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Biology Module 3 > 3.3 Transport in Plants > Flashcards

3.3 Transport in Plants Flashcards

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1
Q

why is water needed in plants

A
  • provides turgor pressure, which helps to support the stems and leaves
  • turgor drives cell expansion, so plants can force way through concrete and tarmac
  • loss of water via evaporation helps to keep plant cool
  • mineral ions and products of photosynthesis can be transported
  • raw material for photosynthesis
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2
Q

why do multicellular plants need transport systems

A
  • need substances like water, minerals and sugars to live, and also get rid of waste substances
    1) have a small SA:V ratio, when accounting features like stems, trunks and roots, not just leaves with large SA
    2) relatively big, so need to move substances from root to tip of leafs
    3) relatively high metabolic rate, as need to transport glucose and O2 to parts of plant that don’t photosynthesise, like underground and internal, and need to transport hormones and mineral ions
  • means that exchanging surfaces via direct diffusion (through outer surface to cells) would be too slow to meet metabolic rates
  • need transport systems to get substances to plants quickly
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3
Q

what is meant by “herbaceous dicotyledonous” plants

A

dicots = make seeds that contain 2 cotyledons (organs that act as food stores for developing embryo of plant and form the first leaves when the seed germinates)

  • soft tissues
  • relatively short life cycle
  • flowering plants
  • non-woody stem
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4
Q

what is the main role of the two transport vessels in plants

A

xylem = water and mineral ions in a solution, up from the roots, stem, to the leaf

phloem = mainly transport sugar in a solution both up and down the plant

  • combine to make vascular system, arranged in vascular bundles
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5
Q

what is the vascular system and what does the structure depend on

A
  • made up of the xylem and phloem, found throughout the plant and transport materials to all parts
  • where they found depend on xylems role of support
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6
Q

explain how the vascular bundle is arranged in the root

A
  • xylem are in the centre (make up like a cross shape)
  • are surrounded by the phloem
  • xylem provides support for the roots as it pushes through the soil, to resist tugging felt on plant due to wind
  • root hairs branch off like hairs
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7
Q

explain how the vascular bundle is arranged in the stem

A
  • the xylem and phloem are found near the outside of the stem to provide a sort of “scaffolding” that reduces bending, providing strength and support
  • phloem are outside the xylem
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8
Q

explain how the vascular bundle is arranged in the leaf

A
  • xylem and phloem make up a network of veins
  • which support the thin leaves
  • like a harry potter thing shape
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9
Q

what is meant by transverse cross sections

A
  • the sections cut through each structure at a right angle to its length (like diagrams you’d expect of the leaves)
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10
Q

what is meant by longitudinal cross sections

A
  • taken along the length of the structure

-e.g. in the stem, the xylem and phloem would just be lines along the outside

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11
Q

break down xylem

A
  • xylem is a tissue made from several types of different cell types
  • xylem vessels are the part of the tissue that actually transports the water and ions
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12
Q

explain how the xylem vessels are adapted for their functions

A

1) very long, tube-like structures formed from cells (vessel elements) joined end to end
2) there are no end walls on these cells, making an interrupted tube that allows water to move through the middle easily
3) the cells are dead, so they contain no cytoplasm
4) walls are thickened with woody substance called lignin, helping to support vessel’s WALLS and stop them from collapsing inwards at lower pressure (can be deposited in xylem walls in different ways, e.g. in a spiral or as distinct rings)
- also waterproofs the walls, improves the adhesion of water molecules (form H bonds with water), and allows flexibility and stretching (prevents stem from breaking)
5) the amount of lignin increases as the cell gets older
6) 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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13
Q

adaptations of root hair cell

A
  • microscopic, so can penetrate between soil particles
  • large SA:V ratio of each hair, and many growing on each tip
  • thin surface layer
  • maintain conc. gradient with soil
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14
Q

explain how water enters the xylem and is drawn up the roots

A
  • enters through root hair cell
  • passes through the root cortex (including the endodermis)
  • reaches the xylem
  • drawn up the roots via osmosis
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15
Q

how does water travel in a plant in terms of osmosis and water potential

A
  • water always moves from areas of higher water potential to areas of lower water potential, down the water potential gradient

1) soil around the roots generally have a high water potential (lots of water there)
2) the leaves have a lower water potential ( water constantly evaporates from them) and the root cells have a lower water potential, due to dissolved solutes such as mineral ions, sugars and amino acids
3) creates a water potential gradient that keeps water moving through the plant in the right direction, from roots to leaves

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16
Q

what are the two paths through which water can move through the roots into the xylem

