Exchange and transport in plantsOxford AQA IGCSE Biology: Revision notes

Section 1

What are xylem and phloem and how do their structures relate to their functions?

Xylem and phloem are two types of vascular tissue that transport different substances through plants.

Xylem structure and function:

Composed of vessel elements and tracheids that form continuous tubes
Cells are dead at maturity with no cell contents
Walls are thickened with lignin for support and to prevent collapse
Functions: Transport water and mineral ions from roots to leaves; provides structural support
Transport is unidirectional (upwards only)

Phloem structure and function:

Composed of sieve tubes (the main transport cells) and companion cells
Cells are living at maturity with active cytoplasm
Companion cells contain many mitochondria to provide energy for active transport
Sieve plates allow movement between sieve tube elements
Functions: Transport sugars (sucrose) and amino acids from leaves to other plant parts
Transport is bidirectional (up and down)

Feature	Xylem	Phloem
Main transport	Water and minerals	Sugars and amino acids
Cell status	Dead	Living
Direction	Upwards only	Both directions
Wall thickening	Lignin	None
Energy required	No	Yes (active transport)

Key termsxylemphloemvessel elementstracheidsligninsieve tubescompanion cellssieve plates

Exam tip

Examiners expect you to link structure to function clearly. Always explain WHY xylem cells are dead (allows water to flow through) and WHY phloem cells are living (need energy for active transport of sugars).

Common mistake

Students often confuse which tissue transports what. Remember: xylem = water (think X for water); phloem = sugars (Think flow of food).

Section 2

How does water move through plants from roots to leaves?

The transpiration stream is the continuous flow of water from the roots to the leaves and out into the atmosphere.

The process step-by-step:

Root absorption: Root hair cells actively absorb mineral ions, lowering water potential
Water uptake in roots: Water moves into root cells by osmosis, creating root pressure
Movement up the stem: Water moves up the xylem vessel elements in a continuous column
Leaf transpiration: Water evaporates from mesophyll cells and diffuses out through stomata
Continuous flow: As water leaves the leaf, more water is pulled up from below (cohesion–tension theory)

Key principles:

Water molecules are cohesive (stick together), forming a continuous column in xylem
The tension created by transpiration pulls water upwards
Water moves from areas of higher water potential (roots) to lower water potential (leaves and atmosphere)
This is a passive process — no energy from the plant is needed once water is in the xylem
Capillary action also helps water move up narrow xylem vessels

The transpiration stream is essential because it:

Transports mineral ions to all parts of the plant
Cools the plant through evaporative cooling
Maintains turgor pressure in cells for support

Key termstranspiration streamroot pressureosmosiscohesion–tension theorywater potentialcapillary actionturgor pressure

Think of it like this

Think of the xylem as a drinking straw: as you suck (transpiration pulls water out at the top), water is pulled up the entire length. The water molecules hold together (cohesion) to form a continuous column.

Example

On a hot day, a plant loses lots of water through transpiration, creating a strong tension. This pulls water up from the roots. If you cut a stem, water in the xylem will rush out because the tension is released, but air enters and breaks the water column — the plant wilts.

Section 3

What is transpiration and which factors affect its rate?

Transpiration is the evaporation of water from plant leaves and other aerial parts, mostly through stomata.

Key factors affecting transpiration rate:

Factor	Effect on Transpiration	Explanation
Temperature	Increases with temperature	Higher temperature increases kinetic energy of water molecules, so more evaporate
Humidity	Increases as humidity decreases	Low humidity creates a larger water potential gradient between leaf and air, so more water diffuses out
Wind speed	Increases with wind speed	Wind removes saturated air around the leaf, maintaining a steep water potential gradient
Light intensity	Increases with light intensity	Stomata open more in light; photosynthesis also increases water demand

Why these factors matter:

All factors work by increasing the water potential gradient between the leaf interior and the surrounding air
A steeper gradient = faster diffusion of water vapour out of the leaf
Temperature, humidity, and wind affect the rate of evaporation and diffusion
Light intensity affects stomatal opening and photosynthetic activity

Stomatal control:

Guard cells control stomatal aperture in response to light
Stomata open in light (for photosynthesis) and close in darkness and drought (to conserve water)
This allows plants to balance water loss with gas exchange for photosynthesis

Key termstranspirationstomataevaporationwater potential gradientguard cellsstomatal aperture

Exam tip

When explaining each factor, always include the mechanism: how does it change the water potential gradient or stomatal opening? Examiners award marks for mechanistic explanations, not just stating the relationship.

Common mistake

Students often say 'high humidity increases transpiration' — this is backwards. HIGH humidity means air is already saturated with water, so the water potential gradient is shallow and transpiration is SLOW.

Section 4

How do we measure transpiration rate using a potometer?

A potometer (or transpirometer) is a piece of apparatus used to investigate the rate of transpiration by measuring water uptake by a plant.

