OxfordAQA International AS and A-level Biology: Mass transport in plants: xylem, transpiration and phloem (9610)

This study guide is aligned to the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, section 3.2.8 (mass transport systems in plants), covering 3.2.8.1 (xylem) and 3.2.8.2 (phloem). It sits in Unit 2, Biological systems and disease, which the specification lists as part of both the International AS and the International A-level. It includes Required practical 6, the simple potometer.

Test yourself afterwards with the revision notes and the practice questions. The course page, the checklist, the free diagnostics and the exam preparation page help you plan.

What this unit covers

Section	What you must be able to do
3.2.8.1	Relate the structure of a dicotyledonous root to the pathway of water from root hairs, through the cortex and endodermis, to the xylem
3.2.8.1	Describe the apoplastic and symplastic pathways
3.2.8.1	Explain the role of root pressure in moving water through the xylem
3.2.8.1	Explain transpiration and the effects of light, temperature, humidity and air movement
3.2.8.2	Explain the mass flow hypothesis for translocation
3.2.8.2	Analyse and interpret experimental evidence on water in the xylem and organic substances in the phloem
Required practical 6	Investigate the rate of water uptake with a simple potometer

The specification says the International AS written papers test knowledge and understanding of the procedures in the required practicals, evaluation of the techniques adopted, and may require interpreting specimen results.

3.2.8.1 Water uptake in a dicot root

A flowering plant moves water and mineral ions in the xylem and organic substances in the phloem. Together they form its mass transport system.

In a dicotyledonous root, the layers from the outside inwards are:

Layer	Features	Role in water movement
Epidermis with root hairs	Root hair cells have a long, thin extension	Large surface area for uptake
Cortex	Thick layer of parenchyma cells with air spaces and porous cellulose walls	Water crosses it by two routes
Endodermis	Single ring of cells around the centre; waterproof band in the walls	Controls what enters the xylem
Xylem	Centre of the root, usually a star or X shape, with phloem between its arms	Carries water upwards

Entering the root hair. Soil water usually has a higher water potential than the cell sap of a root hair. Water enters by osmosis. Mineral ions enter by active transport and diffusion through carrier and channel proteins. Active transport needs ATP from respiration, so it stops if the root is short of oxygen.

Apoplastic and symplastic pathways

• Apoplastic pathway. Water moves through the cell walls and the spaces between cells. It never enters the cytoplasm. The walls are porous, so water moves by mass flow, pulled along by the loss of water above it.
• Symplastic pathway. Water moves through the cytoplasm of connected cells. It passes between neighbouring cells through plasmodesmata, narrow cytoplasmic channels that pass through the cell walls. Water moves from cell to cell by osmosis, down a water potential gradient.

Worked example 1. A root hair has water potential −0.35 MPa. The cortex cell next to it has −0.50 MPa, the next −0.65 MPa and an endodermis cell −0.80 MPa. Which way does water move by osmosis? Water travels towards the more negative water potential. So it moves root hair → −0.50 → −0.65 → −0.80, towards the xylem. A gradient like this keeps water moving inwards.

The endodermis

The endodermis walls contain a band of suberin, a waterproof substance (the Casparian strip). The specification names the endodermis but not this band, which is the usual explanation of its role. It blocks the apoplastic pathway at the endodermis. Water must then pass through the cell surface membrane into the symplast. The membrane controls which ions enter the xylem.

Root pressure

Cells next to the xylem use active transport to move mineral ions into it. The xylem sap then has a lower water potential. Water follows from the cortex by osmosis and builds up a positive pressure that pushes water up the xylem. This is root pressure.

Evidence:

• Sap oozes from the cut stump of a stem.
• Drops of water form at leaf edges at night (guttation) when transpiration is low and the soil is moist.
• Respiratory poisons, cold or lack of oxygen reduce the flow from a stump. Active transport is therefore involved.

Root pressure is a small force. It helps most when transpiration is low. It cannot be the main explanation in tall plants that are transpiring rapidly.

Transpiration

Transpiration is water vapour escaping from the aerial parts of a plant, chiefly via open stomata in the leaves. Water evaporates from the wet walls of mesophyll cells into the air spaces. Vapour then diffuses out through the stomata down a water vapour gradient.

The usual explanation of how this moves water up the xylem has these steps:

1. Evaporation from mesophyll cell walls lowers their water potential. Water moves in from cells nearer the xylem.
2. This pulls water up the xylem, which is under tension (negative pressure).
3. Water molecules stick together (cohesion) through hydrogen bonds. They also stick to the xylem walls (adhesion). So the water column is pulled up without breaking.
4. The thick, lignified xylem walls stop the vessels collapsing inwards under tension.

The flow of water from the soil, up the xylem and out of the leaf is the transpiration stream.

Factors affecting the rate

Factor	Effect on rate	Explanation
Light	Increases	Stomata open in the light, so there is a larger area for diffusion. Stomata are mostly closed in the dark.
Temperature	Increases	Water evaporates faster and vapour molecules have more kinetic energy, so diffusion is faster. Warmer leaf air carries more vapour out, which keeps the gradient steep.
Humidity	Decreases as humidity rises	Moist air outside reduces the gradient between the air spaces and the air.
Air movement	Increases	Moving air removes humid air from around the stomata and keeps the gradient steep.

