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Module 3- exchange and transport > Transport in Plants > Flashcards

Transport in Plants Flashcards

1
Q

Describe why plants need a transport system

A
  • larger plants have smaller surface area to volume ratio- need specialised exchange surfaces and transport system
  • every cell of multicellular plant needs regular supply of oxygen, water, nutrients and minerals
  • plants not very active so respiration rate is low- low demand for oxygen- can be met by diffusion
  • however, demand for sugars still high- plants can absorb water and minerals at the roots, but can’t absorb sugars from the soil
  • the leaves can perform gaseous exchange and manufacture sugars by photosynthesis, but can’t absorb water from the sir
  • therefor plants need transport system to move water and minerals from roots up to the leaves, and sugars from the leaves to the rest of the plant
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2
Q

3 substances plants need, where they come from

A
  • water- obtained by roots (root hair cell) - transported in xylem (1 direction)
  • Minerals- dissolved in soil water and absorbed by the root hair cells transported in xylem (1 direction)
  • sugars- synthesised in the leaves by photosynthesis - transported in both directions by phloem
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3
Q

Name the main general and transport processes in plants

A
  • photosynthesis, respiration, active transport
  • transpiration, translocation
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4
Q

Describe the vascular tissue

A
  • Water and soluble mineral ions travel upwards in xylem tissue
  • assimilates (e.g. sugars) travel up or down in phloem tissue
  • no pump in these tissues, gases not carried by them
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5
Q

Describe the distribution of vascular tissues

A
  • dicotyledonous plants are those that have 2 seed leaves
  • also have very characteristic distribution if vascular tissue- found throughout 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 to help support the plant
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6
Q

Describe the xylem and phloem in the young root

A
  • found at centre if young root
  • central core of xylem, often in shape of X
  • phloem found between arms of X shaped xylem tissue
  • around bundle is special sheath of cells called endodermis- has key role in getting water into xylem vessels
  • just inside epidermis is layer of meristem cells called the pericycle
  • This provides a ‘drill’ like structure
  • This enables the plant to push down into the root
  • Xylem tissues is the strongest so is in the centre – X structure
  • Phloem in four separate sections
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7
Q

Describe the xylem and phloem in
the stem

A
  • found near outer edge of the stem
  • in non-woody plants, the bundles are separate and discrete
  • in woody plants the bundles are spearte in young stems, but become a continuous ring in older stems
  • means there is a complete ring of vascular tissue just under the bark of a tree
  • this arrangement provides strength and flexibility to withstand the bending forces to which stems and branches are exposed
  • 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 us a layer of meristem cells that divide to produce new xylem and phloem
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8
Q

Describe the xylem and phloem in
the leaf

A
  • form the midrib and veins of a leaf
  • a dicotyledons 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 the top of the phloem
  • This provides additional support to the stem
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9
Q

Describe the dissection of plant material

A
  • requires staining to examine distribution of vascular tissue
  • most easily demonstrated in leaf stalk of celery
  • can also be carried out with busy lizzie (impatiens)
  • thin sections can be cut and viewed at low power
  • allow the leafy stem to take up water by transpiration- can then be cuts longitudinally or transversely and examined with a hand lens or microscope
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10
Q

Outline structure and function of xylem

A
  • is a tissue used to transport water and mineral ions from the roots up to the leaves and other parts of the plants
    Xylem tissue consists of:
  • 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 vessels
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11
Q

Describe the development of xylem vessels

A
  • as vessels develop, lignin impregnates the walls of the cells, making the cells waterproof- this kills the cells
  • the end walls and contents of the cells decay, leaving a long column of dead cells with no contents- tube called the Xylem vessel
  • the lignin strengthens the vessel walls and prevents the vessel from collapsing- keeps the vessels open even at times when water may be in short supply
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12
Q

