Define the term transpiration

Transpiration is the loss of water from the leaves of plants by evaporation.

Define the term transpiration stream

The transpiration stream is the upward movement of water in a continuous column up the xylem vessels.

Give three ways in which xylem cells are adapted for water transport

They are hollow tubes with no end walls , allowing a continuous flow of water. They are waterproofed with lignin , preventing loss of water by evaporation. Lignin also strengthens the xylem to reduce breakages.

Describe how the loss of water from the leaves draws water up from the xylem

Water diffuses out of the leaves into the surrounding air via the stomata . This lowers the water potential in the air spaces around the mesophyll cells. Water in the mesophyll cell walls evaporates into the air spaces, lowering the water potential of the mesophyll cells. Water is then drawn from the xylem into the mesophyll cells by osmosis .

State what is described by the cohesion-tension theory

The cohesion-tension theory describes how water is pulled up the xylem as a continuous column by cohesion between water molecules, with the water under tension.

Explain the roles of cohesion and adhesion in moving water up the xylem

Cohesion between water molecules holds them together in a continuous column, so they can be pulled upwards together. Adhesion occurs between the water molecules and the walls of the xylem vessels, helping the water to move upwards.

Which type of bonding between water molecules is responsible for cohesion ?

Hydrogen bonds between water molecules are responsible for cohesion.

True or False? Water in the xylem is under tension , exerting an inward pull on the walls of the xylem vessels

True. The upward pulling force can be so great that the water is under tension, exerting an inward pull on the walls of the xylem vessels. This is known as cohesion tension.

Transpiration is the loss of water from the of a plant by the process of .

Transpiration is the loss of water from the leaves of a plant by the process of evaporation .

What does a potometer measure?

A potometer measures the rate at which a plant shoot takes up water , which provides an estimate of the rate of transpiration.

True or False? A potometer directly measures the rate of transpiration

False. A potometer measures the rate at which a plant shoot takes up water. Most of this water is lost in transpiration, but some enters the cells and is used in photosynthesis.

State four environmental factors that affect the rate of transpiration

Air movement (wind speed). Humidity . Light intensity . Temperature .

Explain the effect of high wind speed on the rate of transpiration

High wind speed increases the rate of transpiration. Wind moves water vapour away from the leaf surface, increasing the water potential gradient between the leaf and the air, so water vapour diffuses out more quickly.

Explain the effect of high humidity on the rate of transpiration

High humidity decreases the rate of transpiration. It reduces the water potential gradient between the leaf and the air, slowing the diffusion of water vapour out of the leaf.

Explain the effect of high temperature on the rate of transpiration

High temperature increases the rate of transpiration. Water molecules gain more kinetic energy , so they evaporate from the mesophyll and diffuse out of the leaf more quickly.

Explain the effect of high light intensity on the rate of transpiration

High light intensity increases the rate of transpiration. The stomata open to allow gas exchange for photosynthesis, so more water can diffuse out of the leaf.

Why are the plant shoot and potometer cut and assembled underwater ?

To prevent air bubbles from entering the xylem , where they could block the movement of water.

Why is the plant shoot cut at a diagonal angle?

A diagonal cut creates a larger surface area for the uptake of water.

Why are the leaves of the shoot dried before starting the experiment?

Water on the leaves would block the stomata and affect the rate of transpiration.

The potometer must be made using petroleum jelly to prevent air entering the system.

The potometer must be made airtight using petroleum jelly to prevent air entering the system.

Define the term translocation

Translocation is the movement of assimilates (organic substances) through the phloem from a source to a sink.

Give three examples of assimilates transported in the phloem

Sucrose . Amino acids . Plant hormones .

In translocation, define a source

A source is the place in a plant where assimilates are produced or stored, e.g. photosynthesising leaf cells.

In translocation, define a sink

A sink is a part of a plant where assimilates are required, e.g. actively growing cells or storage organs.

Describe two adaptations of phloem sieve tube cells

They have reduced cell contents to reduce resistance to the flow of assimilates. They have sieve plates that allow the passage of assimilates between cells.

How are companion cells adapted for their role in the phloem?

They contain many mitochondria to produce the ATP needed for the active loading of sucrose into the phloem.

True or False? Translocation is an active process that depends on energy from ATP

True. Translocation requires energy from ATP , which is why companion cells contain many mitochondria.

True or False? Assimilates can only be translocated upwards through the phloem

False. Assimilates can be translocated either upwards or downwards from a source to a sink.

Assimilates are moved through the phloem from a , where they are produced or stored, to a , where they are required.

Assimilates are moved through the phloem from a source , where they are produced or stored, to a sink , where they are required.

What does the mass flow hypothesis describe?

