Lecrure 11 Flashcards
Why is water essential
• It carries nutrients in the soil to the roots
• Is the solvent for, and medium of, most biochemical reactions within plants
• Its loss from plants is an inevitable consequence of CO2 exchange with the atmosphere
How do plants uptake nutrients
• Soil-water solution
• 98% obtained in soil-water solution
• 2% directly from soil
How many bar at maximal water capacity
0 bar
How many bar at field capacity
-0.33 bar
How many bar at permanent wilting point
-15 bar
What is water potential used to predict
which way water will move
What is water potential measured in
units of pressure called megapascals (MPa)
Potentials are a way to represent what
Free energy
In terms of water potential what happens when a cell is placed in pure water
Water moves into the cell because the water potential of the cell is relatively negative
In terms of water potential what happens when a cell is placed in a solution with different solute potential
water moves in the direction that eventually results in equilibrium – when both cell and solution have the same water potential
Diffusion of water across a semi-permeable membrane is termed what
Osmosis
If a plant cell is placed in a solution with high water potential (low osmotic
concentration) what will happen
-It will become swollen or turgid
If a plant cell is placed in a solution with low water potential (high osmotic
concentration) what will happen
-It will exhibit shrinkage or plasmolysis
Name the two ways water can enter plant roots
•Active Transport
•Passive Transport
Write a short note on ACTIVE transport
•Water is absorbed due to activities going on in roots.
•Absorption of water occurs with the help of energy in the form of ATP. •Absorption takes place against concentration gradient - even when the concentration of cell sap is lower than that of soil water.
Write a short note on PASSIVE transport
•Passive absorption is by osmosis. •Passive absorption takes place along the concentration gradient - when the concentration of cell sap is higher than that of soil water.
•Water is absorbed when transpiration rate is high or soil is dry.
•Due to high transpiration rate, water deficit is created in transpiring cells. •Rapid transpiration removes water and reduces turgor pressure in living cells of the root. The suction force developed is transmitted to root xylem. It pulls water from surrounding root cells to make up water deficit.
Most of the water absorbed by plants comes in through what
The Root Hairs
How is Root Hairs almost always turgid
their water potential is greater than that of soil
What is required for ions to accumulate in root cells
•An expenditure of energy
•Once in the roots the ions are transported via the xylem throughout the plant
How is surface area for water and mineral absorption further increased
•By Mycorrhizal Fungi
•Particularly helpful in PHOSPHORUS uptake
Name the 3 transport routes that exist through cells
•Apoplast route
•Symplast route
•Transmembrane route
Explain Apoplast route
Movement through the cell walls and the space between cells
Explain Symplast route
A cytoplasm continuum between cells connected by plasmodesmata
Explain Transmembrane route
Membrane transport between cells and across the membranes of vacuoles within cells
What is Symplast and Transmembrane route summarised as since they are impossible to separate experimentally
Cell to cell component of transport
Cortical cells have numerous interconnections via what two structures
•Cell walls
•Plasmodesmata
What does the interconnections between cell wall and plasmodesmata in cortical cell link
The Protoplasm of each cells
How can substances move in the cortical cell
•can move across the root, then, either via the cell walls (the apoplastic pathway) or via the cell contents (the symplastic pathway);
•these pathways are assumed to be important in the internal transport of water and nutrients.
On their journey where do molecules reach
Endodermis
Any further passage through the cells walls is blocked by what
The Casparian Strip
What must molecules pass through of endless dermal cells to reach the xylem
must pass through the cell membranes and protoplasts of the endodermal cells
Describe lateral transport of water in roots
•Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls.
•Minerals and water that cross the plasma membranes of root hairs enter the symplast.
•As soil solution moves along the apoplast, some water and minerals are transported into the protoplasts of cells of the epidermis and cortex and then move inward via the symplast.
•Within the transverse and radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks the
passage of water and dissolved minerals.
•Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder.
What are Aquaporins
water channels that exist in vacuole and cell membranes
What do Aquaporins speed up
speed up osmosis, without changing the direction of water movement
Explain water movement in plant from root to stomata
Water first enters the roots and then moves to the xylem, the innermost vascular tissue. Water rises through the xylem because of a combination of factors.
Name the two types of water movement for root to stomata
•Short-distance movement
•Long -distance movement
Describe short-distance movement for water movement from roots to stomata
•Movement of water at the cellular level plays a major role in bulk water
transport.
•Water can diffuse through cell membranes.
Describe long-distance movement for water movement from roots to stomata
•Water movement in most of the vascular plants from roots to leaves. •Sometimes more than 100 m
Water moves from the soil into the roots only if the soil’s what is greater
Water Potential
After water moves from soil into the roots what happens then
then moves along gradients of successively more negative water potentials in the stems, leaves and air
Evaporation of water in a leaf creates what in the xylem
Negative pressure or tension
What is the driving force for transpiration
the gradient in vapor pressure
Name the 3 main forces of water movement through the xylem
• Transpiration Pull
• Cohesion Adhesion Forces (Cohesive Tension Theory)
• Root Pressure
Explain transpiration pull
•It is the pulling force responsible for lifting the water column.
•As water is lost in form of water vapour to atmosphere from the mesophyll cells by transpiration,
•a negative hydrostatic pressure is created in the mesophyll cells which in turn draws water from veins of the leaves.
What happens to the negative tension created during transpiration pull
•negative tension is then gradually transmitted downwards via xylem tissues of the leaf, stem and finally to the roots.
•As a result there is a continuous upward movement of water column in the plant.
Explain cohesion adhesion forces (cohesive tension theory)
•The water molecules in the chain are held together by hydrogen bonds
which exist between neighboring water molecules. (cohesion)
•The chain of molecules is prevented from being pulled down because each water molecule in the chain is attracted to the walls of the xylem by hydrophilic attraction between water and the cellulose in the cell walls. (Adhesion)
•The water column which is held together by cohesion and prevented from lowering by adhesion is pulled up by the tension generated from above by transpiration.
Explain root perssure
•Root pressure is caused by active transport of mineral nutrient ions into the root xylem.
•Without transpiration to carry the ions up the stem, they accumulate in the root xylem and lower the water potential. •Water then diffuses from the soil into the root xylem due to osmosis.
•Root pressure is caused by this accumulation of water in the xylem pushing on the rigid cells.
•Root pressure provides a force, which pushes water up the stem, but it is not enough to account for the movement of water to leaves at the top of the tallest trees.
Name the physical soil factors that affect water absorption in plants
Soil water contents Soil temperature Soil aeration Flooding
Texture and structure Speed of water movement Effective root zone
Name the atmospheric factors that affect water absorption in plants
Temperature
Relative humidity
Vapour pressure deficit (VDP) Wind speed
Name the biological factors that affect water absorption in plants
Plant species
Root length Root system Plant health
Biological stresses
Tolerance to stresses (Drought/Salinity) Plant genotype and phenotype
Growth rate
Plant hormones
