Transport in Plants Flashcards

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1
Q

How is water taken up into plants?

A

Water moves by osmosis from the soil through root hair cells

The soil has a very dilute solution of mineral ions meaning it has a high water potential and the vacuole in the root hair call contains cell sap of ions and sugars lowering water potential.

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2
Q

How are root hair cells adapted to their function?

A

The root hair cell has root hairs which increase the surface area massively meaning water can be taken up quickly.

Root hair cells have thin cell walls which decreases the diffusion pathway and increase the rate of water uptake.

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3
Q

What is the apoplast pathway?

A

Water moves in the cell walls. Cellulose fibres in the cell wall are separated by spaces through which the water moves.

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4
Q

What is the symplast pathway?

A

Water moves through the cytoplasm and plasmodesmata.

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5
Q

What is the vacuolar pathway?

A

Where water moves from vacuole to vacuole

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6
Q

Why does water move across the root?

A

There is a water potential gradient across the root cortex. It is highest in the root hair cells and lowest in the xylem, so water moves down the water potential gradient, across the root.

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7
Q

Describe the structure of the xylem in the roots

A

In roots, the xylem is central and star-shaped with phloem between groups of xylem cells.

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8
Q

Why are the xylem and phloem arranges the way they are in the root?

A

It resists vertical stress (pull) and anchors the plant in the soil

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9
Q

Describe the arrangement of tissues in the stem of a plant

A

The vascular bundles are in a ring at the periphery, with xylem towards the centre and phloem towards the outside .

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10
Q

Why are the xylem and phloem arranges the way they are in the stem?

A

This gives flexible support and resists bending

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11
Q

How are tracheids adapted to their function?

A

Their cell walls contain lignin, which is hard, strong and waterproof.

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12
Q

How does water travel through tracheids?

A

The walls have gaps, called pits, through which the water travels.

Tracheids are spindle-shaped so water takes a twisting rather than a straight path up the plant.

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13
Q

What plants do tracheids occur in?

A

Ferns, conifers and angiosperms (flowering plants)

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14
Q

Why can mosses not grow as tall as other plants?

A

They have no water-conducting tissue and are therefore poorer at transporting water.

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15
Q

What are the two types of conducting cell in the xylem?

A

Tracheids and vessels

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16
Q

What are the two main functions of the xylem?

A

1) Transport of water and dissolved minerals

2) Providing mechanical strength and support

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17
Q

What conducting tissue only occurs in angiosperms?

A

Vessels

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18
Q

How do vessels get the shape of a long hollow tube?

A

As lignin builds up in their cell walls, the contents die, leaving an empty space, the lumen. As the tissue develops, the end walls of the cells break down.

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19
Q

How does water climb in vessels?

A

Due to it having a long hollow fine, like a drainpipe, water climbs straight up the plant.

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20
Q

Why are angiosperms the most dominant plant on earth?

A

Water moving through vessels is so much more efficient than the twisting path through tracheids.

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21
Q

How do you identify xylem in microscope sections?

A

Unlike cellulose of phloem cell walls, the lignin in the xylem stains red.

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22
Q

Why cannot water enter the xylem from the apoplast pathway ?

A

Because lignin makes xylem walls waterproof, water can only pass from the symplast or vacuolar pathways.

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23
Q

How does water leave the apoplast pathway?

A

The endodermis walls are impregnated with a waxy, suberin, forming a distinctive band called the casparian strip. Suberin is waterproof so the Casparian strip prevents water moving further in the apoplast and drives it into the cytoplasm.

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24
Q

How does the plant achieve a lower water potential in the xylem than the endodermis cells?

A

1) The water potential of the endodermis cells is raised by water being driven in by the casparian strip
2) The water potential of the xylem is decreased by active transport of mineral salts, mainly sodium ions from the endodermis and pericycle into the xylem

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25
Q

How do minerals move into the root hair cells?

A

In the soil minerals are present in very low concentrations, so generally, minerals are absorbed into the cytoplasm by active transport.

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26
Q

How do mineral ions move through the root cortex?

A

Along the apoplast pathway, in solution

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27
Q

How do mineral ions enter the xylem from the apoplast pathway?

A

When the minerals reach the endodermis, the casparian strip prevents further movement in the cell walls, so they enter the cytoplasm by active transport, and then diffuse or are actively transported into the xylem.

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28
Q

How is active transport important at the endodermis?

A

It allows the plant to absorb the ions selectively

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29
Q

Describe the cohesion tension theory

A

Water has hydrogen bonding and a dipole structure, therefore water molecules are attracted towards each other which is cohesion.

