3.1.3 Exchange and Transport in plants

Question 1

What are the two vascular tissues in plants, and what are their roles? 2 marks

Answer 1

Xylem tissue - transports water and dissolved mineral ions from the roots to the leaves

Phloem tissue - transports sucrose solution (including some other solutes) from sources to sinks.

Question 2

why do plants need specialised transport systems?

Answer 2

1. Large size - the diffusion distance is too far, so diffusion alone would be too slow
2. Low SA:V ratio, so diffusion would be too slow
3. High metabolic rate

Question 3

What is transpiration?

Answer 3

Transpiration is the loss of water vapour from the leaves of a plant, via evaporation from mesophyll cell wall, followed by diffusion out of open stomata

Question 4

Why is transpiration inevitable? 5 marks

Answer 4

1. When there is sufficient light for photosynthesis (i.e. during the daytime), the plant must open the stomata (pores) on its leaves
2. Open stomata enable the diffusion of carbon dioxide from atmospheric air into the leaf, for use in photosynthesis (i.e. glucose production)
3. Oxygen produced as a waste product from photosynthesis is removed from the leaf by diffusion through the open stomata into the atmosphere
4. However, since the mesophyll cells have a large surface area and cell walls that are moist, there will consequently be much evaporation of water from their cell walls into the air spaces in the spongy mesophyll
5. The resulting water vapour will now diffuse out of open stomata into the atmosphere, down the water (vapour) potential gradient.

Question 5

What are the 5 factors effecting transpiration rate?

Answer 5

1. Light intensity
2. Leaf exposure
3. Temperature
4. Wind/Air currents
5. Air humidity

Question 6

How does light intensity affect transpiration?

Answer 6

A higher light intensity will cause increased opening of stomata; this means there will be an increased area across which water vapour can diffuse out of the leaf; increased loss of water vapour by diffusion through the stomata corresponds to an increased transpiration rate.

Question 7

How to test the affect of light intensity on transpiration?

Answer 7

the effect of light intensity can be investigated by using a lamp with different intensity bulbs from a fixed position

Question 8

How does Leaf exposure affect transpiration?

Answer 8

All leaves have a waxy cuticle on the upper leaf surface, which is relatively impermeable (waterproof) to water; the thicker this waxy cuticle, the lower the rate of evaporation of water directly from the upper leaf surface (the upper epidermis tissue layer), so the lower the transpiration rate.

Question 9

How to test the affect of Leaf exposure on transpiration?

Answer 9

Experimentally, the upper and/or lower leaf surfaces may be smeared with a layer of Vaseline,

Question 10

How does temperature affect transpiration?

Answer 10

As temperature increases, water molecules have more kinetic energy, hence the rate of evaporation from mesophyll cell surfaces increases; the rate of diffusion of water vapour out of open stomata also increases. Additionally, warmer atmospheric air can hold more water vapour before reaching saturation; this results in a steeper water potential gradient between the air spaces in the spongy mesophyll and the atmospheric air, increasing the rates of evaporation and diffusion of water vapour even further. For both of these reasons, increases in temperature will significantly increase transpiration rates.

Question 11

How to test the affect of temperature on transpiration?

Answer 11

A heater with adjustable temperature settings could be used to investigate this experimentally.

Question 12

How does Wind/Air currents affect transpiration?

Answer 12

The movement of atmospheric air past the leaf surface increases transpiration rates by removing the boundary layer of water vapour (humid air) that tends to accumulate around the leaf (in still air conditions); this results in a steeper water (vapour) potential gradient, increasing rates of evaporation and diffusion of water vapour out of open stomata.

Question 13

How to test the affect of Wind/Air currents on transpiration?

Answer 13

To investigate this experimentally, a fan (with different speed settings) could be used.

Question 14

How does Air humidity affect transpiration?

Answer 14

The higher the air humidity, the lower the transpiration rate will be. This is because a higher humidity of the air around the leaf will cause a less steep water (vapour) potential gradient between the air spaces in the spongy mesophyll and the outside atmosphere; this in turn will decrease rates of evaporation and diffusion of water vapour out of open stomata.

Question 15

How to test the affect of Air humidity on transpiration?

