chapter 9 p3

Question 1

The process of translocation

Answer 1

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:

Question 2

Phloem loading:

Answer 2

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.

Question 3

two main ways in which plants load assimilates into the phloem (phloem loading) for transport:

Answer 3

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

Question 4

The symplast route

Answer 4

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.

Question 5

The apoplast route:
p1

Answer 5

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.

Question 6

The apoplast route p2
Active Transport

Answer 6

• 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.

Question 7

The apoplast route p3
Mass Flow and Pressure in Phloem:

Answer 7

• 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.

Question 8

Answer 8

Question 9

Phloem unloading

Answer 9

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.

Question 10

Evidence of translocation: p1

Answer 10

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.

Question 11

Evidence of translocation: p2

Answer 11

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:

Question 12

However questions that remain:

Answer 12

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.

Question 13

Plant adaptations to water availability

Answer 13

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.

Question 14

Xerophytes
p1

Answer 14

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.

Question 15

Xerophytes
p2

Answer 15

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).

Question 16

Xerophytes strategies for conserving water:

Answer 16

A thick waxy cuticle

Sunken stomata

Reduced numbers of stomata

Reduced leaves

Hairy leaves

Curled leaves

Succulents

Leaf loss

Root adaptations

Avoiding the problems

Question 17

A thick waxy cuticle

Answer 17

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.

Question 18

Sunken stomata

Answer 18

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.

Question 19

Reduced numbers of stomata

Answer 19

Many xerophytes have reduced numbers of stomata, which reduce their water loss by transpiration but also reduce their gas exchange capabilities

Question 20

Reduced leaves

Answer 20

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.

Question 21

Hairy leaves

Answer 21

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

Question 22

Curled leaves

Answer 22

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

Question 23

Succulents

Answer 23

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.

Question 24

Leaf loss

Answer 24

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’.

Question 25

Root adaptations

Answer 25

many xerophytes have root adaptations that help them to get as much water as possible from the soil.
Long tap roots growing deep into the ground can penetrate several metres, so they can access water that is a long way below the surface.
A mass of widespread, shallow roots with a large surface area able to absorb any available water before a rain shower evaporates is another adaptation.
Many cacti show both of these adaptations, including the giant saguaro (Carnegiea gigantea), which can get enough water to grow to around 12-18 metres tall and live for around 200 years.
The root system of marram grass consists of long vertical roots that penetrate metres into the sand.
They also have a mat of horizontal rhizomes (modified stems) from which many more roots develop to form an extensive network that helps to change their environment and enable the sand to hold more water.

Question 26

Avoiding the problems

Answer 26

some plants are adapted to cope with the problems of low water availability by avoiding the situation entirely.
Plants may lose their leaves and become dormant, or die completely, leaving seeds behind to germinate and grow rapidly when rain falls again.
Others survive as storage organs such as bulbs (onions, daffodils), corms (crocuses) or tubers (potatoes, dahlias).
A few plants can withstand complete dehydration and recover - they appear dead but when it rains the cells recover, the plant becomes turgid and green again and begins to photosynthesise.
The ability to survive in this way is linked to the disaccharide trehalose, which appears to enable to the cells to survive unharmed.

Question 27

Answer 27

Question 28

Investigating stomatal numbers p1

Answer 28

You can compare the numbers of stomata on the leaves and stems of plants by taking an impression of the epidermis of the leaf or the stem and looking at it under a microscope.
You can use clear nail varnish, Germolene New Skin or DIY water based varnish.

Question 29

Investigating stomatal numbers p2

Answer 29

You need to observe the numbers of stomata, and whether they are open or closed over the same area each time so take care to use the same magnification with your microscope. You may choose to use a graticule.
This technique can be used to compare stomatal numbers in different areas of a plant and in different types of plants.
It can also be used to investigate the opening and closing of the stomata under different conditions.

Question 30

Hydrophytes

Answer 30

Not all plants have to conserve water.
the hydrophytes - plants that actually live in water (submerged, on the surface or at the edges of bodies of water) - need special adaptations to cope with growing in water or in permanently saturated soil.
It is important in surface water plants that the leaves float so they are near the surface of the water to get the light needed for photosynthesis.
Water-logging is a major problem for all hydrophytes.
The air spaces of the plant need to be full of air, not water, for the plant to survive.

Question 31

Examples of hydrophytes include

Answer 31

water lilies (plants of the family Nymphaeaceae) and water cress (Nasturtium officinale), which grow at the surface, duckweeds (genus Lemna), which are submerged or free-floating plants, and marginals such bulrushes (Typha latifolia) and yellow iris (Iris pseudacorus), which grow at the edge of the water.

Question 32

Adaptations of hydrophytes:

Answer 32

Very thin or no waxy cuticle
Many always-open stomata on the upper surfaces
Reduced structure to the plant
Wide, flat leaves
Small roots
Large surface areas of stems and roots under water
Air sacs
Aerenchyma

Question 33

Very thin or no waxy cuticle

Answer 33

hydrophytes do not need to conserve water as there is always plenty available so water loss by transpiration is not an issue.

Question 34

Many always-open stomata on the upper surfaces

Answer 34

maximising the number of stomata maximises gaseous exchange.
Unlike other plants there is no risk to the plant of loss of turgor as there is always an abundance of water available, so the stomata are usually open all the time for gaseous exchange and the guard cells are inactive.
In plants with floating leaves such as water lilies the stomata need to be on the upper surface of the leaf so they are in contact with the air.

Question 35

Reduced structure to the plant

Answer 35

the water supports the leaves and flowers so there is no need for strong supporting structures.

Question 36

Wide, flat leaves

Answer 36

some hydrophytes, including the water lilies, have wide, flat leaves that spread across the surface of the water to capture as much light as possible.

Question 37

Small roots

Answer 37

water can diffuse directly into stem and leaf tissue so there is less need for uptake by roots.

Question 38

Large surface areas of stems and roots under water

Answer 38

this maximises the area for photosynthesis and for oxygen to diffuse into submerged plants.

Question 39

Air sacs

Answer 39

some hydrophytes have air sacs to enable the leaves and/or flowers to float to the surface of the water.

Question 40

Aerenchyma

Answer 40

specialised parenchyma (packing) tissue forms in the leaves, stems and roots of hydrophytes.
It has many large air spaces, which seem to be formed at least in part by apoptosis (programmed cell death) in normal parenchyma. It has several different functions within the plants, including:

Question 41

It has several different functions within the plants, including:

Answer 41

making the leaves and stems more buoyant
forming a low-resistance internal pathway for the movement of substances such as oxygen to tissues below the water. This helps the plant to cope with anoxic (extreme low oxygen conditions) conditions in the mud, by transporting oxygen to the tissues.

Question 42

Future of Aerenchyma:

Answer 42

Aerenchyma is found in crop species that grow in water, such as rice (Oryza sativa and Oryza glaberrima).
Studies suggest that aerenchyma may provide a low resistance pathway by which methane produced by the rice plants can be vented into the atmosphere.
This is part of a major problem.
Atmospheric methane, which contributes of the greenhouse effect and the resulting climate change, has doubled over the past two centuries and flooded rice paddies represent a major source.
In situations where there is plenty of water - for example in mangrove swamps, the roots can become waterlogged.
It is air rather than water that is in short supply.
Special aerial roots called pneumatophores grow upwards into the air.
They have many lenticels, which allow the entry of air into the woody tissue