Several factors act together. A plant short of water also closes its stomata, which cuts transpiration.

Required practical 6: the simple potometer

A potometer records how quickly a cut shoot takes up water. Uptake is a close measure of water loss, but not identical. Some water is used in photosynthesis and some stays in the cells.

Method:

1. Cut the shoot under water at a slant. This stops air entering the xylem.
2. Assemble the potometer under water, so that the apparatus is full of water with no air bubbles.
3. Dry the leaves. Seal the joints with petroleum jelly so they are airtight.
4. Lift the capillary tube end out of the water briefly to draw in one air bubble. Put it back in the beaker.
5. Mark the bubble position and record the distance it moves in a set time. Repeat at least three times. Use the reservoir to reset the bubble.
6. Change one independent variable, such as a fan, and keep the others constant. Allow time for the shoot to adjust.

Calculations. Rate of movement = distance ÷ time. Volume of water = πr² × distance, with r the radius of the capillary tube bore.

Worked example 2. The capillary bore has radius 0.40 mm. In still air the bubble moves 36 mm in 8 minutes. With a fan it moves 62 mm in 8 minutes.

• Still air: 36 ÷ 8 = 4.5 mm min⁻¹.
• With fan: 62 ÷ 8 = 7.75 mm min⁻¹.
• Area = π × 0.40² = 0.503 mm².
• Still-air uptake = 4.5 × 0.503 = 2.26 mm³ min⁻¹.
• Fan uptake = 7.75 × 0.503 = 3.90 mm³ min⁻¹.
• Percentage increase = (7.75 − 4.5) ÷ 4.5 × 100 = 72.2%.

If the shoot has a leaf area of 38 cm², still-air uptake per cm² is 2.26 ÷ 38 = 0.0595 mm³ min⁻¹ cm⁻². Dividing by leaf area lets you compare shoots of different size.

Evaluating the method. Different shoots have different leaf areas. Bubbles may stick. Light also heats the leaf, so a lamp changes temperature as well. The method measures uptake, not loss.

3.2.8.2 Translocation and the mass flow hypothesis

Translocation is the transport of organic substances, mainly sucrose, in the phloem. They move from a source (a region that releases them, such as a photosynthesising leaf) to a sink (a region that uses or stores them, such as a growing root or fruit).

The mass flow hypothesis explains this:

1. At the source, sucrose is actively loaded into the sieve tubes. The loading needs energy from the companion cells.
2. This lowers the water potential of the sieve tube. Water enters by osmosis from the nearby xylem.
3. Water entering raises the hydrostatic pressure at the source end.
4. At the sink, sucrose is taken out and used or stored. The sieve tube water potential rises, so water leaves by osmosis and pressure falls.
5. The pressure difference drives mass flow of the sap from source to sink.

Phloem sap and xylem water move for different reasons. The xylem relies on tension from transpiration (and some root pressure). The phloem relies on pressure from loading.

Analysing the evidence

Evidence	Finding	What it suggests
Ringing: bark ring removed	Sugar builds up above the ring	Sugars travel in the phloem, which lies in the bark
Tracer: radioactive carbon dioxide fed to a leaf	Label later detected in the phloem	Products of photosynthesis move in the phloem
Aphid stylet	Sap flows out of a cut stylet	Sap in the phloem is under pressure
Metabolic inhibitor	Translocation falls	Loading needs energy
Sap analysis	Higher sucrose at the source than the sink	Supports a concentration gradient

Worked example 3. Sections show that a dye in the xylem rises faster in a lit shoot than a shaded one. This supports the idea that light affects the rate of the transpiration stream.

Worked example 4. A tracer pulse is detected at two points on a stem 78 mm apart, 12 minutes apart. Rate = 78 ÷ 12 = 6.5 mm min⁻¹. In m h⁻¹: 6.5 × 60 ÷ 1000 = 0.39 m h⁻¹.

Worked example 5. In a ringing experiment the sucrose concentration above the ring is 74 mg cm⁻³ and below it 11 mg cm⁻³. A control stem shows 41 above and 40 below. In the control, the two values are almost the same (a ratio of 1.03), so the stem has no gradient. In the ringed stem the ratio above:below is 6.7. The blocked flow explains the build-up above the ring.

Limits of the evidence. Evidence from ringing does not show that the phloem alone carries sugar. Evidence for mass flow does not prove that it is the only mechanism. Good answers separate what the data show from what they leave open.

Common errors

• Writing that the apoplast is “through the cell membrane”. It runs through walls and spaces.
• Saying that water moves “up” in the xylem by osmosis. Osmosis moves it across cells in the root.
• Describing root pressure as the main force in tall plants.
• Writing that humidity increases transpiration.
• Giving only “faster” for temperature without a reason.
• Mixing up xylem (water and mineral ions) with phloem (organic substances).
• Calling a potometer a measure of transpiration without saying it measures uptake.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, for International AS exams May/June 2017 onwards and International A-level exams May/June 2018 onwards, published by OxfordAQA. Section 3.2.8 and Required practical 6.