Answer 15

a plastic bag could be placed over the leafy shoot in order to set up a more humid atmosphere around it.

Question 16

what does a potometer measure?

Answer 16

A potometer is a piece of equipment which measures the rate of water uptake by plant or leafy shoot cut from a plant. It may then be assumed that rate of water uptake will be equal to transpiration rate

Question 17

What are the precautions when setting up a potometer?

8 marks

Answer 17

1. Use a healthy leafy shoot
2. Cut the stem of the shoot under water
3. Cut the stem at a sharp angle
4. Assemble the potometer under water
5. Check that all joints in the apparatus are airtight and watertight (and if necessary smear with Vaseline to seal each joint), to avoid leakage of water out of the potometer or entry of air bubbles
6. Allow time for the shoot to acclimatise (adjust) to the conditions before commencing readings, to avoid the initial period when rate of water uptake may be rather erratic (and hence not representative).
7. Insert one air bubble (only) by briefly removing the capillary tube from the water and then submerging it once again; then note the starting position of this bubble on the scale
8. If carrying out replicate readings (from which a mean will be calculated), ensure that each reading is taken under the same conditions, e.g. same temperature, same humidity, same level of air currents, same length of time). (Reliable)

Question 18

why may rate of water uptake not be equal to the transpiration rate?

Answer 18

Some of the water taken up might actually be used up in metabolic reactions such as photosynthesis, and some may be used to increase cell turgor.

Therefore, it may be the case that the real transpiration rate is slightly lower than the rate of water uptake that has been measured by the potometer.

Question 19

How to measure rate of water uptake?

Answer 19

                                   Time

Question 20

What is a Xerophyte?

Answer 20

Xerophytes are plants adapted to arid (dry) environments. In such environments (e.g. the desert), lack of water availability in the soil and excessive water loss by transpiration are strong selection pressures.

Question 21

What is an example of a xerophyte?

Answer 21

Cacti or Marram grass

Question 22

what are the uses for the adaptations of xerophytes?

Answer 22

These features increase their chances of survival in arid habitats by increasing water uptake and/or water storage and ‐ most importantly ‐ decreasing transpirational water loss

Question 23

what adaptaion are also present in this type of plant?

Answer 23

halophytes, plants which are adapted to survive in saline (salty) environments

Question 24

What are the adaptations of Xerophytes? 10 marks

Answer 24

1. Very small surface area of leaves (e.g. the spines of a cactus) OR thick leaves with low surface area to volume ratio
   reduces the surface area from which water can be lost by evaporation, thus reducing transpiration rate and conserving water
2. Thicker waxy cuticle
   waterproof, so makes the leaf surface (almost) impermeable to water, which reduces water loss by evaporation from the epidermal cells, decreasing transpiration rate and thus conserving more water within the leaves.
3. Leaves that curl (roll) e.g. marram grass
   A layer of still air is trapped within the rolled leaf. This air quickly becomes very humid, with a high concentration of water vapour (i.e. high water potential). The water potential gradient between the air spaces in the mesophyll and this trapped air is made less steep. This means the rate of diffusion of water vapour out of open stomata is decreased
4. Sunken stomata
   Air adjacent to the stomata is still and quickly becomes very humid (high water potential). This decreases the steepness of the water potential gradient between the air spaces in the mesophyll and the air outside the stomata. The rate of diffusion of water vapour out of the open stomata decreases, decreasing transpiration and conserving water within the leaf.
5. Fewer stomata
   There is less surface area across which water vapour can diffuse out of the leaf. Hence lower transpiration rate and more water conserved within the leaf.
6. Stomata close during the hottest part of the day When temperature is highest, the rate of diffusion of water vapour out of open stomata would be at its highest (more kinetic energy), giving high transpirational water loss from the leaf. Xerophytes may decrease their transpiration rate by only opening their stomata during cooler times of day, when rates of diffusion of water vapour out of the stomata would be lower (less kinetic energy). This conserves water within the plant BUT does come with a penalty: having stomata closed decreases uptake of carbon dioxide at this time, meaning a decrease in the rate of photosynthesis (just when there is high light intensity which could give high rates of photosynthesis!). Very effective at decreasing transpiration, since most water vapour loss occurs through open stomata
7. Hairy leaves (hirsutism)
   The hairs trap a layer of still air, which quickly becomes humid (high water potential). This decreases the steepness of the water potential gradient, decreasing rates of evaporation and diffusion of water vapour out of open stomata. Transpirational water loss is significantly reduced. Some plants take this adaptation to the extreme: their leaf hairs have a branched structure (as if the hairs themselves are hairy) and are called trichomes.
8. Small size of plant and/or rounded (rather than highly branched) shape
   This ensures there is less surface area from which water can be lost by transpiration or a low surface area to volume ratio.
9. Extensive shallow roots
   These increase the surface area for water uptake by osmosis from the soil/sand, e.g. absorb rainwater as soon as it soaks into the soil/sand
10. Long ‘tap’ root, growing very deep into the soil/sand
    There is increased water availability deeper down in the soil, so this increases the plants ability to absorb water and compensate for transpirational water loss from the aerial (above ground) parts of the plants. A tap root also helps anchor the plant in the soil, so that it is not blown away by strong winds.

