Transport in Plants Flashcards
(97 cards)
1
Q
Why do all multicellular plants need a transport system?
A
Every cell in a multicellular plant needs a regular supply of water and nutrients. In a large multicellular plant, the epithelial cells, which are close to the supply, could gain all they need by simple diffusion. But there are many cells inside the plant that are further from the supply. These cells would not receive enough water or nutrients to survive.
-The problem in plants is that the roots can obtain water fairly easily, but they cannot absorb sugars from the soil. The leaves can produce sugars, but cannot obtain water from the air.
2
Q
Vascular Tissues: What are vascular tissues?
A
It is the transport system in plants. Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally.
3
Q
Vascular Tissues: What are two types of vascular tissues?
A
- Xylem
- Phloem
4
Q
Vascular Tissues: What are vascular bundles?
A
The vascular tissue is distributed throughout the plant but the xylem and phloem are found together in vascular bundles. These bundles often also contain other types of tissue that give the bundle some strength and help support the plant.
5
Q
Vascular Tissues: What is the arrangement of the vascular bundle in a young root?
A
The vascular bundle is found at the centre of a young root, which is good as they are well protected and hard to damage. There is a large central core of xylem, often in the shape of an X. The phloem is found in between the arms of the X-shaped xylem. This arrangement provides strength to withstand the pulling forces to which roots are exposed.
-Around the vascular bundle there is a special sheath of cells called the endodermis, which has a key role in getting water into the xylem vessels. Just inside the endodermis is a layer of meristem cells (that remain able to divide) called the pericycle.
6
Q
Vascular Tissues: What is the arrangement of the vascular bundle in the stem?
A
The vascular bundles are found near the outer edges of the stem.
- In non-woody plants the bundles are separate and discrete.
- In woody plants the bundles are separate in young stems but continuous in older stems. This 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.
7
Q
Vascular Tissues: How is the ring of vascular tissue arranged in the stem?
A
The vascular tissue is arranged in a ring shape around the periphery. The xylem is found towards the inside of each vascular bundle, this supports the plant when full of water. The phloem is found towards the outside of the bundle. In between the xylem and phloem is a layer of cambium. This is a layer of meristem cells that divide to produce new xylem and phloem.
8
Q
Vascular Tissues: Why are the vascular tissues so important for the plant?
A
Without the vascular tissues, the plant will not be able to survive as it will not be possible for them to transport water or sugars.
-If a ring of bark is stripped from a tree, it is likely it will damage the phloem, on the outside of the ring of vascular bundles. This means the plant will not longer be able to transport sugars and so will die.
9
Q
Vascular Tissues: What is the arrangement of vascular bundle in the leaf?
A
The vascular bundles form the midrib and veins of a leaf. These are two main groups of flowering plants, dicotyledons and monocotyledons. These two groups have different patterns of veins. A dicoltyledon leaf has a branching network of veins that can get smaller as they spread away from the midrib. Within each vein, the xylem can be seen on top of the phloem.
10
Q
What is the difference between a root and a stem?
A
Root: -Vascular bundle in central X position -Stomata absent -Endoderm present Stem: -Vascular bundle arranged around periphery. -Stomata present -Endoderm absent
11
Q
What is an epidermis?
A
The outermost layer of a root. In primary root it has root hairs (increases surface area to reabsorb more water and mineral ions, e.g. Mg, and K).
12
Q
What is a cortex?
A
In a root, it is made up of parenchyma cells and can store starch.
13
Q
What is an endodermis?
A
Single layer of cells with a Casparian strip (made of suberin) in cell wall. It is impermeable to water and mineral ions and so when water is going up a root, suberin will direct it to certain channels so it won’t just go anywhere.
14
Q
Xylem: What is the xylem?
A
Xylem is used to transport water and minerals from the root up to the leaves and other part of the plant. Xylem tissue consists of tubes to carry the water and dissolved minerals, fibres help support the plant and living parenchyma cells.
15
Q
Xylem: What are the cell types in the Xylem?
