1. The movement of substances across the plasma membrane occur in a continuous and controlled manner for the survival of a cell.
2. The plasma membrane which is semi-permeable to different substances acts as a :
(i) gatekeeper which regulate what goes in and out of the cell.
(ii)barrier between the the contents of cells and the external environment.
3. Specific substances move across the plasma membrane at certain parts of the membrane.
(i) Oxygen and carbon dioxide move across the lipid bilayers of the plasma membrane by simple diffusion.
(ii) Water molecules move across the lipid bilayers of the plasma membrane by osmosis.
(iii) Glucose, amino acids and charged ions move across the plasma membrane through the carrier proteins and pore proteins by facilitated diffusion.
(iv) Certain molecules and ions which their concentration against the concentration gradient are pumped in or out of the cell through carrier proteins by using energy from ATP (adenosine triphosphate). Example : potassium ions inside the cell and sodium ions outside the cell.
4. The degree of fluidity of the plasma membrane depends on the temperature and the present of specific fatty acids and cholesterol.
5. To maintain the proper functioning of a plasma membrane and a cell as a whole, it is essential for us to take care of our food and water intake. Drinking sufficient water every day will help to hydrate body cells as well as regulate the osmotic pressure of the blood.
6. We should be thankful to God for the phenomenon of movement of substances across the plasma membrane which ensures the survival of the cells.