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AQA BiologyOrganisation

Plant tissues and transport

Explain movement through xylem and phloem and factors affecting transpiration.

Start here

The key idea

Xylem carries water and mineral ions upwards. Phloem translocates dissolved sugars between sources and sinks.

Xylem and phloemThe two transport tissues move different materials.
Xylem and phloemThe two transport tissues move different materials.xylemwater + minerals upphloemsugars both ways
Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

1

Plant tissues

Plant organs such as leaves, stems and roots are made of tissues working together.The epidermis covers the plant surface, the palisade mesophyll is packed with chloroplasts for photosynthesis, and the spongy mesophyll contains air spaces for gas exchange.Xylem and phloem make up the transport tissues, and meristem tissue at growing tips contains cells that can divide.

2

Xylem and phloem

Xylem tissue transports water and dissolved mineral ions from the roots up to the stem and leaves in one direction only.Xylem is made of dead, hollow cells strengthened with a substance called lignin.Phloem transports dissolved sugars (such as glucose) made in the leaves to the rest of the plant for use or storage, in a process called translocation, and can move them in both directions.

3

Transpiration

Transpiration is the loss of water vapour from the leaves, mainly through the stomata.As water evaporates from the leaf, more water is pulled up through the xylem in a continuous transpiration stream from the roots.The rate of transpiration increases with higher temperature, increased air movement (wind), lower humidity and brighter light, which opens the stomata wider.

4

Stomata and guard cells

Stomata are tiny pores, mostly on the lower surface of the leaf, that allow carbon dioxide in and water vapour and oxygen out.Each stoma is surrounded by two guard cells that change shape to open and close the pore.In bright light the guard cells take in water and become turgid, opening the stomata; in darkness or drought they lose water and close the stomata to reduce water loss.

Key terms

Definitions to learn

Xylem

Dead, hollow tissue strengthened with lignin that transports water and minerals up the plant.

Phloem

Living tissue that transports dissolved sugars around the plant by translocation.

Transpiration

The loss of water vapour from a plant's leaves, mainly through the stomata.

Translocation

The transport of dissolved sugars through the phloem to where they are needed or stored.

Stomata

Tiny pores in the leaf that allow gas exchange and water loss, controlled by guard cells.

Guard cell

A cell that changes shape to open or close a stoma depending on conditions.

Worked example

Explain why transpiration rate usually increases on a windy day.

1

Water vapour diffuses out through stomata.

2

Wind removes humid air near the leaf.

3

This maintains a steep water vapour concentration gradient.

Final answer

Wind usually increases transpiration by maintaining a steep gradient.

Exam habit

State which vessel (xylem or phloem) and which substance. Xylem: water and minerals upward.Phloem: sugars between source and sink. Stomata open in light and close in darkness — link to guard cells.

Watch out

Do not confuse xylem and phloem: xylem transports water; phloem transports sugars.