A
- Tracheids
- Xylem vessel elements
- Parenchyma/Collenchyma
- Fibres (e.g. collagen)
16
Q
Xylem: What are tracheids?
A
- Elongated cells with tapered ends
- Have numerous pits through which water can pass freely.
17
Q
Xylem: What are xylem vessel elements?
A
- Shorter and fatter than tracheids
- Long cell walls that have been impregnated with lignin.
- As the xylem develops, the lignin waterproofs the walls of the cells. As a result, the cells die and their end walls and contents decay. This leaves a long column of dead cells with no contents - a tube with no end walls.
- The lignin strengthens the vessel walls and prevents the vessel from collapsing. This keeps the vessel open even at time when water may be in short supply.
18
Q
Xylem: What patterns in the cell wall of xylem vessel elements does lignin form?
A
These may be spiral, annular (rings) or reticulate (a network of broken rings). This prevents the vessel from being too rigid and allows flexibility if the stem or branch.
19
Q
Xylem: Why may some parts of the xylem vessel elements be pitted?
A
In some places, lignification is not complete. It leaves pores in the wall of the vessel, which are called pits or bordered pits. These allow water to leave on vessel and pass into another adjacent vessel, or pass into the living parts of the plant.
20
Q
Xylem: How is the xylem adapted to its function?
A
Xylem tissue can carry water and minerals from roots to the very top of the plant because:
- It is made from dead cells aligned end-to-end to form a continuous column.
- The tubes are narrow so the water column does not break easily and capillary action can be effective.
- Pits in the lignified walls allow water to move sideways from one vessel to another.
- Lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the plant grows and enables the stem or branch to bend.
21
Q
Xylem: Why is the flow of water not impeded?
A
- There are no end walls
- There are no cell contents
- There is no nucleus or cytoplasm
- Lignin thickening prevents the walls from collapsing.
22
Q
Phloem: What is the phloem?
A
The function of the phloem is to transport sugars from one part of the plant to another. This could be up or down the stem.
23
Q
Phloem: What are the cell types in the Phloem?
A
- Sieve tube elements
- Companion cells
- Parenchyma/Collenchyma
- Fibres (e.g. collagen)
24
Q
Phloem: What are sieve tube elements?
A
The sieve tube elements are not true cells as they contain very little cytoplasm and no nucleus. They are lined up end-to-end to form a tube, in which the plant transports sugars (usually sucrose). The sucrose is dissolved in water to form sap. Unlike xylem vessels, this tube contains cross-walls at intervals. These cross-walls are perforated by many pores to allow the sap to flow. Hence the cross walls are called sieve plates and the tubes are called sieve tubes. The sieve tubes have very thin walls and are usually five- or six-sided.
- Living cytoplasm and nucleus disappears as cells differentiates vacuole and organelles, so membranes disappear.
- Inside is hollow so glucose can be transported.
25
Phloem: What are companion cells?
In between the sieve tubes are small cells, each with a large nucleus and a dense cytoplasm. They have numerous mitochondria to produce the ATP needed for active processes. The companion cells carry out the metabolic processes needed by the sieve tube elements. This includes using ATP as a source of energy to load sucrose into the sieve tubes. The cytoplasm of the companion cells and the sieve tub elements are linked through many plasmodesmata. These are gaps in the cell walls, allowing communication and flow of minerals between the cells.
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What are plasmodesmata?
Plasmodesmata are gaps in the cell walls, allowing communication and flow of minerals between the cells. They contain a thin strand of cytoplasm. This allows it to link the contents of adjacent cells.
27
Water: What is water potential
A measure of the tendency of water molecules to diffuse from one place to another.
- Water always moves from a region of higher water potential to a region of lower water potential. The water potential of pure water is zero.
- In a plant cell, the cytoplasm contains salts and sugars (solutes) that will reduce the water potential. This is because there are fewer 'free' water molecules available than in pure water. As a result, the water potential in plant cells is always negative.
