TRANSPORT IN PLANTS

INTRODUCTION

The movement of materials within the body tissues or between tissues in multicellular organisms like high plants and animals occurs in transport systems.

• In unicellular or low plants and animals, such movement is by the transport mechanisms of molecules within a transport medium by diffusion, osmosis, active transport and mass flow, among others.
• In multicellular animals, the medium of transport is blood, while in plants, it is water in which the materials are dissolved and transported.

In case of diseases such as diarrhea, dysentery, malaria, etc, the animal body may undergo dehydration. This causes serious effects on the body’s physiological process, which can lead to death.

Just like dehydration in animals, in plants wilting or excess water loss results, especially during dry seasons or inadequate water supply or absorption by conducting tissues.

Animals and plants have specialized cells or tissues performing special functions of the transport systems apart from transporting materials.

Transporting tissue in plant is xylem and phloem. It involves movement of water, salts and organic molecules (manufactured food).

THE XYLEM

This Consists of xylem vessels and tracheids. Xylem vessels develop from cylindrical cells, arranged end to end, in which the cytoplasm die and cross- walls disappear leaving a dead empty tube. Through this:

Water, mineral salts, move from roots, stems, up to leaves.

Xylem vessels are strengthened by lignin in their walls. This strength gives support to the soft tissue of roots, stems, and leaves: it also prevents collapse of the vessels under tension as sap pressure changes.

Characteristics of xylem tubes

• Consist of dead cells
• They are hollow
• Its walls are lignified
• Has no protein filaments
• Has no cytoplasm
• Transports water and salts

Structure of xylem

PHLOEM TISSUE

This Consists of sieve tubes and companion cells. The sieve tubes are formed from cylindrical cells arranged end to end. Unlike the xylem vessels, the cross walls do not disappear but develop perforations of enlarged pits forming sieve plates.

The protoplasm of the sieve tube element remains living; although its nucleus disintegrates as the cell differentiates. Each sieve tube is closely associated with companion cells which are complete cells. The companion cells regulate metabolic activities of the sieve tubes.

Characteristics of phloem tissue/tube

• Consist of living cells
• Have a thin cytoplasm
• Associated with companion cells
• Consist of sieve cross walls
• Consist of protein filaments
• Transport food materials

Structured comparison between xylem and phloem

Similarities:

• Both are perforated, i.e. xylem is bordered with pits and phloem has sieve pores in the sieve plates.
• Both tissues are surrounded by parenchyma cells as packing.

Differences:

TRANSPORT OF WATER FROM SOIL TO THE LEAVES

 Water absorption by the root hairs to the xylem

Up take of water also called absorption is a continuous stream through the plant. Root hairs in the soil are surrounded by a film of water containing mineral salts/ soil solution.

The soil solution once inside the root hair vacuole is called cell sap and is a strong solution than the soil solution (has a lower osmotic potential) and the cell membrane of the root hair is semi permeable.

The above conditions enable water to move from the soil, pass through the cell membrane in to the vacuole by osmosis.

Root hairs vacuoles contain a high concentration of solute than the surrounding water. Water is absorbed by root hairs by osmosis. This causes the root hair vacuoles to become less concentrated than those of the adjacent cortex cell. Water is then passed into the cortex cell by osmosis and it then enters the xylem tissue.

Water moves through the root cortex from cell to cell by 3 path ways:

1. Some of the water flows along the cell walls (Apoplast).
2. Some water travels in the cytoplasm (Simplest).
3. Most of the water moves from vacuole to vacuole

The inner most region of cortex is made up of the endodermis strip which controls the movement of water from the cortex into the xylem.

The water rises up the xylem by the following forces:

Capillarity:

This is the ability of water to move up the fine tube. It is usually caused by the surface tension but because the capillary tube is narrow, the water rise is limited.

Cohesion – tension forces:

This is a force of attraction between the molecules of the same substance. Cohesion between water molecules allows water in a continuous column without breaking. This occurs because as water is lost by transpiration from the leaves, the water potential at the top of xylem vessels falls below that at the bottom of the xylem in the root. Water is now pulled by this potential difference because of the cohesion of the water molecules.

Adhesion:

This is the force of attraction between molecules of different substances. Adhesion forces between walls of xylem and water molecules support a considerable weight of water within the xylem tissue and prevent the water in the xylem vessels from collapsing.

