Movement Into and Out of Cells: Diffusion, Osmosis and Active Transport

1. A red blood cell is placed in a solution that has a higher water potential than its cytoplasm.
   What will happen to the cell?

A water enters by osmosis and the cell may burst
B water leaves by osmosis and the cell shrinks
C glucose enters by diffusion and the cell becomes turgid
D ions leave by active transport and the cell becomes plasmolysed

2. Which statement correctly describes osmosis?

A movement of water from a low water potential to a high water potential through a fully permeable membrane
B movement of solute from a dilute solution to a concentrated solution through a partially permeable membrane
C net movement of water from a high water potential to a low water potential through a partially permeable membrane
D net movement of water from a concentrated solution to a dilute solution through a cell wall

3. A plant cell is placed in a concentrated sucrose solution.
   Which row correctly describes the movement of water and the final condition of the cell?

A water enters; turgid
B water enters; plasmolysed
C water leaves; turgid
D water leaves; plasmolysed

4. Four potato cylinders are placed in different sucrose solutions for one hour.

Solution 1: mass increases by 12%
Solution 2: mass decreases by 18%
Solution 3: mass does not change
Solution 4: mass increases by 3%

Which solution has the closest water potential to the potato cell sap?

A solution 1
B solution 2
C solution 3
D solution 4

5. Which process requires energy released by respiration?

A oxygen moving from alveoli into blood
B carbon dioxide moving from blood into alveoli
C water entering root hair cells from soil
D nitrate ions entering root hair cells from dilute soil solution

6. A root hair cell contains 25 mmol per dm³ of potassium ions. The soil water contains 4 mmol per dm³ of potassium ions.
   How can potassium ions continue to enter the root hair cell?

A by diffusion down the concentration gradient
B by osmosis through the cell wall
C by active transport using energy
D by diffusion through the vacuole only

7. A dialysis tubing bag contains starch and glucose solution. It is placed in water containing iodine solution. After 30 minutes, the inside of the bag turns blue-black and glucose is found outside the bag.
   Which conclusion is correct?

A starch diffused out and iodine diffused in
B iodine diffused in and glucose diffused out
C glucose diffused in and starch diffused out
D iodine and starch both diffused out

8. Which change would increase the rate of diffusion of oxygen into a cell?

A decreasing the oxygen concentration outside the cell
B increasing the thickness of the cell membrane
C increasing the concentration gradient
D decreasing the surface area of the cell membrane

9. A student says, “Osmosis stops completely when equilibrium is reached.”
   Why is this statement incorrect?

A water molecules continue moving both ways, but there is no net movement
B water molecules stop moving because the membrane becomes impermeable
C solute molecules continue moving, but water molecules stop
D active transport begins once osmosis reaches equilibrium

10. Which situation is an example of diffusion?

A water entering a plant cell through a partially permeable membrane
B mineral ions entering a root hair cell against a concentration gradient
C carbon dioxide moving from a high concentration in cells to a lower concentration in blood
D sucrose moving through phloem using energy from companion cells

11. A plant cell is placed in distilled water.
    Which statement is correct?

A water enters, the vacuole expands and the cell becomes turgid
B water leaves, the vacuole shrinks and the cell becomes flaccid
C water enters, the cell wall bursts and the cell lyses
D water leaves, the cytoplasm pulls away from the cell wall

12. Which structure is partially permeable?

A plant cell wall
B cell membrane
C xylem wall
D starch molecule

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

13. A cell has a water potential of –300 kPa. It is placed in a solution with a water potential of –700 kPa.
    What is the net movement of water?

A into the cell because –700 kPa is higher than –300 kPa
B into the cell because the external solution is more concentrated
C out of the cell because –700 kPa is lower than –300 kPa
D no net movement because both values are negative

14. Which row correctly compares diffusion and active transport?

A diffusion requires energy; active transport does not require energy
B diffusion moves substances down a concentration gradient; active transport can move substances against it
C diffusion only occurs in living cells; active transport occurs in dead cells
D diffusion moves only water; active transport moves only oxygen

15. A student places onion epidermis cells in a strong salt solution and observes them under a microscope.
    Which observation would show plasmolysis?

