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A-Level OCR Biology > Chapter 9 - Transport in Plants > Flashcards

Chapter 9 - Transport in Plants Flashcards

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1
Q

What is the primary function of the xylem?

A

To transport water and minerals, and support the plant structurally

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2
Q

What is the direction of flow in the xylem?

A

From roots to shoots and leaves (up the plant)

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3
Q

What is the xylem mainly made up of?

A

Dead cells

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4
Q

What is the function of lignin in the xylem?

A
  • Supports and strengthens the xylem

- Impermeable so prevents incorrect water loss

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5
Q

What is the function of xylem parenchyma?

A
  • Stores food

- Contains tannin deposits; tannin is a bitter chemical which protects the plant from herbivores

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6
Q

Where is xylem parenchyma found?

A

It is packed around xylem vessels

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7
Q

What occurs at the unlignified bordered pits in the xylem?

A

Water leaves the xylem tube it’s currently in, either entering a new xylem tube, or entering cells

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8
Q

What is the main function of the phloem?

A

To transport food in the form of organic solvents from the leaves (chloroplasts) to the rest of the plant- it supplies cells with the necessary sugars and amino acids

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9
Q

What is the direction of flow in the phloem?

A

Both up and down the plant

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10
Q

What are the transporting vessels of the phloem called?

A

Sieve tubes

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11
Q

What are companion cells?

A

Cells that form with sieve tubes, and are believed to function as a life support system for the sieve tubes.
They contain both mitochondria and a nucleus

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12
Q

What two cell types make up the phloem?

A

Sieve tube elements and companion cells

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13
Q

What is a difference in the cells of the xylem and phloem?

A

Xylem cells are mostly dead, phloem cells are living

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14
Q

What are sieve plates?

A

They are the original cell walls of the sieve tube elements, which have holes in to allow the contents of the phloem to pass through

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15
Q

What are 2 roles of turgor (water) pressure?

A
  • Hydrostatic skeleton (maintains structure and supports stem and leaves)
  • Drives root expansion
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16
Q

Give 2 other roles of water in plants (other than turgor pressure)

A
  • Transport medium for mineral ions and other important substances
  • Water is a raw material required for photosynthesis
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17
Q

What are root hairs?

A

Exchange surface of plants in roots, where water is drawn into the plant

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18
Q

How are root hairs well adapted as exchange surfaces?

A
  • Microscopic size allows them to penetrate easily through soil
  • Large SA:V ratio
  • Thin surface layer = small diffusion distance
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19
Q

How do root hair cells maintain a water potential gradient?

A

There is a high concentration of solutes in the cytoplasm of the root hair cells, which lowers the water potential, meaning there is a water potential gradient, so water enter the cells via osmosis

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20
Q

What is the transport system called in plants?

A

Vascular tissue

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21
Q

What are vascular bundles?

A

The phloem and the xylem together

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22
Q

What is the structure of the vascular tissue in the stems?

A

Passionfruit looking one.

Seeds are the vascular bundles- inside is xylem, outside is phloem

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23
Q

What is the structure of the vascular tissue in the roots?

A

x or + in a circle in the middle

The x inside is the xylem, the circles around the x are the phloem

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24
Q

In roots, why is the vascular bundle in the middle?

A

To help withstand tugging strain as a result of the leaves and stem moving in the wind

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25
Q

What is the structure of the vascular tissue in the leaf?

A

Rain drop looking one.

Circle in middle with top one being xylem and bottom phloem

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26
Q

What are the two different pathways of water from roots to xylem?

A

Symplast and apoplast

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27
Q

What is the symplast pathway?

A

Where water travels through the continuous cytoplasm of cells.
It travels via osmosis, as the root hair cell has a higher water potential than the next cell, and as the water flows into this cell, it then has a higher water potential than the next cell and so on

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28
Q

What is the plasmodesmata?

A

Fine strand of cytoplasm that connects the cytoplasms of different cells, meaning a water can travel through a continuous cytoplasm in symplast

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29
Q

What is the apoplast pathway?

A

Where water travels through the cell walls and intercellular spaces.
The cohesive forces between water molecules means water is being continuously pulled up into the xylem

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30
Q

What eventually happens to the apoplast pathway (in roots) and why is this significant?

A

It cannot go any further due to the impermeable nature of the Casparian strip, so is forced into the cytoplasm to join the symplast pathway.
This is significant, as it means the water had to pass through a selectively permeable membrane, meaning potentially-toxic solutes in the soil water cannot reach the living tissue

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31
Q

What is root pressure?

A

The movement of water up the xylem generated from the pumping of minerals into the xylem (this is independent from any forces of transpiration)

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32
Q

When water reaches the endodermal cells surrounding the xylem, what happens?

