Mass transport in plants Flashcards
What does xylem transport
Water in stems and leaves
theory associated with xylem
Cohesion-tension theory
What does the phloem do
Transports organic substances
Theory associated with phloem
Mass flow hypothesis for the mechanism of translocation in plants
What process is the uptake of water
Passive via osmosis
Uptake of minerals is
Passive or active
Diffusion or active transport
Two pathways that water can take to move across the cortex
Apoplast
Symplast
What structures does the water from the apoplast pathway move through
Spaces though cellulose walls, dead cells and hollow tubes in xylem
Which pathway is faster
Apoplast
What is the casparin strip
Impassible barrier
What happens when water reqaches the endodermis
Casparian strip blocks pathway of apoplast
What does symplast pathway involve
Cytoplasm
Plasmodemata
Vacuole of cells
What does the water move by in symplast and apoplast pathway
Apoplast = diffusion
Symplast = osmosis
How does the water move in the symplast
Water moves via osmosis across the partially permeable cell membrane into vacuole and between cells through plasmodesmata
How does water move in the plant in transpiration
Water potential gradient from soil (high water potential) to the atmosphere (low)
Where do plants lose water
Stomata
What is transpiration
Loss of water via stomata by diffusion
Why is transpiration important
Cool plant via exporative cooling
Supplies plant with water also hekping the transport of ions
What is cohesion
water molecules form h2 bonds between one another and stick together
Whats the transpiration pull
Column of water is pulled up xylem as a result of transpiration
What is the surface area in plants
total area of the organism exposed to external environment
What is the volume of a plant
total internal volume of the organism (space inside an organism)
Interaction between surface area and volume
surface increases volume decreases
why do they interact this way
volume increases more rapidly than surface does
What happens to the water potential of mesophyll cells
when evaporation occurs in their air spaces, they have a lower water potential
so water enters by osmosis from neighbouring cells
Process of how water moves up the stem of the xylem
1) transpiration of mesophyll cells
2) H2 bonds form between water molecules, known as cohesion
3) Water forms unbroken column across mesophyll cells down the xylem
4) Water evaporates from mesophyll cells, drawing water up behind stomata
5) Transpiration pull occurs
6) Transpiration pull puts xylem under tension, cohesion-tension theory
Why is transpiration important
Provides cooling = evaporative cooling
Uptake of mineral ions
Turgor pressure of cells supports leaves and stem (of non-woody plants)
What can cause a water potential gradient and what results in this change
Environmental conditions -> low humidity, high temp
Causes water potential gradient between air inside leaves (high) and air outside (low)
So water vapour diffuses out of the leaves through stomata (transpiration)
Transpiration pull formation
Water vapour lost by transpiration lowers water potential in air spaces around mesophyll cells
As water evaporates out mesophyll cell walls into air spaces a transpiration pull is formed
What does the transpiration pull result in
Water moving out of mesophyll cytoplasm/ walls
Pulls water to leave xylem vessels through pits causing water to be constantly moving up xylem vessels to replace water lost forming a transpiration stream
What pathways move water in which areas
Movement of water through cell walls = apoplast
Movement of water through cytoplasm = sympoplast
What occurs when the rate of transpiration is high
Walls of xylem pulled inwards by faster flow of water
How is transpiration controlled
Guard cells around stomata
Guard cells open when stomata are turgid and close when stomata lose water
Stomata open = greater rate of transpiration and gaseous exchange
Describe the process of transpiration
1) Water vapour diffuses from air spaces through stomata via transpiration, lowering water potential
2) Water evaporates from mesophyll cell walls into air spaces, creating a transpiration pull
3) Water moves through the mesophyll cell (apoplastic pathway) or out of the mesophyll cytoplasm into the cell wall symplastic pathway)
4) Water leaves a xylem vessel to replace the water lost from the leaf
What is the water potential gradient
Loss of water results in potential gradient between leaves (low) and roots high)
What is transpiration
The loss of water vapour from the leaves or stem
What is the transpiration stream
Movement of water through the xylem tissue and mesophyll cells
How does water enter a plant
Not through the leaves, through the roots
Travel through xylem vessels using the transpiration stream where it ends up being released through mesophyll cells via transpiration
Method of potometer experiment (detailed)
1) Cut shoot underwater
2) Place shoot in tube
3) Set up apparatus
4) Make sure it’s airtight using vaseline to seal gaps
5) Dry leaves of shoot
6) Remove capillary tube from water beaker to allow a single air bubble to form and place tube back in water
7) Set up environment investigating
8) Allow plant to adapt to new env
9) Record starting location and end location of air bubble
10) Change intensity of factor being investigated
11) Reset bubble
Potometer exp short of process
1) Record distance of air bubble at start of exp
2) Leave for set period of time
3) Record end location of bubble and calculate distance travelled
4) Reset air bubble using tap of resevoir if neccessary
5) Repeat and change factor being investigated
What does the phloem transport
