Plasma Membrane or Cell Membrane — Long Answer Questions (Class 9 Biology)
Medium Level (Application & Explanation)
Q1. Describe the structure and functions of the plasma membrane. Explain why it is called selectively permeable.
Answer:
The plasma membrane is a thin, flexible layer made mainly of lipids (phospholipids) and proteins arranged in a fluid mosaic. Phospholipid molecules form a double layer (bilayer) whose hydrophobic tails face inward and hydrophilic heads face outward. Proteins are embedded in or attached to this bilayer and act as channels, carriers, receptors, or anchors.
Major functions include: separating the cell’s interior from the external environment, controlling transport of materials into and out of the cell, receiving signals from the environment (via receptor proteins), and helping cells attach to each other or to external structures.
It is called selectively permeable because it allows certain substances (like water, CO₂, O₂, small nonpolar molecules) to cross easily while restricting ions, large molecules, and polar substances unless specialized transport proteins or energy-dependent processes help them pass. Thus the membrane maintains the cell’s internal composition and homeostasis.
Q2. Explain diffusion with examples of CO₂ and O₂ movement in cells. What factors affect the rate of diffusion?
Answer:
Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration until equilibrium is reached. It does not require cellular energy (passive process).
Example with gases: Inside respiring cells, the concentration of carbon dioxide (CO₂) becomes high, so CO₂ diffuses out of the cell into the environment where its concentration is lower. Conversely, oxygen (O₂) in blood or surrounding medium is often at higher concentration than inside cells, so O₂ diffuses in to support cellular respiration. In leaves, O₂ produced during photosynthesis diffuses out through stomata while CO₂ diffuses in for photosynthesis.
Factors affecting diffusion rate include: the concentration gradient (steeper gradient → faster diffusion), temperature (higher temperature → faster movement), surface area (larger surface area → faster diffusion), distance/thickness of the membrane (shorter distance → faster), and size and nature of the diffusing particles (smaller/nonpolar diffuse faster).
Q3. Define osmosis. Explain why water moves from a region of low solute concentration to a region of high solute concentration. Give biological examples.
Answer:
Osmosis is the diffusion of water molecules across a selectively permeable membrane from a region of higher water concentration (or lower solute concentration) to a region of lower water concentration (or higher solute concentration). Water moves this way to balance solute concentrations on both sides of the membrane.
The reason water moves toward higher solute concentration is that solute particles reduce the free water available; to equalize the chemical potential (water potential), water flows into the region with more solute. No external energy is required — it is a passive movement driven by differences in water potential.
Biological examples: In plant roots, water moves from soil (often lower solute) into root hair cells (higher solute) by osmosis, helping maintain turgor pressure in plant cells. In animal cells, when placed in a hypotonic medium water enters and cells may swell; in hypertonic medium water leaves and cells shrink.
Q4. Describe the effects of hypotonic, isotonic, and hypertonic solutions on both animal and plant cells. Explain why these effects differ between cell types.
Answer:
In a hypotonic solution (external solution has higher water concentration than the cell):
Animal cells: Water enters the cell causing swelling and may lead to lysis (bursting).
Plant cells: Water enters and the central vacuole fills, increasing turgor pressure; the cell becomes turgid and is supported by the rigid cell wall.
In an isotonic solution (external water concentration equals that inside the cell):
Animal cells: No net water movement, cell remains normal — this is ideal for many animal cells.
Plant cells: No net movement, cells become flaccid (not fully turgid) and plant may wilt if many cells are flaccid.
In a hypertonic solution (external water concentration lower than inside the cell):
Animal cells: Water leaves the cell, causing it to shrink (crenation).
Plant cells: Water leaves the central vacuole and cytoplasm, causing the plasma membrane to pull away from the cell wall — plasmolysis — and the plant wilts.
These effects differ because plant cells have a rigid cell wall that resists bursting and creates turgor pressure, whereas animal cells lack a cell wall and are more vulnerable to changes in volume.
