Transport Mechanisms (Cambridge (CIE) AS Biology): Flashcards
Exam code: 9700
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How are mineral ions and organic compounds transported within a plant?
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How are mineral ions and organic compounds transported within a plant?
They are dissolved in water and carried through the plant.
Define the apoplast pathway.
The apoplast pathway is the movement of water through the cellulose cell walls and intercellular spaces, without entering the cytoplasm.
Define the symplast pathway.
The symplast pathway is the movement of water through the cytoplasm of cells, passing from cell to cell through plasmodesmata.
Define the Casparian strip.
The Casparian strip is a waterproof band of suberin in the cell walls of the endodermis.
What is the role of the Casparian strip at the endodermis?
The suberin in the Casparian strip is waterproof.
It blocks the apoplast pathway, forcing water into the symplast pathway to cross the endodermis.
The apoplast pathway carries water through the cellulose cell .
The apoplast pathway carries water through the cellulose cell walls.
At the endodermis, the Casparian strip is made waterproof by .
At the endodermis, the Casparian strip is made waterproof by suberin.
True or False?
In the apoplast pathway, water moves through the cytoplasm of cells.
False.
In the apoplast pathway water moves through the cell walls; movement through the cytoplasm is the symplast pathway.
What is the role of lignin in the transport of water through a plant?
Lignin waterproofs and strengthens the walls of the xylem vessels.
This stops water leaking out and stops the vessels collapsing under tension, helping to maintain the continuous water column.
Define transpiration.
Transpiration is the loss of water vapour from the leaves and aerial parts of a plant by evaporation and diffusion.
Describe the process of transpiration.
Water evaporates from the internal surfaces of the mesophyll cells inside the leaf.
The water vapour collects in the air spaces.
It then diffuses out through the stomata to the atmosphere.
Why is transpiration described as evaporation followed by diffusion?
Water first evaporates from the internal leaf surfaces into the air spaces.
The water vapour then diffuses out of the stomata to the atmosphere.
Water vapour diffuses out of the leaf through pores called .
Water vapour diffuses out of the leaf through pores called stomata.
In transpiration, water from the internal surfaces of the leaf.
In transpiration, water evaporates from the internal surfaces of the leaf.
True or False?
Transpiration only involves the evaporation of water.
False.
Transpiration involves evaporation of water from internal leaf surfaces followed by diffusion of water vapour to the atmosphere.
Define cohesion (of water).
Cohesion is the attraction between water molecules caused by hydrogen bonding.
Define adhesion (of water in the xylem).
Adhesion is the attraction between water molecules and the cellulose cell walls of the xylem.
Define transpiration pull.
Transpiration pull is the tension that draws water up the xylem, created by water loss from the leaves.
Explain the cohesion-tension theory of water movement in the xylem.
Water lost by transpiration creates a tension (pull) at the top of the xylem.
Hydrogen bonds hold water molecules together by cohesion, forming a continuous column.
The whole column is pulled up the xylem, aided by adhesion to the cellulose walls.
How does hydrogen bonding help water move up the xylem?
Hydrogen bonds hold water molecules together (cohesion).
This keeps the water column continuous so it does not break as it is pulled up.
The attraction between water molecules, caused by hydrogen bonding, is called .
The attraction between water molecules, caused by hydrogen bonding, is called cohesion.
The attraction between water molecules and the xylem cell walls is called .
The attraction between water molecules and the xylem cell walls is called adhesion.
Define xerophyte.
A xerophyte is a plant adapted to live in dry conditions where water is limited.
How do sunken stomata reduce water loss in a xerophyte?
They trap humid air in pits next to the stomata.
This reduces the water potential gradient, so less water vapour diffuses out.
How does a thick waxy cuticle reduce water loss?
It forms a waterproof barrier over the leaf surface.
This reduces evaporation of water directly through the epidermis.
How do rolled leaves reduce transpiration in a xerophyte?
Rolling traps humid air inside the leaf.
This lowers the water potential gradient and reduces the surface area exposed to the air.
How do hairs (trichomes) on a leaf reduce water loss?
They trap a layer of humid air close to the stomata.
This reduces the water potential gradient, slowing diffusion of water vapour out of the leaf.
A thick waxy reduces evaporation from the leaf surface.
A thick waxy cuticle reduces evaporation from the leaf surface.
Sunken stomata and leaf hairs both trap air to reduce the water potential gradient.
Sunken stomata and leaf hairs both trap humid air to reduce the water potential gradient.
Define assimilates.
Assimilates are organic substances made by the plant, such as sucrose and amino acids.
Define source (in phloem transport).
A source is a part of the plant that loads assimilates into the phloem, such as a photosynthesising leaf.
Define sink (in phloem transport).
A sink is a part of the plant that removes assimilates from the phloem, such as a root or a growing region.
Which assimilates are transported in the phloem, and where do they move?
Sucrose and amino acids, dissolved in water.
They move from sources to sinks in the phloem sieve tubes.
Assimilates move through the phloem sieve tubes from sources to .
Assimilates move through the phloem sieve tubes from sources to sinks.
True or False?
Sucrose is transported in the xylem.
False.
Sucrose and other assimilates are transported in the phloem; the xylem carries water and mineral ions.
Explain how sucrose is loaded into the phloem at the source.
Proton pumps use ATP to actively pump H^+^ ions out of the companion cell.
This builds up a high concentration of H+ outside the cell.
The H+ ions diffuse back in through cotransporter proteins, bringing sucrose with them.
Sucrose then passes into the sieve tube element.
Define cotransporter protein (in sucrose loading).
A cotransporter protein is a membrane protein that moves H^+^ ions and sucrose together into the companion cell.
What is the role of the proton pump in sucrose loading?
It uses ATP to actively pump H^+^ ions out of the companion cell.
This sets up the H+ concentration gradient that drives sucrose uptake.
Why do companion cells contain many mitochondria?
To produce the ATP needed for the active transport of H+ ions by the proton pumps during sucrose loading.
Sucrose loading uses pumps to move H+ ions out of the companion cell.
Sucrose loading uses proton pumps to move H+ ions out of the companion cell.
H+ ions re-enter the companion cell together with sucrose through proteins.
H+ ions re-enter the companion cell together with sucrose through cotransporter proteins.
True or False?
Loading sucrose into the phloem is a passive process.
False.
It requires ATP to power the proton pumps, so it is an active process.
Define mass flow (in phloem).
Mass flow is the bulk movement of phloem sap down a hydrostatic pressure gradient from source to sink.
How is a high hydrostatic pressure created at the source?
Sucrose is loaded into the sieve tube, lowering its water potential.
Water then enters from the xylem by osmosis.
This raises the hydrostatic pressure in the sieve tube at the source.
How is the hydrostatic pressure lowered at the sink?
Sucrose is unloaded from the sieve tube, raising its water potential.
Water then leaves the phloem by osmosis.
This lowers the hydrostatic pressure at the sink.
In which direction does phloem sap move by mass flow?
From the high hydrostatic pressure at the source to the low hydrostatic pressure at the sink.
In mass flow, phloem sap moves down a pressure gradient.
In mass flow, phloem sap moves down a hydrostatic pressure gradient.
Water enters the sieve tube at the source by , raising the hydrostatic pressure.
Water enters the sieve tube at the source by osmosis, raising the hydrostatic pressure.
True or False?
In mass flow, phloem sap moves from the sink to the source.
False.
Phloem sap moves from source to sink, down a hydrostatic pressure gradient.
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