9 Transport system in plants Flashcards

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1
Q

What is the xylem?

A

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

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2
Q

What is the phloem?

A

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

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3
Q

Where is the xylem found?

A
  • 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.
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4
Q

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

A
  • 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.
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5
Q

How is phloem tissue adapted for transporting solutes?

A
  • 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.
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6
Q

What are companion cells?

A
  • The active cells found next to sieve tube elements that supply the phloem vessels with all of their metabolic needs.
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7
Q

How is water drawn into the roots?

A

Via osmosis.

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8
Q

How are root hairs well adapted as exchange surfaces?

A
  • 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.
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9
Q

By which two pathways can water travel through the roots?

A

1) The symplast pathway

2) The apoplast pathway

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10
Q

What is the symplast pathway?

A
  • 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)
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11
Q

What is the apoplast pathway?

A
  • 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)
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12
Q

What is the effect of the Casparian strip?

A
  • 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.
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13
Q

Why is the apoplast pathway the main one?

A

Because it provides the least resistance.

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14
Q

How does cohesion and tension help water move up plants?

A

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.

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15
Q

How is adhesion partly responsible for the movement of water?

A

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.

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16
Q

What is transpiration?

A
  • The loss of water vapour from the leaves and stems of plants.
17
Q

Which 4 factors affect transpiration rate?

A

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.

18
Q

What are xerophytes?

A
  • 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
19
Q

What are some adaptations of xerophytes?

A

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

20
Q

What are hydrophytes?

A
  • Plants with adaptations that enable them to survive in very wet habitats or submerged or at the surface of water.
  • E.g water lillies
21
Q

What are some adaptations of hydrophytes?

A

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

22
Q

What is translocation?

A
  • 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.
23
Q

What is assimilates?

A

The products of photosynthesis that are transported.

24
Q

What are the main sources of assimilates in a plant?

A
  • Green leaves and green stems

- Food stores in seeds when they germinate.

25
Q

What are the main sinks in a plant?

A
  • Roots that are growing and/or actively absorbing mineral ions.
  • Meristems that are actively dividing.
26
Q

What is the mass flow hypothesis?

A
  • 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)
27
Q

What happens at the sink end?

A

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.

28
Q

What happens at the source?

A

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.

29
Q

What happens at the sink?

A

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

30
Q

Why is sucrose passively unloaded at the sink?

A

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.

31
Q

Why is sucrose actively loaded at the source?

A
  • 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.