Movement of water through roots
Special exchnage surface = root hairs = long, thin extension of a root epidermal cell...
> Provide large SA as they are very long and occurs in thousand along each branch of a root
> Have thin surface layer (CSM and celluose cell wall) - across which materials can move easily
When surrounded by damp soil - contains some mineral ions but still has high water potential. Whereas root hairs contain sugars, amino acids and mineral ions = lower water potential. Therefore water moves down water potential gradient from soil solution to root hairs. After being absorbed, water continues journey via one of two routes....
> Apoplastic pathway = water drawn into endodermal cells pulls water behind it due to cohesive properties > creates tension that pulls water along the cell walls of root cortex. No resistance to water movement because the mesh-like structure of cellulose cell wall has many water filled spaces.
> Symplastic pathway - PTO
> Water passes through the cell walls along tiny opening called plasmodesmata.
> Each plasmodesma is filled with thin strand of cytoplasm = continuous strand of cytoplasm from root hair to xylem - water moves along the column by...
> Water entering by osmosis increases WP of root hair cell
> Root hair cell has higher WP than the first cell in the cortex
> Water moves from root hair cell to first cell of cortex down the conc gradient by osmosis
> First cortex cell has higher WP than its neighbour to the inside of the stem
> Water moves to neighbouring cell by osmosis along the conc gradient
> Second cell now has higher WP than third so water moves by O down the CG to 3rd cell
> Loss of water from first cell lowers WP - more water enters by O from root hair cell
> WP gradient set up across all cells of the cortex
Passage of water into the Xylem
Water reaches endodermis by apoplastic pathway > waterproof casparian strip prevents further movement > water forced into living protoplast of the cell - meets water from Symplastic pathway.
For water to enter Xylem must first enter the cytoplasm of endodermal cells . Active transport of mineral ions into the the Xylem by endodermal cells creates lower water potential in Xylem. Water moves into the Xylem by osmosis along the concentration gradient > creates force that helps move water up the plant = root pressure. Evidence of root pressure...
> Pressure increases with a rise in temperature.
> Metabolic inhibitors - prevent most energy release by respiration and cause root pressure to cease
> Decrease in availability of oxygen or respiratory substrates causes a reduction in root pressure.
Movement of water across the cells of a leaf
Water out through stomata - humidity of air usually less than that of air spaces around stomata - if stomata open > water vapour molecules diffuse out > water lost from air spaces replaced by water evaporating from the cell walls of surrounding mesophyll cells. Plants control rate of transpiration by changing size of stomatal pores.
Movement of water across cells of leaf - water lost from mesophyll cells to to the air spaces of the leaf. Replaced by water reaching the mesophyll cells from either of the two pathways. For example, via the symplastic pathway...
> Mesophyll cells lose water into air spaces
> Cells now have lower WP and so water enter by osmosis from neighbouring cells
> Loss of water from these neighbouring cells - lowers their WP - so they take in water from their neighbouring cells. = WP gradient established that pulls water from Xylem to leaf mesophyll and out to the atmosphere.
Movement of water up the stem in the Xylem
Two main factors: Cohesion-tension theory and Root Pressure. Cohesion-Tension Theory...
>Water evaporates from leaves as a result of transpiration
> Water moleceules form H bonds so tend to stick together = Cohesion
> Water forms continuous, unbroken pathway across mesophyll cells and down xylem.
> As water evaporates from mesophyll cells into air spaces beneath stomata - water drawn up behind it as a result of cohesion.
> Water therefore pulled up the xylem as result of transpiration = transpiration pull.
> Transpiartion pull puts xylem under tension - negative pressure within the xylem
Evidence for the Cohesion-tension theory
> Changes in diameter of tree trunks accoridng to rate of transpiration - trunk shrinks in diameter throughout the day when transpiration rate and tension greatest. Trunk diameter increases at night - less tension in xylem - low transpiration rate.
> If Xylem vessel breaks, air enters - tree can no longer draw up water. Continuous column of water broken so water molecules no longer stick together.
> When Xylem vessel broken - water does not leak out - instead air drawn up - under tension
Transpiration pull = passive process - does not require metabolic energy. Xylem vessels are dead so cannot actively move water and their end walls can break down. Xylem must form series of continuous, unbroken tubes from roots to leaves. However energy needed to drive transpiration - in the form of heat from the sun.
Transpiration and factors affecting it
Substances such as mineral ions, sugars and hromones are moved around in the plant dissolved in water > water carried up the plant by transpiration pull > ensures plentiful supply of water and rapid transportation of materials.
Most stomata open in the light to allow diffusion of carbon dioxide in for photosynthesis. When stomata are open water moves out into the atmosphere. Increase in light intensity = increase in transpiration rate.
Affects how much water air can hold and speed at which water molecules move. Rise in temperature > increased KE / speed of molecules > increased rate of evaporation from leaves (transpiration). Increase in temperature also decreases water potential of air.
Factors affecting transpiration
Humidity - Measure of number of water molecules in the air. When air outside the leaf has a high humidity > the WP gradient is decreased > lower rate of transpiration. Low humidity = greater transpiration.
Air movement - Water diffuses through stomata > accumulates as vapour around the stomata on outside of leaf > Increased WP around stomata. Reduces gradient between moist atmosphere in air spaces of leaf and drier air outside > Transpiration rate decreased.
Any movement of air around the leaf will disperse the humid layer at the leaf surface and decrease WP of the air > increased WP gradient > increased rate of transpiration. The faster the air movement > faster the humid air is removed > greater rate of transpiration.
Energy of transpiration comes from evaporation of water from leaves. The factors affecting transpiration are affected by the Sun's energy. Transpiration driven by the Sun.
Measurement of water uptake
Rate of uptake is almost the same as rate of transpiration of water. Measured using a potometer:
1. Leafy shoot cut under water - do not get water on leaves
2. Potometer filled completely with water - no air bubbles
3. Leafy shoot fitted to potometer under water using a rubber tube.
4. Potometer removed from under water - all joints sealed with waterproof jelly
5. Air bubble introduced into the capillary tube
6. Measure distance moved by air bubble in a given time a umber of times - calculate mean
7. Using mean value, calculate volume of water lost. 8. Graph - vol of water lost against time
9. Once air bubble nears junction of reservoir tube and capillary tube - open tap on reservoir and press syringe until bubble back to the start of scale along capillary tube. Can repeate experiment under different conditions.
Limiting water loss in plants
Xerophytes - plants adapted to live in areas where their water losses due to transpiration may exceed their water uptake. Adaptions increase water uptake, store water and reduce transpiration.
Thick cuticle - less water can escape across the cuticle
Rolling up of leaves - protects lower epidermis by trapping a region of still air - becomes saturated with water - no water potential gradient - transpiration reduced - eg marram grass
Hairy leaves - traps moist air next to leaf surface - reduces WP gradient between inside and outside of leaf - less transpiration - eg some heather plants
Stomata in pits or grooves - trap moist air next to leaf - reduce WP gradient eg pine trees
Reduced SA:Vol ratio of leaves - reduced water loss - however must be balanced by need to photosynthesise to meet requirements of plant.
Xerophytes found in; deserts, sand dunes, coastal regions, salt marshes, cold regions.