9.1 transport in plants Flashcards
1
Q
What is the pressure in the phloem?
A
2000kPa
2
Q
Why do multicellular need transport systems?
A
-metabolic demands
-size
-surface area: volume ratio
3
Q
What are dicotyledonous plants (dicots)?
A
make seeds that contain two cotyledons, organs that act as food stores for the developing embryo plant and form the first leaves when the seed germinates.
4
Q
What are the two types of dicotyledonous plants?
A
-herbaceous dicots
-woody dicots
5
Q
What is the vascular system?
A
a series of transport vessels running through the stem, roots and leaves in dicotyledonous plants.
6
Q
What is the vascular system made up of?
A
xylem and phloem
7
Q
How are the xylem and phloem arranged?
A
In a vascular bundle
8
Q
Where are the vascular bundles located in the stem?
A
in the stem, they are around the edge to give strength and support
9
Q
Where are the vascular bundles located in the roots?
A
in the roots, they are in the middle to help the plant withstand the tugging strains that result as the stems and leaves are blown in the wind
10
Q
What are the vascular bundles located in the leaves?
A
in the leaves, the midrib of a dicot leaf is the main vein carrying the vascular tissue through the organ
11
Q
What is the structure of the xylem?
A
- largely non-living tissue
- made out of dead cells
- long/ hollow made by several columns of cells fusing together
- lignified secondary walls to provide more support
12
Q
What is the function of the xylem?
A
- transport of water/ mineral ions
- support
13
Q
What are the two tissues associated with the xylem?
A
- thick walled parenchyma packs around the vessels, storing food, and containing tannin deposits
- tannin is a chemical that protects plant tissues from attack from herbivores
14
Q
In what ways can lignin be laid down in the xylem?
A
- spirals, rings or solid tubes with unlignified areas called bordered pits.
15
Q
What is the structure of the phloem?
A
- main transporting vessels are the sieve tube elements (made up of cells joined end to end to form a long/ hollow structure)
- not lignified
- areas between cell walls become perforated to form sieve plates
- contains companion cells which form with sieve tube elements by plasmodesmata
- phloem tissue contains supporting tissue including fibres and sclereids
16
Q
What are sclereids?
A
cells with extremely thick walls
17
Q
what are the plasmodesmata?
A
microscopic channels through the cellulose cell walls linking the cytoplasm to adjacent cells
18
Q
What are companion cells?
A
very active cells, function as a ‘life support system’ for sieve tube cells, which have lost most of their normal cell functions
19
Q
As large pores appear in the phloem cell walls, what happens?
A
tonoplast, nucleus and other organelles break down.
- the phloem becomes filled with sap and mature phloem cells have no nucleus.
20
Q
What is the function of the phloem?
A
- living tissue that transports food in the form of organic solutes around the plant from the leaves where they are made by photosynthesis
- supplies the cells with sugars and amino acids needed for cellular respiration
- flow of materials can go up and down the plant
21
Q
What are the main transporting vessels of the phloem?
A
- sieve tube elements including sieve plates
- companion cells
- also contain supporting tissues including fibres and sclereids, cells with extremely thick cell walls
22
Q
What are phloem cells made up of?
A
- cells joined end to end to form a long, hollow structure
- unlike xylem, they are not lignified
23
Q
In the phloem, what are in the areas between the cells?
A
- walls become perforated to form sieve plates
24
Q
What do sieve plates in the phloem do?
A
- let phloem contents flow through
25
What happens as large pores appear in the cell walls (become perforated) of the phloem?
- the tonoplast, the nucleus, and some of the other organelles break down.
- phloem becomes a tube filled with sap and mature phloem cells have no nucleus
26
Where do companion cells form in the phloem?
- with sieve tube elements
- linked by many plasmodesmata
27
What are plasmodesmata?
- microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells
- maintain their nucleus and all organelles
28
What are some components of companion cells?
- active cells
- function as a "life support system" for sieve tube cells, which have most of their normal cell functions
29
What is turgor pressure (hydrostatic pressure)?
- result of osmosis in plant cells provides a hydrostatic skeleton to support the stems and leaves
30
What is the turgor pressure in leaf cells?
