Plants gain their nutrients and water via diffusion through the epithelial cells however there are many parts inside the plant where it is further away from the surface and can not obtain these nutrients.
So therefore there is a plant transport system where water is moved in a tissue called vascular tissue.
- Water and soluble minerals travel in the xylem
- Sugars travelorin the phloem tissue
Pholem Tissue consists of 2 types of cells the Sieve tube element & companion cell two cells are linked by PLASMODESMATA.
Sieve tube element
- Sieve tube element consists very little cytoplasm on the lining of the cell with Er, cell wall, plastid and starch grain.
- Sieve tube element transports sugars (sucrose). Sucrose dissolved in water to produce sap.
- Sieve plates consists of pores allows sap to flow in and out.
- very thin walls and no nucleus or ribosome
- large nucleus responsible for control of translocation maintence of Translocation
- Mitrocondria, ribosomes, golgi apparatus, vacuole, rough er, cell wall and dense cytoplasm present. Very Metabolically active.
Xylem- support skeleton of the plant........Dead not metabolically active (tracheids dont have open ends like vessels only pass via pits found in finest branches of the xylem)
- Elongated Lignified cells that are dead and provide support
- long thick walls with lignin as the xylem develops the lignin becomes water-proof so therefore cell contents then die
- Tube - no end walls = xylem vessel
- Lignin strengthens the vessel walls and prevent it from collapsing. Mechanical Strength aloows high pressure of water to flow through and it does easily as theres no cell contents.
- no lignin where the Plasmodesmata is non-lignified (holes) known as pits allow water to pass into another vessel or to the parts of the plant.
Primary Xylem - young root 2 types protoxylem & metaxylem
- PROTOXYLEM 1st xylem developed behind root and shoot tips lignin added in the rings or spirals making spiral vessels or annular vessels
- Metaxylem fully lignified except for pits
Movement of water from root hair cells to xylem
- Root hair cells - thin layer of epidermis and long root hair cells = large surface area
- Minerals enter the cell via active transport the eneergy id needed for atp
- Water potential lower in the the cell, water moves into the cell from soil via osmosis
- water can travel in 3 ways 1.symplast through plasmodesmata cell 2 cell. 2.Apoplast water travels between cell walls NOT through plasma membrane. 3. vacuolar same as symplast pathway but also passes through vacuoles.
- Reaches the epidermis with suberin in casparian ***** blocks apoplast pathway so water is forced to go via symplast
- Endodermal cells moves the water across the endodermis into the xylem via active transport the cortex pumps positive ions into the xylem which lowers the water potential in the xylem so water moves in.
- Water potential gradient is created across the whole cortex
Xylem to leaves
By MASS FLOW - TRANSPIRATION STREAM
- Hydrostatic pressure is high at the bottem and low at the top so liquid moves across this pressure gradient
- Liquid moves up by mass flow. the whole amount of wate flows together. They rely on random movements of individual molecules
- column of water in xylem vessels due to the individual water molecules attracyed together called cohesion
- Water molecules also attract to the inner sides of the xylem vessels this is called adhesion
- Cohesion and adhesion held the whole column of water to flow up the xylem without breaking
From leaf to atmosphere
- Spongey mesophyll has spaces around them filled with air
- the water in cell seeps into the walls so these cell walls always wet
- Some water evaporates into the air spaces so air inside the leaf is saturated with water vapour
- As water collects in these air spaces water ptential rises
- once water vapour is higher than the outside the leaf molecules will diffuse out
- Open stomata allows an easy route for water to pass out from
- stomata open during the day allows gaseous exchange for photosythesis
Potometer measures the water uptake of the plant and for it to be set up there are various stages
- Cut shoot under water, prevents air blocking in the xylem vessels so that water can flow
- allow plant time to adjust to its surroundings
- time how long it takes for the minisus to move a set distance
- Reset the miniscus with water from the syringe and take more readings until water uptake is constant
- continue to record results - calculate the mean of rate of uptake exclude any anamoulas results
Factors affecting the transpiration rate
- Humidity- when low humidity (dry air) greater rates of transpiration theres a steep water potential gradient between leaf and air
- Temperature- when increased temperature there is increased kinetic energy of water molecules which leads to increased evaporation of water from the cell walls into the air spaces and also diffusion of water out of the leaf- greater rates of transpiration
- Light intensity- More light intensity more rate of water uptake, the stomata opens. until a certain high light intensity where the stomata will not open anymore therefore rate of water uptake is constant
- Air movement -no wind little transpiration water molecules collected around the leaf so air is saturated, wind blowing carries water vapour away so then the rate of transpiration is high
Xerophytes are plants which are adpated to reduce the rate of water loss so therefore rates of transpiration are low.
- Small leaves- reduces surface area or rolled up leaves stomata faces inwards so the air around them is saturated with water vapour
- Thick waxy cuticle- waterproof prevents water loss via the epidermis
- Sunken stomata pitts - in the epidermis the moist air gets trapped there which lengthens the diffusion pathway and reduces the evaporation rate
- Hairs - reducing flow of air current around the stomata local humidity next to the leaf decreases air exposure
- Extensive shallow root system- the extensive shallow root systems tend to be in a circular shape which allows quick absorption when it rains
- simillar to xylem vessels bu mass e.g difference in pressure
- requires atp for the plant it has to invest energy its an active transport
- Atp- energy rich compund
- The pressure difference is created by active loading of sucrose into the sieve element
- This decreases the water potential in the sieve element and therefore water follows down the water potential gradient
- When sucrose needs to be removed it is taken from the cell and water again follows by osmosis
- The system creates high hydrostatic pressure in the leaf. low hydrostatic pressure in the roots. Leading to a flow of water that takes solutes with it
SOURCE - an area that loads assimilates into the pholem
SINK - an area that removes assimilates from the pholem
Photosythesis produces sugars (3c triose) these are converted to sucrose
- Sucrose then moves from the palisade cells to the companion cells via the apoplast and symplast pathway.
- H+ ions are pumped out of the companion cell. into the palisade cells ATP is required active process
- However there is now a high water potential gradient in the companion cell so H+ ions will move in with sucrose by the co-transporter H+ will flow down the concentration gardient taking sucrose into the cell
- Sucrose diffuses through the plasmodesmata enters sieve element water follows
RESULT: HIGH HYDROSTATIC pressure in the leaf (source)
- sucrose moves out of the sieve element via diffusion
- Sucrose is quickly broken down by the enzyme intervase
- sucrose to glucose and fructose
- this mechanism creates a concentration gradient down which sucrose flows out of the pholem and into the cell
evidence for translocation
- companion cells pump out sucrose using mitocondria to produce ATP for loading of sucrose
- ph of companion cells are higher than other cells as they are pumping out protons H+ out of their cells (part of loading)
- Aphids insert their stylets into sieve tube elements amd pholem sap enters under pressure
- pholem sap moves much more quickly than diffusion would
- pressures in sieve tubes are more than adequate to move pholem sap