Mass transport in plants

Question 1

What type of process is transpiration

Answer 1

A passive process

Question 2

Why is transpiration a passive process

Answer 2

The energy needed for it to occur is supplied by the sun

Question 3

Describe the stage of transpiration where water moves out of the leaves through the stomata

Answer 3

• The humidity of the atmosphere is usually less than that of the air spaces next to the stomata.
• As a result there is a water potential gradient from the air spaces through the stomata to the air.
• Provided the stomata are open, water vapour diffuses out of the air spaces into the surrounding air.
• Water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.
• By changing the size of the stomatal pores, plants can control their rate of transpiration.

Question 4

Describe the stage of transpiration where water moves across the cells of a leaf via the cytoplasm

Answer 4

• mesophyll cells lose water to the air spaces by evaporation due to 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 these neighbouring cells lowers their water potential.
• they, in turn, take in water from their neighbours by osmosis.
• In this way, a water potential gradient is established that pulls water from the xylem, across the lead mesophyll, and out into the atmosphere.

Question 5

What are the two ways by which water moves across the cells of a leaf

Answer 5

Via cell walls of via the cytoplasm

Question 6

Describe the stage of transpiration where water moves up the stem in the xylem

Answer 6

• Water evaporates from mesophyll cells due to heat from the sun leading to transpiration
• Water molecules form hydrogen bonds between one another and hence stick together. This is cohesion.
• 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, more molecules of water are drawn up behind it as a result of this cohesion.
• A column of water is therefore pulled up the xylem as a result of transpiration.
• This is called the transpiration pull.
• Transpiration pull puts the xylem under tension, that is, there is a negative pressure within the xylem l, hence the name cohesion- tension theory.

Question 7

Describe the pieces of evidence that support the cohesion-tension theory for transpiration

Answer 7

1) The change in the diameter of tree trunks according to the rate of transpiration. During the day, when transpiration is at its greatest, there is more tension in the xylem. This pulls the walls of the xylem inwards and causes the tree trunk to shrink in diameter. At night, when transpiration is at its lowest, there is less tension in the xylem and so the diameter of the trunk increases.
2) If a xylem vessel is broken and air enters it, the tree can no longer draw up water. This is because the continuous column of water is broken and so the water molecules can no longer stick together.
3) When a xylem vessel is broken, water does not leak out, as would be the case if it were under pressure. Instead air is drawn in, which is consistent with it being under tension.

Question 8

What is translocation

Answer 8

The process by which organic molecules and some mineral ions are transported from one part of a plant to another.

Question 9

Briefly describe the structure of the xylem vessels

Answer 9

• Xylem vessels are dead.
• They have no end walls which means the xylem forms a series of continuous, unbroken tunes from roots to leaves, which is essential for the cohesion-tension theory of water flow up the stem.

Question 10

Briefly describe the structure of the phloem

Answer 10

• Phloem is made up of sieve tube elements, long thin structures arranged end to end.
• Their end walls are perforated to form sieve plates.
• companion cells are associated with the sieve tube elements.

Question 11

What are sources

Answer 11

The sites of organic molecule production within the plant

Question 12

What are sinks

Answer 12

The places in the plant where the organic molecules will be stored for later use

Question 13

What is the currently accepted theory for the transport of materials in the plant via the phloem

Answer 13

The mass flow theory

Question 14

What are the three phases the mass flow theory can be divided into

Answer 14

1) Transport of sucrose into sieve elements from photosynthesising tissue
2) Mass flow of sucrose through sieve tube elements
3) Transfer of the sucrose from the sieve tube elements into storage or other sink cells.

Question 15

Describe the first stage in mass flow theory: transfer of sucrose into sieve elements from photosynthesising tissue

Answer 15

• Sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
• The sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells.
• Hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP
• These hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements.
• Sucrose molecules are transported along with the hydrogen ions in a process known as co-transport. The protein carriers are therefore known as co-transport proteins.

