movement in and out of cells prac practice

Define diffusion.

Explain why increasing temperature increases the rate of diffusion. (4)

• Particles gain kinetic energy.

• Particles move faster.

• More frequent collisions.

• Faster net movement from high to low concentration.

Model Answer

Increasing the temperature gives particles more kinetic energy, so they move faster. This causes more frequent collisions and increases the rate at which particles move from a region of high concentration to a region of low concentration.

(4 marks)

Question: A potato cylinder is placed in distilled water. Explain what happens.

• Distilled water has a higher water potential.

• Potato cells have a lower water potential.

• Water moves by osmosis.

• Water passes through a partially permeable membrane.

• Cells become turgid.

• Mass increases.

Model Answer

Distilled water has a higher water potential than the potato cells, so water moves into the cells by osmosis through the partially permeable cell membrane. The cells become turgid and the mass of the potato increases.

(6 marks)

Question: Describe how you would investigate osmosis using potato cylinders

• Cut equal-sized potato cylinders.

• Measure initial mass.

• Place into different concentrations of sugar solution.

• Leave for the same amount of time.

• Remove and blot dry.

• Measure final mass.

• Calculate percentage change in mass.

• Repeat and calculate a mean.

Model Answer

Cut potato cylinders of equal size and measure their initial mass using a balance. Place each cylinder into a different concentration of sugar solution and leave them for the same length of time at the same temperature. Remove the cylinders, gently blot them dry to remove excess solution, and measure the final mass. Calculate the percentage change in mass for each cylinder and repeat the experiment to calculate a mean for more reliable results.

4 marks)

Question: State and explain four factors that affect the rate of diffusion.

• Larger concentration gradient increases diffusion.

• Higher temperature increases particle movement.

• Larger surface area allows more particles to diffuse at once.

• Shorter diffusion distance reduces the time taken for particles to cross.

Model Answer

The rate of diffusion increases when there is a larger concentration gradient because there is a greater difference in concentration between the two regions. Increasing the temperature gives particles more kinetic energy, causing them to move faster. A larger surface area allows more particles to pass through at the same time, while a shorter diffusion distance means particles reach the other side more quickly.

4 marks)

Question: Explain why the alveoli are well adapted for diffusion.

• Large surface area.

• Walls are one cell thick (short diffusion distance).

• Rich blood supply maintains the concentration gradient.

• Moist lining allows gases to dissolve before diffusing.

Model Answer

The alveoli have a very large surface area, allowing more gas exchange to occur at once. Their walls are only one cell thick, providing a short diffusion distance. A rich blood supply continuously removes oxygen and brings carbon dioxide, maintaining a steep concentration gradient. The moist lining allows gases to dissolve before diffusing across the membrane.

Explain why villi are adapted for absorption.

• Large surface area due to villi and microvilli.

• Thin epithelium.

• Good blood supply.

• Maintains concentration gradient.

Model Answer

The villi provide a very large surface area because they are covered with microvilli. Their epithelium is only one cell thick, reducing the diffusion distance. They have a good blood supply that carries absorbed nutrients away, maintaining a steep concentration gradient for rapid absorption.

5 marks)

Question: Explain why root hair cells are adapted for absorbing mineral ions.

• Long extension gives a large surface area.

• Thin cell wall.

• Large vacuole.

• Many mitochondria.

• Active transport is used when soil concentration is lower than inside the root hair cell.

Model Answer

Root hair cells have a long projection that greatly increases their surface area for absorption. Their thin cell wall allows mineral ions to enter easily, while numerous mitochondria provide energy from respiration for active transport. This allows mineral ions to be absorbed even when their concentration in the soil is lower than inside the cell.

Compare diffusion, osmosis and active transport.

• Diffusion moves particles from high to low concentration.

• Osmosis is the movement of water molecules only.

• Osmosis requires a partially permeable membrane.

• Active transport moves substances from low to high concentration.

• Active transport requires ATP.

• Diffusion and osmosis do not require energy.

0 (6 marks)

Question: Describe how you would investigate osmosis using potato cylinders.

• Cut equal-sized potato cylinders.

• Measure initial mass.

• Place in different sugar/salt concentrations.

• Leave for the same time.

• Remove and blot dry.

• Measure final mass.

• Calculate percentage change.

• Repeat and calculate a mean.

Model Answer

Cut potato cylinders so that they are all the same size and measure their initial mass using a balance. Place each cylinder into a different concentration of sugar or salt solution and leave them for the same amount of time at the same temperature. Remove the cylinders, gently blot them dry to remove excess solution, and measure their final mass. Calculate the percentage change in mass and repeat the experiment several times so that a mean can be calculated to improve reliability.

(5 marks)

Question: Why is it important to keep certain variables constant during the potato osmosis experiment?

