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Biology A Level > Transport in Plants > Flashcards

Transport in Plants Flashcards

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1
Q

Why plants need a specialised exchange system?

A
  • To move water and minerals from the roots to the leaves

- To move sugars produced in the leaves during photosynthesis to the rest of the plant

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2
Q

What is transported in the xylem tissue?

A

Water and mineral ions travel upwards in xylem tissue

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3
Q

What is transported in phloem tissue?

A

Sugars and assimilates travel upwards or downwards in phloem tissue

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4
Q

What is a dicotyledonous plant?

A

A dicot is a plant with two seed leaves, they have vascular tissue (xylem and phloem) spread throughout the plant

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5
Q

Where are the phloem and xylem found in young root?

A

In the centre of the young root

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6
Q

What shape is the xylem in the young root?

A

Star shaped, the phloem tissue is found in between the arms of the xylem tissue, this arrangement provides strength to withstand pulling forces to which the roots are exposed to

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7
Q

What is medulla?

A

Tissue in the stems of vascular plants, it is composed of soft spongey parenchyma cells that store and transport nutrients throughout the plant

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8
Q

What is the endodermis?

A

A special sheath of cells around the vascular bundles that play an important role in getting water into the xylem vessels

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9
Q

Which cells are found inside the endodermis?

A

A layer of meristem cells called the pericycle

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10
Q

Where are the vascular bundles found in the stem of non-woody plants?

A

In the outer edge of the stem, the bundles are separate and discreet

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11
Q

Where are the vascular bundles found in the stem of woody plants?

A

In the outer edge of the stem, in younger stems the vascular bundles are separate but become a continuous ring in older stems. This means there is a continuous ring of vascular bundles under the bark of a tree. This provides strength and flexibility to withstand bending forces to which stems and branches are exposed to

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12
Q

In the stem where are the xylem found in vascular bundles in the stem of the plant?

A

Towards the inside of each vascular bundle

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13
Q

In the stem where are the phloem found in vascular bundles in the stem of the plant?

A

Towards the outside of the vascular bundle

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14
Q

What separates xylem and phloem in the vascular bundles in the stem of a plant?

A

Cambium, cambium is a layer of meristem cells that divide to produce new xylem and phloem

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15
Q

What does xylem tissue consist of?

A
  • Vessels to carry the water and mineral ions
  • Fibres to help support the plant
  • Living parenchyma cells that act as packing tissue to separate and support the vessels
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16
Q

Why are xylem cells dead?

A

As the xylem vessels develop, lignin impregnates the walls of the cell making them waterproof but also killing the cell. The end walls and the contents fo the cell decay which leaves a long column of dead cells with no contents. This is called the xylem vessel

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17
Q

What is the function of lignin in the xylem?

A

Strengthens the vessel walls and prevents the vessel from collapsing, this keeps the vessels open at times even when water is in short supply. Lignin thickening creates patterns in the cells this prevents the vessel from being too rigid and allows some flexibility of the stem or branch

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18
Q

How are bordered pits formed?

A

In some places, lignification isn’t complete, this leaves gaps in the cell wall, these are bordered pits. Bordered pits in adjacent xylem vessels are aligned to allow the water to move from one vessel to another. They also allow water to move out of the xylem vessel and to the living parts of the plant

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19
Q

What adaptations odes xylem have to carry out its function

A
  • They are made from dead cells aligned end to end to form a continuous column
  • The xylem vessel is narrow so that the water column doesn’t break and capillary action is effective
  • Bordered pits of two adjacent vessels are aligned to allow water to pas from one vessel to another
  • Lignin forms patterns in the cell walls of the xylem cells, this enables the plant to bend and be flexible
  • There are no cross walls between cells to stop water flow
  • There are no cell contents or cytoplasm
  • Lignin thickening prevents the walls from collapsing even when carrying high volumes of water
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20
Q

What assimilates does phloem mainly transport?

A

Sucrose and amino acids

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21
Q

What does sucrose dissolve in water to form?

A

Sap, sap contains amino acids too

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22
Q

What do sieve tubes consist of?

A

Sieve tube elements and companion cells, sieve tubes have very thin walls

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23
Q

Do sieve tubes contain a nucleus?