As water is drawn up in the root, it evaporated from the leaf pulling up the other water molecules behind it which is called tension.

Adhesion is when water molecules are attracted to the hydrophilic lining of the xylem

These all help water move up the xylem

30
Q

What is root pressure?

A

A consequence of osmotic movement of water going down a concentration gradient at the base of the xylem due to uptake of ions pushing water already there further up.

31
Q

What other mechanisms move water up plant?

A

Capillarity

32
Q

What is a harmful aspect of transpiration?

A

Water loss

33
Q

What are the useful aspects of transpiration?

A

1) water uptake
2) water distribution
3) ion distribution
4) evaporative cooling

34
Q

What factors does transpiration rate depend on?

A

Genetic factors e.g distribution ad size of stomata

Environmental factors e.g temperature, humidity and air movement - affect the water potential gradient between water vapour in leaf and atmosphere

Light intensity

35
Q

How does temperature increase effect transpiration rate ?

A

It increases the kinetic energy of water molecules, meaning they move faster and accelerates their rate of evaporation. The higher temperature causes the water molecules to diffuse away from the leaf more quickly, reducing the water potential around the leaf.

36
Q

How does high humidity effect transpiration rate ?

A

Transpiration leaves a layer of saturated air at the surface of leaves surface. The higher the humidity the smaller the difference in water potential and therefore water will not diffuse down a concentration gradient as quickly so transpiration rate will decrease.

37
Q

What does an increase in air movement do to the transpiration rate?

A

The faster the movement of the surrounding air the faster the concentric shells of water vapour get blown away. This increases the water potential gradient between the inside and the outside of the leaf meaning transpiration rate increases.

38
Q

How does a higher light intensity effect the transpiration rate?

A

As light intensity this increases the width of which the stomata open, increasing the rate of transpiration. So stomata tend to open the widest in the middle of the day increasing transpiration rate.

39
Q

What do you have to make sure you do when you set up a potometer?

A

Cut the leafy shoot/assemble the equipment under water - to make sure no air bubbles enter the xylem as this could block transportation stream

Seal joints with Vaseline to make sure no air enters the equipment

Dry leaves to remove and water from around the leaf which would reduce transpiration rate

40
Q

What is a mesosphyte?

A

A plant living in conditions of adequate water supplies

41
Q

What happens if a mesophyte loses too much water?

A

It wilts and it’s leaves droop. The stomata close and the leaf surface are available for absorbing light is reduced so photosynthesis becomes less efficient.

42
Q

What do mesophytes do to survive unfavourable times of the year when the ground is frozen and liquid water is not available?

A

1) Shed their leaves before winter, so that they do not lose water by transpiration.
2) The aerial parts of many non-Woody plants die off in winter so they are not exposed to cold winds, but their underground organs survive.
3) most annual mesophutes lay over-winter as dormant seeds, with such a low metabolic rate that almost not water is required

43
Q

What are xerophytes?

A

They are plants that live in conditions where water is scarce e.g hot or dry deserts or cold regions where the soil water is frozen, windy locations

44
Q

How are the stomata of marram grass adapted to their environment?

A

Stomata occurs on the upper surface, but not the outer surface of the leaf.

They are in pits and humid air is trapped in the pit, outside the stomata.

45
Q

How do the adaptations of the stomata in marram grass help the plant survive?

A

This reduces the water potential gradient between the inside of the leaf and the outside and so reduces the rate of diffusion of water out through the stomata.

46
Q

How does the adaptations of the shape of marram grass leaves help the plant survive?

A

This reduces the leaf area exposed to air so reduces transpiration.

47
Q

How is the surface of the leaf of a marram grass plant adapted to its surroundings? And how does it help them to survive?

A

It has stiff, interlocking hairs trap water vapour and reduce the water potential gradient between the inside of the leaf and the outside.

48
Q

How has the thickness of the layers in a marram grass plant cell adapted to their surroundings? And how does this help the plant to survive?

A

The plant cells have a thick cuticle and the cuticle is a waxy covering over the outer surface of the leaf. Wax is waterproof and so reduces water loss. The thicker this cuticle, the lower the rate of transpiration through the cuticle

49
Q

What are hydrophytes?

A

They are plants which grow partially or wholly submerged in water.

50
Q

What tissues do hydrophytes not have and why?

A

They have little or no lignified support tissues because water is a supportive medium.

There is little need for transport tissue (xylem is poorly developed) because they are surrounded by water

51
Q

How are hydrophytes’ leaves and stems adapted to their surroundings?

A

Leaves have little or no cuticle, because there is no need to prevent water loss.

