3.1 Movement of Substances Across The Plasma Membrane

Learning Objective:

Analysing the movement of substances across the plasma membrane

Learning Outcomes :
At the end, a student is able to:

• state the substances required by living cells,
• state the substances that have to be eliminated from cells,
• explain the necessity for movement of substances across the plasma membrane,
• describe the structure of the plasma membrane

Substances Transported Into (Enter) The Cells :

1. Oxygen
2. Digested food substances :
   

• Glucose
• Amino acid
• Glycerol
• Fatty acid
  

Substances Eliminated From (Leave) the Cells :

1. Waste products from metabolic processes :
   

• Carbon dioxide
• Urea
• Lactic acid
• Excess water
  

Necessity for Movement of Substances Across The Plasma Membrane

1. A cell is surrounded by a plasma membrane that separates it from the outer environment.
2. For cellular activities to be carried out, the cell has to move substances into and out of the cell.
3. Movement of substances into or out of a cell is important to :

• provide nutrients for metabolism and growth
• supply oxygen for respiration
• regulate solute concentration and suitable pH for maintaining a stable internal environment for optimal enzymatic activities
• maintain an ion concentration gradient required for nerve and muscle cell activities
• secrete useful substances, for example, digestive enzymes and hormones
• eliminate toxic waste products such as urea and carbon dioxide

The Structure of the Plasma Membrane

1. The fluid-mosaic model proposes that the plasma membrane consists of protein molecules scattered in a mosaic pattern on a fluid bilayer of phospholipids molecules.
2. The phospholipid bilayer have different solubility properties of the two ends of phospholipid molecules.
   
3. There are various types of protein which are either partially or fully embedded in the membrane. Pore proteins forms a channel (allow small molecules, either polar or non-polar, to pass through freely) whereas carrier protein acts as a carrier (attach to specific glucose molecules, before transporting the molecules across the plasma membrane).
4. Cholesterol molecules help to stabilise the structure of the plasma membrane.
5. The phospholipid bilayer, proteins and other components are not rigid or static but form a dynamic and flexible structure.