Biology Module 2: Exchange & Transport

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What can & can't roots do?
Can: obtain water. Can't: absorb sugars from the soil.
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What can & can't leaves do?
Can: obtain water. Can't: obtain water from the air.
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Water and soluble minerals travel upwards in what tissue?
Xylem Tissue
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Sugars can travel up or down the plant in what tissue?
Phloem Tissue
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Xylem and Phloem are found where?
In Vascular Bundles.
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Arrangement of xylem & phloem provides?
Strength to withstand the pulling forces that the roots are exposed to.
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What is the endodermis?
A special sheath of cells, role is to get water from the xylem vessels.
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Name one Xylem adaptation Adapt.1.
Narrow Tubes, so water column doesn't break easily and capillary action can be effective.
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Name one Xylem adaptation: Adapt.2.
Pits in the lignified walls allow water to move sideways from one vessel to another.
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Name one Xylem adaptation: Adapt 3.
Lignin in spiral patterns, allows the xylem to stretch and bend as the plant grows. Also it strengthens the walls and prevents them from collapsing.
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Name on Xylem adaptation: Adapt 4.
Made of dead cells that form a continuous column, hence no cell contents, nucleus or cytoplasm, to impede the flow of water
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What does the phloem consist of?
Sieve tube elements and companion cells.
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What are Sieve tube elements?
The contain very little cytoplasm and no nucleus. They're lined up end-to-end to form a sieve tube, to transport sugars (sucrose). The sugar is dissolved in water to from sap.
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The sieve tube contains sieve plates, what do they allow/contain?
They contain many pores to allow the sap to flow.
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What are the companion cells?
Have numerous amount of mitochondria to produce the ATP needed for active process.
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What do companion cells carry out?
They carry out the metabolic processes needed by the sieve tube elements.
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The cytoplasm of the sieve tube elements and companion cells are linked through many?
Plasmodesmata.
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What do the gaps in the cell walls of companion cells allow?
Communication and flow of materials between the cells.
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What is transpiration?
The loss of water vapour from the aerial parts of the plant.
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What three processes does transpiration involve?
Osmosis: From xylem to the mesophyll cells. Evaporation: from the surface of the mesophyll cells into the intercellular spaces. Diffusion: of water vapour down a water vapour potential gradient out of the stomata.
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The movement of water up the stem is called the?
Transpiration stream.
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How is the transpiration stream useful?
It enables leaves to acquire water for photosynthesis. It keeps cells turgid. Evaporation of water can keep the plant cool. It enables cells to acquire water for growth & elongation.
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How is the rate of transpiration measured?
Using a Potometer.
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The factors that affect transpiration. Factor. 1
Number of leaves: the more leaves, the larger the SA over which water vapour can be lost.
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The factors that affect transpiration. Factor. 2
Presence of waxy cuticle: reduces evaporation from the leaf surface.
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The factors that affect transpiration. Factor. 3
The number of stomata: the more there are, the faster the rate of water vapour loss.
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The factors that affect transpiration. Factor. 4
Air movement/Wind: wind will carry away water vapour, maintaining a high water vapour potential gradient.
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What are xerophytes?
Xerophytes are plants well adapted to living in very dry or arid conditions.
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What are the xerophyte adaptations to reduce water loss? Adapt 1.
Pits: Containing stomata at their base which also trap air.
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What are the xerophyte adaptations to reduce water loss? Adapt 2.
Hairs: On the surface of leaves trap a layer of air which can be saturated with moisture to reduce the water vapour potential.
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What are the xerophyte adaptations to reduce water loss? Adapt 3.
Densely packed spongy mesophyll: Which reduces cell SA exposed to air inside the leaves so less water will evaporate into the air spaces.
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What are the xerophyte adaptations to reduce water loss? Adapt 4.
Closing the stomata: when water availability is low., reducing water loss and the need to take up water.
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Describe the Apoplast pathway
The cellulose cell wall have many water-filled paces between the cellulose molecules. Water can move through these spaces and between the cells, the water doesn't pass through any plasma membranes, it can carry dissolved mineral ions and salts.
