Mass Transport In Plants

விளையாடிக் கற்றுக்கொள்ளுங்கள்

ஆற்றல் சம்பாதிக்க இந்த கேள்விகளுக்குப் பதிலளியுங்கள், பின்னர் மீன் பிடித்து ஆராயுங்கள். கணக்கு தேவையில்லை.

கல்வியாளர்களுக்கு: Mass Transport In Plants (Biology, AQA)-க்கான தயாரான பாட ஸ்லைடுகள், திருப்புதல் குறிப்புகள் — உங்கள் பாடத்தில் அவற்றைப் பயன்படுத்தவும், அல்லது கற்பவர்கள் நேரலை விளையாட்டாக விளையாடும் ஊடாடும் வகுப்பு செயல்பாடாக தலைப்பை இயக்கவும்.

பாட குறிப்புகள்

Transpiration and the Xylem

Transpiration is the loss of water from the leaves of plants by evaporation.
Water and dissolved minerals are transported from the soil to the rest of the plant in the xylem.
Xylem cells are specialised for water transport: they are hollow tubes with no end walls, allowing continuous flow of water.
Lignin provides waterproofing to prevent loss of water by evaporation and strengthens the xylem to reduce breakages.
Xylem tissue, together with phloem tissue, makes up the plant's vascular tissue.
Water is required for photosynthesis and to maintain cell structure; minerals are needed for production of important biological molecules, e.g. proteins and chlorophyll.

The Cohesion-Tension Theory

The upward movement of water in the xylem is driven by transpiration.
Water diffuses out of leaves into the surrounding air via the stomata.
Loss of water vapour lowers the water potential in the air spaces surrounding the mesophyll cells.
Water within the mesophyll cell walls evaporates into the leaf air spaces, lowering the water potential of the mesophyll cells.
Water is drawn from the xylem into the mesophyll cells by osmosis.
Water moves up the xylem vessels in a continuous column to replace lost water; this upward movement is the transpiration stream.
Water molecules are pulled upwards due to forces of cohesion (between water molecules) and adhesion (between water and the sides of the xylem).
The upward pulling force can put water under tension, exerting an inward pull on the walls of the xylem vessels; this is known as cohesion-tension.

Factors Affecting Transpiration Rate

High wind speed increases transpiration: wind moves water vapour away from the leaf surface, increasing the water potential gradient between the leaf and the air, increasing diffusion of water vapour.
High humidity decreases transpiration: the water potential gradient between the leaf and the air is reduced, slowing diffusion of water vapour.
High light intensity increases transpiration: the rate of photosynthesis increases, so stomata open to allow gas exchange, allowing more water to diffuse out.
High temperature increases transpiration: particles have more kinetic energy so water molecules evaporate from the mesophyll and diffuse out of the leaf more quickly.

Using a Potometer

A potometer can be used to investigate the effect of environmental factors on the rate of transpiration.
Cut a plant shoot underwater using a diagonal cut: a diagonal cut creates a larger surface area for the uptake of water.
Assemble the potometer underwater to prevent air bubbles from entering the xylem where they could block the movement of water.
Make sure the apparatus is airtight using petroleum jelly to seal any gaps, preventing entry of air into the system.
Dry the leaves using a paper towel: water on the leaves will block the stomata and affect transpiration.
Allow a single air bubble to form inside the potometer and place the end of the tube into a beaker of water; the air bubble allows water movement in the tube to be observed, and the beaker provides a supply of water to replace water lost during transpiration.
Set up the environmental factor to be assessed and allow the plant shoot to adjust to the new environment for 5 minutes.
Record the starting location of the air bubble, leave for a set period (e.g. 15 minutes), record the end location, then reset the bubble by opening the tap below the reservoir.
Repeat steps twice more under the same environmental conditions before repeating under changed environmental conditions.
Note that a potometer measures the rate at which a plant shoot takes up water, not the true rate of transpiration; most of this water is lost in transpiration, but some may enter cells and be used in photosynthesis.