A
  • symplast pathway
  • apoplast pathway
  • via the root cortex
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17
Q

what is the symplast pathway

A
  • water goes through the living part of the cells - the cytoplasm
  • the cytoplasms of neighbouring cells connect through plasmodesmata
  • water moves through this pathway via osmosis, from areas of high to low water potential
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18
Q

what is plasmodesmata

A

small channels in the cell walls

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19
Q

what is the apoplast pathway

A
  • the water goes through the non-living parts of the cells - the cell walls
  • the walls are very absorbent and water can simply diffuse through them, as well as passing through the spaces between them
  • the water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure
  • this is an example of mass flow
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20
Q

explain how the water can change pathways when entering the xylem

A
  • when water in the apoplast pathway gets to the endodermis cells in the root, its pathway is blocked
  • by waxy strip (suberin) in the cell walls, called Casparian strip
  • now water has to take the symplast pathway
  • once past barrier, water moves into the xylem
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21
Q

why is the change in pathway of water in the roots useful

A
  • means that water has to go through a cell membrane
  • which is partially permeable
  • can control whether or not substances in the water get through (nothing toxic gets in)
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22
Q

which is the main pathway water uses to travel through the roots

A

apoplast
- as provides the least resistance

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23
Q

what is root pressure

A

the active pumping of minerals into the xylem to produce movement on water via osmosis
- independent of any effects of transpiration
- not the real major factor here

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24
Q

provide evidence for root pressure

A
  • using poisons like cyanide prevent the production of ATP, and if applied to roots, not root pressure is visible
  • it increases as temperature does and falls as it does as well, suggesting it is linked to chemical reactions
  • if levels of oxygen or respiration substrate fall, so does root pressure
  • xylem sap is forced out of leaves sometimes under certain conditions
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25
Q

where do xylem vessels transport water

A
  • all around the plant
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26
Q

how is water transported in the leaves

A
  • leaves the xylem
  • moves into cells mainly by the apoplast pathway
  • water evaporates from the cell walls of mesophyll cells into the spaces between cells in the leaf
  • when the stomata open, water diffuses out of the leaf, down the water potential gradient, into the surrounding air
  • this loss of water from the plants surface = transpiration
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27
Q

what are stomata

A

tiny pores in the surface of the leaf

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28
Q

what is the transpiration stream

A

the movement of water from the roots to the leaves

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29
Q

why does water need mechanisms to move up the plant

A

it moves UP the plant against the force of gravity

30
Q

what are the mechanisms that move water up the plant

A

cohesion, tension, adhesion

31
Q

how does cohesion and tension help to move water up the plant, against the force of gravity, from roots to leaves

A

1) water evaporates from the leaves at the “top” of the xylem (transpiration)
2) this creates a tension (suction) which pulls more water into the leaf

1) water molecules are cohesive (stick together) so when some molecules are pulled into the leaf, others follow
2) this means the whole column of water in the xylem, from the leaves to the roots, moves upwards
3) water enters the stem through the root cortex cells

  • allow the mass flow of water across long distances up the stem
32
Q

explain how adhesion helps move water up the leaf

A
  • water molecules are attracted to the walls of the xylem vessels too
  • helping the water rise up through the xylem vessels
33
Q

what is the evidence for cohesion-tension theory

A
  • changes in diameter of trees, as when transpiration at height, so is tension, so tree’s diameter shrinks
  • when xylem vessel is broke, air is drawn into xylem, rather than water leaking out
  • plant can no longer pull water up if vessel breaks, as stream is broken
34
Q

what is transpiration

A

the evaporation of water from the plants surface, especially the leaves

35
Q

what is the transpiration stream

A

the movement of water through the plants from the roots until it is lost by evaporation from the leaves

36
Q

what is transpiration the consequence of

A

gas exchange

37
Q

how does gas exchange lead onto transpiration

A
  • a plant needs to open its stomata to let CO2 in so that it can produce glucose
  • however, this also lets out water
  • as there is higher conc. of water inside the leaf than in the air outside
  • so water moves out of the leaf down the water potential gradient when the stomata open
  • so is a side effect of gas exchange needed for photosynthesis
38
Q

explain how the transpiration stream works

A

1) water moves via evaporation from surface of the mesophyll cells to air spaces in leaf, and then diffuses out of stomata down a conc. gradient
2) this loss of water lowers WP of cell, so water moves in from adjacent cells via the pathways
3) this is repeated across stream until it reaches xylem, where water moves out via osmosis again into cell - reduces the HYDROSTATIC PRESSURE AT TOP OF XYLEM
4) adhesion (H bonds with carbs in the walls of narrow xylem) , and cohesion, tend to stick together
5) both contribute towards capillary action: process by which water can rise up narrow tubes against force of gravity
6) water is drawn up in a continuous stream to replace the water lost, contributing to transpiration pull and tension in the xylem, which helps to move water up
7) high HYDROSTATIC PRESSURE AT BOTTOM OF XYLEM, as water moves in down the water potential gradient
- MASS FLOW OF WATER from high to low hydrostatic pressure

