Transport Of Water In The Xylem

Question 1

How is water transported in the xylem?

Answer 1

• water is absorbed through the root hairs and transported through xylem vessels
• energy is supplied by the sun
• it’s a passive process
• water moves through the xylem to the exchange surface where it evaporates through the stomatal pore

Question 2

How is water moved out through the stomata?

Answer 2

• the humidity of the atmosphere is usually less than that of the air spaces next to the stomata
• there is a water potential gradient from the stomata to the air
• if the stomata are open water vapour molecules diffuse out of the air spaces into the air
• water lost by diffusion is replaced by water evaporating from the cell walls of the surrounding mesophyll cells into the air spaces

Question 3

How is water moved across the cells of a leaf?

Answer 3

• water is lost from mesophyll cells by evaporation from their cell walls to the air spaces of the leaf which is replaced from the xylem via cell walls or via the cytoplasm
• a water potential gradient is established that pulls water from the xylem across the leaf of mesophyll and finally out into the atmosphere

Question 4

Why does water movement occur in the cytoplasmic route?

Answer 4

• mesophyll cells lose water to the air spaces by evaporation due to the heat supplied by the sun
• these cells now have a lower water potential and so water enters by osmosis from neighbouring cells
• the loss of water from neighbouring cells lowers they water potential so takes in water from their neighbours by osmosis

Question 5

What is cohesion tension theory?

Answer 5

• water evaporates from mesophyll cells due to heat from the sun leading to transpiration
• water molecules from the hydrogen bonds between one another and hence tend to stick together
• water forms a continuous unbroken column across the mesophyll cells and down the xylem
• as water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata as more molecules of water are drawn up behind it due to cohesion
• a column of water is pulled up the xylem which puts the xylem under pressure so there is a negative pressure

Question 6

What is transpiration pull?

Answer 6

• a column of water is pulled up the xylem through transpiration
• this puts the xylem under tension so there is a negative pressure within the xylem
• it’s a passive process so does not require metabolic energy
• the xylem vessels through which water passes are dead so cannot actively move the water
• the xylem vessels have no end walls which means that xylem forms a series of continuous unbroken tubes from roots to leaves which is essential to the cohesion tension theory of water flow up the stem
• energy is nevertheless needed to drive the process of transpiration which is in the form of heat that evaporates water from the leaves and is ultimately comes from the sun

Question 7

Which evidence supports the cohesion tension theory?

Answer 7

• the diameter of tree trunks according to the rate of transpiration during the day when transpiration is the greatest so there is more tension in the xylem which pulls the walls of the xylem vessels inward and causes the trunk to sink in diameter so at night when transpiration is at its lowest there is less tension in the xylem so the diameter of the trunk increases
• if a xylem vessel is broken and air enters it the tree can no longer draw up water because the continuous column of water is broken so the water molecules can’t stick together
• when a xylem vessel is broken water does not leak out which would happen if it was under pressure so air is drawn in as if it was under tension

Question 8

How does time of day effect the way you hug a tree?

Answer 8

• if you hug a tree trunk in the middle of the day your fingers will touch the far side of the tree
• if you hug the same tree at night your fingers will probably no longer meet

Question 9

What is transpiration?

Answer 9

-the main force that pulls water through the xylem vessels in the stem of a plant is the evaporation of water from the leaves

Question 10

How is the root hair cell specialised?

Answer 10

• each root hair is an extension of a root epidermal cell
• root hairs are the exchange surfaces in plants that are responsible for the absorb toon of water by osmosis and mineral ions by active transport

Question 11

How are the xylem vessels specialised?

Answer 11

• xylem vessels vary in appearance depending on the type and amount of thickening of their cell walls as they mature their walls incorporate a substance called lignin and the cells die
• the lignin often forms rings or spirals around the vessel to add support so the stem stays upright

Question 12

How do you measure water uptake using photometer?

Answer 12

• a leafy shoot is cut under water so care is taken not to get water on the leaves
• the potometer is filled completely with water to make sure there are no air bubbles
• using a rubber tube the leafy shoot is fitted to the potometer under water
• the potometer is removed from under the water and all joints are sealed with waterproof jelly
• an air bubble is introduced into the capillary tube
• the distance moved by the air bubble in given time is measured a number of times and the mean is calculated
• using this mean value the volume of water loss is calculated
• the volume of water lost agains the time in minuets can be plotted
• once the air bubble nears the junction of the reservoir tube and the capillary tube so tap on the reservoir is opened and the syringe is pushed down until the bubble is pushed back to the start of the scale in the capillary tube
• the experiment can be repeated to compare the rates of water uptake under different conditions like different temperatures humidity light intensity or differences in water uptake between different species under the same conditions