Question 25

What is a hydrophyte?

Answer 25

Hydrophytes are plants adapted to growing in water. This can mean that the plant is completely submerged in a pond (e.g. Canadian pondweed), or alternatively the plant may grow with their roots in water or water saturated soil, but leaves on the surface in contact with atmospheric air (e.g. water lilies).

Question 26

what are the main challanges of hydrophyes?

Answer 26

The key challenges for hydrophytes include getting enough light to reach their leaves for photosynthesis and ensuring that there are sufficient air spaces within plant tissues that allow diffusion of gases for photosynthesis and respiration.

Question 27

What are the adaptations of Hydrophytes?

Answer 27

1. Wide, flat leaf shape
   Having very wide leaves floating on the water’s surface increases the surface area for light absorption and hence maximises photosynthesis rate. Thin waxy cuticle (or none)
2. There is no need to conserve water, as transpirational loss of water is not applicable (or not significant), hence an impermeable waxy cuticle on the leaves would not be of benefit. An exception to this might be a waxy deposit specifically around stomata, repelling water from the stomatal pore so that the diffusion of gases is not blocked.
3. Stomata on upper leaf surface, open at all times
   If the plant’s leaves float on the surface of water, having stomata on the upper leaf surface enables carbon dioxide from the atmosphere to diffuse directly into the leaves, for use in photosynthesis. This adaptation is advantageous because there is a higher concentration of carbon dioxide in the air than in the water, and it diffuses more quickly in air than in water. There is no need to close stomata, or restrict them to the lower leaf surface only, since these are only useful adaptations if the transpiration rate needs to be limited – not applicable when water is readily available around the plant to directly move into cells to replace any that is lost
4. Lack of woody stems
   The surrounding water supports the plant’s tissues (e.g. leaves floating on the surface) so there is no benefit in having strong, woody stems for support.
5. Air sacs (‘bladders’)
   In some hydrophytes, air sacs are present, enabling leaves to float to the surface of the
   water, where light absorption and hence photosynthesis rate can be maximised.
6. Aerenchyma tissue present
   Hydrophytes often contain aerenchyma tissue, which is parenchyma (packing tissue) that
   contains large air spaces. The programmed cell death process (apoptosis) plays a role in
   removing cells in order to leave air spaces. The presence of the air spaces makes the tissue
   less dense, such that leaves and stems have increased buoyancy; the air spaces also enable
   rapid diffusion of gases, e.g. enabling sufficient oxygen (for aerobic respiration) to reach
   the submerged tissues that are far below the water’s surface and hence far from the direct
   atmospheric supply of oxygen.
7. Small/absent roots and vascular tissues
   Since water can readily move by osmosis directly into any stems or leaves that are submerged or at least in direct contact with water, there is less need to have a huge root surface area that can uptake sufficient water to supply the whole plant. It would be an inefficient use of resources for a hydrophyte to grow an extensive root system. A similar argument explains why many hydrophytes have reduced vascular tissues: there is little need for xylem to transport water from roots to leaves when water can directly enter any cell to replace that which is lost; it is not a good use of resources to develop extensive vascular tissues.

