Mass Transport In Plants Flashcards
Explain how water enters the xylem
Root hair cells actively transport ions from soil across their membranes
Into their cytoplasm
Via carrier proteins
Lowering water potential so it becomes more negative in cytoplasm than in soil
Water moves into root hair cells via osmosis down a water potential gradient
Water passes from cell, across endodermis and into xylem vessels
How is water transported to leaves
Transpiration from leaves
Creates cohesion tension
Due to hydrogen bonding between water molecules
Adhesion of water molecules binding to xylem creates a continuous column of water
Which is pulled up xylem
Due to negative pressure
Factors that affect transpiration
Light intensity
Temperature
Air movement
Humidity
What causes root pressure
Active transport by endodermis Of ions/salts Into xylem Lowers water potential in xylem So water enters by osmosis
Root adaptations
Hair like extentions: Increase surface area so increase uptake of water and increase the area for channel and carrier proteins
Thin cell wall: Shorter diffusion pathway
Mitochondria: Lots for ATP synthesis to provide the energy needed for active transport of ions/salts
Osmosis
Movement of water from a region of higher water potential to an area of lower water potential across a partially permeable membrane
Via aquaporins
Why do plants need nitrates
DNA
RNA
To make the nitrogenous bases in nucleic acid
Why do plants need magnesium ions
To make chlorophyll
Why do plants need phosphate ions
To make nucleotides of DNA and RNA
To make ATP
To make phospholipids in the phospholipid bilayer
Function of xylem
Transports water from roots up the stem to leaves
Function of phloem
Transports sugar and organic substances
From leaves where they are formed in photosynthesis
To where they are needed (shoots, roots, flowers, fruits)
Xylem adaptations
Dead cells form hollow tubes with no cytoplasm/organelles
~Allows easier water flow with no impediments from organelles
No end walls because end walls break down and form continuous tubes
~Water can form a continuous column
Cell walls are strengthened with lignin
~Xylem waterproof and rigid so provides support and allows them to withstand tension/pressure
Xylem pits that are little holes allowing water to move laterally between xylem vessels
~If one vessel gets blocked water can still get around it so flow of water isn’t slowed or stopped
What are xylem pits
Little holes in xylem vessels
Allowing water to move laterally between xylem vessels
So water can get around blocked vessels
Flow of water isn’t slowed or stopped
What is transpiration
Evapouration of water from a plant
Driving movement of water up a xylem vessel
Explain transpiration and how this leads to a transpiration stream
Stomata open
Water diffuses from airspaces (higher water potential) inside the leaf to (lower water potential) outside of leaf
Loss of water from air space causes movement down a water potential gradient
From mesophyll to air spaces
Lowers water potential of mesophyll cells
Water moves by osmosis from adjacent mesophyll cells
Creating a water potential gradient across the leaf to the xylem vessels
Water enters leaf via osmosis down a water potential gradient
Causing it to be pulled up under tension through xylem from roots
Water forms a continuous column in the narrow xylem vessels
Water molecules form weak hydrogen bonds between each other so stick together (cohesion)
Water attracted to hydrophilic walls of xylem forming forces of adhesion
Pulling force is great
Column of water is under tension
Movement of water through plant from roots to leaves is known as a transpiration stream
What is a transpiration stream
Movement of water through plant from roots to leaves
Explain the cohesion tension theory
Water evaporates from leaves
Lowering water potential in cells
Water is drawn out of xylem
Creating tension/negative pressure
Due to cohesive forces between water molecules due to hydrogen bonding
Adhesion between hydrophilic walls of xylem and water molecules
Water is pulled up as a continuous column
How is a high pressure produced in the leaves
Sugars enter the phloem
So water potential becomes lower/more negative
Water enters leaves by osmosis down a water potential gradient
Increased volume of water in the leaves
Causes an increased pressure in the leaves
Explain the negative pressure in the xylem
Water evaporates from leaves
Lowering water potential in cells
Water is drawn out of xylem
Creating cohesion tension/negative pressure
Due to cohesive forces between water molecules due to hydrogen bonding
Adhesion between hydrophilic walls of xylem and water molecules
Water is pulled up as a continuous column
How does light affect transpiration
Doesn’t affect transpiration directly
Stomata open in the light and close in the dark
Rate of transpiration is higher in the light
How can the factor of temperature be managed to stop transpiration
Use a tank of water to absorb heat from the lamp in experiment
Since heat/temperature is a factor affecting the rate of transpiration
How does temperature affect transpiration
As temperature increases so does the kinetic energy of water molecules
Move more rapidly
Temperature increasing causes an increase in the rate of diffusion
So rate of transpiration increases
How does humidity affect transpiration
Air spaces in the leaf are saturated with water vapour
Air outside contains much less water vapour
Increased humidity means greater difference in humidity between air spaces and air means greater rate of transpiration
Due to a larger water potential gradient
Water leaves leaf down a water potential gradient
How does air movement/wind speed affect transpiration
Air movement over leaf moves water vapour away from the stomatal pores
Increases water potential gradient between inside and outside of the leaf
Greater rate of air movement means faster the movement of water vapour
So the greater the rate of transpiration
Explain the potometer experiment
Leafy shoot cut cut diagonally under the water
potometer filled completely with water so no air bubbles
Rubber tube used to fit leafy shoot to potometer under water
Potometer removed from under water
All joints sealed with waterproof jelly
Air bubble introduced into capillary tube
By slightly tilting a beaker full of water at the end of the tube
As transpiration occurs water moves through the capillary tube, air bubble moving with it
Distance moved over a period of time recorded and a mean is calculated from repeats
