Diffusion, Osmosis & Active Transport

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Notes

Diffusion

**Diffusion** is the net movement of molecules from a region of higher concentration to a region of lower concentration, down a concentration gradient.
It results from the **random movement** (Brownian motion) of particles; energy comes from their kinetic energy.
The **cell membrane** is partially permeable, allowing only some molecules (e.g., small ones) to cross easily.
Examples: O₂ and CO₂ move between cells and capillaries; CO₂ moves out of cells; water vapour diffuses out of leaves.

Factors Affecting Diffusion Rate

**Surface area to volume ratio**: Larger SA:V increases rate; cells like root hair cells and ileum cells have folded surfaces to increase SA.
**Distance**: Shorter distance (e.g., one-cell-thick walls of capillaries and alveoli) speeds up diffusion.
**Temperature**: Higher temperature gives molecules more kinetic energy, increasing diffusion rate.
**Concentration gradient**: A steeper gradient results in faster diffusion.

Water as a Solvent

Water is a **solvent**; many substances dissolve in it, enabling transport and reactions.
Dissolved substances are transported in xylem/phloem (plants) and blood (animals).
Digested food molecules dissolve in water for absorption; urea and excess salts dissolve for removal in urine.
Water is a major component of **cytoplasm**, allowing metabolic reactions to occur.

Osmosis

**Osmosis** is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a **partially permeable membrane**.
Water moves down its concentration gradient; the membrane allows water but not larger solute molecules to pass.
In plant cells, water entry makes the cell **turgid** (firm), providing support; water loss makes it **flaccid** or **plasmolysed** (membrane pulls from wall).
In animal cells, water entry can cause **bursting** (no cell wall); water loss causes **crenation** (shrivelling).

Osmosis Experiments

**Potato cylinder experiment**: Cylinders are weighed, placed in solutions of different concentrations, then reweighed after 20–30 min.
Mass gain → water moved into tissue (solution more dilute); mass loss → water moved out (solution more concentrated); no change → equal concentrations.
**Dialysis tubing experiment**: Tubing filled with concentrated sucrose solution is placed in water; water moves in, causing the tubing to swell or the external water level to drop.
Dialysis tubing is a non-living partially permeable membrane; it allows water and small molecules but not large solutes.

Active Transport

**Active transport** is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, **against** a concentration gradient.
It requires **energy from respiration** (ATP).
Examples: uptake of glucose by epithelial cells of villi and kidney tubules; uptake of mineral ions by root hair cells.
**Carrier proteins** in the membrane bind the substance, change shape using energy, and release it inside the cell.

Diffusion across a partially permeable membrane: molecules move from high to low concentration.

Effect of osmosis on plant and animal cells in different solutions.

Active transport: carrier protein uses ATP to move substrate against concentration gradient.

Practice questions

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1.What is diffusion?
Easy
- AThe movement of molecules from a region of higher concentration to a region of lower concentration
- BThe movement of molecules from a region of lower concentration to a region of higher concentration
- CThe movement of water through a partially permeable membrane
- DThe movement of molecules using energy from respiration
2.Which process involves the movement of water through a partially permeable membrane from a more dilute to a more concentrated solution?
Easy
- ADiffusion
- BActive transport
- COsmosis
- DTranspiration
3.What is the energy source for diffusion?
Easy
- AATP from respiration
- BKinetic energy of random movement
- CLight energy
- DHeat energy from the environment
4.Which statement correctly describes active transport?
Medium
- AMovement of particles down a concentration gradient using energy
- BMovement of particles against a concentration gradient using energy from respiration
- CMovement of water through a partially permeable membrane
- DMovement of particles from high to low concentration without energy
5.A potato cylinder is placed in a concentrated salt solution. After one hour, its mass has decreased. What is the correct explanation?
Medium
- ASalt entered the potato cells by active transport
- BWater moved out of the potato cells by osmosis
- CWater moved into the potato cells by osmosis
- DSalt left the potato cells by diffusion
6.Which of the following is an example of active transport in plants?
Medium
- AMovement of water into root hair cells
- BUptake of mineral ions from soil by root hair cells
- CMovement of carbon dioxide into leaf mesophyll cells
- DLoss of water vapour from leaves
7.A plant cell is placed in a concentrated sugar solution. Which of the following correctly describes the events that occur?
Hard
- AWater enters the cell by osmosis, cell becomes turgid
- BWater leaves the cell by osmosis, cell becomes plasmolysed
- CWater enters the cell by active transport, cell bursts
- DWater leaves the cell by active transport, cell becomes flaccid
8.Which row correctly compares diffusion and osmosis?
Hard
- ABoth require a partially permeable membrane and move particles down a concentration gradient
- BOnly osmosis requires a partially permeable membrane; both move particles down a concentration gradient
- CBoth require energy; diffusion moves particles down a gradient, osmosis moves water up a gradient
- DOnly diffusion requires a partially permeable membrane; osmosis moves water against a gradient

