Objectives
This blog post provides readers with the following objectives. The reader will be able to:
TRANSPORT IN PLANTS
Absorption of Water and Mineral salts
Water is absorbed by the root tips and root hairs of the root. The solutes concentration in the cell sap of the root hair is more than the soil water solution. Water therefore enters the root hairs by osmosis. The root hair sap thus becomes less concentrated than the sap in the surrounding cells. Water then moves from the root hair into the surrounding cells following the concentration gradient until it reaches the xylem vessels. The water moves through the root cortex from cell to cell by three pathways before reaching the xylem vessels:
1. Apoplastic pathway through the cell wall. More rapid - less resistance to the flow of water
2. Symplastic pathway through cytoplasm via plasmodesmata conceptions
3. Vacuolar pathway through the vacuole
Before water enters the xylem, it must cross the endodermis. Cells of the endodermis possess cell walls that are ringed by the Casparian Strip, a waxy layer (composed of suberin). The Casparian Strip prevents the apoplastic flow of water. Water crosses the endodermis by symplastic pathway. The Casparian Strip prevent the backflow of water out of the vascular cylinder into the root cortex. Once the water passes under the Casparian Strip in the endodermal cells, it is free to enter the apoplast again on its way to the xylem
The uptake of minerals by the plant is achieved by diffusion and active transport. Mineral salts are found in ionic form, dissolved in the soil water surrounding the roots. Depending on the mineral salt required by the plant, the roots hairs absorb the salts by diffusion along concentration gradient or against concentration gradient by active transport. Active transport is needed because the concentration of some minerals may be greater in the root hairs than in the growth medium. As a result, energy is needed to make minerals travel against their concentration gradients.
Adaptations of root hairs for absorption of water
1. numerous to provide large surface area
2. lack cuticle
3. thin cell walls
Transport of water up the stem
From the roots, water is transported up the stem through the xylem vessels. The transport involved the following mechanisms; osmosis, root pressure, capillary action, cohesion and adhesion, transpiration and transpiration pull
1. Root pressure: As the water is continuously absorbed into the xylem vessels in the root, it builds up a hydrostatic pressure, (called root pressure) that pushes the water up the root. This is illustrated by exudation of water from stumps of the stem after a tree has been cut.
2. Transpiration pull: Transpiration is the loss of water from the leaves through the stomata, cuticle or lenticels. As water evaporates, a tension is set up in the leaves and in xylem vessels. This creates a continuous pull of water from the roots to the leaves. This is termed as transpirationpull.
3. Cohesion and Adhesion: The movement of the water in the xylem vessels is due to cohesion (attraction between water molecules) and adhesion (attraction of water molecules to the vessel walls).
4. Capillary action: The long, narrow tubes of the xylem vessels also enable water to move up stem, a process known as capillary action.
Experiment to Show Water Uptake from the Soil into the Plants
Aim: To show that roots absorb
water.
Material: Test-tube, Young Leafy shoot with intact root, mustered oil, test-tube stand
Procedure
1. Fill two test tubes with water
2. Insert leafy shoot with roots intact into one of the test tubes.
3. Pour few drops of mustered oil in both test-tubes to prevent loss of water by evaporation.
4. Observe the level of water in both test tubes for 24 hours
Observation: At the end of the experiment, there
will be drastic fall in level of water in test tube with plant indicating that
roots absorb water which helps the entire plant.
There will be little fall in level of water in the control, indicating there wasn’t absorption of water in absence of root.
Inference: Roots of the plant absorbs water and helps the entire plant.
Experiment to Show the Path Taken by Absorbed Water
Aim: To show water moves through the xylem tissues
Material: beaker, Young herbaceous plant, eosin solution or colored liquid, razor blade
Procedure
1. Fill a beaker with dilute eosin solution.□
2. Place the young herbaceous plant with the root washed inside a beaker containing the eosin solution.
3. Allow the set-up to stand for 1-6 hours.
4. Cut longitudinal or cross sections of a root, stem, petiole and leaf.
5. Examine and observe it under microscope or with hand lens.
Observation: The color of the solution will stain the xylem vessels.
Conclusion: It is claimed that xylem is responsible for the conduction of water.
