Factors affecting the rate of transpiration

1) Temperature
Increase in temperature increases the rate of transpiration. This is because high temperatures provide latent heat of vaporization which increases the evaporation of the water leading to more water to be lost.
Temperatures also increases the kinetic energy of the air molecules around the leaf which causes them to move further apart and this increases rate of diffusion from the leaf

2) Relative humidity
Humidity is the amount of water vapour in the atmosphere. As humidity increases, the rate of transpiration decreases. This is because the environment becomes saturated with the water vapour. The water then can be absorbed from the plant decrease which reduces the rate of transpiration

3) Wind
Rate of transpiration is higher in windy air than in still air. This is because wind helps / assists to remove water vapour in the air around the leaf and creates more spaces that can take up more water vapour
However, if the wind speed becomes too high transpiration stops due to mechanical closure of the stomata and the cooling effect the wind has on the leaf.

4) Light intensity
Rate of transpiration is high during the presence of light and low in the dark.This is because high light intensity result in high rate of photosynthesis which increase the sugar concentration in the guard cells which lead to wide opening of the stomata leading to more evaporation from the plant ( also light provide heat which increase evaporation from the leaf stomata.

5) Availability of water
This affects the turgidity of the guard cells i.e. more water more turgidity which leads to opening of stomata and enable more water loss and to high transpiration rate.

6) Atmospheric pressure
Humidity decreases with decrease in atmospheric pressure. Hence decrease in atmospheric pressure greatly increases the rate of transpiration due to decreased humidity.

Non environmental factors
7) Distribution of stomata
The rate of transpiration is low when more stomata are on the lower side and is higher when more stomata are on the upper side of the leaf.

8) Number of stomata
The greater the number of stomata, the higher the rate of transpiration because more water vapour is lost through the stomata.

9) Surface area for transpiration
Plants with wide/broad leaves have a larger surface for transpiration thus they experience a higher rate of transpiration. But that with small leaves e.g. desert plants have a small surface area hence low rate of transpiration.

10) Thickness of the plant cuticle
The rate of transpiration decreases with increase in thickness of the cuticle. For that reason, plants found in deserts have extremely thick cuticle than those in tropical regions.

MECHANISM OF STOMATAL OPENNING AND CLOSURE
Stomata open during day and close at night.
During the day, photosynthesis takes place in the guard cells in the presence of sunlight. This leads to accumulation of sugars in the guard cells which lowers their water potential.
As a result, water moves into the guard cells by osmosis from the neighbouring epidermal cells. Turgor pressure of the guard cells increases which causes their outer thin elastic walls to expand and pull the inner thick inelastic walls outwards, hence opening the stoma.

Diagram showing a stoma open

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At night, there is no photosynthesis due to absence of light. Osmotic pressure inside guard cells decreases/ water potential increases. This causes the guard cells to lose water to the neighbouring epidermal cells by osmosis. Turgor pressure
inside guard cells lowers, making the inner walls to move closer together and the stoma closes.

EXPERIMENTS TO MEASURE THE RATE OF TRANSPIRATION

  1. THE WEIGHING METHOD
    This is where a potted plant is weighed on the balance to determine the difference in weight before and after transpiration. The difference in weight shows the amount of water lost by the plant in a given period of time.
  2. POTOMETER METHOD
    This is done using an instrument called a potometer. The potometer works on assumption that water lost from the leaves during transpiration equals water absorbed by the plant.
    Therefore the potometer:
    Directly measures the rate of water uptake/ absorption of the shoot and
    Indirectly measures rate of water loss / evaporation of water/ transpiration from the leaves.
    Set up of a potometer
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Procedure:
a) A leafy shoot of a plant is cut under water to prevent air bubbles from entering as these would block the xylem vessels.
b) The potometer is filled with water.
c) The leafy shoot is fixed into the cork and then fitted into the mouth of the potometer vessel.
d) Vaseline is smeared at the interface of the shoot and the cock to prevent entry of air into the apparatus.
e) A single air bubble is introduced at the open end of the capillary tube by touching the open end briefly under water and then release.
f) At a given mark V1, reached by the air bubble, a clock is started and after a given time t, the new position of the air bubble V2, is noted and recorded.

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g) In any given set of environmental conditions, about 3 experiments can be performed, resetting the air bubble after each experiment by opening the tap and then close.
h) Average rate is then calculated and taken as the rate of transpiration in that environment.
i) The set up can be moved to different environmental conditions and rate of transpiration determined in the same way.

