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Higher biology- maintaining water balance in plants
SQA notes
- The study of transpiration should include reference to the movement of water from the soil through the root hairs, cortex and xylem to the leaves and thenceto the atmosphere without mention of the endodermis or Casparian strip.
- Mention should be made of different factors affecting the rate of transpiration.
- The concept of cohesion and adhesion of water molecules should be dealt with briefly. The uptake and transport of nutrient ions and the cooling effect of the evaporation of water from the leaves should also be considered.
- The opening and closing of stomata should be explained in terms of changes in turgor. The underlying mechanism need not be explained.
- Candidates should be able to give an explanation of the effect of each adaptation.
transpiration stream
The transpiration stream is the continuous flow of water from roots to leaves.
- water enters root hair cells by osmosis (down a concentration gradient) - root hairs offer a large surface area for this absorbtion
- water continues down a concentration gradient into a neighbouring cortex cell and so on, across the root to the Xylem vessels.
- often this water tranfer is made outwith the cells, in the cell wall's many water spaces.
- water leaves the xylem vessel once it reaches the leaves
- It moves into the spongy measophyll and then in to air spaces arounf the stomata
- it is lost into the atmosphere by evaporation, this process is transpiration.
As water is lost from cells, the reduced water content draws more water into root hairs and through the plant- this is called the transpiration pull- and is due to cohesion, an attraction between water molecules.
Adhesion is the attraction between water molecules and unlike particles ie. a glass capilary tube of xylem vessel.
this may seem insignificant but is important because the transpiration stream
- carries water for photosynthesis to the palisade cells in the leaves
- carries essential mineral salts in solution
- cools the plants due to evaporation on leaves
- it maintinas turgor and is essential for support in non woody plants
Stomata
On plants such as grass - monocotyledonus plants- stomata are found equally distributed over the surface.
On dicotyledonous plants, stomata are restriced to the lower epidermis.
guard cells differ form normal epidermal cells in three ways
- 1. sausage shaped
- 2. contain chloroplasts
- 3. have a thicker and less elastic inner wall, creating bending when turgid
Click here to view a picture of stomata.
stomata open = turgid
stomata closed= flaccid
problems
- potential water balance problem in that stomata are open during the day when water loss is at its height.this is counter balanced with osmoregulation techniques.
Potometer
A potometer is used to measure rate of water uptake by a leafy shoot.
An atmometer is the control for this experiment, and a porous pot is used instead of a leafy cutting.
note:
- stems must be cut under water to prevent air bubbles in the xylem
-scale is measured by teh progress of an air bubble in glass tubing
- the plant must be given time to equilibriate to each environmental condition.
Click here to view a diagram of a potometer.
results
- wind increases rate of transpiration (due to surrounding air constantly being replaced with new, drier air)
- increased humidity decreases transpiration rate due to the concentration gradient being reduced.
- transpiration increases with increased temperature due to increased evaporation
- airp pollution can slow transpiration by blocking stomata
Adaptaions to Environmental Conditions
mesophytes
- normal plants, in environments where water is abundant and excessive transpiration does not occur, have little adaptaions.
Xerophytes
- plants that live in habitats where transpiration rate is excessively high ie. hot, dry conditions with little soil moisture. and in very windy areas.
- structural adaptations to reduce water loss
- reduced number of stomata
- thick waxy cuticle covering
- often leaves are rolled to trap air
- hairs onleaves help to trap humid air
- Sunken stomata create a moisture rich atmospher around the stomata and prevent excessive evaporation
- surface area of leaves reduced ie. as needles/ spines
- Structural adaptations for drought resistance
- long roots
- superficial roots at soils surface to absord maximum water in rainfall
- succulent tissue stores water
- Physiological adaptations
- reversed stamatal rythm (open at day and closed at night)
- existance in a desicatted state, only germinating once drought ceases.
click here or here to see examples of xerophytes.
Hydrophytes
- plants which live completely or partially submerged in water
- Adaptations
- Air spaces provide boyancy (allowing the plant to be closer to the light) and allow oxygen from photosynthesis to be stored for respiration.
- little xylem, any xylem is found in the centre of the stem for maxium flexibility.
- specialised leaves ie. lily pads with long stalks and large surface areas as the hold all of the stomata for the plant. submerged leaves are small and narrow to prevent damage from water currents.
Comments
All images from BBC bitesize