39
Q

what are the 4 factors that affect the transpiration rate

A
  • temperature
  • humidity
  • wind
    (all alter the water potential gradient)
  • light
40
Q

how does light effect the rate of transpiration

A
  • lighter = faster rate of transpiration
  • the stomata open when it gets light so CO2 can diffuse into the leaf for photosynthesis
  • when its dark, the stomata are usually closed so there is little transpiration
41
Q

how does temperature effect the rate of photosynthesis

A
  • higher the temp = higher rate
  • warmer water molecules have more kinetic energy
  • so evaporate from the spongy mesophyll cells inside the leaf faster
  • increases the water potential gradient between the inside and outside of the leaf
  • making water diffuse out of the leaf faster
  • (also decreases humidity of air, by increasing amount of water it can hold before saturated)
42
Q

how does humidity affect the rate of transpiration

A
  • lower the humidity = faster rate
  • if air around the plant in dry, the water potential gradient between the leaf and the air is increased
  • increases transpiration
43
Q

how does the wind affect the rate of transpiration

A
  • the windier = the faster rate of transpiration
  • lots of air movement blows away water molecules from around the stomata
  • increases the water potential gradient
  • increases the rate
44
Q

what is a potometer used for

A

used to measure transpiration rates

45
Q

what is the assumption made when using a potometer

A

the water uptake from a plant is equal (directly related to) the water loss by the leaves

  • not 100% as water is used up in photosynthesis
46
Q

PAG: how can you use a potometer to measure the factors effect on the rate of transpiration

A

1) cut shoot underwater to prevent air from entering the xylem, and cut at a slant to increase the surface area available for water uptake
2) assemble the potometer in water and insert the shoot under water, so no air can enter
3) remove the apparatus from the water, but keep the end of the capillary tube submerged in a beaker of water
4) check that the apparatus is airtight and watertight
5) dry the leaves, and allow time for the shoot to acclimatise and then shut the tap
6) remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the breaker
7) record the starting position of the air bubble
8) start the stopwatch and record the distance moved by the bubble per unit time - the rate of air bubble movement is an estimate of the transpiration rate
9) only change one variable, but keep all the other conditions the same

  • when using, do not allow air bubble to move too far - enter xylem
  • place open end in water - no new air bubbles
  • keep shoot still
47
Q

what does phloem transport

A

solutes, mainly sugar sucrose

  • products of photosynthesis
48
Q

basic similarity and difference between xylem and phloem

A
  • both arranged as tubes
  • phloem only used to transport tissue, and isn’t used for support
49
Q

what makes up phloem

A
  • phloem fibres
  • phloem parenchyma
  • sieve tube elements
  • companion cells
50
Q

explain sieve tube elements

A
  • living cells that form the tube for transporting solutes through the plant
  • joined end to end to form sieve tubes
  • the -sieve- parts are the end walls, which have lots of holes in them to allow solutes to pass through, perforated walls
  • the elements have no nucleus, very thin layer of cytoplasm and few organelles
  • the cytoplasm of adjacent cells in connected through holes in the sieve plates
51
Q

what are companion cells

A
  • as the sieve tube elements have no nucleus, they cannot survive on their own
  • one companion cell for every sieve tube element, joined to sieve tube elements by many plasmodesmata
  • carry out living functions for both themselves and the sieve cells, as contain nucleus
  • provide energy for the active transport of solutes
52
Q

how do you prepare a plant stem for dissection

A
  • use scalpel to cut a cross section of the stem, as thinly as possible for better viewing under a microscope
  • use tweezers to place section under water until used, to prevent it from drying out
  • place into a dish containing stain, like toluidine blue O, which stains lignin in xylem vessels blue green
  • rinse of section in water after minute of stain and mount each one onto a slide
53
Q

what is translocation

A

the movement of dissolved substances, e.g. dissolved sugars and amino acids, to where they needed

  • from source to sink
  • can occur in both directions, and is energy requiring
54
Q

what are assimilates

A

dissolved substances which are translocated by the phloem

55
Q

what is a source and what is a sink

A
  • source: where a substance is made (has a high conc. of it, and where it is loaded into the phloem)
  • e.g. green leaves, stems, storage organs like tap roots and tubers, seeds when germinating
  • sink: where the substance is used up ( has a low conc. of it, and is unloaded out of the phloem)
  • e.g. roots that are growing or actively absorbing minerals, meristems that are actively dividing and seeds, fruits and storage organs
56
Q

what is the main assimilate in plants?
where is the source, sink and why is it used?

