Plasma Membrane or Cell Membrane

The plasma membrane is the outer layer of a cell. It separates the inside of the cell from the outside environment. It controls what enters and exits the cell. This makes the plasma membrane selectively permeable.

How Substances Move Into and Out of the Cell

Diffusion

1. Definition: Diffusion is the movement of substances from a high concentration area to a low concentration area.
2. Example:
   • Carbon dioxide (CO2) is produced by cells and needs to exit. Inside the cell, CO2 concentration is high. Outside, it is low. CO2 moves out by diffusion, from high to low concentration.
   • Oxygen (O2) enters the cell when the concentration of O2 is lower inside the cell compared to outside.

Osmosis

1. Definition: Osmosis is the movement of water molecules through a selectively permeable membrane.
2. Direction of Movement: Water will move from an area with a low concentration of solutes to an area with a high concentration of solutes.

Types of Solutions

1. Hypotonic Solution:

   • Definition: A solution that has a higher water concentration than the cell.
   • Outcome: Water enters the cell, making it swell.

2. Isotonic Solution:

   • Definition: A solution with the same concentration of water as the cell.
   • Outcome: No net movement of water. The cell remains the same size.

3. Hypertonic Solution:

   • Definition: A solution with a lower concentration of water than the cell.
   • Outcome: Water leaves the cell, causing it to shrink.

Activities to Demonstrate Osmosis

Activity: Osmosis with an Egg

1. Step 1: Remove the shell of an egg using dilute hydrochloric acid.
2. Step 2: Place the egg in pure water.
   • Observation: The egg swells as water enters by osmosis.
3. Step 3: Place a de-shelled egg in concentrated salt solution.
   • Observation: The egg shrinks because water leaves the egg into the salt solution.

Making Observations with Dried Fruits

• Step 1: Soak dried raisins or apricots in plain water.
  • Outcome: The fruits gain water and swell.
• Step 2: Place them in a concentrated sugar or salt solution.
  • Outcome: The fruits lose water and shrink.

Summary of Key Points

• Unicellular freshwater organisms and plant cells often gain water through osmosis.
• Plant roots absorb water this way.
• Diffusion helps cells exchange gases (O2 and CO2) and obtain nutrients.
• The plasma membrane is flexible and made of lipids and proteins.
• Cells can engulf food and other materials through a process called endocytosis.

Questions and Answers

1. What is the plasma membrane?

   • The plasma membrane is the outer layer of a cell. It separates the cell's inside from the outside.

2. What does selectively permeable mean?

   • It means the plasma membrane allows some substances to enter or exit while blocking others.

3. What is diffusion?

   • Diffusion is the movement of substances from high concentration to low concentration.

4. How does CO2 leave the cell?

   • CO2 moves out of the cell by diffusion when its concentration is higher inside than outside.

5. What is osmosis?

   • Osmosis is the movement of water across a selectively permeable membrane.

6. What happens in a hypotonic solution?

   • Water enters the cell, causing it to swell.

7. What is an isotonic solution?

   • It is a solution where water concentration is equal inside and outside the cell, so the cell size stays the same.

8. What happens in a hypertonic solution?

   • Water leaves the cell, causing it to shrink.

9. What is endocytosis?

   • It is a process where the cell engulfs food and other materials from the environment.

10. Why is osmosis important for plants?

    • Osmosis helps plants absorb water from the soil through their roots.