3.3.4.2 Mass Transport in Plants

where is water absorbed in plants?

1. roots
2. through root hairs

how is water transported?

through hollow, thick-walled tubes called xylem vessels

what is the xylem?

a type of plant tissue responsible for transporting water and nutrients from the roots to the rest of the plant

what type of process is transpiration?

1. passive - does not require metabolic energy

2. vessels are dead cells so cannot actively move the water

structure of xylem

1. dead cells

2. no end walls - continuous, unbroken column

3. contain pits - these allow water to move sideways between the vessels

4. lignin - tough substances which thickens vessel; waterproof

what is transpiration?

the loss of water vapour via the stomata by diffusion

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

what is transpiration pull?

the movement of water from the roots to the leaves

why is transpiration passive?

energy for this is supplied by the sun

describe the process of transpiration

1. Water vapour evaporates out of stomata on leaves. This loss in water volume creates a lower pressure

2. When this water is lost by transpiration more water is pulled up the xylem to replace it (moves due to negative pressure).

3. Due to the hydrogen bonds between water molecules, they are cohesive (stuck together). This creates a column of water within the xylem.

4. Water molecules also adhere (stick) to the walls of the xylem. This helps to pull the water column upwards.

5. As this column of water is pulled up the xylem it creates tension, pulling the xylem in to become narrower

why is there a water potential gradient from air spaces to the air and what does this allow?

1. humidity of the atmosphere is less than that of the air spaced next to the stomata
2. therefore there is a water potential gradient
3. when stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air
4. this water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells

how to plants control rate of transpiration?

by changing the size of the stomatal pores

how can water reach mesophyll cells from the xylem?

via cells walls or the cytoplasm

how does water move across the cells of leaf by the cytoplasmic route?

1. mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
2. these cells now have a lower water potential and so water enters by osmosis from neighbouring cells
3. the loss of water from these neighbouring cells lower their water potential
4. they take water from their neighbours by osmosis

- this establishes a water potential gradient that pulls water from the xylem, across the leaf mesophyll and finally out into the atmosphere

how is water moved up the stem?

1. water evaporated from mesophyll cells due to heat from the sun leading to transpiration

2. water molecules form hydrogen bonds between one another and hence stick together - known as cohesion

3. water forms a continuous unbroken column across the mesophyll cells and down the xylem

4. as water evaporates from mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result

5. a column of water is therefore pulled up the xylem as a result of transpiration - this is called transpiration pull

6. transpiration pull puts the xylem under tension - there is a negative pressure within the xylem hence the name cohesion tension theory

describe how water is moved through a plant according to the cohension-tension hypothesis (4)

1. water evaporation / transpires from leaves

2. reduced water potential in cell / osmotic pressure gradient

3. water drawn out of xylem

4. creates tension

5. cohesive forces between water molecules

6. water pulled up as column

what evidence is there to support the cohesion tension theory?

1. change in diameter according to tree trunk according to the rate of transpiration - trunk shrinks in diameter during day and increases in diameter during night

2. if xylem vessel is broken and air enters it, the tree can no longer draw up water as continuous column of water is broken

3. when a xylem vessel is broken, water does not leak out, instead air is drawn in

how sucrose transports from the source to the sieve tube element:

1. photosynthesis occurring in the chloroplasts of leaves creates organic substances, e.g. sucrose.

2. sucrose is actively transported into the sieve tube element, using the companion cell

3. this creates a high concentration of sucrose at the site of production, therefore sucrose diffuses down its concentration gradient into the companion cell via facilitated diffusion

4. active transport of H+ occurs from the companion cell into the spaces within the cell walls using energy

5. this creates a concentration gradient and therefore the H+ move down their concentration gradient via carrier proteins into the sieve tube elements.

6. co-transport of sucrose with the H+ ions occurs via protein co-transporters to transport the sucrose into sieve tube element

movement of sucrose within the phloem sieve tube element:

1. the increase of sucrose in the sieve tube element lowers the water potential

2. water enters the sieve tube elements from the surrounding xylem vessels via osmosis.

3. the increase in water volume in the sieve tube element increase the hydrostatic pressure causing the liquid to be forced towards the sink

transport of sucrose to the sink (respiring cells):

1. sucrose is used in respiration at the sink, or stored or stored as insoluble starch

2. more sucrose is actively transported into the sink cell, which causes the water potential to decrease.

3. this results in osmosis of water from the sieve tube element into the sink cell (some water also returns to the xylem).

4. the removal of water decreases the volume in the sieve tube element and therefore the hydrostatic pressure decreases.

5. movement of soluble organic substances is due to the difference in hydrostatic pressure between the source and sink end of the sieve tube element

how is pressure generated in the phloem?

1. sucrose actively transported into phloem

2. lowers water potential

3. water moves into phloem from xylem by osmosis

investigating translocation - tracers

1. tracing involves radioactively labelling carbon

2. plants are provided with only radioactively labelled carbon dioxide and over time this is absorbed into the plant and used in photosynthesis to create sugars which all contain radioactively labelled carbon

3. thin slices from the stems are then cut and placed on x-ray film that turns black when exposed to radioactive material.

4. when the stems are placed on the x-ray film the section of the stem containing the sugars turn black, and this highlights where the phloem are and shows sugars are transported in the phloem

investigating translocation - tree ringing experiments

1. a ring of bark and phloem are peeled and removed off a tree trunk

2. the result of removing the phloem is that the trunk swells above the removed section

3. analysis of the liquid in this swelling shows it contain sugars

4. this shows that when the phloem is removed, the sugars cannot be transported and therefore proves the phloem transports sugars