MOVEMENT OF SUBSTANCES IN AND OUT OF CELLS

Objectives

The readers will be able to:
o     Explain the need for movement of substances across the cell surface membrane.
o     Perform experiment to demonstrate diffusion, osmosis and plasmolysis
o     Explain the concepts of plasmolysis, active transport, endocytosis (phagocytosis and pinocytosis) and exocytosis.

Movement of Substances in and Out of Cells

The cells of multicellular organisms are surrounded by a watery fluid. The cytoplasm is also fluid, so various substances (both organic and inorganic) in solution pass into and out of the cells. The need for substances to move in and out of cell across the surface membrane;

o to obtain nutrients

o to eliminate waste metabolic products

o to secrete vital substances such as enzymes, hormones, defensive chemicals etc.

o to generate ionic gradient for the activities of muscles and nerves

o to maintain suitable medium for effective functioning of enzymes

Two ways by which substances move into and out of the cell are;

1. Passive transport: refers to movement of particles across a membrane from the side with a higher concentration of particles to the side with a lower concentration. Because movement follows a concentration gradient, energy is not supplied by the cell. The main types of passive transport are diffusion, osmosis and filtration.

2. Active transport: involves the use of energy supplied by the cell to move materials across the membrane against a concentration gradient. Transport proteins are used and energy is supplied by ATP.

Osmosis

Osmosis is defined as the movement of water molecules from a region of lower concentration to a region of high concentration through a semi permeable membrane.

Osmosis is movement of water molecules from a dilute solution to a more concentrated solution through a semi permeable membrane.

1. A semi permeable membrane is a membrane which allow certain types of materials to pass across it e.g., pig’s bladder, membrane around red blood cells, and yam tissues.

2. Freely permeable membrane: is a membrane which allows the passage of all materials across it.

Experiment to Demonstrate Osmosis

1. Demonstrating Osmosis in Non-living Tissues

Materials

Visking tube or cellophane, thistle funnel, sugar solution, distilled water, beaker, stand/clamp

Procedure

1. Distilled water is poured in a beaker.

2. The mouth of thistle is tightly covered with a visking tube or cellophane.

3. Thistle funnel is filled with a strong sugar solution.

4. The thistle funnel is lowered into the beaker with distilled water and clamped vertically.

5. The level of sugar solution in the funnel is marked as the first level.

Observation

o The sugar level in the thistle funnel rises as water in beaker falls

Conclusion

Water molecules moved from the beaker into the sugar solution in the thistle funnel via semi–permeable membrane by osmosis causing a risen in the level of solution in the thistle funnel.

Demonstration of Osmosis in non-living Material

2. Demonstrating Osmosis in Living Tissues

Materials: 3 Irish potato or yam tuber, 3 Petri-dishes, sugar or salt, Knife or scalpel, water

Procedure

o Obtain three yam tubers and peeled them with a knife.
o The middle cavities of the tubers are scooped out using knife
o One of the cut tubers is placed in boiling water for about 5 minutes to kill the cells
o The three prepared tubers are placed in dishes containing water
o One of the fresh tubers is filled with water. A tea spoonful of sugar is placed in other raw tuber and in the cavity of the boiled tuber as shown below

o The set-up is left to stand for about 6 to 12 hours

Observation

In the cavity of the boiled tuber C, there was no water while in the cavity of A; there was water which dissolved sugar crystal. The level of water in the cavity of B remain the same.

Conclusion

In A, osmosis took place because there was a difference in water potential between cavity and the water in the Petri-dish and semi permeable membrane of yam cells was living.

In B, there were no solute molecules to enable osmotic flow of water

In C, osmosis did not occur because the semi permeable membrane of yam cells was destroyed by boiling.

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Terms used in connection with osmosis

Tonicity: refers to the relative concentration of solute on either side of a membrane.

Isotonic solution: Is a solution which has the same concentration as another solution with which it is being compared. i.e., solutions with same osmotic pressure.

Hypotonic solution: is a solution which has a lower concentration than another solution with which it is being compared. A hypotonic solution has lower osmotic pressure and is generally termed as less concentrated.

Hypertonic solution: is a solution which has higher concentration than another solution with which it is being compared. A hypertonic solution has a higher osmotic pressure and is generally termed as more concentrated solution.

Osmosis and Animal Cells

Red Blood Cells

When red blood cells are placed in dilute solution (hypotonic solution) e.g., distilled water, the cells absorb water, swell up and eventually burst i.e., haemolysis.