Question 28

describe the xylem tissue

4 marks

Answer 28

many parallel, hollow tubes called xylem vessels

Each xylem vessel is itself comprised of many individual xylem vessel elements, joined end‐to‐end

As xylem tissue matures, significant changes occur as the cells differentiate: the cells elongate to become tube‐like xylem vessel elements; the xylem vessel elements lose their nucleus and cytoplasm and the end‐walls between adjoining elements completely break down; this leaves continuous, hollow xylem vessels, which run from the roots all the way to the leaves; the walls of the vessels become lignified, i.e. strong, hydrophobic, impermeable lignin is deposited in the cell wall in addition the cellulose. Mature xylem is a dead tissue

There may be living parenchyma tissures between the strucutres

Question 29

what is the function ox xylem vessels?

Answer 29

carry water and mineral ions from roots to leaves

also provides support to stems and leaves.

Question 30

what is the transpiration stream?

Answer 30

The transpiration stream is the upward flow of water (and dissolved mineral ions) in the xylem; the main driving force for this upward flow is the loss of water vapour from the leaves by transpiration.

Question 31

Describe the Cohesion-Tension theory

7 marks

Answer 31

1. Water evaporates from the moist cell walls of leaf mesophyll cells, which are in contact with the air spaces in the spongy mesophyll. This water, now a gas ‐ water vapour ‐ will diffuse out of the stomata, which need to be open during the day for uptake of carbon dioxide for photosynthesis. This loss of water vapour from the leaves is called transpiration.
2. Transpiration sets up a water potential gradient, such that water then moves by osmosis from cell to cell across the mesophyll tissue in the leaf, to replace that being lost.
3. Ultimately, this causes water to move out of the top of the xylem via osmosis into the adjoining mesophyll cells, down the water potential gradient.
4. Since water molecules form continuous columns up the length of each xylem vessel, the loss of water at the top of the xylem exerts a pulling force (a negative pressure) on the rest of the column beneath it. This pulling force is called tension: we say that the columns of water in each xylem vessel are under tension, meaning that they are effectively being pulled
   upwards. This idea is termed transpiration pull (or transpirational pull), since it is ultimately caused by the transpiration of water vapour from the leaves.
5. The columns of water remain intact due to the cohesion between the water molecules (which are polar), i.e. the hydrogen bonding of water molecules to each other.
6. Given that the water is under tension, with lowest pressure at the top of each water column, we can say that there is a hydrostatic pressure gradient stretching the length of each column of water, in each xylem vessel. The pressure is highest at the base (in the roots) and lowest at the top of the xylem vessels (in the leaves). This means that the water will move against gravity in an upwards direction (but note that the water is moving down the hydrostatic pressure gradient, from the region of higher pressure to the region of lower pressure).
7. The flow of the water ‘en masse’ in this way can be called: ‘mass flow’ or ‘bulk flow’. It is different from osmosis because the water is moving as a block, simply carrying any dissolved solutes with it at the same rate of movement. These solutes are mainly mineral ions (nitrate, phosphate, magnesium etc.). Remember that the function of xylem tissue is to transport both water and the dissolved mineral ions from the roots to the leaves.

Question 32

What is root pressure?

Answer 32

1. In the roots, the xylem is surrounded by a ring of tissue called the endodermis. The cells in the tissue, called endodermal cells, actively transport mineral ions into the xylem vessels (against their concentration gradient, requiring ATP from respiration and specific carrier proteins).
2. This will lower the water potential of the water in the xylem, and therefore cause water to move by osmosis into the xylem vessels, following the mineral ions, down the water potential gradient which has been established.
3. This inward flux of water into the xylem at the roots increases the hydrostatic pressure at the base of the xylem vessels. This causes an upwards ‘pushing’ force on the column of water in each xylem vessel. This upwards force is called ‘root pressure’.

Question 33

what is the significane of root pressure on the upwartd movement of water?

Answer 33

Root pressure does not have a significant effect on the upward water of movement by itself BUT it contributes (is synergistic) to the hydrostatic pressure gradient which stretches the length of each column of water and which is primarily established by transpiration (as Hence, water will move up the xylem vessels as continuous columns, held intact by cohesion (the hydrogen bonding between the water molecules).

Question 34

What is capillary action?

Answer 34

‘the tendency of water to rise up a narrow tube’. The narrower the tube, the greater the effect.