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Water: What happens if you place a plant cell in pure water
If you place a plant cell in pure water, it will take up water molecules by osmosis. This is because the water potential in the cell is lower (more negative) that the water potential of the water. But the cell will not continue to absorb water until it bursts. This is because the cell has a strong cellulose wall. Once the cell is full of water it is described as being turgid. The water inside the cell starts to exert pressure on the cell wall, called the pressure potential. As the pressure potential builds up it reduces the influx of water.
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Water: What happens if you place a plant cell a concentrated salt solution
If a plant cell is placed in a solution with a very low water potential (a concentrated salt solution), it will lose water by osmosis. This is because the water potential of the cell s higher than the water potential of the solution. So water diffuses down its water potential gradient out of the cell. The cell loses its turgidity. If water loss continues the cytoplasm and vacuole shrink. Eventually the cytoplasm no longer pushes against the cell wall. This is called incipient plasmolysis. If water continues to leave the cell, the plasma membrane will lose contact with the wall, a condition known as plasmolysis.
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Water: How does water move between cells
When plant cells are touching each other, water molecules can pass from one cell to another. The water molecules will move from the cell with the higher water potential (less negative) to the cell with a lower water potential (more negative).
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Water: What are the routes water can take between cells
- Apoplast Pathway
- Symplast Pathway
- Vacuolar Pathway
32
Water: What is the apoplast pathway
The cellulose cell walls have many water-filled spaces (intercellular spaces) between the cellulose molecules. Water can move through these spaces and between the cells. In this pathway, the water does not pass through any plasma membranes. This means that dissolved mineral ions and salts can be carried with the water.
- The Casparian strip blocks this pathway, forcing the water to pass into the cytoplasm through cell membranes.
- 90% of water travels through this pathway.
33
Water: What is the symplast pathway
Water enters the cell cytoplasm through the plasma membrane. It can then pass through the plasmodesmata from one cell to the next. The plasmodesmata are gaps in the cell wall that contain a thin strand of cytoplasm, therefore the cytoplasm of adjacent cells is linked. Once inside the cytoplasm, water an move through the continuous cytoplasm from cell to cell.
-9% of water travels through this pathway.
34
Water: What is the vacuolar pathway?
This is 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.
-A small amount of water travels through this pathway.
35
Water: How is water taken up from the soil?
Plant roots are surrounded by soil particles. The outermost layer of cells (the epidermis) contains root hair cells that increase the surface area of the root. These cells absorb minerals from the soil by active transport using ATP for energy. The minerals reduce the water potential of the cell cytoplasm. This makes the water potential in the cell lower than that in the soil. Water is taken up across the plasma membrane by osmosis as the molecules move down the water potential gradient.
36
Water: What minerals are taken up from the soil with the water?
- Nitrates (to make proteins)
| - Magnesium (to make chlorophyll)
37
Water: What is the movement of water across the root driven by?
-The movement of water across the root is driven by an active process that occurs at the endodermis. The endodermis is a layer of cells surrounding the xylem. It is also known as the starch sheath as it contains granules of starch - a sign that energy is being used. The endodermis consists of special cells that have a waterproof strip in some of their walls, called the Casparian strip. This blocks the apoplast pathway, forcing water into the symplast pathway.
38
Water: How is water moved across the root?
- The endodermis cells move minerals by active transport from the cortex into the xylem. This decreases the water potential in the xylem. As a result, water moves from the cortex through the endodermal cells to the xylem by osmosis.
- This reduces the water potential in the cells just outside the endodermis. This, combined with water entering the root hair cells, creates a water potential gradient across the whole cortex. Therefore water is moved along the symplast pathway from the root hair cells, across the cortex and into the xylem.
- At the same time, water can move through the apoplast pathway across the cortex. This water moves into the cells to join the symplast pathways just before passing through the endodoermis.
39
Water: What is the Casparian strip?
The Casparian strip are waterproof cells in the endodermis. It is made out of suberin.
40
Water: What is the role of the Casparian strip?
The Casparian strip blocks the apoplast pathway between the cortex and the xylem. This ensures that water and dissolved nitrate ions have to pass into the cell cytoplasm through cell membranes.
