Carbohydrates manufactured by the leaves must move through the plant to the roots for respiration, growth and storage.

The movement of dissolved solutes through a plant is called translocation.

To understand transport in plants better, the knowledge of the internal structure of plant roots, stems and leaves is essential.

Also necessary is knowledge of the mechanisms by which materials enter or leave cells i.e. diffusion, osmosis and active transport.

INTERNAL STRUCTURE OF STEMS:

The internal structure of stems is made of the epidermis, the cortex and the pith. Between the cortex and the pith are the vascular bundles.

The epidermis: This is a layer of rectangular cells one cell thick on the outside of the stem.

They are fitted without air spaces between them.

Their outer walls are thickened into a cuticle which is impermeable to gases or to water. However, in older stems there are small gaps in the epidermis called lenticels. Gaseous exchange occurs through these lenticels.

The cells contain no chloroplasts except in green stems where the stomata are also     found.

The cortex:

This is a region inside the epidermis but outside the conducting part of the stem.

In dicotyledons like sunflower, it is made of three layers of tissues.

The outer one is the collenchyma for strengthening to support the stem.

-          The middle one is the parenchyma for packing up remaining space and storing food. In green stems it has chloroplasts.

Beneath the endodermis is the sclerenchyma which consists of very hard and tough tissue.

The internal structure of a dicolyledonous stem:

                                

The vascular bundles:

They contain the conducting part of the stem. In the dicotyledonous stem the vascular bundles are arranged cylindrically in the cortex. In the monocotyledonous stem the vascular bundles are scattered all over the cortex and there is no cambium.

 

 Internal structure of a monocotyledonous stem:

 

                       

The phloem :

The phloem carries manufactured food substances from the leaves to the other parts of the plant.

It consists of three types of cells, all with cell walls:

‑           The sieve tubes which are cylindrical cells arranged end to end in long rows.

‑     The companion cells are narrow cells filled with dense cytoplasm containing a nucleus and mitochondria for respiration to release energy required to transport food substances across cells. There is one companion cell beside each sieve tube cell.

‑     The parenchyma which are large thin‑walled cells found among sieve tubes. They act as storage organs.

NB: The horizontal cross-walls of the sieve tubes have perforations called sieve pores.

                                              

The xylem:

The xylem vessels carry water and mineral salts from the soil to the leaves. The xylem vessels are formed from cells whose horizontal cross walls are broken down and the vertical walls are impregnated with lignin. Between the xylem vessels are the trachieds which also give the tree its woody characteristic.

The cambium:

This produces cells of the phloem and the xylem tissue by carrying out cell division. These cell divisions continue for a long time resulting into secondary thickening.

THE INTERNAL STRUCTURE OF ROOTS:

On the outside of a young root is the piliferous layer which has cells that give rise to the root hairs. Each root hair is a long structure projecting from a single cell, which is able to absorb water, and mineral salts from the soil.                     

Much of the root is made of large thin‑walled cells forming the cortex and these cells store food. Lateral roots grow from the pericycle towards the centre of the root.

Internal structure of a dicolyledonous root:

The xylem is found in the centre of the root and is star‑shaped and the phloem stands in the middle of the xylem rays. So there is no pith.

                                     

 

The internal structure of a monocotyledonous root:

The xylem vessels and the phloem tubes form a ring with the same radius leaving a pith in the centre.

 

                                   

DIFFUSION

All substances are made of small particles called molecules.

In solids the molecules are packed together with very little or no freedom of movement.

In liquids the molecules are also packed together but are free to move.

In gases the molecules are farther apart and they move at random colliding with each other and with the container walls. So molecules of gases tend to distribute themselves evenly through any space in which they are confined.

Also, molecules of substances that dissolve in a liquid tend to distribute themselves evenly throughout the liquid.

The movement of molecules of liquids or gases from a region of high concentration to a region of low concentration which tends to result in uniform distribution is called diffusion.

Experiment to demonstrate diffusion of a solid in a liquid:

Materials required: Potassium permanganate pellets, water, beaker.

 Method: Drop one pellet of potassium permanganate at the bottom of water in a beaker. Do not shake but leave for 20 minutes.

Observation: The water will turn purple.

Conclusion: The molecules of potassium permanganate spread evenly from a region where they were highly concentrated to regions in the water where there was low concentration till uniform distribution; so diffusion took place.

Experiment to demonstrate diffusion in gases:

Materials required: Wet, red litmus paper, glass tube, glass rod, cork, cotton wool, strong ammonia solution, clock.

Method:

‑           Cork one end of the glass tube.