Q5. Describe in detail the egg osmosis experiment and the dried fruit activity. Explain the observations and the conclusions about osmosis.
Answer:
Egg osmosis experiment (steps and explanation):
First, remove the eggshell using dilute hydrochloric acid (HCl); this dissolves the shell and leaves a semipermeable membrane around the egg.
Place the deshelled egg in pure water. Water moves into the egg by osmosis because the egg’s interior has higher solute concentration. Observation: the egg swells and may appear larger and firmer.
Next, place a deshelled egg in a concentrated salt solution. Here the external solution has higher solute concentration than the egg interior, so water moves out of the egg. Observation: the egg shrinks and may become flaccid.
Dried fruit activity (steps and explanation):
Soak dried raisins or apricots in plain water. They have low water content and relatively higher internal solute; water moves in by osmosis and the fruits swell and soften.
Place them in a concentrated sugar or salt solution; water moves out of the fruit into the surrounding solution causing the fruit to shrink.
Conclusions: Both activities show that water moves across a semipermeable membrane from regions of higher water concentration to lower water concentration. These simple experiments visually demonstrate osmosis and how solute concentration differences drive water movement.
High Complexity (Analytical & Scenario-Based)
Q6. A person drinks seawater in an emergency. Analyze the effect of seawater on the body's cells and explain why drinking seawater is harmful.
Answer:
Seawater has high salt concentration and is therefore hypertonic compared to the fluids inside human cells. When a person drinks seawater, it increases the salt concentration in the blood and extracellular fluid.
Due to osmosis, water moves from body cells (higher water concentration) into the blood (now lower water concentration due to excess salt). This causes cells throughout the body to lose water and shrink, impairing their function. In the brain, cell shrinkage can lead to headaches, confusion, and severe neurological problems.
Kidneys attempt to remove excess salt by producing urine, but kidneys require water to excrete salts; drinking seawater forces kidneys to work harder and can lead to dehydration because more water is lost than gained.
Overall, seawater intake worsens dehydration, disrupts electrolyte balance, can cause kidney damage, and is therefore dangerous. The correct response is to find fresh water or use desalination/electrolyte replacement methods rather than drinking seawater.
Q7. Design an experiment using dialysis tubing to demonstrate selective permeability and osmosis. Describe the procedure, controls, expected results, and interpretation.
Answer:
Procedure:
Soak a piece of dialysis tubing to make it pliable. Tie one end to form a bag and fill it with a sugar solution (e.g., sucrose) and indicator like glucose indicator or use starch and iodine. Close the other end.
Place the tubing in a beaker of pure water. Set up a control: a tubing bag filled with plain water placed in pure water.
Leave for several hours and observe any changes in the surrounding water and contents of the tubing. Test the outside solution periodically for the presence of glucose with glucose test strips or Benedict’s test; test for starch with iodine. Measure mass or volume change of the tubing bag.
Expected results and interpretation:
If sucrose cannot pass through the membrane but water can, water will move into the bag by osmosis and the bag will swell and gain mass. The outside water should remain clear.
If small molecules like glucose are used and they can pass, the outside solution will test positive for glucose, showing selective permeability.
Controls show that without solute, no osmotic swelling occurs. This experiment demonstrates that membranes can be permeable to water but not to larger solute molecules, illustrating selective permeability and osmosis.
Q8. Explain how root hair cells absorb water and mineral ions from the soil. Distinguish between the roles of osmosis and active transport.
Answer:
Root hair cells increase the surface area of roots, allowing greater contact with soil water and dissolved minerals. Water absorption primarily occurs by osmosis: soil water usually has a higher water potential than the root hair cell sap (which contains dissolved minerals), so water moves into the root hair cell through the selectively permeable plasma membrane. This water then moves from cell to cell towards the xylem, creating an upward flow to the plant.
Mineral ions (e.g., nitrates, potassium) are often present in low concentration in the soil. Root hair cells use active transport — energy-consuming ...
