AQA A-Level Biology (7402)

## Learning Outcomes

- Explain why large plants need mass transport systems rather than relying on diffusion alone
- Describe the structure of xylem and phloem and relate structure to function
- Explain how water moves from the soil into the xylem and then up the plant by the cohesion-tension theory
- Distinguish clearly between transpiration, the transpiration stream, and translocation
- Explain the mass flow hypothesis for phloem transport from source to sink
- Interpret and evaluate evidence from ringing, radioactive tracer, and aphid stylet experiments
- Apply knowledge to potometer practical questions, including calculations and limitations
- Explain how xerophytic adaptations reduce water loss

## AQA A-Level Biology (7402) Syllabus

For the AQA A-Level Biology (7402), you are expected to understand mass transport in plants, with a focus on the following syllabus points:

- Know that xylem transports water and mineral ions, while phloem transports organic substances such as sucrose and amino acids
- Explain how water enters root hair cells and moves through root tissues by apoplast and symplast pathways
- Understand the role of the endodermis and Casparian strip in selective uptake into the xylem
- Describe the cohesion-tension theory for the movement of water in xylem
- Define translocation and explain the movement of assimilates from sources to sinks in phloem
- Explain the mass flow hypothesis, including active loading, osmosis, and hydrostatic pressure gradients
- Interpret evidence from ringing, radioactive tracer, and aphid stylet experiments
- Evaluate potometer measurements and explain the effects of environmental factors on transpiration rate

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why is diffusion alone not enough to move substances through a large plant?
2. How does the Casparian strip affect the movement of water and ions into the xylem?
3. How does transpiration create a force that pulls water up the xylem?
4. What is the difference between a source and a sink in phloem transport?
5. Why does a potometer only estimate transpiration rather than measure it directly?

## Introduction

Large multicellular organisms have a common problem: useful substances must move over distances far too great for diffusion to be efficient on its own. Plants are no exception. Even though they do not have a heart or a blood circulation system, they still need to move water, mineral ions, and the products of photosynthesis from one region to another.

In plants, this is done by two transport tissues: **xylem** and **phloem**. Xylem is mainly linked to water movement from roots to leaves. Phloem is mainly linked to the movement of organic substances such as sucrose and amino acids between different parts of the plant. In exams, students often know the names of these tissues but lose marks by mixing up their contents, direction of flow, or mechanism of transport.

> **Key Term: mass transport**
> The efficient movement of substances over large distances between exchange surfaces and the cells that need those substances.

### Why plants need mass transport systems

A small organism can rely on diffusion because the diffusion distance is short and its surface area to volume ratio is high. Large plants cannot. Cells in the centre of roots, stems, and leaves are too far from the external environment for diffusion alone to supply enough water or solutes at a useful rate.

Plants solve this by using vascular tissue. Xylem and phloem maintain gradients that allow exchange to continue at cell surfaces. In other words, mass transport does not replace diffusion entirely. Instead, it gets substances close enough for diffusion, osmosis, or active transport to finish the job.

This is a useful synoptic point. In biology, transport systems and exchange surfaces work together. Roots absorb water and ions. Leaves exchange gases and lose water vapour. Xylem and phloem link these regions into one working whole.

> **Key Term: xylem**
> Plant tissue that transports water and mineral ions from roots to leaves, mainly in an upward direction.
>
> **Key Term: phloem**
> Plant tissue that transports organic substances such as sucrose and amino acids by translocation.

A direct comparison helps:

- **Xylem**: dead cells, hollow tubes, lignified walls, transport water and mineral ions, mainly one-way upward
- **Phloem**: living tissue, sieve tube elements with companion cells, transport sucrose and amino acids, movement can be in either direction in different sieve tubes

Students often confuse “two-way transport” with “back and forth in the same tube at the same time”. The safer wording is that translocation can occur in different directions in different sieve tubes, depending on source and sink positions.

> **Test Tip:** If a question asks about transport of sugars, amino acids, or assimilates, the answer must refer to phloem, not xylem. This is a very common error.

### Water uptake and movement in xylem

Water enters the plant at the roots. Usually, soil water has a higher water potential than the cytoplasm of root hair cells, so water enters root hair cells by **osmosis** across their partially permeable membranes. Mineral ions may also be actively transported into root cells, which helps lower water potential and encourages further water uptake.

Once in the root, water moves across the cortex towards the xylem by two pathways:

- **Apoplast pathway**: through cell walls and intercellular spaces
- **Symplast pathway**: through the cytoplasm of cells connected by plasmodesmata
  > **Key Term: apoplast pathway**
  > Movement of water through the cell walls and intercellular spaces of plant tissues.
  >
  > **Key Term: symplast pathway**
  > Movement of water through the cytoplasm of plant cells via plasmodesmata.

