1.2- S6: Transport in plants

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What is the xylem?
Plant transport tissue carrying water from the roots to the rest of the plant. Hollow columns of dead cells lined end to end and lignin.
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What is the phloem?
Plant transport tissue carrying products from photosynthesis to the rest of the plant. Sieve tube elements and companion cells.
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How are xylem vessels adapted to transport water and mineral ions?
Long vessel elements joined together- dead cells due to lignin on walls of dead cells being waterproof. No cytoplasm, hollow tubes. Lignin thickening forms patterns spiral patterns in wall- support and flexible. Pits in walls- water/ions transport.
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How is the phloem tissue adapted for transporting solutes?
Sieve tube elements- living cells joined end to end. Little cytoplasm and no nucleus. Pores allow sap to flow and plasmodesmata between sieve tube elements. Companion cells- large nucleus and many mitochondria produce ATP needed for active transport
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How does water enter root hair cells?
Soil around roots has a higher water potential and leaves have a lower water potential due to transpiration. The water potential gradient keeps water moving in the correct direction.
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How can water move from the root into the xylem tissue?
Symplast pathway- Through living parts of cell- cytoplasm and transported through plasmodesmata. Apoplast pathway- non-living parts of cells- through cell walls and spaces between cell walls.
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What is the role of the Casparian strip?
endodermis layer between the root cortex and xylem. It blocks apoplast pathway- ions must diffuse through cell membranes via transporter proteins. Concentration of substances controlled. Ions into xylem reduces water potential- water in by osmosis.
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How does water move up the stem of plants?
Root pressure: water always moving into xylem by osmosis- pushes water up. Cohesion holds water molecules together in chain, when lost from above, molecules move up in chain. Tension created pulls water up stem. Adhesion- attracted to walls of xylem
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How does water leave the leaves?
Water evaporates from the cell walls into spaces between cells in the leaf. When stomata open, water vapour diffuses out of leaf down water vapour concentration gradient. Transpiration.
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What are the 3 steps to transpiration?
Osmosis from the xylem to mesophyll cells. Evaporation from the surface of the mesophyll cells into the intercellular spaces. Diffusion of water vapour from the intercellular spaces out through stomata.
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How does the number of leaves, light and stomata affect water loss?
Leaves- higher SA, more water vapour can diffuse out. Light- stomata open for the gaseous exchange for photosynthesis. Stomata- larger, more water vapour diffuses out of leaf. Stomata on lower surface of leaf- water loss is slower.
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How does temperature and humidity affect water loss?
Temperature- increase rate of evaporation from surface of cells. More kinetic energy so rate of diffusion through stomata rises.Humidity- higher decreases rate of water loss- smaller water vapour potential gradient.
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How are xerophytic plants adapted to reduce water loss and slow transpiration?
Stomata- pits or on underside of leaf to shelter from wind and sun. Hairs on epidermis trap water vapour, decreasing WVP gradient. Curled and small leaves trap water vapour and shelter against wind. Spines reduce SA. Waxy cuticle waterproof.
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What is translocation?
The transport of assimilates throughout the plant in the phloem tissue.
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How does sucrose enter the phloem?
ATP used by companion cells -> H+ ions out of cytoplasm into surrounding tissue. Diffusion gradient is created, H+ ions diffuse back into companion cells with sucrose molecules, through cotransporter proteins. Sucrose diffuse into phloem- C gradient.
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How does sucrose move along the phloem at the source?
Sucrose enters sieve tube elements, decreasing water potential, so water moves into phloem by osmosis, increasing the hydrostatic pressure
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How does sucrose move along the phloem at the sink?
Sucrose molecules diffuse or are actively transported into surrounding tissue for storage or respiration. This increases water potential in the phloem, so water moves out of the phloem by osmosis, decreasing the hydrostatic pressure.
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How does sucrose move along the phloem from source to sink?
Water moves along the source from high hydrostatic pressure in the source to lower hydrostatic pressure in the sink. This flow of water transports sucrose and assimilates along the phloem- mass flow.
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What is evidence for the mass flow mechanism?
Concentration of sucrose higher in source than sink. End of aphid stylet found in the phloem tissue. When a tree is cut, sugar collects above the cut and acts as a sink, not further growth below the cut.
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Card 2

Front

What is the phloem?

Back

Plant transport tissue carrying products from photosynthesis to the rest of the plant. Sieve tube elements and companion cells.

Card 3

Front

How are xylem vessels adapted to transport water and mineral ions?

Back

Preview of the front of card 3

Card 4

Front

How is the phloem tissue adapted for transporting solutes?

Back

Preview of the front of card 4

Card 5

Front

How does water enter root hair cells?

Back

Preview of the front of card 5
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