Transport in plants
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| Two types of tissue involved in transport in plants | Xylem tissue transports in water and mineral ions in solution - substances move up from the plant to the leaves. Phloem tissue transports sugars - both up and down the plant
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| Root - vascular system | Xylem is in the centre and surrounded by the phloem to provide support for the root as it pushes through the soil
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| Stems - vascular system | Xylem and phloem are near the outside to provide a sort of 'scaffolding' that reduces bending.
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| Leaf - vascular system | Xylem and phloem make up a network of veins which support the thin leaves
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| Xylem vessels - adaptation for transporting water and mineral ions | Long tube-like structures made from vessel elements joined end to end. No end walls -H2O can pass through. Cells are dead -no cytoplasm. walls-thick- lignin to support xylem - increases as plant gets older. H2O and ions move in & out- pits with no lignin
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| Phloem tissue - adaptation fro transporting solutes | Transports solutes - mainly sucrose around plant. Cells arranged in tubes. Purely a transport tissue - isn't used for support as well. Phloem tissue contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells.
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| Sieve tube elements | Living cells that form the tube for transportation. Joined end to end to form sieve tubes. The 'sieve'parts are the end walls - have lots of holes - allows solutes to pass through. No nucleus, very thin cytoplasm, few organelles. Cytoplasm's are connected
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| Companion cells | The lack of a nucleus and other organelles in sieve tube elements means that they can't survive on their own - companion cell for every sieve tube element. Companion cells carry out the living functions for both themselves and their sieve cells.
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| How does water enter a plant? | H2O - soil - root - xylem. It enters through root hair cell and then passes through root cortex, including the endodermis, to reach the xylem, H2O drawin into roots by osmosis- travels down water potential gradient.
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| Water potential | H2O moves from HWP to LWP. Soil around roots have HWP and leaves have a LWP (water constantly evaporates from them). This creates a water potential gradient that keeps H2O moving through the plant in the right direction, from roots (high) to leaves (low)
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| Symplast pathway | Goes through living part of the cell - the cytoplasm. The cytoplasm's of neighbouring cells connect through the plasmodesmata. Water moves through the symplast pathway via osmosis
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| Apoplast pathway | Goes through the non- living part of the cell - the cell wall. Walls are very absorbent and water can diffuse through and pass through spaces between them. Water can carry solutes and move from areas of HHP to LHP
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| Apoplast to Symplast pathway | H2O gets to the endodermis cell in the root its path is blocked by the casparian strip - H2O has to move to the symplast pathway. Useful as it the water has to go through a cell membrane -partially permeable - control if a substance in H2O gets through
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| Up xylem and out of the leaves - water transport | Xylem vessels transport water all around the plant. Leaves - water leaves xylem and moves into cells mainly by apoplast pathway. H2O evaporates from cell wall into spaces between cells in the leaf. Stomata open -H2O diffuses out of leaf -down WPG into air
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| Transpiration stream | Movement of water from roots to leaves
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| Cohesion and Tension | Help H2O move against gravity (roots to leaves) H2O evaporates from leaves, creates tension (suction) pulling more H2O into leaf. H2O molecules - cohesive (stick together)- some pulled, others follow. H2O moves up. H2O enters through root cortex cells
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| Adhesion | Partly responsible for movement of water. Water molecules attracted to each other and walls of the xylem vessels. Helps water rise up through xylem vessels.
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| Transpiration | Loss of water from a plants surface especially the leaves -result of gas exchange
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| How is transpiration a consequence of gas exchange? | Plant opens stomata - lets CO2 in - produces glucose (photosynthesis) - this lets H2O out. Higher concentration in leaf than air. Moves out leaf down WPG when stomata open. Transpiration - side effect of gas exchange needed for photosynthesis
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| Light - factors effecting transpiration rate | Lighter it is the faster the transpiration rate. Due to stomata opening when it gets light, so CO2 can diffuse into leaf for photosynthesis. When dark stomata re usually closed so little transpiration.
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| Temperature - factors effecting transpiration rate | Higher the temperature the faster the transpiration rate. Warmer H2O molecules - more energy - evaporate from cells inside leaf faster. increases water potential gradient between in and outside of leaf - H2O diffuse out faster
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| Humidity - factors effecting transpiration rate | Lower the humidity the faster the transpiration. If the air around the plant is dry the water potential gradient between the leaf and the air is increased, increasing transpiration.
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| Wind - factors effecting transpiration rate | Windier it is the faster the transpiration. Lots of air movement blows away H2O molecules from around the stomata. Increases water potential gradient increasing rate of transpiration.
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| Xerophytic plants - adapted to reduce water loss | Marram grass (MG)- stomata deep in pits (wind shelter) layer of hairs on endodermis & rolls leaves (traps moist air, reduces SA) - reducing WPG slowing transpiration. Cacti & MG have thick waxy layer- reduces water loss. Cacti have spines & close stomata
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| Hydrophillic plants - adapted to survive in water | Air spaces help plant to float - can act as O2 store (respiration) & increases amount of light received. Stomata usually only present on the upper surface of floating leaves - maximise gas exchange. Flexible leaves & stems - prevent damage by H2O currents
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| Translocation | Movement of dissolved substances - e.g. sugars (sucrose) & amino acids to where they're needed in a plant. Dissolved substances in are sometimes called assimilate. Energy requiring (happened in phloem) moves substances from 'sources' to 'sinks'
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| Sink and Source | Source- where a substance is made up (it's at a high concentration there) Sink- where it's used up (LC). Some parts of a plant can be both e.g. sucrose can be stores in leaves and roots. Enzymes maintain a CG from source to sink - always LC at sink
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| Phloem Transport (mass flow hypothesis) | AT-loads solutes into sieve tubes. lowers WP in sieve tube- H2O enters-osmosis from xylem to companion cell- high pressure at source end. Sink end- solutes removed-high WP- H2O leaves- low pressure. Result-pressure gradient- pushes solutes to needed place
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| Active loading | Used to move substances into companion cells (CC)into sieve tubes against CG. ATP transports H+ out of cell. Sets up concentration gradient-more H+ ions in tissue than cell. H+ & sucrose binds to a co-transporter in CC -sucrose move to sieve plates
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| ATP | Product of respiration. The breakdown of ATP supplies the initial energy needed for the active transport of the H+ ions.
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