A

No they have no nucleus and very little cytoplasm, this allows mass flow of sap to occur

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24
Q

What are sieve plates?

A

Sieve plates are perforated cross walls at the end of each sieve tube element, the perforations allow the movement of sap from one element to the next

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25
Q

What is the main function of the sieve plate?

A

After injury or infection, the pores in the sieve plate rapidly become blocked by deposition of callose, this prevents loss of sap and inhibits transport of pathogens around the plant

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26
Q

What are companion cells?

A

In between sieve tubes are small cells with a large nucleus and dense cytoplasm. They have a large nucleus ad dense cytoplasm, they also contain many mitochondria to produce the ATP required for active processes. Companion cells carry out the metabolic processes required to load assimilates into the sieve tube elements

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27
Q

What are the three pathways water can take to move through cells?

A
  • Apoplast
  • Symplast
  • Vacuolar
28
Q

Describe the apoplast pathway

A

Water passes through the gaps between two cells it doesn’t pass through any plasma membranes meaning the water moves by mass flow rather than osmosis

29
Q

Describe the symplast pathway

A

Water passes into the cell through the permeable cell walls and the partially permeable plasma membrane into the cell cytoplasm. From here it moves into the next cell through the plasmodesmata

30
Q

Describe the vacuolar pathway

A

Water passes into the cell through the permeable cell walls and the partially permeable plasma membrane into the cell cytoplasm. The water can also move through the vacuoles of the cells, like the symplast pathway, water moves from one cell to the next through the plasmodesmata

31
Q

How does the cell wall prevent a plant cell from bursting?

A

Water moves down its water potential gradient until the cell becomes full of water, the cell is now turgid. The water inside the cell then starts to exert pressure on the cell wall called the pressure potential. As the pressure potential builds up the influx of water is reduced

32
Q

What happens to the cell cytoplasm when the cell is placed in a solution with a salt solution with a very negative water potential?

A

Water leaves the cell cytoplasm and the cytoplasm and vacuole shrink, this causes the cytoplasm to move away from the cell wall. The cell is plasmolysed and the tissue is flaccid

33
Q

What is transpiration?

A

The loss of water vapour from the upper parts fo the plant, particularly the leaves. The majority of water leaves through the stomata as they are open to allow gaseous exchange for photosynthesis to occur

34
Q

What prevents water evaporating from the leave’s surface?

A

The waxy cuticle

35
Q

What time of day does most water leave the plant?

A

During the middle of the day as the stomata are open to allow gaseous exchange for photosynthesis and rate of photosynthesis is highest when light intensity is higher meaning more CO2 is required or it will become a limiting factor

36
Q

What is the typical water pathway for water leaving the leaf of a plant?

A
  • Water enters the leaf through the bordered pits of the xylem and moves into the spongey mesophyll cells by diffusion
  • The water evaporates from the spongey mesophyll cells and moves out of the leaf through stomata by diffusion
  • For the water to diffuse out of the leaf there has to be a a higher water potential inside the leaf than outside
37
Q

Why is transpiration essential for the plant to survive?

A

As water is lost from the leaf, it must be replaced from below, this draws water up the stem as a transpiration stream, this is useful as:

  • mineral ions are transported up the plant
  • cell turgidity is maintained
  • water is supplied for growth, cell elongation and photosynthesis
  • supplies water to the leaves that evaporates, this is useful on a hot day as it cools the plant as energy is required to evaporate water due to its high specific latent heat
38
Q

How does light intensity affect transpiration rate?

A

As light intensity increases, so does transpiration rate as the stomata open further when light intensity increases to allow more CO2 to enter the plant for photosynthesis to ensure CO2 doesn’t become a limiting factor

39
Q

How does temperature affect transpiration rate?

A

As temperature increases rate of transpiration also increases this is due to three factors:

  • The water molecules in the stomata have more kinetic energy meaning they have a faster rate of diffusion
  • Higher temperature decreases relative water potential in the air meaning there is a steeper water potential gradient
  • Rate of evaporation from the cell surfaces increases meaning water potential in the leaf rises as more water evaporating from the spongey mesophyll cells
40
Q

How does relative humidity affect the rate of transpiration?