Describe patterns of lignin in Xylem vessels

A
  • the lignin thickening forms patterns in the cell wall- maybe spiral, annular (rings), or reticulate (network of broken rings)- 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, also allow water to leave the Xylem and pass into the living parts of the plants
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13
Q

Describe adaptations of xylem to its function

A

Xylem vessels can carry water and mineral ions from the roots to the very top of the plants because:
- 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 deposits in the walls in the spiral, annular or reticulate patterns allows Xylem to stretch as the plant grows, and enables this the stem or branch to bend

the flow of water is not impeded, because:
- there are no cross walls
- there are no cell contents, nucleus or cytoplasm
- lignin thickening prevents the walls from collapsing

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14
Q

Describe the structure and function of phloem

A
  • 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 cells
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15
Q

Describe sieve tube elements in phloem

A
  • elongated sieve tube elements are lined up end to end to form sieve tubes
  • they contain no nucleus and very little cytoplasm, leaving space for the 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 movements of the sap from one element to the next
  • the sieve elements are lined up end to end to form sieve tubes
  • they contain no nucleus and very little cytoplasm, leaving space for the 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 movements of the sun from one element to the next
  • the safe have very thin walls, and when seen in transverse section are usually five or six sided
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16
Q

Describe companion cells in phloem

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 process is
  • the companion cells carry out the metabolic processes needed to load assimilates actively into the sieve tubes
  • the companion cells and sieve tube elements in the phloem are linked by fine strands of cytoplasm, through gaps in the cell walls- allows communication and flow of substances between the cells- plasmodesmata
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17
Q

Outline pathways water takes through plant cells, name 3

A
  • cellulose cell walls fully permeable to water - water molecules can move freely between the cellulose molecules or even gaps between the cells
  • water can also pass across the cell wall and through the partially permeable plasma membrane into the cell cytoplasm or the vacuole
  • many plant cells are joined by special cytoplasmic bridges- cell junctions at which the cytoplasm of one cell is connected to that of another through a gap in the cell walls- junctions are called the plasmodesmata
  • apoplast, symplast, vacuolar pathways
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18
Q

Describe the apoplast pathway

A
  • water passes through the spaces in the cell walls and the spaces between the cells
  • it doesn’t pass through any plasma membrane membranes into the cells
  • this means that the water moves by mass flow rather than osmosis
  • dissolved mineral ions and salts can be carried with the water
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19
Q

Describe 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
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20
Q

Describe the vacuolar pathway

A
  • similar to the symplast pathway, but the water is not confined to the cytoplasm of the cells
  • it is able to enter and pass through the vacuoles as well
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21
Q

Describe water potential, uptake and loss in plant cells

A
  • the cytoplasm contains mineral ions and sugars- solutes- will reduce the water potential as there are fewer free water molecules available than in pure water- always negative
  • if placed in pure water, will take up water molecules by osmosis as water potential in cell is lower- down water potential gradient into cell- won’t burst due to strong cellulose cell wall- becomes turgid
  • once full- 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
  • if placed in a salt solution with a very negative water potential, it will lose water by osmosis as water potential of sale is less negative done part of a solution- water moves down water potential gradient out of cell
  • as water loss continues, the cytoplasm and vacuole shrink
  • eventually, the cytoplasm no longer pushes against the cell all, and the cell is no longer turgid
  • if water continues to leave the cell, then the plasma membrane will lose contact with the wall- plasmolysis- the issue is now flaccid
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22
Q

Describe movement of water between plant cells

A
  • when plant cells are touching each other, water molecules can pass from one cell to another- will move from higher to lower water potential- osmosis
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23
Q

What is the transpiration stream

A
  • the movement of water from the soil, through the plants, to the earth surrounding the leaves
  • main driving force is the water potential gradient between the soil and the earth in the leave spaces
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24
Q

