Plant Transport Flashcards
1
Q
How is water taken up the plant?
A
It travels from the soil through the roots and is transported to the leaves where it maintains turgidity and is a reactant in photosynthesis.
2
Q
What process is a lot of water lost by?
A
Transpiration via the stomata. The water loss must be offset by a constant replacement from the soil.
3
Q
Which region has the greatest uptake?
A
The root hair cell
4
Q
What features does the root hair cell have?
A
Large surface area for water to enter via osmosis. Cellulose cell wall which is freely permeable to water.
A large number of mitochondria to provide ATP for active transport of mineral ions.
A large number of protein carriers in bedded in the membrane for active transport of mineral ions.
5
Q
Does soil water have a dilute or concentrated solution of mineral ions?
A
It contains a very dilutes solution so has a high water potential.
6
Q
Does the vacuole and cytoplasm of the root hair cell have a dilute or concentrated solution of mineral ions?
A
It contains a concentrated solution of solutes therefore has a low water potential. Water moves into the root hair cell down water potential gradient by osmosis.
7
Q
What is the apoplast pathway?
A
It’s when the source solution soaked into the walls of the epidermal cells and travels across the cortex to the cell walls or through spaces between cells, drawn by the transpiration stream.
8
Q
What is the symplast pathway?
A
It’s when water moves to the cytoplasm of cells via the plasmodesmata. The plasmodesmata are strands of cytoplasm through pits in the cell wall joining adjacent cells so that the simplest route is continuous across the root cortex.
9
Q
What is the vacuolar pathway?
A
It’s when water travels through the cell vacuoles.
10
Q
What are the two main pathways?
A
The apoplast and symplast pathway is with the apoplast past rate being faster and more significant.
11
Q
Why can’t water into the xylem from the apoplast route?
A
The ligin makes the xylem waterproof. Water can only pass from the symplast or vacuolar pathways into the xylem so must leave the apoplast pathway.
12
Q
Describe the structure of the epidermis?
A
The vascular tissue in the centre of the root is surrounded by the pericycle where the pericycle is surrounded by a single layer of the cells called endodermis.
13
Q
What are the endodermis cell walls impregnated by?
A
A waxy cuticle called the Suberin.
14
Q
What does the Suberin form?
A
A distinctive band on the radical and tangential walls called the casparian band.
15
Q
What does the casparian band do?
A
It blocks the apoplast route so drives the water into the cytoplasm. At the band water passes across the plasma membrane and continues along the symplast route. As the xylem lacks cell contents the water is transferred to the apoplast in the pericycle.
16
Q
Why does most water enter the cytoplasm of the root hair cell?
A
By osmosis because the active uptake of mineral ions lowers the solute potential.
17
Q
How does water cross the cortex of the plant?
A
Via the cell walls. Water molecules are attracted to cellulose (adhesion) and to each other (cohesion)
18
Q
How far can water and ions travel along the apoplast route?
A
It can travel along that route until it reaches the casparian band which prevents it moving over, so must cross into the symplast route.
19
Q
How are ions transported across the membrane?
A
By active transport, with water following into the symplast route down a WP gradient.
20
Q
What happens when water and ions enter the pericycle?
A
They enter from the epidermis where the ions are actively pumped into the xylem and water follows by osmosis.
21
Q
Describe the movement of water from the root endodermis to the xylem?
A
It moves via osmosis across the endodermal cell membrane. Water potential in the xylem need to be more negative than the water potential in the endodermal cells.
22
Q
How is a lower water potential in the xylem than the endodermal cells activated?
A
The water potential in the endodermis cells is raised by water driven in by the casparian strip.
The water potential in the xylem is lowered by active transport of mineral salts mainly sodium ions from the endodermis and the pericycle into the xylem.
23
Q
How does water move into the xylem?
A
Down the water pressure gradient via osmosis. Water coming into the xylem generates an upward push of root pressure and water already in the xylem.
24
Q
How are minerals absorbed into the cytoplasm of root hair cells?
A
By active transport against a concentration gradient.
25
Describe the uptake of nitrogen?
Nitrogen usually enters the plan is nitrate/ammonium ions which diffuse along a concentration gradient into the apoplast stream but enters the symplast by active transport against a concentration gradient and then flows in the cytoplasm through the plasmodesmata. Active transport allows the plan to absorb the iron selectively at the endodermis.
26
Describe the movement of water from the roots to the leaves?
