What do diffusion and osmosis have in common?
A. They only happen in living cells.
B. They require transport proteins in the membrane.
C. They are passive transport mechanisms.
D. Net movement of substances is against the concentration gradient.
▶️Answer/Explanation
Answer: C. They are passive transport mechanisms.
Explanation:
- Both diffusion and osmosis involve the movement of substances from an area of higher concentration to lower concentration without the use of energy. This makes them passive transport processes.
- Diffusion is the movement of solutes, while osmosis specifically refers to the movement of water across a selectively permeable membrane.
Answer Evaluation:
A. Incorrect – Diffusion and osmosis occur in both living and non-living systems.
B. Incorrect – Transport proteins are involved in facilitated diffusion or active transport, not in simple diffusion or osmosis.
C. Correct – Both processes move substances passively, requiring no energy.
D. Incorrect – Substances move down the concentration gradient, not against it.
What is the difference between simple diffusion and facilitated diffusion?
▶️Answer/Explanation
Answer: D
Explanation:
| Feature | Simple Diffusion | Facilitated Diffusion |
|---|---|---|
| Energy Required? | No | No |
| Membrane Involvement? | Yes | Yes |
| Transport Mechanism | Molecules move directly through the phospholipid bilayer | Molecules move through protein channels or carriers |
| Type of Molecules | Small, non-polar (e.g., O₂, CO₂) | Larger or polar (e.g., glucose, ions) |
| Direction of Movement | High to low concentration (down the gradient) | High to low concentration (down the gradient) |
Why Other Options Are Incorrect:
A. Rate decreases with increasing concentration gradient → False. For both simple and facilitated diffusion, the rate generally increases with concentration gradient.
B. Faster movement of molecules — Slower movement of molecules → Misleading. The speed depends on the type of molecule and the presence of channels, not a fixed rule.
C. Always involves a membrane — Never involves a membrane → False. Both types occur across membranes.
What route is used to export proteins from the cell?
A. Golgi apparatus → rough endoplasmic reticulum → plasma membrane
B. Rough endoplasmic reticulum → Golgi apparatus → plasma membrane
C. Golgi apparatus → lysosome → rough endoplasmic reticulum
D. Rough endoplasmic reticulum → lysosome → Golgi apparatus
▶️Answer/Explanation
Answer: B. Rough endoplasmic reticulum → Golgi apparatus → plasma membrane
Explanation:
- Proteins that are meant to be exported from the cell follow a specific path. They are first synthesized by ribosomes on the rough endoplasmic reticulum (RER). Then, they are transported to the Golgi apparatus, where they are modified, sorted, and packaged into vesicles. Finally, these vesicles fuse with the plasma membrane, releasing the proteins outside the cell via exocytosis.
Answer Evaluation:
A. Incorrect – The Golgi apparatus does not come before the RER in the pathway.
B. Correct – This is the correct sequence for exporting proteins: RER → Golgi → plasma membrane.
C. Incorrect – Lysosomes are used for digestion inside the cell, not for protein export.
D. Incorrect – Proteins are not transported from the RER to lysosomes and then to the Golgi; this sequence is biologically incorrect.
Why do crop plants dry out when a field is irrigated with water contaminated by sea water?
A. The plants lose water by active transport.
B. The plants gain salt by osmosis.
C. The plants gain salt by diffusion.
D. The plants lose water by osmosis.
Answer/Explanation
Answer: D. The plants lose water by osmosis
Explanation:
- When irrigation water is contaminated with sea water, it contains a high concentration of salt. This makes the soil solution hypertonic compared to the water inside the plant cells.
- As a result, water moves out of the plant cells into the salty soil by osmosis (movement of water from a region of higher water concentration to a region of lower water concentration across a partially permeable membrane).
- This loss of water causes the plant cells to become dehydrated, leading to wilting and drying out of the crop.
Answer Evaluation:
A. Incorrect – Active transport requires energy and moves solutes, not water.
B. Incorrect – Salt moves by diffusion, not osmosis.
C. Incorrect – While salt may diffuse into plant roots, this does not directly cause water loss; osmosis is the key process.
D. Correct – Water leaves plant cells by osmosis when the soil is more concentrated with salt.
Question
The diagram refers to questions 2 and 3. It shows a heterotrophic, unicellular, freshwater organism that has been placed in distilled water. The short arrows show movement of water and the long arrows show a sequence of steps.
What would happen if the unicellular organism was placed in a solution slightly less concentrated than the cytoplasm of the cell, rather than in distilled water?
A. The cell would become larger.
B. More water would be expelled from the cell.
C. X would fill more slowly.
D. X would not appear.
▶️Answer/Explanation
Answer: C. X would fill more slowly.
Explanation:
The diagram shows a unicellular freshwater organism (such as a paramecium) in distilled water a hypotonic environment. Water enters the cell by osmosis because the concentration of solutes is higher inside the cell.
- The structure labeled X is a contractile vacuole, responsible for pumping out excess water to prevent the cell from bursting.
- In distilled water, there is a high influx of water, so X fills rapidly and expels water frequently.
- If the organism is placed in a solution slightly less concentrated than the cytoplasm (i.e., a less hypotonic solution), then:
- Less water will enter the cell by osmosis.
- Therefore, the contractile vacuole (X) will fill more slowly.
Why Other Options Are Incorrect:
A. The cell would become larger → Not likely; some water enters, but not as much as in distilled water.
B. More water would be expelled from the cell → Incorrect; less water would enter, so less would be expelled.
D. X would not appear → False; the contractile vacuole is still needed, just less active.
Question
Onion (Allium cepa) epidermis was placed in pure water and observed with a light microscope using high magnification.
