mass transport in plants Flashcards
what is the role of the xylem and how is adapted for its function.
-transports water and minerals in solution which moves up the plant from root to leaves
-long cells joined with no end walls to allows a continuous water column
-cells contain no cytoplasm to remove any obstructions to water flow
-thick cell walls containing lignin which strengthens the walls to withstand pressure
-has pits in the side walls which allow lateral movement of water to get around the blocked vessels
explain the cohesion-tension theory
-water evaporates/ transpires from the leaves
-reduces water potential in the spongy mesophyll cells
-water is drawn out of the xylem
-creates tension
-cohesive forces between water molecules due to hydrogen bonds
-water is pulled up in an unbroken, continuous column
-the outermost layer of roots contain root hair cells which absorb mineral ions from the soil by active transport so that water potential in the root hair cells is lower than the soil thus minerals and water moves in
what evidence is there for the cohesion-tension theory
-there is a change in tree trunk diameter as in daylight the transpiration rates would be higher so there is higher tension in the trunks which pulls the walls of the xylem in causing shrinkage
-if xylem is broken an air bubble will enter which breaks the continuous water column hence water is no longer drawn up
-when a xylem vessel is broken the water does not leak out but instead air is drawn in. this presents that there is a hydro static pressure gradient from the base to the top of the plant which draws air in
what is the function of the phloem?
transports assimilates up and down a plant
what are the three components of phloem vessels
-sieve tube elements form a tube to transport the solutes. they have no organelles, they have little cytoplasm and thick walls which means easier flow and stronger resistance to pressure
-companion cells -carry out metabolic processes to load assimilates into the sieve tube elements. they have numerous mitochondria to produce ATP for active transport of solutes
-plasmodesmata cells - smaller pores in side walls which allow assimilates to be passed between the companion cells and sieve tube elements
recall the mass flow theory
-at the source, hydrogen ions are actively transported from the companion cells into the surrounding photosynthesising cells
-this means the hydrogen ions can move down their concentration gradient along with sucrose molecules by co transport into the companion cells
-sucrose is loaded into the phloem sieve tubes through the plasmodesmata which lowers the water potential in the source end of the phloem
-water enters the tubes by osmosis from the xylem, creating a high hydrostatic pressure at the source end
-this causes mass flow towards the sink
-sucrose actively transported by companion cells into sink for respiration or storage as starch
-increases the water potential in the sieve tubes so water leaves by osmosis lowering the hydrostatic pressure
-produces a hydrostatic gradient so there is a continuous mass flow
give evidence for the mass flow mechanism
-pressure in the sieve tubes as sap is released when they are cute
-sucrose concentration is higher in leaves than the roots
-downward flow occurs in daylight only as photosynthesis is occuring
-companion cells have mitochondria for ATP production
give evidence against the mass flow mechanism
-not all solutes move at the same speed as they would do in mass flow
-sucrose is delivered at the same rate independent of the concentration gradient
-the structure of sieve plates creates a barrier for mass flow
describe the ringing experiment to show mass flow in plants.
-removing a ring of bark from a woody stem makes a bulge above the ring
- the bulge has a higher concentration of sugars than below the ring
-this shows that there is a downward flow of sucrose
describe how radioactive tracing is used to support the mass flow theory
-radioactive Carbon-14 can be used as a radioactive tracer to track the movement of organic substance in a plant
describe how a metabolic inhibitor is used to support the mass flow theory
-a metabolic inhibitor stops ATP production
-it can be placed in a phloem so translocation stops
-this evidences active transport is involved
describe how aphids can be used to show mass flow in plants
-pressure in the phloem can be investigated using aphids to analysis sap flow
-aphids pierce the phloem and the sap flows out fasters nearest the leaves
-this suggests there is a pressure gradient
why is transpiration important for plants?
-transpiration allows evaporative cooling which brings down the temperatures of the leaves
-due to the transpiration it maintains turgor pressure which helps keep the plant cells turgid in order to maintain plant structure and allow the functioning of guard cells which are necessary for photosynthesis
how does temperature affect transpiration?
-the higher the temperature the faster the rate of transpiration
-warmer water molecules have more kinetic energy thus they evaporate from the cells inside the leaves faster
-this increases the concentration gradient between the inside and the outside of the leaf making water diffuse out of the leaf faster
how does wind affect transpiration?
-the windier it is the faster the transpiration rate
-air movement outside the leaf will carry water away which maintains a high water vapour potential gradient so transpiration is higher
