Plant Organisation & Transport

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CognitoPlant transport — xylem and phloem

CognitoTranspiration and stomata

Key facts & methods

Plant tissues and organs

Epidermal tissue: outer protective layer of leaf and root.
Mesophyll tissue: contains chloroplasts — site of photosynthesis in leaves.
Xylem: transports water and mineral ions upward from roots. Dead cells with lignified walls forming hollow tubes.
Phloem: transports sugars (sucrose) both up and down from leaves to rest of plant (translocation). Living cells.
Meristem tissue: found at root and shoot tips. Undifferentiated cells that divide and differentiate.

Transpiration

Transpiration: loss of water vapour from leaves through stomata.
Water evaporates from mesophyll cells → diffuses out through stomata → replaced by water drawn up xylem.
This creates the transpiration stream — a continuous column of water from roots to leaves.
Factors increasing transpiration rate: increased light intensity, increased temperature, increased wind speed, decreased humidity.
Stomata: pores in leaf surface controlled by guard cells. Open in light (for CO₂ entry), close in dark.

Guard cells and stomata

Guard cells control opening and closing of stomata.
In light: guard cells take up water by osmosis → become turgid → stomata open.
In dark/drought: guard cells lose water → become flaccid → stomata close → reduces water loss.
Most stomata on lower (abaxial) surface of leaves — reduces direct sunlight and water loss.
Measurement: potometer measures transpiration rate by tracking water uptake.

Translocation

Phloem transports dissolved sugars (sucrose) from sources to sinks.
Source: where sugar is made or stored (e.g. leaf, storage organ).
Sink: where sugar is used or stored (e.g. growing root tip, fruit, storage root).
Translocation can go up or down the plant (unlike xylem which is one-way upward).
Aphids insert stylets into phloem — evidence that phloem is under pressure.

Exam questions — 5 questions · 11 marks · AQA 8461 style

1 markXylem vs phloemAQA 8461 style

Which of the following correctly describes the function of phloem?

A Transports water and minerals from roots to leaves, upward only

B Transports dissolved sugars from leaves to all other parts of the plant, in both directions

C Provides structural support to the stem through lignified walls

D Transports water only, not minerals

3 marksTranspiration factorsAQA 8461 P1 style

A student measures transpiration rate using a potometer on a warm, windy day, then again on a cool, still day. The rate is higher on the warm windy day. Explain why. (3 marks)

Hint: Explain two factors: temperature (kinetic energy/evaporation rate) and wind (concentration gradient). Link each to the rate of water loss.

+30 XP

1 markGuard cellsAQA 8461 style

What causes stomata to open in the light?

A Guard cells produce glucose by photosynthesis which is converted to starch, making them rigid

B Guard cells absorb water by osmosis, become turgid and their shape changes to open the pore

C Guard cells lose water and become flaccid, pulling apart to open the pore

D Guard cells die in the light, leaving a gap

4 marksXylem structure and functionAQA 8461 style

Describe the structure of xylem vessels and explain how this structure is suited to their function. (4 marks)

Hint: Structure: dead cells, hollow, lignified walls, end walls removed. Function: link each structural feature to how it helps transport water.

+40 XP

1 markTranspiration directionAQA 8461 style

In which direction does translocation in phloem move?

A Upward only, from roots to leaves

B Downward only, from leaves to roots

C In both directions, from sources to sinks

D Sideways across the stem only

Plant Organisation & Transport

Plant tissues and organs

Transpiration

Guard cells and stomata

Translocation

Module complete! 🎉