More Exchange and Transport Systems - Transport in Plants - Xylem

Question 1

How does light affect transpiration rate?

Answer 1

Light and Transpiration Rate
POSITIVE CORRELATION

the stomata open when there’s light to let in CO2, so transpiration occurs

Question 2

How does temperature affect transpiration rate?

Answer 2

Temperature and Transpiration Rate
POSITIVE CORRELATION

when the water is warmer,
the molecules have more energy
so they move faster,
so a higher rate of evaporation.
As the water evaporates from the leaf,
there's a higher concentration gradient,
and so diffusion occurs at a higher rate

Question 3

How does humidity affect transpiration rate?

Answer 3

Humidity and Transpiration Rate
NEGATIVE CORRELATION 
when there is less humidity, 
the concentration gradient is higher, 
so a higher rate of transpiration

Question 4

How does wind affect transpiration rat

Answer 4

Wind and Transpiration Rate 
POSITIVE CORRELATION 
when there is more wind, 
there's a higher conc. gradient 
so a higher transpiration rate

Question 5

What is transpiration and how does it occur?

Answer 5

Transpiration is the loss of water from a plant’s surface, especially the leaves.

• Water evaporates from the moist cell walls and accumulates between cells in a leaf
• When the stomata open, it moves out of the leaf down the concentration gradient

Question 6

In short, what are the four factors of transpiration?

Answer 6

Light
Temperature
Humidity
Wind

Question 7

What is a potometer?

What does it measure?

Answer 7

A piece of apparatus used to estimate transpiration rates.

It measures water uptake by a plant,
but it’s assumed that water uptake by a plant is directly related to water loss by the leaves.

Question 8

Describe the practical that investigates how different factors affect the transpiration rate.

Answer 8

1) Cut a shoot underwater to prevent air entering the xylem.
Cut it at a slant angle to increase surface area available for water uptake.

2) Assemble the potometer in water and insert the shoot underwater, so no air can enter.
3) Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water.
4) Check that the apparatus is watertight and airtight.
5) Dry the leaves, allow time for the shoot to acclimatise, and then shut the tap.
6) Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water.
7) Record the starting position of the air bubble.

8) Start a stopwatch and record the distance moved by the bubble per unit time (e.g. hour).
The rate of air bubble movement is an estimate of the transpiration rate.

9) Remember, only change one variable (e.g. temperature) at a time. Keep all others (e.g. light, humidity) constant.