More questions

9.By which process do oxygen and carbon dioxide move between cells and capillaries?
Easy
- AActive transport
- BOsmosis
- CDiffusion
- DTranspiration
10.Which factor would increase the rate of diffusion of a gas across a membrane?
Medium
- ADecreasing the temperature
- BDecreasing the concentration gradient
- CIncreasing the surface area of the membrane
- DIncreasing the thickness of the membrane
11.Red blood cells are placed in pure water. What happens to them?
Hard
- AThey shrink because water leaves by osmosis
- BThey burst because water enters by osmosis
- CThey remain unchanged because the cell membrane is impermeable
- DThey shrink because salt leaves by diffusion
12.What is the role of carrier proteins in active transport?
Medium
- AThey allow water to pass through the membrane
- BThey bind to specific molecules and transport them against a concentration gradient using energy
- CThey form pores for diffusion of ions
- DThey provide structural support to the cell membrane
13.A student places a cylinder of potato tissue in a 0.5 mol/dm³ salt solution. After 24 hours, the mass of the cylinder increases. Which statement explains this change?
Medium
- AWater moved into the potato cells because the salt solution has a higher water potential than the cells.
- BWater moved into the potato cells because the salt solution has a lower water potential than the cells.
- CWater moved out of the potato cells because the salt solution has a higher water potential than the cells.
- DWater moved out of the potato cells because the salt solution has a lower water potential than the cells.
14.The table shows the results of an osmosis experiment using potato cylinders in different sucrose solutions. | Sucrose concentration (mol/dm³) | Initial mass (g) | Final mass (g) | |---------------------------------|-----------------|----------------| | 0.0 | 5.0 | 5.9 | | 0.2 | 5.0 | 5.4 | | 0.4 | 5.0 | 5.0 | | 0.6 | 5.0 | 4.6 | At which concentration is the water potential of the potato cells equal to that of the sucrose solution?
Hard
- A0.0 mol/dm³
- B0.2 mol/dm³
- C0.4 mol/dm³
- D0.6 mol/dm³
15.A group of students investigated the effect of temperature on the rate of diffusion of potassium permanganate crystals in water. They measured the time for the colour to spread a fixed distance. Which temperature would give the fastest rate?
Medium
- A10°C
- B20°C
- C30°C
- D40°C
16.In an experiment, a piece of dialysis tubing containing a 10% sucrose solution is placed in a beaker of distilled water. After 30 minutes, the tubing has increased in mass. What is the best explanation for this?
Hard
- ASucrose moved out of the tubing by diffusion.
- BWater moved into the tubing by osmosis.
- CSucrose moved into the tubing by active transport.
- DWater moved out of the tubing by osmosis.
17.Which of the following structures is best adapted for diffusion due to its large surface area and thin walls?
Easy
- AArtery
- BCapillary
- CBronchiole
- DAlveolus
18.A plant cell is placed in a concentrated salt solution. Which of the following correctly describes the events that occur?
Medium
- AWater moves out of the cell by osmosis; the cell becomes turgid.
- BWater moves out of the cell by osmosis; the cell becomes flaccid.
- CWater moves into the cell by osmosis; the cell becomes turgid.
- DWater moves into the cell by active transport; the cell becomes plasmolysed.
19.The diagram shows a partially permeable membrane separating two solutions. Side A contains 5% sucrose, side B contains 15% sucrose. Which of the following will occur?
Hard
- ANet movement of water from A to B.
- BNet movement of water from B to A.
- CNet movement of sucrose from A to B.
- DNo net movement of water.
20.Which of the following statements about diffusion and osmosis is correct?
Medium
- ABoth require a partially permeable membrane.
- BBoth involve the movement of water molecules.