- There are transporter proteins in the cell membranes, this means nitrates can be actively transported, from the cytoplasm of the cortex cells, into the xylem. This lowers the water potential in the xylem so water from cortex cells follows into the xylem by osmosis.
- Once the water has entered in the xylem it cannot pass back into the cortex as the apoplast pathway of the endodermal cells is block.
41
Water: What are the three processes that help to move water up the stem?
- Root pressure
- Transpiration pull
- Capillary action
42
Water: How does root pressure help move water up the stem?
The action of the endodermis moving minerals into the xylem by active transport dries water into the xylem by osmosis. This forces water into the xylem and pushes water up the xylem.
-Root pressure can push water a few meters up a stem, but cannot account for water getting to the top of tall trees.
43
Water: How does transpiration pull help move water up the stem?
The loss of water by evaporation from the leaves must be replaced by water coming up from the xylem. Water molecules are attached to each other by forces of cohesion (hydrogen bonding). These cohesion forces are strong enough to hold the molecules together in a long chain or column. As molecules are lost at the top of the column, the whole column is pulled up as one chain, this creates the transpiration stream.
-The pull from above can create tension in the column of water. This is why the xylem vessels must by strengthened by lignin. The lignin prevents the vessel from collapsing under tension. Because this mechanism involves cohesion between the water molecules and tension in the column of water, it is called the cohesion-tension theory. It relies on the plant maintaining an unbroken column of water all the way up the xylem. If the water column is broken in one xylem vessel, the water column can still be maintained through another vessel, via the pits.
44
Water: How does capillary action help move water up the stem?
The same forces that hold water molecules together also attract the water molecules to the sides of the xylem vessel. This is call adhesion. Because the xylem vessels are very narrow these forces of attraction can pull the water up the sides of the vessel.
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Water: How does water leave the leaf?
Most of the water that leaves the leaf exits through the stomata, tiny pores in the epidermis. Only a tiny amount leaves through the waxy cuticle. Water evaporates from the cells lining the cavity immediately below the guard cells. This lowers the water potential in these cells, causing water to enter them by osmosis from neighbouring cells. Water then enters these neighbouring cells from cells deeper in the leaf, and so on until water leaves the xylem and enters into the innermost leaf cells.
46
Transpiration: What is transpiration?
Transpiration is the loss of water vapour from the upper parts of the plant, particularly the leaves.
47
Transpiration: What three processes does transpiration involve?
- Osmosis from the xylem to the mesophyll cells
- Evaporation from the surface of the mesophyll cells into the intercellular spaces
- Diffusion of water vapour from the intercellular spaces out through the stomata.
48
Transpiration: What happens in the process of transpiration?
- Water enters the leaves in the xylem and passes to the mesophyll cells by osmosis.
- The water evaporates from the surface of the mesophyll cells to form water vapour.
- The spongy mesophyll cells have large air spaces between them that help the water vapour to diffuse through the leaf tissue.
- As the water vapour collects in these air spaces, the water vapour potential rises. Once the water vapour potential inside the leaf is higher than outside, water molecules will diffuse out of the leaf. Open stomata provide an easy route for the water vapour to leave the leaf. The stomata are open during the day to allow gaseous exchange for photosynthesis.
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Transpiration: What drives the process of transpiration?
The evaporation of water from the leaves.
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Transpiration: What is the transpiration stream?
As water leaves the xylem in the leaf, it must be replaced from below. Water moves up the xylem from the roots to replace the water lost.
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Transpiration: Why is the movement of water up the stem due to transpiration useful to the plant?
- Water is required in the leaves for photosynthesis
- Water is required to enable cells to grow and elongate
- Water keeps the cells turgid
- The flow of water can carry useful minerals up the plant
- Evaporation of water can keep the plant cool
52
Transpiration: What does a potometer measure?
A potometer measures the rate of water uptake by a cut stem.
-It is possible to study the effect of different environmental conditions on the rate of transpiration, by placing the whole apparatus in a different setting.