A cell wall pulls away from the cell membrane
B cell membrane and cytoplasm pull away from the cell wall
C chloroplasts move into the vacuole
D nucleus bursts and disappears

16. Which condition would make diffusion across an exchange surface fastest?

A small surface area, steep concentration gradient, thick barrier
B large surface area, steep concentration gradient, thin barrier
C large surface area, no concentration gradient, thin barrier
D small surface area, shallow concentration gradient, thin barrier

17. A cube-shaped cell has sides of 2 cm. Another cube-shaped cell has sides of 4 cm.
    Which statement is correct?

A the 4 cm cell has the larger surface area to volume ratio
B the 2 cm cell has the larger surface area to volume ratio
C both cells have the same surface area to volume ratio
D neither cell can exchange substances by diffusion

18. Which cell is most likely to use active transport at a high rate?

A mature red blood cell transporting oxygen
B root hair cell absorbing nitrate ions
C xylem vessel carrying water
D dead skin cell at the surface of the body

19. A plant cell is flaccid.
    Which statement best describes it?

A it has lost enough water that the cytoplasm has completely pulled away from the cell wall
B it has gained water until pressure prevents further net entry
C it has less water than a turgid cell but is not plasmolysed
D it has burst because too much water entered

20. A bag made from partially permeable membrane contains 10% sucrose solution. It is placed in 2% sucrose solution.
    Which change occurs?

A water enters the bag by osmosis
B sucrose leaves the bag by osmosis
C water leaves the bag by osmosis
D sucrose enters the bag by active transport

21. Which statement about active transport is correct?

A it only moves water molecules
B it can only occur through cell walls
C it uses energy to move particles against a concentration gradient
D it stops if oxygen concentration outside the cell becomes high

22. Which process explains the uptake of water by root hair cells from soil water?

A active transport of water down its concentration gradient
B diffusion of mineral ions into the vacuole
C osmosis of water through a partially permeable membrane
D transpiration of water through the cell wall

23. A student investigates potato strips in sucrose solutions.
    Which variable should be controlled to make the investigation valid?

A final mass of each potato strip
B percentage change in mass
C concentration of sucrose solution
D initial length or size of potato strips

24. Which substance can pass directly through a partially permeable membrane during osmosis?

A starch
B water
C protein
D glycogen

25. A cell is placed in a solution. The cell becomes plasmolysed.
    Which statement must be true?

A the solution has a higher water potential than the cell sap
B the solution has the same water potential as the cell sap
C the solution has a lower water potential than the cell sap
D the solution contains no solute particles

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

26. Which movement is against a concentration gradient?

A oxygen from alveoli into blood
B carbon dioxide from respiring cells into blood
C mineral ions from dilute soil water into root hair cells with a higher ion concentration
D water from dilute soil water into root hair cells

27. Which statement about diffusion is correct?

A it requires a partially permeable membrane
B it only happens in living organisms
C it is the net movement of particles from higher concentration to lower concentration
D it always uses energy from respiration

28. An animal cell and a plant cell are both placed in distilled water.
    Why does the animal cell burst but the plant cell does not?

A animal cells have no cytoplasm
B plant cells have a cell wall that resists pressure
C animal cells have a higher water potential than distilled water
D plant cells do not allow water to enter

29. A plant cell is turgid. Which condition caused this?

A water moved out by osmosis into a concentrated solution
B water moved in by osmosis from a dilute solution
C mineral ions moved out by diffusion
D glucose moved in by active transport

30. Which statement correctly describes the role of respiration in active transport?

A respiration produces water, which pushes ions across membranes
B respiration releases energy needed by carrier proteins in membranes
C respiration makes the cell wall partially permeable
D respiration creates osmosis by removing all solutes

31. A student places a potato cylinder of mass 5.00 g into a solution. After one hour, its mass is 4.40 g.
    What is the percentage change in mass?