Movement in the Phloem

Plants also need to transport organic substances, e.g. carbohydrates produced in the leaves by photosynthesis, for respiration in other parts of the plant.
Organic substances, also known as assimilates, are transported in the phloem; examples include sucrose, amino acids and plant hormones.
Phloem sieve tube cells have reduced cell contents to reduce resistance to flow of assimilates, and sieve plates allow passage of assimilates between cells.
Companion cells contain many mitochondria to produce ATP for the active loading of sucrose into the phloem tubes.
Movement of assimilates through the phloem is called translocation.
Translocation moves assimilates either upwards or downwards from cells in the source to cells in the sink.
A source is the place in which assimilates are produced or stored, e.g. cells in photosynthesising leaves are a source of sugars; cells in storage organs during early spring may be a source of carbohydrates for new growth.
A sink is the part of a plant where assimilates are required, e.g. cells in parts of a plant that are actively growing, or cells in plant storage organs.
Translocation is an active process, dependent on energy from ATP.

The Mass Flow Hypothesis

The process by which phloem sap moves in one direction along phloem sieve tubes is known as the mass flow hypothesis.
The direction may differ depending on the location of sources and sinks in the plant.
Sucrose loading: companion cells use ATP to actively pump hydrogen ions out of the cytoplasm into their cell walls; hydrogen ions move down their concentration gradient back to the cytoplasm via a cotransporter protein, carrying sucrose molecules; sucrose then moves into the sieve tubes via plasmodesmata.
The high concentrations of solutes in the phloem lower the water potential and cause water to move into the phloem vessels by osmosis; water can move in from neighbouring xylem vessels.
This results in increased hydrostatic pressure and generates a hydrostatic pressure gradient between the source and the sink; the contents of the phloem move towards the sink down a pressure gradient.
At the same time sucrose is being unloaded from the phloem at the sink, lowering the water potential of the cells of the sink.
Water follows by osmosis, maintaining the hydrostatic pressure gradient between the source and the sink.
Movement of sucrose in the phloem is an active process, requiring energy from ATP to generate a hydrostatic pressure gradient.

Evaluating the Mass Flow Hypothesis

The mass flow hypothesis is widely accepted, but it is a theory for which evidence is still being collected.
Evidence for: when the phloem sieve tube is punctured, phloem sap oozes out, showing the contents exert pressure on the phloem walls.
Evidence for: phloem sap extracted near a source has a higher sucrose concentration than sap extracted near a sink; water would move into the phloem by osmosis near a source and out near a sink.
Evidence for: metabolic inhibitors stop translocation, showing ATP is required and it is an active process.
Evidence for: removal of a ring of bark from trees results in a bulge above the ring, and fluid from the bulging region has a higher sugar concentration than fluid from below the ring; removal of the phloem tissue prevents the passage of sugars.
Evidence against: amino acids appear to travel more slowly than sucrose in the phloem, but the hypothesis states they should flow at the same rate.
Evidence against: experiments have detected different substances, within the same sieve tube, moving in opposite directions, but the hypothesis states everything should be flowing in one direction.
Evidence against: sieve plates are present in the phloem, creating a barrier to mass flow, so there is no reason for them to have evolved.

Tracer and Ringing Experiments

Tracers are chemicals that can be traced as they move through an organism; a common example is radioactive carbon dioxide, 14CO2.
14CO2 is readily absorbed by the leaves and used in photosynthesis to produce sucrose; the sucrose formed will be radioactive so its movement around the plant via translocation can be traced.
Ringing experiments involve removing a ring of tissue from the outside of a plant stem.
As the phloem is located towards the outside of the stem and the xylem towards the centre, the ring removes the phloem only, with the xylem remaining intact.
In an example ringing experiment: plants were ringed at different locations on the stem, a control plant did not have a ring removed, plants were supplied with 14CO2, and after a period of time the levels of radioactive carbon in different parts of the plant were measured.
Results show that the phloem is involved in the transport of sucrose, and not the xylem.
No radioactive sucrose is detected past the ringing point on the stems, due to the break in the phloem at this point.
In the phloem, the transport of sucrose occurs both upwards and downwards; sucrose is translocated from source tissues in the leaves to sink tissues above and below.

