Water transport in plants

Explain how water enters xylem from the endodermis in the root and is then transported to the leaves.

(In the root) 1. Casparian strip blocks apoplast pathway, allows only symplast pathway 2. Active transport BY ENDODERMIS 3. Of ions in to xylem 4. Lowers water potential in xylem. 5. Water enters xylem by osmosis down a water potential gradient. (Xylem to leaf) 1. Evaporation from leaves 2. Creates cohesion between water molecules = transpiration pull 3. Create tension between water molecules = negative pressure 4. Adhesion between water molecules + xylem 5. Creates continuous water column.

Explain how the cuticle reduces water loss.

(waxy so) impermeable to water/waterproof/stops water passing through

Sodium chloride and potassium chloride had different effects on the opening and closing of stomata. Suggest an explanation for this

1) K + (ions/chloride) cause stomata to open/ Na + (ions/chloride) cause stomata to close; 2) more K + (ions/chloride) in (guard) cells/ Less Na + (ions/chloride) in (guard) cells; 3) K + (ions/chloride) lowers water potential inside/ Na + (ions/chloride) lowers water potential outside

Transpiration pull is...

1. A passive process = does not require metabolic energy to take place 2. The xylem vessels which water passes through are dead = thus cannot actively move water. 3. As they are dead = their end walls can break down, this means that the xylem forms a series of continuous, unbroken tubes from root to leaves. = This is essential to the cohesion-tension theory of water flow up the stem. 4. Energy is nevertheless needed to drive the process of transpiration = ENERGY IN THE FORM OF HEAT FROM THE SUN, EVAPORATING WATER.

How + why does water move into the root hair cells

1. A soil solution which contains small quantities of mineral ions. 2. Contains mostly water = very high water potential only slightly less than 0 3. Root hairs have sugars + amino acids + mineral ions dissolved inside of them = LOW WATER POTENTIAL OF CELL 4. Water therefore moves by osmosis form the soil solution into the root hair cells down this water potential gradient. 5. After being absorbed into the root hair cell, water continues its journey through the root and into the xylem in two ways.

Describe what causes root pressure

1. Active transport BY ENDODERMIS 2. Ions/salts into xylem 3. Lowers water potential in the xylem 4. Water enters by osmosis

How is root pressure produced

1. Active transport of mineral ions into the xylem. 2. FROM ENDODERMAL CELLS 3. Casparian strip prevents return of water

Problem with this experiment

1. Assumes water taken up is the water lost in transpiration. 2. BUT some of the water taken up by be being used in PS, hydrolysis (e.g. starch to glucose) and making plant cells turgid, to ensure plant remains upright, doesn't wilt.

The diameter of the trunk decreased during the same period, reaching its minimum when the flow rate was highest. Use your knowledge of the cohesion-tension theory to suggest an explanation for this decrease.

1. Attraction/adhesion of water molecules to xylem walls 2. Results in tension as water pulled up stem 3. Pulling in walls

How does the endodermis affect movement of water through apoplas pathway

1. Casparian bands 2. Impermeable = waterproof barrier 3. LOWER WATER POTENTIAL IN CYTOPLASM OF ENDODERMIS CELL 4. Water enters symplast pathway down water potential gradient 5. BY OSMOSIS

Why does water move from apoplast to symplast pathway when it reaches the tissue labelled W

1. Casparian strip 2. Impermeable barrier to water movement 3. Water enters cell along WP gradient

The endodermis controls the entry of substances into the xylem. Explain how

1. Casparian strip 2. So water/mineral ions cannot move through apoplast/cell wall 3. Only move through symplast, HAVE TO CROSS CELLE MEMBRANE. 4. Active transport of mineral ions into xylem 5. AT BY ENDODERMIS

Whole exam question: explain how water enters the xylem from the endodermis in the root and is then transported to the leaves. = FROM THE ENDODERMIS IS A KEY POINT!

