Transpiration

Question 1

What is transpiration?

Answer 1

The evaporation of water from a plants surface

Question 2

Why does transpiration occur?

Answer 2

Thermal energy from the sun (temperature) causes evaporation
Stomata provide a route for diffusion of water vapour out of leaves

Question 3

Transpiration process?

Answer 3

Water evaporates from cells surrounding the air spaces due to the energy from the sun, reducing their water potential
Water moves into these cells from neighbouring cells, diffusing down the water potential gradient
This produces a water potential gradient from the xylem (high) to cells adjacent to air spaces (low)
Water lost from leaf cells replaced by water from xylem

Question 4

Cohesion-tension theory?

Answer 4

Water evaporates from the leaves at the top of the xylem (transpiration) which creates a negative pressure
this creates tension, which pulls more water into the leaf
Water molecules are cohesive and can form hydrogen bonds so the whole column of water in the xylem (from the leaves down to the roots) moves upwards to counter the negative pressure
Water enters the stem through the roots
The pull of water creates tension in the xylem- the walls are pulled in slightly

Question 5

The two types of tissue?

Answer 5

Xylem tissue- water and mineral ions in solution
Move up the plant from the roots to the leaves

Phloem tissue- organic substances (e.g. sugar) in solution
Move up and down the plant

Question 6

Rate of transpiration increases with?

Answer 6

Increased temperature
Decreased humidity
Increased light intensity
Increased wind speed

Question 7

Effect of light on transpiration?

Answer 7

Stomata open during light to let in CO2 for photosynthesis- allows for more gas exchange
When it’s dark, the stomata are closed- little transpiration
POSITIVE CORRELATION

Question 8

Temperature?

Answer 8

Higher the temp = increase in thermal energy = water molecules have more kinetic energy
Water molecules evaporate from the cells inside the leaf faster- increases the conc gradient between the inside & outside of leaf- water diffuses out of the leaf faster
POSITIVE CORRELATION

Question 9

Humidity?

Answer 9

If the air around the plant is dry, the conc gradient between the leaf and the air is increased- increases transpiration
NEGATIVE CORRELATION

Question 10

Wind speed?

Answer 10

Lots of air movement blows away water molecules from around the stomata
Increases conc gradient
POSITIVE CORRELATION