Experiment to Show Root Pressure Affect the Rise of Water in the Xylem of a plant
Aim: To show that root pressure affects the ascent of water in the xylem.
Material: potted plant, glass tube, eosin solution or colored liquid
Procedure
1. Cut across the stem of a well-watered potted plant, just above the soil level and observe the sap exuding from the cut surface.
2. Attach a glass tube to the cut end of the stem using a rubber tube.
3. Attach the glass tube to a retort stand.
4. Pour a colored solution such as eosin solution into the glass tube and note the level.
Observation: The level of eosin solution in glass tube rises above that of the original level
Conclusion: The rise in the level of eosin solution is due to root pressure resulting in the continuous absorption of water by the root hairs.
Transpiration
Transpiration is the loss of water in the form of water vapor from the aerial parts of the plant mainly through the leaves. Water is lost in plants usually by diffusion because there is usually more water in the cells of the plant than in the atmosphere.
Types
of Transpiration:
Transpiration is of three types:
1. Stomatal: transpiration takes place through stomata in the leaves. About 90% to 95 % of transpiration of plant takes place through stomata. The stomata occur in the lower epidermis of a leaf bordered by guard cells. Water vapor escapes from the leaf into the atmosphere due to changes in the turgor pressure of the guard cells.
2. Cuticular: there are Cutine coatings in young stems and leaves called Cuticle. At the cuticle water vapor diffuses across the cuticle of the upper epidermis of the leaf into the atmosphere along diffusion gradients.
3. Lenticular: due to secondary growth in stem, small pores are developed by rupturing of the epidermal layer. These pores are called Lenticels. The cells of lenticel are loosely packed. Some portion of transpiration takes place through these lenticels.
Importance of Transpiration
3. Evaporation of water helps to cool the leaves
4. Supply water to all the cells for metabolic processes and for turgidity
5. It helps in removal of excess water in the plant
Disadvantages of Transpiration
Structural Adaptations of Plants to Reduce Transpiration
1. Curled up leaves, so it creates a humid environment around the leaf, hence less transpiration occurs.
2. Presence of hairs and scales on the surface of the leaves, to trap escaping water molecules, to reduce the rate of transpiration.
3. Sunken stomata; stomata of some leaves e.g. xerophytes are sunk deep into the surface to reduce water loss.
4. Having stomata closed during the day time.
5. Leaf spines: Some plants have spines instead of leaves. Spines usually have thicker cuticles and a very small surface area, which decreases transpiration.
6. Some plants that occur in dry places have a thick cuticle that reduces transpiration.
7. Reduction of leaf size: small leaves have a smaller surface area for transpiration to occur.
Factors Affecting the Rate of Transpiration
1 Temperature: high temperatures cause an increase in transpiration rate. This is because high temperatures dry the air around the leaf and also increases the kinetic energy of water molecules. At low temperatures, the air around the leaf gets saturated and this lowers the transpiration rate.
2. Humidity: is the amount of water vapor in the air. The higher the humidity of the surrounding air, the lower the rate of transpiration and vice versa.
3. Light: affects transpiration because stomata usually open in the light and closes in darkness. As the light intensity increases the degree of opening of stomata also increases, hence transpiration also increase.
4. Number and distribution of stomata: the more the stomata, the faster the rate of transpiration. Plants with more stomata on the upper surfaces of the leaves transpire at a higher rate.
5. Leaf size: the rate of transpiration also depends upon the surface area of leaf. More surface area provides more stomata and there is more transpiration.
6. Soil water: If availability of soil water is less the rate of transpiration will decrease.
7. Internal surface of leaf: thin cuticle, thin cell walls, exposed stomata, and well-developed spongy parenchyma favor transpiration. On the other hand, leaves with thick cuticle, thick cell walls, well-developed palisade, sunken stomata etc. will have reduced transpiration rate.
Measurement
of the Rate of Transpiration
Weighing method
1. A potted plant is well watered and the pot is enclosed within a polythene bag to prevent direct evaporation of water from the soil.