1. Casparian strip blocks the apoplast path way/only allows water to move through the symplast pathway 2. Ions pass into the xylem 3. By active transport 4. Reduces water potential in the xylem, allowing water to enter by osmosis. 5. As water evaporates from the leaf/transpiration = more water molecules get pulled up the xylem. 6. As water molecules form a continuous column 7. Due to hydrogen bonds between them = COHESION

Evidence for cohesion-tension theory:

1. Change in diameter of tree trunks according to rate of transpiration = during the day when transpiration rate is greatest = there is more tension = more negative pressure in the xylem. 2. Water pulled up trunk/moves up at fast rate 3. Water column under tension 4. Adhesion between water + cells walls/xylem 5. Pulls xylem in 6. This is more so when TR is greater = causes trunk to shrink in diameter 7. At night, when transpiration is at its lowest, there is less tension in the xylem and so the diameter of the runk increases.

What one must keep the same

1. Circumference of capillary tube = bigger circumference = molecules of water would not move as far 2. Time 3. Species of plant = transpire at different rates 4. Surface area of the leaves

Measurement of water uptake using a potometer = used to estimate transpiration rate = assumed that water uptake is directly related to water loss by the leaves.

1. Cut a shoot underwater to prevent air getting in the xylem + cut it at a slant to increase SA available for water uptake. 2. Assemble the potometer in water and insert the shoot under water so no air can enter. 3. Remove apparatus from the water but keep end of the capillary tube submerged in a beaker of water. 4. Dry the leaves, allow time for shoot to acclimatise, shut the tap. 5. Remove the end of the capillary tube from the beaker until one air bubble has formed, then put the end of the tube back in water. 6. Record the starting position of the bubble. 7. As the plant takes up water, the air bubbles moves along the scale. 8. Time it. 9. Record the distance moved by the bubble per unit time, e.g. cm3 per minute 10. Repeat the experiment to increase the reliability of your results. = you can repeat for different conditions.

The student wanted to calculate the rate of water uptake by the shoot in cm3 per minute. What measurements did she need to make?

1. Distance and time; 2. Radius/diameter/area (of capillary tube);

Passage of water into the xylem/ROOT PRESSURE

1. Endodermal cells actively transport salts/mineral ions into the xylem. = this process requires energy and therefore can only occur within living tissue. 2. Takes place along carrier proteins in the cell surface membrane. 3. The active transport of mineral ions in tho the xylem by endodermal cells creates a LOWER WP IN THE XYLEM. 4. Water now movies into the xylem, by osmosis, down a water potential gradient. 5. This creates a force that helps to move water up the plant = root pressure 1. Ions move into the xylem from the endodermis 2. By active transport 3. Lowers water potential in the xylem 4. Allowing water to enter the xylem by osmosis

Describe the role of tissue X in the entry of water and mineral ions into the xylem.

1. Endodermis has waterproof/casparain strip 2. SO water + mineral ions must cross cell membranes + go via cytoplasm/symplast = apoplast blocked 3. Mineral ions move by AT into xylem 4. Create WP gradient for water to diffuse into xylem

Describe how water moves up through the trunk of a tree to the leaves

1. Evaporation from the leaves 2. THROUGH THE STOMATA 3. Cohesion of water molecules = water drawn up as column/continuous stream 4. Leaf cells = lower water potential 5. More water enters by osmosis 6. ALSO adhesion of water to the walls 7. Root pressure also forces some water up

Symplastic pathway (symplast) = Goes through the cytoplasm = Cytoplasm of neighbouring cells connect through plasmodesmata = small channels in the cell walls = Continuous column of cytoplasm extending through the root hair cell to the xylem as the centre of the root = This is how water passes through

1. Goes through the cytoplasm 2. Water entering by osmosis increases the water potential of the root hair cell. 3. Root hair cell now has a higher water potential than the first cell in the cortex. 4. Water therefore moves from the root hair cell to the first cell in the cortex by osmosis, down a WP gradient 5. First C cell now has a higher WP that its neighbour, then 2nd 6. Water moves from 1 to 2 via osmosis, WP gradient. 7. 2nd cell now has higher WP than third, water moves by osmosis into third, etc 8. At the same time, loss of water from the first cortical cell lowers its WP = causes more water to enter by osmosis from the root hair cell. 9. WP is therefore set up across all the cells of the cortex, which carries water along the cytoplasm fro the root hair cell to the endodermis.