A

The water potential of water in xylem cells is lower than in the cytoplasm of endodermal cells, so water enters the xylem via osmosis

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33
Q

How is active transport believed to be involved in the process of water transport/root pressure?

A

Endodermal cells are believed to actively move mineral ions into the xylem by active transport. This creates a steeper water potential gradient, meaning the water can enter efficiently via osmosis

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34
Q

What is evidence for the role of active transport in root pressure?

A
  • Cyanide affects the production of ATP; when cyanide is applied to root cells (so there is no energy supply), root pressure disappears
  • Root pressure increases and decreases with increases and decreases in temperature, indicating chemical reactions being involved
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35
Q

What are stomata opened and closed by?

A

Guard cells

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36
Q

What 2 things happen when stomata are open?

A
  • Oxygen can enter and CO2 can leave

- Water is lost via transpiration

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37
Q

Why do some stomata have to be open at night?

A

To take in oxygen for cellular respiration

38
Q

Summarise the transpiration stream

A

Water enter through roots
Transported along apoplast or symplast pathway to xylem
Transported up xylem until it reaches the leaves
It then leaves through the stomata

39
Q

Why is it important that the xylem tubes are narrow enough?

A

So the water can flow in a continuous stream

40
Q

What is cohesion-tension theory?

A

The model in which water moves from the soil, up the xylem and across the leaves in a continuous stream

41
Q

What is capillary action and what causes it in plants?

A

Capillary action is where water can rise up a narrow tube against the force of gravity.
This occurs as a result of the combined effects of the adhesion and cohesion of water moving up the xylem

42
Q

What causes adhesion in water moving up the xylem?

A

Water molecules forming hydrogen bonds with the carbohydrates in the walls of the xylem vessels

43
Q

What happens when the water reaches the leaves?

A

It either goes along the apoplast pathway, in which it will diffuse into the air spaces,
or along the symplast pathway, in which it travels through the mesophyll cells, until it evaporates from the freely permeable cell wall of the mesophyll cells into the air spaces.
It then leaves into the external air down a concentration gradient through the stomata

44
Q

What causes cohesion in water moving up the xylem?

A

Water molecules forming hydrogen bonds between one another, meaning they tend to stick together

45
Q

What is transpiration pull?

A

The continuous stream of water drawn from the roots to replace the water lost by evaporation

46
Q

Where are stomata located?

A

In the lower epidermis of leaves (underside)

47
Q

How do stomata open?

A

When guard cells have low water potential, so water enters the cell and makes it a turgid cell. Due to their inner cell wall being thicker and more rigid, the guard cells bend away from each other when they fill with water and this opens a pore - the gates open.

48
Q

What happens to stomata when water becomes scarce?

A

Hormonal signals can trigger turgor loss from the guard cells, which close the stomatal pores and so conserve water

49
Q

4 factors that affect transpiration

A
  • Light (lack of=no photosynthesis=no need for co2=stomata close)
  • Relative humidity of air (less humid=higher concentration gradient=more transpiration)
  • Temperature (more kinetic energy = faster evaporation from mesophyll + increases volume of water external air can hold)
  • Soil-water levels (more water = more transpiration
50
Q

Where is the Casparian strip found?

A

Runs around endodermal cells, forming a waterproof layer

51
Q

What are xerophytes?

A

Plants that have developed a wide range of adaptions that enable them to live in areas of very low water availability

52
Q

How does an especially thick waxy cuticle help conserve water?

A

It helps minimise the volume of water lost via transpiration through the cuticle

53
Q

How do sunken stomata help conserve water?

A

Many xerophytes have their stomata located in pits, which reduce air movement, producing a microclimate of still, humid air, which reduces the water potential gradient, so reduces the rate of transpiration

54
Q

How does a reduced number of stomata help conserve water?

A

Less stomata = less water loss via transpiration

55
Q

How do reduced leaf areas help conserve water?

A

Less leaf area = lower SA:V ratio = lower rate of transpiration

56
Q

How does hairy and/or curled leaves help conserve water?

A

It creates a microclimate of still, humid air, which reduces the water potential gradient, so reduces the rate of transpiration

57
Q

What are succulents?

A

Plants that store water in specialised parenchyma tissue- the water is then used in times of drought

58
Q

How does leaf loss help conserve water?

A

Some plants lose their leaves when water is not available, so as to stop transpiration from occurring

59
Q

Examples of root adaptations in xerophytes

A
  • Long tap roots that can penetrate deep into soil, accessing deep underground water
  • Shallow roots with a large surface area that can absorb water from a rain shower before it evaporates
60
Q

What are hydrophytes?

A

Plants that have developed a wide range of adaptions that enable them to live in areas of very high water presence

61
Q

What is the major problem that hydrophytes must prevent?

A

Water-logging- air spaces filling up with water, rather than air

62
Q

How does very thin/no waxy cuticle help hydrophytes?

