Support & Transport in Plants

Two transport systems, one plant

Plants have two separate transport tissues, both found in the roots, stems, and leaves:

Tissue	Transports	Direction
Xylem	Water and dissolved minerals	Upward — roots to leaves
Phloem	Manufactured food (sugars) — translocation	From leaves to all other parts

Transpiration — the engine that pulls water up

Transpiration is the loss of water vapour from a plant's surface, mostly through tiny pores (stomata) in the leaves. As water evaporates from the leaf, it creates a "pull" that draws more water up through the xylem, all the way from the roots — a bit like sucking on a straw.

The pathway is: water uptake by root hairs → movement up the xylem → evaporation from the leaves (transpiration). This constant flow also carries dissolved minerals to where they're needed.

What affects the rate of transpiration?

Temperature — higher temperature increases the rate of evaporation, speeding up transpiration.
Light intensity — more light means stomata open wider (for photosynthesis), increasing water loss.
Humidity — high humidity in the air reduces the concentration gradient for evaporation, slowing transpiration.
Wind — moving air sweeps away water vapour near the leaf surface, increasing the rate of transpiration.

Scientists measure transpiration rate in the lab using a potometer, which tracks how quickly a leafy shoot draws up water.

Growth rings — a record in the wood

As a tree undergoes secondary growth each year, it produces a new ring of wood. Counting these annual rings in a cut tree trunk can reveal both the tree's age and clues about past climate conditions — wider rings generally indicate a favourable growing season.

Answer in your exercise book.

Question 1 · (2 marks)

Define transpiration.

Question 2 · (4 marks)

Explain how transpiration helps draw water up through the xylem, from the roots to the leaves.

Question 3 · (4 marks)

State FOUR factors that affect the rate of transpiration, and explain how EACH one affects the rate.

Question 4 · (2 marks)

Name the instrument used to measure the rate of water uptake in a leafy shoot.

Question 5 · (3 marks)

Explain what annual rings in a tree trunk can tell us, and why they form.

Question 6 · (6 marks)

A learner uses a potometer to measure the rate of water uptake by a leafy shoot, by tracking how far an air bubble moves along a calibrated scale over time. The experiment is repeated with a fan blowing air gently over the leaves:

Time (min)	Bubble distance, still air (mm)	Bubble distance, with fan (mm)
0	0	0
5	12	25
10	24	50
15	36	74

(a) Calculate the rate of water uptake, in mm/min, in still air between 0 and 10 minutes. Show your working. (2)
(b) Calculate the rate of water uptake, in mm/min, with the fan running, between 0 and 10 minutes. Show your working. (2)
(c) Using the concept of transpiration, explain why the rate with the fan is so much higher than in still air. (2)

Support & Transport in Plants

Two transport systems, one plant

Transpiration — the engine that pulls water up

What affects the rate of transpiration?

Growth rings — a record in the wood

Transpiration Rate Simulator

Water movement through the plant