Makindo Medical Notes

Related Subjects: |DNA replication |The Nucleus |Mitosis and Meiosis |Ribosomes |Microtubules |Mitochondria |Smooth and Rough Endoplasmic Reticulum |Sodium Potassium ATPase pump |The Cell membrane

The cell membrane, or plasma membrane, is a dynamic and selectively permeable barrier that surrounds each cell in the human body. It regulates the movement of substances in and out of cells, maintains cellular integrity, and facilitates communication between cells.

Structure

• Lipid Bilayer: The cell membrane is primarily composed of a double layer of phospholipids.
  • Hydrophilic (water-attracting) heads face outward toward the cellular environment.
  • Hydrophobic (water-repelling) tails face inward, creating a nonpolar interior that prevents free passage of polar molecules.
• Proteins: Embedded within the membrane are integral and peripheral proteins.
  • Integral proteins span the entire membrane, acting as channels or carriers.
  • Peripheral proteins are attached to the outer or inner surface, providing structural support and aiding in cell signaling.
• Cholesterol: Molecules of cholesterol interspersed within the membrane add fluidity and stability.
  • Prevents the membrane from becoming too rigid or too fluid, especially in varying temperatures.
• Carbohydrates: Carbohydrates attached to proteins (glycoproteins) and lipids (glycolipids) form the glycocalyx.
  • Involved in cell recognition, protection, and adhesion.
  • Plays a role in immune response and communication between cells.

Functions

• Selective Permeability: The membrane controls what enters and exits the cell, allowing essential molecules such as nutrients and ions while blocking harmful substances.
• Transport: Specialized proteins facilitate the movement of molecules across the membrane.
  • Passive transport (e.g., diffusion, facilitated diffusion) moves molecules down their concentration gradient without energy.
  • Active transport requires ATP to move molecules against their gradient (e.g., Na+/K+ pump).
• Cell Communication: Receptor proteins on the cell surface receive signals (e.g., hormones), allowing the cell to respond to external stimuli.
• Cell Recognition: Carbohydrates on the cell surface identify the cell type, which is important in immune function and tissue organization.
• Structural Support: The cell membrane helps maintain cell shape and is anchored to the cytoskeleton for structural integrity.

Clinical Relevance

• Membrane Disruptions: Conditions like cystic fibrosis result from faulty membrane proteins (e.g., CFTR protein), leading to abnormal ion transport.
• Cell Signaling Disorders: Dysfunctions in membrane receptors can lead to various diseases, including diabetes and certain cancers.
• Immune Response: The glycocalyx is essential for cell recognition, allowing the immune system to distinguish self from non-self.

Summary

The cell membrane is a crucial component of cellular function, providing a selectively permeable barrier, facilitating communication, and maintaining structural integrity. Its complex composition allows it to carry out diverse roles essential to the survival and function of cells.