A

It means water can be easing lost through the cuticles via transpiration, helping prevent waterlogging

63
Q

How do many always open stomata help hydrophytes?

A

It maximises gaseous exchanges, and the do not need to worry about turgor loss, as they live in an abundance of water

64
Q

Where are stomata found on water lillies?

A

On the upper side, as the bottom is in the water

65
Q

Why do hydrophytes have a reduced structure?

A

As water supports the leaves and flowers, so there is no need for strong supporting structures

66
Q

How do wide, flat leaves help hydrophytes?

A

They spread across the greatest surface area, so as to capture as much light as possible

67
Q

Why do hydrophytes have small roots?

A

As water can diffuse directly into the stem and leaf tissue

68
Q

Why do hydrophytes have air sacs?

A

To allow them to float on the surface of water

69
Q

What is aerenchyma?

A

Specialised parenchyma that forms in hydrophytes, which makes leaves and stems more buoyant, and forms a low-resistance internal pathway for the movement of substances (e.g. water).

70
Q

What are anoxic conditions?

A

Extreme low oxygen conditions

71
Q

What is translocation?

A

The transport of organic compounds in the phloem from sources to sinks

72
Q

What are assimilates?

A

The products of photosynthesis that are transported

73
Q

What is the main assimilate transported around the plant?

A

Sucrose

74
Q

What happens to glucose before being translocated and why?

A

It is converted into sucrose before being transported, as this is because sucrose is less likely to be used in metabolism than glucose and so more likely to reach its destination

75
Q

What are the main sources of assimilates in plants?

A
  • Green leaves and green stems
  • Storage organs
  • Food stores in seeds when they germinate
76
Q

What are the main sinks in a plant?

A
  • Roots that are growing/actively absorbing mineral ions
  • Meristems that are actively dividing
  • Any parts of a plant that need energy to grow (such as developing seeds)
77
Q

What is phloem loading?

A

When the soluble plants from photosynthesis are actively moved into the phloem from the sources

78
Q

What are the two main ways in which plants load assimilates into the phloem?

A
  • The symplast route

- The apoplast route

79
Q

How does the symplast route work phloem loading?

A

Sucrose moves through the cytoplasm of mesophyll cells and into the sieve tubes via diffusion

80
Q

Where are proton pumps found, and what do they do?

A

They are found in companion cells; they pump H+ ions into the outside cell area, creating a concentration gradient of H+ cells from outside to inside the companion cells

81
Q

Where are co-transporters found, and what do they do?

A

They are found in companion cells; the H+ ions pumped out by the proton pumps want to re-enter the companion cell, and they do this through the co-transporter. However to enter through the co-transporter, they must also take with them sucrose, therefore sucrose also enters into the companion cells

82
Q

What happens when sucrose enters the companion cells during the apoplast route?

A

The sucrose will enter the sieve tube elements by diffusion through the plasmodesmata

83
Q

What happens when sucrose enters the sieve tube elements?

A

The water potential in the phloem decreases, causing water to enter via osmosis.
This leads to a build up of turgor pressure, and causes mass flow to occur

84
Q

How does mass flow work in the phloem?

A

Assimilates flow from the area of higher turgor pressure (the sources) down the concentration gradient to the areas of lower turgor pressure (the sinks)

85
Q

What type of process is phloem loading and why?

A

It is an active process, as ATP is used to pump the H+ ions out of companion cells during the apoplast route

86
Q

How do the assimilates enter the sinks?

A

The assimilates will enter via diffusion from the phloem into the sinks

87
Q

How is the concentration gradient maintained from phloem to sink and why?

A
  • Sucrose either rapidly moves onto other cells by diffusion, or is converted into another substance (e.g. glucose or starch)
  • This is to make sure the sinks continue to receive sucrose
88
Q

What happens to water in the phloem after the assimilates enter the sink, and why is this important?

A

Assimilates entering the sink will make the water potential in the phloem higher, which will cause water to often follow the assimilates into the sink via osmosis, or sometimes be drawn into the transpiration stream in the xylem
This is important because it maintains a low turgor pressure at the sinks

89
Q

Proof of the involvement of active transport in translocation

A
  • Microscopy allows us to see adaptions in the companion cells for active transport
  • If mitochondria are poisoned by cyanide, translocation stops
  • The flow of the phloem is 10,000 faster than lone diffusion would be, indicating active transport is driving the mass flow
90
Q

How is phloem and xylem transport similar?

A
  • Both involve mass flow

- Solutes carried in solution in both

91
Q

How can some parts of a plant act as both a source and a sink (with an example)?

A

Certain parts can store and then release carbohydrates when needed- an example is a leaf, which can act as a sink or source at different times of the year

A-Level OCR Biology (21 decks)

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