Makindo Medical Notes

Structure of the Cell Membrane

• The cell membrane, also known as the plasma membrane, is a thin, flexible barrier that surrounds the cell. It plays a crucial role in maintaining the integrity of the cell and controlling the movement of substances in and out of the cell. The cell membrane's structure is often described by the fluid mosaic model, which emphasizes its dynamic and flexible nature.
• Phospholipid Bilayer: The fundamental structure of the cell membrane is the phospholipid bilayer. This bilayer is made up of two layers of phospholipid molecules, with their hydrophobic (water-repelling) tails facing inward and their hydrophilic (water-attracting) heads facing outward, towards the cell's interior and exterior.
• Membrane proteins are embedded in the phospholipid bilayer. These proteins serve various functions:
  • Integral proteins: Span the entire membrane, acting as channels, carriers, or receptors for molecules.
  • Peripheral proteins: Attach to the surface of the membrane, playing roles in cell signaling, structure, and maintaining the cell’s shape.
• Cholesterol molecules are scattered within the phospholipid bilayer, adding rigidity and fluidity to the membrane. Cholesterol helps maintain the membrane’s stability and prevents it from becoming too rigid or too fluid.
• Carbohydrate chains are often attached to proteins or lipids on the outer surface of the membrane, forming glycoproteins and glycolipids. These carbohydrates play key roles in cell recognition, signaling, and interactions with other cells.
• The term fluid mosaic model describes the cell membrane's structure, where the phospholipids, proteins, cholesterol, and carbohydrates are not static but move fluidly within the membrane. This allows the cell membrane to be flexible and adapt to different conditions while maintaining its integrity.
• In summary, the cell membrane is a dynamic structure composed of a phospholipid bilayer, proteins, cholesterol, and carbohydrates. It regulates the movement of substances in and out of the cell, provides protection, and allows communication with the cell's external environment.

Diffusion

When a moving molecule, A, approaches a stationary molecule, B, the electrostatic and other nuclear forces of molecule A repel molecule B, transferring some of the energy of motion of molecule A to molecule B. Consequently, molecule B gains kinetic energy of motion, whereas molecule A slows down, losing some of its kinetic energy. A single molecule in a solution bounces among the other molecules—first in one direction, then another, then another, and so forth—randomly bouncing thousands of times each second. This continual movement of molecules among one another in liquids or gases is called diffusion. Diffusion through the cell membrane is divided into two subtypes, called simple diffusion and facilitated diffusion.

• Simple diffusion means that kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without interaction with carrier proteins in the membrane. The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move.
• Facilitated diffusion requires interaction of a carrier protein. The carrier protein aids passage of molecules or ions through the membrane by binding chemically with them and shuttling them through the membrane in this form.
• Simple diffusion can occur through the cell membrane by two pathways: (1) through the interstices of the lipid bilayer if the diffusing substance is lipid-soluble; and (2) through watery channels that penetrate all the way through some of the large transport proteins, as shown to the left in Figure 4-2

Difference in Electrolyte Concentration