- CBoth are passive processes that do not require metabolic energy.
- DBoth move molecules against a concentration gradient.
21.A student set up an experiment using dialysis tubing filled with starch solution and placed it in a beaker of iodine solution. After 1 hour, the contents of the tubing turned blue-black. What does this indicate?
Hard
- AStarch moved out of the tubing by diffusion.
- BIodine moved into the tubing by diffusion.
- CWater moved into the tubing by osmosis.
- DIodine moved into the tubing by active transport.
22.Which process is responsible for the uptake of mineral ions by root hair cells?
Easy
- ADiffusion
- BOsmosis
- CActive transport
- DTranspiration
23.A red blood cell is placed in a solution and it bursts. What can be concluded about the solution?
Medium
- AIt is isotonic to the cell.
- BIt has a higher solute concentration than the cell.
- CIt has a lower solute concentration than the cell.
- DIt is distilled water.
24.The graph below shows the change in mass of potato cylinders placed in different concentrations of sugar solution over 30 minutes. [Graph: x-axis = sugar concentration (%), y-axis = percentage change in mass. Points: 0% → +20%, 5% → +10%, 10% → 0%, 15% → -10%, 20% → -20%] What is the approximate concentration of the potato cell sap?
Hard
- A0%
- B5%
- C10%
- D15%
25.A student investigates the effect of temperature on the rate of diffusion of potassium permanganate in water. They place a crystal at the bottom of a beaker of water at 20°C and measure the time for the colour to spread. They repeat at 40°C. Which factor is the independent variable?
Medium
- ATemperature
- BRate of diffusion
- CSize of crystal
- DVolume of water
26.In an osmosis experiment using dialysis tubing, a student fills the tubing with a 20% sucrose solution and places it in a beaker of distilled water. After 30 minutes, the tubing has increased in mass. Which statement explains this?
Medium
- AWater moved into the tubing by osmosis because the sucrose solution has a lower water potential than distilled water.
- BSucrose moved out of the tubing by diffusion because the concentration gradient is from high to low.
- CWater moved out of the tubing by osmosis because the sucrose solution has a higher water potential than distilled water.
- DSucrose moved into the tubing by active transport because energy is required.
27.A student places five potato cylinders of equal mass into sucrose solutions of different concentrations. After 24 hours, the cylinders are reweighed. The results are shown: Sucrose concentration (mol/dm³): 0.0, 0.2, 0.4, 0.6, 0.8 Percentage change in mass: +15%, +8%, 0%, -5%, -12% At which concentration is the water potential of the potato cells equal to that of the solution?
Hard
- A0.4 mol/dm³
- B0.2 mol/dm³
- C0.6 mol/dm³
- D0.0 mol/dm³
28.Which of the following is an example of active transport in the human body?
Easy
- AUptake of glucose by epithelial cells of the small intestine
- BMovement of oxygen from alveoli into the blood
- CMovement of carbon dioxide from cells into capillaries
- DMovement of water from the small intestine into the blood
29.A student sets up an experiment to investigate the effect of surface area on the rate of diffusion. They use agar blocks containing phenolphthalein and place them in sodium hydroxide solution. Which block will have the highest rate of diffusion?
Medium
- AA cube of side 1 cm
- BA cube of side 2 cm
- CA sphere of diameter 2 cm
- DA rectangular block of dimensions 1 cm × 1 cm × 2 cm
30.A red blood cell is placed in a solution and observed under a microscope. It swells and bursts. What can be concluded about the solution?
Medium
- AIt has a higher water potential than the cell.
- BIt has a lower water potential than the cell.
- CIt has the same water potential as the cell.
- DIt is a concentrated sugar solution.
31.In an investigation, a student places a plant cell in a concentrated salt solution. After some time, the cell becomes plasmolysed. Which of the following correctly describes the appearance of the cell?
