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

These revision notes follow the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, section 3.2.8 (3.2.8.1 xylem and 3.2.8.2 phloem) and Required practical 6. Unit 2 is listed as part of both the International AS and the International A-level. Read the study guide first for full explanations.

Then try the practice questions. The course page, the checklist and the free diagnostics help you plan your revision.

Key definitions

• Xylem: tissue that carries water and mineral ions up the plant. Walls are lignified and the vessels are dead.
• Phloem: tissue that carries organic substances, mainly sucrose, in sieve tubes with companion cells.
• Apoplastic pathway: water moves through cell walls and the spaces between cells, without entering the cytoplasm.
• Symplastic pathway: water moves through the cytoplasm of linked cells, through plasmodesmata.
• Root pressure: positive pressure that pushes water up the xylem, produced when ions are actively moved into the xylem and water follows by osmosis.
• Transpiration: escape of water vapour from the plant, chiefly via stomata.
• Translocation: movement of organic substances in the phloem from sources to sinks.
• Source: a region that releases sucrose into the phloem. Sink: a region that uses or stores it.

Root pathway in order

1. Soil water enters the root hair cell by osmosis.
2. It crosses the cortex by the apoplastic pathway or the symplastic pathway.
3. At the endodermis, the waterproof suberin band in the walls blocks the apoplastic route. Water enters the symplast.
4. Water leaves the endodermis and enters the xylem, which is in the centre of the root.

Feature	Apoplastic	Symplastic
Route	Cell walls and spaces	Cytoplasm through plasmodesmata
Crosses cell surface membranes in the cortex?	No	Only on entering the first cell
Mechanism	Mass flow	Osmosis
Blocked at endodermis?	Yes	No

The specification names the endodermis and not the suberin band. The band is the usual explanation of why water must enter the symplast there.

Root pressure

Point	Detail
Cause	Active transport of ions into the xylem by cells around it
Effect	Lower water potential in the xylem, so water enters by osmosis
Evidence	Sap oozes from a cut stump. Guttation. Flow falls with cold or respiratory poison
Limit	A small push. Cannot explain water reaching the top of a tall tree

Transpiration

Water evaporates from mesophyll cell walls, vapour diffuses out through stomata, and water is pulled up the xylem under tension. Cohesion keeps the water column unbroken. Adhesion helps it cling to the walls.

Factor	Change	Rate	Reason
Light	Brighter	Up	Stomata open
Temperature	Higher	Up	Faster evaporation and diffusion
Humidity	Higher	Down	Smaller vapour gradient
Air movement	More	Up	Humid air swept away from stomata

Required practical 6: potometer

Method in steps:

1. Cut the shoot under water at a slant.
2. Assemble the apparatus under water with no air bubbles.
3. Dry the leaves. Seal joints with petroleum jelly.
4. Introduce one air bubble. Record its start position.
5. Time the bubble over a fixed distance or measure the distance in a fixed time.
6. Repeat and calculate a mean. Change one variable only.

Quantity	Equation
Rate of bubble movement	distance ÷ time
Capillary cross-section	πr²
Volume taken up	πr² × distance
Volume per minute	πr² × distance ÷ time
Percentage change	(new − old) ÷ old × 100

Worked reminder. The bubble moves 30 mm in 4.0 minutes in a tube of radius 0.50 mm. Rate = 7.5 mm min⁻¹. Area = π × 0.50² = 0.785 mm². Uptake = 7.5 × 0.785 = 5.89 mm³ min⁻¹.

Limits: measures uptake not loss; leaf area varies; the shoot has no root; a lamp also warms the leaf; the bubble can stick.

Mass flow hypothesis

1. Sucrose is actively loaded into sieve tubes at the source, with energy from the companion cells.
2. Water potential of the sieve tube falls. Water enters by osmosis from the xylem.
3. Hydrostatic pressure rises at the source end.
4. Sucrose is removed at the sink. Water potential rises, water leaves, pressure falls.
5. Sap flows down the pressure gradient from source to sink.