What would happen to these cells if they were transferred to a hypertonic solution?
A. Cells would gain mass.
B. Cells would take in water by osmosis and swell.
C. Cells would burst open, releasing their content.
D. Cell membranes would detach from walls at some points.
▶️Answer/Explanation
Answer: D. Cell membranes would detach from walls at some points.
Explanation:
- Hypertonic solution: A solution that has a higher solute concentration than the cytoplasm of the cells.
- When onion cells are placed in such a solution, water leaves the cells via osmosis (from an area of low solute concentration to high).
- As water exits, the cytoplasm and cell membrane shrink, pulling away from the cell wall.
- This process is called plasmolysis.
In the image you provided, you see healthy, turgid onion epidermal cells in pure water. When placed in a hypertonic solution, they would lose water, and the cell membrane would pull away from the rigid cell wall but the cell wall stays intact.
Why Other Options Are Incorrect:
A. Cells would gain mass → False; they lose water and lose mass.
B. Cells would take in water by osmosis and swell → That happens in hypotonic, not hypertonic, solutions.
C. Cells would burst open, releasing their content → Plant cells have a rigid cell wall that prevents bursting. Animal cells might burst, but not plant cells.
Question
Red blood cells from a small mammal were immersed in NaCl (sodium chloride) solutions of different concentrations for 2 hours. The graph shows the percentage of hemolysed (ruptured) red blood cells at each concentration.
What can be deduced from the graph?
A. At Y, the net movement of Na ions between red blood cells and the NaCl solutions is zero.
B. At X, Na and Cl ions disrupt the structure of cell membranes.
C. At Y, the hypertonic NaCl solutions diffuse into the red blood cells.
D. At X, water has moved by osmosis into the red blood cells.
▶️Answer/Explanation
Answer: D. At X, water has moved by osmosis into the red blood cells.
Explanation:
Interpretation of the Graph:
The graph shows % hemolysis (rupture of red blood cells) versus NaCl concentration.
- Region X (0.00–0.06 M NaCl): 100% hemolysis occurs → Red blood cells burst due to water entering by osmosis (hypotonic conditions).
- Region Y (0.12–0.14 M NaCl): 0% hemolysis → Cells remain intact → No net water entry (isotonic or hypertonic conditions).
- In region X, the NaCl solution is hypotonic (lower solute concentration than inside cells), so water moves into the red blood cells by osmosis, causing them to swell and eventually burst (hemolysis).
- In region Y, the solution is isotonic or hypertonic. There’s no net gain of water, so the cells stay intact, and no hemolysis occurs.
Why the Other Options Are Incorrect:
A. The net movement of Na⁺ isn’t the cause of hemolysis it’s the osmosis of water that matters here.
B. Na⁺ and Cl⁻ do not disrupt cell membranes directly. It’s the osmotic pressure from water influx that causes bursting.
C. In hypertonic solutions, water leaves the cells, not the other way around. NaCl does not diffuse in significantly to cause bursting.
Question
Suggest a reason for the difference in the isotonic points for the potato and the carrot tissues.
▶️Answer/Explanation
OR
may have been grown in different soils giving their tissues different contents
OR
may have been stored under different conditions
OR
may be more dehydrated / different water content
OR
different types of tissue / different age
Question
Which process(es) occur(s) by osmosis?
I. Uptake of water by cells in the wall of the intestine
II. Loss of water from a plant cell in a hypertonic environment
III. Evaporation of water from sweat on the skin surface
A. I only
B. I and II only
C. II and III only
D. I, II and III
▶️Answer/Explanation
Answer: B. I and II only
Explanation:
Osmosis is the passive movement of water molecules across a semi-permeable membrane from a region of higher water potential (low solute concentration) to lower water potential (high solute concentration).
Analysis of each statement:
I. Correct – This involves water moving into cells from the gut lumen where solute concentrations are higher inside the cells, so water enters by osmosis.
II. Correct – In a hypertonic solution, the external environment has more solutes, so water moves out of the plant cell by osmosis, causing plasmolysis.
III. Incorrect – Evaporation is a physical process, not a biological transport mechanism like osmosis. It involves water changing from liquid to gas, not movement across membranes.
Conclusion:
- Only statements I and II describe processes that occur by osmosis.
- Statement III describes evaporation, which is not osmosis.
Question
In an experiment on osmosis, red blood cells were immersed in a salt solution for two hours. The micrographs show the appearance of these cells before and after immersion in the salt solution.
What explains the observed changes?
A. The salt solution was hypertonic and entered the red blood cells.
B. The salt solution was hypotonic and disrupted the membranes of the red blood cells.
C. The salt solution was hypertonic and water moved into it from the red blood cells.
D. The salt solution was hypotonic and mineral salts were lost from the red blood cells.
▶️Answer/Explanation
Answer: C. The salt solution was hypertonic and water moved into it from the red blood cells.
Explanation:
- Before immersion: Red blood cells are round and biconcave — their normal shape in an isotonic environment.
- After immersion: Many cells appear spiky and shriveled — indicating crenation caused by water leaving the cells due to osmosis.
- This happens when the external solution has a higher solute concentration (hypertonic), causing water to move out of the cells.
Why the Other Options Are Incorrect:
A. The salt solution did not enter the red blood cells instead, water left the cells.
B. A hypotonic solution would cause swelling or bursting, not shrinkage.
D. There is no evidence that mineral salts left the cells it’s the water movement that caused the observed changes.