Stems and leaves have large air spaces, continue down to their leaves, forming a reservoir of o2 and co2, which provide buoyancy.

52
Q

How are the stomata in hydrophytes adapted to their function?

A

Stomata are on the upper surface of floating leaves, because the lower surface is on the water.

53
Q

How are the end walls of the phloem different than in the xylem?

A

The end walls do not break down. Instead, the end walls and sometimes parts of the side walls are perforated in areas called sieve plates.

54
Q

What types of cells and tissue is the phloem made up of?

A

It is a living tissue and consists of several types of cells, including sieve tubes and companion cells.

55
Q

What adaptation do sieve tube elements have to make them suited to their function?

A

They lose their nucleus and most of their other organelles during development, allowing space for transporting materials.

56
Q

How is the metabolism of a sieve tube element controlled?

A

It is controlled by at least one neighbouring companion cell.

57
Q

What features of a companion cell suggest they are very biochemically active?

A

This is indicated by the large nucleus, dense cytoplasm containing much RER and many mitochondria.

58
Q

How are companion cells and sieve tube elements connected?

A

Plasmodesmata

59
Q

How is transport in the xylem different to in the phloem?

A

Products of photosynthesis are translocated from “source” to “sink”. Means unlike xylem which can only transport upwards, phloem can translocate up, down and sideways.

60
Q

What main materials does the phloem transport?

A

Sucrose and amino acids

61
Q

What is the technique for a ringing experiment?

A

Cylinders of outer bark tissue are removed from all the way around a Woody stem. This would remove the phloem. The plant would then be left some time, while it photosynthesised, then the contents above and below the ring would be analysed.

62
Q

What are the results of a ringing experiment?

A

Above the ring, there was a lot of sucrose, suggesting it had been translocated in the phloem from source to sink. Below the ring, there was no sucrose, suggesting it had been used by plant tissues but not replaced, because the ring prevented it moving downwards.

63
Q

How is a radioactive tracer and autoradiography experiment set up to investigate organic substances translocated in phloem?

A

A stem section is placed on a photographic film which is exposed if there is a radiation source, producing an autoradiograph.

63
Q

What are the results of the autoradiography experiment?

A

A plant photosynthesis in the presence of 14C in carbon dioxide 14CO2. The position of exposure and therefore the radioactivity, coincides with the position of the phloem, because it is the phloem that translocates the sucrose made of 14CO2 in photosynthesis.

64
Q

What is the technique for using aphids to investigate the organic substances transported in phloem.

A

An aphid has a styler, this is inserted into a sieve tube and the phloem contents exudes under pressure into the stylet. The aphid was anaesthetised and removed.

65
Q

What are the results of the aphid experiment?

A

As the sap in the phloem is under pressure it exuded from the styles and was collected and analysis showed the presence of sucrose.

66
Q

What did aphid and radioactive tracer experiments show?

A

The radioactivity and therefore, the sucrose made in photosynthesis, moved at a speed of 0.5-1 m h^-1.

67
Q

Why were the results of the aphid and radioactive tracer experiment unusual?

A

The speed that the sucrose moves at is much faster than the rate of diffusion alone so some additional mechanism had to be considered.

68
Q

What is the mass flow hypothesis?

A

Suggests a passive flow of sugars from the phloem of the leaf, where there is the highest concentration (the source), to growing tissues, where there is a lower concentration (the sink).

69
Q

Describe process of mass flow?

A

In leaf cells, sucrose is made by photosynthesis. The sucrose makes the water potential very negative and water passes into the cell by osmosis.

As water enters the leaf hydrostatic pressure builds up, forcing sucrose in solution into the phloem joining source to sink.

The pressure pushes the sucrose solution down the phloem and this movement is called mass flow.

In the sink the sucrose is converted to insoluble starch so water potential is higher and then water is forced up the xylem back to the source.

70
Q

What aspects of translocation does mass flow not explain?

A

1) Rate of phloem transport is 10000 times faster than diffusion.
2) Does not take into account sieve plates
3) Sucrose and amino acids move at different rates in different directions
4) Phloem has a high oxygen consumption, translocation is slowed at low temperatures or when cyanide is applied.
5) Companion cells are very active but mass flow does not suggest a role for them.

71
Q

What do other theories suggest instead of mass flow?

A

1) Active process - cyanide and low temperatures inhibit translocation - indicates energy generated by respiration is used.
2) Protein filaments - pass through sieve pores so different solutes are carried along different routes though same sieve tube.
3) Cytoplasmic streaming - could be responsible for movement in different directions.