Apoplastic pathway (apoplast) = goes through cell walls = walls are very absorbent

1. Goes through the walls 2. What water in the apoplast pathway reaches the endodermis cell though, it is blocked by a waxy strip in the cell walls = the Casparian strip. 3. Casparian strip = a waterproof band 4. At this point, water is forced into the cytoplasm where it joins water that has arrived there by the symplastic pathway.

How do hairs reduce water loss

1. Hairs trap a layer of air/trap water vapour/maintain humidity 2. REDUCES WP GRADIENT

Link between humidity and water loss

1. Humidity increases, rate of trans decreases 2. High humidity = lots of water in air, increased saturation 3. Increased water potential 4. Reduced diffusion gradient/reduced water potential gradient 5. Slower rate of evaporation

Evidence for cohesion-tension theory:

1. If an xylem vessel is broke and air enters it, tree can no longer draw up water = BECAUSE the continuous column of water is broken and so water molecules can no longer stick together. 2. When a xylem vessel is broken, water does not leak out = under pressure. = Instead air is drawn in, which is consistent with it being UNDER TENSION.

Explain the increase in the flow rate between 1000 and 1400 hours. increase in transpiration rate/evaporation due to increase in temperature ; increased (kinetic) energy of water molecules; OR increase in light (intensity) increases transpiration rate/evaporation; greater stomatal aperture / more stomata open; increase in flow rate due to cohesion/attraction of water molecules;

1. Inc in transpiration rate, evaporation due to (A) temp increase = increase kinetic energy of water molecules (B) increase in light intensity = greater stomatal aperture, more stomata open = INCREASE IN FLOW RATE DUE TO COHESION/TENSION/ATTRACTION OF WATER MOLECULES.

Why does branch get thinner

1. Increase in transpiration produces higher tensions 2. Reduces the pressure in the xylem 3. Negative pressure reduces the diameter 4. Adhesive forces between water molecules and xylem walls

Suggest an explanation for the difference in transpiration rate between conditions A and C = low and high temperature

1. Increases kinetic energy 2. Water molecules move MORE AND FASTER 3. Increases diffusion/evaporation

TRANSPIRATION PULL

1. Increasing transpiration from leaves. 2. TRANSPIRATION INCREASE TRANSPIRATIONAL PULL, CREATES TENSION 3. Waters molecules are cohesive, stick together 4. PRODUCES WATER COLUMN.

Explain why the values for the pressure in the xylem are negative

1. Inside xylem is lower than atmospheric pressure. = because water is under tension

Give one environmental factor that the student should have kept constant during this investigation.

1. Light (intensity) / temperature / air movement / humidity;

Explain the evidence from the diagram that the rate of diffusion of water vapour is greater in the direction of arrow A than arrow B

1. Lines closer together between A and stoma 2. So a steeper concentration gradient

Explain how xylem tissue is adapted for its function

1. Long cells, tubes with no end walls = continuous water column 2. No cytoplasm/organelles = to impede, restrict, obstruct water flow 3. Thick wall = support, withstand pressure, water proof, keeps water in cells

Describe how this apparatus could be used to measure the rate of water uptake by a leafy shoot in cm3 per minute.

1. Measure time taken 2. For bubbles to move a given distance 3. Calculate cross sectional area of capillary tubing 4. Calculate volue of water taken up AND SO LOST, per unit time.