At the endodermis, the **Casparian strip** blocks the apoplast route. This matters because water and ions can no longer simply move through cell walls into the stele. They must cross a cell-surface membrane and enter the symplast. That gives the plant selective control over which mineral ions enter the xylem.

> **Key Term: Casparian strip**
> A waterproof band of suberin in the endodermis that blocks the apoplast pathway and forces water and ions through cell membranes.

A common mistake is to say that the Casparian strip “stops water entering the xylem”. It does not. It blocks one route, not all movement. Water still reaches the xylem, but only after crossing membranes where selective uptake can occur.

Xylem vessels are adapted for transport:

- vessel elements are dead, so there is no cytoplasm blocking flow
- cells are joined end to end to form continuous tubes
- there are no end walls, or very reduced end walls, so resistance is low
- lignin strengthens the walls and prevents collapse under tension

The mechanism that moves water upward is the **cohesion-tension theory**.

Step by step:

1. Water evaporates from mesophyll cell walls into the air spaces of the leaf.
2. Water vapour diffuses out through open stomata. This is **transpiration**.
3. As water leaves mesophyll cells, their water potential becomes lower.
4. Water moves from the xylem into the mesophyll by osmosis.
5. This pulls water from the xylem in the leaf, creating tension, or negative pressure.
6. Because water molecules cohere through hydrogen bonding, a continuous column of water is pulled upward.
7. Adhesion between water and xylem walls helps maintain the column.
8. Water enters the roots from the soil to replace what is lost.
   > **Key Term: transpiration**
   > The loss of water vapour from the aerial parts of a plant, mainly through the stomata.
   >
   > **Key Term: cohesion-tension theory**
   > The explanation of xylem transport in which transpiration creates tension that pulls a continuous column of water upward because of cohesion between water molecules.

Why does this work? Water has cohesion because hydrogen bonds form between its molecules. Without cohesion, the column would break. Without lignified xylem walls, the vessels could collapse under the tension created by transpiration.

Environmental factors affect transpiration rate in predictable ways:

- **Higher light intensity** usually increases transpiration because stomata open for gas exchange
- **Higher temperature** increases evaporation from mesophyll cell walls and speeds diffusion
- **Higher air movement** removes humid air near the leaf, maintaining a steep water vapour gradient
- **Higher humidity** reduces the water vapour gradient, so transpiration falls

Students should always explain these changes using the water potential or water vapour gradient, not just say “it speeds it up”.

> **Test Tip:** In “explain” questions, do not stop at naming a factor. Link the factor to evaporation, the diffusion gradient, and then the change in transpiration rate.

### Translocation in phloem and the mass flow hypothesis

Phloem transport is called **translocation**. It is the movement of organic solutes, especially sucrose and amino acids, from where they are made or released to where they are used or stored.

> **Key Term: translocation**
> The movement of organic substances in the phloem from sources to sinks.

A **source** is a region with a net production or release of assimilates. A photosynthesising leaf is a common source. A storage organ can also become a source when it releases stored carbohydrate.

A **sink** is a region that uses or stores assimilates. Examples include roots, developing fruits, seeds, and meristems.

> **Key Term: source**
> A region of a plant where assimilates are produced or released and loaded into the phloem.
>
> **Key Term: sink**
> A region of a plant where assimilates are removed from the phloem for use or storage.

Phloem structure explains its function. **Sieve tube elements** are living cells arranged end to end. Their end walls form sieve plates. They have very little cytoplasm and no nucleus at maturity, which reduces resistance to flow. Each sieve tube element has an associated **companion cell**, rich in mitochondria, which provides ATP and metabolic support.

> **Key Term: sieve tube element**
> A living phloem cell joined end to end with others to form tubes for translocation.
>
> **Key Term: companion cell**
> A metabolically active cell associated with a sieve tube element that provides ATP and support for phloem transport.

The accepted explanation is the **mass flow hypothesis**, also called the pressure flow hypothesis.

At the source:

1. Sucrose is loaded into companion cells and sieve tube elements.
2. This loading uses active processes and ATP supplied by companion cells.
3. The sucrose lowers the water potential inside the phloem.
4. Water moves into the phloem from the nearby xylem by osmosis.
5. This raises hydrostatic pressure at the source.

At the sink:

1. Sucrose is removed from the phloem into sink cells.
2. It may be used in respiration or converted into storage products such as starch.
3. Removing sucrose raises the water potential of the phloem at the sink.
4. Water leaves the phloem, often moving back into the xylem.
5. Hydrostatic pressure at the sink becomes lower.

The difference in hydrostatic pressure drives bulk flow of phloem sap from source to sink.

> **Key Term: mass flow hypothesis**
> The explanation of phloem transport in which active loading at the source and unloading at the sink create a hydrostatic pressure gradient that drives bulk flow.

