Cell Membrane Structure and Function: A Comprehensive Review

Posted on Feb 7, 2025 in Biology

Cell Membrane Structure: The Fluid Mosaic Model

Singer and Nicholson proposed the fluid mosaic model in 1972.

Under an electron microscope, the plasma membrane appears as two dark bands separated by a clear layer.

  • The membrane is not rigid but fluid.
  • The membrane is a fluid mosaic where the lipid bilayer cements the network. Proteins are embedded within or interact with each other. Both lipids are amphipathic substances.
  • Both proteins and lipids can move laterally.
  • Lipids and integral proteins are arranged in a mosaic pattern.
  • Membranes are asymmetric in the distribution of their components.

Functions of the Cell Membrane

  • Limits the cell as a selective barrier for the exchange and transport of substances.
  • Produces and controls electrochemical gradients (difference in charge and concentration on both sides of the membrane). Charges tend to equalize by ion exchange, creating a gradient used to produce energy.
  • Facilitates the exchange of signals.
  • Involved in cell division.
  • Plays a role in cellular immunity.
  • Mediates endocytosis and exocytosis.

Cell Membrane Chemistry

The cell membrane is composed of approximately 60% protein and 40% lipids.

Lipids

Lipids are asymmetrically distributed and heterogeneous, including phospholipids, glycolipids, and sterols.

Lipids can perform the following movements:

  • Rotation
  • Lateral diffusion
  • Flip-flop

Proteins

Proteins give the membrane its specific functions. They can play a structural role, have recognition and adhesion functions, or be involved in transport and cellular metabolism. Most are globular.

Proteins are classified according to their position in the membrane and the type of bond:

  • Integral proteins: Strongly associated with the membrane by hydrophobic or covalent bonds.
    • Transmembrane proteins (intrinsic)
    • Peripheral proteins (extrinsic)
  • Peripheral proteins: Attached to the membrane through ionic or hydrogen bonds and are easily separated from it.

Carbohydrates

Oligosaccharides are attached to proteins or lipids.

Located on the outer surface of the membrane, the glycocalyx has important functions.

Transport of Molecules Across the Cell Membrane

Low Molecular Weight Molecules

Passive Transport

Passive transport can occur through:

  • Simple Diffusion:
    • Soluble substances slide between the phospholipids.
    • Channel proteins allow passage of ions and polar substances.
  • Facilitated Diffusion:
    • Transport proteins facilitate this process.
    • Carrier proteins bind to the molecule being transported and undergo a conformational change that allows passage.

Active Transport

Active transport occurs against the concentration gradient and involves osmotic or electric power consumption.

Sodium-Potassium Pump: The differences in concentration of Na+/K+ are due to the activity of the Na+/K+ pump, which pumps three Na+ ions outward and two K+ ions inward against the gradient simultaneously.

High Molecular Weight Molecules

Endocytosis

The cell captures particles from the external environment.

  • Pinocytosis: Fluid intake of dissolved particles.
  • Phagocytosis: Ingestion of large solid particles.
  • Receptor-mediated endocytosis: A receptor on the membrane binds to the substance being ingested.

Exocytosis

Exocytosis produces the union of the membrane of the gallbladder and the plasma membrane, generating a pore through which the contents of the vesicle are released.

Transcytosis

Transcytosis is a set of phenomena that allows a substance to pass through the cytoplasm.

The Plant Cell Wall

Chemistry

  • Cellulose
  • Amorphous matrix mainly composed of pectin and hemicellulose

Structure

Middle Lamella

  • Outer and first layer
  • Can be shared with adjacent cells
  • May present cell intercom bridges: plasmodesmata
  • Composed primarily of pectin and protein

Primary Wall

  • Situated below the middle lamella
  • Fibrillar aspect
  • Characteristic of growing cells

Secondary Wall

  • Innermost layer
  • Present in support cells or vascular tissues
  • Formed when growth ceases
  • Consisting mainly of cellulose fibers arranged in layers

Functions

  • Confers rigidity and resistance
  • Responsible for the cell maintaining an upright position
  • Unites adjacent cells
  • Involved in maintaining osmotic balance, allowing plant cells to live in the hypotonic medium of the plant, preventing them from swelling and bursting
  • Waterproofs the cell, avoiding water loss
  • Serves as a barrier to pathogens