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OCR AS/A Level Biology Context > module 3.3: transport in plants > Flashcards

module 3.3: transport in plants Flashcards

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1
Q

what are dicotyledonous plants

A

plants with two seed leaves and a branching pattern of veins the leaf

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2
Q

what is a meristem

A

a layer of dividing cells, here it is called the pericycle

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3
Q

what is a vascular tissue

A

consists of cells specialised for transporting fluids by mass flow

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4
Q

why do plants need a transport system

A

to move:
- water and minerals from the roots up to the leaves
- sugars from the leaves to the rest of the plant

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5
Q

what does the vascular tissue consist of

A

xylem and phloem

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6
Q

describe the distribution of vascular tissue in dicotyledonous plants

A

the vascular tissue is distributed throughout the plant. the xylem and phloem are found together in vascular bundles. these bundles may also contain other types of tissue (such as collenchyma and sclerenchyma) that give the bundle some strength and help to support the plant

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7
Q

describe the distribution of the vascular tissues in young root

A
  • the vascular bundle is found at the centre of a young root
  • there is a central core of xylem, often in the shape of an X
  • the phloem is found in between the arms of the X-shaped xylem tissue
  • this arrangement provides strength to withstand the pulling forces to which roots are exposed
  • around the vascular bundle is a special sheath of cells called the endodermis
  • the endodermis has a key role in getting water into the xylem vessels
  • just inside the endodermis is a layer of meristem cells (cells that remain able to divide) called the pericycle
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8
Q

describe the distribution of vascular tissue in the stem

A
  • the vascular bundles are found near the outer edge of the stem
  • in non-woody plants the bundles are separate and discrete
  • in woody plants the bundles are separate in young stems, but become a continuous ring in older stems
  • the xylem is found towards the inside of each vascular bundle and the phloem towards the outside
  • in between the xylem and phloem is a layer of cambium. the cambium is a layer of meristem cells that divide to produce new xylem and phloem
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9
Q

describe the distribution of the vascular tissue in the leaf

A
  • the vascular bundles form the midrib and veins of a leaf
  • a dicotyledonous leaf has a branching network of veins that get smaller as they spread away from the midrib
  • within each vein, the xylem is located on top of the phloem
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10
Q

what are the best plants to dissect and stain to view the vascular tissue

A

celery and busy lizzies

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11
Q

what are companion cells

A

the cells that help to load sucrose into the sieve tubes

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12
Q

what are the sieve tube elements

A

make up the tubes in phloem tissue that carry sap up and down the plant. the sieve tube elements are separated by sieve plates

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13
Q

what is the xylem

A

a tissue used to transport water and mineral ions from the roots up to the leaves and other parts of the plant

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14
Q

what is the xylem tissue consist of

A
  • vessels to carry the water and dissolved mineral ions
  • fibres to help support the plant
  • living parenchyma cells which act as packing tissue to separate and support the vessels
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15
Q
A
  • as xylem vessels develop, lignin impregnates the walls of the cells, making the walls waterproof. this kills the cells
  • the end walls and contents of the cells decay, leaving a long column of dead cells with no contents — a tube called the xylem vessel
  • the lignin strengthens the vessel walls and prevents the vessel from collapsing. this keeps vessels open even at times when water may be in short supply. the lignin thickening forms patterns in the cell wall
  • this prevents the vessel from being too rigid and allows some flexibility of the stem or branch
  • in some places lignification is not complete, leaving gaps in the cell wall. these gaps form pits or bordered pits. the bordered pits in two adjacent vessels are aligned to allow water to leave one vessel and pass into the next vessel
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16
Q

what are the adaptations of xylem to its function

A
  • they are made from dead cells aligned end to end to form a continuous column
    The tubes are narrow, so that the water column does not break easily and capillary action can be effective
  • bordered pits in the lignified walls allow water to move sideways from one vessel to another
  • lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the plant grows, and enables the stem or branch to bend
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17
Q

why is the flow of water is not impeded

A
  • there are no cross-walls
  • there are no cell contents, nucleus or cytoplasm
  • lignin thickening prevents the walls from collapsing
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18
Q

describe the structure and function of phloem

A
  • it consists of cells that perform the function of transporting sucrose. the main cell types are the sieve tube elements and the companion cells
  • phloem is a tissue used to transport assimilates (mainly sucrose and amino acids) around the plant. the sucrose is dissolved in water to form sap. phloem tissue consists of sieve tubes — made up of sieve tube elements — and companion cell
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19
Q

describe the sieve tube elements

A
  • they contain no nucleus and very little cytoplasm, leaving space for mass flow of sap to occur
  • at the ends of the sieve tube elements are perforated cross-walls called sieve plates
  • the perforations in the sieve plate allow movement of the sap from one element to the next
  • the sieve tubes have very thin walls and when seen in transverse section are usually five- or six-sided
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20
Q

