Plant transport: F211: 1.2.3

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F211 Biology Mind Map on Plant transport: F211: 1.2.3, created by m.c.ridley on 02/23/2014.

Created by m.c.ridley over 5 years ago
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Plant transport: F211: 1.2.3
1 Transpiration
1.1 Constant evaporation of water from aerial parts of a plant. Transpiration stream = constant flow of water from roots to shoots.
1.2 Rate = lower in afternoon - plant is wilting; stomata may be closed; more humid (humidity decreases the water VAPOUR potential gradient). However, it is unavoidable during daylight since stomata need to be open to allow for gas exchange for photosynthesis. Rate = fastest in WARM and WINDY conditions (WARM = photosynthesis faster so more need for water; WINDY = maintains a high water VAPOUR potential gradient.
1.3 Contributes to water movement up stem in: a) leaf - water loss in leaf is replaced by apoplast/symplast/vacuolar pathway down a water potential gradient; b) xylem - loss of water at the top of the leaf means hydrostatic pressure is less at the top of the stem, therefore water moves down a pressure gradient; there is cohesion/adhesion of water molecule. [cohesion = attraction with other water molecules; adhesion to sides of xylem].
1.4 Potometer = estimate of transpiration rate since it measures uptake of water NOT loss. To use a potometer, you need to use a healthy shoot; cut the shoot 2cm from the bottom (to ensure no air in xylem); watertight seals (to ensure water uptake is accurate); use dry leaves (so evaporation from stomata not affected). Repeats = reliability.
1.5 Constant flow of water from roots to leaves = transpiration stream. Achieved by a) water moving from a higher water potential to a lower water potential; b) high hydrostatic pressure at bottom of xylem; c) transpiration at the leaves (i.e. giving a lower hydrostatic pressure here); d) water under tension; e) cohesion between water molecules; f) adhesion between water and xylem
2 Phloem = sieve tubes and companion cells. Moves SUCROSE from sources (where sucrose is loaded) to sinks (where sucrose is deposited). Leaves = sources; DEVELOPING buds/GROWING tips = sinks.
2.1 Sieve tubes = living; moves sucrose in BOTH directions (unlike xylem with water); no lignin; no bordered pits (but does have PLASMODESMATA to allow sucrose to move from companion cell to sieve tube).Cross walls allow sap to flow.
2.2 Companion cells - contain mitochondria to produce ATP required for active transport. Contain plasmodesmata to allow communication between companion cells and sieve tubes.
2.2.1 Loading assimilates (sucrose) into phloem is the role of the companion cell. It involves a) active transport of H+ out of the companion cell. b) This creates a H+ concentratio gradient so H+ diffuses back into the companion cell. c) As it does this, sucrose moves in with the H+. d) Sucrose then diffuses via the plasmodesmata into the sieve tube element Evidence for = companion cells have lots of mitochondria. Evidence against = role of sieve plates unclear
3 Xerophytes = plants which survive hot, dry conditions
3.1 Have a thick cuticle for waterproofing; rolled leaves (to trap water VAPOUR - this reduces the water potential gradient meaning water is less likely to evaporate); reduced SA (so less water evaporation); hairs (to trap water VAPOUR); fewer stomatal pores (so less water evaporation)
4 Uptake of minerals from the soil is by active uptake. A consequence of this is that the water potential in the root is lower than the soil. Water is able to move in by osmosis from a high water potential to a lower one down a water potential gradient. Once in, water moves by the APOPLAST (in between cells - can also carry minerals with it); SYMPLAST (through the cytoplasm - using plasmodesmata to get from one cell to the next) and VACUOLAR (through the vacuoles).
4.1 CASPARIAN STRIP blocks the APOPLAST pathway so the water potential in the xylem is lowered meaning water will enter by OSMOSIS from higher water potential to lower water potential.
5 Plants need transport systems due to low SA: vol rations. They are XYLEM and PHLOEM
5.1 Xylem = water/minerals up a plant to leaves for photosynthesis. Made of continuous column of DEAD cells; contains BORDERED PITS (to allow water to move in/out and supply water to other parts of the plant) and LIGNIN (strength - stops it collapsing; flexibility - stops stem from breaking; c) waterproofing (to reduce water loss).

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