A 12% decrease
B 12% increase
C 0.60% decrease
D 60% decrease

32. Which explanation best describes why a raisin swells when placed in distilled water?

A water enters by osmosis because the raisin has a lower water potential than distilled water
B sugar leaves by diffusion because the raisin has a higher sugar concentration
C water leaves by osmosis because distilled water has a lower water potential
D sugar enters by active transport because distilled water has no sugar

33. A membrane allows water molecules to pass through but not sucrose molecules. Side X contains 5% sucrose and side Y contains 20% sucrose.
    What is the net movement?

A water from X to Y
B water from Y to X
C sucrose from X to Y
D sucrose from Y to X

34. Which feature of root hair cells increases the rate of water uptake?

A thick waterproof cuticle
B long extension increasing surface area
C absence of a cell membrane
D many chloroplasts

35. The concentration of oxygen inside a respiring cell is lower than outside the cell.
    How does oxygen enter the cell?

A active transport
B osmosis
C diffusion
D transpiration

36. A plant cell is placed in a solution and becomes turgid.
    Which row is correct?

A external solution more dilute; water enters cell
B external solution more concentrated; water enters cell
C external solution more dilute; water leaves cell
D external solution more concentrated; water leaves cell

37. Which statement about cell walls is correct?

A they are partially permeable and control osmosis
B they are freely permeable and provide support
C they actively transport ions using ATP
D they prevent all diffusion into plant cells

38. Which movement would stop if respiration in a root hair cell stopped?

A water entering by osmosis
B oxygen diffusing into the cell
C nitrate ions entering against a concentration gradient
D carbon dioxide diffusing out of the cell

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

39. A student places plant cells in three solutions.

Solution X: cells become turgid
Solution Y: cells become flaccid
Solution Z: cells become plasmolysed

Which row shows the solutions from highest water potential to lowest water potential?

A X → Y → Z
B Z → Y → X
C Y → X → Z
D X → Z → Y

40. Which statement correctly explains why villi in the small intestine help absorption by diffusion?

A they reduce the concentration gradient
B they increase surface area
C they make the membrane thicker
D they prevent blood flow

41. In an experiment, methylene blue dye is placed at the bottom of a beaker of water. After several hours, the whole beaker becomes blue.
    What has happened?

A dye molecules moved randomly from high concentration to low concentration
B water molecules moved by active transport into the dye
C dye molecules moved by osmosis through a membrane
D dye molecules stopped moving once they reached the top

42. A student says, “Water moves by osmosis from a region with more solute to a region with less solute.”
    Which correction is best?

A water moves from lower water potential to higher water potential
B water moves from higher water potential to lower water potential
C water moves only if solute molecules can also move
D water moves only by active transport

43. A solution causes an animal cell to shrink.
    What is the solution compared with the cytoplasm?

A more dilute, with higher water potential
B more concentrated, with lower water potential
C same concentration, with equal water potential
D pure water, with no solute

44. Which movement is not diffusion?

A oxygen moving into a leaf through stomata
B carbon dioxide moving out of a respiring muscle cell
C perfume particles spreading through air
D mineral ions entering a root hair cell against their concentration gradient

45. Which statement about concentration gradients is correct?

A a steeper gradient usually increases the rate of diffusion
B a steeper gradient stops osmosis
C concentration gradients are only found in animal cells
D concentration gradients require energy to exist

46. A plant cell has a solute concentration of 0.8 mol per dm³. It is placed in a solution with a solute concentration of 0.2 mol per dm³.
    What happens?

A water enters the cell by osmosis
B water leaves the cell by osmosis
C solute enters the cell by osmosis
D solute leaves the cell by active transport

47. Which row correctly identifies the process and direction?

Situation: glucose concentration is higher in the small intestine than in the blood.