Water always moves down the water tension gradient. Air has a very low water potential and the soil water has a very high water potential due to the dilute solution of solutes. Therefore water moves up the soil through the plants into the air it passes from the route to the xylem up to the leaves when most is transpired
27
What are the three main mechanisms for the movement of water?
Cohesion tension, capillarity, root pressure.
28
Where does water evaporate from and what does this cause?
Evaporates from the leaves cells into the air spaces and defuses out through the stomata. They stored water across the cells of the leaf in the apoplast, symplast and vacuolar pathways from the xylem.
29
What is cohesion?
It's when the water molecules are attracted to each other due to the polar nature of the hydrogen bonds between the water molecules, which then 'cling' to each other as they move up the xylem.
30
What does cohesion cause?
It causes a continuous pole which produces tension in the water column. The columns of water in the xylem and held open by the cohesive forces between water molecules and the adhesive forces between the water molecules and the hydrophilic lining of the xylem vessels.
31
What is adhesion?
The charges of the molecules with water being polar it adheres to the ligin which lines the inner surface of the xylem vessels which contributes to the movement up the xylem.
32
How does transpiration cause tension?
As transpiration is the loss of water from the leaves this creates a negative pressure which gives rise to the transpiration stream. They continued removal of water molecules from the top of the xylem vessels results in tension causing a pull in the xylem column.
33
What does the cohesion tension theory describe?
The movement of water up the xylem by a combination of adhesion of water molecules and tension in the column resulting in their cohesion.
34
What is capillarity?
It's the movement of water up narrow tubes (xylem) by capillary action. It only operates at short distances i.e. up to 1 m.
35
What is root pressure?
ROOT pressure is when mineral ions are actively transported into the xylem, decreasing the water potential within the xylem. Water moves into the xylem by osmosis increasing the hydrostatic pressure.
36
In the transpiration stream what is water drawn up by?
Cohesive forces between the water molecules. Adhesive forces between the water molecules and the hydrophilic lining in the xylem vessels.
37
What are the two main types of water conducting tissues in the xylem?
Xylem vessels and tracheids.
38
What do you xylem vessels and tracheids form?
Continuous tubes (no end walls where cells join) a column of water can only travel up one direction.
39
What is the role of dead cells within the xylem?
These dead cells have no cell contents, so passive with no impediment to fly so it's easier for water to flow up. They are dead due to the decomposition of ligin on the cellulose cell walls which makes them impermeable.
40
How is the ligin deposited?
As spring/spirals which provide mechanical strength as it prevents the xylem from collapsing due to tension (negative pressure) this supports the plant and allows adhesion. Pit where there are no ligin is deposited = plasmodesmata (allows sideways movement between vessels)
41
What are xylem vessels?
They form main conduct in tubes with wide cells that have reduced or absent end walls so form a continuous tube. The ligin builds up their cell wall as the contents die which leaves an empty space. As a tissue develops the end walls break down leaving a hollow tube which water can travel up (cellulose stains red in the xylem so its easily identifiable)
42
What are xylem tracheids?
They are slightly narrower with perforated end walls so water flow is more obstructed than vessels but provides more support. They are less adapted than vessels.
43
Where are tracheids found?
They are found in ferns, conifers and angiosperms. They are found in the finest branches of the xylem (leaves and roots)
44
What are the other cell types in the xylem?
Fibres - support role only
| Parenchyma- packaging tissue, which keeps all xylem elements together in one place.
45
What are the functions of the types of tissue within the xylem?
To transport materials and water
| Provide mechanical strength and support.
46
What are the two main types of cells in the phloem?
Sieve tubes and companion cells
47
How are sieve tubes formed?
They are formed by parenchyma cells called sieve tube elements which are stacked on top of each other. The cell walls at the end where they join are perforated and from sieve plates which allows the flow through and cytoplasmic strands to link cells.
48
What are sieve tubes the only developed component for?
The flow of materials.
49
Describe this cells within the phloem?
They are living cells which lack a nucleus, have little cytoplasm or organelles for easier flow.
50
What does the phloem consist of?
Sieve tubes and companion cells which are linked by plasmodesmata with fibres and parenchyma. These cells act as a support unit for the sieve element with very dense cytoplasm containing many mitochondria and ribosomes as they are metabolically active.
51
What are sieve elements?
They form main conducting tubes for transport of soluble organic materials made by photosynthesis – sucrose and amino acid which can flow up and down the phloem.
