Mass transport in plants
Xerophyte
A plant that is adapted to reduce water loss so that it can survive in very dry conditions
Name the two types of vascular tissues found in plants
Phloem and xylem
Name the 2 important cells in phloem tissue
Sieve tube elements and companion cells
Explain how a xerophyte reduces water loss by having stomata sunk in pits
This traps moist air, increasing humidity and slowing diffusion of water vapour from the stomata
What substance can be used to track the movement of organic substances in a plant
A radioactive tracer
A leaf is an organ. What is an organ?
A group of tissues
Explain how tracer experiments prove the mass flow hypothesis through the phloem
A plant is grown in a lab and one leaf exposed to a carbon dioxide which contains radioactive isotope 14C. This 14CO2 is taken up by the plant during photosynthesis, and then incorporated into sucrose and glucose. Later on, the plant gets killed by freezing it with liquid nitrogen and then placed on a photographic film in the dark. The results show an autoradiograph outlining the compound location containing the 14C. The experiment shows how organic compounds are transported downwards from the leaves to the roots in a plant. The technique can be used to trace the ions, sugars or water.
What evidence is there to prove that there is a pressure gradient in mass flow
Aphids can be used to investigate pressure in the phloem.Aphids pierce the phloem leaving the mouthpart behind which allows the sap to flow out.The sap flows quicker nearer the leaves t than further down the stem and so this is evidence that there's a pressure gradient.
Explain why the values for the pressure in the xylem are negative.
Cohesion tension. Pressure inside xylem lower than atmospheric pressure.
Companion cells
Control the activities of the sieve tube elements.
The student measured the water loss in milligrams. Explain the advantage of using ten leaves when taking measurements in milligrams.
Decreases chance of error. Improves accuracy.
Explain how a xerophyte reduces water loss by deep roots
Deep roots enables it to reach water far underground
One of the two plant species used by the student in this investigation was a xerophyte. Other than the distribution of stomata, suggest and explain the xerophytic features the leaves of this plant might have.
Hairs so 'trap' water vapour and water potential gradient decreased. Stomata in pits so 'trap' water vapour and water potential gradient decreased.Thick layer so increases diffusion distance. Waxy cuticle so reduces transpiration.Curled leaves so 'trap' water vapour and water potential gradient decreased. Spines instead of leaves so reduces surface area to volume ratio.
Explain how a xerophyte reduces water loss by having hairs on its stem
Hairs trap moist air. This increases humidity and slows diffusion of water vapour from stomata.
Give three factors that would result in a high rate of diffusion of molecules of a gas such as carbon dioxide through a membrane.
High concentration gradient. High temperature. High pressure increased. Large surface area. Short diffusion path.
Explain how water enters xylem from the endodermis in the root and is then transported to the leaves. detlteeeeDELETTTE
In the root Casparian strip blocks apoplast pathway and only allows symplast pathway.Active transport by endodermis of ions into xylem. Lower water potential in xylem so water enters xylem by osmosis down a water potential gradient. Xylem to leaf Transpiration from leaves creates cohesion tension water molecules bind to xylem. Creates continuous water column.
How does temperature affect transpiration?
Increasing temperature increases the kinetic energy of the water molecules so they evaporate from the stomata faster. This increases the concentration gradient between the inside and outside of the leaf making water diffuse out faster. This increases the transpiration rate.
Root pressure
Influx of water from the soil. This creates a positive hydrostatic pressure pushing the water into the xylem vessel.
What are the exchange systems in plants?
Leaves and roots
Structure of the xylem
Long continuous hollow tube which has no resistance to water flow. Narrow lumen. Wall made out of lignin which is strong, waterproof, and adhesive. Wall contains pores so that water and minerals can leave.
How does water move up the xylem?
Loss of water at the leaves by transpiration. Water moves from the top of the xylem into the leaf by osmosis (transpirational pull). This applies TENSION to the column of water in the xylem. The column of water moves up as one as the water particles stick together, COHESION. This is the cohesion tension theory. It is supported by capillary action, adhesion and root pressure.