A key exam distinction is this: the actual bulk flow through the sieve tube is passive, but the process as a whole depends on active loading and unloading, so translocation is affected by ATP supply, oxygen availability, temperature, and metabolic poisons.

A classic example is the potato plant. Sucrose moves to tubers, where it is converted to starch. Because starch is insoluble, sucrose concentration in the sink remains low. That helps maintain continued movement from source to sink.

Common student errors here include:

- saying sucrose moves by diffusion all the way through the phloem
- forgetting the role of water entering from the xylem
- describing pressure gradient instead of hydrostatic pressure gradient
- mixing up transpiration in xylem with translocation in phloem

### Evidence, practical work, and exam application

Several experiments support phloem translocation and the mass flow hypothesis.

**Ringing experiments** involve removing a ring of bark from a woody stem. This removes phloem but leaves xylem largely intact. After some time, sugars accumulate above the ring, causing a bulge. Tissues below the ring may die from lack of assimilates. This shows that sugars move in the phloem, not the xylem.

**Radioactive tracer experiments** use **carbon-14** in carbon dioxide. The plant incorporates the radioactive carbon into sugars during photosynthesis. The labelled sugars can then be tracked as they move from the leaf to other parts of the plant. This shows that assimilates move from sources to sinks in phloem tissue.

**Aphid stylet experiments** are especially useful for the mass flow idea. Aphids feed from phloem using a long stylet. If the stylet is cut, phloem sap continues to flow out. This shows that phloem sap is under positive hydrostatic pressure.

These experiments are evidence, not absolute proof. Some observations still raise questions about the model. For example:

- sieve plates seem as if they should slow mass flow
- not all solutes move at exactly the same rate
- delivery to sinks is not always exactly what a simple pressure model predicts

That is why the mass flow hypothesis is best described as the best-supported explanation, not as unquestionable fact.

A required practical link is the **potometer**, used to estimate transpiration rate by measuring water uptake.

Worked example:

A capillary tube has radius 0.25 mm. In 5 minutes, the bubble moves 40 mm.

Step 1: Calculate volume of water taken up.

Volume = πr²d

= π × (0.25)² × 40  
= π × 0.0625 × 40  
= π × 2.5  
= 7.85 mm³

Step 2: Calculate rate.

Rate = volume ÷ time  
= 7.85 ÷ 5  
= 1.57 mm³ min⁻¹

So the estimated transpiration rate is **1.57 mm³ min⁻¹**.

This practical has key evaluation points:

- cut the shoot under water to prevent air entering xylem
- fill apparatus completely with water
- ensure the apparatus is airtight
- control variables such as leaf area, temperature, light intensity, humidity, and air movement
- repeat measurements and calculate a mean

Most importantly, a potometer does **not** measure transpiration directly. It measures water uptake. These are close, but not identical, because some water is used in photosynthesis, growth, and maintaining turgor.

> **Test Tip:** If asked to evaluate a potometer, always include the point that it estimates transpiration from water uptake rather than measuring water loss directly.

Xerophytes are another common exam angle. These are plants adapted to dry conditions. Their features reduce water loss by reducing evaporation or by trapping humid air near stomata. Adaptations include:

- thick waxy cuticle
- rolled leaves
- hairs
- sunken stomata
- reduced leaf area

Marram grass is a standard example. Rolled leaves and hairs trap still, moist air inside the leaf roll. That reduces the water vapour gradient between the leaf and the surrounding air, so transpiration is reduced.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Large plants require mass transport because diffusion alone is too slow over long distances
- Xylem transports water and mineral ions mainly upward from roots to leaves
- Phloem transports organic substances such as sucrose and amino acids by translocation
- Water moves across roots by apoplast and symplast pathways
- The Casparian strip blocks the apoplast pathway at the endodermis and forces selective membrane crossing
- The cohesion-tension theory explains xylem transport using transpiration, cohesion, adhesion, and tension
- Transpiration is water vapour loss from aerial parts, mainly through stomata
- Translocation moves assimilates from sources to sinks in phloem
- The mass flow hypothesis depends on active loading, osmosis, and a hydrostatic pressure gradient
- Ringing, radioactive tracer, and aphid stylet experiments provide evidence for phloem transport and mass flow
- A potometer measures water uptake as an estimate of transpiration rate
- Xerophyte adaptations reduce water loss by lowering evaporation or reducing the water vapour gradient

## Key Terms and Concepts

- mass transport
- xylem
- phloem
- apoplast pathway
- symplast pathway
- Casparian strip
- transpiration
- cohesion-tension theory
- translocation
- source
- sink
- sieve tube element
- companion cell
- mass flow hypothesis


Exchange with the environment - Mass transport in plants