describe companion cells

A
  • in between the sieve tubes are small cells, each with a large nucleus and dense cytoplasm
  • these are the companion cells. they have numerous mitochondria to produce the ATP needed for active processes. The companion cells carry out the metabolic processes needed to load assimilates actively into the sieve tubes
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21
Q

what is the plasmodesmata

A

gaps in the cell wall containing cytoplasm that connects two cells

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22
Q

what are the 3 pathways for the way water travels

A
  • apoplast pathway
  • symplast pathway
  • vacuolar pathway
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23
Q

what is the apoplast pathway

A

water passes through the spaces in the cell walls and between the cells. water moves by mass flow rather than by osmosis. also, dissolved mineral ions and salts can be carried with the water

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24
Q

what is the symplast pathway

A

water enters the cell cytoplasm through the plasma membrane. it can then pass through the plasmodesmata from one cell to the next
this pathway requires water to cross partially permeable membranes

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25
Q

what is the vacuolar pathway

A

this is similar to the symplast pathway, but the water is not confined. it is able to enter and pass through the vacuoles as well

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26
Q

what is water potential

A

a measure of the tendency of water molecules to move from one place to another. water always moves from a region of higher water potential to a region of lower water potential. the water potential of pure water is zero. as a result, the water potential in plant cells is always negative

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27
Q

as the cytoplasm contains mineral ions and sugars, what does this do to the water potential

A

it reduces it

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28
Q

describe water uptake in a plant cell

A
  • the water potential in a plant cell is more negative (lower) than the water potential of the water, so water molecules will move down the water-potential gradient into the cell
  • the cell has a strong cellulose cell wall. once the cell is full of water it is described as being turgid
  • the water inside the cell starts to exert pressure on the cell wall, called the pressure potential. as the pressure potential builds up, it reduces the influx of water
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29
Q

describe water loss in a plant cell

A
  • if a plant cell is placed in a salt solution with a very negative (low) water potential, then it will lose water by osmosis. this is because the water potential of the cell is less negative (higher) than the water potential of the solution, so water moves down the water potential gradient out of the cell
  • as water loss continues, the cytoplasm and vacuole shrink. eventually, the cytoplasm no longer pushes against the cell wall, and the cell is no longer turgid
  • if water continues to leave the cell, then the plasma membrane will lose contact with the wall — a condition known as plasmolysis. the tissue is now flaccid
30
Q

describe the movement of water between cells

A
  • when plant cells are touching each other, water molecules can pass from one cell to another
  • the water molecules will move from the cell with the less negative (higher) water potential to the cell with the more negative (lower) water potential - osmosis
31
Q

what is transpiration

A

the loss of water vapour from the aerial parts of a plant, mostly through the stomata in the leaves

32
Q

describe the process of transpiration

A
  1. water enters the leaf through the xylem, and moves by osmosis into the cells of the spongy mesophyll. it may also pass along the cell walls via the apoplast pathway
  2. water evaporates from the cell walls of the spongy mesophyll
  3. water vapour moves by diffusion out of the leaf through the open stomata. this relies on a difference in the concentration of water vapour molecules in the leaf compared with outside the leaf. this is known as the water vapour potential gradient. there must be a less negative (higher) water vapour potential inside the leaf than outside
33
Q

what is the importance of transpiration

A
  • transports useful mineral ions up the plant
  • maintains cell turgidity
    ▪ supplies water for growth, cell elongation and photosynthesis
  • supplies water that, as it evaporates, can keep the plant cool on a hot day
34
Q

what are the factors affecting transpiration

A
  • light intensity
  • temperature
  • relative humidity
  • air movement
  • water availability
35
Q

explain how light intensity is a factor affecting transpiration

A
  • in light, the stomata open to allow gaseous exchange for photosynthesis. higher light intensity increases the transpiration rate
36
Q

explain how temperature is a factor affecting transpiration

A
  1. increase the rate of evaporation from the cell surfaces so that the water-vapour potential in the leaf rises 2. increase the rate of diffusion through the stomata because the water molecules have more kinetic energy 3. decrease the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf
37
Q

explain how relative humidity is a factor affecting transpiration

A
  • higher relative humidity in the air will decrease the rate of water loss. this is because there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside
38
Q

explain how air movement is a factor affecting transpiration

A

air moving outside the leaf will carry away water vapour that has just diffused out of the leaf. this will maintain a high water vapour potential gradient

39
Q

explain how water availability is a factor affecting transpiration

A

if there is little water in the soil, then the plant cannot replace the water that is lost. if there is insufficient water in the soil, then the stomata close and the leaves wilt

40
Q

what is a potometer

A

a device that can measure the rate of water uptake as a leafy stem transpires

41
Q

what happens when using the potometer to measure the rate of water uptake

A

water vapour lost by the leaves is replaced from the water in the capillary tube. the movement of the meniscus at the end of the water column can be measured

