THE CELL AND ITS ENVIRONMENT

DIFFUSION

Diffusion is defined as the process by which molecules or ions of a substance (i.e. gases and liquids) move from a region of high concentration to a region of low concentration until they are evenly distributed. The substance involved in diffusion may be liquid, gases or solid.

 

Factors Affecting or Controlling Diffusion

The rate or speed of diffusion is controlled by a number of factors which include:

State of matter: Diffusion varies with the three state of matter. The diffusion of gases is much faster than that of liquids because the gas molecules are freer and therefore faster than liquid molecules.

Molecular size: The nature or the size of the molecules affect diffusion. In general, the smaller the molecules, the faster the rate of diffusion while the larger the molecules, the slower the rate of diffusion.

Differences in concentration: For diffusion to take place in a medium, there must be differences in the concentration of the substance in two areas. The greater the differences in the concentration of the molecules, the greater the rate of diffusion.

Temperature: High temperature increases the speed at which molecules move. Thus, the higher the temperature, the faster the rate of diffusion.

 

Experiment to Demonstrate Diffusion in Liquids

Take a beaker and fill it with distilled water (Fig. 4.1). Use pipette to deliver small quantity of potassium permanganate solution gently at the bottom of the beaker and leave it to stand for few minutes. The purple colour of the potassium permanganate solution starts to spread outside.

Eventually, the colour spreads evenly throughout the water medium so that the water have the same shade of purple colour.

 

Experiment to Demonstrate Diffusion in Gases

Take a bottle of ammonia solution, open the bottle and move some distance away from the bottle and wait for some time. Then smell the air to perceive the odour. The smell of the ammonia gas shows that diffusion of ammonia gas has taken place.

 

Importance of Diffusion to Animals

Diffusion plays important roles in the life of animals through the following processes:

1. There is intake of oxygen or nutrients from mother to foetus (embryo) through placenta.
2. Gaseous exchange in mammals occurs in the lungs during respiration.
3. Gaseous exchange in many cells and organisms, e.g. Amoeba takes in oxygen and gets rid of carbon dioxide by diffusion.
4. There is movement of carbon dioxide from the lung capillaries into the air sac.

 

Importance of Diffusion to Flowering Plants

Diffusion is important to flowering plants in the following ways:

1. Movement of carbon dioxide through the stomata of the leaves during respiration.
2. There is movement of carbon dioxide through the stomata into the leaves during photosynthesis.
3. Water vapour leaving the leaves during transpiration.
4. Movement of oxygen into the leaves through the stomata during respiration.

 

Diffusion in Nature or Non-living Conditions

Diffusion is also very important in nature or non-living conditions through the following processes:

1. The spread of the smell or odour of perfume from a person or a corner of a room.
2. Diffusion of molecules (gases and liquids) in iodine, potassium permanganate and copper sulphate solutions.
3. The spread of insecticide in a room.
4. The spread of the smell of gases released from the anus.

 

OSMOSIS

Osmosis is defined as the flow of water or solvent molecules from a region of dilute or a weaker solution to a region of concentrated or stronger solution through a selectively or differentially permeable membrane. It should be noted that osmosis is a special form of diffusion.

 

Conditions Necessary for Osmosis to Take Place

There are three major conditions which are necessary for osmosis to take place. These are:

1. Presence of a stronger solution, e.g. sugar or salt solution.
2. Presence of a weaker solution, e.g. distilled water
3. Presence of a selectively or differentially permeable membrane.

 

Living Cells as Osmometer

In osmosis, there are usually two solutions which are separated by a differentially permeable membrane. The weaker solution is said to be hypotonic while the stronger solution is said to be hypertonic. When both solutions have the same concentration, they are said to be isotonic.

Hypotonic: When a cell of a living plant or animal is surrounded by pure water or solution whose solute concentration is lower, water passes into the cell by osmosis. The solution is therefore said to be hypotonic.

Isotonic: When the solute concentration of the cell and its surrounding medium are the same, the solution is said to be isotonic.

Hypertonic: When the cell is surrounded by a stronger solution, water will be lost by the cell. The shrinking of the cell is as a result of the surrounding solution being hypertonic (Fig. 4.2).

In living cells, when water moves across the membrane into a solution of a higher concentration, a pressure is created in the cell. This pressure is called osmotic pressure. The solution is said to exert a higher osmotic pressure than the weaker solution. Osmotic pressure is a force that draws in water into the cell. The pressure which a solution can potentially exert is called its osmotic potential. Osmoregulation is the control of fluctuations in the concentration of substances in cell fluids by special devices such as the contractile vacuoles in Amoeba and Paramecium.

 

 

Experiment I

• Aim: To demonstrate osmosis using a non-living material.
• Materials required: Two thistle funnel, beakers, sugar solution, water, pig bladder or cellophane paper
• Method: Pour equal quantity of water into the beakers, then cover the bottom of the thistle funnels with cellophane paper (selectively permeable membrane). Then pour sugar solution into thistle funnel A and water into thistle funnel B (control experiment) and mark their levels. Then, immerse the two funnels into the beakers containing water (fig. 4.3). Allow the experiment to remain for 2-3 hours.
• Observation: At the end of the experiment, the volume of sugar solution will rise in the thistle funnel A while the water level in the beaker will reduce. At the same time, the volume of water in funnel B and beaker remain at the same level.
• Conclusion: The rise of sugar solution in thistle funnel A and a decrease in the water level in the beaker show that osmosis has taken place.