52
What are the other cell types within a phloem?
Phloem fibres - support, NO transport
| Parenchyma - packaging tissue, keeps all phloem elements in one place.
53
Describe the xylem vessel?
ONE way only
Water and minerals
No end walls between cells
Thick walls stiffened by ligin
54
Describe the phloem vessel?
Water and food
TWO way flow
Cells have end walls with perforations
55
What does vascular tissue do?
Transports materials around the body
56
Describe the distribution of the vascular tissue?
The vascular tissue is the xylem and phloem which is found adjacent to each other in vascular bundles. They have different distributions at different parts of the plant.
57
Describe the distribution of the vascular tissue in the roots?
The xylem is central and star shaped with phloem between groups and xylem cells. This arrangement resist vertical stresses (Paul) and anchors the plant in the soil.
58
Describe the distribution of the vascular tissue in the stems?
Vascular bundles are in a ring at the periphery with silence towards the centre and phloem towards the outside. This gives flexible support and resist bending.
59
Describe the distribution of the vascular tissue in the leaves?
The vascular tissue is in the midrib and in a network of veins giving flexible strength and resistance to tearing.
60
What is the transpiration stream?
It's the continuous flow of water in at the roots and goes up the stem and then out the atmosphere.
61
What is the percentage of water absorbed by the plant which is lost by continuous evaporation from the leaves?
99%
62
What do plants need to do in terms of balance with water?
They need to balance water uptake by water loss.
63
What happens if a plant loses more than it absorbs?
It will wilt
64
What happens if only a small volume of water is lost?
Then a plant can recover when water is available
65
What happens if a excessive amount of water is lost?
Then the plant is unable to regain turgor after wilting so will die.
66
What is transpiration?
It's the evaporation of water vapour from the leaves or other aboveground parts of the plant out through the stomata into the atmosphere.
67
What does the rate of transpiration depend on?
Genetic factors – controlling the number, distribution and size of the stomata. Environmental factors – temperature, humidity, movement, light intensity.
68
What effect does temperature, humidity and air movement have?
They affect the WP gradient between the water vapour in the leaf and the atmosphere so will affect the rate.
69
What effect does temperature have on the rate of transpiration?
An increase in temperature lowers the water potential of the atmosphere. It increases the kinetic energy of water molecules accelerating the rate of transpiration from the walls of the mesophyll cells. If the stomata is open then it speeds up the rate of diffusion out into the atmosphere. Higher the temperature more water molecules will diffuse away from the leaf more quickly reducing the water potential around the leaf.
70
What effect does humidity have on the rate of transpiration?
The air inside the leaf is saturated with water vapour so it's relative humidity is 100% the humidity of the atmosphere surrounding the plant varies but is never greater than 100%. There is a water potential gradient between the leaf and the atmosphere and when the stomata are open water diffuses out of the leaf down the water potential gradient. The humidity the higher the water potential, water diffuses down its gradient of relative humidity away from the leaf.
71
What effect does air movement have on the rate of transpiration?
This maintains a diffusion gradient. Fast moving air = increased transpiration. When water is lost from the leaf it forms a thin layer outside the leaf this reduces the water potential between the leaf and the atmosphere outside. When wind is present it blows away the layer of humid air at the leaf's surface. The water potential gradient between inside and outside the leaf consequently increases and water vapour diffuses out through the stomata more quickly. The faster the movement of air the faster the concentric shells of water vapour are blown away the faster the transpiration rate.
72
What effect does light intensity have on the rate of transpiration?
It affects the rate by controlling the degree of stomatal opening. Stomata open wider as light intensity increases, increasing the transpiration rate. So stigmata tend to be open the widest at the middle of the day and less wide in the morning and evening and closed at night.
73
On what type of day is more water lost?
On a dry windy day than humid still day as the spongy mesophyll cells are saturated with water which evaporates and moves down a water potential gradient from a leaf to the atmosphere which has a lower humidity, the wind having reduced the thickness of the saturated air layer at the leaf surface.
74
What does a potometer measure?
The uptake of water as most water taken up by the shoot is lost by transpiration the rate of uptake is almost equal to the transpiration rate. The potometer can be used to compare different environments with the same officiate or can be used to compare the uptake of different species under the same conditions.
75
How to set up the potometer experiment?
