Permeability

Define permeability.

Permeability refers to the ability of substances to cross the membranes.

How does the cell membrane act as a barrier?

The cell surface membrane is selectively permeable to water and some solutes. Lipid soluble (non-polar) substances can move through the membrane more easily than water-soluble (polar) substances

State the factors which affect the permeability of cell membranes.

• Temperature

• Solvents: ethanol (alcohols)

• pH

• NaCl concentration

How does temperature affect the cell membrane permeability?

As the temperature increases, membrane permeability increases.

At higher temperatures, there is greater kinetic energy. More kinetic energy means the phospholipids have more energy and move more, creating gaps between the phospholipids. As a result, membrane permeability increases.

How are the phospholipids arranged when the temperature is 0°C and below?

Phospholipids are packed closely together and the membrane is rigid.

Channel and carrier proteins are still able to change shape / deform which increases permeability.

Describe the permeability of the cell membrane at very high temperatures.

At temperatures above 40°C, the cell membrane and tonoplast become increasingly unstable. Increased heat energy leads to increases kinetic energy. The phospholipids vibrate more and more and move further apart. This increases the permeability of the membrane.

The proteins within the membrane denature at high temperatures.

Explain how to investigate the effect of temperature on cell membrane permeability.

The effect of temperature on membrane permeability can be investigated using beetroot cells.

Beetroot cells contain a pigment called betalin. We can observe the effect of temperature on cell membranes in beetroot by observing the leakage of this pigment, indicating the weakening of the cell membrane. Betalains display as a dark purple colour in this case.

State the independent, dependent, and control variables relating to the beetroot practical investigating membrane permeability.

Independent variable: Temperature (°C )

Dependent variable: Absorbance (AU)

Control variable(s):

• Mass of beetroot

• Length of beetroot

• Same species of beetroot

• Same volume of distilled water

Explain the results collected relating to the beetroot practical investigating membrane permeability.

The results shows that as the temperature increases, the absorbance increases.

• As temperature increases, the phospholipids and proteins gain more kinetic energy.

• This causes larger gaps to appear between the phospholipids.

• The membrane (tonoplast and the plasma) become more permeable.

• Therefore, more pigment will leak out the cell.

How does the ethanol concentration affect the cell membrane permeability?

As the ethanol concentration increases, membrane permeability increases.

Organic solvents such as ethanol are able to dissolve phospholipids; this causes the phospholipid bilayer to lose its ability to control the movement of substances.

How does the sodium chloride concentration affect the cell membrane permeability?

As the sodium chloride concentration increases, membrane permeability decreases.

Sodium ions (Na+) attach to the oxygen atoms on the hydrophilic (phosphate) heads of the phospholipid bilayer. This reduces mobility of the phospholipid molecules. The permeability of the cell membrane will decrease.

Suggest why the presence of unsaturated fatty acids in the phospholipid would increase fluidity of the membrane (past paper question).

• Unsaturated fatty acids contain carbon carbon double bonds.

• This produces kinks in the side chains.

• The fatty acids are less tightly packed together, weakening the intermolecular forces between molecules.

• Less energy is required to break bonds, increasing fluidity.