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பதிவு செய்க — இலவசம்உள்நுழைக

பயிற்சி கேள்விகள்

இலவச முன்னோட்டம் — 61-இல் 8 கேள்விகள். அனைத்தையும் பார்க்க பதிவு செய்யவும்.

1.Which of the following is the best definition of transpiration?
Medium
- AThe loss of water from the leaves of plants by evaporation
- BThe upward movement of water in the xylem
- CThe active transport of sugars into the phloem
- DThe movement of water from the soil into the roots by osmosis
2.Which of the following is a feature of xylem vessels that makes them specialised for the transport of water?
Medium
- AThey contain many mitochondria to provide ATP for active transport
- BThey have no end walls and are hollow tubes
- CThey have sieve plates to allow passage of assimilates
- DThey have a large number of chloroplasts for photosynthesis
3.Which of the following are examples of assimilates transported in the phloem? (select all that apply)
Medium
- ASucrose
- BAmino acids
- CPlant hormones
- DWater
- ELignin
4.Translocation is the movement of water and minerals in the xylem.
Easy
True or false?
5.The mass flow hypothesis states that phloem sap moves in one direction along phloem sieve tubes.
Easy
True or false?
6.Which of the following is evidence that supports the mass flow hypothesis?
Medium
- AAmino acids travel more slowly than sucrose in the phloem
- BDifferent substances move in opposite directions within the same sieve tube
- CPhloem sap oozes out when a sieve tube is punctured
- DSieve plates are present in the phloem
7.Put the steps of the mass flow hypothesis in the correct order.
Medium
- Sucrose is actively loaded into the phloem at the source
- Water enters the phloem by osmosis, increasing hydrostatic pressure
- A hydrostatic pressure gradient drives mass flow towards the sink
- Sucrose is unloaded from the phloem at the sink
8.Match each term with its correct description.
Medium
- Transpiration
- Translocation
- Cohesion-tension theory
- Loss of water vapour from leaves
- Movement of assimilates in the phloem
- Explains upward water movement in xylem