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Transpiration: How is the rate of water uptake measured using a potometer?
Speed f movement of air bubble (mm/s) x cross-sectional area of capillary tube (mm^2) = rate of water uptake (mm^2)/s)
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Transpiration: How is a potometer set up?
- Take a healthy shoot
- Cut at a slant underwater (the slant increases surface area, and being underwater prevents the xylem from taking up air).
- Check apparatus to make sure it is full of water, and there are no air bubbles
- Remove potometer from water, put on clamp stand and check that it is airtight so no water can escape.
- Dry leaves to remove any water vapour so not to affect transpiration.
- Keep conditions constant while allowing shoot to adjust to the climate, and keep screw shut (if there is one).
- Attach ruler
- Measure how far air bubble moves in a certain period of time
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Transpiration: What are the main factors that affects the rate of water loss?
- Light intensity
- Temperature
- Humidity
- Air movement
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Transpiration: How does light intensity affect the rate of water loss?
-Light affects transpiration indirectly.
Stomata are mostly open during daylight, so more water is evaporated through them, creating a higher water vapour gradient, and they are closed at night.
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Transpiration: How does temperature affect the rate of water loss?
When the temperature is increased, the kinetic energy of water molecules increase and there is faster evaporation, which creates a higher water vapour gradient.
- Increases the rate of evaporation from the cell surfaces so that the water vapour potential in the leaf rises.
- Increases the rate of diffusion through the stomata because the water molecules have more kinetic energy.
- Decreases the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf.
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Transpiration: How does humidity affect the rate of water loss?
The higher the humdity, the slower the rate of transpiration. This is because their is a smaller water vapour gradient between spongly mesophyll cell spaces and the atmosphere, due to their being more water vapour in the air surrounding the leaf.
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Transpiration: How does air movement affect the rate of water loss?
An increase in wind speed will blow away the humidity that was formed outside the stomata. As the humid air is replaced by dry air, the water vapour gradient increases and so more water is lost as their is less water vapour in the surrounding air.
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Transpiration: What are the other factors that affects the rate of water loss?
- Number of leaves
- Number, size and position of stomata
- Presence of cuticle
- Water availability
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Transpiration: How does the number of leaves affect the rate of water loss?
A plant with more leaves has a larger surface area over which water vapour can be lost, meaning water loss rate increases.
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Transpiration: How does the number, size and position of stomata affect the rate of water loss?
- If the leaves have many large stomata, then water vapour is lost more quickly.
- If the stomata are on the lower surface, water vapour loss is slower.
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Transpiration: How does the presence of a cuticle affect the rate of water loss?
A waxy cuticle reduces evaporation from the leaf surface.
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Transpiration: How does water availability affect the rate of water loss?
If there is little water in the soil, then the plant cannot replace the water that is lost. Water loss in plants is reduced when stomata are closed or when the plants shed leaves in winter.
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Transpiration: What happens if the plant loses too much water?
If water loss by transpiration is greater than water uptake from the soil, the plant cells will lose turgidity.
- Non-woody plant will wilt and eventually die.
- The leaves of woody plants will also wilt and the plant will eventually die.
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Transpiration: Why is the loss of water by transpiration unavoidable?
Plants exchange gases with the atmosphere via their stomata. During the day, plants take up a lot of carbon dioxide to be used in photosynthesis. They must also remove oxygen, which is a by-product 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.
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Transpiration: How can some plants reduce water loss by structural or behavioural changes?
- An impermeable waxy cuticle on the leaf will reduce water loss due to evaporation through the epidermis, as it is waterproof.
- The stomata are often found on the undersurface of leaves, not on the top surface where there is direct heat from the sun, which increases evaporation.
- Most stomata are closed at night, where there is no light for photosynthesis.
- Deciduous plant lose their leaves in winter, when the ground may be frozen (making water less available) and when temperatures may be too low for photosynthesis.
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Xerophytes: What are xerophytes?
Plants which have adapted to reduce water loss so it can survive in dry conditions.