42
Q

how do you study the different affects of different environmental factors on transpiration

A

you can place the whole apparatus under different sets of conditions. remember to vary only one factor at a time, in order to determine the effect on the transpiration rate

43
Q

what are the precautions taken to ensure that the potometer shows valid results

A
  1. set it up under water to make sure there are no air bubbles inside the apparatus
  2. ensure that the shoot is healthy
  3. cut the stem under water to prevent air entering the xylem
  4. cut the stem at an angle to provide a large surface area in contact with the water
  5. dry the leaves
44
Q

how do you find the transpiration rate from the results of the potometer

A
  1. measuring the volume of water taken up by the shoot involves calculating the volume of a cylinder (the length of capillary tube)
  2. the volume of a cylinder is given by the formula: v= πr^2l (where r is the radius of the capillary tube and I is the length of the capillary tube)
  3. the rate of transpiration is the volume calculated, divided by the time taken. rate = volume/time
45
Q

what is meant by adhesion

A

the attraction between water molecules and the walls of the xylem vessel

46
Q

what is meant by cohesion

A

the attraction between water molecules caused by hydrogen bonds

47
Q

what is the transpiration stream

A

the movement of water from the soil, through the plant, to the air surrounding the leaves

48
Q

describe the water uptake and movement across the root

A
  • the outermost layer of cells (the epidermis) of a root contains root hair cells. these are cells with a long extension (root hair) that increases the surface area of the root. these cells absorb mineral ions and water from the soil. the water then moves across the root cortex down a water-potential gradient to the endodermis of the vascular bundle. water may also travel through the apoplast pathway as far as the endodermis, but must then enter the symplast pathway, as the apoplast pathway is blocked by the Casparian strip
49
Q

what is the endodermis

A

a layer of cells surrounding the medulla and xylem. this layer of cells is also known as the starch sheath, as it contains granules of starch — a sign that energy is being used

50
Q

where does the movement of water across the root occur and what type of process is it

A

in the endodermis
active process

51
Q

what does the Casparian strip do

A
  • blocks the apoplast pathway between the cortex and the medulla. this ensures that water and dissolved mineral ions (especially nitrates) have to pass into the cell cytoplasm through the plasma membranes
  • this makes the water potential of the medulla and xylem more negative, so that water moves from the cortex cells into the medulla and xylem by osmosis
  • once the water has entered the medulla, it cannot pass back into the cortex, as the apoplast pathway of the endodermal cells is blocked by the Casparian strip
52
Q

what are the 3 processes that help to move water up the stem

A
  • root pressure
  • transpirational pull
  • capillary action
53
Q

describe root pressure

A
  • the action of the endodermis moving minerals into the medulla and xylem by active transport draws water into the medulla by osmosis
  • pressure in the root medulla builds up and forces water into the xylem, pushing the water up the xylem
  • root pressure can push water a few metres up a stem, but cannot account for water getting to the top of tall trees
54
Q

describe transpirational pull

A
  • the loss of water by evaporation from the leaves must be replaced by water coming up from the xylem
  • water molecules are attracted to each other by forces of cohesion
  • these cohesion forces are strong enough to hold the molecules together in a long chain or column. as molecules are lost at the top of the column, the whole column is pulled up as one chain. the pull from above creates tension in the column of water
  • this is why the xylem vessels must be strengthened by lignin. the lignin prevents the vessel from collapsing under tension. because this mechanism involves cohesion between the water molecules and tension in the column of water, it is called the cohesion—tension theory
  • it relies on the plant maintaining an unbroken column of water all the way up the xylem. If the water column is broken in one xylem vessel, then the water column can maintained through another vessel via the bordered pits
55
Q

describe capillary action

A
  • the same forces that hold water molecules together also attract water molecules to the sides of the xylem. this is called adhesion
  • as xylem vessels are very narrow, these forces of attraction can pull water up the sides of the vessel
56
Q

how does water leave the leaf

A
  • most water leaves the plant via the stomata and only a tiny amount of water leaves the plant through the waxy cuticle
  • water evaporates from the cells lining the cavity immediately above the guard cells (the sub-stomatal air space). this lowers the water potential in these cells, causing water to enter them by osmosis from neighbouring cells
  • in turn, water is drawn from the xylem in the leaf by osmosis. water may also reach these cells by the apoplast pathway from the xylem
57
Q

what is a hydrophyte

A

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

58
Q

what is a xerophyte

A

a plant adapted to living in dry conditions

59
Q

what do plants living on land must be adapted to

A
  • reduce this loss of water
  • replace the water that is lost
60
Q

what are the physical and behavioural adaptations of most terrestrial plants

A
  • a waxy cuticle on the leaf will reduce water loss due to evaporation through the epidermis
  • the stomata are often found on the under-surface of leaves, not on the top surface — this reduces the evaporation due to direct heating from the sun
  • most stomata are closed at night, when there is no light for photosynthesis
  • deciduous plants lose their leaves in winter, when the ground may be frozen (making water less available) and when temperatures may be too low for photosynthesis
61
Q

