Mod 2: 1.2.2 Transport in Plants

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Explain the need for transport systems in multicellular plants in terms of size and surface area:volume ratio
Because they are multicellular they have a small surface area:vol ratio. Exchange of substances via direct diffusion is too slow. A transport system is needed to move substances to and from cells quickly.
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What two tissues are involved in transport in plants and what do they transport.
Vascular bundles, Xylem- transports water and mineral ions. Phloem- transports dissolved substances, like sugars.
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Describe the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants.
In a root, the xylem and phloem are in the centre to provide support for the root. In stems, the xylem and phloem are near the outside to reduce bending. In a leaf, xylem and phloem make up a network of veins which support thin leaves.
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What is the structure and function of xylem vessels?
Xylem vessels, long hollow tube. Cells are dead so contain no cytoplasm. Walls contain lignin (woody substance) makes it thicker and supports the vessels to stop inward collapsing. Water/ions move in and out of vessels through small pits.
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What are the structures and functions of sieve tube elements and companion cells?
Living cells that form the tube for transporting solutes, joined end to end. Sieve's = end walls, contain holes for solutes to pass through. No nucleus, few organelles. Companion cells provide energy for active transport of solutes and the cell.
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What is transpiration?
The evaporation of water from a plants surface (leaves).
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How is transpiration a consequence of gaseous exchange?
Plant opens stomata to let in co2, to make glucose (photosynthesis). Water also gets out, higher concentration of water inside leaf than air, water moves out down water potential gradient
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What are the factors that affect the transpiration rate?
Light- faster rate, stomata open when light. Temp- higher temp=faster rate, water evaporates faster, increases water potential gradient. Humidity-lower=faster rate, increased water potential gradient. Wind-windier=faster, blows away water molecules.
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How is a potometer used to estimate transpiration rates?
Measures the plants water uptake. (Explain experiment step by step)
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Explain in terms of water potential, the movement of water between plant cells and their environment.
Water moves down a water potential gradient. Soil generally has higher water potential, leaves have a lower water potential (evaporation) Creates water potential gradient that keeps water moving through plant in right direction, from roots to leaves.
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What are the 3 pathways water can take to reach the xylem?
Apoplast- through spaces and cell walls (simply diffuse as cell walls are absorbent) Symplast- through cells cytoplasm (cytoplasm of neighbouring cells connect by plasmodesmata (small channels) Vacuolar- through the cells vacuole.
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Which pathway is blocked before reaching the xylem and by what?
Apoplast pathway- when water reaches the endodermis cells in the root, Casparian strips (waxy) stop it entering the xylem so it takes the Symplast route. Cell membrane can now control what substances in the water go through.
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Once the water is in the xylem, how is it transported to the leaves?
Water leaves xylem at the leaves by the apoplast route. Water evaporates from the cell walls into the spaces between cells in the leaf. When stomata open, water moves out down water potential gradient into air.
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What is the mechanism by which water is transported from the root cortex to the air surrounding the leaves?
Transpiration stream- Cohesion, tension & adhesion. Water evaporates from leaves, creates a tension (suction) pulling water up, water=cohesive (they stick together) so move up altogether. Adhesive xylem walls, water is attracted to them, so rises.
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How are the leaves of some xerophytes adapted to reduce water loss by transpiration?
Sunk in stomata, sheltered from wind. Less stomata, fewer places water is lost. Thick, waxy epidermis layer, waterproof. Spines instead of leaves, less surface area for water loss. Curled leaves + Hairs, trap moist air around stomata (lower WPG)
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How and why is there a lower concentration of substances in the 'sink' of a plant than in the 'source' of a plant?
Ensures there is a constant supply of new substances e.g. sucrose reaching the sink from the phloem. Enzymes maintain a concentration gradient from source to sink by changing or breaking down dissolved substances at the sink. E.g sucrose into starch
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Describe the mass flow hypothesis and how it explains phloem transport.
Active transport loads dissolved solutes (at source) into sieve tubes in phloem. Lowers water potential inside sieve tubes, water enters (osmosis). Creates high pressure at source end. Opposite at sink end. Pressure gradient- moves solutes along.
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What is the evidence for and against for translocation / mass flow hypothesis?
For- Ringing, Aphids and Radioactive isotopes (lookup) Against- sugar travels to many different sinks, not just one with higher water potential. Sieve plates would create barrier to mass flow, a lot of pressure needed.
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Other cards in this set

Card 2

Front

What two tissues are involved in transport in plants and what do they transport.

Back

Vascular bundles, Xylem- transports water and mineral ions. Phloem- transports dissolved substances, like sugars.

Card 3

Front

Describe the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants.

Back

Preview of the front of card 3

Card 4

Front

What is the structure and function of xylem vessels?

Back

Preview of the front of card 4

Card 5

Front

What are the structures and functions of sieve tube elements and companion cells?

Back

Preview of the front of card 5
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