Describe water uptake and movement across the root

A
  • the outermost layer of cells (epidermis) of a root contains root hair cells- 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 mineral ions that have been actively absorbed make the water potential of the cytoplasm more negative, causing water from the soil to enter the root sound by osmosis
  • the water then moves across the root cortex by osmosis and via apoplast pathway 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
  • mineral ions are actively transported into the medela making the water potential negative so more water flows by osmosis
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25
Q

Describe the role of the endodermis in the transpiration stream

A
  • the movement of water across the route is driven by an active process that occurs at the endodermis
  • endodermis is a layer of cells surrounding the medulla and Xylem, also known as the starch sheath, as contains granules of starch- sign that energy is being used
  • the casparian strip blocks the apoplast pathway between the cortex and the medula
  • this ensures that water and dissolved mineral ions, especially nitrates, have to pass into the cell cytoplasm through the plasma membrane (symplast pathway)
  • the plasma membranes contain transporter proteins, which actively pump mineral ions from the cytoplasm of the cortex cells into the medulla and Xylem
  • 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 endodermal cells is blocked by the casparian strip
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26
Q

What is the nature of movement of water up through the Xylem, name 3 processes that help

A

Mass flow- a flow of water and mineral ions in the same direction- root pressure, transpiration pull, and capillary action helps move water up the stem

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27
Q

Describe root pressure as part of the transpiration stream

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 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 all trees
28
Q

Describe transpiration pull as part of the transpiration stream

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
  • ask 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- 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 plants maintaining an unbroken column of water all the way up the xylem, but if the water column is broken in one xylem vessel, the water column can still be maintained through another vessel via the bordered pits
29
Q

Describe capillary action as part of the transpiration stream

A
  • the same forces that hold water molecules together also attract the water molecules to the sides of the xylem vessel- adhesion
  • because the Xylem vessels are very narrow, these forces of attraction can pull the water up the sides of the vessel
30
Q

Water uptake across the root diagram

A
31
Q

Transpiration stream diagram

A
32
Q

Describe how water leaves the leaf as part of the transpiration stream

A
  • most of the water that leaves the leaf exits as vapour through the stomata
  • only a tiny amount leaves through the waxy cuticle
  • water evaporates from the cells lining the cavity immediately above the guard cells (sub-stomatal air space)
  • this lowers the water potential in these cells, causing quarter 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
33
Q

Why is access to water a problem for most terrestrial plants

A
  • issue for most plants living on land- water is lost by transpiration because plants exchange gases with the atmosphere via their stomata
  • during the day combat plants take up a lot of carbon dioxide for use in photosynthesis, they must also remove oxygen (byproduct of photosynthesis) so the stomata must be open during the day
  • while the stomata are open, there is an easy route for water to be lost- this water must be replaced
  • plants living on lands must be adapted to reduce this loss of water and replace the water that is lost- based terrestrial plants can reduce their water losses by structural and behavioural adaptations
34
Q

Adaptations of terrestrial plants to reduce water losses

A

Behavioural and structural:
- a waxy cuticle on the leaf will reduce water loss due to evaporation through the epidermis
- the stomata are often found on the undersurface of leaves, not on the top surface- 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 grounds may be frozen(making water less available), ends when temperatures may be too low for photosynthesis

35
Q

Name four types of plants which are adapted two availability of water

A
  • terrestrial plants
  • marram grass
  • cacti
  • hydrophytes
36
Q

Describe the characteristics of marram grass

A
  • marram grass (ammophilia) specialises in living on sand dunes
  • the conditions are particularly harsh, because any water in the sand drains away quickly, the sons may be salty and the leaves are often exposed to very windy conditions
  • xerophyte- plant adapted to living in arid conditions
37
Q

Adaptations of marram grass

A
  • the leaf is rolled longitudinally say that air is trapped inside- this air becomes humid, which reduces water loss from the leaf- the 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 airspace
  • 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
38
Q

Describe cacti adaptations to overcome arid conditions

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- 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 wide spread, in order to take advantage of any rain that does fall
39
Q