When red blood cells are placed in a more concentrated solution (hypertonic solution) e.g., a strong sugar or salt solution, water moves out of the cells to the surrounding solution by osmosis. As a result, the cells shrink, a process called crenation. However, when red blood cells are placed in isotonic solution, they neither gain water and burst nor lose water and shrink. This indicates that the blood plasma is isotonic to solution in red blood cells.

Osmosis and Plant Cells

Plant cells are surrounded by an inextensible, resistant and completely permeable cellulose cell wall. The center of cells contains vacuoles, which contains sap. Sap is a solution of salt, sugars and organic acid. Sap is surrounded by a semi-permeable membrane called tonoplast.

o When a plant cell is placed in hypotonic solution e.g., distilled water, water enters the vacuole of the cell by osmosis. The sap vacuole enlarges and pushes the cytoplasm against the cell wall. The cell swells up without bursting due the thick cell wall. This makes the cell turgid.

N/B: The pressure exerted out wards by vacuole is called turgor pressure.

o When a plant cell is placed in a hypertonic solution e.g., strong sugar solution, water moves out of the vacuole of the cell by osmosis. The vacuole shrinks causing the cell membrane to pull away from the cell wall. The cell becomes flaccid or plasmolyzed.

o Plasmolysis is the shrinkage of the protoplasm away from the cell wall due to loss of water from the plant cell by osmosis when the cell is placed in a hypertonic solution.

Demonstration of Plasmolysis using Spirogyra Filament

1. Filament of spirogyra in pond water was placed on a glass slide

2. It was observed under low power of the microscope

3. The cells were found to be turgid.

4. The pond water was absorbed using filter paper

5. A drop of strong salt solution was added and left for a few minutes.

6. It was observed that the protoplasm shrank and pulled away from cell wall.

7. Leaving a gap between the cell wall and the plasma membrane.

Factors Affecting the Rate of Osmosis

1. Temperature: the rate of osmosis increases with increasing temperature.

2. Concentration gradient: The greater the difference in concentration between the two solutions, the faster the rate of osmosis.

3. Presence of semi-permeable membrane

Significance of Osmosis in Living Organisms

1. Enhance absorption of water by root hairs from the soil.
2. Enhance the movement of water from roots hairs through the root cortex to the xylem
3. Gives turgidity to plant cells
4. Controls the opening and the closure of the stomata by guard cells
5. It enables movement of water from gut into blood streams via gut walls
6. It enables re-absorption of water into blood streams via kidney tubules.
7. Entry of water into unicellular organisms.

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Diffusion

Diffusion is the movement of particles or molecules from a region of higher concentration to a region of lower concentration until the molecules are evenly dispersed. The difference of concentration between the two regions is often termed as the concentration gradient, and diffusion will continue until this gradient has been eliminated. A dynamic equilibrium is established when the concentration of the regions is equal, and the rate of diffusion or movement of substance in either direction becomes equal.

Factors that Affect the Rate of Diffusion

1. Size of molecules: smaller particles diffuse faster than larger particles.

2. Temperature: the rate of diffusion increases as temperature increases because the particles acquire more kinetic energy and therefore move faster.

3. Concentration gradient: the steeper the difference in concentration, the faster the rate of diffusion.

4. Membrane permeability: diffusion of different substances is affected by membrane permeability. E.g., cell membranes are selectively permeable and therefore different molecules will have different rates of diffusion.

5. Membrane surface area: the rate of diffusion through a membrane is directly proportional to the surface area of the membrane.

Experiment to Demonstrate Diffusion in Liquid

Material: beaker, pipette, water, copper sulphate solution or potassium permanganate (dye)

Method

□ Fill a large beaker with water.

□ Using a pipette carefully drop a crystal of potassium permanganate at the bottom of the beaker near to one side of the beaker.

□ Leave the beaker undisturbed for at least 30 minutes and observe it at regular interval.

Observation

□ Molecules of potassium permanganate gradually move away from the crystal where they are highly concentrated.

□ Eventually they become evenly distributed throughout the water in the beaker.

Conclusion

Uniform coloration of the content in the beaker is as result of diffusion

Factors that Affect the Rate of Diffusion

1. Size of molecules: smaller particles diffuse faster than larger particles.

2. Temperature: the rate of diffusion increases as temperature increases because the particles acquire more kinetic energy and therefore move faster.

3. Concentration gradient: the steeper the difference in concentration, the faster the rate of diffusion.

5. Membrane surface area: the rate of diffusion through a membrane is directly proportional to the surface area of the membrane.

Experiment to Demonstrate Diffusion in Liquid

Material: beaker, pipette, water, copper sulphate solution or potassium permanganate (dye)

Method

□ Fill a large beaker with water.