A glucose enters blood by diffusion down the concentration gradient
B glucose enters blood by osmosis down the concentration gradient
C glucose leaves blood by active transport against the concentration gradient
D glucose enters blood by diffusion against the concentration gradient

48. A student boils root hair cells and then places them in soil water containing nitrate ions at a lower concentration than inside the cells.
    What will happen to nitrate ion uptake?

A it increases because boiling makes membranes more permeable to active transport
B it stops or greatly decreases because active transport requires living cells
C it continues normally because diffusion uses no energy
D it changes into osmosis because nitrate ions dissolve in water

49. Why can oxygen diffuse rapidly across the alveoli?

A alveoli have thick walls and low blood supply
B alveoli have a large surface area and thin walls
C alveoli actively transport oxygen using cilia
D alveoli contain cell walls to support diffusion

50. Which statement is correct?

A diffusion, osmosis and active transport all require living cells
B diffusion and osmosis are passive, but active transport requires energy
C osmosis and active transport only occur in plant cells
D active transport moves particles only from high concentration to low concentration

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

1. Answer: A
   A is correct because water moves into the red blood cell by osmosis from higher water potential to lower water potential, so the cell may burst.
   B is wrong because water would leave only if the external solution had lower water potential.
   C is wrong because animal cells do not become turgid; turgid is used for plant cells.
   D is wrong because plasmolysis happens in plant cells, not red blood cells.

2. Answer: C
   A is wrong because osmosis is from high water potential to low water potential, not the reverse.
   B is wrong because osmosis is movement of water, not solute.
   C is correct because osmosis is the net movement of water through a partially permeable membrane from high water potential to low water potential.
   D is wrong because the cell wall is freely permeable and osmosis needs a partially permeable membrane.

3. Answer: D
   A is wrong because water does not enter when the outside solution is concentrated.
   B is wrong because plasmolysis happens when water leaves, not enters.
   C is wrong because a cell cannot become turgid after losing water.
   D is correct because water leaves the plant cell by osmosis and the cytoplasm pulls away from the cell wall.

4. Answer: C
   A is wrong because a 12% mass increase means water entered, so the solution is more dilute than the potato cell sap.
   B is wrong because an 18% mass decrease means water left, so the solution is more concentrated.
   C is correct because no change in mass means no net movement of water, so the water potential is closest to the potato cell sap.
   D is wrong because a 3% increase is close but still shows net water entry.

5. Answer: D
   A is wrong because oxygen enters blood by diffusion, which does not require energy.
   B is wrong because carbon dioxide moves by diffusion.
   C is wrong because water enters root hair cells by osmosis.
   D is correct because nitrate ions often enter root hair cells by active transport against a concentration gradient.

6. Answer: C
   A is wrong because potassium ions cannot diffuse from 4 mmol per dm³ to 25 mmol per dm³; that is against the gradient.
   B is wrong because osmosis is movement of water, not potassium ions.
   C is correct because active transport uses energy to move ions against the concentration gradient.
   D is wrong because diffusion through the vacuole is not how ions enter from soil water.

7. Answer: B
   A is wrong because starch is too large to diffuse out through the dialysis tubing.
   B is correct because iodine diffused into the bag and glucose diffused out.
   C is wrong because starch did not diffuse out.
   D is wrong because iodine moved into the bag, not out.

8. Answer: C
   A is wrong because decreasing oxygen outside reduces the concentration gradient.
   B is wrong because increasing membrane thickness slows diffusion.
   C is correct because a steeper concentration gradient increases the rate of diffusion.
   D is wrong because decreasing surface area slows diffusion.

9. Answer: A
   A is correct because at equilibrium, water molecules still move both ways, but there is no net movement.
   B is wrong because the membrane does not become impermeable.
   C is wrong because water molecules do not stop moving.
   D is wrong because active transport does not automatically begin at equilibrium.

10. Answer: C
    A is wrong because water moving through a partially permeable membrane is osmosis.
    B is wrong because movement against a concentration gradient is active transport.
    C is correct because carbon dioxide moves from a higher concentration to a lower concentration by diffusion.
    D is wrong because sucrose translocation in phloem involves energy and is not simple diffusion.

