Transpiration and translocation

Question 1

What is transpiration?

Answer 1

the loss of water vapour via the stomata

Question 2

What is the transpiration stream?

Answer 2

the stream of water flowing up through a plant

Question 3

Is the movement of water in a plant passive or active?

Answer 3

passive

Question 4

Describe and explain the structure of the xylem?

Answer 4

xylem tissue contains tubes called vessels
dead cells are arranged end to end
their walls become impregnated with lignin which is impermeable to water
the living parts of the cell die, leaving a water filled space called the lumen

Question 5

What direction/s does the xylem transport?

Answer 5

one direction ONLY

Question 6

How does water move up the xylem?

Answer 6

due to cohesive-tension theory

Question 7

Explain cohesive-tension theory?

Answer 7

-water evaporates from the stomata, which creates a water potential gradient
-this loss of water by transpiration means that more water molecules are pulled up the xylem to replace it
-due to hydrogen bonds between water molecules (cohesion) a column of water is created
-as the column of water is pulled up it creates tension, pulling the xylem in to be narrower
-water molecules also adhere to the sides of the xylem which helps to pull the column upwards
-finally water enters the roots via osmosis to replace the water moving up the xylem

Question 8

What are the factors affecting transpiration rate and why?

Answer 8

wind (air movement)- increases transpiration- more wind will blow away humid air containing water vapour, so maintains the water gradient

humidity-decreases transpiration- increases water vapour in the air so the water potential gradient is not as steep

temperature-increases transpiration- more heat=more kinetic energy, faster moving molecules, more evaporation

light-increases transpiration- more light=more stomata to open, larger surface area for evaporation

Question 9

How can we measure the rate of transpiration?

Answer 9

using a potometer

Question 10

What is translocation?

Answer 10

the movement of organic substances around a plant via the phloem

Question 11

What is the most important organic substance moved around a plant?

Answer 11

sucrose (disaccharide, non-reducing sugar, inert)

Question 12

What is a source?

Answer 12

where substances are being produced, so have a high concentration

Question 13

What is a ‘sink’?

Answer 13

where substances are being used up, or converted to other molecules, so have a lower concentration

Question 14

How can radioactive isotopes be used in translocation?

Answer 14

take carbon-14 for example- will be used as normal in photosynthesis, so the c-14 will be incorporated into near sugars, and as c-14 is radioactive it can be traced

Question 15

Describe and explain the phloem structure

Answer 15

SIEVE TUBE- THE BIT IN THE MIDDLE

it has sieve tube elements (a whole part of the phloem tube, looks like the whole of one cell on a diagram) which are living cells that form the tubes for transporting solutes. they have no nucleus and few organelles

it also has companion cells which are the cells surrounding the sieve tube, and they carry out living functions for sieve cells. they have a lot of mitochondria

Question 16

What is the direction of flow in translocation?

Answer 16

always from the source to where its being used, but can go in any direction

Question 17

Explain the two parts to the mass flow hypothesis

Answer 17

at the source-
-active transport is used to load the sugars into the phloem from companion cells
-water potential is lowered in the sieve cells of the phloem, so water moves in too via osmosis from the xylem
-this creates a high hydrostatic pressure at the source at the end of the phloem

at the ‘sink’-
-sugars are removed from the phloem to be used up
-water returns to xylem by osmosis due to an increase in water potential
-hydrostatic pressure is hence reduced in the phloem at the sink end

Question 18

What causes the flow of solute in the phloem?

Answer 18

the hydrostatic pressure gradient

Question 19

Why is there cohesion between water molecules?

Answer 19

-water is a dipolar molecule
-enables hydrogen bonds to form
-creates cohesion as they stick to each other

Question 20

What 3 things make up cohesive-tension theory?

Answer 20

cohesion/adhesion/root pressure

Question 21

What are the adaptions of sieve tube elements?

Answer 21

living cells, but no nucleus and few organelles

Question 22

What are the adaptions of companion cells?

Answer 22

provide ATP for the active transport of organic substances