Examiner tips

How to score full marks

  • 1Xylem carries water UP only; phloem carries sugars in BOTH directions — do not mix these up.
  • 2When explaining transpiration rate, always link the factor (temperature, wind, humidity, light) to the rate of evaporation or diffusion of water vapour.
  • 3Remember xylem cells are DEAD and phloem cells are LIVING — examiners often test this directly.
Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1State two factors that increase transpiration rate.
Mark scheme
  1. 1.Think about evaporation and diffusion.
Any two from: higher temperature, greater light intensity, more air movement, lower humidity.
2What is translocation?
Mark scheme
  1. 1.Identify the substance and tissue.
The movement of dissolved sugars through phloem.
3How are root hair cells adapted for absorption?
Mark scheme
  1. 1.Link shape to surface area.
Their long projections create a large surface area for water and mineral ion uptake.
4Name the tissue that transports water through a plant.[1 mark]
Mark scheme
  1. 1.Recall which transport tissue carries water.
Xylem (1).
5State two substances transported by the phloem and where they are made.[2 marks]
Mark scheme
  1. 1.Think about the products of photosynthesis.
Dissolved sugars / glucose (1) made in the leaves by photosynthesis (1).
6Explain why most stomata are found on the lower surface of a leaf.[3 marks]
Mark scheme
  1. 1.Consider light, temperature and water loss.
The lower surface is cooler / more shaded (1), so less water evaporates from the leaf (1), reducing water loss while still allowing carbon dioxide in for photosynthesis (1).
7Describe how the structure of xylem makes it suited to its function.[4 marks]
Mark scheme
  1. 1.Link each structural feature to transporting water.
Made of dead, hollow cells with no end walls (1) forming a continuous tube for water to flow through (1); strengthened with lignin (1) which supports the plant and stops the tubes collapsing under pressure (1).
8A student investigates transpiration and finds the rate is higher on a hot, windy day than on a cool, still day. Explain this difference.[5 marks]
Mark scheme
  1. 1.Explain the effect of temperature and wind separately on water movement.
Higher temperature increases the rate of evaporation of water from the leaf cells (1), so water diffuses out of the stomata faster (1); wind blows away the water vapour around the leaf (1), maintaining a steep concentration gradient (1) so more water diffuses out, increasing the transpiration rate (1).
9Explain how guard cells open the stomata in bright light.[3 marks]
Mark scheme
  1. 1.State what guard cells take in and how this changes their shape.
In bright light, guard cells absorb water by osmosis and become turgid (1); the inner wall is thicker so the cells bend outwards (1); the pore opens, allowing carbon dioxide in for photosynthesis and water vapour and oxygen out (1).
10A student measures transpiration using a potometer. Describe how a potometer measures the rate of transpiration and state one assumption made.[3 marks]
Mark scheme
  1. 1.Explain how water uptake is measured.
  2. 2.State the assumption linking uptake to transpiration.
A potometer measures the rate at which water is taken up by the cut shoot (1) by recording the movement of an air bubble along a capillary tube over time (1); the assumption is that water uptake equals water lost by transpiration (1).
11Explain why a plant wilts if the soil becomes very dry.[4 marks]
Mark scheme
  1. 1.Link soil water availability to osmosis in roots.
  2. 2.Connect to turgidity of cells.
When the soil is dry the water concentration in the soil water is lower than or equal to that in root cells (1); water cannot move into the root by osmosis (1); cells lose water and become flaccid (lose turgor) (1); without turgor pressure to support the plant tissues, the plant wilts (1).
12Suggest and explain two ways a plant adapted to hot desert conditions might reduce water loss.[4 marks]
Mark scheme
  1. 1.Think about adaptations to stomata, leaf surface or roots.
  2. 2.Link each adaptation to reducing transpiration.
Any two from: having fewer stomata (1) reduces the number of pores through which water can evaporate (1); having a thick waxy cuticle on the leaf surface (1) reduces evaporation from the leaf surface (1); closing stomata during the day (1) prevents evaporation when temperature and sunlight are high (1); having small or rolled leaves (1) reduces the surface area for evaporation (1).
13Describe the role of the palisade mesophyll layer in a leaf and explain how its position and structure are suited to photosynthesis.[4 marks]
Mark scheme
  1. 1.State its function.
  2. 2.Link its position near the top of the leaf and its high chloroplast density.
The palisade mesophyll is the main site of photosynthesis in the leaf (1); it is positioned near the upper surface where light intensity is greatest (1); the cells are packed with chloroplasts to maximise light absorption (1); cells are column-shaped and tightly packed to intercept as much light as possible (1).
14Compare translocation and transpiration in terms of the tissue used, the substance transported and the direction of transport.[4 marks]
Mark scheme
  1. 1.Use xylem/phloem and state what each carries.
  2. 2.State the direction(s) for each.
Transpiration uses xylem tissue and transports water (and mineral ions) (1) only upwards from roots to leaves (1); translocation uses phloem tissue and transports dissolved sugars (1) in both directions, from leaves to all other parts of the plant (1).
Official exam-board sources
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