Describe the structure of the xylem
Made up of long narrow cells with cellulose cell walls. As they grow the walls are impregnated with lignin, a non living impermeable substance.
Why does the diameter of a tree decrease during the day?
More light and higher temperature. Increase rate of transpiration. Increase transpirational pull. Water pulled up xylem by cohesion tension because the water particles stick to the wall of the xylem. The walls of the xylem are pulled inwards as a result.
Describe how the students would have returned the air bubble to the start of the capillary tube in this investigation.
Open tap or add water from reservoir.
Explain how the palisade cells are adapted for photosynthesis.
Palisade cells are located near top of leaf, closer to light. They have a large surface area for light. Thin cell wall for short diffusion distance for carbon dioxide. Contains many chloroplasts, the site of photosynthesis.Has a large vacuole, pushes chloroplast to the edge of the cell closer to light.
What is a phloem?
Phloem is a tissue that transports organic substances in plants such as sugar up and down the plant.
Explain the role of the phloem in plants
Phloem is the tissue that transports organic substances in plants.
Name a piece of apparatus that can be used to estimate transpiration rate
Potometer
Explain how a xerophyte reduces water by having less stomata than a mesophyte.
Reduced number of stomata reduces the amount of water vapour that can diffuse out of the stomata and so it reduces the transpiration rate
Explain how a xerophyte reduces water loss by having spines instead of leaves
Reduces surface for water loss by transpiration.
Some plants that live in deserts are xerophytes. Explain three adaptations that these plants might have which reduce water loss.
Rolled leaves traps humid layer. Sunken stomata trap humid layer. Hairy surface traps humid layer. Fewer leaves so fewer stomata; Thick waxy cuticle so less evaporation.
Why is transpiration faster during the day?
Stomata open so photosynthesis increases. More water pulled up by cohesion tension.
Temperature is a factor that affects the transpiration rate. Explain how
The higher the temperature the faster the transpiration rate. Warmer molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the concentration gradient between the inside and outside of the leaf making water diffuse out of the leaf faster.
Light is a factor that affects the transpiration rate. Explain how
The lighter it is the faster the transpiration rate. This is because when it is light the stomata open and carbon dioxide can enter the leaf for photosynthesis.
Humidity is a factor that affects the transpiration rate. Explain how
The lower the humidity the faster the transpiration rate. This is because if the air around the plant is dry then the concentration gradient between the leaf and the air increases which increases transpiration.
Students investigated the effect of removing leaves from a plant shoot on the rate of water uptake. Each student set up a potometer with a shoot that had eight leaves. All the shoots came from the same plant. The potometer they used is shown in the diagram.The students' results are shown in the table.Explain the relationship between the number of leaves removed from the plant shoot and the mean rate of water uptake.
The mean rate of water uptake decreases as the number of leaves are reduced. Less surface area.Fewer stomata so less evaporation by transpiration. Less cohesion tension.
Wind is a factor that affects the transpiration rate. Explain how
The more windier it is the faster the transpiration rate. This is because lots of air movement blows away water molecules from around the stomata. This increases the concentration gradient which increases the transpiration rate.
A student investigated the rate of transpiration from a leafy shoot. She used a potometer to measure the rate of water uptake by the shoot. The diagram shows the potometer used by the student. The student measured the rate of water uptake three times. Suggest how the reservoir allows repeat measurements to be made.
The reservoir would allow water to be added. This will move the position of the bubble to the start.
State three types of evidence that prove the mass flow hypothesis.
The use of radioactive tracers, ringing experiments and from the use of sap sucking insects.
Potometers estimate transpiration rates by measuring the water uptake by the plant. Suggest an assumption that must be made when using this apparatus.
The water uptake by the plant is directly linked to water loss by the leaves
How does wind affect transpiration?
The windier it is the faster the rate of transpiration. This is because the increased air movement blows away water molecules from the stomata. This increases the concentration gradient between the leaf and the air so transpiration rate increases.
How does light affect transpiration?