A

As relative humidity increases, the rate of transpiration decreases as the water potential gradient becomes less steep as there is a higher water potential outside the leaf meaning the water potential gradient is smaller so rate of transpiration decreases

41
Q

How does wind (air movement) affect rate of transpiration?

A

As air movement increases, water vapour is moved away from the outside of the plant meaning the water potential gradient is increased and the rate of transpiration increases

42
Q

How does water availability (amount of water in the soil) affect rate of transpiration?

A

As water availability is decreased, the plant can’t replace the water that evaporates from the leaf, therefore transpiration rate decreases as water availability decreases. If there is no water in the soil at all, the stomata close and the leaves wilt.

43
Q

How do roots take up water from the soil?

A
  • The epidermis (the outermost layer of cells) of the root contains root hair cell that increase the surface area of the root, this means that the root can take up more water and mineral ions
  • Water then moves down a water potential gradient towards the endodermis of the vascular bundle
  • Water can also travel to the endodermis via the apoplast pathway but has to enter the symplast pathway when the water reaches the endodermis as the apoplast pathway is blocked by casparian strip
44
Q

What s a root hair cell?

A

A root hair cells are cells in the epidermis of the root with a long extension (root hair), that increase the surface are of the root

45
Q

What is the role of the endodermis?

A
  • Movement of water across the root is driven by an active process that occurs at the endodermis. The endodermis is a layer of cells surrounding the medulla and xylem. This layer of cells is also known as the starch sheath as it contains starch granules (meaning energy is being used as the monomer for starch is glucose)
  • Casparian strip blocks the apoplast pathway between the cortex and the medulla
  • This ensures that water and mineral ions pass into the cytoplasm of the cells through the plasma membrane
  • The plasma membranes contain transporter proteins that actively transport mineral ions from the cytoplasm of the cortex cells into the medulla and xylem
  • This makes the water potential of the xylem ad medulla more negative and therefore water moves into the medulla and xylem from the cortex cells by osmosis
  • Once water has entered the medulla it can’t enter the cortex again as the casparian strip is blocking the apoplast pathway
46
Q

How does root pressure enable water to be drawn up the xylem in the stem?

A

The action of the endodermis actively transporting mineral ions into the medulla and xylem by active transport lowers the water potential in the medulla and xylem and therefore water is drawn into the medulla by osmosis. Pressure builds up in the medulla and water is forced into the xylem, this pushes water up the xylem by a few metres but can’t account for getting water to the top of tall trees

47
Q

How does the transpiration pull enable water to be drawn up the xylem in the stem?

A

The loss of water from the leaves by evaporation must be replaced by water coming up the xylem. Water molecules are attracted to each other by forces of cohesion (hydrogen bonding). These cohesion forces are strong enough to hold the molecules together in a long chain/column. As molecules are lost at the top of the column as the water leaves the leaf by evaporation, the whole column is drawn up as one chain.

48
Q

Why do the xylem vessels need to be strengthened by lignin?

A

As when the water column is drawn up the xylem as water leaves the leaf, the pull from above creates tension in the column of water. The lignin prevents the vessel from collapsing under tension

49
Q

How does capillary action help to draw water up the xylem in the stem of the plant?

A

The same forces that hold water molecules together attract water molecules to the side of the xylem vessel, this is adhesion. Xylem vessels are very narrow meaning that forces of attraction can pull the water up the sides of the vessel

50
Q

What creates the tension in the xylem?

A

Movement of water out of the xylem at the top of the vessel into the leaf creates low hydrostatic pressure and therefore tension in the column of water

51
Q

How does water evaporate through the stomata?

A

Water evaporates from the cells lining the cavity immediately above the guard cells, this causes water to enter these cells by osmosis as their water potential becomes more negative when water evaporates from them. In turn, the water diffusing into the cells lining the cavity from the neighbouring cells is replaced by water drawn in from the xylem

52
Q

What is a hydrophyte?

A

A plant adapted to living in water or where the ground is very wet

53
Q

What is a xerophyte?

A

A plant adapted to living in dry conditions

54
Q

How are terrestrial plants adapted to reduce water loss?

