Water uptake and movement Flashcards
what is the importance of water in plants ?
Water provides the cellular solute for metabolism and cell function
What does tugor pressure do in plants ?
Tugor pressure derived from water content of vacuoles generates tugor of cells and gives plant tissue rigidty and architecture
How does tugor pressure help young cells ?
Tugor pressure not only confers rigidty on mature plant structures but also drives expansion of young cells during unfolding and expansion of structures such as leaves and flowers
Where do root hairs differentate from ?
Differentiate from files of epidermal cells in the differentiation zone
how does root hair cells differentiate ?
The hair part of the cell extends by tip growth and is initiated at the lower end of the epidermal cell - the nucleus moves into the hair during development
How long do root hairs function for ?
Root hairs are only functional for around 48 hours and then die or the hair is sloughed by the soil
where do root hairs get generated from ?
New root hairs are constantly being generated by the differentiation zone as the root extends
Where does most water uptake by the entire root system occur ?
in the root hair zone of each root
What are 3 functions of root hairs ?
- Supplement older parts of root system which are becoming inefficient due to suberisation and secondary thickening - these reduce permeability
- Allow invasion of new areas of soil-important because hydraulic conductivity falls rapidly as soil dries, leading to formation of drying zones around roots
- Enable penetration of soil particles and soil crevices
What are the two radial pathways for water uptake ?
Water can be taken up either through cells, which requires crossing plasma membranes and moving between cells ( SYMPLASTIC PATHWAYS) or water can flow through the cell walls and intercellular spaces at least up until the endodermis ( APOPLASTIC PATHWAY )
How does water loss the plasma membrane ?
1- Aquaporins are a group of membrane proteins which provide a water channel in the protein pore
2- Water movement is entirely driven by osmosis and water can move in either direction according to differences in osmotic potential. Rate of water movement is roughly 3 billion water molecules per second.
what can be used
to show aquaporin distribution in barely root tissue ?
A fluroscent antibody probe against aquaporins in sections of barely root tissue shows aquaporin distribution.
How can aquaporins can close due to environmental stress ?
1- During flooding a histidine residue is protonated
2- During drought, specific serine residues are desphosphorylated
3- In both cases, this causes the aquaporin pore to close
How does water move between cells in the symplastic pathway ?
1- Plasmodesmata are structures interlinking plant cells through their joined cell wall
2- An average cell has between 1000 and 100000 plasmodesmata connecting it with adjacent cells
3- Movement through plasmodesmata is controlled in various ways
What is the Caparian Strip in Endodermis layer ?
The endodermis layer of cells have a ring of suberin in their cell walls which prevents the passage of water through the apoplastic pathway - this suberinised layer is called the CASPARIAN STRIP
How does the plant control the apoplastic pathway of water uptake ?
Water must be imported into the endodermal cytoplasm and out again to move to the xylem - This gives plants control over the apoplastic pathway of water uptake
How does water move from cell to cell in the symplastic pathway ?
by osmosis down a gradient of water potential until it reaches the xylem cell in the vascular tissue.
What are the 2 types of cells which transport water upwards throughout the above ground parts of the plants ?
Tracheids and vessel elements
What are the only plumbing cells in gymnosperms ?
tracheids
What plumbing cells are angiosperms ?
Tracheids and vessel elements
What are Tracheids - give the structural features ?
1- Tracheids are long spindle-shaped cells which form an overlapping network
2- They have pits in their sidewall where the cell wall is thin and these pits join up between adjacent cells to form pit-pairs, forming a low resistance pathway for water flow.
What are vessel elements ?
Are shorter than tracheids but have pits in their side walls and peforated end walls. Thus they can stack into long xylem vessels up to several metres long.
What does lignification do to the walls of vessel elements ?
Lignification in the side walls can either be as spirals or hoops, both giving significant strength to the vessels wall
How do tracheids and vessels elements allow hydraulic conductivity of water through them ?
They both start out as living cells but undergo programmed cell death resulting in a network of dead cells with high strengthened cell walls that allow hydraulic conductivity of water through them.
How can water move between cells ?
water can move between cells either through adjoining pits in the cell wall or through end wall perforations.
How is most water moved up the stem of a plant ?
By cohesion-tension
What is cohesion tension ?
Cohesion-tension is the way most water moves up the stem, water is dragged up through the xylem cell system because of water loss from the leaf canopy, via transpiration.
What is the loss of water from the lead by transpiration driven by ?
Its driven by the large differences in water concentration between the leaf and the air
Why is water syicky and how does it help with movement up the stem ?
Water is sticky because of hydrogen bonding between water molecules and thereby can be pulled under tension through the system
Where are stomata located in arabidopsis ?
Stomata are abundant on the leafs lower surface and developmental patterning means they are always seperated by at least one epidermal cell
What is caviation and how is it caused ?
1- Caviation is the formation of air bubbles in vessels and tracheids as a result of a breakage in the hydrostatic column. This stops water flow in the cell
2- Because of sideways movement of water through pi-pit junctions, water can pass around air-filled cells and water flow is moderately unhindered.
How can air bubbles be dispersed in vessels and tracheids ?
1- Air bubbles can be dispersed at night by lower temperatures or to some extent by root pressure
when does root pressure occur ?
1- Happens in some species
2- High water levels in soil
3- When transpiration rates are low
4- Plant is metabolically active
5- Ions concentrate in the root xylem
6- Water moves into the plant up osmotic potential gradient
What can root pressure lead to ?
May result in guttation - water exuded from leaf
What are hydathodes ?
are structures to allow water exudation at leaf margin