1.5MPa (11251mmHg)
31
What does turgor drive?
- cell expansion
- a force that enables plant roots to force their way through tarmac and concrete
32
How are plants kept cool?
- loss of water by evaporation
33
In which solution are mineral ions and products of photosynthesis transported?
- aqueous
34
What are root hair cells and how do they link to water?
- exchange surface in plants where water is taken into the body of the plant from the soil
- root hair is a long, thin extension from a root hair cell, a specialised epidermal cell found near the growing root tip
35
How are root hairs well adapted as exchange surfaces?
- microscopic size = can penetrate easily between soil particles
- each microscopic hair has a large SA:V ratio and there are thousands on each tip
- each hair has a thin surface layer through which diffusion and osmosis can take place quickly
- concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell
36
Why does soil water have a high water potential?
- has a very low concentration of dissolved minerals
- cytoplasm and vacuolar sap of the root hair cell contain many different solvents so the water potential IN THE CELL is lower
- as a result water moves into the root hair cells via osmosis
37
How does water move across the root into the xylem?
- symplast pathway
- apoplast pathway
- vacuolar pathway
38
What is the symplast pathway with water movement across the root?
- water moves through the symplast by osmosis
- the root hair cell has a higher water potential than the next cell along (due to water diffusing in from the soil, making the cytoplasm more dilute), so water moves from the root hair cell into the next door cell by osmosis
- process continues from cell to cell across the root until the xylem is reached
39
What is the symplast?
- continuous cytoplasm of the living plant cells that is connected via the plasmodesmata
40
What happens when water leaves the root hair cell by osmosis?
- water potential of the cytoplasm falls again, maintaining a steep water potential gradient to ensure that water continues to move into the cell from the soil
41
What is the apoplast pathway in water movement across the root?
- movement of water through the apoplast
- water fills this space between the loose, open network of fibres in the cellulose cell wall
- as water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to the cohesive forces between the water molecules
- the pull from the water moving into the xylem and up the plant along with the cohesive forces between the water molecules creates a tension that means there is a continuous flow of water through the open structure of the cellulose wall, which offers little or no resistance
42
What is the apoplast?
- cell walls and intercellular spaces
43
How far along does water move in the pathways?
- until it reaches the epidermis
44
What is the epidermis?
- layer of cells surrounding the vascular tissue of the roots
45
Why is the epidermis particularly noticeable in the roots?
because of the effect of the Casparian Strip
46
What is the Casparian Strip?
- band of waxy material called Suberin that runs around each of the endodermal cells forming a waterproof layer
47
Where must the water flow go after being blocked by the Casparian strip?
- it must leave the apoplast pathway and join the symplast pathway
48
How are all the potentially-toxic solutes "filtered" off during osmosis?
- water must pass through selectively permeable cell surface membranes, with no carrier proteins to admit the solutes.
49
Why is the water potential of the xylem tissue much lower than the water potential of the endodermal cells?
- solute concentration in the cytoplasm of the endodermal cells is relatively dilute compared to cells in the xylem
- endodermal cells move mineral ions into the xylem by active transport
50
What happens when water is inside the vascular bundle?
- returns to the apoplast pathway to enter the xylem itself and move up the plant
51
What is root pressure?
- where active pumping of minerals into the xylem produces movement of water by osmosis
- independent of any effects of transpiration
52
What can root pressure also help with?
pushing water up the plant
- not the major factor of this though
53
What evidence can be given to support the role of active transport in root pressure?
- poisons affect mitochondria and prevent production of ATP (if cyanide is applied to root cells so there is no energy supply, the root pressure disappears)
- root pressure increases with a rise in temperature, suggesting chemical reactions are involved
- if levels of oxygen or respiratory substrates fall, root pressure falls
- xylem sap may exude from the cut end of stems at certain times (xylem sap is forced out of special pores at the end of leaves in some conditions - known as guttation)
54
How are leaves adapted to prevent losing water rapidly and through constant evaporation from their surfaces?
- large surface area for capturing sunlight and carrying out photosynthesis
- surfaces are covered in a waxy cuticle that makes them waterproof
55
What are stomata?
microscopic pores in the leaf
- can be opened and closed by guard cells, which surround the stomatal opening
56
How does gas exchange occur in the leaf?