‑     Use the glass rod to push squares of the wetted red litmus paper along the inside of the glass tube at even intervals.

‑     Close the open end of the glass tube with cork carrying a plug of cotton wool saturated with strong ammonia solution.

-          Note the time taken for each red litmus paper to turn blue.

                               Observation: The wetted red litmus paper turns blue at time intervals up to the last.

Conclusion: The alkaline ammonia vapour has diffused along the inside of the tube from a region of high concentration to a region of low concentration till uniform distribution.

NB: Rates of diffusion depending on concentration of ammonia solution can be compared by having different tubes.

Experiment to show diffusion in a solid:

Materials required:  Irish potato, knife, ruler, potassium permanganate solution, beaker, clock, filter paper, pair of tongs.

Method:

‑           Use the knife to shape two cubes out of the irish potato, one of 1cm and the other of ½ cm

‑     Drop both in a beaker containing potassium permanganate and leave them there for 15 minutes.

‑     Use the pair of tongs to remove the cubes and wipe them dry using filter paper.

      ‑     Cut through the middle of each cube and take measurements of penetration of                   potassium permanganate in each.

Observation

The smaller cube will have a deeper penetration of potassium permanganate solution than the larger cube.

Conclusion

Diffusion takes place in smaller objects which have a higher surface area to volume ratio than in larger objects with lower surface area to volume ratio.

Factors affecting the rate of diffusion:

The size of the diffusing molecules: The larger the size of the diffusing molecules the slower the rate of diffusion and the smaller the size of the diffusing molecules the faster the rate of diffusion. So rate of diffusion is inversely proportional to the size of the diffusing molecules.

The temperature of a substance: The higher the temperature of a substance the faster the rate of diffusion in it and the lower the temperature of the substance the slower the rate of diffusion in it. So the rate of diffusion is directly proportional to the temperature of the substance.

The concentration gradient:

This refers to the difference in concentration between the molecules diffusing and the medium through which they are diffusing. The higher the concentration gradient the higher the rate of diffusion and the lower the concentration gradient the lower the rate of diffusion. So rate of diffusion is directly proportional to the concentration gradient.

So in organisms there must be constant supply of a fresh substance diffusing on     one side and on the other side, there must be rapid removal of the substance diffusing.

The surface area over which diffusion occurs: The larger the surface area over which diffusion occurs the faster the rate of diffusion and the smaller the surface area over which diffusion occurs the slower the rate of diffusion. So rate of diffusion is directly proportional to the surface area over which diffusion occurs. The alveoli in lungs and the filaments in gills increase the area over which diffusion of oxygen and carbon dioxide occurs.

The distance over which diffusion occurs: The shorter the distance between the two regions of varying concentrations the faster the rate of diffusion and the longer the      distance between the two regions of varying concentrations the slower the rate of diffusion. So rate of diffusion is inversely proportional to the distance between the two regions of varying concentrations.

Cell membranes and epithelial layers across which diffusion regularly occurs          must therefore be thin.

Examples to illustrate diffusion gradient:

In the intercellular spaces near the palisade cells, there is an accumulation of carbon dioxide. When photosynthesis is taking place in the palisade cell, all the carbon dioxide in the palisade cell is being used up i.e. it is zero; whereas that in the intercellular space is high. So a diffusion gradient is set up between the air in the intercellular space and the palisade cell. As a result carbon dioxide diffuses into the palisade cell from the intercellular space.

When photosynthesis is taking place, oxygen is produced at a high rate in the palisade cell while any in the intercellular space is being sent out through the stoma. So a diffusion gradient is set up between the palisade cell and the air in the intercellular space. As a result oxygen diffuses out of the palisade cell into the intercellular space.

   Application of diffusion:

OSMOSIS

When a solute dissolves in water, the solute molecules attract the water molecules forming weak chemical bonds. If the sugar is dropped into a beaker with water, the sugar molecules will move from where they are highly concentrated to where there is a low concentration by diffusion. But also because of chemical attraction, even the water molecules will move from low concentration to join the sugar molecules. So the sugar molecules are moving from a high concentration to a low concentration by diffusion and the water molecules are moving from a low concentration to a high concentration by chemical attraction.

If the sugar molecules and the water molecules are separated by a membrane which allows water molecules to pass through to the sugar molecules due to chemical attraction but which does not allow the sugar molecules to pass across to the water molecules, then only the water molecules will move across from a low concentration to a high concentration across the selective membrane.

Such a movement of molecules from a region of low solute concentration to a region of high solute concentration through a membrane that allows certain molecules to pass through but not others is called osmosis.