Precautions taken when using a potometer in order to ensure accurate results

  1. A leafy shoot should be used to ensure significant water loss.
  2. The shoot must be cut under water to prevent air from entering and blocking the xylem vessels.
  3. The whole apparatus must be full of water.
  4. A single air bubble must be present in the capillary tube fir each experiment.
  5. Air bubble must be reset to zero mark before each experiment
  6. A graduated capillary tube must be used in order to clearly read results.
  7. Air bubble should not cross the T- function at the reservoir

ADAPTATIONS OF PLANTS TO REDUCE TRANSPIRATION RATE
i) Shedding off of leaves in deciduous plants to reduce transpirations since most of it occur from the leaves
ii) Reducing the number, size and distribution of the stomata and only on lower epidermis
iii) Structural adjustments in stomata i.e. some plants have sunken stomata and others have hairy stomata which reduces evaporation from them.
iv) Reduction in leaf structure i.e. some plant leaf are reduced to narrow or thorny / spines structures that reduce surface area over which transpiration occurs.
v) Rolling of leaves to create a humid atmosphere around the stomata in order to reduce water loss.
vi) Possession to thick cuticle of the leaves to prevent water loss through it.
vii) Thick leaves that store water
viii) Changes in the rhythm of stomata opening i.e. they close during day and open at night when temperatures are very low.
ix) They shed off their leaves in extremely hot environment to cut down water loss.
x) Reversed opening and closing of stomata. Stomata open at night and close during the day when it’s rate of transpiration is likely to be higher.

IMPORTANCE OF TRANSPIRATION (FUNCTIONS / ADVANTAGES)
a) Results in the absorption of water and its movement up the plant to aid processes like photosynthesis.
b) Contribution to maintenance of continuous stream of water throughout the plant.
c) Transported water keeps the plant cells turgid and cools the plant.
d) Results in the movement of mineral salts up the plants to where they are needed.
DISADVANTAGES / DANGERS OF TRANSPIRATION
a) Excessive water loss from the plant may lead to wilting, drying and even death of the plant.
b) Water may lead to over cooling which affect metabolic activities
c) Over absorption of mineral salts with water lead to soil exhaustion.

TRANSPORT OF MATERIALS IN ANIMALS
The transport system in animals consists of a number of routes through which specific materials are distributed
Smaller organisms (protozoa) that have large surface area to volume ratio carry out transport by simple diffusion.
Transport system is important in large organisms (multicellular) because the increased size of the organisms and the great distance over which materials are supposed to move makes diffusion rate slow which in turn make it inadequate for the distribution of these materials
To overcome the physical limitation on size placed by diffusion, multicellular animals have the major adaptations.
They have organs that provide a large surface area for absorption of nutrients such as small intestines and exchange of gases such as lungs/ gills, without a great
increase in total body volume. They have a transport (circular) system within the body, so that substances can be carried to cells that need them and waste products removed more quickly than in diffusion.
The circulatory system in mammals consist of closed tubes and heart which provide the forces that drives the fluid (blood) in these which include Arteries, capillaries and veins.

Plants do not use a circulatory system because:

  • The oxygen requirement of the plant is very low as compared to mammals.
  • Plants have a continuous series of airspaces throughout the body opening to the atmosphere by the stomata and ventricles.
  • In plants oxygen from the air diffuses through the stomata opening in to the airspaces and from the air spaces in to the cells by diffusion. And the oxygen dissolved in the soil water also diffuses through the root hairs in to the plant sap.
  • The carbon dioxide produced during respiration is used up during photosynthesis.

Types of transport systems in animals

  1. Closed circulatory system:
    Closed circulatory system e.g. in earthworm, fish and mammals have blood enclosed in tubes
    Here blood is pumped by the heart to tissues through the arteries and return to the heart through the veins.
    The arteries and veins are connected by capillaries which are thin walled
    The body cells do not come in to direct contact with blood but are bathed in the tissue fluids. Substances diffuse out of the blood which is confined to blood vessels in to the tissue fluid and then across to cells membrane in to the cell.
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Advantages of closed circulatory system

  • Distribution of blood/materials is easily controlled.
  • Blood moves or flows very fast leading to quick supply of materials.
  • Blood flows at a high pressure leading to an effective system.

Demerits of closed circulatory system
It requires a special heart whose pumping action provides pressure for movement of blood.
Blood movement meets a high resistance within vessels.

  1. Open circulatory system e.g. in molluses and arthropods
    Here the artery that leaves the heart is very short and blood empties in a large blood filled space called haemocoel. Then blood from these spaces return to the heart through the short veins.
    The organism cells are directly bathed in blood and materials diffuse out of the blood in to each cell across the cell membrane.

Advantages of open circulatory system
Easy diffusion of materials due to absence of vessel barriers.
It does not require special pumping hearts since blood is flowing through cavities with less resistance.

Disadvantages of open circulatory system
Blood flows sluggishly/slowly leading to slow supply of materials.
Blood flows at a low pressure.
There is little control over distribution of materials or blood.

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