- carbon dioxide moves from the air in the leaf and oxygen moves out of the leaf by diffusion down a concentration gradient through the stomata
57
What also happens when stomata are open to allow gas exchange?
water vapour also moves out by diffusion and is lost
- called transpiration, an inevitable consequence of gaseous exchange
58
Why does some stomata need to be open all the time?
- during the day, a plant needs to take in carbon dioxide for photosynthesis
- at night, when no oxygen is being produced by photosynthesis it needs take in oxygen for cellular respiration
59
What is the transpiration stream?
- once in the leaves, water moves by osmosis across membranes and by diffusion in the apoplast pathway from the xylem through the cells of the leaf where it evaporates from the freely permeable cellulose cell walls of the mesophyll cells in the leaves into the air spaces.
- the water vapour then moves into the external air through the stomata along a diffusion gradient
60
How is water transported up the height of a plant through xylem vessels?
- water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a concentration gradient
- the loss of water by evaporation from a mesophyll cell lowers the water potential of the cell, so water moves into the cell from an adjacent cell by osmosis, along both apoplast and symplast pathways
- this is repeated across the leaf into the xylem , water moves out of the xylem by osmosis and into the cells of the leaf
- water molecules form hydrogen bonds with the carbohydrates in the walls of the narrow xylem vessels (adhesion). Also form hydrogen bonds with each other and stick together (cohesions)
The combined effects of adhesion and cohesion result in water exhibiting CAPILLARY ACTION
61
What is capillary action?
- process by which water can rise up a narrow tube against the force of gravity.
- water is drawn up the xylem in a continuous stream to replace the water lost by evaporation (transpiration pull)
62
What does the transpiration pull result in?
- in a tension in the xylem, which in turn helps to move water across the roots from the soil
63
What is the cohesion-tension theory?
- model of water moving from the soil in a continuous stream up the xylem and across the leaf
64
What evidence is there for the cohesion-tension theory?
- changes in diameter of trees
- when a xylem vessel is broken
65
How does a change in the diameter of trees provide evidence for the tension-cohesion theory?
- when transpiration is at its height during the day, the tension in the xylem vessels is at its highest too, as a result the tree shrinks in diameter
- at night, transpiration is at its lowest and the tension in the xylem vessels is at its lowest, the diameter of the tree increases
- this can be tested by measuring the circumference of a suitably sized tree at different times of the day
66
How does a broken xylem vessel provide evidence for the cohesion-tension theory?
- in most circumstances air is drawn in to the xylem rather than water leaking out
- the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has be broken
67
How can transpiration be a problem for the plant?
- amount of water available is often limited
- when their is high intensity sunlight, the plant photosynthesises rapidly, so their will be a high rate of gaseous exchange, stomata will all be open. plant may lose so much water through transpiration that the supply cannot meet the demand
68
How can transpiration be measured?
- via the uptake of water by the plant
- using a potometer
- making sure all joints are sealed with waterproof jelly to stop water loss from anything other than transpiration
69
How can the rate if water uptake be measured (formula)?
distance moved by air bubble/ time taken for air bubble to move that distance (cm s-1)
70
How is a fresh shoot prepared when measuring transpiration using a potometer?
- stem is cut under water and transferred to the apparatus to avoid introducing air bubbles to the stem
- don't get water on the leaves
71
Why is a reservoir used in a potometer, measuring the rate of water uptake?
- water can be let into the capillary tube, pushing the air bubble back to the start of the scale
72
What kind of process is the opening/ closing of the stomata?
turgor-driven process
73
What happens to the stomata when turgor is low?
- the asymmetric configuration of guard cell walls closes the pore
74
How does turgor increase in the stomata?
- when the environmental conditions are favourable, guard cells pump in solutes by active transport, increasing their turgor
75
What happens when stomata increase in size/ open and turgor increases?
- cellulose hoops prevent the cells from swelling in width so they extend lengthways
- inner wall of the guard cell is less flexible than the outer wall, cells become bean-shaped and open the pore
76
What happens to the stomata when water becomes scarce?
