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2.8Structures and Functions in Living Organisms

Transport Systems — Edexcel Linear - OL Revision Notes & Topical Questions

15 min read 25 parts Lesson 23 of 25 Unit quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Lesson summary

Transport in Plants: Xylem, Phloem, Roots and Transpiration — EDEXCEL LINEAR - OL notes: Explain why organisms need transport systems, the roles of xylem and phloem, how root hair cells absorb water and ions, and the factors that affect transpiration.To function, organisms must e…

  • translocation
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Worked example 5

Part 1 of 7The need for a transport system

To function, organisms must exchange substances such as food molecules and waste with their environment.

The need for a transport system
Unicellular organisms have a large SA:V ratio and a short diffusion distance, so diffusion is enough.
Large multicellular organisms have a small SA:V ratio and long diffusion distances, so they need specialised transport systems.
Xylem
In plants, the vascular system transports substances: the xylem moves water and minerals from roots to shoots,
  • and the phloem distributes sugars and amino acids from the leaves to the rest of the plant.

Part 2 of 7Xylem and phloem

XylemPhloem
TransportsWater and mineral ionsSucrose and amino acids
DirectionRoots to leaves (upwards)Both directions (translocation)
StructureDead cells with no end walls, forming hollow tubes strengthened with ligninLiving cells (sieve tubes with companion cells)

The movement of sucrose through the phloem is called translocation; sucrose moves from sources (leaves) to sinks (growing regions, roots and storage organs).

Part 3 of 7Root hair cells

Root hair cells are tiny, single-celled extensions of epidermis cells in the root.

1
The long extension gives a large surface area for absorbing water by osmosis and mineral ions by active transport.
2
The root hair cell has a lower water potential than the soil water, so water enters by osmosis.
3
Mineral ions such as nitrate are at a lower concentration in the soil than in the cell,
  • so they are taken in by active transport using energy from respiration.
    • (hence many mitochondria)
4
Water passes across the root cortex into the xylem and is drawn up the plant.

Part 4 of 7Transpiration

Transpiration is the evaporation of water vapour from the surface of a plant, mainly from the leaves through open stomata. It creates a pull that draws water up through the xylem (the transpiration stream), which brings minerals, supplies water for photosynthesis, keeps cells turgid and cools the leaf.

FactorEffect on transpiration rateReason
Light intensityIncreasesStomata open wider
TemperatureIncreasesFaster evaporation and diffusion
HumidityDecreases as humidity risesSmaller concentration gradient
Wind speedIncreasesRemoves the water vapour layer, steepening the gradient

Part 5 of 7Why plants and large animals need transport: the surface area link

As an organism increases in size, its volume grows faster than its surface area, so the surface area to volume ratio falls.

1
A plant such as a tree cannot get water to its highest leaves by diffusion alone,
  • and its roots cannot get sugars from leaves by diffusion either;
    • they need the mass-flow systems of xylem and phloem.
2
In a root, the cells of the cortex pass water on by osmosis from cell to cell and through the cell walls to the xylem in the centre of the root.
3
The endodermis then ensures that water and ions enter the xylem.
4
The pull of the transpiration stream, helped by cohesion between water molecules (they stick together) and adhesion to the xylem walls, draws a continuous column of water up the plant,
  • so the plant does not need to use energy to lift the water.

Part 6 of 7Exam focus on transpiration

Graphs of transpiration against light intensity, temperature or wind speed rise and then level off or fall.

Exam focus on transpiration
Describe the pattern and explain it: more light opens the stomata wider; more heat gives more kinetic energy for evaporation; wind removes saturated air;
  • but at very high temperatures or in drought, stomata close and the rate falls.
Wilting occurs when transpiration exceeds the uptake of water: the cells lose turgor and the plant droops.

Part 7 of 7Worked examples

  1. State two differences between xylem and phloem. [2]2 marks

  2. Explain how root hair cells are adapted. [3]3 marks

  3. Define transpiration. [1]1 mark

  4. Explain why windy conditions increase transpiration. [2]2 marks

  5. Why do mineral ions enter by active transport? [2]2 marks

Key terms

TermMeaning
XylemTissue carrying water and mineral ions upwards.
PhloemTissue carrying sucrose and amino acids.
TranslocationMovement of sucrose in the phloem.
TranspirationEvaporation of water vapour from the plant surface.
Root hair cellRoot epidermal cell with a long extension for absorption.

Avoid these mistakes

1
Saying the phloem carries water.
2
Saying water enters root hair cells by active transport.
3
Thinking transpiration is the same as photosynthesis.
4
Saying humidity raises transpiration.

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