Plasma Membrane (Cell Membrane) – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain how the plasma membrane’s selective permeability helps a cell maintain balance.
Answer:
- The plasma membrane separates the inside from the outside.
- It is selectively permeable, so it controls entry and exit.
- Oxygen (O2) enters when it is lower inside.
- Carbon dioxide (CO2) leaves when it is higher inside.
- Water moves as needed by osmosis to avoid swelling or shrinking.
- It lets in needed nutrients and blocks harmful substances.
Q2. Describe diffusion with suitable examples. Explain why it is important for cells.
Answer:
- Diffusion is movement from high concentration to low concentration.
- It happens because particles spread out on their own.
- CO2 leaves the cell by diffusion when it is high inside.
- O2 enters the cell by diffusion when it is low inside.
- Diffusion is faster when the difference in concentration is large.
- It helps in quick gas exchange and supports cell life.
Q3. What is osmosis? Explain the role of hypotonic, isotonic, and hypertonic solutions.
Answer:
- Osmosis is movement of water through a selectively permeable membrane.
- Water moves from low solute to high solute concentration.
- In a hypotonic solution, water enters the cell and it swells.
- In an isotonic solution, there is no net movement and size stays same.
- In a hypertonic solution, water leaves the cell and it shrinks.
- These ideas explain why cells change size in different solutions.
Q4. Explain the egg osmosis activity. What does it show about the cell membrane?
Answer:
- First, remove the shell with dilute hydrochloric acid.
- The egg membrane remains and acts like a selectively permeable membrane.
- In pure water, the egg swells as water enters by osmosis.
- In concentrated salt solution, the egg shrinks as water leaves.
- This shows water movement depends on solution concentration.
- It proves that membranes let water pass but limit other substances.
Q5. How does the raisin (or dried fruits) activity demonstrate osmosis?
Answer:
- Soak dried raisins in plain water and they swell.
- Water moves into the raisin by osmosis as outside solute is lower.
- Place them in concentrated sugar or salt solution, and they shrink.
- Water now moves out because the outside is hypertonic.
- The raisin skin acts like a selectively permeable membrane.
- This shows how water movement depends on solute concentration.
High Complexity (Analysis & Scenario-Based)
Q6. A plant cutting is placed first in pure water and then in concentrated salt solution. Predict and explain the changes.
Answer:
- In pure water, the outside is hypotonic to the cells.
- Water enters the cells by osmosis, and the cutting becomes firm.
- In concentrated salt, the outside is hypertonic.
- Water leaves the cells, and the cutting becomes limp and shrinks.
- If kept in an isotonic medium, there is no net change in size.
- These changes happen due to direction of water movement across the membrane.
Q7. A freshwater unicellular organism is suddenly moved to salty water. What will happen to it? Explain using osmosis.
Answer:
- In freshwater, the outside is usually hypotonic, so water tends to enter.
- In salty water, the outside becomes hypertonic.
- Water leaves the cell by osmosis, and the cell shrinks.
- The organism may lose shape and slow its movements.
- Diffusion of gases still occurs, but stress remains high.
- It may not survive long unless returned to a safer medium.
Q8. During heavy activity, CO2 builds up inside cells. Explain how diffusion and the plasma membrane help maintain balance.
Answer:
- Inside the cell, CO2 becomes high during activity.
- By diffusion, CO2 moves out to the lower concentration outside.
- If O2 is lower inside, it moves in by diffusion.
- The plasma membrane is selectively permeable to small gases.
- It is flexible and made of lipids and proteins, so exchange is smooth.
- This keeps the cell’s internal balance during changing needs.
Q9. Compare endocytosis with diffusion and osmosis. When would a cell use endocytosis?
Answer:
- Endocytosis is when the cell engulfs material from outside.
- It is used for large particles that cannot pass through pores.
- Diffusion moves small substances like O2 and CO2.
- Osmosis moves only water across the membrane.
- In endocytosis, the membrane makes a pocket and pulls material in.
- Cells use it to take in food or needed materials from the environment.
Q10. A farmer irrigates with salty water. Predict the effect on roots using osmosis and solution types.
Answer:
- Roots absorb water by osmosis from the soil.
- Salty water makes the soil hypertonic to the root cells.
- Water leaves the root cells and moves into the soil.
- Roots and leaves may shrink and wilt over time.
- If soil becomes less salty, water can enter again and help recovery.
- Adding salt disturbs the plant’s water balance, so it should be avoided.