11. Answer: A
    A is correct because distilled water has a higher water potential, so water enters, the vacuole expands and the plant cell becomes turgid.
    B is wrong because water does not leave in distilled water.
    C is wrong because the plant cell wall prevents bursting.
    D is wrong because cytoplasm pulls away from the cell wall during plasmolysis, not in distilled water.

12. Answer: B
    A is wrong because the plant cell wall is freely permeable.
    B is correct because the cell membrane is partially permeable.
    C is wrong because xylem walls are not partially permeable membranes for osmosis.
    D is wrong because starch is a molecule, not a membrane.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

13. Answer: C
    A is wrong because –700 kPa is lower water potential than –300 kPa.
    B is wrong because water does not enter a cell from a more concentrated external solution.
    C is correct because water moves from –300 kPa to –700 kPa, so it leaves the cell.
    D is wrong because negative values can still be compared; the more negative value is lower.

14. Answer: B
    A is wrong because diffusion does not require energy, while active transport does.
    B is correct because diffusion is down a concentration gradient, while active transport can move substances against it.
    C is wrong because diffusion can occur in living and non-living systems, but active transport needs living cells.
    D is wrong because diffusion is not only water and active transport is not only oxygen.

15. Answer: B
    A is wrong because the cell wall does not pull away; it stays rigid.
    B is correct because during plasmolysis, the cell membrane and cytoplasm pull away from the cell wall.
    C is wrong because chloroplasts do not move into the vacuole.
    D is wrong because the nucleus does not burst during plasmolysis.

16. Answer: B
    A is wrong because small surface area and thick barrier slow diffusion.
    B is correct because large surface area, steep concentration gradient and thin barrier all increase diffusion rate.
    C is wrong because no concentration gradient means no net diffusion.
    D is wrong because small surface area and shallow gradient slow diffusion.

17. Answer: B
    A is wrong because larger cubes have a smaller surface area to volume ratio.
    B is correct because the 2 cm cube has the larger surface area to volume ratio.
    C is wrong because the ratios are different.
    D is wrong because both can exchange substances by diffusion, but the smaller cube does it more efficiently.

18. Answer: B
    A is wrong because mature red blood cells mainly transport oxygen using haemoglobin and lack mitochondria.
    B is correct because root hair cells use active transport to absorb mineral ions.
    C is wrong because xylem vessels are dead and cannot carry out active transport.
    D is wrong because dead skin cells cannot actively transport substances.

19. Answer: C
    A is wrong because that describes a plasmolysed cell, not a flaccid cell.
    B is wrong because that describes a turgid cell.
    C is correct because a flaccid cell has lost some water but is not fully plasmolysed.
    D is wrong because plant cells do not burst easily due to their cell walls.

20. Answer: A
    A is correct because the bag has a more concentrated sucrose solution, so water enters by osmosis.
    B is wrong because osmosis is water movement, not sucrose movement.
    C is wrong because water moves into the 10% sucrose solution, not out.
    D is wrong because the membrane is not living, so active transport cannot occur.

21. Answer: C
    A is wrong because active transport moves ions and molecules, not only water.
    B is wrong because active transport occurs through cell membranes, not cell walls.
    C is correct because active transport uses energy to move particles against a concentration gradient.
    D is wrong because active transport depends on respiration inside the cell, not simply high oxygen outside.

22. Answer: C
    A is wrong because water is not actively transported.
    B is wrong because diffusion of ions does not explain water uptake.
    C is correct because water enters root hair cells by osmosis through a partially permeable membrane.
    D is wrong because transpiration is evaporation/loss of water from leaves.

23. Answer: D
    A is wrong because final mass is measured, not controlled.
    B is wrong because percentage change is calculated, not controlled.
    C is wrong because concentration of sucrose solution is the independent variable being changed.
    D is correct because initial size/length/mass of potato strips should be controlled for validity.

24. Answer: B
    A is wrong because starch molecules are too large.
    B is correct because osmosis is the movement of water molecules.
    C is wrong because proteins are large molecules.
    D is wrong because glycogen is a large carbohydrate molecule.

