the soil into the root hair cell, down the water potential gradient.

[Extended] Water pathway through the root

Once water enters the root hair cell, it doesn't jump straight into the xylem. It moves across the root cortex, passing from cell to cell by osmosis. Each cell along the path has a slightly more concentrated cell sap than the one before it, so water keeps moving inward. The pathway runs: soil water enters root hair cell, crosses cortex cells, reaches the xylem vessels in the centre of the root.

At each step, the same principle applies. Water always moves from where it is more concentrated (as a solvent) to where it is less concentrated, through the partially permeable membranes of each cell.

Transpiration: the engine that pulls water upward

Transpiration is the loss of water vapour from the leaves of a plant, mainly through the stomata. It sounds like a waste of water, but it is actually the driving force behind the entire upward movement of water through the plant.

Here is the sequence, step by step:

1. Water evaporates from the surfaces of mesophyll cells inside the leaf into the air spaces.
2. This water vapour diffuses out of the leaf through open stomata.
3. The loss of water from mesophyll cells draws more water out of the leaf xylem by osmosis.
4. This creates a pulling effect (transpiration pull) that draws a continuous column of water up through the xylem from root to leaf.
5. As water leaves the root xylem, more water is drawn in from the root cortex cells, and then from the soil.

The whole system works like a chain: pull from the top (evaporation at the leaf) transmits all the way down to the bottom (absorption at the root). No pump required.

Factors affecting transpiration rate

Four environmental factors change how fast a plant loses water. Each one works by affecting how quickly water vapour diffuses out of the stomata.

Notice the pattern: three of the four factors increase transpiration when they increase. Only humidity works in the opposite direction. That is because humidity reduces the concentration gradient, while the other three either increase the energy of water molecules or open up the exit route.

Investigating transpiration: the potometer

The standard practical for measuring transpiration rate uses a potometer. A leafy shoot is sealed into a glass tube filled with water, and a scale or air bubble tracks how fast water is taken up. Because almost all the water a plant absorbs is lost through transpiration, the rate of water uptake closely matches the rate of transpiration.

To test the effect of different conditions, you change one variable at a time (for example, placing a fan next to the shoot to increase wind speed) while keeping everything else constant, then measure how far the air bubble moves in a set time period.

The celery experiment: seeing transport in action

One of the simplest ways to demonstrate xylem transport is the celery-and-dye experiment. You stand a celery stalk in a beaker of water coloured with blue or red food dye and leave it for several hours. When you cut the stalk open, you can see coloured lines running through it. Those coloured lines are the xylem vessels: the dye has been carried up through them by the transpiration stream.

If you cut a cross-section, you see the dye concentrated in the vascular bundles around the outside of the stem. This confirms the arrangement pattern discussed earlier: in a stem, the vascular bundles sit in a ring near the outer edge.

The experiment also works well for testing variables. A celery stalk placed in warm water will show dye rising faster than one in cold water, because heat increases transpiration rate. Two stalks with and without leaves demonstrate that removing leaves (and therefore stomata) dramatically slows water uptake.

Translocation: moving food around the plant

While xylem handles water, the phloem handles food. Translocation is the transport of sucrose and amino acids in the phloem from where they are made (sources) to where they are needed (sinks).

• Sources are parts of the plant that produce or release sugars. Leaves are the main source during the day because photosynthesis produces glucose, which is converted to sucrose for transport. Storage organs can also be sources when they release stored starch (converted back to sucrose).
• Sinks are parts of the plant that consume or store sugars. Growing tips, developing fruits, roots and storage organs are all sinks.

The key difference from xylem transport: translocation moves substances in both directions. Sucrose can travel downward from a leaf to a root for storage, or upward from a storage root to a growing shoot in spring. The direction depends on where the source and sink are relative to each other.

Translocation requires energy (it is an active process), which is why phloem cells are alive and have companion cells that supply ATP through aerobic respiration. For your IGCSE exam, remember that translocation is active and bidirectional, while transpiration is passive and unidirectional.

Worked exam question

Question: A plant is placed in a warm, bright room with a fan blowing across its leaves. Explain why this plant will wilt faster than an identical plant in a cool, dark, still room. [4 marks]

Model answer:

1. In the warm, bright, windy conditions, stomata are open wider because of the high light intensity. [1 mark]
2. Higher temperature increases the kinetic energy of water molecules, so evaporation from mesophyll cell surfaces is faster. [1 mark]
3. The fan removes the layer of humid air from around the leaves, maintaining a steep concentration gradient for water vapour, so diffusion out of stomata is faster. [1 mark]
4. Water is lost through transpiration faster than it can be replaced by absorption at the roots, so cells lose turgor and the plant wilts. [1 mark]

Notice the structure: each point names a specific factor, states its effect on a specific process, and connects that to water loss. That is the pattern examiners reward. Vague statements like "the plant loses water faster" without explaining the mechanism will not score full marks.

Common exam mistakes

1. Saying xylem transports "food": xylem carries water and mineral ions only. Food (sucrose and amino acids) travels in the phloem. Mixing these up is one of the most common errors.
2. Describing transpiration as "the movement of water up the plant": transpiration is specifically the loss of water vapour from leaves through stomata. The upward movement of water is called the transpiration stream or transpiration pull, driven by transpiration but not transpiration itself.
3. Forgetting that root hair cells absorb water by osmosis, not active transport: water entry into root hairs is passive (osmosis). Active transport is used for mineral ion uptake, not water uptake.
4. Confusing the direction of phloem transport: students often write that phloem only moves substances downward. Phloem moves sucrose and amino acids in both directions, from source to sink, wherever those happen to be.
5. Not linking structure to function for xylem [Extended]: when asked to "relate structure to function," you must explicitly connect each structural feature to a specific transport advantage. Listing structures without explaining how each one helps water move gets zero credit for the explanation marks.
6. Writing that transpiration "sucks" water up: the mechanism is a pull created by evaporation and cohesion of water molecules. "Suction" implies a vacuum pump, which is not what happens. Use "transpiration pull" instead.

Self-check questions

1. State two functions of xylem tissue in a plant.
2. Draw a simple labelled diagram of a root cross-section showing the positions of xylem and phloem.
3. Explain how the structure of root hair cells is adapted for efficient water absorption.
4. Describe the pathway of water from the soil to the xylem, naming the process by which water moves at each stage.
5. A potometer experiment shows that a leafy shoot takes up 5 cm of water in 10 minutes in still air, but 14 cm in 10 minutes with a fan. Calculate the percentage increase in uptake rate and explain the result.
6. [Extended] Explain three structural features of xylem vessels and state how each feature relates to the function of transporting water.
7. Define translocation and explain how it differs from transpiration.
8. A student removes all the leaves from a plant and measures its water uptake. Predict and explain what will happen to the rate of water uptake compared to the intact plant.