Question

Discuss the factors affecting the permeability of the plasma membrane. Cite references

Answer 1

The permeability of a cell membrane is affected by the polarity, electric charge and molar mass of the molecules that diffuse through it. The phospholipid layers that make up the cell membrane also affect its permeability. A cell membrane consists of two phospholipid layers. Each layer has an electrically charged and hydrophilic head, while the tail is uncharged and hydrophobic. The electrically charged heads of these layers face toward the water. The uncharged tails face each other. This makes it easier for small, neutrally-charged molecules to pass through the cell membrane as opposed to charged and larger molecules. The phospholipid layers also prevent non-lipid soluble substances from passing through the cell membrane.Cell membranes are selectively permeable, allowing some substances to pass through while restricting the passage of others, says Physiology Web. This is essential for providing a cell with nutrients, eliminating waste and preventing unwanted molecules from entering a cell. The double phospholipid layers of a cell membrane include polar heads and non-polar tails, says WiseGeek. Cell membranes are very permeable to non-polar molecules, such as oxygen, nitrogen, carbon dioxide and steroids, says Physiology Web. Conversely, membranes are less permeable to small polar molecules, such as water, glycerol, urea and ethanol, and highly impermeable to large polar molecules, such as glucose and sucrose.

1. Temperature and kinetic energy:

Increasing temperature gives molecules move kinetic energy so they move faster.

Decreasing temperature lowers kinetic energy so they move more slowly.

2. Phospholipids and changing temperature:

Many organisms do not generate heat to maintain body temperature and so their temperature varies with their environment.

3. When the temperature drops

saturated fatty acids become compressed.
-unsaturated fatty acid become compressed and the kinks of their tails push adjacent phospholipid molecules away - this maintains the membrane fluidity.
-cholesterol in the membrane buffers the effect of lower temperature to prevent the reduction in the membrane fluidity

4. When temperature increases.

phospholipids acquire more kinetic energy and move around more in a random way. This increases membranes fluidity.

Permeability increases.
- Affects the way membrane-embedded proteins are positioned and may function
- increase in membrane fluidity may affect the infolding of the plasma membrane during phagocytosis. May also change the ability of cells to signal other cells by releasing chemicals.

5. Proteins and temperature

Proteins are not as stable as lipids

- high temperatures cause atoms within their large molecules to vibrate, and this breaks the hydrogen bonds and ionic bonds that hold their structure together - they unfold.
- their shape changes and can't change back - they're denatured

6. Effect of solvents on phospholipids

Organic solvents e.g. acetone and ethanol damage cell membranes as they dissolve lipids