A
  • sucrose (made of glucose and fructose)
  • source: usually the leaves
  • sink: food storage organs and meristems in the roots, stems and leaves
  • used as it is soluble and metabolically inactive, so doesn’t get used up during transport
57
Q

explain how some parts of the plant can be both source and sink

A

ROOTS:
- sucrose can be stored in the roots (source)
- in growing season, can be transported from roots to the leaves to provide energy for growth (sink)

58
Q

how is a concentration gradient maintained from source to sink

A
  • use of enzymes, which change the assimilate at the sink (e.g. by breaking them down into something else)
  • so always a lower conc. at sink
59
Q

what is sucrose used for at the sink, and how does this help with translocation

A
  • converted to starch at the sink (in potatoes)
  • so always lower conc. at sink vs in phloem
  • so constant supply of new sucrose reaches sink
  • at other sinks, sucrose can be converted into glucose and fructose using enzyme invertase, so lowering conc.
60
Q

which hypothesis is widely believed by scientists as being the method of translocation

A

mass flow hypothesis

61
Q

explain the overall mass flow of translocation

A
  • active transport is used to actively load solutes into the sieve tubes of the phloem at the source
  • this lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from xylem and companion cells
  • creates high hydrostatic pressure inside the sieve tubes at source end
  • at sieve end, solutes are removed from the phloem to be used up
  • increases the water potential in sieve tubes, so water leaves via osmosis
  • this lowers the hydrostatic pressure here
  • results in a pressure gradient from source end to sink end
  • gradient pushes solutes along sieve tubes to where they’re needed
62
Q

using which route do assimilates mainly move into the sieve tube element

A

-the apoplast pathway
- ACTIVE LOADING

  • move substances from surrounding tissue into companion cells
  • from companion cells to sieve tube
  • AGAINST conc gradient
63
Q

why do active processes need to be used to move substances into the phloem

A
  • there is a higher conc. of sucrose in companion cells than surrounding tissue cells
  • higher conc. in the sieve tubes than in the companion cells
64
Q

how is succrose actively loaded into the phloem

A

1) in companion cell, ATP is used to actively transport H+ ions out of the cell into surrounding tissue cells - energy supplied via breakdown of ATP
2) sets up a conc. gradient, with more H+ in surrounding tissue > companion cell
3) H+ ion binds to a co-transport protein in the companion cell membrane and re-enters cell (down conc. gradient)
4) sucrose molecule binds to the co-transport protein at the same time ( the movement of H+ ions is used to move sucrose into cell) against conc. gradient
5) sucrose molecule is then transported out of companion cell into sieve tubes via the same process - diffusion through plasmodesmata

65
Q

how is phloem unloaded

A
  • at any point when cells need it
  • mainly diffusion of sucrose
66
Q

what is the evidence for the translocation of substances

A
  • microscopy lets us see adaptations of companion cells for active transport
  • if mitochondria of companion cells is poisoned, translocation stops
  • flow of sugar is too fast to be left to diffusion alone, so must be active
  • aphids help to show the pressure differences at sink vs source
67
Q

what are xerophytes

A

plants that have adapted to live in hot, dry, breezy conditions

e.g marram grass on sand dunes and cacti

68
Q

why do xerophytes need to be adapted

A

to prevent them from losing too much water by transpiration, as water availability is already low

69
Q

how are xerophytes adapted

A

1) stomata in sunken pits, so are sheltered from wind, so reduce air movement and produce microclimate of still, humid air (reduce WP gradient, so reduce transpiration)
2) thick waxy cuticle on epidermis, to reduce water loss by transpiration as layer is waterproof, so water cannot move through
3) reduce SA of leaves by forming spikes instead of leaves, so less water loss
4) have layer of hairs on the epidermis, trapping moist air around stomata and reducing WP gradient
5) leaves can curl and roll, trapping moist air in still microclimate and slowing transpiration, but also reducing exposed SA for losing water and protecting from wind
6) close stomata at hottest times where transpiration in highest, or have less stomata in general
7) succulents store water in stem and root parenchyma tissue
8) losing leaves

70
Q

what are hydrophytes

A

plants that grow and live in water, submerged or on surface and edge

  • e.g. water lilies, water cress and duckweeds
71
Q

why do hydrophytes need adaptations

A

need to cope with low O2 levels

72
Q

what are adaptations of hydrophytes

A

1) thin or no waxy cuticle, as don’t need to conserve water
2) many always open stomata usually only on upper surfaces of floating leaves, maximising gas exchange
3) have flexible leaves and stems as supported by water around them, so dont need rigid stems to support, and being flexible helps damage by water currents
4) wide flat leaves to capture as much of light as possible
5) air spaces to help the plant float and act as an O2 store for respiration ( water lilies have large air spaces in leaves to help float and capture light, and in roots and stems to allow O2 to move from leaves down to parts underwater, via a low resistance internal pathway) - e.g in the aerenchyma tissue (formed of parenchyma tissue which must have gone through apoptosis)

Biology Module 3 (3 decks)

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