- hormonal signals from the roots can trigger turgor loss from the guard cells, which close the stomatal pore and so conserve water
77
What factors affect transpiration?
- light intensity
- relative humidity
- temperature
- air movement
- soil-water availability
78
How does light intensity affect transpiration?
- in the dark, most of the stomata will close
- increasing light intensity gives increasing numbers of open stomata, increasing the rate of water vapour diffusing out and therefore increasing the evaporation from the surfaces of the leaf
- so increasing light intensity increases rate of transpiration
79
How does relative humidity affect transpiration?
- relative humidity is the amount of water vapour in the air compared to the total concentration of water the air can hold
- a very high relative humidity will lower the rate of transpiration because of the reduced water vapour potential gradient between the inside of the leaf and the outside air
- very dry air has the opposite effect and increases the rate of transpiration
80
How does temperature affect transpiration?
- an increase in temperature increases the kinetic energy of the water molecules and therefore increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf
- in increase in temperature increases the concentration of water vapour that the external air can hold before it becomes saturated (so decreases its relative humidity and water potential)
- both factors increase the diffusion gradient between the air inside and outside the leaf, thus increasing the rate of transpiration
81
How does air movement affect transpiration?
- each leaf has a layer of still air around it trapped by the shape of the leaf and features such as hairs on the surface of the leaf decrease air movement close to the leaf
- the water vapour that diffuses out of the leaf accumulates here and so the water vapour potential around the stomata increases, in turn reducing the diffusion gradient
- anything that increases the gradient, also increases the rate of transpiration
- so air movement or wind will increase the rate of transpiration and vice versa
82
How does soil water availability affect transpiration?
- the amount of water available in the soil can affect transpiration rate
- if it is very dry the plant will be under water stress and the rate of transpiration will be reduced
83
What is translocation?
- the process by which plants transport organic compounds in the phloem from sources to sinks (tissues that need them)
- in many plants, this is an active process and happens in both directions
84
What are products of photosynthesis that are transported up and down the plant known as?
- assimilates
85
What are products of photosynthesis that are transported up and down the plant known as?
- assimilates
86
What is the main assimilate transported around the plant?
- although glucose is made in the process of photosynthesis, sucrose is the main assimilate transported around the plant via translocation
87
What are the main sources of assimilates in a plant?
- green leaves and green stems
- storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period
- food stores in seeds when they germinate
88
What are the main sinks in a plant?
- roots that are growing and/or actively absorbing mineral ions
- meristems that are actively dividing
- any parts of the plant that are laying down food stores, such as developing seeds, fruits or storage organs
89
What is phloem loading?
- process by which plants load assimilates into the phloem for transport
90
What are the two ways in which phloem loading can occur?
- symplast route
- apoplast route
91
What is the symplast route in terms of phloem loading?
- sucrose from the source moves through the cytoplasm of the mesophyll cells and on into the sieve tubes by diffusion through the plasmodesmata
- although phloem loading and translocation are referred to as active processes, this route is largely passive
- the sucrose ends up in the sieve elements and water follows by osmosis
- creates a pressure of water that moves the sucrose through the phloem by mass flow
92
What is the apoplast route in terms of phloem loading?
- in many plant species, sucrose from the source travels through the cell walls and inter-cell spaces to the companion cells and sieve elements by diffusion down a concentration gradient, maintained by the removal of sucrose into the phloem vessels.
- in the companion cells sucrose is moved into the cytoplasm across the cell membrane in an active process
- hydrogen ions are actively pumped out of the companion cell into the surrounding tissue using ATP
- the hydrogen ions return to the companion cell down a concentration gradient via a co-transport protein
- increases the sucrose concentration in the companion cells and in the sieve elements through the many plasmodesmata between the two linked cells
- as a result of the build up of sucrose in the companion cell and sieve tube element, water moves in by osmosis (build up of turgor pressure in walls)
- the water carrying assimilates moves into the tubes of the sieve elements, reducing pressure in companion cells and moves up/ down the plant by mass flow to areas of lower pressure (sinks)
93
Why do companion cells have many infoldings in their cell membranes?