Such a membrane that allows certain molecules to pass through but not others is called a selectively permeable membrane or a semi‑permeable membrane.

Experiment to demonstrate osmosis in non‑living material:

Materials required: Cellophane tubing, beaker of water, string bands, sugar or salt, capillary tube, clamp stand. Method:

-          A cellophane tubing tied at one end is filled with sugar and is fitted over the end of a capillary tube using a string band.

-          The cellophane is then lowered into a beaker of water and the tube is supported by a clamp stand.

-          The experiment is observed for 30 minutes.

 

Observation: The water level in the capillary tube rises. There may be a slight decline in the water level in the beaker.

Conclusion: Water moved from a region of low solute concentration to a region of high solute concentration; so osmosis occurred through the non‑living material.

NB: Practically, if the water in the beaker was tasted or tested for sugar, it would be found that sugar molecules too, diffused out of the cellophane tubing into the water in the beaker. This is so because cellophane is not that efficient as a semi-permeable membrane.

Experiment to demonstrate osmosis in living tissue:

Materials required: Irish potato, cork borer, salt solution, distilled water, beakers, ruler, filter paper, pair of tongs.

Method:

Bore out cylinders of Irish potatoes of equal lengths.

-          Drop some in salt solutions and others in distilled water.

-          Leave for 15 minutes.

-          Remove the cylinders using a pair of tongs and dry them using filter paper.

-          Take measurements of their lengths and try to bend them.

Observation: The cylinders placed in distilled water had become long, hard and flexible while those placed in salt solution were short, soft and could be easily bent.

Conclusion: Water entered the Irish potato in distilled water by osmosis; and water left the Irish potato in the salt solution by osmosis.

The molecular sieve theory :

The semi‑permeable membrane acts as a molecular sieve having tiny pores in it which allow small water molecules to pass through but not large sugar molecules to pass through.

NB:

‑ The cell walls of plant cells are freely permeable but the cytoplasm is selectively permeable so it plays a great role in movement of water into and out of cells by osmosis.

‑ Since selectivity of a semi‑permeable membrane in cells depends on the cytoplasm, then if the cytoplasm were killed by boiling then the selectivity of the semi‑permeable membrane would be destroyed; so osmosis would not take place.

Application of osmosis:

-          For movement of water from the soil to the xylem vessels in a root through the root hair and the cells of the cortex.

-          For movement of water from the xylem vessels in leaves to the palisade cells.

-          For movement of water from blood to the tissues in man.

-          For movement of water from blood to the kidney tubules during excretion.

Osmotic potential:

A dilute solution is said to be hypotonic and it has more free water molecules than a concentrated solution which is hypertonic. Since a dilute (hypotonic) solution has more free water molecules ready to move, we say it has a higher water potential or osmotic potential than a concentrated (hypertonic) solution which has fewer free water molecules ready to move. So pure water has the highest possible osmotic potential. Solutions of equal concentration are said to be isotonic.

Turgidity:

If a plant cell were put in a hypotonic solution or in distilled water:

A plant with turgid cells is resilient and strong and the leaves are held out firmly. In young plants support is mainly due to turgidity.

Plasmolysis:

If a plant cell is placed in a hypertonic solution i.e. one which is more concentrated than the plant cell sap in the vacuole:

 Flaccidity:

Plasmolysis does not usually take place in nature since few cells are subjected to hypertonic conditions. However, when plants lose water to the atmosphere faster than they obtain it from the soil, their vacuoles lose water. So the turgor pressure of the cells decreases and the cells become flaccid. A plant with flaccid cells is weak, flabby, the stem droops and the leaves are limp. Such a plant is said to be wilting.

NB: Plasmolysis is due to loss of water from the vacuoles due to hypertonic conditions whereas flaccidity is due to loss of water from the vacuoles without due replacement.

Osmosis and red blood cells:

o   If a red blood cell were put in a hypotonic (dilute) solution, the osmotic potential of the red blood cell would be less than that of the solution; so water would enter the red blood cell.

o   Since the red blood cell has a thin cell membrane, it would end up bursting.

o   If a red blood cell were put in a hypertonic solution, the osmotic potential of the          red blood cell would be higher than that of the solution; so water would pass out of the red blood cell and it would shrink.

How water passes from one cell to the next by osmosis:

If a cell A and B have varying sugar concentrations with A having a weaker solution than B the osmotic potential of A would be higher than the osmotic potential of B; so water would pass across their cytoplasms from A to B by osmosis.

It should be noted that most water that moves from one cell to the other passes by diffusion along the cells walls as shown in the diagram above.