மேலும் கேள்விகள்

9.Which of the following are functions of water in plants, not including transpiration? (select all that apply)
Medium
- AIt is a reactant in photosynthesis
- BIt maintains cell structure
- CIt is used to produce ATP in respiration
- DIt is a solvent for minerals and other solutes
- EIt is a product of the light-dependent reaction
10.Match each term related to the mass flow hypothesis with its correct description.
Medium
- Source
- Sink
- Translocation
- Mass flow hypothesis
- The part of a plant where assimilates are required
- The process by which phloem sap moves in one direction along phloem sieve tubes
- The place in which assimilates are produced or stored
- The movement of assimilates through the phloem
11.Put the steps of the mass flow hypothesis in the correct order, starting from sucrose loading at the source.
Medium
- Sucrose is actively loaded into the phloem at the source
- Water moves into the phloem by osmosis, lowering the water potential
- Hydrostatic pressure increases, generating a pressure gradient
- Phloem sap moves towards the sink down the pressure gradient
- Sucrose is unloaded from the phloem at the sink
12.Which of the following are evidence AGAINST the mass flow hypothesis? (select all that apply)
Hard
- AAmino acids appear to travel more slowly than sucrose in the phloem
- BPhloem sap oozes out when the phloem sieve tube is punctured
- CDifferent substances within the same sieve tube can move in opposite directions
- DMetabolic inhibitors stop translocation
- ESieve plates are present in the phloem
13.The mass flow hypothesis states that translocation is an active process requiring ATP.
Easy
True or false?
14.Match each environmental condition with its effect on the rate of transpiration.
Medium
- High wind speed
- High humidity
- High light intensity
- High temperature
- Increased rate because water vapour is removed from the leaf surface
- Decreased rate because the water potential gradient is reduced
- Increased rate because stomata open for gas exchange
- Increased rate because particles have more kinetic energy
15.In a potometer experiment, why is it important to cut the plant shoot underwater using a diagonal cut?
Medium
- ATo prevent air bubbles from entering the xylem and to increase the surface area for water uptake
- BTo prevent water loss from the shoot and to reduce transpiration
- CTo allow air to enter the xylem and help water movement
- DTo increase the rate of photosynthesis in the leaves
16.Which of the following are adaptations of xylem vessels for mass transport? (select all that apply)
Medium
- AHollow tubes with no end walls
- BLignin in cell walls
- CCompanion cells with many mitochondria
- DSieve plates between cells
- EPits in the cell walls
17.A student sets up a potometer and measures the movement of the air bubble over 20 minutes. The bubble moves 45 mm. What is the rate of water uptake in mm min⁻¹?
Medium
- A2.25
- B0.44
- C900
- D4.5
18.In a ringing experiment, a ring of bark is removed from a tree stem. Which of the following observations would provide evidence that sucrose is transported in the phloem?
Medium
- AA bulge forms above the ring with a higher sugar concentration than below the ring.
- BThe xylem continues to transport water and minerals normally.
- CThe leaves above the ring wilt and die.
- DThe roots below the ring grow more rapidly.
19.Which of the following statements are evidence AGAINST the mass flow hypothesis? (select all that apply)
Hard
- AAmino acids travel more slowly than sucrose in the phloem.
- BDifferent substances within the same sieve tube can move in opposite directions.
- CSieve plates are present in the phloem.
- DPhloem sap oozes out when a sieve tube is punctured.
- EMetabolic inhibitors stop translocation.
20.In a potometer, the rate of water uptake is always exactly equal to the rate of transpiration.
Easy
True or false?
21.Put the steps of the mass flow hypothesis in the correct order, starting with sucrose loading at the source.
Medium
- Sucrose is actively loaded into the phloem at the source.
- Water enters the phloem by osmosis, increasing hydrostatic pressure.
- Hydrostatic pressure gradient drives mass flow towards the sink.
- Sucrose is unloaded at the sink, lowering water potential.
- Water follows by osmosis, maintaining the pressure gradient.