 

 

Experiment II

• Aim: To demonstrate osmosis using a living tissue
• Materials required: Yam tuber, sugar solution, water, knife, dishes.
• Method: Peel the yam tuber, cut it into two parts, make a cavity with the aid of the knife into the two cut yam tubers. Pour water into the two petri-dishes. Place each half of the yam tubers with base down into the petri-dishes containing water (fig. 4.4). Add small quantity of sugar solution to yam tissue A and allow yam tissue B to serve as control experiment. The set-up is allowed to stand for 4-6 hours.
• Observation: At the end of the experiment, it is observed that the level of sugar solution in A has risen resulting in a decrease in water level in the petri-dish while the water level in B remains the same both in the tuber and in the petri-dish.
• Conclusion: Since the sugar solution has risen in yam tissue A, it shows that osmosis has taken place.

Note: Materials that can be used as living tissues (selectively permeable membrane) are yam tubers, potato, pawpaw, cocoyam etc.

 

 

Differences Between Diffusion and Osmosis

 

Diffusion

1. Diffusion occurs in gases and liquids.
2. Differentially permeable membrane is not required.
3. It occurs in living and non-living organisms.

 

Osmosis

1. Osmosis occurs in liquid medium only.
2. Differentially permeable membrane is required.
3. It occurs naturally in living organisms.

 

PLASMOLYSIS

Plasmolysis is defined as the outward movement or flow of water from living cells when they are placed in a hypertonic solution. Plasmolysis is often regarded as the opposite of osmosis.

The process of plasmolysis involves the withdrawal of water from living cells up to the extent that it will result in the pulling away of the cytoplasm from the cell membrane or cell wall. As a result of this, the cytoplasm will shrink and the whole cell will collapse. When this happens, the cells are said to be plasmolysed. This will eventually lead to wilting or death of the plant.

 

Process of Plasmolysis in Plant Cell

When a living plant cell is placed or surrounded by a sugar or salt solution, a more concentrated or hypertonic solution than the cell sap, water will be lost from the cell to the stronger solution resulting in exosmosis. As a result of this, the vacuole will shrink, pulling the cytoplasm away from the cell wall or membrane.

 

Experiment to Demonstrate Plasmolysis Using Spirogyra Filament

Place a piece of Spirogyra filament on a glass slide containing few drops of water, covered with cover slip. Observe the set up under the microscope. The cells are noticed to be normal or turgid.

Add few drops of concentrated salt or sugar solution on the tissue. Leave it for 15 minutes. Observe under the microscope. It will be observed that the cytoplasm is drawn away from the cell wall showing that exosmosis has occurred or the cells have been plasmolysed (Fig. 4.5).

 

 

HAEMOLYSIS

Definition: Haemolysis is defined as the process by which red blood cells or corpuscles become split or burst as a result of too much water passing into it. This situation will occur when a red blood cell is placed in a weaker or hypotonic solution where the red blood cell takes in water and become swollen and may even burst.

 

Experiment to Demonstrate Haemolysis Using the Red Blood Cell

When the red blood cell is placed or surrounded by distilled water (hypotonic solution), water passes into the cell showing that osmosis has taken place. This results in the increase in size of the cell or the cell becomes turgid or swollen (fig. 4.6). For the fact that the distilled water is hypotonic or less concentrated than the blood cell, water is absorbed by the cell. This will make the cell to swell and burst.

 

Similarities and Differences Between Plasmolysis and Haemolysis Similarities

1. They both occur in living cells.
2. Both processes can lead to the death of the cells concerned.
3. Cells expand initially as more water comes into the cells in both processes.

 

Differences

Plasmolysis

1. It occurs in plant cells.
2. Plant cell shrinks.
3. It occurs in a hypotonic solution.

 

Haemolysis

1. It occurs in red blood cells.
2. Red blood cell bursts.
3. It occurs in hypertonic solution.

 

TURGIDITY

Turgidity is defined as the condition in which cells absorb plenty of water up to a point where the cell is fully stretched. At this point, the cell is said to be turgid. Turgidity occurs when a cell is placed in hypotonic solution (distilled water). As a result of the fact that the cytoplasm solution is stronger than the water, the cell absorbs water and becomes turgid.

Turgidity is useful to the plants because it makes them stand erect, gives support to the stem, leaves, flowers and guards cells.

 

FLACCIDITY

Flaccidity is defined as the condition in which plants lose water to their surroundings faster than they can absorb. When plant looses more water, it is said to be flaccid. Flaccidity normally occurs when there is no water in the soil or during drought. Such continuous loss of water to the surroundings may cause the plant to wilt or even die if it continues for a very long time.