End of chapter exercises
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End of chapter exercises
The water loss in two plants was recorded over a 12 hour period. Study the information below and then answer the questions that follow. The results for the two plants were recorded in the following table.
Time of Day | 06:00 | 08:00 | 10:00 | 12:00 | 14:00 | 16:00 | 18:00 |
Water loss in Plant A (\(\text{cm$^{3}$}\)) | \(\text{0,0}\) | \(\text{0,4}\) | \(\text{1,6}\) | \(\text{6,0}\) | \(\text{9,0}\) | \(\text{8,0}\) | \(\text{7,6}\) |
Water loss in Plant B (\(\text{cm$^{3}$}\)) | \(\text{0,2}\) | \(\text{0,7}\) | \(\text{4,0}\) | \(\text{14,0}\) | \(\text{19,0}\) | \(\text{18,2}\) | \(\text{17,7}\) |
Guttation | Transpiration |
occurs early morning and at night | occurs during the day when it is hot and light |
takes place through hydathodes | takes place through the stomata |
Water is lost in liquid form | Water is lost as vapour |
caused by root pressure | caused by high water potential |
Water droplets are found on the margin of the leaf | Water vapour transpiration takes place mostly in the lower surface of the leaf where stomata are located |
Give a short explanation for the following:
- Capilliarity
- Dicotyledon
- Transpiration
- Water potential gradient
- Capillarity is the tendency of water molecules to cling to each other (cohesion) and cling to the sides of xylem vessels (adhesion). These two forces together cause capillarity – the make a water column in xylem continuous, so it ‘creeps up’ the xylem and is easily moved up by transpiration pull.
- Dicotyledons are plants in one class of the Angiosperms / flowering plants. They have seeds with 2 cotyledon or seed lobes, net veins, petioles, flower parts in multiples of 4 or 5, tap root systems and vascular bundles in a ring in the stem.
- Transpiration is the loss of water vapour from the aerial parts of plants. It occurs mainly through stomata on the leaves. It is fastest during the hottest times of the day and is increased in low humidity and when the wind blows.
- Water potential gradient is the difference in the water potential of two liquids. Water potential is the potential of water to move from one area to another due to differences in pressure, solutes dissolved in the water and other factors. The water potential gradient between two solutions has a direct effect on the rate of osmosis – the bigger the gradient, the faster osmosis occurs in the direction of the gradient, i.e. always from a high to low water potential.
- Dry air / low humidity
- high air temperature
- high wind speed
- bright light
- Leaves with parallel veins
- Leaves with leaf sheaths, not petioles
- Fibrous root system
- Flower parts in multiples of 3
- Seeds with one cotyledon
- Vascular bundles scattered in the stem, not in a circle
- Small or narrow leaves to reduce the surface area over which water vapour is lost
- Leaves in a rosette arrangement to shade lower leaves and trap water vapour
- Curled up leaves to trap water vapour inside the leaf cavity
- Thick cuticles to prevent evaporation of water
- Sunken stomata to trap water vapour in pits near the leaf
- Very few stomata or stomata only on the lower surface to reduce water loss
- Trichomes to make leaves reflective and trap water vapour
During transpiration, the movement of water through the xylem is largely due to:
- mitosis
- capillary action
- osmosis
- all of the above
Stomata:
- are found in plant roots
- permit the intake of carbon dioxide
- prevent the intake of oxygen
- all of the above
Water can be lost by a plant through which process(es)?
- guttation
- transpiration
- condensation
- a and b
What environmental condition(s) always lead to an increase in transpiration rate in each plant tested?
- heat
- wind
- light
- all of the above
Wind appears to increase the rate of transpiration in a plant. This is most likely due to the fact that:
- humidity increased
- evaporation increased
- stomata were forced to close
- all of the above
Describe how light intensity is responsible for an increase or decrease in the transpiration rate.
Light intensity increases rate of photosynthesis. This increases the rate of glucose production and storage in guard cells. This leads to movement of water into the guard cells as a result of a lower water potential with respect to outside the guard cells. The increase in turgor pressure of the guard cells results in the opening up of the stomata which results in increased transpiration.
Study the two graphs below which show water loss from a plant over a period of time, and answer the following question:
Which graph could show water loss under increasing external humidity? Give reasons for your answer.
Graph 2. As the humidity increases over time, the transpiration rate slows down. When the humidity is low water is lost more rapidly because there is a steep water potential gradient between water vapour inside the stomata and the outside atmosphere. As humidity increases, the water potential gradient across the inside and outside the leaf become more equal, and so the rate of transpiration decreases. At high humidity, the water loss is reduced to zero.
Describe how each of the following adaptations results in a decrease in the transpiration rate:
Complete the following sentences:
Draw a table showing how the structure of root hairs is adapted for their function.
Structure | Function |
Thin and long root hairs | Increase surface area over which absorption of water and mineral salts can occur |
Thin cell wall | Makes osmosis faster |
No cuticle | Cuticle would hinder water absorption |
Large vacuole | Allows for storage of water and mineral salts |
Mineral salts in vacuole | Creates a concentration gradient to encourage osmosis |
Several root hairs | Increased surface area |
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