Volume lost can be calculated over a period of time
Precautions for potometer experiment
Seal joints Cut shoot underwater Cut shoot diagonally Dry leaves No air bubbles present other than one introduced Note the start point of air bubble
Why are joints sealed in potometer experiment
Sealed with a waterproof jelly
Stops water getting out and air getting in
Making potometer airtight
Why is the shoot cut underwater in the potometer experiment
Stops air bubbles being taken into xylem and breaking the cohesion tension
Why is the shoot cut diagonally in the potometer experiment
Stops the end of xylem closing
Why do you dry the leaves in the potometer experiment
Water on leaves lowers the water potential gradient and blocks the stomata opening
Limitations of photometer experiment
Only a proxy measure of transpiration
Absence of roots
Volume of water taken up isn’t equal to the volume of water lost in transpiration
Because water used for other processes like photosynthesis, hydrolysis, support and turgidity
Mass flow hypothesis
In source (leaf) sugars are actively transported into phloem
By companion cells
Lowering water potential of sieve tubes
So water enters via osmosis
Increase in volume of water causes an increase in pressure
Causing a mass movement towards sink (roots) due to hydrostatic pressure gradient
Sugars converted in roots for respiration or storage
Explain what happens at a source
Active transport is used to load solutes (e.g. sucrose from photosynthesis)
From companion cells into the sieve tubes of the phloem at the source
Lowering the water potential inside the sieve tubes
Water enters the sieve tubes by osmosis from the xylem and companion cells
Creating a high pressure inside the sieve tubes at a source end of phloem
Resulting in a hydrostatic pressure gradient from a source to sink
Which pushes solutes along sieve tubes towards sink
Explain what ringing experiments are
A ring of bark (containing phloem but not xylem) is removed from a woody stem
Causing a bulge to form above ring
Fluid from bulge has a higher sugar concentration than the fluid below the ring
Because sugars can’t move past since the bark removed contains phloem
Evidence of a downward flow of sugars
Evidence for mass flow from ringing experiment
Accumulation of sucrose/amino acids in bulge above cut phloem but a lower sugar concentration below
Sucrose below cut carries on moving to sink until it runs out
Evidence against mass flow hypothesis from ringing experiments
Sieve plates would create a barrier to mass flow
Sugar travels to many different sinks not just the one with highest water potential as the model suggests
Explain what happens at a sink
Solutes are removed from the phloem to be used in respiration for energy for active transport or to be stored as starch
Increasing the water potential inside sieve tubes
So water leaves the tubes by osmosis
Into the xylem
Lowering pressure inside sieve tubes
And creating a hydrostatic pressure gradient from source to sink
Gradient pushes solutes along sieve tubes towards sink
What are radioactive tracer experiments
Supplying leaves with radioactive carbon 14 (carbon dioxide)
Which gets converted to Glucose in photosynthesis
Then sucrose or other organic substances
Allowing radioactive carbon 14 to be tracked as it is transported throughout the plant
What do radioactive tracers show
Movement of radioactive carbon 14 through the plant
When converted into glucose in photosynthesis
Then sucrose
Shows translocation
What could potentially hinder mass transport of sugars
Sieve plates
How are sieve cells adapted for mass transport of sugars
Living cells that have no nucleus and few organelles so no obstructions to mass flow
Each sieve tube element has a companion cell to carry out living functions for sieve cell
Connected to eachother through sieve plates
Pores allow sucrose/amino acids to move through
How are companion cells adapted for mass flow of sugars
Each companion cell carries out living functions for one sieve cell
Contain many mitochondria for ATP synthesis through aerobic respiration for the active transport of solutes
Plasmodesmata connecting companion cells and sieve tubes allow movement of substances like ATP, proteins, solutes
How can heat treatment affect a plant
Damages/denatured enzymes involved in ATP synthesis to produce energy needed for active transport of solutes
So damages phloem
Some water is transported in the phloem but most in xylem
Flow of water not really affected since it is a passive process
Osmosis occurs down a water potential gradient which doesn’t require ATP
How is pressure generated in the phloem
Active transport used to load solutes from companion cells Into sieve tubes of phloem At the source Lowering the water potential in sieve tubes Water enters via osmosis Down a water potential gradient From xylem and companion cell Increased volume of water in phloem Increases the hydrostatic pressure At the source
How does the mass movement of water relate to ion dispersion
Mass movement of water ensures mineral ions are transported around plant
Advantage of evaporation other than in xylem and water transport
Has a cooling effect
Reduces chance of denaturation of enzymes
Graph for rate of transpiration vs temperature
Directly proportional
Passes through the origin
Straight line
Increase in kinetic energy increases speed water molecules diffuse at
Graph of rate of transpiration against light intensity
Positive y intercept
Proportional increase for a bit
Begins to plateau
Increase in light intensity causes increase in stomata opening (increase photosynthesis)
Plateaus when all stomata open
Stomata limiting factor
Explain the graph of rate of transpiration against humidity
Positive y intercept
Straight line with negative gradient
Increasing humidity decreases water potential gradient
Structure of phloem
Phloem tissue formed from cells arranged in tubes
Sieve tubes
Sieve plates
Companion cells
How can aphids be used to show mass flow
To investigate pressure in phloem They pierce phloem Then their bodies removed Mouth left behind Allowing sap to flow out Flows out quicker nearer leaves than further down stem Evidence of a pressure gradient
How do metabolic inhibitors show mass flow
Stop ATP production Put into phloem Translocation stops Therefore active transport involved Can be tracked using a technique called autoradiography
Sinks
Where sugars and organic substances are needed
Roots
Shoots
Flowers
Fruits