ACTIVE TRANSPORT:

Diffusion and osmosis are called passive transport because they move molecules through cells without the use of energy. Active transport is when molecules are moved against a concentration gradient from a low concentration to a high concentration, and so energy is used. The molecules may then be transferred with the assistance of co‑enzymes.

In the root hair cell, there is a higher concentration of salts than in the surrounding soil solution. So only active transport can move the mineral salts from the soil solution into the root hair cell against a concentration gradient. Since energy is required, roots must be constantly supplied with oxygen in the soil air.

During absorption of digested foods in the ileum, active transport plays a part.

During selective re‑absorption of glucose and salts in the kidney tubules, active transport plays a part.

Movement of water from the soil through a plant to the leaves:

This is in three sections:

Movement from the soil to the xylem vessels in the root.

Movement through the xylem vessels upwards through the stem.

Movement from the xylem vessels through the leaf's mesophyll cells.

Movement of water from the soil to the xylem vessels:

Root hairs are thin‑walled extensions of cells of the piliferous layer of the root. These root hairs push between soil particles and are surrounded by a moisture film of soil water.

Soil water forms a weak solution with mineral salts; and cell sap in the vacuole of the root hair cell is more concentrated than the soil solution.

So the osmotic potential of the soil water solution is higher than the osmotic potential of the root hair cell sap in the vacuole.

So water passes by osmosis from the soil to the root hair cell vacuole across the thin cytoplasmic lining.

This extra water into the root hair cell vacuole increases the osmotic potential of the root hair cell so that it is higher than the osmotic potential of the neighboring cortical cell marked A.

So by osmosis water will pass from the‑root hair cell vacuole to the neighbouring cortical cell A's vacuole across their cytoplasms.

This extra water in the vacuole of cortical cell A in turn increases its osmotic potential so that it is higher than the osmotic potential of another neighboring cortical cell B.

So by osmosis water will pass from the vacuole of cell A to the vacuole of cell B across their cytoplasms.

Thus by osmosis, water passes from cell to cell until it reaches the xylem vessels.

NB: Mineral salts enter the root hair cell against a concentration gradient by active transport which requires energy from respiration. They may also move along the cell walls up to the xylem vessels by diffusion.

How water is taken up the stem through the xylem vessels:

Transpiration stream (pull): This is the pull exerted on water in the xylem vessels by transpiration so that there results a withdrawal of water from the xylem            vessels creating a tension that pulls along more water upwards along the xylem vessels.

Transpiration pull is the main force that takes water and mineral salts up the xylem vessels in a stem.

Capillary attraction: This is where adhesive forces exist between water molecules and the xylem vessels so that water is pulled up the stem through the xylem vessels.

Root pressure: This is the pressure exerted by a root to pump water up the stem in the xylem vessels.

Root pressure could be demonstrated by fitting a capillary tube with water on a cut stem using rubber tubing. A rise in the water in the capillary tube will be witnessed.

 It is root pressure that is also responsible for guttation where droplets of water are released from leaves through cell pores called hydathodes which are at the edges of leaves.

Movement of water through the leaves:

Most water moves along the cell walls by diffusion but some moves across the cytoplasm by osmosis.

When mesophyll cell A loses water by evaporation through its cell wall, its cell sap in the vacuole becomes more concentrated than the cell sap in the vacuole of neighbouring cell B.

So the osmotic potential of mesophyll cell A will be less than that of neighboring cell B.

So by osmosis water will leave cell B and enter mesophyll cell A.

This will in turn make the osmotic potential of cell B which is neighbouring the xylem vessels to be lowered below the osmotic potential of the xylem vessels themselves.

So again by osmosis, water will leave the xylem vessels and enter cell B.

Translocation of manufactured food through a plant:

Food manufactured by photosynthesis like sucrose passes via the sieve tubes in the phloem upwards going to growing regions, maturing fruits or seeds; or downwards to roots and other storage organs. Unlike the xylem vessels, sieve tubes of the phloem contain living cells.

Evidence that phloem is made of living cells:

If oxygen supply to the phloem is cut off, then translocation of sugars ceases.

If a jet of steam is applied to the stem in a region below a leaf, the phloem there is killed and so no sugars from that leaf appear below the scald.

Also if the jet of steam was applied to the stem in a region above the leaf, the        phloem there would be killed and so no sugars from that leaf would appear above the scald.

NB: Heat treatment of the stem has no effect on mineral salt uptake through the stem. So if a plant stem were treated with heat, the plant would die because the roots are lacking food for respiration which is supposed to be supplied by the phloem. Otherwise the leaves continue getting water through the xylem vessels.