The mass flow hypothesis describes the mechanism by which phloem sap moves in one direction along sieve tubes, from a source to a sink.

Describe how sucrose is loaded into the phloem at the source

Companion cells use ATP to actively pump hydrogen ions out of their cytoplasm into their cell walls. Hydrogen ions move back into the cytoplasm down their concentration gradient via a cotransporter protein , carrying sucrose molecules with them. Sucrose then moves into the sieve tubes via plasmodesmata .

Explain why water enters the phloem at the source

The high concentration of sucrose lowers the water potential of the phloem. Water therefore moves into the phloem by osmosis , e.g. from neighbouring xylem vessels.

How is a hydrostatic pressure gradient generated in the phloem?

Water entering the phloem at the source increases the hydrostatic pressure there. This creates a pressure gradient between the source and the sink, and the phloem contents move towards the sink down this pressure gradient .

What happens to sucrose at the sink ?

Sucrose is unloaded from the phloem at the sink. This lowers the water potential of the sink cells, so water follows by osmosis , helping to maintain the hydrostatic pressure gradient.

Which type of transport is used to load sucrose into the phloem at the source?

Active transport , which requires energy from ATP .

From where can water move into the phloem at the source?

From the neighbouring xylem vessels, by osmosis .

True or False? The bulk movement of sap in the phloem is driven by diffusion alone

False. The bulk movement of sap is driven by a hydrostatic pressure gradient (mass flow), not by diffusion alone.

At the source, sucrose is loaded into the phloem by active transport, which lowers the of the phloem so that water enters by .

At the source, sucrose is loaded into the phloem by active transport, which lowers the water potential of the phloem so that water enters by osmosis .

The contents of the phloem move from the source to the sink down a pressure gradient.

The contents of the phloem move from the source to the sink down a hydrostatic pressure gradient.

How does phloem sap oozing out of a punctured sieve tube support the mass flow hypothesis?

It shows that the contents of the phloem are under pressure , exerting pressure on the phloem walls.

Phloem sap near a source has a higher sucrose concentration than sap near a sink. How does this support the mass flow hypothesis?

It supports the idea that water moves into the phloem by osmosis near the source and out of the phloem by osmosis near the sink.

How does the fact that metabolic inhibitors stop translocation support the mass flow hypothesis?

It shows that ATP is required, confirming that mass flow is an active process .

Removing a ring of bark produces a bulge above the ring with a higher sugar concentration. How does this support the mass flow hypothesis?

Removing the phloem (located just below the bark) prevents the passage of sugars , so they accumulate above the ring.

How does the observation that amino acids travel more slowly than sucrose challenge the mass flow hypothesis?

The hypothesis predicts that all solutes should flow at the same rate , but they do not.

Different substances have been observed moving in opposite directions within the same sieve tube. Why does this challenge the mass flow hypothesis?

The hypothesis states that everything should flow in one direction , so this observation contradicts it.

Why does the presence of sieve plates provide evidence against the mass flow hypothesis?

Sieve plates create a barrier to mass flow , so there appears to be no reason for them to have evolved.

True or False? There is evidence both for and against the mass flow hypothesis

True. Although widely accepted, the mass flow hypothesis is supported by some evidence and challenged by other evidence, so data is still being collected.

The observation that different solutes can move in directions within the same sieve tube is evidence the mass flow hypothesis.

The observation that different solutes can move in opposite directions within the same sieve tube is evidence against the mass flow hypothesis.

A LevelBiologyAQAFlashcardsExchange & TransportMass Transport in Plants

Mass Transport in Plants (AQA A Level Biology): Flashcards

Exam code: 7402

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Define the term transpiration

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Cards in this collection (63)