Root pressure:

This is regarded as the pressuring force of the water up the stem from the roots.

It is affected by the same factors that affect respiration in living cells like oxygen supply, temperature, starch supply and the presence of respiratory poison like cyanide.

The root pressure theory has been suggested as a result of a common observation that water tends to exude from the cut stem indicating that some pressure in a root is actually pushing the water up. However, like capillary, root pressure is not sufficient on its own to push water to the leaves of the plant at the top of the tree and can slowly cause guttation in transpiring herbaceous plants.

Transpiration pull:

This is the pulling force generated by the evaporation of water from the leaves. This is caused when the cells of the spongy mesophyll layer in the leaf lose water by evaporation into the air spaces causing their cell sap to become more concentrated and as a result they draw the water from the surrounding cells by osmosis. These cells in turn get water from the xylem in the veins and then water from the xylem moves to replace the lost water by evaporation. This evaporation sets up the passing action on water in the xylem called transpiration pull.

Adaptations of the root hair to water absorption

• The root hair is slender and flexible and can therefore flow between the soils
• They are numerous which increase the surface area available for water
• They lack the cuticle which would restrict water.
• They are long and narrow which increases surface area to volume ratio that increases the rate of water absorption.
• The cytoplasm of the root hair contains numerous mitochondria where respiration occurs to release ATP needed for active transport of mineral salts from the soil solution to the cytoplasm of the root.
• At the center of the root hair is a vascular tissue which transports water and mineral salts to the rest of the plant.
• The cell sap of the root hair contains sugars, amino acids and salts, and so its concentrated than the soil solution and this low osmotic potential enables water to enter it by osmosis.

Importance of water to the plant

• Raw material for photosynthesis
• Solvent for mineral salts and oxygen that enable them to diffuse into the
• It is a constituent of the cytoplasm and all sap of the growing plants
• Provides turgidity which provides support in non woody plants
• Cools the leaves of the plants during transpiration

Absorption of mineral salts by the root hairs

Mineral salts are moved in the plant in the xylem in solution with water. Roots absorb mineral salts in form of ions by diffusion and active transport. Active transport is the movement of the materials against the concentration gradient by the use of energy released from respiration.

TRANSPORT OF THE PRODUCTS OF PHOTOSYNTHESIS

The process by which the soluble products of photosynthesis are carried in plants is called translocation. Translocation is the movement of manufactured food from the side of photosynthesis. Throughout the plant, sugars and amino acids are transported in the phloem from the leaves to the growing parts of the plant or storage organs. Food substances may also move from the storage organs to the growing regions of the plants. In the phloem, food substances may move upwards/down wards.

The Translocation process

The process of photosynthesis leads to accumulation of food substances in leaves. This causes a high turgor pressure within the leaves.

Food substances in the roots are used for respiration or they are stored in the storage organs and these results in the low turgor pressure in the root cells. The difference between turgor pressure in the roots and leaves enables the food substances to move from leaves to other parts of the plant by a process called mass flow which is the major process of translocation.

There is also a minor process i.e. active transport where the sugars e.g. sucrose are actively transported from leaves to the storage organs.

ACTIVE TRANSPORT

Active transport is the net movement of particles against a concentration gradient. Energy is therefore required. During active transport, molecules are transported from a low concentration to a high concentration.

They are said to move against the concentration gradient, such a process requires energy.

In active transport, the cell must use its own energy to move the molecules against a concentration gradient. The mitochondria in the cells supplies the energy required, thus active transport can only take place in living cells.

Importance of Active Transport

Active transport facilitates absorption of mineral salts and ions from the surrounding soil even when concentration of these mineral salts is already higher in the cells than in the soil.

The cells lining the small intestines continue to absorb food molecules by active transport even when the concentration of these molecules is higher in the cells than the intestine lumen.

Nerve cells need sodium ions and potassium ions to function. the concentration of sodium ions outside a nerve cell is higher than the concentration on the inside. The concentration of potassium ions on the outside is lower than on the inside. The nerve cells maintain these concentrations by active transport.

When urine is first formed in the kidney, it contains useful substances like glucose in addition to waste products. The useful substances are reabsorbed into the blood stream by active transport.