22.In a tracer experiment using radioactive carbon dioxide, which of the following results would support the mass flow hypothesis?
Hard
- ARadioactive sucrose is detected in the phloem above and below the leaves.
- BRadioactive sucrose is detected only in the xylem.
- CRadioactive sucrose is detected only in the leaves.
- DRadioactive sucrose is detected in the roots but not in the leaves.
23.A student sets up a potometer but forgets to seal the joints with petroleum jelly. What is the most likely effect on the measured water uptake?
Medium
- AAir enters the apparatus, so the air bubble moves faster than the true rate of water uptake
- BAir enters the apparatus, so the air bubble moves more slowly than the true rate of water uptake
- CWater leaks out of the apparatus, so the air bubble moves faster than the true rate of water uptake
- DThere is no effect, because petroleum jelly only prevents the shoot from drying out
24.In a potometer experiment, a student records the air bubble moving 32 mm in 8 minutes. What is the rate of water uptake?
Medium
- A0.25 mm min⁻¹
- B4 mm min⁻¹
- C256 mm min⁻¹
- D0.4 mm min⁻¹
25.Put the following steps of a potometer experiment in the correct order.
Medium
- Cut a plant shoot underwater using a diagonal cut
- Assemble the potometer underwater
- Seal joints with petroleum jelly
- Dry the leaves with a paper towel
- Allow a single air bubble to form and place the end of the tube into a beaker of water
- Record the starting position of the air bubble
26.What is the percentage decrease in mean rate of water uptake when all 7 leaves are covered compared to when none are covered?
Medium
- A90.9%
- B9.1%
- C99.1%
- D0.10%
27.In a ringing experiment, a ring of bark is removed from a tree stem. Which observation would provide evidence that sucrose is transported in the phloem?
Hard
- AA bulge forms above the ring, and fluid from the bulge has a higher sugar concentration than fluid below the ring
- BA bulge forms below the ring, and fluid from the bulge has a higher sugar concentration than fluid above the ring
- CThe xylem becomes blocked, preventing water transport to the leaves
- DRadioactive sucrose is detected in the xylem above the ring
28.What is transpiration?
Easy
- AThe loss of water from the leaves of plants by evaporation
- BThe upward movement of water in the xylem
- CThe transport of sugars in the phloem
- DThe uptake of water by the roots
29.Which of the following is the main sugar transported in the phloem?
Easy
- ASucrose
- BGlucose
- CStarch
- DFructose
30.Transpiration is the loss of water from the leaves of plants by evaporation.
Easy
True or false?
31.The transpiration stream is the loss of water vapour from the leaves.
Easy
True or false?
32.Which of the following is a function of the vascular bundles in a plant stem?
Medium
- ATransport of water, minerals and assimilates
- BSupport only
- CPhotosynthesis only
- DStorage of starch only
33.Put the following steps of the cohesion-tension theory in the correct order.
Medium
- Water evaporates from the mesophyll cell walls into the leaf air spaces
- Water vapour diffuses out of the leaf through the stomata
- Water is drawn from the xylem into the mesophyll cells by osmosis
- Water moves up the xylem as a continuous column due to cohesion and tension
34.Which environmental condition would result in the highest rate of transpiration?
Medium
- AHigh wind speed, high light intensity, high temperature, low humidity
- BLow wind speed, low light intensity, low temperature, high humidity
- CHigh wind speed, low light intensity, low temperature, high humidity
- DLow wind speed, high light intensity, high temperature, high humidity
35.Which of the following are xerophytic adaptations of leaves to reduce water loss? (select all that apply)
Medium
- AThick waxy cuticle
- BSunken stomata
- CReduced number of stomata
- DLarge surface area
- EThin cuticle
36.Place the following steps of the cohesion-tension theory in the correct order.
Medium
- Water evaporates from mesophyll cell walls into leaf air spaces
- Water diffuses out of leaves via stomata
- Water is drawn from xylem into mesophyll cells by osmosis
- Water moves up xylem as a continuous column
- Water potential in air spaces decreases
37.Which of the following best describes the role of hydrogen bonds in the cohesion-tension theory?
Medium
- AThey cause water molecules to adhere to xylem walls