There is a positive correlation between light intensity and transpiration rate because the stomata open when its light to let in carbon dioxide for photosynthesis. The higher the light intensity the more stomata open which means more water can diffuse out of the leaf increasing transpiration rate.
Suggest and explain one other way in which companion cells are adapted for the transport of sugars between cells
They contain mitochondria which releases energy for active transport.
A potometer measures the rate of water uptake rather than the rate of transpiration. Give two reasons why the potometer does not truly measure the rate of transpiration.
Water used for turgidity. Water used in photosynthesis. Water produced in respiration. Apparatus not sealed.
What is a xylem?
Xylem tissue transports water and mineral ions in solution up the plants from the roots to the leaves
Compare and contrast the xylem and the phloem
Xylem transports water and minerals whilst the phloem transports glucose. Xylem transports in one direction from roots to leaves, phloem transports in both directions.
Explain how a xerophyte reduces water loss by having shallow spreading roots
shallow spreading roots allows them to easily collect occasional rainfall
Name the two types of tissue involved in transport in plants
xylem and phloem
Clover leaves have three leaflets at the top of a vertical stalk. During daylight these leaflets are normally extended. At night the leaflets fold down like an umbrella, as shown in the diagram. The extending and folding is due to changes in the water content of certain cells at the junction between the three leaflets. The leaflets extend when the cells gain potassium ions from neighbouring cells in the stalk. Folding results when these potassium ions pass back again.Suggest an explanation of the method by which the leaflets extend.
Active transport of potassium ions into cells. Cells have a lower water potential so water enters by osmosis. The increased hydrostatic pressure extends leaflets.
A scientist placed plant cells in solutions containing different concentrations of calcium ions. She measured the rate uptake of calcium ions by plant cells. Figure 7 shows her results. What can you conclude from figure 7 about the processes involved in the uptake of calcium ions by these plant cells. use evidence from figure 7 to support your answer.
Between 0.0 and 0.1 calcium ions cannot enter by facilitated diffusion as there is no diffusion gradient for entry into the cell. Between 0.1 and 0.3 calcium ions enter by facilitated diffusion as calcium ions enter without oxygen.
What evidence is there to prove that active transport is involved in mass flow
If a metabolic inhibitor, which stops ATP production, is put into the phloem, then translocations stops. This is evidence that active transport is involved because active transport requires ATP
Difference between the cell wall in xylem and the cell wall in sieve tube elements
Sieve tube elements have walls made of cellulose without any lignin whilst xylem have lignin in their cell walls
Briefly describe translocation
Sucrose moves into phloem by active transport. Water moves into phloem by osmosis. Hydrostatic pressure builds up. Pressure moves down its pressure gradient taking sucrose with it. Sucrose at the sink is used up in respiration or stored as starch.
State the 4 main factors that affect transpiration
Temperature, humidity, Light and wind
Explain how a xerophyte reduces water loss by having a waxy cuticle
Waxy cuticle is impermeable to water preventing evaporation
Transpiration
Evaporation of water through the stomata
Describe and explain three ways in which the sieve tubes are adapted for their transport function
Gaps in sieve plates for large molecules to pass through which allows for unrestricted mass flow. Elongated sieve tubes means there are fewer sieve plates to restrict flow. No organelles to allow rapid and unrestricted mass flow.
The mass flow hypothesis of translocation consists of three stages. Briefly describe these stages
Organic compounds are loaded into the sieve tube elements in the leaves, resulting in an increase in hydrostatic pressure caused by osmotic influx. The increase in hydrostatic pressure pushes the contents of the sieve tube elements down a pressure gradient. Organic compounds are unloaded into cells at the sinks resulting in a decrease in hydrostatic pressure.
Explain the role of the roots
Roots absorb water by osmosis and minerals by active transport. Plants need water for photosynthesis, cytoplasm hydration, turgidity of cells. Plants need magnesium to make chlorophyll, nitrate to make amino acids, phosphate to make phospholipid, ATP and DNA.
Give six precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot.
Seal joints to ensure airtight. Cut shoot under water to prevent air bubbles. Cut shoot at a slant to increase surface area. Dry off leaves. Shut tap. Note where bubble is at start.