Define the term transpiration
Transpiration is the loss of water from the leaves of plants by evaporation.
Define the term transpiration stream
The transpiration stream is the upward movement of water in a continuous column up the xylem vessels.
Which tissue transports water and dissolved minerals from the roots to the rest of a plant?
The xylem.
Give three ways in which xylem cells are adapted for water transport
They are hollow tubes with no end walls, allowing a continuous flow of water.
They are waterproofed with lignin, preventing loss of water by evaporation.
Lignin also strengthens the xylem to reduce breakages.
Through which structures does water vapour diffuse out of the leaf during transpiration?
The stomata.
Describe how the loss of water from the leaves draws water up from the xylem
Water diffuses out of the leaves into the surrounding air via the stomata.
This lowers the water potential in the air spaces around the mesophyll cells.
Water in the mesophyll cell walls evaporates into the air spaces, lowering the water potential of the mesophyll cells.
Water is then drawn from the xylem into the mesophyll cells by osmosis.
State what is described by the cohesion-tension theory
The cohesion-tension theory describes how water is pulled up the xylem as a continuous column by cohesion between water molecules, with the water under tension.
Explain the roles of cohesion and adhesion in moving water up the xylem
Cohesion between water molecules holds them together in a continuous column, so they can be pulled upwards together.
Adhesion occurs between the water molecules and the walls of the xylem vessels, helping the water to move upwards.
Which type of bonding between water molecules is responsible for cohesion?
Hydrogen bonds between water molecules are responsible for cohesion.
True or False?
Water in the xylem is under tension, exerting an inward pull on the walls of the xylem vessels
True.
The upward pulling force can be so great that the water is under tension, exerting an inward pull on the walls of the xylem vessels. This is known as cohesion tension.
Transpiration is the loss of water from the of a plant by the process of .
Transpiration is the loss of water from the leaves of a plant by the process of evaporation.
What does a potometer measure?
A potometer measures the rate at which a plant shoot takes up water, which provides an estimate of the rate of transpiration.
True or False?
A potometer directly measures the rate of transpiration
False.
A potometer measures the rate at which a plant shoot takes up water. Most of this water is lost in transpiration, but some enters the cells and is used in photosynthesis.
State four environmental factors that affect the rate of transpiration
Air movement (wind speed).
Humidity.
Light intensity.
Temperature.
Explain the effect of high wind speed on the rate of transpiration
High wind speed increases the rate of transpiration.
Wind moves water vapour away from the leaf surface, increasing the water potential gradient between the leaf and the air, so water vapour diffuses out more quickly.
Explain the effect of high humidity on the rate of transpiration
High humidity decreases the rate of transpiration.
It reduces the water potential gradient between the leaf and the air, slowing the diffusion of water vapour out of the leaf.
Explain the effect of high temperature on the rate of transpiration
High temperature increases the rate of transpiration.
Water molecules gain more kinetic energy, so they evaporate from the mesophyll and diffuse out of the leaf more quickly.
Explain the effect of high light intensity on the rate of transpiration
High light intensity increases the rate of transpiration.
The stomata open to allow gas exchange for photosynthesis, so more water can diffuse out of the leaf.
Why are the plant shoot and potometer cut and assembled underwater?
To prevent air bubbles from entering the xylem, where they could block the movement of water.
Why is the plant shoot cut at a diagonal angle?
A diagonal cut creates a larger surface area for the uptake of water.
Why are the leaves of the shoot dried before starting the experiment?
Water on the leaves would block the stomata and affect the rate of transpiration.
The potometer must be made using petroleum jelly to prevent air entering the system.
The potometer must be made airtight using petroleum jelly to prevent air entering the system.
Define the term translocation
Translocation is the movement of assimilates (organic substances) through the phloem from a source to a sink.
In which tissue are organic substances (assimilates) transported in a plant?
The phloem.
Give three examples of assimilates transported in the phloem
Sucrose.
Amino acids.
Plant hormones.
In translocation, define a source
A source is the place in a plant where assimilates are produced or stored, e.g. photosynthesising leaf cells.
In translocation, define a sink
A sink is a part of a plant where assimilates are required, e.g. actively growing cells or storage organs.
Describe two adaptations of phloem sieve tube cells
They have reduced cell contents to reduce resistance to the flow of assimilates.
They have sieve plates that allow the passage of assimilates between cells.
How are companion cells adapted for their role in the phloem?
They contain many mitochondria to produce the ATP needed for the active loading of sucrose into the phloem.
True or False?
Translocation is an active process that depends on energy from ATP
True.
Translocation requires energy from ATP, which is why companion cells contain many mitochondria.
True or False?
Assimilates can only be translocated upwards through the phloem
False.
Assimilates can be translocated either upwards or downwards from a source to a sink.
Assimilates are moved through the phloem from a , where they are produced or stored, to a , where they are required.
Assimilates are moved through the phloem from a source, where they are produced or stored, to a sink, where they are required.
What does the mass flow hypothesis describe?
The mass flow hypothesis describes the mechanism by which phloem sap moves in one direction along sieve tubes, from a source to a sink.
Describe how sucrose is loaded into the phloem at the source
Companion cells use ATP to actively pump hydrogen ions out of their cytoplasm into their cell walls.