- BThey hold water molecules together in a continuous column
- CThey actively pump water up the xylem
- DThey break to release energy for water movement
38.Transpiration is the loss of water vapour from the leaves of plants by evaporation.
Easy
True or false?
39.Which of the following are evidence FOR the mass flow hypothesis? (select all that apply)
Medium
- APhloem sap oozes out when sieve tube is punctured
- BSucrose concentration is higher near source than sink
- CAmino acids travel more slowly than sucrose
- DMetabolic inhibitors stop translocation
- EDifferent substances move in opposite directions in same sieve tube
40.A student uses a potometer to measure water uptake in a leafy shoot. The air bubble moves 48 mm in 12 minutes. What is the mean rate of water uptake in mm min⁻¹?
Medium
- A4.0
- B0.25
- C576
- D0.4
41.Complete the sentences to accurately describe the mechanism of sugar transport in plant stems.
Easy
Sucrose is actively transported into the phloem by a group of specialised cells called ____. The increased solute concentration causes the water potential to ____ in the phloem and so water enters by ____. This results in a high ____ gradient causing a mass flow of water and sugar towards the ____ of the plant where the sugars are ____.
42.Name the main sugar that is found in the phloem.
Easy
43.Identify cell type B labelled in Figure 1.
Easy
44.In Figure 1 the starting mean weight of the leaves in set X was 0.1 grams. After 90 minutes the mean weight was 0.096 grams. Calculate the percentage change in weight.
Medium
45.Complete the sentences describing the cohesion-tension theory.
Medium
Water molecules are pulled upwards in the xylem due to forces of ____ between water molecules and ____ between water and the sides of the xylem. This upward movement can create ____ within the xylem vessels.
46.A potometer was used to measure water uptake. The air bubble moved 30 mm in 10 minutes. Calculate the rate of water uptake in mm min⁻¹.
Medium
47.Name the main sugar transported in the phloem.
Easy
48.A potometer was used to measure water uptake. The air bubble moved 60 mm in 15 minutes. Calculate the rate of water uptake in mm min⁻¹.
Medium
49.Complete the sentence about the potometer method.
Medium
When setting up a potometer, the plant shoot should be cut underwater to prevent ____ from entering the xylem.
50.Complete the sentences describing the movement of water in the xylem.
Medium
Water diffuses out of leaves into the surrounding air via the ____. The loss of water vapour lowers the water potential in the air spaces surrounding the mesophyll cells. Water is then drawn from the xylem into the mesophyll cells by ____.
51.A potometer was used to measure water uptake. The air bubble moved 72 mm in 12 minutes. Calculate the rate of water uptake in mm min⁻¹.
Medium
52.Name the cell type in the phloem that contains many mitochondria and is responsible for the active loading of sucrose.
Easy
53.Describe the role that hydrogen bonding plays in the cohesion-tension theory of water transport in the xylem of plants.
Medium
54.Explain why a potometer measures water uptake rather than the true rate of transpiration.
Hard
55.Explain why the rate of transpiration decreases at high humidity.
Hard
56.A student investigated the effect of covering leaves with grease on water uptake. The results showed that as more leaves were covered, the mean rate of water uptake decreased. Explain these results.
Challenge
57.Explain how the cohesion-tension theory accounts for the movement of water up the xylem of a tall tree.
Hard
58.In Figure 1 the starting mean weight of the leaves in set X was 0.1 grams. After 90 minutes the mean weight was 0.096 grams. Calculate the percentage change in weight.
Medium
- A-5 %
- B-2 %
- C-3 %
- D-4 %
59.A potometer was used to measure water uptake. The air bubble moved 30 mm in 10 minutes. Calculate the rate of water uptake in mm min⁻¹.
Medium
- A3 mm min⁻¹
- B2 mm min⁻¹
- C4 mm min⁻¹
- D5 mm min⁻¹
60.A potometer was used to measure water uptake. The air bubble moved 60 mm in 15 minutes. Calculate the rate of water uptake in mm min⁻¹.
Medium
- A3 mm min⁻¹
- B5 mm min⁻¹
- C4 mm min⁻¹
- D6 mm min⁻¹
61.A potometer was used to measure water uptake. The air bubble moved 72 mm in 12 minutes. Calculate the rate of water uptake in mm min⁻¹.
Medium
- A7 mm min⁻¹
- B5 mm min⁻¹
- C6 mm min⁻¹
- D8 mm min⁻¹

