water transport in plants

Question 1

what is root pressure

Answer 1

can be demonstrated by cutting off the shoot of a plant at soil level. fluid will exude from the xylem

• the pressure of the fluid being pushed out of the cut xylem can be measured
• we know it is an active process

Question 2

what is the endodermis

Answer 2

• waxy, water repellent band called the casparian strip in its cell wall. this forces water in the root travelling towards the xylem to leave the vacuolar pathway and pass through the endodermis cell membrane

Question 3

how is root pressure created

Answer 3

• active transport of mineral ions into xylem by endodermal cells creates less negative water potential in xylem
• due to casparian strip all water is passing through the endodermal cells
• this water now moves into xylem by osmosis down the water potential gradient
• this creates force that helps move water up then xylem of the plant
• this is root pressure

Question 4

what is cohesion

Answer 4

the forces of attraction between like molecules

Question 5

what is adhesion

Answer 5

the forces of attraction between ‘unlike’ molecules

Question 6

what is capillary action

Answer 6

if a narrow tube is placed in a beaker of water the water rises up the tube by capillary action

Question 7

what is cohesion-tension theory

Answer 7

a wide capillary tube has a small surface area to volume ratio; a relatively small number of molecules in contact with the wall of the tube are trying to pull (cohesion) a relatively large number of molecules up the tube
a small capillary tube has a large surface area to volume rating this means a large number of molecules in contact with the wall of the tube (adhesion) are pulling a small number of water molecules up the tube

Question 8

what are xylem vessels

Answer 8

very narrow and form continuous columns up the stem due to the loss of their end walls
they act as capillary tubes which support long columns of water
if air bubbles form in a xylem vessel (which breaks the column of water) it can move through a pit into an adjacent vessel
water being lost from the leaves pull the water out of the xylem in the leaves and this force is passed down the xylem due to cohesion

Question 9

what is a transpiration stream

Answer 9

movement of water vapour down its concentration gradient from the air spaces in the leaf through the stomata
the water has evaporated from the surface of the mesophyll cells
water in the xylem of the leaf travels to the spongy mesophyll cells by osmosis down the water potential gradient

Question 10

what is transpiration

Answer 10

• movement of water vapour out of a leaf down a water potential gradient through the stomata
  water evaporates from the surface of the spongy mesophyll cells into the air spaces in the leaf
  this means there is higher concentration of water vapour in the leaf than outside the leaf
  water vapour infuses through the stomata

Question 11

what is a potometer

Answer 11

used to measure the transpiration rates from plant shoots
as water vapour transpires from the leaves it is replaced by water from the capillary tube
this can be recored as the movement of the air bubble in a known period of time
opening the tap on the reservoir allows the air bubble to be returned to the end of the capillary tube so repeats can be taken