Marram grass has specialised cells within its leaves that enable the leaf to roll in on itself. In conditions of water shortage the leaf rolls with the lower surface inwards. This adaptation enables marram grass to withstand very dry conditions. Explain how.

1. Most stomata on inner lower surface 2. Rollinng protects stomata from wind + increases humidity around stomata 3. Reduces diffusion/WP gradieny 4. Also reduces exposures to SUN 5. SO LESS WATER LOST BY EVAPORATION/TRANSPIRATION.

Apoplast pathway

1. Movement of water through cell walls

The results of this investigation support the cohesion tension theory. Explain how.

1. Movement starts/peaks earlier in branches/higher up; 2. Creates tension/'negative pressure'/'pull'

Evidence for the existence of root pressure

1. Pressure increases with a rise in temperature = active transport increases as rate of respiration increases = decreases at lower temps 2. Metabolic inhibitors like cyanide prevent most energy release by respiration = RP ceases. 3. A decrease in the availability of oxygen/respiratory substrates causes a reduction in root temperature.

Explain the relationship between the number of leaves removed from the plant shoot and the mean rate of water uptake.

1. Reduced surface area 2. Fewer stomata 3. Leads to less evaporation/transpiration 4. (WATER UPTAKE!!!) = Less cohesion/tension/pulling force

Give three structural adaptations and explain how each could help these plants survive periods of water shortage.

1. Reduced surface area of leaf for water loss 2. Thick water proof cuticle = increase DD 3. Inrolling of leaf = reduce WP gradient/air movement across stomata/traps moist, humid air which becomes saturated/reduces surface area 4. Sunken stoma = reduce WP gradient = traps air = reduces surface area 5. No stoma on upper surface = reduces sa for evap 6. Succulent, swollen stem = water sotrage 7. Deep long roots = absorb water deepr in the soil 8. Shallower roots = absorb surface water. 9. Extense root system = large SA for water absorption.

Advantage of having no root hairs during dry season

1. Reduced water loss 2. From large root hair surface into the soil

Suggest an explanation for the difference in transpiration rate between conditions A and B. = still air and moving air

1. Removes water vapour/moisture/saturated air; 2. Increases water potential gradient/more diffusion/more evaporation;

Explain rate of water flow DURING THE DAY

1. Rise + fall, maximum at midday 2. Rise related to increasing temp/light intensity = inc rate of evaporation 3. Fall related to stomatal closure = explanation of this being to do with light = reduces water loss, less evap, transp less, rate of flow less

Water entering roots overall

1. Root hair cells absorb water 2. Once absorbed water has to get through the cortex 3. Including the endodermis 4. Before it can reach the xylem

Describe how water is taken up by root hairs and reaches the xylem in the root.

1. Root hair cells have lower WP than soil 2. Due to active uptake of ions 3. So water enters root hair cells by osmosis 4. Water passes along apoplast pathway 5. Through water filled spaces ofcells walls 6. Passes along symplast pathway 7. Through cytoplasm/plasmodesmata, water moves from cells to cell by osmosis. 8. Casparian strip forces wwater from apoplast into cytoplasm/symplast 9. Water moves from cell to cell/enters xylem by osmosis. 10. Ions actively transported by endodermis into xylem + low is water potential.

Movement of water up the stem = the main force that pulls water up the stem of a plant is the evaporation of water from leaves = transpiration.

