RP4 Cell-surface membrane permeability Questions

Specification route: RP4
Question bank: 14 questions
Course stage: AS / Year 12

Sample questions

QUESTION 1 · RP4 · LEVEL 2

When investigating the permeability of beetroot cell-surface membranes, why does placing the beetroot cylinders in ethanol cause the surrounding solution to turn red?

Ethanol is an organic solvent that dissolves the phospholipid bilayer of the cell membranes, allowing the red pigment to leak out.
Ethanol denatures the red pigment proteins, fundamentally changing their colour from white to red.
Ethanol causes massive osmosis into the cells, bursting them entirely.
Ethanol acts as an enzyme that actively pumps the pigment out of the vacuole.

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Cell membranes are primarily made of lipids.

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Answer: Ethanol is an organic solvent that dissolves the phospholipid bilayer of the cell membranes, allowing the red pigment to leak out.

Required practical 4. Betalain pigment is trapped inside the vacuole. Ethanol dissolves lipids, damaging the cell-surface and vacuole membranes (phospholipid bilayers), increasing permeability and releasing the pigment.

QUESTION 2 · RP4 · LEVEL 2

Before putting cut beetroot cylinders into test tubes of different temperatures, they are washed thoroughly in distilled water. Why is this step necessary?

To wash away any pigment that was released from cells damaged during the cutting process.
To ensure the beetroot cells are completely fully turgid before the experiment begins.
To sterilise the beetroot and prevent bacterial enzymes from breaking down the pigment.
To instantly freeze the cell membranes, creating a zero-permeability baseline.

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The scalpel slices right through millions of cells, spilling their contents instantly.

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Answer: To wash away any pigment that was released from cells damaged during the cutting process.

Required practical 4. Cutting the beetroot physically breaks open cells, releasing pigment. If not washed off, this pigment would artificially inflate the absorbance readings, regardless of the membrane permeability caused by the independent variable.

QUESTION 3 · RP4 · LEVEL 3

When using a colorimeter to measure the concentration of red betalain pigment leaked from beetroot cells, which colour filter should be used?

A green or blue filter, because these colours are absorbed by the red pigment, providing the most sensitive reading.
A red filter, because the red light will easily pass through the red pigment.
A clear filter, to allow all wavelengths of white light to hit the sensor.
A yellow filter, to neutralise the red colour and measure the turbidity of the water.

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A colorimeter measures absorbance. You want to use a light color that the pigment 'eats up', not one it reflects.

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Answer: A green or blue filter, because these colours are absorbed by the red pigment, providing the most sensitive reading.

Required practical 4. A colorimeter measures absorbance. A red solution reflects red light but absorbs complementary colours (like blue/green). Using a complementary filter maximises absorbance readings, increasing sensitivity.

QUESTION 4 · RP4 · LEVEL 2

In the beetroot permeability experiment, placing the cylinders in a water bath at $70^\circ C$ causes a massive release of pigment. Why?

The high temperature denatures the membrane proteins and increases the kinetic energy of the phospholipids, creating gaps in the cell-surface and vacuole membranes.
The high temperature rapidly increases the rate of active transport of the pigment out of the cell.
The high temperature causes the water inside the vacuole to boil, bursting the cell wall.
The high temperature activates enzymes that digest the betalain pigment into smaller, soluble pieces.

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Membranes contain a lot of proteins, which are highly sensitive to thermal energy.

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Answer: The high temperature denatures the membrane proteins and increases the kinetic energy of the phospholipids, creating gaps in the cell-surface and vacuole membranes.

Required practical 4. High temperatures disrupt the fluid-mosaic model. Membrane proteins denature and the phospholipid bilayer becomes highly fluid and leaky, releasing the vacuolar contents.

QUESTION 5 · RP4 · LEVEL 2

When cutting cylinders of beetroot to investigate membrane permeability, why is it essential to wash the cylinders thoroughly in distilled water before starting the experiment?

To remove any betalain pigment released from cells that were physically broken open by the scalpel during cutting.
To ensure the cells are fully turgid so that osmosis does not interfere with the results.
To actively dissolve the waxy cuticle on the outside of the beetroot, allowing alcohol to penetrate.
To sterilise the surface of the beetroot to prevent bacterial enzymes from digesting the membrane.

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If you don't do this, the water will instantly turn pink before you even apply your independent variable.

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Answer: To remove any betalain pigment released from cells that were physically broken open by the scalpel during cutting.

Required practical 4. Cutting physically shears cell-surface and vacuolar membranes, leaking pigment. Washing removes this 'background' pigment so that any subsequent leakage measured is solely due to the independent variable (e.g., temperature or alcohol).

QUESTION 6 · RP4 · LEVEL 3

In the beetroot experiment, what biological mechanism explains the massive increase in pigment leakage when temperatures exceed $60^\circ C$?

Membrane proteins denature and the phospholipid bilayer gains too much kinetic energy, becoming highly fluid and creating gaps that allow the pigment to escape.
The high temperature activates enzymes that actively pump the pigment out of the cell vacuole.
The water inside the cells boils, bursting the cellulose cell walls and scattering the tissue.
The betalain pigment denatures and physically shrinks, allowing it to easily slip through normal membrane pores.

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Consider the components of the fluid-mosaic model and how heat affects them.

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Answer: Membrane proteins denature and the phospholipid bilayer gains too much kinetic energy, becoming highly fluid and creating gaps that allow the pigment to escape.

Required practical 4. High temperatures disrupt the cell-surface and vacuolar membranes. Hydrogen bonds in transport proteins break (denaturation), and lipids gain kinetic energy, separating from each other and making the membrane completely permeable.

QUESTION 7 · RP4 · LEVEL 2

If a student uses different concentrations of ethanol instead of temperature as the independent variable, why does a higher ethanol concentration lead to greater pigment release?

Ethanol is an organic non-polar solvent that dissolves the phospholipid bilayer of the cell membranes, destroying their structure.
Ethanol is an acidic chemical that denatures the channel proteins in the membrane.
Ethanol forces the beetroot cells to respire anaerobically, producing toxic byproducts that burst the cell.
Ethanol causes massive osmotic influx of water, bursting the cells open.

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Lipids do not dissolve in water, but they do dissolve in this type of chemical.

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Answer: Ethanol is an organic non-polar solvent that dissolves the phospholipid bilayer of the cell membranes, destroying their structure.

Required practical 4. The cell membrane is primarily composed of a phospholipid bilayer. Organic solvents like ethanol dissolve lipids, rapidly breaking down the membrane boundary and allowing the vacuole contents to leak out.

QUESTION 8 · RP4 · LEVEL 3

When using a colorimeter to measure the absorbance of the beetroot solution, why should the student choose a blue or green filter rather than a red filter?

The red pigment reflects red light but heavily absorbs complementary colours like blue and green, providing a wider range of measurable absorbance readings.
Red light carries too much thermal energy and would denature the pigment inside the colorimeter.
Blue light actively reacts with the betalain pigment to create a measurable precipitate.
Red filters are only used for measuring transmission, whereas blue filters measure absorbance.

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If you shine red light at a red liquid, most of it passes straight through, making the machine read a very low absorbance.

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Answer: The red pigment reflects red light but heavily absorbs complementary colours like blue and green, providing a wider range of measurable absorbance readings.

Required practical 4 / AT a. Solutions appear a certain colour because they reflect that wavelength and absorb others. To get sensitive, high-resolution absorbance data, you must use a filter of a colour that the solution strongly absorbs.

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