Hard
- AThe cell membrane has pulled away from the cell wall.
- BThe cell wall has shrunk away from the cell membrane.
- CThe vacuole has expanded, pushing the cell membrane against the cell wall.
- DThe cell has burst open.
32.Which statement about diffusion is correct?
Easy
- AIt is the movement of particles from a region of higher concentration to a region of lower concentration.
- BIt is the movement of particles from a region of lower concentration to a region of higher concentration.
- CIt requires energy from respiration.
- DIt only occurs in liquids.
33.A student investigates the effect of concentration gradient on the rate of diffusion. They use a glass tube with a cotton wool plug soaked in hydrochloric acid at one end and another plug soaked in ammonia solution at the other. A white ring of ammonium chloride forms. Which factor is the dependent variable?
Medium
- ADistance of the white ring from each end
- BConcentration of hydrochloric acid
- CConcentration of ammonia solution
- DTemperature of the room
34.In an experiment, four cylinders of potato tissue are placed in sucrose solutions of different concentrations. The initial mass of each cylinder is 5.0 g. After 24 hours, the masses are: Solution A: 5.5 g Solution B: 5.0 g Solution C: 4.5 g Solution D: 4.0 g Which solution has the highest water potential?
Hard
- ASolution A
- BSolution B
- CSolution C
- DSolution D
35.Which of the following correctly describes the role of carrier proteins in active transport?
Medium
- AThey use energy from respiration to change shape and move substances against a concentration gradient.
- BThey provide a channel for substances to diffuse freely across the membrane.
- CThey bind to water molecules and facilitate osmosis.
- DThey break down ATP to release energy for diffusion.
36.Which process is responsible for the uptake of mineral ions by root hair cells in plants?
Easy
- AActive transport
- BDiffusion
- COsmosis
- DTranspiration
37.Which of the following is an example of diffusion in living organisms?
Easy
- AMovement of water into root hair cells
- BMovement of oxygen from alveoli into blood capillaries
- CUptake of glucose by epithelial cells of the small intestine
- DMovement of mineral ions into root hair cells
38.A group of students carried out an investigation into osmosis using cylinders of potatoes. The potatoes were put into a solution of salt. The results from their experiment are shown below. Concentration of salt solution / mol dm⁻³: 0.2 Mass of potato cylinder at start / g: 2.5 Mass of potato cylinder after 24 hours / g: 2.7 Which statement below correctly explains this change in mass?
Hard
- AWater moved out of the potato cells because the salt solution has a lower water potential than the potato cells
- BWater moved into the potato cells because the salt solution has a higher water potential than the potato cells
- CSalt entered the potato cells by active transport, increasing the mass
- DWater moved into the potato cells because the salt solution has a lower water potential than the potato cells
39.Which of the following factors would increase the rate of diffusion of a gas across a membrane?
Medium
- ADecreasing the surface area of the membrane
- BIncreasing the thickness of the membrane
- CIncreasing the concentration gradient across the membrane
- DDecreasing the temperature
40.In an experiment using dialysis tubing, a student fills the tubing with a 20% sucrose solution and places it in a beaker of distilled water. After 30 minutes, the tubing has increased in mass. Which row correctly describes the net movement of water and the process involved?
Hard
- AWater moves from tubing to beaker by osmosis
- BWater moves from beaker to tubing by osmosis
- CWater moves from beaker to tubing by active transport
- DSucrose moves from tubing to beaker by diffusion