So the fact that xylem tissue is made of many parallel vessels, each with a small diameter, will increase capillarity (compared to have just one, wider vessel).

Capillarity is due to cohesion plus adhesion (the hydrogen bonding of water molecules to the cellulose and lignin in the walls of the xylem vessels).

Question 35

What happens if the transpiration stream is interuppted?

Answer 35

If an air bubble enters a xylem vessel, that vessel is effectively blocked (in the short term at least), because the continuous column of water in that vessel has been broken. The lower half of the column of water may collapse and the vessel is rendered useless. The breaking of columns in this way (often caused by an excessively high transpiration rate, e.g. in hot,
windy weather), is known as cavitation.

The problem is partly solved by the fact that water may be able to move laterally (sideways) from the blocked vessel into an adjacent vessel. This is because even vessels whose walls contain a large amount of waterproof lignin will contain pits (unlignifiedregions which act as pores through which water can move sideways). The lateral movement of water between xylem vessels reduces the consequences of an individual vessel being blocked with an air bubble.

However, the plant will have serious problems if multiple parallel vessels all become blocked: transpirational water loss from the leaves will continue as normal, but insufficient water is now being supplied to the leaves via the xylem. The rate of water vapour loss by evaporation and diffusion out of the leaves is now exceeding the rate of water supply from the xylem. This causes the mesophyll cells in the leaf to lose turgor and become flaccid (soft), since they still lose water by osmosis, but do not gain sufficient water to replace this and allow them to remain turgid. If cells in the leaf become flaccid in this way, the entire leaf wilts (droop). This is bad news for the plant because the leaf will no longer be held up to intercept light for photosynthesis.

Wilting can also (and more commonly) be caused by the rate of water uptake via the root hair cells being lower than the rate of transpirational loss. This will happen if the soil is too dry and/or if the weather conditions are warm and/or windy and/or there is high light
intensity. It can also happen if the root hairs have been damaged or broken off (by the plant being pulled up by a gardener wanting to move it, or by a fungus): this will reduce the surface area across which water can be taken up.

Question 36

what is the apoplast pathway?

Answer 36

The apoplast pathway is used by the greatest proportion of the water crossing the cortex. It has the lowest resistance and hence the fastest flow rate. The water (and minerals dissolved in it) moves through the gaps between the cellulose microfibrils that make up the cell
walls of the cortical cells; the water never actually enters the cytoplasm of any cortical cell, and does not cross any cell surface membranes.

In the apoplast, the water is moving by mass flow, with dissolved solutes being carried at the same rate; the main driving force is a hydrostatic pressure gradient: in the outer regions of the cortex, the hydrostatic pressure has been increased due to entry of water into root hair cells by osmosis; however, in the region of the cortex nearest to the xylem, the hydrostatic pressure is lower due to the transpiration pull effect set up by loss of water vapour from the leaves. Water will move down the hydrostatic pressure gradient, through the spaces in the cell walls that comprise the apoplast pathway. When the water in theapoplast pathway reaches the endodermis, this route is blocked by the presence of the Casparian Strip

Question 37

what is the symplast pathway?

Answer 37

The symplast is made up of the cytoplasm of the cortical cells; the symplast pathway therefore refers to the movement of water from cell‐to‐cell, passing through the cytoplasm. The water is moving down the water potential gradient: water potential is highest in the outer regions of the cortex due to entry of water into root hair cells, and lowest in the innermost regions of the cortex, where water is entering the xylem and being carried away. The water in the symplast can move from cell‐to‐cell by crossing the cell surface membranes of the cortical cells; alternatively, (some) water can pass into the next cell through plasmodesmata (pores that connect the plant cells together). The water in the symplast is moving across the cortex by osmosis (if cell surface membranes are being crossed) or diffusion (if water is using plasmodesmata).

syMplast = Membrane or plasModesmata

Question 38

what is the vacuolar pathway?

Answer 38

Some of the water using the symplast route actually passes into (and then out of) the large/permanent vacuole found in each cortical cell. To do so, the tonoplast membrane must be crossed. Like the cell surface membrane, the tonoplast is partially permeable; water molecules are small enough that they can readily pass through. Water passing through the vacuole of each cortical cell is said to be using the vacuolar pathway; it is moving down the water potential gradient, by osmosis.