A
  • The have a waxy cuticle on their leaf to reduce water loss due to evaporation through the epidermis
  • The stomata are closed at night when CO2 isn’t required for photosynthesis as there is no light
  • The stomata are located on the under-side of the leaf, this reduces the volume of water by evaporation through the plant due to direct heating from the sun
  • Deciduous plants such as trees, lose their leaves in winter when the ground may be frozen (water is less available) and when temperatures may be too low for photosynthesis
55
Q

What are the adaptions of Marram grass (a xerophyte that lives on sand dunes)?

A
  • Leaf is rolled longitudinally so that air gets trapped inside and becomes humid, this means that the water potential of the air rolled into the leaf will have a higher water potential than inside the leaf
  • Thick waxy cuticle on the outer side of the rolled leaf so that evaporation is reduced
  • Stomata are on the inside of the rolled leaf meaning that they have a lower water potential than the humid air inside the rolled leaf
  • Stomata are in pits in the lower epidermis which is also covered and protected by hairs, this helps to reduce air movement so that the water potential is higher outside the stomata than inside the leaf
  • Spongy mesophyll is very dense with few air spaces so there is less surface area for evaporation of water
56
Q

How are cacti adapted to overcome their arid conditions?

A
  • Cacti are succulents meaning they store water in their stems which become fleshy and swollen. Their stem is often ribbed or fluted meaning it can expand when water is available
  • Their leaves are reduced to spines meaning the surface area of the leaves is smaller. When the total surface area of the leaves is reduced, transpiration rate is decreased as diffusion rate is decreased
  • Their stem is green for photosynthesis meaning it has more chloroplasts which contain chlorophyll which make the plant green
  • Roots are very wide-spread allowing the plant to take advantage of any rain that does fall
57
Q

What other adaptations do xerophytes have?

A
  • Xerophytes may maintain a low water potential inside their leaf cells by maintaining a high salt concentration inside the cells. This reduces the evaporation of water from the leaf as the water potential gradient is reduced between the outside of the leaf and inside the leaf
  • Stomata is closed when water availability is low, this reduces water loss and need to take up water
  • They have a very long tap rot that can reach water deep underground
58
Q

What adaptations do hydrophytes have?

A

Many large air spaces in the leaf keeps the leaf afloat so they are in the air and can absorb sunlight

  • Stomata are on the upper epidermis, this means the are exposed to air and gaseous exchange can occur
  • The stem has many air spaces to allow oxygen to diffuse quickly to the roots for aerobic respiration
59
Q

What are hydathodes?

A

Structures at the tips of leaves of hydrophytes that release droplets of water to keep the transpiration stream going

60
Q

What is translocation?

A

Movement of assimilates throughout the plant, assimilates are substances made by the plant using substances absorbed from the environment

61
Q

What is a source ?

A

A part of a plant that loads assimilates into the sieve tubes

62
Q

What is a sink?

A

A part of a plant that removes assimilates from phloem sieve tubes

63
Q

Describe the process of active loading

A

ATP is used to actively transport H+ ions out of the companion cells. This reduces the concentration of H+ inside the companion cells, creating a concentration gradient. This causes H+ to diffuse back into the companion cell through special co-transporter proteins which only allow H+ ions back through if they are accompanied by a sucrose molecule, this is secondary active transport as the active transport of the H+ out of the companion cell moves sucrose into the companion cell against its concentration gradient. Once the sucrose is in the companion cell, it diffuses dow its concentration gradient into the sieve tube elements

64
Q

How does sap move through the sieve tube?

A
  • As sucrose diffuses into the sieve tube elements, it lowers the water potential meaning water follows by osmosis and increases hydrostatic pressure
  • Sap moves down the sieve tube from higher hydrostatic pressure at the source to lower hydrostatic pressure at the sink cell
  • Sucrose is removed from the sieve tube at the sink cell meaning the water potential is increased and water moves out of the sieve tube which reduces the hydrostatic pressure
  • If the source cell is at the bottom of the plant then the process works in the same way and sap moves up the sieve tube instead of down
65
Q

Does the source cell have to be a leaf cell?

A

No, in early spring the source cell can be the roots as starch can be converted to sucrose and transported to other parts of the plant for growth

66
Q

How does sucrose leave the sieve tube?

A

By diffusion through the plasmodesmata or it may be actively transported out of the sieve tube into the surrounding sink cells

Biology A Level (21 decks)

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