Suggest and explain the ways in which sieve cells are adapted for mass transport.
Sieve cells have few organelles, very little cytoplasm, large vacuole, and thick walls. This leads to more flow because strong walls resist pressure so flow is unrestricted.
A student investigated the rate of transpiration from a leafy shoot. She used a potometer to measure the rate of water uptake by the shoot. The diagram shows the potometer used by the student. The student cut the shoot on a slant and put it into the potometer under water. Explain why.
Slant increases surface area. Prevent air entering. Allows for a continuous water column.
Scientists measured the rate of water flow and the pressure in the xylem in a small branch. Their results are shown in the graph. Use your knowledge of transpiration to explain the changes in the rate of flow in the xylem shown in the graph.
Stomata open. Transpiration highest around midday. Middle of day warmer.Increased water potential gradient. Cohesion between water molecules.
Name the main carbohydrate transported by phloem tissue.
Sucrose
Explain what is meant by translocation
Translocation is the movement of solutes from 'source', that is where the solute is made, to 'sink' that is where they're needed in a plant. Solutes are sometimes called assimilate. It's an energy requiring process that happens in the phloem
Name four processes that cause water to move up a plant stem.
Transpiration, root pressure, cohesion tension and capillary action.
Describe the function of the phloem
Transports organic material (e.g. sucrose) up and down a plant
Function of sieve tube elements
Transports organics substances from source to sink
Describe the structure of a leaf
Upper layer is called the upper epidermis and it is coated with a waxy cuticle which acts as a barrier to reduce water loss. Beneath the upper epidermis are palisade cells which is where photosynthesis takes place. Beneath palisade cells are Spongy mesophyll cells which are loosely packed leaving air spaces to allow ease of gas exchange. The lower layer called the lower epidermis.
Root pressure
Water absorbed at the roots pushes the column of water up slightly by hydrostatic pressure.
Capillary action
Water automatically moves up narrow lumen of xylem.
Explain the cohesion tension theory of water transport.
Water evaporates from the leaves by transpiration. This creates tension which pulls more water into the leaves. In the xylem, there's a continuous column of water. Each water molecule is joined by hydrogen bonds. This means when one water molecules is pulled out of the xylem by osmosis through the leaves, it pulls all the other water molecules behind it up through the xylem. This creates tension drawing water up towards the xylem against gravity. Water enters the stem through the roots.
A student investigated the rate of transpiration from a leafy shoot. She used a potometer to measure the rate of water uptake by the shoot. The diagram shows the potometer used by the student. The student assumed that water uptake was equivalent to the rate of transpiration. Give three reasons why this might not be a valid assumption.
Water is used to provide turgidity and support. Water is used in photosynthesis and produced in respiration. The apparatus may not be sealed.
Adhesion
Water particles stick to lignin in wall of xylem.
Organic compounds made in the leaves of a plant and can be transported to the plant's roots by translocation in the phloem. One theory of translocation states that organic substances are pushed from a high pressure in the leaves to a lower pressure in the roots. Describe how a high pressure is produced in the leaves.
Water potential becomes lower as sugar enters phloem. Water enters phloem by osmosis. Increased volume of water causes increased pressure.
Explain the mass flow hypothesis as a mechanism for translocation
Active transport is used to actively load the sucrose from photosynthesis from companion cells into the sieve tubes of the phloem at the leaves.This lowers the water potential in the sieve tube so water enters the tubes by osmosis from the xylem and companion cells.This creates a high pressure inside the sieve tubes at the source end of the phloem. At the sink end solutes are removed from the phloem used up. This increases the water potential inside the sieve tubes so water also leaves the tubes by osmosis.This lowers the presser inside the sieve tube at the sink end of the phloem.This results in a pressure gradient from the source end to the sink end. And this gradient pushed solutes along the sieve tubes towards the sink where they will be used in respiration or stored
Clover leaves have three leaflets at the top of a vertical stalk. During daylight these leaflets are normally extended. At night the leaflets fold down like an umbrella, as shown in the diagram.The extending and folding is due to changes in the water content of certain cells at the junction between the three leaflets. The leaflets extend when the cells gain potassium ions from neighbouring cells in the stalk. Folding results when these potassium ions pass back again. Suggest an explanation of the method by which the leaflets extend.