Hydrogen ions move back into the cytoplasm down their concentration gradient via a cotransporter protein, carrying sucrose molecules with them.
Sucrose then moves into the sieve tubes via plasmodesmata.
Explain why water enters the phloem at the source
The high concentration of sucrose lowers the water potential of the phloem.
Water therefore moves into the phloem by osmosis, e.g. from neighbouring xylem vessels.
How is a hydrostatic pressure gradient generated in the phloem?
Water entering the phloem at the source increases the hydrostatic pressure there.
This creates a pressure gradient between the source and the sink, and the phloem contents move towards the sink down this pressure gradient.
What happens to sucrose at the sink?
Sucrose is unloaded from the phloem at the sink.
This lowers the water potential of the sink cells, so water follows by osmosis, helping to maintain the hydrostatic pressure gradient.
Which type of transport is used to load sucrose into the phloem at the source?
Active transport, which requires energy from ATP.
From where can water move into the phloem at the source?
From the neighbouring xylem vessels, by osmosis.
True or False?
The bulk movement of sap in the phloem is driven by diffusion alone
False.
The bulk movement of sap is driven by a hydrostatic pressure gradient (mass flow), not by diffusion alone.
At the source, sucrose is loaded into the phloem by active transport, which lowers the of the phloem so that water enters by .
At the source, sucrose is loaded into the phloem by active transport, which lowers the water potential of the phloem so that water enters by osmosis.
The contents of the phloem move from the source to the sink down a pressure gradient.
The contents of the phloem move from the source to the sink down a hydrostatic pressure gradient.
How does phloem sap oozing out of a punctured sieve tube support the mass flow hypothesis?
It shows that the contents of the phloem are under pressure, exerting pressure on the phloem walls.
Phloem sap near a source has a higher sucrose concentration than sap near a sink. How does this support the mass flow hypothesis?
It supports the idea that water moves into the phloem by osmosis near the source and out of the phloem by osmosis near the sink.
How does the fact that metabolic inhibitors stop translocation support the mass flow hypothesis?
It shows that ATP is required, confirming that mass flow is an active process.
Removing a ring of bark produces a bulge above the ring with a higher sugar concentration. How does this support the mass flow hypothesis?
Removing the phloem (located just below the bark) prevents the passage of sugars, so they accumulate above the ring.
How does the observation that amino acids travel more slowly than sucrose challenge the mass flow hypothesis?
The hypothesis predicts that all solutes should flow at the same rate, but they do not.
Different substances have been observed moving in opposite directions within the same sieve tube. Why does this challenge the mass flow hypothesis?
The hypothesis states that everything should flow in one direction, so this observation contradicts it.
Why does the presence of sieve plates provide evidence against the mass flow hypothesis?
Sieve plates create a barrier to mass flow, so there appears to be no reason for them to have evolved.
True or False?
There is evidence both for and against the mass flow hypothesis
True.
Although widely accepted, the mass flow hypothesis is supported by some evidence and challenged by other evidence, so data is still being collected.
The observation that different solutes can move in directions within the same sieve tube is evidence the mass flow hypothesis.
The observation that different solutes can move in opposite directions within the same sieve tube is evidence against the mass flow hypothesis.
The fact that inhibitors stop translocation supports the idea that mass flow is an process requiring ATP.
The fact that metabolic inhibitors stop translocation supports the idea that mass flow is an active process requiring ATP.
In plant transport studies, define a tracer
A tracer is a chemical that can be traced as it moves through an organism.
Name a common tracer used to study transport in plants
Radioactive carbon dioxide (¹⁴CO₂).
Explain how radioactive carbon dioxide can be used to trace translocation
The radioactive ^14^CO~2~ is absorbed by the leaves and used in photosynthesis to produce sucrose.
The sucrose formed is radioactive, so its movement around the plant by translocation can be traced.
What is done in a ringing experiment?
A ring of tissue is removed from the outside of a plant stem.
Why does removing a ring of tissue from a stem remove only the phloem?
The phloem is located towards the outside of the stem, while the xylem is towards the centre, so ringing removes the phloem but leaves the xylem intact.
What is the purpose of the control plant in a ringing experiment?
The control plant has no ring of tissue removed, providing a comparison to show the effect of removing the phloem.
What do tracer and ringing experiments show about the transport of sucrose?
They show that sucrose is transported in the phloem and not the xylem.
No radioactive sucrose is detected past the ringing point, due to the break in the phloem.
What do ringing experiments reveal about the direction of sucrose transport in the phloem?
Sucrose is translocated both upwards and downwards, from source tissues in the leaves to sink tissues above and below.
True or False?
In a ringing experiment, removing the ring of tissue leaves the xylem intact
True.
The xylem is located towards the centre of the stem, so it remains intact while the phloem is removed.
Radioactive carbon dioxide is absorbed by the leaves and used in to produce radioactive , which can then be traced.
Radioactive carbon dioxide is absorbed by the leaves and used in photosynthesis to produce radioactive sucrose, which can then be traced.
In a ringing experiment, no radioactive sucrose is detected past the ring because there is a break in the at that point.
In a ringing experiment, no radioactive sucrose is detected past the ring because there is a break in the phloem at that point.

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