Question 39

Crossing the endodermis

Answer 39

The water that has crossed the cortex must now move across the endodermis before it canreach the xylem, located in the very centre of the root. When the water that has crossed the cortex reaches the endodermis (the ring of tissue surrounding the xylem and phloem in the centre of the root), all water and dissolved minerals which were travelling in the apoplast pathway must now enter the symplast. This is because the apoplast pathway is now blocked due to the presence of the Casparian Strip in the cell walls of the endodermal cells. The Casparian Strip consists of a layer of suberin (a waxy, waterproof polymer), that fills in the spaces between cellulose microfibrils in the cell walls.

Question 40

function of the casparian strip

Answer 40

The function of the Casparian Strip in the endodermis is to force all water and mineral ions to cross the cell surface membrane of an endodermal cell (i.e. to enter the symplast). The endodermal cell surface membranes are partially permeable and have a carefully regulated selection of ion channels and carrier proteins. In this way, the endodermis acts as a control point, allowing only certain minerals to pass through into the xylem beyond, in carefully controlled quantities and proportions.

Question 41

what is the phloem function?

Answer 41

The phloem is a vascular tissue responsible for transport of sucrose solution from source to sink. The mechanism by which the sucrose solution moves from a source towards a sink in a phloem is called translocation

Question 42

what is a source?

Answer 42

a plant organ or tissue that is a net producer of sugars and so which can load
sucrose into the phloem for export to sinks

Question 43

what is a sink?

Answer 43

a plant organ or tissue that is a net consumer of sugars and so which must unload sucrose from the phloem, which has come from a source.

Question 44

what is translocation?

Answer 44

the movement of sucrose solution in the phloem, by mass flow from source
to sink.

Question 45

what are the two cells types in the phloem?

Answer 45

sieve tube elements and companion cells

Question 46

What are sieve tube elements?

Answer 46

Sieve tube elements are joined end‐to‐end to form long, cylindrical sieve tubes, allowing the flow (translocation) of sucrose solution from source to sink. The end walls between adjoining sieve tube elements are referred to as sieve plates: they contain numerous holes called sieve pores, which allow sucrose solution to flow from one element into the next. The sieve tube elements have some cytoplasm (containing only a few organelles, such as mitochondria) but this is found only at their periphery (edges); the central region of each element is the space in which the sucrose solution is able to flow. The fact that there is little cytoplasm and few organelles means low resistance to the flow of this solution. Sieve tube elements have no nucleus, hence more space is available for the flow of sucrose solution. However, their lack of nucleus means that each sieve tube element is dependent on its adjoining companion cell to keep it alive (e.g. supply of proteins and ATP). A sieve tube element is directly connected to a companion cell via small pores called plasmodesmata, which allow diffusion directly between the two cells (with no need for a membrane to be crossed). Sucrose, amino acids, proteins and ATP can diffuse from cell to the other, according to the direction of the concentration gradient.

Question 47

what are companion cells?

Answer 47

Companion cells support the metabolic needs of sieve tube elements, and play a key role in the loading or unloading of sucrose to/from the sieve tube elements. Unlike sieve tube elements, companion cells have their own nucleus and are completely filled with cytoplasm. The nucleus in a companion cell is responsible for controlling the metabolism of itself and the adjoining sieve tube element; for example, genes in the nucleus are transcribed and then translated in order for proteins to be produced, according to the needs of both cells. Companion cells are smaller in size than sieve tube elements; however, they have very dense cytoplasm contain many organelles and are very metabolically active. For example, there are a large number of mitochondria, which provide ATP via aerobic respiration; this ATP is needed to provide energy for active transport.

Question 48

what is transloaction?

Answer 48

Translocation is the movement of sucrose solution (which may also include other solutes) in the phloem from source to sink. Translocation is an energy requiring process; specifically, the loading of sucrose into the phloem at a source requires energy from ATP. The proposed mechanism by which translocation occurs is called the Mass Flow Hypothesis. Whilst sucrose is considered to be the main solute transported, other compounds are also detected in phloem sap, including amino acids. Most types of solute molecule found in the phloem sap are examples of assimilates: this term refers to organic compounds that are directly (or indirectly) the products of photosynthesis; all types of sugar, including sucrose, and all types of amino acid, are assimilates.