Slides

Diffusion in Biology

Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
Molecules move down a concentration gradient as a result of their random movement (Brownian motion).
The cell membrane is partially permeable – it allows some molecules to cross easily, but not others.
Examples: oxygen and carbon dioxide moving between cells and capillaries.

What is diffusion?

Movement of molecules from low to high concentration using energy
Movement of water through a partially permeable membrane
Movement of molecules from high to low concentration down a concentration gradient
Movement of molecules against a concentration gradient

Factors that Influence Diffusion

Surface area to volume ratio: larger surface area speeds up diffusion (e.g. root hair cells, villi).
Distance: shorter distance increases rate (e.g. thin walls of capillaries and alveoli).
Temperature: higher temperature gives molecules more kinetic energy, increasing rate.
Concentration gradient: steeper gradient increases rate.

Which of these factors increase the rate of diffusion?

Increasing the surface area
Increasing the distance molecules travel
Increasing the temperature
Decreasing the concentration gradient

Water as a Solvent

Water is a solvent – many substances dissolve in it.
Dissolved substances can be transported around organisms (e.g. in blood, xylem and phloem).
Digested food molecules need to dissolve to be absorbed.
Toxic substances like urea dissolve in water for removal in urine.

Which of the following is NOT a role of water as a solvent in living organisms?

Transporting dissolved food molecules in the blood
Providing energy for active transport
Dissolving urea for removal in urine
Allowing metabolic reactions to occur in cytoplasm

Osmosis

Osmosis is the diffusion of water molecules from a dilute solution (high water potential) to a concentrated solution (low water potential) across a partially permeable membrane.
Water moves down its concentration gradient.
The cell membrane is partially permeable – it allows water through but not larger solute molecules.

Osmosis is the movement of ____ molecules from a dilute solution to a concentrated solution across a ____ permeable membrane.

Osmosis Experiments

Potato cylinders are placed in solutions of different concentrations.
If the cylinder gains mass, water moved into the tissue (solution more dilute than tissue).
If it loses mass, water moved out (solution more concentrated than tissue).
No change means concentrations are equal.

A potato cylinder is placed in a salt solution. After 1 hour its mass decreases. What does this indicate?

Water moved into the potato by osmosis
Salt moved into the potato by active transport
Water moved out of the potato by osmosis
Salt moved out of the potato by diffusion

Osmosis in Plant Cells

In a dilute solution, water enters the cell, vacuole expands, and the cell becomes turgid (firm).
The cell wall prevents bursting.
In a concentrated solution, water leaves, cell becomes flaccid (limp) and may plasmolysis (membrane pulls away from wall).
Turgor pressure supports the plant.

Which statements about osmosis in plant cells are true?

In a dilute solution, plant cells become turgid.
The cell wall prevents the cell from bursting.
Plasmolysis occurs when the cell gains too much water.
Flaccid cells provide support for the plant.

Osmosis in Animal Cells

Animal cells have no cell wall, so they are more affected by osmosis.
In a dilute solution (e.g. distilled water), water enters and the cell may burst (lysis).
In a concentrated solution, water leaves and the cell shrivels (crenation).

What happens to a red blood cell placed in distilled water?

It shrivels up
It bursts
It becomes turgid
It remains unchanged

Active Transport

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration.
It requires energy from respiration.
It moves substances against a concentration gradient.
Examples: uptake of glucose by epithelial cells in the small intestine and kidney tubules; uptake of mineral ions by root hair cells.

Active transport moves particles from a region of ____ concentration to a region of ____ concentration, using energy from ____.

Proteins & Active Transport

Carrier proteins in the cell membrane pick up specific molecules.
Energy from respiration changes the shape of the carrier protein, moving the molecule across the membrane.
This allows cells to absorb substances even when they are in low concentration outside.

Which of the following processes requires carrier proteins and energy?

Diffusion
Osmosis
Active transport
Evaporation

Which of these are examples of active transport?

Uptake of glucose by epithelial cells in the small intestine
Movement of oxygen from alveoli into blood
Uptake of mineral ions by root hair cells
Movement of water into a plant cell

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