1. Cut a healthy shoot underwater, to stop air entering the xylem vessels
2. Cut the shoot at a slant (increasing SA)
3. Underwater fill the potometer so there is no air bubbles
4. Check the apparatus to make sure its full of water and insert the shoot with rubber tubing underwater to prevent air locks in the xylem or apparatus.
5. Shut the screw clip and remove the potometer and ensure air tight joints around the shoot, so seal with Vaseline. Dry the leaves.
6. Introduce an air bubble or meniscus into the capillary tube
7. Measure the distance of the air bubble which moves in a given time
8. Use the water reservoir to bring the air bubble back to the start point, repeat the measurement several times and calculate the mean distance.
9. Repeat, so that you can compare rates of water uptake under different conditions.
76
What is translocation?
It is the movement of soluble products of photosynthesis such as sucrose and amino acid through the flowing from the sources to the sinks. Products of photosynthesis are translocated in the phloem away from the source (site of synthesis in the leaves) to other parts of the plant the sinks which is used for growth and storage. The phloem can translocate both up and down and sideways to where ever the products are needed.
77
What are the different experiments which were tested to show translocation through the phloem?
Ringing experiments, aphid experiments, radioactive traces and autoradiography and aphids and radioactive traces.
78
What is the ringing experiment?
It was when cylinders of outer bark tissue were removed from all the way round a woody stem in a ring which also removed the phloem. The plant was left to photosynthesise, where the phloem contents were analysed and it was found that a bulge of sugar formed away from the ring at the top which suggested that sugar is translocated down the stem of the phloem as there was no sucrose below the rain as it prevented it moving downwards.
79
What is the aphid experiment?
An aphid has a hollow needle like mouthparts called a stylet which they use to penetrate phloem tubes. It inserts into a sieve tube and the phloem contents, sap, exude under pressure into a the aphids stylet. If aphids are anaesthetised with CO2 and the stylet is cut off which remains in the phloem. Pure phloem can be collected through the stylet for analysis. Sap always flows OUT the stylets showing its under pressure.
80
What is the radioactive and autoradiography experiment ?
A plant photosynthesises in the presence of a radioactive isotope 14 CO2. A stem selection is placed on a photographic film where the 'source' and 'sink' are placed in the dark for 24 hours. When the film is developed the presence of the radioactivity shows up as fogging of the negatives. The radioactivity coincides with the position of the phloem indicating that sucrose is transported both up and down the stem.
81
What is the aphid and radioactive tracer experiment?
Radioactive labelled CO2 (14 CO2), is placed in a bad surrounding an illuminated leaf. The CO2 is incorporated into sugars and transported in the phloem. Aphids feeding on the sugar in the phloem can be used to trace the movement of sugar in the plant from the source to the sink. Result : speed of movement is faster than that would be possible with diffusion.
82
What does the mass flow hypothesis suggest?
There is a passive flow of sucrose from the source to the sink. The source has the highest concentration and the saint has a lower concentration.
83
What is the evidence against mass flow?
It doesn't take into account sieve plates
Sucrose and amino acids move at different rates and in different directions in the same tissue.
Phloem has a high oxygen consumption, translocation is slowed or stopped at low temperatures or if respiratory poisons are used e.g potassium cyanide.
Companion cells contain many mitochondria they are biochemically active but the hypothesis doesn't suggest a role for them
The rate of phloem is about 10,000 times faster than substances moving by diffusion
Companion cells along the whole length of the phloem ?
84
What new theories are there?
An active process - loading sucrose into the phloem. H+ions pumped out to re-enter by diffusion via co-transport which allows the entry of sucrose. Cyanide and low temps inhibit translocation indicating that ATP is used.
Protein filaments- Pass through the sieve pores and suggest that different solutes are transported along different filaments (along different routes through the same sieve tube element)
Cytoplasmic streaming could be responsible for the bidirectional movement in individual sieve tube elements providing there was some mechanism to support transportation of solutes through sieve pores.
85
Describe the translocation of sugars?
1. H+ ions are pumped out of the companion cells
2. H+ ions return to the companion cells with sucrose down the diffusion gradient which occurs through co-transport proteins.
3. Sucrose diffuse into the sieve tube elements through the plasmodesmata
4. WP inside the sieve tube decreases, as water moves into the sieve tube element via osmosis
5. Hydrostatic pressure in the sieve tube at the source increases. Sugary fluid moves down the sieve tube from the higher hydrostatic pressure to a lower hydrostatic pressure i.e sugary fluid moves from the source to the sink
6. Sucrose molecules move from the sieve tubes into the surrounding cells by AT or FD. Sucrose enters the root cell (sink) to be used in respiration or to be converted into starch for storage.