It is for the same reason that a plant dries up when a ring of bark is removed from the stem. The phloem which is found near the bark is thus cut and so food does not reach the roots. Food thus just accumulates into a bulge above the scar.

 

TRANSPIRATION:

This is the process by which plants lose water vapour to the atmosphere. It takes place mostly through the stomata of leaves and is called stomatal transpiration but could also occur through the leaf cuticle (cuticular transpiration) or through the lenticels on stems (lenticular transpiration). Transpiration can also occur through flowers.

Transpiration can lead to wilting, if plant cells lose more water to the atmosphere than they are gaining from the soil.

Benefits of transpiration:

Uptake of water by transpiration stream: Transpiration of water from the mesophyll cells results in a withdrawal of water from the xylem vessels which creates a tension that pulls more water along the xylem vessels from the roots. This leads to further absorption of water from the soil. So it aids in water uptake.

Uptake of salts: The transpiration stream where water uptake is encouraged results in uptake of the dissolved salts too.

Cooling the plant: Evaporation of water from the leaf surface results in escape of latent heat which cools the plant.

Environmental factors affecting transpiration:

              
Humidity: When the atmosphere is very humid i.e. saturated with water vapour, little more can be absorbed from plants; so transpiration rate will be reduced. In a dry atmosphere transpiration rate is high.

Air movements (wind): In still air transpiration is reduced because the region around the leaf is saturated with water vapour. In windy air, water vapour is swept away from the leaf as fast as it diffuses out of the leaf. So transpiration rate is high and continuous.

Plant structure factors controlling transpiration :

Surface area of leaves: The higher the surface area of leaves the higher the rate of transpiration. Many plants control transpiration by reducing leaf surface area by having needle‑like or rolled leaves for example pine. Deciduous trees reduce their overall leaf area by shedding the leaves in the dry season to reduce transpiration since roots can absorb no water from the dry soil.

Condition of the leaves: Some leaves are succulent so that they store water for long periods; while others have a thick cuticle on the leaves which reduces transpiration; and yet others have a hairy cover on the leaves which also reduces transpiration.

Number and condition of stomata: The more the stomata on a leaf the higher the rate of transpiration. Apart from having few stomata, some leaves have sunken stomata deep in the epidermal pits and this too reduces transpiration.

To compare rates of transpiration by water uptake:

Materials required: Potometer, leafy shoots, water, rubber tubing, stop clock.

Method:

• Use a potometer which uses very small quantities of water so that temperature changes have negligible interference with the results.
• Place the cut shoot in water immediately after cutting it from the tree so that air is not taken into the water vessels of the stem.
• Fill the potometer with water and fit the cut end of the shoot into rubber tubing carefully not to include any air bubbles.
• Note the time taken for the meniscus in the capillary tube to move a fixed distance.
• Transfer the potometer to different environmental situations and compare the time taken for the meniscus to move that fixed distance.

Always allow the apparatus to adjust to new conditions by leaving it there for a few minutes.

For better vision, use a coloured liquid.

Observation:

As water evaporates from the cut shoot, more water is drawn from the potometer tubes and the capillary tube.

Conclusion:

In windy, warm and less humid conditions, the rate of transpiration is high unlike in cool, humid and still air conditions.

To measure water loss from potted plant:

Materials required: Beam balance, potted plant, weights, water, polythene bag, string.

Method:

• Cover the pot of the potted plant with polythene bag and tie tightly with string to prevent evaporation directly from the soil.
• Place the potted plant on one pan of the beam balance.
• Balance it with weights on the other pan.
• Keep adding weights at regular intervals to the scale pan with the plant to maintain balance (or keep reducing weights off the scale pan with weights).

    

Observation:

The pan with potted plant keeps reducing in weight.

Conclusion:

Weights added (or reduced) after start of the experiment equal to water lost.

 

Experiment to show that water vapour is given off during transpiration:

Materials required: Potted plant, polyphone bag, string, cobalt chloride paper or anhydrous copper sulphate crystals.

Method:

Enclose the shoot in a transparent polythene bag and ties tightly around the base of the stem so that no water enters or leaves the polythene bag.

Leave for an hour in sunlight.

Remove the polythene bag and test the condensed liquid in the bag with blue cobalt chloride paper or white anhydrous copper sulphate crystals.

Set up a control in similar conditions but using a plant stump with all leaves removed.

Observation:

Droplets of liquid appear on the inside the polythene bag and on testing, the cobalt chloride paper changes from blue to pink.  If anhydrous copper sulphate is used, then it changes from white to blue.

In the control, no water condenses on the inside of the polythene bag.

Conclusion: Water vapour is given out during transpiration.