Active transport of potassium ions into cells. Cells have a lower water potential than neighbours so water enters cells by osmosis down water potential gradient. This increases pressure. The turgidity extends leaflets.
They measured the rate of photosynthesis in a single leaf, L, before and after ringing the stem. They also measured the rate of respiration in a single pod, P, before and after ringing. The pod contained many developing seeds. The scientists carried out the same measurements on a control plant which was not ringed. The diagram shows the positions of the leaves and pods on the stem and where the stem was ringed Describe and explain these results.
After ringing rate of respiration remains constant in Pod P.Energy requirements the same in Pod P) In leaf L After ringing rate of photosynthesis decreases and then levels off. Pod P is the only sink. Less sugar used / sugar accumulates. Rate at which sugars are used controls rate of photosynthesis
Describe the processes involved in the transport of sugars in plant stems.
At source, sucrose is actively transported into the sieve tubes by companion cells. This lowers water potential in phloem so water enters by osmosis.This produces high hydrostatic pressure. Mass flow towards the sink.At sink sugars are unloaded and either used in respiration or stored as starch.
A scientist placed plant cells in solutions containing different concentrations of calcium ions. She measured the rate uptake of calcium ions by plant cells. Figure 7 shows her results. Suggest one way in which the scientist could have ensured the solutions she used for curve X contained no oxygen.
Boil water as the gases will evaporate. Add oil ontop of solution.
A student investigated the rate of transpiration from a leafy shoot. She used a potometer to measure the rate of water uptake by the shoot. The diagram shows the potometer used by the student. The student wanted to calculate the rate of water uptake by the shoot in cm³ per minute. What measurements did she need to make?
Distance and time. Radius of capillary tube
Describe the role of enzymes in translocation
Enzymes maintain a concentration gradient from the source to the sink by changing the solutes at the sink by breaking them down or by making them into something else. This makes sure that there is always a lower concentration at the sink than at the source.
During photosynthesis, plants take in carbon dioxide. As plants need light to photosynthesise plants open their stomata during the day and close them at night. Suggest why a xerophyte would open their stomata at night and close them at midday
Evaporation rate would be slower at night because in the dessert its very cold. Evaporation rate would be much higher midday because it would be very hot and the plant could lose water by transpiration very rapidly.
The leaflets may fold down during the day if the soil becomes dry. The stomata are situated on the underside of the leaflets. Suggest how folding helps the clover plant to reduce the effects of water shortage.
Humid air trapped, so less transpiration of water from stomata.
Mass flow hypothesis
Hydrogen Ions are actively transported from companion cells into source. This lowers the hydrogen ion concnettration in the companion cells so hydrogen ions therefore diffuses back into companion cells from source and as they do, they pull in sucrose with them by co-transport. sucrose then diffuses into sieve tube. This lowers the water potential of sieve tube so water follows by osmosis. This water will carry the sucrose by hydrostatic pressure. Sucrose unloaded from phloem at sink. sucrose moves out of phloem/sieve tube into sink by diffusio water follows by osmosis. Sucrose is used for respiratior or stored as starch.
What evidence is there to show that in mass flow there's a downward flow of sugars.
If a ring of bark which includes the phloem and not the xylem is removed from a woody stem, a bulge forms about the ring. The fluid from the bulge has a higher concnetration of sugars than the fluid from below the ring, This is evidence that there's a downward flow of sugars.
Describe the mass flow hypothesis for the mechanism of translocation in plants.
In leaf, sugars are actively transported into phloem by companion cells. This lowers water potential of sieve tube and water enters by osmosis. Increase in pressure causes mass movement towards root. Sugars converted in root for respiration or for storage.
