OCR GCSE Biology A: Transport in plants: water uptake, transpiration and translocation (J247)

This guide covers the plant half of statement set B2.2 (“The challenges of size”) in the OCR GCSE (9-1) Gateway Science Biology A specification (J247), Version 4.0: statements B2.2f to B2.2j. It sits inside topic B2, Scaling up. None of these five statements is marked as Higher Tier only, so the content is for both tiers. For the whole of topic B2 see the Scaling up study guide; this page goes deeper on plant transport. Related pages: the revision notes and the practice questions. Course hub: OCR GCSE Biology. Printable checklist. Not sure where you stand? Try a free diagnostic.

What you must be able to do

Statement	What the specification asks
B2.2f	Explain how plants absorb water and mineral ions, linking root hair cell structure to function
B2.2g	Describe transpiration and translocation (including the structure and function of stomata)
B2.2h	Explain how xylem and phloem are adapted to their functions
B2.2i	Explain the effect of environmental factors on the rate of water uptake (light intensity, air movement, temperature)
B2.2j	Describe how a simple potometer is used to investigate those factors, including calculating rate and percentage gain or loss of mass

The maths skills listed for this set are rate calculations and compound measures, ratios and percentages, and plotting and interpreting graphs. Expect data questions as well as explanations.

B2.2f: Taking in water and mineral ions

A plant is a large organism with a small surface-area-to-volume ratio overall, so it needs a transport system. The roots take in water and mineral ions from the soil. The entry points are root hair cells, which are specialised cells of the root surface.

Structure and function of a root hair cell

Feature	Why it helps
Long, thin extension (the root hair)	Gives a very large surface area in contact with soil water
Thin cell wall and membrane at the hair	Short distance for water and ions to cross
Large vacuole containing dissolved sugars and salts	Keeps the water concentration inside lower than in the soil, so water moves in
Many mitochondria	Supply energy from respiration for active transport
Lots of carrier proteins in the membrane	Move specific mineral ions in

Water enters by osmosis. Soil water is usually a dilute solution, so it has a higher water concentration than the cytoplasm and vacuole of the root hair cell. Water moves down this gradient through the partially permeable membrane. Having a large surface area makes the process faster.

Mineral ions such as nitrate and magnesium are usually at a lower concentration in the soil than inside the cell. They move in by active transport: against the concentration gradient, using carrier proteins and energy from respiration. Diffusion alone would carry them the wrong way. Active transport is why a waterlogged soil, with little oxygen for respiration, slows ion uptake.

After entering the root hair cell, water and ions pass inwards through the root to the xylem.

B2.2g and B2.2h: Transpiration and the xylem

Transpiration is the loss of water vapour from a plant, mainly from the leaves through the stomata. The stages are:

1. Water evaporates from the damp surfaces of mesophyll cells, and the vapour collects in the air spaces of the leaf.
2. Water vapour diffuses out through open stomata, down a concentration gradient into the drier air outside.
3. Water leaving the leaf is replaced by water pulled up the xylem from the roots. Water molecules stick together, so the water forms a continuous column, and this transpiration stream carries mineral ions with it.

Stomata are pores, mostly on the lower surface of a leaf. Each pore is bordered by two guard cells. When the guard cells take in water they swell and become turgid, and the pore opens, allowing gas exchange for photosynthesis. When they lose water they become flaccid and the pore closes. This lets the plant limit water loss. The waxy cuticle covering the leaf reduces evaporation from the rest of the surface.

Xylem adaptations (B2.2h)

• Xylem vessels are made from dead cells with no cytoplasm and no end walls between them, so they form hollow tubes with little resistance to flow.
• The walls are strengthened with a tough material called lignin. This stops the tubes collapsing under the tension of the pulled water column and also gives support to the plant.
• Xylem carries water and mineral ions in one direction only: up from the roots.

In an explanation, always link each feature to its function.

B2.2g and B2.2h: Translocation and the phloem

Translocation is the movement of dissolved food, mainly sucrose, from the leaves where it is made (the sources) to the parts of the plant that use or store it (the sinks), such as growing tips, roots, fruits and storage organs. Unlike the xylem, it moves in both directions, depending on where the sugar is needed, and it uses energy.

Phloem adaptations

• The main tubes are made from living cells called sieve tube elements. The end walls between them are perforated (sieve plates), letting the sugar solution pass from cell to cell.
• They contain very little cytoplasm and have no nucleus, which leaves room for flow.
• Each sieve tube element is supported by a companion cell, which has many mitochondria. The companion cell supplies energy for loading sugars into the phloem.