Evidence to show that food made in leaves is Translocated by the phloem

The Ring Experiment:

Remove a ring of the bark from the stem at a point between the ground and the upper leaves. Leave another plant with the ring on. The plants are left to stand for one week after which the observation is made.

Observation

The upper part of the stem of the ring plant swells immediately above the ring while the lower part of the stem remains unswollen. The unringed plant remains unchanged.

Conclusion: The phloem transports manufactured food.

Explanation

When a ring of a base is cut, the phloem tissue is removed along with it since it’s found within the bark. This cuts off the supply of manufactured food to the lower parts of the plant as a result, the phloem in the upper part of the stem will transport the food to the part just above the ring. The food will then accumulate in this part hence it will swell. When the ring is removed, the tree or plant also dries because the food supply to the root is cut off therefore the stored food in the roots gets exhausted then the roots die.

Feeding Aphids:

When the proboscis of the sucking aphid is cut, it is found to have penetrated into the phloem tube and when its contents of the proboscis are analyzed, it is found to contain products of photosynthesis (sucrose) which are transported to the bark through the phloem.

Radio Active Tracers:

If a plant is exposed to CO2 labeled with radioactive C-14, the C-14 becomes incorporated into the end products of photosynthesis which are subsequently detected in the stem. That these substances are confined to the phloem and can be shown by cutting sections of the stem, placing the sections in contact with photographic film and making auto radiographing it is found that the sites of radioactivity correspond precisely to the positions of the phloem.

TRANSPIRATION

This is a process by which plants lose water in form of water vapor mainly through leaves to the atmosphere. Transpiration can also occur from flowers.

Types of transpiration

1. Stomatal transpiration: This is the transpiration through the stomatal opening. This contributes up to 80-90% of water
2. Cuticular transpiration: This occurs through the leaf cuticle which amounts for about 20% of the water
3. Lenticular transpiration: This occurs through the stem pores called lenticels and accounts for about 0.1% of the water

Water can also be lost from the plants as water droplets in a process called guttation through special structures called hydrates found on leaf types or margins.

An experiment to show that water is lost mainly from leaves during transpiration

Apparatus:

Potted plant, Polythene paper, String and Cobalt (II) chloride paper or anhydrous copper (II) sulphate.

Procedure

1. Tie polythene around the tin of the potted plant. Using a string to avoid evaporation of water from the soil surface.
2. Tie transparent polythene around the leafy shoot of the
3. Set up another similar control experiment but with leaves removed and dry
4. Leave the experiment to settle for 3 hours in bright
5. Remove the polythene around the leafy shoot and test the drops of liquid inside the polythene using anhydrous copper (ii) sulphate / cobalt (ii) chloride

Observation

A vapor forms inside the polythene and turns into drops / liquid which turn anhydrous copper (ii) sulphate from white to blue or blue cobalt (ii) chloride paper to pink.

No vapour is observed from experiment with no leaves / dry plant.

Conclusion: Transpiration occurs from the leaves

Note: A control experiment may also be a covered pot where the plant shoot has been cut off.

Experiment to compare transpiration rates on both surfaces of a leaf

Apparatus

• Potted plant,
• glass slide
• Cobalt (ii) chloride paper
• Rubber bands

Procedure

1. Fix pieces of Cobalt (ii) chloride paper on the upper and lower surfaces of a leaf still to the plant with glass slides.
2. Tie the slides using the rubber bands
3. Note the time taken for the Cobalt (ii) chloride paper on each slide to turn / change colour from blue to pink.

Observation

The lower surface cobalt (ii) chloride paper turns pink faster than that on the upper surface.

Conclusion

The lower surface has a higher transpiration rate than the upper surface. This is due to numerous stomata on the lower surface of the leaf.

Factors that affect the rate of transpiration

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

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

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

Light intensity:

• Rate of transpiration is high in 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.
  

Atmospheric pressure:

• Humidity decreases with decrease in atmospheric pressure. Hence decrease in atmospheric pressure greatly increases the rate of transpiration due to decreased

Non environmental factors

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

Number of stomata:

• The greater the number of stomata, the higher the rate of transpiration because more water vapour is lost through the

Surface area for transpiration:

• Plants with wide/broad leaves have a larger surface for transpiration thus they experience a higher rate of

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

Experiments to measure the rate of transpiration

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.

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

Set up of a potometer