7. Water moves out of the sieve tube by osmosis. Hydrostatic pressure at the sink drops.
86
What are the three types of plants which can be classified depending on the prevailing water supply?
Mesophytes, xerophytes, hydrophytes
87
What are mesophytes?
They are plants which the living conditions of adequate water supplies
88
What are xerophytes?
They are plants which live in conditions where water is scarce.
89
What are hydrophytes?
Water plants
90
What happens to mesophytes when water is lost?
It is readily replaced by the uptake from the soil, so requires no special needs and conserving it. If a leaf loses too much water it will wilt and the leaves will droop. The stomata will close to prevent any further water loss. This will reduce the surface are of the leaf which is available for absorbing light so photosynthesis is less efficient.
91
How are mesophytes adapted?
They are adapted to grow best in well-drained soil is a moderate moderately dry water uptake at night compensates the water loss during the day. Excess water loss is prevented because the stomata generally close at night when it's dark.
92
What adaptations to mesophytes have?
Many shed their leaves before winter so they don't lose any water via transpiration when water is scarce.
The aerial parts of many nonwoody plants die off in the winter but their underground organ survive i.e. bulbs and corns.
Many annual mesophytes survive winter as dormant seeds with a low metabolic rate where no water is required.
93
How are xerophytes adapted?
They are adapted to living with low water availability with modified structures to prevent excessive water loss.
94
Where are xerophytes found?
They may live in hot, dry, desert regions, cold regions where soil water is frozen for match of the year or exposed to windy conditions.
95
Describe ammophila arenaria ?
It is a Marram grass which colonises in the sand dunes. It has no soil with the rainwater draining away rapidly, there are high wind speeds salt spray and a lack of shade from the Sun. The high winds can increase the rate of transpiration as well as being hot.
96
What does rolled leaves do for xerophytes?
Large thin walled epidermal cells called hinge cells at the bases of grooves become plasmolysed when they lose water from excessive transpiration and the leaf rolls with its upper surface (adaxial) inwards this reduces the leaves are exposed to air so reduces transpiration.
97
How does a sunken stomata help xerophytes?
Stomata are found in grooves on the adaxial but not the outer surface (abaxial) of the leaf. They are in pits so that the humid air is trapped outside the stomata. This reduces the water potential gradient between the inside and the outside of the leaf so reduces the rate of transpiration.
98
What do hairs (trichomes) do for xerophytes?
They are stiff and interlocking which trap water vapour and reduce the WP gradient, between the leaf and atmosphere.
99
What does a waxy cuticle do for xerophytes?
A thick waxy cuticle covering on the outer (abaxial) leaf surface, which is water proof so reduces water loss. The thicker the cuticle the lower the rate of cuticular transpiration.
100
What does fibres of sclerenchyma do for xerophytes?
Stiff to maintain leaf shape even when the cells become flaccid.
101
What do hinge cells do for xerophytes?
When flaccid they collapse so they roll the leaf.
102
Which features have the same purpose of trapping water vapour near the stomata and reducing the diffusion gradient reducing the rate of diffusion from the leaf?
Rolled leaves, sunken stomata, hairs (trichomes)
103
Why do pine trees have needle like leaves?
To reduce the SA at which water can be lost at
104
Explain a cactus adaptations?
They have succulent stems for storage of water, with their leaves being spines. Many cacti can close the stomata during the day, in which they contain fewer stomata per unit area.
105
Where do hydrophytes grow?
Partially or wholly submerged in water e.g. waterlily which is rooted to the mud at the bottom of the pond and has a lead floating on the surface.
106
Adaptations for hydrophytes
As water is a supportive medium they have little or no lignified support tissues.
Xylem is poorly adapted as there is little need for transport tissue as the water as a plant is surrounded in water.
Leaves have little or no cuticle as there is no need to prevent water loss.
Stomata is found on the upper surface as the lower surface is in water (CO2 and during the day and O2 out).
Stem and leaves have large airspaces (tissue called aerenchyma) continuous down to the roots there is rapid diffusion of gases to the roots and underwater parts the airspaces act as reservoirs for CO2 and O2 which assists in buoyancy.