where is marram grass (ammophila ) specialised to live

A

it lives in sand dunes

62
Q

what are the adaptations of marram grass

A
  • the leaf is rolled longitudinally so that air is trapped inside air becomes humid, which reduces water loss from the leaf he leaf can roll more tightly in very dry conditions
  • there is a thick waxy cuticle on the outer side of the rolled leaf (upper epidermis), to reduce evaporation
  • the stomata are on the inner side of the rolled leaf (lower epidermis), so they are protected by the enclosed air space
  • the stomata are in pits in the lower epidermis, which is also folded and covered by hairs. these adaptations help to reduce air movement and therefore loss of water vapour
  • the spongy mesophyll is very dense, with few air spaces — so there is less surface area for evaporation of water
63
Q

what are some adaptations of cacti

A
  • cacti are succulents — they store water in their stems which become fleshy and swollen. the stem is often ribbed or fluted so that it can expand when water is available
  • the leaves are reduced to spines. this reduces the surface area of the leaves. when the total leaf surface area is reduced, less water is lost by transpiration
    -the stem is green for photosynthesis
  • the roots are very widespread, in order to take advantage of any rain that does fall
64
Q

what is an example of a hydrophyte

A

water lilies (family Nymphaeales)

65
Q

what are the adaptations of water lilies (family Nymphaeales)

A
  • many large air spaces in the leaf. this keeps the leaves afloat so that they are in the air and can absorb sunlight
  • the stomata are on the upper epidermis, so that they are exposed to the air to allow gaseous exchange
  • the leaf stem has many large air spaces. this helps with buoyancy, but also allows oxygen to diffuse quickly to the roots for aerobic respiration
66
Q

describe translocation

A
  • occurs in the phloem, and is the movement of assimilates throughout the plant - assimilates are substances made by the plant, using substances absorbed from the environment. These include sugars (mainly transported as sucrose) and amino acids
67
Q

what is the source

A

a part of the plant that loads materials into the transport system

68
Q

what is the sink

A

a part of the plant where assimilates are removed from the transport system

69
Q

describe active loading (proton pump)

A
  • sucrose is loaded into the sieve tube by an active process. this involves the use of energy from ATP in the companion cells. the energy is used to actively transport hydrogen ions (H+) out of the companion cells
  • as a result, a concentration gradient is created. the hydrogen ions diffuse back into the companion cells through special cotransporter proteins. these proteins only allow the movement of the hydrogen ions into the cell if they are accompanied by sucrose molecules. this is known as cotransport. It is also called secondary active transport
  • as it results from the active transport of the hydrogen ions out of the cell and moves the sucrose against its concentration gradient
  • as the concentration of sucrose in the companion cell increases, it can diffuse through the plasmodesmata into the sieve tube
70
Q

describe what happens at the source

A
  • sucrose entering the sieve-tube element, makes the water potential inside the sieve tube more negative (lower). as a result, water molecules move into the sieve-tube element by osmosis from the surrounding tissues. this increases the hydrostatic pressure in the sieve tube at the source
  • a source is any part of the plant that loads sucrose into the sieve tube. in early spring, this could be the roots, where energy stored as starch is converted to sucrose and moved to other parts of the plant in order to enable growth in the spring. the most obvious source is a leaf
  • sugars made during photosynthesis are converted to sucrose and loaded into the phloem sieve tubes. this occurs during late spring, summer and early autumn, whilst the leaves are green. the sucrose is transported to other areas of the plant that may be growing (meristems), or to areas such as the roots — for storage
71
Q

describe what happens at the sink

A
  • the sucrose could be used for respiration and growth in a meristem, or it could be converted to starch for storage in a root
  • where sucrose is being used in the cells, it can diffuse out of the sieve tube via the plasmodesmata
  • it may also be removed by active transport
  • the removal of sucrose from the sap makes the water potential less negative (higher), so that water moves out of the sieve tube into the surrounding cells. this reduces the hydrostatic pressure in the phloem at the sink
72
Q

what happens along the phloem

A
  • water entering the sieve tube at the source increases the hydrostatic pressure. water leaving the sieve tube at the sink reduces the hydrostatic pressure. therefore a pressure gradient is set up along the sieve tube, and the sap flows from higher pressure to lower pressure
  • this could be in either direction, depending upon where sucrose is being produced and where it is needed. it is even possible that sap could be flowing in opposite directions in different sieve tubes at the same time
  • since the sap In one tube is all moving in the same direction, this is mass flow

OCR AS/A Level Biology Context (24 decks)

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