• Ensures a fair test.

• Only the independent variable affects the results.

• Keep potato size constant.

• Keep time constant.

• Keep temperature and solution volume constant.

Model Answer

Variables must be kept constant to ensure the investigation is a fair test and that only the concentration of the solution affects the results. The potato cylinders should be the same size, left in the solutions for the same amount of time, and kept at the same temperature with equal volumes of solution.

(4 marks)

Question: Why should potato cylinders be dried before measuring their final mass?

• Removes excess solution.

• Prevents inaccurate measurements.

• Ensures only water inside the potato is measured.

• Improves accuracy.

Model Answer

The potato cylinders should be gently dried to remove any solution on their surface. This prevents extra liquid from increasing the recorded mass and ensures that only the water absorbed into the potato is measured, making the results more accurate.

(5 marks)

Question: Explain what happens to a plant cell when it is placed in distilled water.

• Distilled water has a higher water potential than the cell sap.

• Water enters the cell by osmosis.

• Water passes through the partially permeable cell membrane.

• Vacuole increases in size.

• Cell becomes turgid.

• Cell wall prevents the cell from bursting.

Model Answer

Distilled water has a higher water potential than the cell sap, so water moves into the cell by osmosis through the partially permeable cell membrane. The vacuole fills with water and expands, causing the cytoplasm to press against the cell wall. The cell becomes turgid, but it does not burst because the rigid cell wall resists the pressure.

(5 marks)

Question: Explain what happens to a plant cell when it is placed in a concentrated sugar solution

• Outside solution has a lower water potential.

• Water leaves the cell by osmosis.

• Vacuole shrinks.

• Cytoplasm shrinks.

• Cell membrane pulls away from the cell wall.

• Cell becomes plasmolysed.

Model Answer

The concentrated sugar solution has a lower water potential than the cell sap, so water moves out of the cell by osmosis. As water leaves, the vacuole and cytoplasm shrink, causing the cell membrane to pull away from the cell wall. The cell becomes plasmolysed.

15 (5 marks)

Question: Explain what happens to an animal cell in a dilute solution and in a concentrated solution.

• Water enters by osmosis.

• Cell swells.

• Cell may burst (lysis/haemolysis).

Concentrated solution

• Water leaves by osmosis.

• Cell shrinks.

• Cell becomes crenated.

Model Answer

In a dilute solution, water enters the animal cell by osmosis, causing it to swell and possibly burst because it has no cell wall. In a concentrated solution, water leaves the cell by osmosis, causing it to shrink and become crenated.

4 marks)

Question: Why do animal cells burst in distilled water but plant cells do not?

• Water enters both cells by osmosis.

• Animal cells have no cell wall.

• Plant cells have a rigid cell wall.

• Cell wall prevents bursting.

Model Answer

Water enters both plant and animal cells by osmosis. Animal cells do not have a cell wall, so they may swell until they burst. Plant cells have a strong cellulose cell wall that resists the pressure of the water entering, preventing them from bursting.

(6 marks)

Question: Explain why active transport is important in root hair cells.

• Mineral ion concentration in soil may be lower than inside the cell.

• Diffusion cannot occur in this situation.

• Mineral ions move against the concentration gradient.

• Carrier proteins are used.

• ATP from respiration provides energy.

• Many mitochondria are present.

Model Answer

Root hair cells often absorb mineral ions when their concentration in the soil is lower than inside the cell. Since diffusion cannot move particles against the concentration gradient, active transport is required. Carrier proteins in the cell membrane use energy released from respiration in the form of ATP, and the many mitochondria in root hair cells provide this energy.

(5 marks)

Question: Explain why active transport is important in the small intestine.

• Glucose and amino acids may be at a lower concentration in the intestine than in the blood.

• Active transport moves them against the concentration gradient.

• Uses ATP from respiration.

• Ensures maximum absorption.

Model Answer

Active transport allows glucose and amino acids to be absorbed from the small intestine into the blood even when their concentration is lower in the intestine than in the blood. This process uses energy released from respiration, ensuring that as many nutrients as possible are absorbed.

19 (4 marks)

Question: Explain why large organisms need transport systems.

• Small surface area to volume ratio.

• Long diffusion distance.

• Diffusion alone is too slow.

• Transport systems deliver substances efficiently.

Model Answer

Large organisms have a small surface area to volume ratio and long diffusion distances. As a result, diffusion alone is too slow to supply all cells with oxygen and nutrients or remove waste products. Transport systems are therefore needed to move substances quickly throughout the body.

5 marks)

Question: Explain why organisms with a large surface area to volume ratio do not usually need transport systems.

(4 marks)

Question: Explain why a concentration gradient is necessary for diffusion.

• Diffusion occurs from high to low concentration.

• Particles move randomly.