திருப்புதல் குறிப்புகள்

## Transpiration - **Transpiration** = loss of water from leaves by evaporation. - **Transpiration stream** = upward movement of water in the xylem. - Water is needed for photosynthesis and cell structure; minerals for proteins and chlorophyll. - **Xylem adaptations:** hollow tubes, no end walls → continuous flow; lignin waterproofs and strengthens. - **Sequence:** water diffuses out via stomata → lowers water potential in air spaces → water evaporates from mesophyll cell walls → water drawn from xylem by osmosis → continuous column moves up (transpiration stream). - **Cohesion-tension theory:** water is pulled up due to cohesion between water molecules and adhesion to xylem walls; tension pulls xylem walls inward. ## Potometer Practical - Measures **water uptake**, not true transpiration (some water used in photosynthesis). - **Method key points:** cut shoot underwater (diagonal cut → larger surface area); assemble underwater (prevents air bubbles blocking xylem); seal with petroleum jelly (airtight); dry leaves (water blocks stomata); allow one air bubble; record bubble movement over time; reset bubble; repeat. - **Rate = distance moved ÷ time.** - **Environmental effects:** - High wind speed → increases (removes water vapour, steepens water potential gradient). - High humidity → decreases (reduces water potential gradient). - High light intensity → increases (stomata open for gas exchange). - High temperature → increases (more kinetic energy, faster evaporation). ## Movement in the Phloem - **Translocation** = movement of assimilates (sucrose, amino acids, plant hormones) from **source** to **sink**. - Source = where produced/stored (e.g. photosynthesising leaves); sink = where required (e.g. growing root tip). - **Sieve tube cells:** reduced cell contents (less resistance), sieve plates allow passage. - **Companion cells:** many mitochondria → ATP for active loading of sucrose. - Translocation is an active process, requires ATP. ## The Mass Flow Hypothesis 1. Sucrose actively loaded into phloem at source: companion cells pump H⁺ out; H⁺ return via cotransporter carrying sucrose; sucrose enters sieve tubes via plasmodesmata. 2. High solute concentration lowers water potential → water enters by osmosis from xylem. 3. Increased hydrostatic pressure → pressure gradient from source to sink → mass flow. 4. Sucrose unloaded at sink, lowering water potential there. 5. Water follows by osmosis, maintaining pressure gradient. ## Evaluating the Mass Flow Hypothesis - **For:** phloem sap oozes when punctured (pressure); higher sucrose near source than sink; metabolic inhibitors stop translocation (ATP needed); ringing causes bulge above ring with higher sugar. - **Against:** amino acids travel slower than sucrose; different substances move in opposite directions in same sieve tube; sieve plates create a barrier (why evolve?). ## Tracer & Ringing Experiments - Tracer = radioactive CO₂ (¹⁴CO₂) absorbed by leaves → radioactive sucrose traced. - Ringing removes phloem (outside) leaving xylem intact. - Results: no radioactive sucrose past ring; transport occurs both upwards and downwards; phloem (not xylem) transports sucrose.

Transpiration: The loss of water from the leaves of plants by evaporation.
Transpiration stream: The upward movement of water in the xylem vessels in a continuous column to replace water lost by transpiration.
Cohesion-tension theory: The mechanism of water movement in the xylem where water is drawn upward due to cohesion between water molecules and adhesion between water and the sides of the xylem; this upward movement can create tension within the xylem vessels.
Xylem: Specialised cells that transport water and dissolved minerals from the roots to the leaves in vascular plants.
Phloem: Tissue that transports organic substances (assimilates) such as sucrose, amino acids and plant hormones from source to sink.
Translocation: The movement of assimilates through the phloem from a source to a sink; an active process dependent on energy from ATP.
Source: The place in which an assimilate has been produced or stored, e.g. photosynthesising leaf cells.
Sink: The part of a plant where assimilates are required, e.g. actively growing cells or storage organs.
Mass flow hypothesis: The process by which phloem sap moves in one direction along phloem sieve tubes, driven by a hydrostatic pressure gradient between source and sink.
Potometer: Apparatus used to investigate the effect of environmental factors on the rate of transpiration by measuring water uptake.
Companion cells: Phloem cells containing many mitochondria to produce ATP for the active loading of sucrose into the phloem tubes.
Sieve tube cells: Phloem cells with reduced cell contents to reduce resistance to flow of assimilates and sieve plates that allow passage of assimilates between cells.
Tracer: A chemical that can be traced as it moves through an organism, e.g. radioactive carbon dioxide (¹⁴CO₂) used to trace sucrose movement.
Ringing experiment: An experiment involving removal of a ring of tissue from the outside of a plant stem, removing the phloem while leaving the xylem intact, to study translocation.

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இந்த தலைப்பிற்கான ஒவ்வொரு கேள்வியையும், ஸ்லைடுகளையும், ஃப்ளாஷ் கார்டுகளையும் மற்றும் திருப்புதல் குறிப்புகளையும் பார்க்க இலவச கணக்கை உருவாக்கவும்.

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இந்த தலைப்பிற்கான முந்தைய தேர்வு பயிற்சி விரைவில் வருகிறது.

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