1. STOMATA = Stomata are open = Water molecules diffuse out of the air spaces into the surrounding air. = Water lost from air spaces is replaced by water evaporating from the cell walls of surrounding mesophyll cells. = By changing the size of their stomatal pores, plants can control their rate of resp. 2. MOVEMENT OF WATER ACROSS THE CELLS OF LEAF = Water is lost of mesophyll cells by evaporation from their surfaces int0 the air spaces = This is replaced by water reaching the mesophyll cells from the xylem by either the apoplastic of symplastic pathways. = Symplastic pathway = water movement occurs because: (a) mesophyll cells lose water to air spaces (b) these cells now have a lower water potential (c) so water enters by osmosis from neighbouring cells (d) loss of water from these neighbouring cells lowers their water potential (e) they in turn take in water from their neighbours by osmosis. = WP is established that pulls up water fro mthe xylem, across the leaf mesophyll, out into the atmosphere. 3. MOVEMENT OF WATER UP THE STEM IN THE XYLEM = Two mains factors responsible for movement of water from roots to leaves: cohesion-tension and root pressure

Advantage of using same plant in every experiment

1. Same num of leaves 2. Same SA of leaves 3. Same age/thickness of cuticle

Give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot.

1. Seal joints with vaseline / ensure it is airtight / water tight 2. Cut shoot under water 3. Cut shoot at a slant 4. Dry off leaves 5. Insert into apparatus under water 6. Ensure no air bubbles are present 7. Shut the tap 8. Note where bubble is at start.

Size of stomata + water loss

1. Smaller stomata have faster water loss 2. Diffusion gradient not as steep as diameter increases

Why is less water lost through the upper surface of the leaf?

1. Stomata on lower epidermis 2. THICKER WAXY CUTICLE ON UPPER SURFACE

Covered stomata with grease

1. Stomata on upper surface covered 2. Less evaporation/transpiration/diffusion out 3. OR CLOSE DUE TO LACK OF LIGHT grease provides.

Explain the change in the rate of water movement through the xylem between 06.00 and 12.00 hours.

1. Stomata open 2. PS increases/transpiration increases 3. This creates tension (suction) which pulls more water into the leaf 4. Cohesion between water molecules means when some molecules are pulled into the leaf others follow. 5. Means the whole column of water in the xylem moves upwards. 6. Cohesion tension

Give two features of the Hakea leaf shown in the drawing that reduce the rate of transpiration. In each case explain how the feature contributes to a lower rate of transpiration. Sunken stomata/description; Reduces water potential/diffusion/concentration gradient / traps humidity/water molecules;

1. Sunken stomata 2. Reduces water potential gradient = traps humidity/water molecules above stomata AND 1. Thick cuticle 2. Impermeable to water (Accept waterproof) / increases diffusion pathway/distance

Two shoots were kept in the same environmental conditions. Give one measurement that you would need to make to be able to compare water uptake by the shoots.

1. Surface area of leaves/number of stomata/number of leaves/size of leaves 2. As larger number/area means more water loss 3. Diameter of steam = thicker stem = more xylem

Transpiration flow continues, even when plant stem is put in solution which kills plant cells

1. Water moves in dead cells = xylem is non living tissue 2. Process is passive = no energy needed

Describe and explain how water in the mesophyll cells passes out of the leaf (pathway from cells) along cell walls / through spaces and out through stoma(ta); by diffusion (disqualify if osmosis mentioned); down a WP/diffusion/concentration gradient;

1. Water passes from cells 2. A long cell walls 3. Through air spaces 4. Out through stomata 5. BY DIFFUSION DOWN A DIFFUSION GRADIENT 6. OR DOWN A WATER POTENTIAL GRADIETN

The diameter was less at 12.00 than it was at 03.00 hours. Explain why the diameter was less at 12.00 hours.

1. Water pulled up the trunk 2. Moves up at a fast rate 3. Creates tension (which pulls more water up into leaf) 4. Water column under tension 5. Sticking/adhesion between water and cells/walls/xylem 6. Pulls xylem in/negative pressure

