9 Transport system in plants

Question 1

What is the xylem?

Answer 1

A largely non-living tissue that carries water and minerals from the roots to the other parts of the plant.

Question 2

What is the phloem?

Answer 2

Phloem is a living tissue that transports food in the form of organic solvents e.g sugars around the plant from the leaves.

Question 3

Where is the xylem found?

Answer 3

• In a root, the xylem is in the centre surrounded by phloem to provide support as it pushes through the soil.
• In the stems, the xylem and phloem are near the outside to provide support and to refrain from bending.
• In a leaf, xylem and phloem make up a network of veins which supports the thin leaves.

Question 4

How are xylem vessels adapted to transporting water and mineral ions?

Answer 4

• They are long, tube like structures formed from cells joined end to end.
• There are no end walls on these cells, making an uninterrupted tube that allows water to pass up through the middle easily.
• The cells are thickened with lignin, which helps to support the xylem vessels and stops them collapsing inwards.
• Water and ions move into and out of the vessels through small pits in the walls where there’s no lignin.

Question 5

How is phloem tissue adapted for transporting solutes?

Answer 5

• The sieve tube elements are living cells that form the tube for transporting solutes through the plant.
• In areas between the cells. the walls become perforated to form sieve plates which let the phloem contents flow through.

Question 6

What are companion cells?

Answer 6

• The active cells found next to sieve tube elements that supply the phloem vessels with all of their metabolic needs.

Question 7

How is water drawn into the roots?

Answer 7

Via osmosis.

Question 8

How are root hairs well adapted as exchange surfaces?

Answer 8

• Their microscopic size means they can penetrate easily between soil particles.
• Each microscopic hair has a large surface area:volume ratio.
• Each hair has a thin surface layer through which diffusion and osmosis can take place quickly.
• The concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between soil water and the cell.

Question 9

By which two pathways can water travel through the roots?

Answer 9

1) The symplast pathway

2) The apoplast pathway

Question 10

What is the symplast pathway?

Answer 10

• Movement of water and solutes through the cytoplasm of the cells via plasmodesmata by osmosis.
• It goes through the living parts of the cell. (cytoplasm)

Question 11

What is the apoplast pathway?

Answer 11

• Movement of substances through the cell walls and cell spaces by diffusion and into the cytoplasm by active transport.
• It goes through the non-living parts of the cell. (cell wall)

Question 12

What is the effect of the Casparian strip?

Answer 12

• When water in the apoplast pathway reaches the endodermis cells in the root, its path is blocked by a waxy strip in the cell walls called the Casparian strip.
• So the water has to take the symplast pathway.
• This is useful because it means the water has to go through a cell membrane which can filter out toxins as it controls which substances in the water can go through.
• Once past this barrier, the water moves into the xylem.

Question 13

Why is the apoplast pathway the main one?

Answer 13

Because it provides the least resistance.

Question 14

How does cohesion and tension help water move up plants?

Answer 14

1) Water evaporates from the leaves at the top of the xylem.
2) This creates a tension which pulls more water into the leaf.
3) Water molecules are cohesive so when some are pulled into the leaf others follow. This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards.
4) Water enters the stem through the root cortex cells.

Question 15

How is adhesion partly responsible for the movement of water?

Answer 15

1) As well as being attracted to each other, water molecules are attracted to the walls of the xylem vessels.
2) This helps water to rise up through the xylem vessels.

Question 16

What is transpiration?

Answer 16

• The loss of water vapour from the leaves and stems of plants.

Question 17

Which 4 factors affect transpiration rate?

Answer 17

1) Light- the lighter it is, the faster the transpiration rate. This is because the stomata open where it gets light, so CO2 can diffuse into the leaf for photosynthesis.
2) Temperature- the higher the temperature, the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.
3) Humidity- the lower the humidity, the faster the transpiration rate. If the air around the plant is dry, the water potential gradient between the leaf and air is increased, which increases transpiration.
4) Wind- the windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata This increases the water potential gradient, which increases the rate of transpiration.

Question 18

What are xerophytes?

Answer 18

• Plants with adaptations that enable them to survive in dry habitats or habitats where water is in short supply in the environment.
• E.g cactus, marram grass

Question 19

What are some adaptations of xerophytes?

Answer 19

1) A thick waxy cuticle that is waterproof / (relatively) impermeable to minimise water vapour loss
2) Sunken stomata- many xerophytes have their stomata located in pits, which reduce air movement, producing humid air that reduces the water vapour potential gradient and reduces transpiration/ traps water vapour
3) Thin needles/ small leaves: smaller surface area, reducing transpiration as greater sa: vol
4) Hairy leaves- traps water vapour
5) fewer stomata- reduces diffusion (of water vapour)
6) most stomata on lower surface ;less exposure to sun

Question 20

What are hydrophytes?

Answer 20

• Plants with adaptations that enable them to survive in very wet habitats or submerged or at the surface of water.
• E.g water lillies

Question 21

What are some adaptations of hydrophytes?

Answer 21

1) Very thin or no waxy cuticle- they dont need to conserve water as there is always plenty available
2) Many always-open stomata on the upper surfaces- maximises gas exchange
3) Reduced structure to the plant- water supports the leaves and flowers so there is no need for strong supporting structures.
4) Wide,flat leaves- to capture much light as possible
5) Small roots
6) Large surface areas of stems and roots- to maximise area for photosynthesis

Question 22

What is translocation?

Answer 22

• The movement of organic solutes around a plant in the phloem.
• Transports from ‘sources’ to ‘sinks’.
• Sources is where a substance is made.
• Sink is the area where a substance is used up.

Question 23

What is assimilates?

Answer 23

The products of photosynthesis that are transported.

Question 24

What are the main sources of assimilates in a plant?

Answer 24

• Green leaves and green stems

- Food stores in seeds when they germinate.

Question 25

What are the main sinks in a plant?

Answer 25

• Roots that are growing and/or actively absorbing mineral ions.
• Meristems that are actively dividing.

Question 26

What is the mass flow hypothesis?

Answer 26

• In xylem, mass flow of contents is achieved passively due to transpiration creating differences in water potential between soil and air
• In phloem, mass flow of contents is achieved actively as phloem cells use energy to create pressure differences that push contents along (the apoplast route)

Question 27

What happens at the sink end?

Answer 27

1) Solutes are removed from the phloem to be used up.
2) This increases the water potential inside the sieve tubes, so water also leaves the tube by osmosis.
3) This lowers the pressure inside the sieve tubes.

Question 28

What happens at the source?

Answer 28

Loading

1) active transport of H+ pumped out of companion cell into leaf cell, creating a hydrogen ion concentration gradient
2) H+ diffuse back into companion cell with sucrose (co-transporter protein used)
3) Sucrose diffuses through plasmodesmata from companion cell to sieve tube.
4) Water potential powered in phloem, drawing in water to drive mass flow.

Question 29

What happens at the sink?

Answer 29

Unloading

1) Sucrose enters companion cells through plasmodesmata and then enters root cells by facilitated diffusion
2) Sucrose converted to other carbs e.g starch which lowers sucrose concentration and encourages more sucrose to leave the phloem

Question 30

Why is sucrose passively unloaded at the sink?

Answer 30

At a sink, sucrose is passively unloaded into cells that require it as they have a low concentration of sucrose and allows water to move out of the phloem, decreasing pressure in the phloem.

Question 31

Why is sucrose actively loaded at the source?

Answer 31

• At a source, sucrose is actively loaded into sieve elements which lowers water potential and allows water to move into the phloem, increasing pressure in phloem.