• Net movement only occurs when concentrations differ.

• No net movement at equilibrium.

Model Answer

A concentration gradient is necessary because diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. If the concentrations are equal, particles still move randomly, but there is no overall net movement.

Question (7 marks)

Design an investigation to determine the effect of sugar concentration on osmosis in potato cylinders.

(Mark Scheme)

Aim

• Investigate the effect of sugar concentration on osmosis in potato cylinders.

Independent Variable

• Concentration of sugar solution (%).

• Use 0%, 5%, 10%, 15%, 20%.

Dependent Variable

• Percentage change in mass.

• Measure initial and final mass using a digital balance.

Standardised Variables

• Length and diameter of potato cylinders.

• Volume of sugar solution (e.g. 20 cm³).

• Time left in solution (e.g. 30 minutes).

• Temperature (keep constant).

• Use the same potato.

Method

• Measure the initial mass of equal-sized potato cylinders.

• Place each cylinder into a different sugar concentration.

• Leave for the same time.

• Remove and blot dry.

• Measure the final mass.

• Calculate the percentage change in mass.

Repeat

• Repeat each concentration twice.

• Calculate the mean.

Control

• Use distilled water (0% sugar solution) as the control.

Safety

• Handle the cork borer or scalpel carefully.

• Cut away from your body.

Model Answer

Investigate the effect of sugar concentration on osmosis in potato cylinders. Prepare sugar solutions of 0%, 5%, 10%, 15% and 20%. Cut potato cylinders of equal length and diameter using a cork borer and measure their initial mass using a digital balance. Place one cylinder into each solution and leave them for 30 minutes at the same temperature. Remove the cylinders, blot them dry and measure their final mass. Calculate the percentage change in mass. Keep the size of the potato cylinders, the volume of solution, the temperature and the time constant throughout the investigation. Repeat each concentration twice and calculate the mean. Use distilled water as the control. Handle the cork borer carefully and cut away from your body.

7 marks)

Design an investigation to determine the effect of temperature on the rate of diffusion using agar cubes.

Aim

• Investigate the effect of temperature on diffusion.

Independent Variable

• Temperature (°C).

• Use 20°C, 30°C, 40°C, 50°C and 60°C.

• Maintain temperatures using water baths.

Dependent Variable

• Time taken for the agar cube to become colourless.

• Measure using a stopwatch.

Standardised Variables

• Size of agar cubes.

• Concentration of acid.

• Volume of acid.

• Type of agar.

Method

• Prepare equal-sized agar cubes.

• Place one cube into acid at each temperature.

• Measure the time taken for the colour to disappear completely.

Repeat

• Repeat each temperature twice.

• Calculate the mean.

Control

• Use room temperature (20°C) as the control.

Safety

• Wear goggles.

• Handle hot water baths carefully.

Model Answer

Investigate the effect of temperature on the rate of diffusion using agar cubes. Prepare water baths at 20°C, 30°C, 40°C, 50°C and 60°C. Cut agar cubes so they are all the same size. Place one cube into the same concentration and volume of acid at each temperature and measure the time taken for the cube to become colourless using a stopwatch. Keep the size of the agar cubes, the concentration and volume of acid, and the type of agar constant. Repeat the investigation twice at each temperature and calculate the mean. Use 20°C as the control. Wear safety goggles and take care when handling hot water baths.

7 marks)

Design an investigation to determine the effect of surface area to volume ratio on the rate of diffusion.

Key Points (Mark Scheme)

Aim

• Investigate the effect of surface area to volume ratio on diffusion.

Independent Variable

• Size of agar cubes.

• Use cubes of 1 cm³, 2 cm³, 3 cm³, 4 cm³ and 5 cm³.

Dependent Variable

• Time taken for each cube to become colourless.

• Measure using a stopwatch.

Standardised Variables

• Acid concentration.

• Volume of acid.

• Temperature.

• Type of agar.

Method

• Prepare agar cubes of different sizes.

• Place each cube into acid.

• Measure the time taken for each cube to lose its colour completely.

Repeat

• Repeat each cube size twice.

• Calculate the mean.

Control

• Use the smallest cube (1 cm³) as the comparison control.

Safety

• Wear goggles when handling acid.

• Handle the scalpel carefully.

Model Answer

Investigate the effect of surface area to volume ratio on the rate of diffusion. Prepare agar cubes with side lengths of 1 cm, 2 cm, 3 cm, 4 cm and 5 cm. Place each cube into the same concentration and volume of acid and measure the time taken for the cube to become colourless using a stopwatch. Keep the concentration of acid, the volume of acid, the temperature and the type of agar constant. Repeat the investigation twice for each cube size and calculate the mean. Use the smallest cube as the comparison. Wear safety goggles when handling acid and use the scalpel carefully.