Phloem sap containing sucrose and water dissolved with substances such as amino acids, hormones and minerals
Source of assimilates (substances transported into tissue that will become tissue)
Green leaves that photosynthesis (glucsoe)
Storage organs (unload substances)
Food stores in seeds (which germinate)
Where are assimilates required
Tissues and sinks
What are some examples of sinks that require assimilates
Meristems that are actively dividing
Roots that are growing or absorbing mineral ions
Parts where assimilates are stores -> developing seeds, fruits, storage organs
Where do assimilates move
Upwards or downwards in the phloem sieve tubes as they move from source to sink
How do assimilates move by
Symplastic (passive process by diffusion through cytoplasm)
apoplastic pathway (active process through cell walls)
How do sucrose molecules move via the apoplastic pathway (loading mechanism)
1) Companion cells pump H+ out of the cytoplasm using proton pumps in cell walls (requires ATP)
2) H+ move down concentration gradient back to the cytoplasm of the companion cell through a cotransporter protein
3) H+ carries sucrose molecules into companion cells against the concentration gradient
4) Sucrose molecules move into the sieve tubes via the plasmodesmata (holes) from the companion cells
5) Allows plants to build up sucrose in the phloem
6) High concentration of sucrose decreases the water potential in the phloem and water enters by osmosis
7) Translocation of sugars towards sink cells because of the high pressure water moves into the phloem at, where sugar is unloaded, from an area of high hydrostatic pressure to an area of low hydrostatic pressure
Where does the unloading of assimilates occur
Sinks
What type of reaction unloads sucrose
Active transport
How are assimilates unloaded
1) Sucrose actively transported out of companion cells and move out of the phloem tissue via apoplast or symlpastic pathway
2) To maintain the concentration gradient in the sink tissue, sucrose is converted to storage molecules using enzymes
How is the unloading mechanism adapted
-The intercellular space has very few organelles so phloem sap can flow easily by moving through open corridor that has low resistance
-Sieve cells have thick cell walls to help them withstand pressure exerted by the mass flow of sugars
How does phloem sap move
By mass flow up and down the plant
Why are carbohydrates transported in the form of sucrose
Efficient energy transfer and increased energy storage
Less reactive than glucose (because it’s a non-reducing sugar) no immediate reactions occur as it is being transported
What is the advantage of mass flow
Moves organic solutes faster
Mass flow in the xylem
Xylem tissue has a pressure differences that causes mass flow to occur because of a water potential gradient between soil and leaf
When is ATP required in mass flow
In phloem tissue NOT the xylem
How is a pressure difference generated during mass flow in the phloem
Pressure difference generated by actively loading sucrose into the sieve elements at the source which lowers the water potential in the sap
This allows water to move into the sievve elements as it travels down the water potential gradient by osmosis
Overall how mass flow works
1) Sucrose is loaded into the phloem through companion cells (at the leaf)
2) Water flows into the phloem from the xylem vessel (using the transpiration stream)
3) Sucrose moves via translocation through sieve tubes in the water moving down the hydrostatic pressure gradient
4) Sucrose is unloaded from the phloem into the sink (at the root cell)
5) Water moves back into the xylem by osmosis
https://cdn.savemyexams.com/cdn-cgi/image/w=1280,f=auto/uploads/2021/01/Phloem_-Mass-flow-from-source-to-sink.png
WHat is the rle of H2 bonding in trhe transport of water in the xylem
Water is lost from the lead due to transpiration
Polar water molecules stick together by H2 bonds by cohesion
Forms a continuous water column
The adhesion of water to xylem walls
So water can be pulled up the xylem vessels
Why do you cut the shoots underwater
To prevent air entering the xyle
Features of the xylem vessels that adapt them for the transport of water
Contain no cytoplasm
Cell walls reinforced with lignin
Water can move between vessels via small gaps called pits in the side walls
End walls are broken down so the celsl form continuous pipes
Functions of the vascular bundles
Transport water from the roots to the leaves
Transport minerals from the roots to the leaves
Transport sucrose from leaves or sink to where they are stored
How is sucrose transported from lower leaves to higher one
1) Active transport of sucrose from the leaf into the phloem
2) Causing the water potential to decrease in the phloem and so water enters by osmosis
3) Results in a high hydrostatic pressure
4) Mass flow upwards to lead buds down the pressure gradient from area of high pressure to low
5) At the higher leaves sugars are unloaded which lowers the water potential of surrounding cells
6) Water moves out of the sieve tube by osmosis and hydrostatic pressure in phloem decreases
7) When water leaves the phloem at the sink celsl it maintains the pressure gradient
Why does sucrose get transported mostly upwards when the plant is growing
Direction of sucrose depends on the location of sink
When a plant is growing the areas undergoing cell division are the main sinks
These areas are at the top of the plant or leaf buds are above leaves
Why is it a benefit that excess sucrose in sink tissues are converted to starch
Starch is insoluble
Starch doesnt cause water to leave the phloem by osmosis so pressure gradient is maintained
