transport of water

Question 1

summarise water transport in plants in 6 steps:

Answer 1

1. water uptake near root tips
2. water enters xylem
3. water moves up the xylem
4. water moves from the xylem to leaf cells
5. evaporation of water into leaf air spaces
6. transpiration of water vapour through open stomata into air

Question 2

what are root hair cell?

Answer 2

many cells in the roots of plants have very thin, single-celled extensions that make up the outer layer (epidermis) of the root. They are called root hairs, and they are a specialised exchange surface for the uptake of water and mineral ions

Question 3

what features of root hair cells adapt them for water uptake?

Answer 3

• root hair cells provide a large surface area
• they have a thin wall - short diffusion distance
• they are permeable
• they undergo rapid growth - allows response to water
• they have a low water potential - which allows them to absorb water via osmosis

Question 4

how is water transported from soil to root hair?

Answer 4

root hair cells absorb minerals from the soil by active transport using ATP for energy
the minerals reduce the water potential of the cell cytoplasm
water then enters the cell down the water potential gradient - via osmosis

Question 5

where does water taken up by the root hairs need to get through ?

Answer 5

water that has been taken up by the root hairs, needs to get through 2 tissue
- the cortex
- the endodermis
water has to pass through these 2 tissues to reach the xylem

Question 6

how many possible ways can water molecules move through the cortex?

Answer 6

3 ways

Question 7

what is the apoplast pathway

?

Answer 7

• water soaks into the layers of cellulose fibres in the walls
• it then seeps across the root from the cell wall to cell wall and through the spaces between cells without entering a cell

Question 8

describe the symplast pathway?

Answer 8

• water enters the cell cytoplasm through the plasma membrane
• it then moves from cell to cell by osmosis
• or moves from cell to cell via the plasmodesmata

Question 9

what is the vacuolar pathway?

Answer 9

the vacuolar pathway is similar to the symplas pathway, but water is not confined to the cytoplasm of the cells. Water is able to enter and pass through vacuoles as well

Question 10

what is the problem with the vacuolar pathway?

Answer 10

there is high resistance and therefore little flow

Question 11

on reaching the endodermis what does the waxy casparian strip prevent?

Answer 11

the waxy Casparian strip prevents any further movement of water between or along cell walls - thus preventing the apoplast
water is then forced to pass through the plasma membrane and cytoplasm/vacuole of endodermal cells - symplast/vacuolar pathway

Question 12

why have the casparian strip?

Answer 12

This thick, waterproof, waxy substance known as the casparian strip forms an impenetrable barrier to water in the walls of the endodermis cells
this gives the plant control of what ions pass into the xylem vessels, as everything must pass the plasma membrane

Question 13

how many processes help water move through the xylem vessels i.e. up the stem?

Answer 13

there are 3 processes that help water move up the stem against the force of gravity

Question 14

describe what is meant by root pressure?

Answer 14

• The action of the endodermis moving minerals into the xylem by active transport drives water into the xylem by osmosis
• this pushes water into and up the xylem creating high pressure at the bottom

Question 15

what is the casparian strip made from?

Answer 15

suberin

Question 16

what is root pressure able to achieve?

Answer 16

root pressure can push water a few metres up a stem but cannot account for water getting to the top of tall trees.

Question 17

how can the magnitude of root pressure be demonstrated?

Answer 17

the magnitude of root pressure can be demonstrated by attaching a mercury nanometre to the cut stump of a plant

Question 18

describe the cohesion-tension theory?

Answer 18

water is a polar molecule
this means that water molecules are attracted to each other by forces of cohesion
these forces are strong enough to hold the molecules together in a long chain or column
this means that as some water molecules leave a plant through the stomata, others are pulled up behind them

Question 19

what can the force of cohesion create in the column of water?

Answer 19

the force of cohesion can create tension in the column of water - this is why xylem vessels must be strengthened by lignin

Question 20

what is meant by capillary action?

Answer 20

capillary action happens when water is confined to relatively long tubes of narrow diameter. This causes the water to rise through the tube: the narrower the tube the greater the height reached by the water molecules

Question 21

what does capillary action result from?

Answer 21

capillary action results from the strong intermolecular attractions that occur between water molecules (cohesion) and also the attraction between the water molecules and the side of the vessel (adhesion)

Question 22

why can capillary action or capillarity alone account for the upward transport of water molecules?

Answer 22

although the xylem vessels are extremely narrow tubes, the maximum height reached, even in such a small diameter vessel is approximately 50 mm

Question 23

what is the movement of water from leaf to atmosphere known as?

Answer 23

transpiration

Question 24

describe the process of transpiration?

Answer 24

• when water reaches the leaves it exits the xylem and moves into the cells mainly via the apoplast pathway
• evaporation of water then occurs from the cell walls into the air spaces between the cells in the leaves
• when the stomata are open water is able to diffuse out of the leaf, down the water potential gradient into the surrounding air (transpiration)
• loss of water from these mesophyll cells lowers their water potential such that they absorb water from adjoining cells of higher water potential
• the deficit of water in these mesophyll cells creates a pull on the contents in the xylem
• thus due to cohesion and adhesion water is pulled up the xylem vessels in a continuous uninterrupted column known as the transpiration stream

Question 25

define the term transpiration?

Answer 25

the loss of water by evaporation from the aerial parts of the plant (leaves) via stomata

Question 26

what happens in low light to guard cells?

Answer 26

in low light guard cells lose water and become flaccid causing the stomata to close