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Xerophytes: What adaptations do xerophytes have?
- Smaller leaves
- Densely packed spongy mesophyll
- A thicker waxy cuticle
- Closing of the stomata when water is less available
- Hairs on the surface of the leaf
- Pits containing sunken stomata at the base
- Rolled up leaves
- High salt concentration
- Shallow but extensive fibrous root system
- Deep root system
- Thick, round stem
- Thick epidermis
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Xerophytes: How is having smaller leaves a benefit?
Having smaller leaves, especially leaves shaped like needles, reduces the total surface area of the leaves. The total leaf surface area is also reduced, so that less water is lost by transpiration, for example a pine tree.
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Xerophytes: How is having a densely packed spongy mesophyll a benefit?
Having a densely packed spongy mesophyll reduces the cell surface area that is exposed to the air inside the leaves. Less water will evaporate into the leaf air spaces, reducing the rate of water loss.
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Xerophytes: How is having a thicker waxy cuticle a benefit?
A thicker waxy cuticle on the leaves reduces evaporation further. This is because it is impermeable and so therefore waterproof, preventing water from escaping.
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Xerophytes: How is the closing of the stomata when water is less available a benefit?
Closing the stomata when water is low will reduce water loss and so reduce the need to take up water.
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Xerophytes: How is having hairs on the surface of the leaf a benefit?
Hairs on the surface of the leaf trap a layer of air close to the surface. This air can become saturated with moisture and will reduce the diffusion of water vapour out through the stomata. This because the gradient of the water vapour potential between the inside of the leaf and the outside has been reduced.
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Xerophytes: How is having pits containing sunken stomata at the base a benefit?
Pits containing stomata at their base also trap air that can become saturated with water vapour. This will reduce the gradient in the water vapour potential between the inside and outside the leaf, so reducing loss by diffusion.
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Xerophytes: How is having rolled up leaves a benefit?
Rolling the leaves so that the lower epidermis is not exposed to the atmosphere can trap air that becomes saturated. This is another way to reduce or even eliminate the water vapour potential gradient.
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Xerophytes: How is having a high salt concentration a benefit?
Some plant have a low water potential inside their leaf cells. This is 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 air spaces is reduced.
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Xerophytes: How is having a shallow but extensive fibrous root system a benefit?
A shallow and extensive root system is good because the plant can collect occasional rainfall to help collect some water when it can.
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Xerophytes: How is having a deep root system a benefit?
A deep root system allows the plant to reach water far underground.
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Xerophytes: How is having a thick, round stem a benefit?
A thick stem stores more water and surface area is reduced by having a squat rounded shape.
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Xerophytes: How is having a thick epidermis a benefit?
Having a thick epidermis means there is a larger diffusion distance and so reduces the amount of evaporation.
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Xerophytes: What is marram grass?
Marram grass (Ammophilia) specialise in living on snad dunes. The conditions are particularly harsh because any water in the sand drains away quickly, the sand may be salty and the leaves are often exposed to very windy conditions.
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Xerophytes: What adaptations do marram grass have?
- Leaf rolled up to trap air inside
- Thick waxy cuticle to reduce water evaporation from surface
- Trapped air in centre with high water vapour potential
- Hairs on lower surface reduce movement of air
- Stomata in pits to trap air with moisture close to the surface.
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Translocation: What is translocation?
The movement of assimilates (sugars and other chemicals made by the plant cells) in the phloem. Sugars are transported in phloem in the form of sucrose.
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Translocation: How does sucrose enter the phloem?
Sucrose is loaded into the phloem by an active process. ATP is used by the companion cells to actively transport hydrogen ions (protons) out of their cytoplasm and into the surrounding tissue. This sets up a diffusion gradient and the hydrogen ions diffuse back into the companion cells. This diffusion occurs through special cotransporter proteins. These proteins enable the hydrogen ions to bring sucrose molecules into the companion cells. As the concentration of sucrose molecules builds up inside the companion cells, they diffuse into the sieve tube elements through numerous plasmodesmata.
