Cellular Membrane Transport Physiology

1. The Cell Membrane

• Structure: The cell membrane is a thin structure (around 7.5-10nm), encapsulating the cell, separating the intracellular fluid from the extracellular fluid. It is composed of the following major components:
  • Proteins (55%): Mostly glycoproteins, floating within the lipid bilayer of the membrane.
    • Integral (transmembrane) proteins: Span the entire thickness of the membrane, forming channels for ions, water, and water-soluble substances to pass through. These channels are selectively permeable due to their gates that can open and close.
    • Peripheral proteins: Located on one side of the cell, often attached to one end of the integral protein.
  • Glycolipids (25%): Lipids with attached sugar molecules.
  • Cholesterol (13%): A sterol that helps stabilize the membrane structure, maintaining the appropriate viscosity.
  • Lipids (4%): Mainly phospholipids, forming the lipid bilayer, creating a hydrophobic barrier for the cell membrane.
  • Carbohydrates (3%): Usually in the form of glycoproteins and glycolipids, forming the glycocalyx layer on the outer surface of the cell membrane.
• Functions:
  • Separates intracellular and extracellular fluids, creating a stable environment for cell function.
  • Controls the transport of substances into and out of the cell.
  • Participates in metabolic processes, signal reception, and immune responses.

2. The Lipid Barrier

• Composition: Primarily composed of cholesterol and phospholipids.
  • Hydrophobic portion: Consists of fatty acids and steroid nuclei located in the center of the lipid bilayer, forming a barrier separating the intracellular and extracellular fluids.
  • Hydrophilic portion: Consists of phosphate and OH- groups located on both sides of the membrane, in contact with the intracellular and extracellular fluids.
• Functions:
  • Creates a hydrophobic barrier, preventing the diffusion of water-soluble substances across the cell membrane.
  • Protects the cell from harmful agents from the external environment.

3. Membrane Proteins

• Integral (transmembrane) proteins:
  • Span the entire thickness of the membrane, forming channels or passageways.
  • Highly selectively permeable, allowing specific ions, water, and some water-soluble substances to pass through.
  • Protein channels can be regulated by gates that open and close, controlling the transport of substances across the membrane.
• Peripheral proteins:
  • Located on one side of the cell membrane, often attached to one end of an integral protein.
  • Perform various functions, including:
    • Receiving signals from the external environment.
    • Binding to enzymes or other proteins.
    • Participating in the transport of substances across the membrane.

4. Membrane Carbohydrates

• Structure: Usually in the form of glycoproteins and glycolipids, forming the glycocalyx layer on the outer surface of the cell membrane.
• Functions:
  • Participate in cell recognition and immune responses.
  • Create the negative charge of the cell membrane, helping to repel substances with the same charge, creating conditions for other substances to pass through.
  • Participate in cell adhesion.

5. Cellular Membrane Transport of Substances

There are two main forms:

• Passive diffusion:
  • Does not require energy.
  • Occurs along the electrochemical gradient (from high concentration, pressure, or potential to low concentration, pressure, or potential).
  • Includes the following forms:
    • Simple diffusion across the lipid bilayer:
      • Substances with lipid structures are easily transported across the membrane.
      • Substances that do not have lipid structures but are lipid-soluble are also transported quickly.
      • Small-sized substances can still pass through the membrane.
      • Water is also transported across the membrane, partly through membrane proteins.
      • Na+, K+, and H+ ions are not transported across the membrane due to the hydrated ion size being larger than the channel size or the cell membrane carrying a negative charge.
    • Simple diffusion through protein channels:
      • Protein channels are open passages from outside to inside the cell.
      • Protein channels have high selectivity, allowing only water, a few specific ions, or specific molecules to pass through.
      • The specificity of protein channels depends on the shape, charge, and internal diameter of the channel.
    • Facilitated diffusion:
      • Requires the participation of carrier proteins.
      • Substances transported by facilitated diffusion are usually monosaccharides (glucose, mannose, galactose…) and most amino acids.
      • Insulin increases the rate of facilitated diffusion of glucose by 10-20 times.
• Active transport:
  • Occurs against the electrochemical gradient (from low concentration, pressure, or potential to high concentration, pressure, or potential).
  • Requires energy and carrier proteins.
  • Includes two types:
    • Primary active transport (PAT):
      • Energy released from ATP hydrolysis or from high-energy phosphate compounds.
      • Typical example is the Na+-K+ pump.
    • Secondary active transport (SAT):
      • Energy obtained from the concentration gradient of ions from PAT and requires carrier proteins.
      • Includes symport and antiport.

Note:

• Net diffusion rate: Is the difference in the diffusion rates of two transport streams in opposite directions across the membrane.
• Factors affecting the net diffusion rate: Membrane permeability, concentration difference, pressure difference, potential difference.
• Nernst equation: Used to calculate the electromotive force (EMF) of the membrane, representing the potential difference between the two sides of the membrane:
  • EMF = +-61*log(Ci/Co)
  • EMF: Electromotive force across the membrane.
  • Ci: Intracellular ion concentration.
  • Co: Extracellular ion concentration.
  • Sign of potential (+): negative ions, (-) positive ions.
• Osmosis: Is the net diffusion of water across the membrane.
• Osmotic pressure: Is the pressure required to stop osmosis.
• Saturation of active transport: When the concentration of the transported substance is high, the transport rate reaches a maximum (Vmax), called saturation.
• Causes of saturation: The rate of chemical reaction when the substance binds to or detaches from the carrier, the time required for the carrier protein to change its shape.

6. Endocytosis and Exocytosis

• Endocytosis: The phenomenon of cells engulfing substances into their interior.
  • There are two types: phagocytosis and pinocytosis.
  • It is an active transport process that requires energy.
  • Phagocytosis: Cells engulf bacteria, neutrophils, dead tissue, etc., large-sized substances.
  • Pinocytosis: Cells engulf liquids and dissolved substances of small size.
• Exocytosis: Waste products from digestion inside the phagosomes are transported to the cell membrane, phagosomes fuse with the cell membrane, and waste products are released from the cell.

7. Examples of Substance Transport Across Membranes

• Na+-K+ pump: Helps regulate cell volume and create the resting membrane potential.
• Ca++ pump: There are two types, one pumps Ca++ from the cytoplasm to the outside, the other pumps Ca++ from the cytoplasm to organelles.
• Symport: Na+ is pumped out, increasing the concentration of Na+ outside the membrane, creating potential energy for Na+ to tend to move back into the membrane. This diffusion brings other substances into the cell, requiring a carrier protein to bind to Na+ and the substance being transported. The mechanism of substance absorption mainly occurs in the renal tubules and digestive tract.
• Antiport: Na+ moves into the cell while Ca++ moves out, or Na+ moves into the cell while H+ moves out, bound to a carrier protein. The mechanism of excretion and regulation of H+.

8. Role of Substance Transport Across the Cell Membrane

• Maintains a stable environment for cell function.
• Provides energy and essential nutrients for the cell.
• Removes waste products and excess cell products.
• Participates in metabolic processes, signal reception, and immune responses.

9. Conclusion

Transport of substances across the cell membrane is a complex process that plays an essential role in the life of the cell. Understanding these transport mechanisms will help us grasp the fundamental principles of physiology and apply them to various fields such as medicine, pharmacology, and agriculture.

Note:

• This article provides a general overview of cellular membrane transport physiology. To understand this topic more deeply, you should consult further specialized materials.
• The physiology of substance transport across the cell membrane is an area that is constantly being researched and developed. Many aspects are not fully understood.