Question 16

Describe the second stage in the mass flow theory: Mass flow of sucrose through sieve tube elements

Answer 16

• The sucrose produced by photosynthesising cells (source) is actively transported into the sieve tubes.
• this causes the sieve tubes to have a lower water potential
• As the xylem had a much high water potential, water moves from the xylem into the sieve tubes by osmosis, creating high hydrostatic pressure within them.
• At the respiring cells (sink), sucrose is either used up during respiration or converted to starch for storage.
• These cells therefore have a low sucrose content and so sucrose is actively transported into them from the sieve tubes.
• This lowers their water potential
• Due to this, water also moves into the respiring cells from the sieve tubes by osmosis.
• The hydrostatic pressure of the sieve tubes in this region is therefore lowered.
• As a result of water entering the sieve tube elements at the source and leaving the sink, there is a high hydrostatic pressure at the source and a low one at the sink.
• There is therefore a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes.

Question 17

Describe the third stage in the mass flow theory: Transfer of sucrose from the sieve tube elements into storage or other sink cells

Answer 17

The sucrose is transported, by active transport in the companion cells, out of the sieve tubes and into the sink cells

Question 18

Explain why Mass flow is an active process

Answer 18

• While Mass flow itself is a passive process, it occurs as a result of the active transport of sugars.
• Therefore the process as a whole is active.

Question 19

Describe the evidence that supports the mass flow hypothesis

Answer 19

• There is a pressure within sieve tubes, as shown by sap being released when they are cut.
• The concentration of sucrose is higher in leaves (source) than in roots (sink)
• downwards flow in the phloem occurs in daylight, but ceases when leaves are shaded, or at night.
• increase in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem a little later.
• metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem.
• companion cells possess many mitochondria and readily produce ATP

Question 20

Describe the evidence that questions the mass flow hypothesis

Answer 20

• the function of the sieve plates is unclear, as they would seem to hinder mass flow
• not all solutes move at the same speed- they should do so if movement is by mass flow
• sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to the ones with the lowest sucrose concentration, which the mass flow theory would suggest

Question 21

Describe what a ringing experiment is and the observations we see

Answer 21

• Woody stems have an outer protective layer of bark on the inside of which is a layer of phloem that extends all around the stem.
• Inside the phloem layer is xylem
• At the start of a ringing experiment, a section of the outer layers is removed around the complete circumference of a woody stem while it is still attached to the rest of the plant.
• After a period of time, the region of the stem immediately above the missing ring of tissue is seen to swell.
• Samples of the liquid that has accumulated in this swollen region are found to be rich in sugars and other dissolved organic substances.
• Some non-photosynthetic tissues in the region below the ring are found to wither and die whereas those above the ring continue to grow.

Question 22

What two things do the observations of a ringing experiment suggest removing phloem around the stem had led to:

Answer 22

• the sugars of the phloem accumulating above the ring, leading to swelling in this region.
• the interruption of flow of sugars to the region below the ring and the death of tissues in this region.

Question 23

Explain the conclusion that can be drawn from a ringing experiment

Answer 23

The phloem, rather than the xylem is the tissue responsible for translocating sugars in plants.

Question 24

How does a ringing experiment prove that the phloem is responsible for translocating sugars, not the xylem

Answer 24

• As the ring of tissue removed had not extended into the xylem, it’s continuity had not been broken.
• If the xylem were the tissue responsible for translocating sugars you would not have expected sugars to accumulate above the ring nor tissues below it die.

Question 25

List the key evidence that shows that organic molecules are transported in the phloem

Answer 25

• When the phloem is cut, a solution of organic molecules flow out.
• Plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in the phloem after a short time.
• Aphids are a type of insect that feed on plants. They have needle like mouthparts which penetrate the phloem. They can therefore be used to extract the contents of the sieve tubes. These contents show daily variations in the sucrose content of the leaves that are mirrored a little later by identical changes in the sucrose content of the phloem.
• the removal of a ring of phloem from around the whole circumference of a stem leads to the accumulation of sugars above the ring and their disappearance from below it.