Cell Membrane

Plasma Membrane (Cell Membrane)

By definition, a cell is a discrete entity surrounded by a plasma membrane (cell membrane).

1. Membrane Functions

  • Regulation of Material Movement: The plasma membrane regulates the movement of materials in and out of the cell through selective permeability.

    • Known as "transport" when materials move through the membrane.

  • Boundary Maintenance: Maintains the boundary of a cell, keeping internal conditions stable (homeostasis).

  • Attachment to Internal Components: Attaches the membrane to internal cellular components as well as extracellular components and other cells.

  • Cell Communication: Allows a cell to receive signals from other cells, facilitating communication.

2. Membrane Structure

  • Fluid-Mosaic Model: Proposed by Singer and Nicolson in 1972, this theory describes the plasma membrane's structure as a mosaic of diverse components, with a fluid nature that allows for movement.

    • Fluid: Refers to how movable the components of the membrane are.

    • Mosaic: Indicates that there are many different types of components that can create patterns or exist without a specific structure.

A. Lipid Bilayer

  • Function: Forms the basic boundary around the cell and is characterized by its hydrophobic nature.

    • Phospholipids: The most common lipids in membranes, consisting of:

    • A hydrophilic (water-attracting) head.

    • Two hydrophobic (water-repelling) tails.

    • Amphipathic Molecules: These molecules have both a hydrophilic portion (the head) and a hydrophobic portion (the tails).

    • Membrane Sac Formation: Phospholipids can form structures such as micelles, bilayers, and liposomes based on their behavior in water.

B. Membrane Fluidity

  • Fatty Acid Tails: The degree of saturation and length of fatty acid tails affect fluidity.

    • Shorter Tails: Increase movement and fluidity within the membrane.

    • C=C double bonds: Introduce kinks in the fatty acid chain, further enhancing fluidity.

    • Sterols: Such as cholesterol (in animals), phytosterols (in plants), and ergosterol (in fungi) add stability and change membrane fluidity.

C. Lipid Diversity

  1. Cholesterol/Phytosterols:

    • Structurally composed of four rings and a hydrocarbon tail that is hydrophobic, with an -OH group that is hydrophilic.

  2. Glycolipids:

    • Amphipathic lipids with carbohydrates attached, consisting of few to several sugar molecules, which are found on the extracellular monolayer.

D. Movement of Lipids in Membrane

  • Types of Movement:

    • Lateral Movement: Lipids can move side-to-side within the same layer.

    • Rotational Movement: Lipids can rotate around their axes.

    • Flip-Flop: Moving from one monolayer to another requires proteins (enzymes) and energy; this is not likely to occur spontaneously.

  • Distribution of Lipids: Certain regions, known as lipid rafts, contain specialized lipids and proteins that serve specific functions.

E. Membrane Proteins

  • Integral Proteins:

    • Embedded within the phospholipid bilayer, requiring detergents for removal due to their strong attachment.

    • Transmembrane Proteins: These extend fully through the membrane, protruding on both sides (cytosolic and extracellular).

  • Peripheral Proteins:

    • Attached loosely to membrane components, easily removable without the use of detergents.

F. Movement of Proteins in Membrane

  • Protein Movement Characteristics:

    • Integral proteins can spin and move laterally but cannot flip-flop across the membrane.

    • Peripheral proteins can also move laterally but are restricted by tight junctions or anchoring to structures.

G. Membrane Carbohydrates

  • Carbohydrate Components:

    • Present in the extracellular monolayer, these carbohydrates play a significant role in cell recognition and protection.

  1. Glycolipids: Sugars attached to membrane lipids.

  2. Glycoproteins: Sugars attached to membrane proteins.

  3. Glycocalyx: A sugar coat on the surface of animal cells that protects cells and aids in cell recognition.

3. Membrane Asymmetry

  • Definition: Refers to the unequal distribution of different types of lipids and proteins between the two layers of a biological membrane, such as the plasma membrane of a cell.

  • Monolayer Differences: The structure of the two monolayers is not identical in composition and function.

    • Glycocalyx carbohydrates extend from the extracellular monolayer but not from the cytosolic side.

    • Transmembrane proteins have distinct parts and functions depending on which monolayer they extend from.

    • Peripheral proteins are only attached to one monolayer, and certain phospholipids may be more prevalent in one layer compared to the other.