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Describe the Symplast pathway
Water enters the cytoplasm through the plasma membrane, and pass form one cell to the next through the plasmodesmata. Once inside the cytoplasm, water can move through the continuous cytoplasm from cell to cell.
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What is the function of the root hair cells, in the roots?
Use ATP to absorb minerals via active transport.
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What do the minerals absorbed do?
They reduce the water potential of the cell cytoplasm, making it lower than that in the soil. Result = water is taken up across the plasma membrane by osmosis, as molecules move down the water potential gradient.
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What is the movement of water across the root?
The movement is driven by an active process that occurs at the endodermis
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What does the Casparian strip consist of?
Consists of special cells that have a waterproof strip in some of their walls.
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What's the function of the Casparian strip?
To block the apoplast pathway, forcing water into the symplast pathway.
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What is ensured when the apoplast pathway is blocked?
This ensure that water and idissolved nitrates have to pass into the cell cytoplasm through cell membranes.
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What's the functon of transporter proteins?
They can actively transport nitrates from the cytoplasm of cortex cells into the xylem. This lower the water potential in the xylem so water from cortex cells follows into the xylem by osmosis.
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What are the three processes that help move water up the stem?
Root pressure, Transpiration Pull & Capillary Action.
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What is root pressure?
The action of the endodermis moving minerals into the xylem by active transport, drives water into the xylem by osmosis. This forces water into the xylem and pushes it upwards.
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What is Transpiration Pull?
Water molecules are attracted by forces of cohesion. Forces are strong enough to hold molecules together in long chain. Molecules are lost at the top of the column, column pulled up creating the transpiration stream.
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What does the pull of the column of molecules create?
It creates tension in the column of water, hence lignin on the xylem walls prevent the vessel from collapsing.
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What is the coheison tension theory?
Cohesion between water molecules and tension in the column of water.
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What can be done if the column of water is broke in one xylem vessel?
It can still be maintained through another vessel via the pits.
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What is Capillary Action?
The same forces, holding water molecules together also attract the water molecuels to the sides of the xylem vessel. This is adhesion. Due to the narrow xylem vessels, the forces pull water up the sides of the vessel.
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What is translocation?
Is the transport of assimilates throughout the plant in the phloem tissue.
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What does the source release?
The source releases sucrose into the phloem
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What does the sink do?
Removes sucrose from the phloem.
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Describe the process of translocation, Pt.1
Sucrose enters the phloem by active loading. Companion cells use ATP to actively transport H+ ions out of their cytoplasm and int othe surrounding tissue. This set's up a diffusion gradient and the H+ ions diffuse back into the companion cells.
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Describe the process of translocation, Pt.2
This diffusion occurs through special contransporter proteins which enable the H+ ions to bring the sucrose back.
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What happens as the concenctration of sucrose builds up inside the companion cells?
The sucrose diffuses into the seive tube elements through numerous plasmodesmata.
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What happens to the water potential gradient as sucrose enters the sieve tube element?
Reduces water potenetial, as a result water follows by osmosis and icreases the hydrostatic pressure in the sieve tube element.
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Evidence for translocation? Ev. 1
Plant supplied with radioactive carbon dioxide - used in photosynthesis. Carbon soon appears in the phloem.
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Evidence for translocation? Ev. 2
Ringing a tree to remove the phloem, results in sugars collecting above the ring.
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Evidence for translocation? Ev. 3
Aphid feeding on a plant stem can be used to show that mouthparts are taking food from the phloem.
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Evidence against translocation? Ev.1
Not all solutes in the phloem sap move at the same rate.
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Evidence against translocation? Ev.2
The role of sieve plates is unclear.
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Other cards in this set

Card 2

Front

What can & can't leaves do?

Back

Can: obtain water. Can't: obtain water from the air.

Card 3

Front

Water and soluble minerals travel upwards in what tissue?

Back

Preview of the front of card 3

Card 4

Front

Sugars can travel up or down the plant in what tissue?

Back

Preview of the front of card 4

Card 5

Front

Xylem and Phloem are found where?

Back

Preview of the front of card 5
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