- to increase the surface area for active transport of sucrose into the cell cytoplasm
- also have many mitochondria to supply ATP needed for transport pumps
94
What does solute accumulation in source phloem lead to?
- increase in turgor pressure that forces sap to regions of lower pressure in the sinks
95
What is the pressure generated in the phloem?
- 2 MPa
- much more than in the human arteries (0.016 MPa)
- pressure differences in plants allows transport of water and solutes rapidly over many metres
96
When is sucrose unloaded in the phloem?
- at any point into the cells that need it
97
What is the main mechanism of phloem unloading?
- diffusion of the sucrose from the phloem into surrounding cells
- sucrose rapidly moves on into other cells or is converted into another substance so that a concentration gradient of sucrose is maintained between the contents of the phloem and the surrounding cells
98
What does the loss of solutes from the phloem leading to a rise in the water potential of the phloem result in?
- water moves out into the surrounding cells by osmosis
- some of the water that carried the solute to the sink is drawn into the transpiration stream in the xylem
99
What evidence is there to support the process of translocation?
- advances in microscopy allows us to see the adaptations of the companion cells for active transport
- if mitochondria in companion cells are poisoned, translocation stops
- flow of sugars in the phloem is about 10000 times faster than it would be by diffusion alone, suggesting an active process is driving the mass flow
- aphids can be used to determine the translocation of organic solutes in the phloem, shows that there is a positive pressure in the phloem that forces the sap out through the stylet. The pressure and therefore the flow rate in the phloem is lower closer to the sink than it is near the source
100
What are the two main species of plants that have adapted to water availability?
- Xerophytes
- Hydrophytes
101
What are Xerophytes?
- plants adapted to conserve water
- water availability is very low
- hot conditions, particularly dry, hot and breezy, water evaporated from the leaf surfaces very quickly
- can also be found in very cold and icy conditions, where the water on the ground is not freely available to them because it is frozen
102
What are some types of Xerophytes?
- conifers
- marram grass
- cacti
103
What are the main ways xerophytes adapt to conserve water?
- thick waxy cuticle (minimise water loss)
- sunken stomata (reduce air movement, creates a microclimate of still, humid air)
- reduced number of stomata
- reduced leaves (reduced SA:V ratio)
- hairy leaves (create microclimate of still, humid air)
- curled leaves (confines stomata in microclimate)
- succulents (store water in specialised parenchyma tissue, swollen/ fleshy appearance)
- leaf loss (losing leaves when water is not available)
- root adaptations (long tap roots grow deep into the soil to access water further down, shallow and widespread roots have a large surface area to absorb water before it evaporates near the surface)
- avoiding the problems (become dormant/ die to leave seeds behind to germinate when water becomes available through rainfall, etc)
104
What are hydrophytes?
- plants that live in water
- water logging is a major problem , as air spaces need to be full of air, not water, for the plant to survive
105
What are some types of hydrophytes?
- water lilies
- water cress
- duckweeds
- bulrushes
- yellow iris
106
How are hydrophytes adapted to survive in water?
- very thin/ no waxy cuticle (don't conserve water)
- many always open stomata on upper surfaces (max gas exchange, inactive guard cells, in contact with air)
- reduced structure of plant (water supports plant)
- wide, flat leaves (capture as much light as possible)
- small roots (direct diffusion of water into stem/ leaf)
- large SA of stems/ roots underwater (max photosynthesis)
- air sacs (enable leaves/ flowers to float on water surface)
- aerenchyma (specialised parenchyma tissue forms on leaves, stems and roots, with many air spaces, formed partly by apoptosis in normal parenchyma
107
What is apoptosis?
- programmed cell death
108
What are the functions of aerenchyma (specialised parenchyma/ packing tissue)?
- making leaves and stems more buoyant
- forming a low-resistance internal pathway for the movement of substances below the water, helping the plant cope with anoxic (extreme low oxygen conditions) conditions in the mud, by transporting oxygen to the tissues
109
What happens in special situations where roots of a plant become waterlogged?
- air rather than water that is in short supply
- special aerial roots called pneumatophores grow upwards into the air
- have many lenticels, which allow the entry of air into the tissue