25. Answer: C
    A is wrong because higher external water potential would cause water to enter.
    B is wrong because equal water potential would cause no net movement.
    C is correct because plasmolysis occurs when water leaves the cell into a solution with lower water potential.
    D is wrong because a solution with no solute would have very high water potential and would make the cell turgid.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

26. Answer: C
    A is wrong because oxygen moves from higher concentration in alveoli to lower concentration in blood.
    B is wrong because carbon dioxide moves down its concentration gradient.
    C is correct because mineral ions moving from dilute soil into a root hair cell with higher ion concentration is against the gradient.
    D is wrong because water movement from soil into root hair cells is osmosis.

27. Answer: C
    A is wrong because diffusion does not require a partially permeable membrane.
    B is wrong because diffusion can happen outside living organisms, such as perfume spreading in air.
    C is correct because diffusion is the net movement of particles from higher concentration to lower concentration.
    D is wrong because diffusion is passive and does not require energy.

28. Answer: B
    A is wrong because animal cells do have cytoplasm.
    B is correct because the plant cell wall resists pressure and prevents bursting.
    C is wrong because distilled water has a higher water potential than the cell cytoplasm.
    D is wrong because water does enter plant cells.

29. Answer: B
    A is wrong because water leaving into a concentrated solution causes flaccidity or plasmolysis.
    B is correct because water moving into a plant cell from a dilute solution makes it turgid.
    C is wrong because mineral ion diffusion does not cause turgidity.
    D is wrong because glucose active transport does not directly make a plant cell turgid.

30. Answer: B
    A is wrong because respiration does not push ions across membranes using water.
    B is correct because respiration releases energy used by carrier proteins for active transport.
    C is wrong because respiration does not make the cell wall partially permeable.
    D is wrong because respiration does not remove all solutes.

31. Answer: A
    Mass change = 4.40 g – 5.00 g = –0.60 g.
    Percentage change = 0.60 ÷ 5.00 × 100 = 12% decrease.
    A is correct.
    B is wrong because the mass decreased, not increased.
    C is wrong because 0.60 g is the mass change, not the percentage change.
    D is wrong because 0.60 ÷ 5.00 is 12%, not 60%.

32. Answer: A
    A is correct because distilled water has a higher water potential than the raisin, so water enters by osmosis.
    B is wrong because sugar leaving is not the main reason the raisin swells.
    C is wrong because distilled water has a higher water potential, not lower.
    D is wrong because sugar does not enter by active transport in this situation.

33. Answer: A
    A is correct because water moves from the 5% sucrose side to the 20% sucrose side by osmosis.
    B is wrong because water does not move from the more concentrated solution to the more dilute one.
    C is wrong because sucrose cannot pass through the membrane.
    D is wrong because sucrose cannot pass through the membrane.

34. Answer: B
    A is wrong because a waterproof cuticle would reduce water uptake.
    B is correct because the long root hair increases surface area for water absorption.
    C is wrong because root hair cells have cell membranes.
    D is wrong because root hair cells are underground and usually lack chloroplasts.

35. Answer: C
    A is wrong because oxygen moves down its concentration gradient and does not need active transport.
    B is wrong because osmosis is water movement.
    C is correct because oxygen enters by diffusion from higher concentration outside to lower concentration inside.
    D is wrong because transpiration is water loss from leaves.

36. Answer: A
    A is correct because a more dilute external solution has higher water potential, so water enters the cell.
    B is wrong because a more concentrated external solution would make water leave.
    C is wrong because water enters from a dilute solution.
    D is wrong because water leaves in a concentrated solution, causing flaccidity or plasmolysis.

37. Answer: B
    A is wrong because cell walls are freely permeable, not partially permeable.
    B is correct because cell walls are freely permeable and provide support.
    C is wrong because cell walls do not actively transport ions.
    D is wrong because cell walls do not prevent all diffusion.

