A7 Mass transport in plants

Question 1

What are potometers used to measure?

Answer 1

Transpiration

Question 2

What are the steps in using a potometer?

Answer 2

1. Cut a shoot underwater to prevent air getting into xylem and cut at a slant to increases the surface area for water uptake.
2. Assemble the potometer under water and insert the shoot
3. Keep the end of the capillary tube in water
4. Check the apparatus is water and airtight

Question 3

State and explain the 5 properties of water.

Answer 3

1. Water is cohesive which means it sticks to other water molecules due to its hydrogen bonds. It is also adhesive which means it sticks to other substances forming a hydration shell
2. Water can act as a solvent so it can dissolve many different kinds of molecules.
3. Water is a reactive molecule as it is used in condensation reactions and released in hydrolysis reactions. It is also a metabolite as it is involved in important metabolic reactions ( photosynthesis and respiration).
4. Water has high latent heat for vaporisation ( requires heat energy to convert from a liquid to a gas which is called evaporation which involves sweat)
5. Water has high specific heat capacity as it acts as a temperature buffer (maintains temp change)

Question 4

What is the theory called in the xylem where water molecules are cohesive and that there is a pull force from evaporation and diffusion?

Answer 4

Cohesion-tension theory

Question 5

What is transpiration?

Answer 5

The evaporation and diffusion of water molecules out of the stomata

Question 6

Describe the Cohesion- tension theory. 5 marks

Answer 6

-Water lost from leaf because of transpiration / evaporation and diffusion of water out of the stomata
-Lowers water potential of mesophyll / leaf cells
-Water pulled up xylem (creating tension)
-Water molecules cohere / ‘stick’ together by hydrogen bonds;
-(forming continuous) water column;
-Adhesion of water (molecules) to walls of xylem;