	Xylem	Phloem
Carries	Water and mineral ions	Organic substances
Direction	Upwards	Source to sink, either direction
Driving force	Tension from transpiration (plus some root pressure)	Pressure from loading at the source
Cells	Dead	Living sieve tube cells

Interpreting evidence

Evidence	Point to make
Ringing	Sugar accumulates above the ring, so the phloem carries it down
Radioactive tracer	The label appears in the phloem, so assimilates move there
Aphid stylet	Sap flows out under pressure, as the hypothesis predicts
Inhibitor	Translocation falls, so loading needs energy
Sap analysis	Sucrose concentration is higher at the source than the sink

When you read data:

1. Describe the pattern with figures.
2. Compare with the control.
3. State what the pattern suggests.
4. Say what it does not show.

Must-know distinctions

• Transpiration and translocation. Transpiration is water vapour escaping from leaves. Translocation is movement of organic substances in the phloem. The words look alike, so write each in full.
• Uptake and loss. A potometer measures uptake. Most of the water taken up is lost, but some is used in photosynthesis and some keeps cells turgid.
• Root pressure and transpiration pull. Root pressure pushes from below by osmosis. Transpiration pulls from above, so the xylem is under tension. Both can operate, but tension is the larger effect in a transpiring plant.
• Source and sink. These are roles, not organs. A storage organ is a sink while it fills and can later be a source.
• Water potential direction. Water follows the fall towards the more negative value.

Worked reminders for data

Water potential gradient. Soil water is at −0.15 MPa and the root hair at −0.40 MPa. Water enters the root hair because it has a lower (more negative) water potential than the soil water.

Percentage change. Uptake rises from 2.0 to 3.1 mm³ min⁻¹ when the lamp is moved closer. Change = (3.1 − 2.0) ÷ 2.0 × 100 = 55%.

Ringing. Above the ring the sucrose is 52 mg cm⁻³ and below it 9 mg cm⁻³. Write the figures, name the control, and say the phloem carries sucrose downward and was blocked by the ring.

Quick self-test

1. Name the layers a water molecule crosses from a root hair to the xylem.
2. State one difference between the apoplastic and symplastic pathways.
3. Why is the apoplastic route blocked at the endodermis?
4. State two pieces of evidence for root pressure.
5. Explain why a fan increases the rate of transpiration.
6. A bubble moves 45 mm in 6.0 minutes. Calculate the rate in mm min⁻¹.
7. The capillary radius is 0.60 mm. Calculate the cross-sectional area to 2 significant figures.
8. Combine the bubble rate from question 6 with the radius in question 7. Calculate the volume of water taken up per minute.
9. A stem has a ring of bark removed. Sucrose builds up above the ring. What does this suggest?
10. A sink uses sucrose. How does this help to maintain mass flow?
11. A tracer moves 54 mm in 9.0 minutes. Calculate the rate in m h⁻¹.

Answers

1. Epidermis (root hair), cortex, endodermis, then xylem.
2. The apoplastic pathway runs through walls and spaces. The symplastic pathway runs through cytoplasm via plasmodesmata.
3. The endodermis cell walls contain a waterproof suberin band, so water must pass through the membrane into the cytoplasm.
4. Any two: sap oozes from a cut stump; guttation; flow falls with a respiratory poison or cold.
5. A fan removes humid air near the stomata, so the vapour gradient is steeper and vapour diffuses out faster.
6. 45 ÷ 6.0 = 7.5 mm min⁻¹.
7. π × 0.60² = 1.131, so 1.1 mm².
8. Using the unrounded area 1.131 mm²: 7.5 × 1.131 = 8.5 mm³ min⁻¹ (to 2 significant figures).
9. The phloem lies in the bark and carries sucrose downwards, and the ring has blocked it.
10. Removing sucrose at the sink keeps the sieve tube water potential higher there, so water leaves and pressure stays lower than at the source.
11. 54 ÷ 9.0 = 6.0 mm min⁻¹; 6.0 × 60 ÷ 1000 = 0.36 m h⁻¹.

Where marks are usually lost

• Saying the apoplast goes “through” cells. It goes through walls and spaces.
• Writing “water moves by osmosis up the xylem”. Say tension, cohesion and adhesion.
• Writing “sucrose is pumped by active transport at the sink”. It is loaded at the source.
• Treating higher humidity as a cause of faster transpiration.
• Quoting temperature “speeds it up” with no mention of evaporation or diffusion.
• Not converting units: mm, mm³, minutes and hours.
• Using the capillary diameter in πr².
• Quoting data without figures from the table, or ignoring the control.
• Calling the potometer a measure of transpiration rather than water 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.