2. It is then weighed and the mass is recorded.
3. The plant is taken outside for a few hours after which it is weighed again.
4. The difference in weight represents the amount of water lost through transpiration.
5. The rate of transpiration is then calculated as the amount of water lost per unit time.
Potometer
Potometer is the instrument used to measure the rate of water uptake in a plant. When a Potometer is used to calculate the rate of transpiration, it is assumed that all the water taken up is lost by transpiration. It measures how factors such as light, temperature, humidity, light intensity and wind will affect the rate of transpiration.
Procedure
2. Insert the leafy shoot through the hole of the stopper provided with the potometer.
3. Fill the potometer with water and fit the stopper holding the leafy shoot to the apparatus.
4. Use Vaseline to seal all the connections of the apparatus.
5. Trap an air bubble in the capillary tube by the following procedures:
7. Transpiration rate can be expressed in terms of water transpired per unit time per unit area of leaf surface.
Limitations of Potometer
1. It only measures the rate of water uptake, rather than the actual transpiration rate.
2. It uses a cut shoot rather than a whole plant. Twig may receive shocks and may not function normally.
3. The presences of bubbles may prevent continuous flow of water.
Experiment to Demonstrate Transpiration
Aim: To show that water vapor is given off during transpiration
Apparatus: A potted plant, 2 bell-jars, polythene bag, anhydrous copper sulphate
Procedure
1. Take a potted plant and cover the pot and base of stem with polythene bag.
2. Place the potted plant on a glass plate and invert a dry bell-jar over it.
3. Leave the apparatus in sunlight.
4. Set up a control experiment with no plant.
Observation
After an hour, drops of colorless liquid are seen inside the bell-jar with the plant. To show that these drops are water, touch them with anhydrous copper sulphate (white) and its color changes to blue. No drops of water are found in the control experiment.
Conclusion
The water droplets on the inside of the jar containing the plant came from the leaves because the rest of the plant body and the soil were covered with polythene bag. Thus the potted plant present in the bell-jar showed the phenomenon of transpiration.
Experiment to Show that Leaves have more Stomata on
their Lower Surfaces
Aim: To show that there is more transpiration from the lower leaf surface as compared to the upper.
Procedure
1. Take a potted plant. Water the plant and leave it for an hour.
2. Take two equal size cobalt chloride papers and with the help of forceps place one on the upper surface and the other on the lower surface of a leaf.
3. Place dry glass sides on the upper and the lower cobalt chloride papers and fix them with a rubber band. (The glass slides will prevent the cobalt chloride papers to come in contact with atmospheric humidity.)
4. Note changes in the color of the two cobalt chloride papers.
Guttation
Guttation is the exuding of drops of water on the tips or edges of leaves of some vascular plants such as grasses. At night, transpiration does not occur since the stomata are closed.
Water enters the plant roots because the water potential of the roots is lower than the surrounding soil. Thus, water accumulates in the plant, resulting in root pressure.
The root pressure forces some water to exit the leaf tip or edge structures called hydathodes or water glands, forming drops. Root pressure is what drives the flow of water, rather than transpirational pull.
Conditions for Guttation
1.. High humidity
2. Low temperature
3. Very high root pressure
4. High moisture content of the soil
5. Still air
5. Absence of transpiration
Differences between Guttation and Transpiration
|
Guttation |
Transpiration |
|
Water is loss in liquid
state |
Water is loss in gaseous
state |
|
Occurs
only through large specialized pores in leaves called hydrothodes |
Occurs through the
stomata, lenticel and cuticle |
|
Hydrothodes remain open
all the time |
Stomata opens during the
day |
Similarities between Guttation and Sweating
1. Both processes involve loss of water
2. Both result in cooling
4. The rate of both is affected by environmental condition6. Water is loss through pores
Difference between Transpiration and Sweating
|
Transpiration |
Sweating |
|
Occurs in plants through
stomata, lenticel and cuticle |
Occurs in mammal through
sweat pores in the skin |
|
Involves
only loss of water |
Involve loss of water,
salt and nitrogenous waste |
|
Water is lost in the form
of vapor |
Water is lost in liquid
form |
|
Occur
during the day |
Occur in the day and
night |
|
No glands involved |
Special glands are
involved |
Translocation of Organic Food
Translocation is the movement of organic food, mainly
sucrose through the phloem from regions of production to regions of
storage or utilization. The glucose formed during photosynthesis in mesophyll
cells, is used in respiration and the excess is converted into sucrose.