How a potometer works:

A cut plant shoot is placed in a sealed tube filled with water
A capillary tube with a graduated scale measures water movement
As the plant transpires, water is drawn up through the xylem
This creates a water deficit, and water is pulled from the potometer into the plant
The air bubble in the capillary moves along the scale, indicating water uptake
The distance the bubble moves is proportional to the rate of transpiration

Measuring transpiration rate:

Record the initial position of the air bubble
Measure the distance moved in a set time period (e.g., 10 minutes)
Calculate rate = distance moved / time taken
Repeat and calculate a mean for accuracy

Variables to investigate:

Change one environmental factor (temperature, humidity, wind speed, or light intensity)
Keep all other factors constant (controlled variables)
Measure water uptake at each condition
Plot a graph of the results to show the relationship

Important considerations:

Ensure an airtight seal so only transpiration causes water loss (not direct evaporation)
Keep the apparatus in a constant position to avoid false readings
A potometer measures water uptake, not transpiration directly, but assumes all water taken up is transpired

Key termspotometertranspiration ratecapillary tubegraduated scaleair bubblewater uptakecontrolled variables

Exam tip

Exam questions often ask you to 'describe how to investigate the effect of [factor] on transpiration using a potometer.' Your answer must identify the independent variable (the factor you change), dependent variable (water uptake/distance moved), and at least three controlled variables (e.g. same plant, same time period, same initial conditions).

Example

If investigating temperature: set up identical potometers in water baths at 15°C, 25°C, and 35°C. Measure bubble movement every 5 minutes for 30 minutes. Record distance moved. Calculate mean rate for each temperature. You should find transpiration rate increases with temperature.

Section 5

How are sugars transported through phloem by translocation? (Higher Tier)

Translocation is the transport of dissolved sugars (mainly sucrose) and amino acids through phloem sieve tubes from areas of production (source) to areas of use or storage (sink).

Source and sink:

Source: A region of sugar production (e.g., mature photosynthetic leaves)
Sink: A region of sugar use or storage (e.g., growing shoots, roots, fruits, seeds)
Direction of translocation changes depending on the season and plant needs

The mechanism of translocation (mass flow hypothesis):

Active loading at the source: Sugars are actively transported from photosynthetic cells into sieve tube elements
- Requires ATP from companion cell mitochondria
- Lowers the water potential in the sieve tube
Water uptake: Water moves into the sieve tube by osmosis (from high to low water potential)
- Creates turgor pressure (hydrostatic pressure) in the sieve tube
Mass flow: The increased turgor pressure pushes the solution of sugars and water along the sieve tube towards the sink
- This is a passive process once the pressure difference is created
Active unloading at the sink: Sugars are actively transported out of the sieve tube into sink cells
- Raises water potential in the sieve tube
- Water follows by osmosis, maintaining the pressure gradient

Key points:

Translocation is bidirectional — phloem can transport sugars up, down, or sideways depending on source and sink locations
Unlike xylem, phloem transport requires living cells and active transport
The companion cells provide the ATP needed for active transport of sugars
Both sugars and amino acids are transported by the same mechanism

Key termstranslocationsucroseamino acidssieve tubessourcesinkactive loadingmass flow hypothesisturgor pressureactive unloading

Exam tip

Examiners expect you to explain the ENTIRE process step-by-step, linking each stage to osmosis, water potential, and pressure. Don't just say 'sugars move through phloem' — show how active loading, osmotic water uptake, and pressure gradients work together.

Think of it like this

Think of phloem like a pressurised pipeline: sugars are pumped in at one end (active loading), water follows by osmosis, building pressure that pushes the whole mixture along. At the other end, sugars are removed (unloading), water leaves by osmosis, and pressure drops ready for the cycle to continue.

Must Know

Xylem transports water and mineral ions upwards; cells are dead, lignified, and passive. Phloem transports sugars and amino acids in both directions; cells are living and require active transport.
The transpiration stream moves water from roots to leaves by cohesion–tension forces — transpiration (evaporation at leaves) creates tension that pulls water up through xylem, with water molecules held together by cohesion.
Transpiration rate increases with: higher temperature (more kinetic energy), lower humidity (steeper water potential gradient), higher wind speed (removes saturated air), and higher light intensity (stomata open more). All factors work by increasing the water potential gradient.
A potometer measures water uptake via a moving air bubble in a capillary tube; to investigate a factor's effect, change one independent variable (e.g. temperature) and keep all others constant (controlled variables).
Translocation of sugars occurs by mass flow: active loading at the source (uses ATP, lowers water potential), osmotic water uptake creates turgor pressure, which pushes solution passively along phloem towards the sink where sugars are unloaded. This requires living, respiring companion cells to provide energy.

That's the notes covered.

Carry on to the next subtopic.