The structure of the plasma membrane

Brief Account

The plasma membrane and its structure

1. The plasma membrane is vitally important in regulating the passage of materials into and out of the cell.

2. The plasma membrane is a semi permeable membrane , that is, some molecules such as water can pass through but others cannot.

3. Living cells carry out life processes such as absorbing water and nutrients from surroundings, excreting waste products such as urea that are not needed by cells and exchanging respiratory gases during respiration.

4. Those processes are essential to a cell because they help the cell to:
• Maintain optimal pH value and ion concentration that are suitable for cells activities.
• Obtain nutrients for metabolism
• Eliminate toxic waste products from the cell
• Secrete useful substances such as hormones and enzymes.

5.Plasma membrane regulates the exchange of substances between the content of the cell and the external environment.

6. Plasma membrane also control the movement of substances in and out of the cells occurs across it.

7.Plasma membrane composed mainly of phospholipids and proteins

8. At the plasma membrane, phospholipids arrange themselves in a double layer, called the phospholipid bilayer.

9. The phospholipid molecules are able to move within the membrane, giving flexibility and fluidity to the membrane.

10. Some membrane proteins that have carbohydrates attached to them are called glycoproteins.

11. The proteins are widely spread out between the phospholipid bilayer.

12. The protein molecule that forms a channel or a pore is known as channel or pore protein, while the protein that acts as a carrier is known as carrier protein. Both are transport proteins which regulate the movement of water-soluble molecules and ions through the plasma membrane.

Important Terms:

Carrier protein
· a protein which transports specific molecules or ions pass through the plasma membrane in which it is embedded and into the cell.

Glycolipids
· Combinations of lipis and polysaccharides, function as to help cells to recognize each other.

Glycoproteins
· Combinations of proteins and polysaccharides function as to help cells to recognize each other.

Cholesterol is a steroid which is a common component of animal cell membranes and functions to help stabilize the membrane. It is a crucial molecule in animals although high levels of it in the blood may contribute to atherosclerosis.

SPM 2006 Paper 2- Essay Question

7(a) Plasma membrane encircles the protoplasm of a cell. As a whole plasma membrane exhibits fuidity as if it is a layer of oil in a beaker of water.

What is meant by plasma membrane exhibits fluidity? [4 marks]

SUGGESTED ANSWER:

Plasma membrane exhibits fluidity because :

• it is very soft
• extremely thin
• consist of two layers of lipid (phospholipids)
• various types of protein molecules are inserted within the two layers of lipids
• form a membrane that is not static/ free moving/dynamic
• it is strong structure / not easily broken

[ Any 4 please...]

Question:

What is the difference between the behavior of Channel Protein and Pore protein?

Answer:

Channel proteins do not require the binding of a molecule and conformational change to open.

Channel proteins allow molecules to diffuse across the membrane through diffusion .
whereas

Carrier protein allow specific molecules to cross the cell membrane by undergoing a conformational change upon the binding of the molecule.

The conformational change opens a hole through which the molecule can enter or exit a cell.

SPM 2007-Paper 2 Question 2(b) State two characteristicsof the phospholipid bilayer. [2 marks]

Suggested Answer:

1. It consists of hydrophilic head and hydrophobic tail

2.Hydrophilic head is attracted to water

3.Hydrophobic tail is attracted to lipid/ tail is facing to each other

4. The bilayer is mobile/ not static/ dynamic