The phloem contains a very concentrated solution of dissolved solutes, mainly sucrose. This solution is called the sap. It transports through the phloem from source (regions of production) to sink (regions of storage or utilization e.g. roots, tubers, fruits, leaves, growing regions).
Mechanisms of Translocation
Pressure or mass flow theory
The main mechanism is thought to be the mass flow of fluid up the xylem and down the phloem, carrying dissolved solutes. The mass flow is driven by a combination of active transport and evaporation. This is called the pressure or mass flow theory, and it works like this:
1. Sucrose produced by photosynthesis is actively pumped into the phloem vessels by the companion cells.
This decreases the water potential in the leaf phloem, so water diffuses from the xylem vessels by osmosis.
2. This increases the hydrostatic pressure in the phloem, so water and dissolved solutes are forced downwards to relieve the pressure. This is mass flow (the flow of water together with its dissolved solutes due to a force).
3. In the roots the solutes are removed from the phloem by active transport into the cells of the root. The mass-flow explains the fast speed of solute translocation.
Cytoplasmic streaming
There is a circular movement of cell protoplasm when viewed under the microscope. This is known as cyclosis. The cytoplasmic streaming theory states that;
1. as cyclosis occurs it stirs up the sap of the vacuoles and by so doing the sap drips through the sieve pores along the concentration gradient into the next sieve element
2. Therefore, translocation of solutes in opposite direction due to concentration gradient.
Translocation Experiments
1. Puncture
Experiments
If the phloem is punctured with a hollow tube then the sap oozes out, showing that there is high pressure (compression) inside the phloem (this is how maple syrup is tapped). If the xylem is punctured then air is sucked in, showing that there is low pressure (tension) inside the xylem. Water is pulled up in the xylem, sap is pushed down in the phloem.
2. Ringing Experiments
Since the phloem vessels are outside the xylem vessels, they can be selectively removed by cutting a ring in a stem just deep enough to cut the phloem but not the xylem. After a week there is a swelling above the ring, reduced growth below the ring and the leaves are unaffected. This was early evidence that sugars were transported downwards in the phloem.
.webp "Ring experiment")
3. Radioactive Tracer Experiments
Radioactive isotopes can be used to trace transported compounds. They can
be traced using autoradiograph or a GM tube. This technique can be used to
trace sugars or ions or even.
A plant is grown in the lab and one leaf is exposed to carbon dioxide
containing the radioactive isotope 14C. This 14CO2
is taken up by photosynthesis and the 14C is fixed into organic
products (glucose) of photosynthesis which are then translocated to other parts
of the plant.
The autoradiograph analysis shows that organic compounds (presumably sugars) are transported downwards from the leaf to the roots and is found entirely in the phloem.
4. Aphid Stylet Experiments
Aphids, such as greenfly, have specialized mouthparts called stylets,
which they use to penetrate phloem tubes and sup of the sugary sap therein. If
the aphids are anaesthetized with carbon dioxide and cut off, the stylet
remains in the phloem so pure phloem sap can be collected through the stylet
for analysis.
EXCRETION IN PLANTS
Excretion is the removal of waste products of metabolism and other non-useful materials from the body of an organism, which would become toxic if allowed to accumulate. In plants, breakdown of substances is much slower than in animals. Hence accumulation of waste is much slower and less toxic. Plants have no special organs for removal of wastes. The waste products of respiration and photosynthesis are used as raw materials for each other. Oxygen gas produced as a by-product of photosynthesis is used up during respiration and carbon dioxide produced during respiration is used up during photosynthesis.
Excretory Products of Plants
Excretion is carried out in the plants in the following ways:
1. The gaseous wastes, oxygen, carbon dioxide and water vapour are removed through stomata of leaves and lenticels of stems.
2. Some waste products collect in the leaves and bark of trees. When the leaves and bark are shed, the wastes are eliminated.
3. Some waste products are rendered harmless and then stored in the plant body as solid bodies. Raphides, tannins, resins, gum, rubber and essential oils are some such wastes.
4. Water is also removed from plant through hydathodes present in the leaves.
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