Describe the mass flow hypothesis for the mechanism of translocation in plants
In source sugars actively transported into phloem by companion cells. This lowers water potential of sieve cells and so water enters phloem by osmosis. This Increase in hydrostatic pressure causes mass movement towards sink. Sugars are used in root for respiration or are converted to starch for storage.
Sucrose is moved through phloem by mass flow. Describe how.
Sucrose loaded into phloem by active transport; Involving companion cells. Reduced water potential. Water moves in by osmosis.High hydrostatic pressure. Lower at sink so sucrose pushed towards sink.
The products of starch breakdown are transported and used for the growth of new shoots.Explain how it is transported and used.
Sucrose transported in phloem by mass flow. Glucose converted to cellulose.
Describe the adaptations of xerophytes
Sunken pits, waxy cuticle, Rolled leaves, Spines instead of leaves, Hairy stem, Reduced number of stomata, Deep roots, Shallow spread roots,
Some of the energy in producers is used to move mineral ions from cells in the root into the root xylem. Explain how this energy is used.
The ATP is used in active transport. Carrier proteins.
How does humidity affect transpiration?
There's a negative correlation between humidity and transpiration rate. This is because having dry air around the plant will increase the concentration gradient between the inside and outside of the leaf making water diffuse out faster, increasing transpiration.
Xylem transports water through a plant. Describe and explain how the cells of xylem are adapted for this function.
Thick cell walls withstand tension.Lignin in cell walls.Walls waterproof. Xylem cells have no end walls so this allows for a continuous column of water. Xylem vessels are stacked on top of each other so a continuous column of water. Have no cytoplasm so reduces resistance to flow of water. Xylem cells have pores in side walls which enables sideways flow.Narrow tubes increased surface area for adhesion. Molecules in cell walls allows adhesion.
Avoiding excess water loss is one of the major problems facing plants and small rodents living in a desert.Describe and explain four structural adaptations of desert plants which limit water loss.
Thick waxy cuticle impermeable to water. Stomata on lower surface and so out of direct sunlight which reduces evaporation rate. Sunken stomata, rolled up leaves, and hairs keeps saturated air near leaf and so reduces concentration gradient. Reduced leaves so less surface area so diffusion is slower.
Translocation of solutes in plants can be modelled in an experiment using radioactive tracers. Explain how
This can be done by supplying part of a plant with an organic substance that has a radioactive label e.g. carbon dioxide with the radioactive isotope carbon 14. The radioactive carbon will be incorporated into organic substances produced by the leaf which will move around the plant by translocation. A technique called autoradiography can be used to track the movement of these substances. The plant is killed and placed under a photographic film to reveal where the radioactive tracer has spread to in a plant.
The ringing experiment involves removing a selected part of a stem tissue in the form of a ring. Explain how ringing experiments prove the mass flow hypothesis through the phloem.
Three plants are selected in the ringing experiment. In plant A , only the xylem is removed, in plant B only the phloem is removed, and plant C is the control. The three plants are kept under sunlight to allow photosynthesis to take place. After some time, it is noticed that in plant B, the portion of the cut stem immediately bulges above the ring while plants A and C have no sign of bulging. This works on the foundations of downward translocation taking place in the phloem as plant's B phloem was removed. Such instances didn't happen in plants A or C as the phloem was still intact. This suggests that the removal of xylem had no effect on the translocation.
Scientists investigated the rate of water movement through the xylem of a twig from a tree over 24 hours. The scientists also measured the diameter of the trunk of the tree on which the twig had been growing. The diameter was less at 12.00 than it was at 03.00 hours. Explain why the diameter was less at 12.00 hours.
Water pulled up trunk moves up at fast rate.Water column under tension. Adhesion between water and xylem which pulls xylem in.
The student then compared the rate of transpiration from the two species of plant. She did this by measuring the rate of water uptake by each plant species. Suggest four reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration.
Water used for turgidity. Water used in photosynthesis. Water used in hydrolysis. Water produced during respiration;
The stomata on the leaves of pine trees are found in pits below the leaf surface. Explain how this helps to reduce water loss.
Water vapour accumulates so diffusion gradient reduced so less water diffuses out of stomata.