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Translocation: What is the source?
At the end of the sieve tube where the assimilates are being produced is called the source.
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Translocation: What is the sink?
At the end of the sieve tube where the assimilates are going is called the sink.
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Translocation: Where are the source and sink?
Perhaps the most obvious source is a leaf. Sugars made during photosynthesis are converted to sucrose and loaded into the phloem. This occurs during late spring, summer and early autumn, while the leaves are green. In early spring, when the leaves are growing they need energy. This energy is supplied from stores in other parts of the plant, and the leaves act as a sink.
-Other sources include the roots, where stored carbohydrates are released into the phloem. This occurs particularly in spring, when other parts of the plant need energy for growth. During summer and autumn the roots store sugars as starch. Thus the roots can also act as a source at some times of year and as a sink at other times.
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Translocation: What is the movement of sucrose at the source?
Sucrose entering the sieve tube element reduces the water potential inside the sieve tube. As a result, water molecules move into the sieve tube element by osmosis from surrounding tissues. This increases the hydrostatic pressure in the sieve tube at the source.
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Translocation: What is active loading?
Active transport moving organic compounds into the phloem using energy.
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Translocation: What is the movement of sucrose along the phloem?
Water entering the phloem at the source, moving down the hydrostatic pressure gradient and leaving the phloem at the sink, produces a flow of water along the phloem. This flow carries sucrose and other assimilates along the phloem. This is called mass flow. It can occur in either direction - up or down the plant - depending on where sugars are needed.
-Mass flow may occur up or down the plant in the same phloem tube at different times. It may be moving assimilates up the plant in some tubes and down the plant in other tubes at the same time.
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Translocation: What is the movement of sucrose at the sink?
Sucrose is used in the cells surrounding the phloem. The sucrose may be converted to starch for storage, or may be used in metabolic processes such as respiration. This reduces the sucrose concentration in these cells. Sucrose molecules move by diffusion or active transport from the sieve tube elements into the surrounding cells. This increases the water potential in the sieve tube element, so water molecules move into the surrounding cells by osmosis. This reduces the hydrostatic pressure in the phloem at the sink.
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Translocation: What is the process of mass flow?
1. Sucrose is actively loaded into the sieve tube element and reduces the water potential.
2. Water follows by osmosis and increases the hydrostatic pressure in the sieve tube element.
3. Water moves down sieve tube from higher hydrostatic pressure at source to lower hydrostatic press at sink
4. Sucrose is removed from the sieve tube by the surrounding cells and increases the water potential in the sieve tube
5. Water moves out of sieve tube and reduces the hydrostatic pressure
94
Translocation: What is the evidence that the phloem is used in translocation?
- If a plant is supplied with radioactively labelled carbon dioxide (which will be used in photosynthesis), the labelled carbon soon appears in the phloem.
- Ringing a tree to remove the phloem results in sugars collecting above the ring. This prevents the downward movement of sucrose, and so can only accumulate above the ring.
- An aphid feeding on a plant stem can be used to show that the mouthparts are taking food from the phloem. If their stylet (mouthpiece) is removed, it can be seen in the phloem, and the fluid is collected from the aphid and is analysed, showing it contains many sugars.
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Translocation: What is the evidence that translocation need metabolic energy (ATP)?
- The companion cells have many mitochondria.
- Translocation can be stopped by using a metabolic poison that inhibits the formation of ATP.
- The rate of flow of sugars in the phloem is so high that energy must be needed to drive the flow. It has been calculated that sugars move up to 10,000 times more quickly by mass flow than they could through diffusion alone.
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Translocation: How do we know translocation is used?
- The pH of the companion cells is higher than that of surrounding cells.
- The concentration of sucrose is higher in the source than in the sink.
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Translocation: Is there any evidence against translocation?
- Not all the solutes in the phloem sap move at the same rate.
- Sucrose is moved to all parts of the plant at the same rate, rather than going more quickly to areas with a low concentration.
- The role of sieve plates is unclear.