Question 49

what are properties of sucrose making it good for transport?

Answer 49

Sucrose molecules are (relatively) small and highly soluble (having lots of hydroxyl groups that hydrogen bond with water molecules) – sucrose can therefore reach very high concentrations in solution without crystallising;

Sucrose is relatively inert (metabolically unreactive) so is not significantly used up or converted into other products when en route from source to sink.

Question 50

what is sucrose?

Answer 50

disaccharide made of glucose plus fructose

Question 51

what is the mass flow hypothesis?

Answer 51

1. Sucrose is loaded into the phloem companion cells at the source, using an active (i.e. ATP‐requiring) mechanism (see below for detail).
2. Sucrose molecules then diffuse down their concentration gradient into the adjoining phloem sieve tube elements, through the plasmodesmata that directly link the two cell types.
3. The build‐up of sucrose in the sieve tubes decreases their water potential; hence water now enters the sieve tubes (from surrounding tissues), moving by osmosis down the water potential gradient, through the partially permeable cell surface membranes of the sieve tube elements.
4. The influx of water into sieve tubes causes an increase in the hydrostatic pressure of the phloem sap at the source end of the phloem sieve tubes.
5. Meanwhile at a sink, sucrose may be actively unloaded from the phloem, causing an increase in the water potential of the sap in the sieve tubes, such that water leaves the sieve tubes by osmosis; this decreases the hydrostatic pressure of the phloem tissue at the sink.
6. Since the hydrostatic pressure of the sap in the phloem is now higher at the source that at the sink, the phloem sap starts to move (translocate) from the source towards the sink, flowing down the hydrostatic pressure gradient by mass flow; solutes such as sucrose are carried with the water.

Question 52

how is sucrose loaded into the phloem?

Answer 52

1. Specific carrier proteins (called proton pumps), located in the cell surface membrane of companion cells, move hydrogen (H+) ions by active transport from their cytoplasm to the outside of the cell. This requires energy from ATP hydrolysis.
2. This sets up a steep concentration gradient of H+ ions (i.e. a big difference in concentration) between the inside and outside of the companion cell; this difference in the concentration of H+ is also referred to as a proton gradient.
3. H+ ions now diffuse (passively) back into the companion cell, moving down their concentration gradient by facilitated diffusion. They cross the cell surface membrane using a specific type of carrier protein called a co transport protein. This cotransport protein also allow sucrose across the membrane into the companion cell; the sucrose is being moved into the companion cell against its concentration gradient, yet this step does NOT involve energy from ATP hydrolysis; the uptake of sucrose is possible only because its movement into the cells is being coupled to the diffusion of H+ ions, which are moving into the cell down their (steep) concentration gradient.

Question 53

What is Evidence for the active loading of sucrose into the phloem and the mass flow hypothesis?

Answer 53

ATP is detected in phloem sap, suggesting that it is produced in significant quantities in phloem tissue and hence that an active (energy‐requiring) process is involved in transport.

Metabolic inhibitors (e.g. cyanide), which block ATP production by preventing respiration occurring, result in a significant decrease to the rate of translocation in the phloem.

The flow rate of sucrose solution in the sieve tubes is thousands of times higher,than the rate which diffusion alone could achieve, suggesting an active process is involved.

Experiments using the stylets (needle‐like mouthparts) of aphids have demonstrated that the sap in the phloem sieve tubes has a positive pressure (i.e. a pressure higher than the atmospheric pressure or the pressure of the surrounding tissue). Also, that the pressure is highest at the end of the sieve tubes closer to the source, and lower in the regions closer to the sink, i.e. there is a hydrostatic pressure gradient in the sieve tubes, in the direction of source (higher pressure) to sink (lower pressure).

removal of a section of tree bark (which contains phloem) results in swelling above the cut. This is due to sucrose building up as the phloem has been interrupted, lowering water potential and causing water to move into the affected tissue by osmosis.

Question 54

can you draw the roots?

Answer 54

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Question 55

can you draw the stem?

Answer 55

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Question 56

can you draw the leaf?

Answer 56

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