Describe other xerophytic features to reduce water loss

A
  • closing the stomata when water availability is low will reduce water loss and so reduce the need to take up water
  • some plants have a low water potential inside the leaf cells- achieved by maintaining a high salt concentration in the cells
  • the low water potential reduces the evaporation of water from the cell surfaces as the water potential gradient between the cells and the leaf spaces is reduced
  • a very long tap root that can reach water deep underground
40
Q

What are hydrophytes, what issues are they faced with

A
  • plants that live in water such as water lilies (nymphaeles)
  • have easy access to water, but faced with other issues such as getting oxygen to their submerged tissues and keeping afloat- need to keep their leaves in the sunlight for photosynthesis
41
Q

Describ the adaptations of a water Lily

A
  • many large air spaces in the leaf- keeps the leaf afloat so that they are in the air and can absorb sunlight
  • this 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- helps with buoyancy, allows oxygen to diffuse quickly to the roots for aerobic respiration
42
Q

Describe issues with hydrophytes transpiring, how they do it

A
  • transpiration is the loss of water vapour from the surfaces of the leaves, but the water will not evaporate into water or into air that has a very high humidity
  • if water cannot leave the plants, then the transpiration stream stops and the plants cannot transport mineral ions up the leaves
  • many plants contain specialised structures at the tips or margins of the leaves called hydathodes- can release water droplets which may then evaporates from the leaf surface
43
Q

Define transpiration

A

The loss of water vapour from the upper (aerial) parts of the plant- particuarly the leaves

44
Q

Describe how most water leaves the plant through transpiration

A
  • some may evaporate through upper leaf surface, but this loss is limited by the waxy cuticle
  • next water vapour leaves through the stomata, which opened to allow gushes exchange for photosynthesis
  • since photosynthesis occurs only when there is sufficient light, the majority of water vapour is lost during the day
45
Q

Describe the typical pathway taken by most water leaving the leaf

A
  • water enters the leaf through the Xylem, and moves by osmosis into the cells of me spongey mesophyll- may also pause along the cell walls via the apoplast pathway
  • water evaporates from the cell walls of the spongy mesophyll
  • water vapour moves by diffusion out of the leaf through the open stomata- relies on a difference in the concentration of water vapour molecules in the leaf compared with outside the leaf- known as the water vapour potential gradient- must be a less higher water paper potential inside the leaf than outside
46
Q

Describe the importance of transpiration

A
  • inevitable consequence of gas exchange, but also essential for the plant to survive
  • as water vapour is lost from the leaf, it must be replaced from below- draws water up the stem as a transpiration stream- at this movement:
  • transports useful mineral ions up the plant
  • maintain’s 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
47
Q

Name five environmental factors that affect transpiration rate

A
  • light intensity
  • temperature
  • relative humidity
  • air movement (wind)
  • water availability
48
Q

Describe how light intensity affects transpiration rate

A
  • in light, the stomata open to allow gaseous exchange for photosynthesis
  • higher light intensity increases the transpiration rate
49
Q

Describe how temperature intensity affects transpiration rate

A
  • a higher temperature will increase the rate of transpiration in three ways-
  • increase the rate of evaporation from the cell surfaces so that the water vapour potential in the leaf rises
  • increase the rate of diffusion through the stomata because the water molecules have more kinetic energy
  • decrease the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leave
50
Q

Describe how relative humidity affects transpiration rate

A
  • higher relative humidity in the air will decrease the rate of water loss as there is a smaller water vapour potential gradient between the air spaces in the leaf and the air outside
51
Q

Describe how air movement (wind) affects transpiration rate

A
  • air moving outside the leaf will carry away water vapour that has just diffused out of the leaf- maintains high water vapour potential gradient
52
Q

Describe how water availability affects transpiration rate

A
  • if there is little water in the soil, then the plants cannot replace the water that is lost- if there is insufficient water in the soil, the stomata close and the leaves wilt
53
Q

