chapter 9 p3 Flashcards
The process of translocation
Translocation is a vital and effective process - a large tree can transport around 250kg of sucrose down its trunk in a year, and substances move at speeds of around 0.15-7 metres per hour.
The details of how substances are moved in the phloem of plants are still the subject of active investigation but the main steps are described here:
Phloem loading:
In many plants the soluble products of photosynthesis are moved into the phloem from the sources by an active process.
Sucrose is the main carbohydrate transported - it is not used in metabolism as readily as glucose and is therefore less likely to be metabolised during the transport process.
two main ways in which plants load assimilates into the phloem (phloem loading) for transport:
One is largely passive, the other is active.
Active phloem loading by the apoplast route is the most widely studied.
A Figure 1 The active movement of sucrose (S) into a companion cell or sieve tube across the cell membrane
The symplast route
In some species of plants the sucrose from the source moves through the cytoplasm of the mesophyll cells and on into the sieve tubes by diffusion through the plasmodesmata (known as the symplast route).
Although phloem loading and translocation are often referred to as active processes, this route is largely passive.
The sucrose ends up in the sieve elements and water follows by osmosis.
This creates a pressure of water that moves the sucrose through the phloem by mass flow.
The apoplast route:
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In many plant species sucrose from the source travels through the cell walls and inter-cell spaces to the companion cells and sieve elements (known as the apoplast route) 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.
The apoplast route p2
Active Transport
- Hydrogen ions (Ht) are actively pumped out of the companion cell into the surrounding tissue using ATP.
- The hydrogen ions return to (outside cell] - high H+ concentration the companion cell down a concentration gradient via a co-transport protein.
- Sucrose is the molecule that is co-transported.
- This increases the sucrose concentration in the companion cells and in the sieve elements through the many plasmodesmata between the two linked cells.
- Companion cells have many infoldings in their cell membranes to give an increased surface area for the active transport of sucrose into the cell cytoplasm.
- They also have many mitochondria to supply the ATP needed for the transport pumps.
The apoplast route p3
Mass Flow and Pressure in Phloem:
- As a result of the build up of sucrose in the companion cell and sieve tube element, water also moves in by osmosis.
- This leads to a build up of turgor pressure due to the rigid cell walls.
- The water carrying the assimilates moves into the tubes of the sieve elements, reducing the pressure in the companion cells, and moves up or down the plant by mass flow to areas of lower pressure (the sinks).
- Solute accumulation in source phloem leads to an increase in turgor pressure that forces sap to regions of lower pressure in the sinks.
- The pressure generated in the phloem is around 2 MPa (15 000mm Hg) - considerably higher than the 0.016 MPa (120mm Hg) of pressure in a human artery.
- These pressure differences in plants can transport solutes and water rapidly over many metres.
- Solutes are translocated either up or down the plant, depending on the positions of the source.
Phloem unloading
The sucrose is unloaded from the phloem at any point into the cells that need it.
The main mechanism of phloem sucrose is loaded into the phloem unloading seems to be by diffusion of the sucrose from the phloem into the surrounding cells.
The sucrose rapidly moves on into other cells by diffusion or is converted into another substance (for example glucose for respiration, starch for storage) so that a concentration gradient of sucrose is maintained between the contents of the phloem and the surrounding cells.
The loss of the solutes from the phloem leads to a rise in the water potential of the phloem.
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.
Evidence of translocation: p1
Advances in microscopy allow us to see the adaptations of the companion cells for active transport.
If the mitochondria of the companion cells are poisoned, translocation stops.
The 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.
Evidence of translocation: p2
Aphids can be used to demonstrate the translocation of organic solutes in the phloem.
Using evidence from aphid studies, it has been shown 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.
The concentration of sucrose in the phloem sap is also higher near to the source than near the sink.
However questions that remain:
However questions that remain:
Not all solutes in the phloem move at the same rate.
On the other hand, sucrose always seems to move at the same rate regardless of the concentration at the sink.
And no one is yet completely sure about the role of the sieve plates in the process.
Plant adaptations to water availability
Land plants exist in a state of constant compromise between getting the carbon dioxide they need for photosynthesis and losing the water they need for turgor pressure and transport.
They must have a large SA:V ratio for gaseous exchange and the capture of light for photosynthesis, but this greatly increases their risk of water loss by transpiration.
Xerophytes
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Most plants have adaptations to conserve water.
These include a waxy cuticle to reduce transpiration from the leaf surfaces, stomata found mainly on the underside of the leaf that can be closed to prevent the loss of water vapour, and roots that grow down to the water in the soil.
However, in habitats where water is often in very short supply, this is not enough.
Xerophytes
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In hot conditions, particularly hot, dry, and breezy conditions - water will evaporate from the leaf surfaces very rapidly.
Plants in dry habitats have evolved a wide range of adaptations that enable them to live and reproduce in places where water availability is very low indeed.
They are known as xerophytes.
Examples are conifers (class Pinopsida), marram grass (Ammophila spp.) and cacti (members of the plant family Cactaceae).
Xerophytes strategies for conserving water:
A thick waxy cuticle
Sunken stomata
Reduced numbers of stomata
Reduced leaves
Hairy leaves
Curled leaves
Succulents
Leaf loss
Root adaptations
Avoiding the problems
A thick waxy cuticle
in most plants up to 10% of the water loss by transpiration is actually through the cuticle.
Some plants have a particularly thick waxy cuticle to help minimise water loss.
This adaptation is common in evergreen plants and helps them survive both hot dry summers and cold winters when water can be hard to absorb from the frozen ground.
Holly (Ilex spp.) is an example commonly seen in the UK.
Sunken stomata
many xerophytes have their stomata located in pits, which reduce air movement, producing a microclimate of still, humid (moist) air that reduces the water vapour potential gradient and so reduces transpiration.
These are seen clearly in xerophyes such as marram grass, cacti, and conifers.
Reduced numbers of stomata
Many xerophytes have reduced numbers of stomata, which reduce their water loss by transpiration but also reduce their gas exchange capabilities
Reduced leaves
by reducing the leaf area, water loss can be greatly reduced.
The leaves of conifers are reduced to thin needles.
These narrow leaves, which are almost circular in cross-section, have a greatly reduced SA:V ratio, minimising the amount of water lost in transpiration.
Hairy leaves
some xerophytes have very hairy leaves that, like the spines of some cacti, create a microclimate of still, humid air, reducing the water vapour potential gradient and minimising the loss of water by transpiration from the surface of the leaf.
Some plants - such as marram grass - even have microhairs in the sunken stomatal pits
Curled leaves
another adaptation that greatly reduces water loss by transpiration, especially in combination with other adaptations, is the growth of curled or rolled leaves.
This confines all of the stomata within a microenvironment of still, humid air to reduce diffusion of water vapour from the stomata.
Marram grass is a good example of a plant with this strategy
Succulents
succulent plants store water in specialised parenchyma tissue in their stems and roots.
They get their name because, unlike other plants, they often have a swollen or fleshy appearance.
Water is stored when it is in plentiful supply and then used in times of drought. Salicornia spp. (edible samphire). which grows on UK salt marshes, and desert cacti are examples of succulents, as are aloes, which include Aloe vera, a plant often used in cosmetics.
Leaf loss
some plants prevent water loss through their leaves by simply losing their leaves when water is not available.
Palo verde (Parkinsonia spp.) is a desert tree that loses all of its leaves in the long dry seasons.
The trunk and branches turn green and photosynthesise with minimal water loss to keep it alive.
Its name is derived from the Spanish words meaning ‘green pole’.