Factors affecting transpiration rate

1. Temperature, affects two factors: = how much water the air can hold, wp of air = the speed at which water molecules move - A rise in temperature = increases the KE and hence the movement of water molecules. = Move more + faster = Faster evaporation = This means that the water evaporates more rapidly from the leaves, so ROT increases. = Also decreases the humidity (number of water molecules in the air) of the air outside the leaf, decreases its water potential, steeper gradient = faster rate of evaporation = inc ROT, (reduction in temperature has a reverse effect, it reduces transpiration rate. 2. Light = The lighter it is the faster the ROT = Because stomata opens when it gets light. = When stomata are open, water moves out of leaf down WP gradient and into atmosphere. = Inc in light intensity = inc in ROT. 3. Humidity = the lower the humidity, faster the ROT. = Greater the water potential gradient between inside + outside of leaf = increase conc gradient = Faster water will move out = greater the rate of transpiration. or... = more humid, reduce conc gradient. = slows rate of diffusion 4. Wind = The windier it is, the faster the ROT, movement of air around the leaf will disperse the humid layer at the leaf surface. = Lots of air movement blows away water molecules around the stomata. = Increases the WP gradient/maintains WP gradient = Increases the ROT. = The faster the wind, the more rapidly humid air is removed and the greater the ROT. (less windy) = As water diffuses out through the stomata, it accumulates as vapour around the stomata on the outside of the leaf. = WP around the stomata is therefore increased, reducing the WP gradient, reducing ROT.

What is transpiration

1. The evaporation of water from a plant's surface, especially the leaves. 2. Water evaporates from the moist cell walls and accumulates in the air spaces between cells in the leaf. 3. When the stomata open, it moves out of the leaf down a water potential gradient as there is less water outside.

Xylem transports water through a plant. Describe and explain how the cells of xylem are adapted for this function.

1. Thick walls = withstand tension/negative pressure 2. Xylem cells have no end walls = continuous column of water 3. Xylem vessels stacked on top of each other = continuous column of eater 4. Have no cytoplasm = hollow, reduces resistance to flow of water, cont column 5. Narrow tubes = increased surface area for adhesion 6. Molecules in cell walls = allow adhesion

2 measurements you need to make

1. Time 2. Distance moved by air bubbles

Use your knowledge of transpiration to explain the changes in the rate of flow in the xylem shown in the graph.

1. Transpiration highest around midday 2. Stomata open 3. Middle of day = warmer + lighter SINCE IT IS RATE OF FLOW YOU HAVE TO SAY: 4. Increased water potential gradient 5. Increased tension 6. Cohesion between water molecules

Remember:

1. Untreated plants release O2 in the light by PS. 2. Untreated plants use oxygen in dark, respiring (negative O2 uptake 3. Amitrole treated plants do not release, only use oxygen 4. So must not be able to PS only respire

Cohesion-tension theory = helps water move up plants from roots to leaves AGAINST THE FORCE OF GRAVITY. =

1. Water evaporates from leaves as a result of transpiration 2. Water molecules form hydrogen bonds between one another, thus sticking together = cohesion. 3. Water forms a continuous unbroken pathway across the mesophyll cells and down the xylem 4. As water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata (creates tension/suction) = more molecules of water are drawn up behind it as a result of cohesion. 5. Water is hence pulled up the xylem as a result of transpiration = transpiration pull. 6. Transpiration pull puts the xylem under tension.

- WATER LOSS = TALK ABOUT WATER POTENTIAL GRADIENT, DIFFUSES OUT VIA WP GRADIENT OUT OF THE STOMATA - WHEN OBSERVING GRAPH, LOOK AT WHOLE PATTERN.

1. Water evaporates/transpired/DIFFUSES OUT 2. Via a water potential gradient 3. Stomata open, via the stomata 4. Water potential gradient reduces during the investigation 5. Because the water IS NOT REPLACED 6. Stomata close/closing = prevents further water loss

Use your knowledge of the cohesion-tension theory to explain how water in the xylem in the roots moves up the stem. ***

1. Water evaporates/transpires 2. Creates WP gradient 3. Water moves via apoplast pathway 4. Water drawn out of xylem 5. CREATES TENSION/PULLING EFFECT/NEGATIVE PRESSURE 6. Cohesive forces/H bonding between water molecules = water moves as a column up the xylem as continuous stream/column.