□ Using a pipette carefully drop a crystal of potassium permanganate at the bottom of the beaker near to one side of the beaker.

□ Leave the beaker undisturbed for at least 30 minutes and observe it at regular interval.

Observation

Molecules of potassium permanganate gradually move away from the crystal where they are highly concentrated.

Eventually they become evenly distributed throughout the water in the beaker.

Conclusion

Uniform coloration of the content in the beaker is as result of diffusion.

Experiment to Demonstrate Diffusion in Gases

Material: A bottle of perfume with high scent

Method

□ Close all the windows and doors of a room with reasonable size.

□ Spray the perfume in one corner of the room.

□ Quickly move to the opposite corner and smell the scent of the perfume.

□ Walk about in the room and smell the scent of the perfume.

Observation: The scent of the perfume gets to the opposite corner of the room.

Conclusion: The spread of the scent of the perfume throughout the room indicates that diffusion has occurred.

Importance of Diffusion in Living Organism

1. Excess water diffuses out of leaves surface through transpiration.
2. Mineral salts are absorbed from the soil into root hairs of plants by diffusion.
3. Diffusion of carbon dioxide into the leaf and oxygen out of the leaf through the stomata.
4. Removal of waste products from plants and animals occurs by diffusion.
5. Gaseous exchange in unicellular organisms such Amoeba occurs mainly by diffusion.
6. The end product of digestion absorbed into the blood stream by diffusion.
7. Oxygen diffuses into the blood and carbon dioxide diffuse out of the blood in the lungs of mammals.

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Active Transport

Active transport is the movement of a substance across a cell membrane against its concentration gradient (from low concentration to high concentration) using energy from the cell. Movement against a concentration gradient requires energy. The energy is supplied in the form of ATP which is released by breaking a phosphate bond to produce ADP. Cells that use a lot of active transport have many mitochondria to produce the ATP needed.

Factors that Affecting Active Transport

□ Temperature: the rate of substance absorption increases with increasing temperature.

□ Oxygen and glucose concentration: oxygen and glucose are required for cellular respiration to produce energy for this process. Therefore, low glucose and oxygen concentration reduce the rate of active transport.

Importance of Active Transport

□ Absorption mineral salts from the soil by root hairs.
□ Absorption of digested food in the small intestine.
□ Movement of glucose into the phloem cells of plants.
□ Selective re-absorption by the kidney nephrons.

Bulk (Vesicular) Transport

It involves the movement of large molecules into and out of cell through cell membrane by using vesicles. Vesicles are small, membranous sacs formed from a section of the cell membrane that move around the cell. There are two types of bulk transport: endocytosis and exocytosis.

Endocytosis

Endocytosis is the movement of large molecules and droplets of fluid into the cell in the form of vesicle. Vesicle formation is an energy requiring process. There are three types of endocytosis; pinocytosis, phagocytosis and receptor-mediated endocytosis.

Phagocytosis

Phagocytosis or ‘‘cellular eating’’ is process of engulfing and ingesting particles such as bacteria, duct and cellular debris. In phagocytosis the plasma membrane surrounds a macromolecules or even entire cell from the extracellular environment and bud off to form a food vacuole or phagosome. The newly-formed phagosomes then fuse with a lysosome whose hydrolytic enzymes digest the content.

Pinocytosis

Pinocytosis or ‘‘cellular drinking’’: the cell engulfs drops of fluid by pinching in and forming vesicles that are smaller than phagosomes in phagocytosis. It is non-specific process in which the cell takes in whatever solutes that are dissolved in the liquid it envelops.

Receptor-mediated endocytosis

Receptor-mediated endocytosis is an extremely selective process of importing materials into cell. Cell membrane contains depressed regions called coated pit where specific receptor proteins are located. The cell only takes in an extracellular molecule if it binds to specific receptor protein on the cell’s surface. Once bound, the coated pit pinches in, to form a coated vesicle. This coated vesicle then fuses with a lysosome to digest the coat and release molecule into the cytosol. E.g., Mammalian cells use receptor-mediated endocytosis to take cholesterol into cells.

Exocytosis

Exocytosis: in exocytosis materials are exported out of the cell via secretory vesicles. In this process, the Golgi package macromolecules into transport vesicles that travel to and fuse with the plasma membrane. This causes the vesicle to spill its contents out of the cell. Exocytosis is important in expelling waste materials out of the cell and in the secretion of cellular products such as digestive enzymes or hormones.

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MOVEMENT OF SUBSTANCES IN AND OUT OF CELLS

Movement of Substances in and Out of Cells