What is translocation, where does it happen, briefly describe

A
  • occurs in the phloem
  • is the movement of assimilates through the plant
  • assimilates all substances made by the plants, using substances absorbed from the environment including sugars (mainly transported as sucrose) and amino acids
  • a part of the plant that loads assimilates into the phloem sieve tubes is called a source, a part of the plant that removes assimilates from the phloem sieve tubes is called a sink
54
Q

Name the process by which sucrose is loaded into the sieve tube

A

active loading/cotransport/secondary active transport

55
Q

Describe the loading of sucrose into the sieve tube

A
  • loaded by active process- involves the use of energy from ATP in the companion cells
  • the energy is used to actively transport hydrogen ions out of the companion cells- increases their concentration outside of the cells and decreases their concentration inside 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- cotransport
  • AKA 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
  • once the concentration of sucrose in the companion cell increases, it can diffuse through the plasmodesmata into the sieve tube
56
Q

Describe movement of sucrose, what it is caused by, name two parts of the plants involved in this

A
  • movement of sucrose along the phloem is by mass flow
  • a solution of sucrose, amino acids and other assimilates flows along the tube
  • the solution is called sap, and it can be made to flow either up or down the plant as required
  • the flow is caused by a difference in hydrostatic pressure between the two ends of the tube, which produces a pressure gradient
  • water enters the tube at the source, increasing the pressure, and it leaves the tube at the sink, reducing the pressure- therefore the SAP flows from the source to the sink
57
Q

Describe translocation- pressures/ cause of movement

A
  • sucrose is actively loaded into the sieve tube element and reduces the water potential- water follows by osmosis and increases the hydrostatic pressure in the sieve tube element
  • sap moves down the sieve tube from higher hydrostatic pressure at source to lower hydrostatic pressure at sink
  • sucrose is removed from the sieve tube by the surrounding cells and increases the water potential in the sieve tube
  • water moves out of the sieve tube and reduces the hydrostatic pressure
  • called the mass flow hypothesis
58
Q

Describe the source in translocation

A
  • sucrose entering the makes the water potential inside the sieve tube more negative
  • as a result, water molecules move into the sieve tube element by osmosis from the surrounding tissue- increases the hydrostatic pressure in the sieve tube at the source
59
Q

Describe what the source in translocation can be

A
  • 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 into sucrose and moved to other parts of the plant in order to enable growth in the spring
  • most obvious source is a leaf- sugars made during photosynthesis are converted to sucrose and loaded into the phloem sieve tubes- occurs during late spring, summer, and early autumn- whilst the leaves are green- sucrose is transported to the other areas of the plants that may be growing (meristem), or to areas such as the roots for storage
60
Q

Describe the sink in translocation

A
  • a sink is anywhere that removes sucrose from the phloem sieve tubes
  • the sucrose could be used for respiration and growth in a meristem, or could be converted to starch for storage in a root
  • where sucrose is being used by the cells, it can diffuse out of the sieve tubes via the plasmodesmata
  • it may also be removed by active transport- the removal of sucrose from the sap makes the water potential higher, so that water moves out of the sieve tube into the surrounding cells- reduces the hydrostatic pressure in the phloem at the sink
61
Q

Describe movement along at 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 the 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
62
Q

Movement of sap in the phloem diagram

A
63
Q

Equipment used to measure transpiration

A

potometer

64
Q

Precautions to take when using a potometer

A
  • set it up underwater to make sure there are no air bubbles inside the apparatus
  • ensure the shoot is healthy
  • cut the stem under water to prevent air entering the xylem
  • cut the stem at an angle to provide a large surface area in contact with the water
  • dry the leaves
  • ensure apparatus is air tight
65
Q

How to calculate transpiration rate from potometer

A
  • calculate the volume of the capillary tube (volume of a cylinder- πr^2l )
  • volume/time
    or speed of movement of air bubble x cross sectional area of capillary tube

Module 3- exchange and transport (3 decks)

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