	Xylem	Phloem
Carries	Water and mineral ions	Dissolved sugars (sucrose)
Direction	Upwards only	Up and down
Cells	Dead, lignified, hollow	Living sieve tubes with companion cells
Process	Transpiration stream	Translocation

Evidence for translocation. If a ring of bark (which includes the phloem) is removed from a stem, a swelling forms above the cut, because sugars from the leaves collect there. Tissue below the ring is starved of sugar. The xylem in the centre of the stem is untouched, so the leaves still receive water. A tracer such as radioactive carbon in carbon dioxide can also be followed from the leaves into the phloem.

B2.2i: Environmental factors

Transpiration is driven by the concentration gradient of water vapour between the inside of the leaf and the air, and by how open the stomata are. Water uptake follows the rate of loss, so the same factors affect both.

Factor	Effect on rate	Explanation
Light intensity	Increases	Stomata open in the light for photosynthesis, so more vapour can leave. In the dark most stomata close and the rate is low.
Temperature	Increases	Water evaporates faster, and vapour diffuses faster. Warm air can also hold more vapour, so the gradient is steeper.
Air movement	Increases	Draughts sweep vapour off the leaf surface, so the gradient stays steep. In still air, vapour builds up near the leaf.
Humidity (extension)	Decreases when higher	A smaller gradient between leaf and air. This follows from the same gradient idea.

Always link the factor to either the gradient or the stomata. “Faster” alone does not explain anything.

B2.2j: The potometer

A potometer measures the rate of water uptake by a leafy shoot. Uptake is a good estimate of water lost by transpiration, because nearly all the water taken in is lost. A small amount is used in photosynthesis and to keep cells turgid, so the two values are similar but not identical.

Method

1. Cut a leafy shoot while it is submerged, at a slant, and connect it to the apparatus without lifting it out of the water, so that no air enters the xylem.
2. Make all joints watertight and airtight, with petroleum jelly if needed. Dry the leaves.
3. Allow a single air bubble into the capillary tube and note where it sits on the scale.
4. Time the bubble over a fixed interval and record how far it travels. Reset it and take more readings so you can average them.
5. Alter one factor only (a fan, a lamp or a warmer room, for example) and run the readings again, holding everything else steady.

Variables to control: the same shoot or a similar one, the same time, and the other environmental factors. Airtight joints matter because a leak changes the bubble movement.

Worked example 1: rate of bubble movement and volume taken up

A bubble moves from the 12 mm mark to the 66 mm mark in 6.0 minutes. The capillary has internal radius 0.60 mm.

distance = 66 - 12 = 54 mm
rate     = 54 / 6.0 = 9.0 mm per minute
area     = pi x 0.60^2 = 1.131 mm^2
volume rate = 9.0 x 1.131 = 10.18 mm^3 per minute  (10 mm^3 per minute to 2 s.f.)

Rate = distance ÷ time. To turn this into a volume, multiply by the cross-sectional area of the tube. One cubic millimetre equals one microlitre.

Worked example 2: comparing two conditions

Three readings of distance moved in 5.0 minutes: still air 15, 17, 16 mm; with a fan 29, 31, 30 mm.

mean still = 16 mm   -> 16 / 5 = 3.2 mm per minute
mean fan   = 30 mm   -> 30 / 5 = 6.0 mm per minute
percentage increase = (30 - 16) / 16 x 100 = 87.5%

The fan raised the rate by 87.5%. The reason is that moving air takes the vapour away from the stomata, so the gradient stays steep.

Worked example 3: percentage mass change

A leafy shoot stands in a tube of water with a layer of oil on top, to stop evaporation from the water. The whole tube starts at 212.6 g and is 208.1 g after 3.0 hours.

loss = 212.6 - 208.1 = 4.5 g
percentage loss = 4.5 / 212.6 x 100 = 2.1% (2 s.f.)
rate = 4.5 / 3.0 = 1.5 g per hour

The mass lost is almost entirely transpired water. Percentage change is always calculated from the starting value.

Worked example 4: stomatal density

An impression of a leaf is made with clear nail varnish and viewed under a microscope. Five fields of view, each 0.50 mm in diameter, show 14, 12, 15, 13 and 16 stomata.

mean count = (14 + 12 + 15 + 13 + 16) / 5 = 14
field area = pi x (0.25)^2 = 0.196 mm^2
density    = 14 / 0.196 = 71 stomata per mm^2

Common errors

• Saying roots take in minerals by osmosis. Water enters by osmosis; mineral ions mostly by active transport.
• Saying phloem is made of dead cells. Xylem is dead; phloem is living.
• Saying “plants lose water through their roots” for transpiration. It happens mainly at stomata.
• Calling translocation one-way. Sugars move up or down, depending on source and sink.
• Forgetting to cut the shoot under water, or to dry the leaves, in the potometer method.
• Calculating percentage change from the final value instead of the starting value.
• Writing a rate without units, or giving a volume rate when only a distance rate was asked for.

Official syllabus

OCR, GCSE (9-1) Biology A (Gateway Science) J247 Specification, Version 4.0 (August 2026), first assessment 2018, statements B2.2f to B2.2j.