mass transport in plants Flashcards
what are the properties of water and explain them? (6)
● water is cohesive (sticks to other water particles) and adhesive (sticks to other substances) - water is polar (partially negative oxygen and partially positive hydrogen)
so hydrogen bonds form between lone pair of oxygen and hydrogen
● water can act as a solvent - hydration shells when water reacts with something that can dissolve - allows metabolic reactions to occur
● water is a reactive molecule - used in condensation and hydrolysis reactions
● water is a metabolite - involved in important metabolic reactions (photosynthesis and respiration)
● has high latent heat of vaporisation - requires heat energy to convert from liquid to gas
so provides cooling effect through evaporation (sweat)
● high specific heat capacity - acts as a temp buffer (means water is a good habitat) as it takes a lot of heat to change its temperature
what is transpiration?
the evaporation and diffusion of water out of the open stomata which pulls on the column of water in the xylem - due to waters cohesive property
what is happening at the roots of the plant?
● water enters roots via osmosis - lower water potential in root hair cell
minerals enter roots via active transport - minerals lower wp of root hair cell
● root hair cell contains many mitochondria - respiration - ATP production - active transport of mineral ions
root hair cells - increase surface area
● water passes from root hair cells, across the cells of cortex to the xylem
what is the cortex of the roots?
root and stem tissue found between the epidermis and vascular tissue of a plant.
explain transpiration in a plant / cohesion tension theory (8)
● transpiration is evaporating water
● from spongy mesophyll through stomata
● lowers water potential of mesophyll - creates a water potential gradient
● water moves out of xylem via osmosis
● water molecules / hydrogen bonds stick together (cohesion)
● so, water pulled up continuous column
● water is pulled up creating tension
● column doesn’t break because of adhesion between water molecules and xylem walls
what is tension?
pull force generated on the continuous column of water due to the evaporation and diffusion of water out of the stomata
what are features of the xylem? (8)
● have no cytoplasm / hollow - reduces resistance to flow of water to ensure a continuous column of water
● xylem vessels stacked on top of each other - to ensure a continuous column of water
● lignin (a protein) in cell walls - makes walls waterproof and allows them to withstand tension
● thick cell walls - allows them to withstand tension
● narrow tubes - increases surface area for adhesion
● pores - enables sideways flow of water and entry / exit of water
● no end walls - to ensure continuous column of water
● dead
what factors affect the rate of transpiration?
● light intensity - high
● temperature - high
● wind speed - high
● humidity - low
how does light intensity affect rate of transpiration?
● light intensity does not affect transpiration directly
● high light intensity - more open stomata
● more diffusion of water out as increased photosynthesis in guard cells to produce glucose - solute
● this lowers water potential so guard cells become turgid so open
how does high temperature affect rate of transpiration?
● high temperature - more kinetic energy at higher temp
● so, increased evaporation and increased diffusion
how does low humidity affect rate of transpiration?
● less water vapour in air
● high concentration gradient of water
● so, higher rate of diffusion
how does high wind speed affect rate of transpiration?
● wind carries away diffused water molecules
● which creates a high concentration gradient
what are xerophytes?
● dry plants
● have adaptations which help them to reduce water loss in their dry environments
● water loss is reduced by reducing transpiration
how are xerophytes adapted to their dry environments (to reduce transpiration) ?
● sunken stomata - reduces conc gradient between the leaf and the air - reducing evaporation
● leaf hairs - around stomata to trap water vapour
● rolled leaf - with stomata on inside (wind protection)
● fewer stomata - less diffusion of water out of the spongy mesophyll
● waxy cuticle - waterproof cuticle on leaves prevents evaporation
what is a potometer used for?
● investigating transpiration
● enables measurement of water uptake by plants
● unit of volume (cm3) per unit of time (hour/min)
how do you prepare a potometer?
● cut shoot underwater to prevent air getting into xylem
● cut at slant to increase surface area for water uptake
● assemble potometer underwater and insert shoot
● keep end of capillary tube in water
● check apparatus is water tight and air tight
how does a potometer work?
● an air bubble is introduced into the capillary tube
● as transpiration occurs, water moves through the capillary tube, and bubble of air moves with it
● distance moved over a period of time is recorded and the mean is calculated of a number of repeats
● volume of water lost over a period of time can be calculated
why are results from a potometer an approximate value?
● some water is used in photosynthesis
● water is stored in vacuole
● water is produced via respiration
● water is used in hydrolysis and made in condensation reactions
what is translocation?
the movement of sugars and ions through the phloem
describe the mass flow hypothesis
● in source cells (leaf), sucrose (disaccharide of glucose and fructose) is actively transported into phloem by the companion cells (part of the phloem - keeps phloem alive by doing respiration)
source cell is doing photosynthesis or could be hydrolysing starch
● this lowers water potential of the sieve tube as sucrose dissolves in water
so, water moves in from the xylem by osmosis
● increase in volume of water creates an increase in hydrostatic pressure - creates hydrostatic pressure gradient between source and sink cell
● this causes movement of water towards the sink cell
● the sucrose then is actively transported across companion cell into the sink cell and sucrose used by cell e.g. in respiration / photosynthesis or building up starch
● creates high water potential around sink cell - due to less sucrose dissolved in water
so water moves back into xylem by osmosis as it has a low water potential (due to minerals)
explain radioactive tracers
● radioactive carbon (C14) is used by plant to make 14CO2
●14CO2 taken up by photosynthesis which incorporates C14 into glucose and then sucrose
exam question - explain 5 properties that make water important for organisms
● metabolite in condensation / hydrolysis / photosynthesis / respiration
● a solvent so metabolic reactions can occur
● high heat capacity so buffers changes in temperature
● large latent heat of vaporisation so provides cooling effect (through evaporation)
● cohesion between water molecules so supports column of water in plants
● cohesion between water molecules provides surface tension so supporting small organisms
exam question - describe the cohesion tension theory of water
● water is evaporated through open stomata
● which lowers water potential of mesophyll
● creating a water potential gradient
● water pulled up xylem creating tension
● water molecules are cohesive due to hydrogen bonding
● forming continuous water column
● adhesion of water molecules to walls of xylem
exam question - describe the mass flow hypothesis for the mechanism of translocation in plants
● in source cell, sucrose is actively transported into sieve tube of phloem
● by companion cells
● lowers water potential of sieve tube and water from xylem enters by osmosis
● increase in hydrostatic pressure causes mass movement towards sink
● sucrose used in sink cell for respiration / storage