Describe how water moves through a plant concerning the cohesion-tension theory

1. Water evaporates/transpires from leaves 2. Reduces water potential gradient across cells 3. WATER DRAWN OUT OF XYLEM 4. CREATES TENSION 5. COHESIVE FORCES BETWEEN WATER MOLECULES 6. WATER PULLED UP AS A COLUMN

Explain the changes in transpiration rate and water uptake from midnight to midday

1. increased stomatal aperture/light/temperature (increases transpiration rate); 2. decreases water potential in root / increased uptake by osmosis

How does the closing of stomata reduce water loss

1. stoma can close; 2. reduces area for evaporation or transpiration;

The student assumed that water uptake was equivalent to the rate of transpiration. Give two reasons why this might not be a valid assumption.

1.(used to provide) turgidity/support/description of; 2. (used in) photosynthesis / (produced in) respiration; 3. Apparatus not sealed/leaks 4. Water produced in respiration;

Why are root hairs efficient surfaces for the exchange of water + mineral ions?

1.Provide a larger surface area = very long extensions and occur in the thousands 2. Thin surface layer of of only a cell membrane and wall = across which materials move easily

Leafy shoot?

= leafs are needed so transpiration can occur

Scale (usually mm)

= needed so you can precisely measure the uptake of water by the plant.

Why cut the shoot underwater?

= prevent air entering the stem/xylem. = important to maintain a continuous column of water in the xylem.

Why are all the parts sealed with vaseline?

= prevents air bubbles entering water flow/stem = continuous column of water required for expeiriment to work.

Reason for the reservoir

= used to return bubble to the start for repeats. = allows to to reset potometer.

Cohesion

As water is drawn into the endodermal cells, it pulls more water along behind it due to the cohesive properties of the water molecules. = This creates a tension that draws water along the cell walls of the cells of the root cortex. = Mesh like structure of cellulose walls has many water filled spaces so there is very little resistance to this pull of water along the cell walls.

Why is it important to keep environmental conditions constant?

Because environmental conditions affect rate of transpiration

Tissue Y

Endodermis

Explain one piece of evidence from the graph that shows the uptake of chloride ions by roots is by active transport

Idea of uptake against a concentration gradient Explain the difference in the rate of uptake for 4 °C and 18 °C = Rate of respiration increases; So more ATP produced / energy released (for active uptake); More chance of collision with carriers (due to higher kinetic energy

In dry conditions, the rate of transpiration is affected by an increase in temperature. Describe and explain how.

Increase in transpiration/evaporation/diffusion; Higher kinetic energy / faster movement of molecules/ particles/steeper water potential gradient OR Decreases in transpiration/evaporation/diffusion; Due to closing of stomata;

When talking about rate of flow of water

Must always state = increased tension (negative pressure) = cohesion between water molecules = transpiration pull

Describe how the students would have returned the air bubble to the start of the capillary tube in this investigation.

Open/use tap / add water from reservoir;

The student cut the shoot and put it into the potometer under water. Explain why.

Prevent air entering / continuous water column;

Give one environmental factor that should be kept constant in this investigation. Explain how allowing this factor to vary would affect the results.

Temperature (no mark) Higher temperature give molecules more kinetic energy; increase rate of evaporation; Air movement (no mark) In still air a layer of water vapour build up around stomata/reduces W.P. gradient; reduces the rate of evaporation; Humidity (no mark) High humidity reduces the water potential gradient; reduces the rate of evaporation; Light (intensity) (no mark) High light (intensity) causes stomata to open; increases rate of transpiration;

What ultimately drives transpiration?

The Sun, all factors are directly or indirectly the result of the Sun's energy.

Xerophyte adaptations

few stomata, to reduce SA for diffusion; small leaves, reduce SA for diffusion;

Stomata close

in the dark

Stomata open

in the light