BIOLOGY LESSON NOTE ON OSMOSIS

Topic: Osmosis

Class: Senior Secondary School (SSS 2)

Duration: 80 minutes

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Specific Objectives

By the end of the lesson, students should be able to:

1. Define osmosis and explain its significance in biological systems.
2. Describe the process of osmosis using real-life examples.
3. Differentiate between osmosis, diffusion, and active transport.
4. Explain the factors affecting osmosis.
5. Describe the effects of osmosis on plant and animal cells.
6. Conduct simple experiments to demonstrate osmosis.
7. Explain the importance of osmosis in plants and animals.

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Lesson Content

Introduction to Osmosis

Osmosis is a fundamental biological process that plays a crucial role in the survival of living organisms. It is a type of passive transport that allows the movement of water molecules across a selectively permeable membrane. Understanding osmosis is important for grasping key biological concepts, such as cell function, nutrient absorption, and waste removal.

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Definition of Osmosis

Osmosis is the movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) across a selectively permeable membrane. This movement continues until equilibrium is reached.

Key Terms:

• Selective permeability – The ability of a membrane to allow certain molecules to pass through while restricting others.
• Solvent – The liquid in which a substance dissolves (e.g., water).
• Solute – The substance dissolved in the solvent (e.g., sugar, salt).
• Hypotonic solution – A solution with a lower solute concentration than the cell’s cytoplasm, leading to water entering the cell.
• Hypertonic solution – A solution with a higher solute concentration than the cell’s cytoplasm, causing water to leave the cell.
• Isotonic solution – A solution with an equal concentration of solute as the cell’s cytoplasm, leading to no net movement of water.

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How Osmosis Works

Osmosis occurs due to the difference in water potential between two regions separated by a selectively permeable membrane. Water moves passively down its concentration gradient, requiring no external energy. The process continues until equilibrium is reached.

Example:

• Plant Roots and Water Absorption: Plant roots absorb water from the soil through osmosis. The root cells have a higher solute concentration than the surrounding soil, so water moves into the cells.

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Osmosis vs. Diffusion vs. Active Transport

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Factors Affecting Osmosis

1. Concentration Gradient: The greater the difference in solute concentration, the faster the rate of osmosis.
2. Temperature: Higher temperatures increase the kinetic energy of water molecules, speeding up osmosis.
3. Surface Area: Larger surface areas allow more water molecules to pass through, increasing osmosis.
4. Membrane Permeability: The nature of the membrane affects how easily water molecules move through.
5. Pressure: Pressure differences between solutions can affect the rate of osmosis.

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Effects of Osmosis on Cells

1. In Animal Cells:

   • Hypotonic Solution: Water enters the cell, causing it to swell and potentially burst (lysis).
   • Hypertonic Solution: Water leaves the cell, causing it to shrink (crenation).
   • Isotonic Solution: No net water movement; the cell remains stable.

2. In Plant Cells:

   • Hypotonic Solution: Water enters, causing the cell to become turgid (firm and swollen).
   • Hypertonic Solution: Water leaves, causing plasmolysis (shrinking of the cytoplasm away from the cell wall).
   • Isotonic Solution: No net movement of water; the cell remains normal.

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Importance of Osmosis in Living Organisms

1. Plant Water Absorption: Plants absorb water from the soil through osmosis.
2. Cell Stability: Helps maintain cell turgidity, preventing wilting in plants.
3. Nutrient Transport: Aids in the movement of nutrients in and out of cells.
4. Waste Removal: Assists in the excretion of waste materials.
5. Regulation of Body Fluids: Maintains the balance of fluids in animal cells.

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Experiment: Demonstrating Osmosis Using a Potato

Materials Needed:

• Raw potato
• Salt solution (hypertonic)
• Distilled water (hypotonic)
• Knife
• Two beakers

Procedure:

1. Cut the potato into two equal pieces.
2. Place one piece in a beaker containing distilled water.
3. Place the second piece in a beaker containing a concentrated salt solution.
4. Leave for 30 minutes and observe the changes.

Observations:

• The potato in distilled water becomes swollen (turgid).
• The potato in salt solution shrinks due to water loss (plasmolysis).

Conclusion:

Water moves by osmosis from a region of high concentration to a region of low concentration, demonstrating the principles of osmosis.

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Evaluation

1. Define osmosis.
2. State two differences between osmosis and diffusion.
3. List three factors affecting osmosis.
4. Explain what happens to a red blood cell placed in a hypertonic solution.
5. Why is osmosis important in plants?

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Objective Questions on Osmosis

1. Osmosis is the movement of ___ across a selectively permeable membrane.
   a) Glucose
   b) Water
   c) Oxygen
   d) Carbon dioxide

2. In a hypotonic solution, a plant cell becomes ___.
   a) Flaccid
   b) Crenated
   c) Turgid
   d) Plasmolyzed

3. What happens to an animal cell in a hypertonic solution?
   a) It swells
   b) It shrinks
   c) It bursts
   d) It remains unchanged

4. The process of osmosis requires ___.
   a) Energy
   b) Oxygen
   c) A selectively permeable membrane
   d) A carrier protein

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Conclusion

Osmosis is a vital process in both plants and animals, playing a crucial role in maintaining cell stability, nutrient transport, and waste removal. Understanding osmosis helps explain many biological functions, from water absorption in plants to kidney function in humans. Through experiments and real-life applications, we can see the importance of osmosis in everyday life.