38. Answer: C
    A is wrong because osmosis does not directly require energy from respiration.
    B is wrong because diffusion does not require respiration.
    C is correct because nitrate ions entering against a concentration gradient requires active transport and energy from respiration.
    D is wrong because diffusion of carbon dioxide does not require respiration energy, even though respiration produces carbon dioxide.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

39. Answer: A
    A is correct because turgid cells are in the highest water potential solution, flaccid cells are in a lower/equal-ish water potential solution, and plasmolysed cells are in the lowest water potential solution.
    B is wrong because plasmolysed cells are in the lowest water potential solution, not highest.
    C is wrong because turgid cells show higher external water potential than flaccid cells.
    D is wrong because plasmolysed cells should come after flaccid, not before it.

40. Answer: B
    A is wrong because villi help maintain or increase concentration gradients, not reduce them.
    B is correct because villi increase surface area for faster absorption by diffusion.
    C is wrong because thicker membranes slow diffusion.
    D is wrong because blood flow helps maintain the concentration gradient.

41. Answer: A
    A is correct because dye molecules move randomly from a region of higher concentration to lower concentration.
    B is wrong because active transport does not occur in a beaker of water.
    C is wrong because osmosis requires a partially permeable membrane and involves water.
    D is wrong because particles continue moving randomly even when evenly spread.

42. Answer: B
    A is wrong because it reverses the direction of osmosis.
    B is correct because water moves from higher water potential to lower water potential.
    C is wrong because solute molecules do not need to move for osmosis to occur.
    D is wrong because osmosis is passive, not active transport.

43. Answer: B
    A is wrong because a more dilute solution would make water enter the animal cell.
    B is correct because a more concentrated solution has lower water potential, so water leaves the animal cell and it shrinks.
    C is wrong because equal water potential would cause no net movement.
    D is wrong because pure water would make the animal cell swell and possibly burst.

44. Answer: D
    A is wrong because oxygen entering a leaf through stomata is diffusion.
    B is wrong because carbon dioxide leaving a respiring cell is diffusion.
    C is wrong because perfume spreading through air is diffusion.
    D is correct because mineral ions entering against their concentration gradient is active transport, not diffusion.

45. Answer: A
    A is correct because a steeper concentration gradient usually increases the rate of diffusion.
    B is wrong because a steeper gradient can increase osmosis if water potential difference is greater.
    C is wrong because concentration gradients occur in plants, animals and non-living systems.
    D is wrong because concentration gradients do not always require energy to exist.

46. Answer: A
    A is correct because the cell has a higher solute concentration than the outside solution, so water enters by osmosis.
    B is wrong because water would leave only if the external solution were more concentrated.
    C is wrong because osmosis is movement of water, not solute.
    D is wrong because this situation describes osmosis, not active transport.

47. Answer: A
    A is correct because glucose moves from higher concentration in the small intestine to lower concentration in the blood by diffusion.
    B is wrong because osmosis is water movement, not glucose movement.
    C is wrong because the direction is wrong; glucose enters the blood.
    D is wrong because diffusion cannot occur against a concentration gradient.

48. Answer: B
    A is wrong because boiling kills cells and destroys normal membrane function; it does not improve active transport.
    B is correct because active transport requires living cells and energy from respiration.
    C is wrong because nitrate ions cannot diffuse into cells if the concentration is lower outside than inside.
    D is wrong because osmosis is water movement, not nitrate ion movement.

49. Answer: B
    A is wrong because thick walls and low blood supply would slow diffusion.
    B is correct because alveoli have a large surface area and thin walls for rapid diffusion.
    C is wrong because oxygen is not actively transported by cilia.
    D is wrong because animal alveoli do not have cell walls.

50. Answer: B
    A is wrong because diffusion and osmosis can occur without living cells.
    B is correct because diffusion and osmosis are passive processes, while active transport requires energy.
    C is wrong because osmosis and active transport occur in animal cells too.
    D is wrong because active transport moves particles against the concentration gradient, from low to high concentration.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.