Cell Membrane – Structure and Function, Cell Surface specialization

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Cell Membrane – Structure and Function, Cell Surface specialization


All eukaryotic cells are enveloped by a limiting membrane composed of the following components:

1.       Lipids:

o   Phospholipids

o   Cholesterol

o   Glycolipids

2.       Proteins

3.       Chains of Oligosaccharides:

o   These are small numbers of simple sugars (monosaccharides) linked to phospholipid and protein molecules.

Characteristic of Biological Membrane:

  • Asymmetric
  • Fluid Mosaic

Membranes range from 7.5 to 10 nm in thickness and are visible only in electron microscopes. They exhibit a trilaminar organization with two hydrophilic heads and a sandwiched layer of hydrophobic tails.

  • Membrane Phospholipids:
    • Consist of two non-polar hydrophobic tails linked to a charged polar hydrophilic head group, providing stability to the membrane (amphiphilic characteristic).
    • Cholesterol is also present (in a 1:1 ratio with phospholipids), inserted among the phospholipid fatty acids, restricting their movement.
  • Integrin Proteins:
    • These proteins are linked to both cytoplasmic cytoskeletal filaments and extracellular matrix components. Through these linkages, there is a constant exchange of influences in both directions between the extracellular matrix (ECM) and the cytoplasm.

Proteins of the Membrane, Synthesized by RER, Can Be Divided Into:


1.       Integral Transmembrane Proteins:

o   Facilitate the transfer of chemical substances by channels, transporters, or pumps.

o   Examples:

§  Transporters (Carriers) and Channels: Allow diffusion of small ions, molecules, and water through gap junctions.

§  Pumps: Actively transport specific ions across the membrane using energy (e.g., Na+/K+ pump).

§  Transport Using Vesicles: Involved in endocytosis and exocytosis.

§  ABC Transporters: Use ATP for molecule transfer and are connected to multi-drug resistance (MDR).

2.       Peripheral Proteins:

o   Exhibit looser association with one of the two membrane surfaces (inner or outer).

o   The distribution of membrane proteins differs between the two surfaces of the cell membrane (asymmetric characteristic).

External Surface of the Cell:

  • Carbohydrate-rich region called the glycocalyx:
    • Made of carbohydrate chains linked to membrane proteins and lipids.
    • Plays a role in cell recognition and attachment to other cells and extracellular molecules.

Membrane Characteristics:

  • The membrane exhibits a fluid mosaic appearance due to the combination of the fluid nature of the lipid bilayer and the mosaic appearance of the membrane proteins.

Cell Membrane Functions:

1.       Selective Barrier:

o   Regulates the passage of materials into and out of the cell to maintain constant ion concentration and regulate the intracellular environment.

2.       Specific Recognition and Regulatory Functions:

o   Carries out various recognition and regulatory processes.

3.       Interactions with the Environment:

o   Includes cell adhesion and cell-to-cell signaling.

Endocytosis: Materials can cross the plasma membrane through a general process called endocytosis, which involves folding and fusion of a membrane to form vesicles enclosing the transported material.

  • Phagocytosis (“Cell Eating”):
    • White blood cells (such as macrophages and neutrophils) engulf and remove bacteria, protozoa, and dead cells.
  • Pinocytosis (“Cell Drinking”):
    • Small particles are brought into the cell, forming invaginations and being suspended within vesicles that break down by lysosomes.
  • Receptor-Mediated Endocytosis (“Absorptive Pinocytosis”):
    • Cells absorb metabolites, hormones, and proteins by inward budding of plasma membrane vesicles containing proteins with receptor sites.

In Exocytosis: In exocytosis, a vesicle fuses with the plasma membrane, resulting in the release of its contents into the extracellular space without compromising the integrity of the plasma membrane (opposite to endocytosis).


                                                                                                                                                                                                                                       




Cell Surface Specialization: The cell surfaces exhibit three distinct domains:

1.       Apical Domain and Its Modifications:

o   Microvilli: These finger-like cytoplasmic projections are approximately 1 μm high and 0.08 μm wide. The appearance of microvilli varies widely among cells. The number and shape of microvilli correlate with the cell’s absorptive capacity:

§  Cells with tall, closely packed microvilli are likely involved in the transportation and absorption of metabolites.

§  Cells with small, irregularly shaped microvilli are less active in transportation and absorption.

o   Within microvilli, clusters of actin filaments are cross-linked to each other and to the surrounding plasma membrane by several other proteins.

o   Stereocilia/Stereovilli: These are extremely long, immotile microvilli that facilitate absorption. They are limited to specific locations, such as the epididymis and sensory hair cells of the inner ear.

o   Cilia and Flagella: These are motile processes covered by the cell membrane and have a highly organized microtubule core. Cilia’s main function is to sweep fluid along the surface of cell sheets. Both cilia and flagella have the same structure, composed of 9 peripheral microtubules surrounding two central microtubules (9+2 pattern – axoneme).

2.       Lateral Domain:

                                                                                                                     


o   Seals to Prevent Material Flow:

§  Tight (Occluding) Junctions: These junctions, made by occludin proteins, prevent the passive flow of materials between adjacent cells.

o   Sites of Adhesion:

§  Adhesive or Anchoring Junctions: These junctions, made by cadherin proteins, play a role in cell adhesion.

o   Channels for Communication:

§  Gap Junctions: These junctions, made by connexin proteins, allow intercellular communication by forming channels between adjacent cells.

o   The order of these junctions from apical to basal ends of the cells is as follows: Tight junctions → Adhering junctions → Desmosomes → Gap junctions → Hemidesmosomes.

o   The tight junction (zonula occludens) and adherent junction (zonula adherens) are typically close together and form continuous bands around the cell’s apical end. Adhering junctions stabilize and strengthen these circular bands, helping to hold the layer of cells together. Desmosomes provide strong attachment points, bound to intermediate filaments, and play a major role in maintaining the unity of an epithelium. Gap junctions serve as intercellular channels for the flow of molecules.

3.       Basal Domain:

o   Basement Membrane:

§  The basement membrane (term used in light microscopy) mediates the attachment of epithelial cells to underlying connective tissue. It provides support, nourishment, and binds the cell to neighboring structures.

o   Cell-to-ECM Junctions:

§  Focal Adhesions: These anchor actin filaments of the cytoskeleton into the basement membrane, creating a dynamic link between the actin cytoskeleton and extracellular matrix proteins.

§  Hemidesmosomes: These anchor the intermediate filaments of the cytoskeleton into the basement membrane.

o   Basal Cell Membrane Folding:

§  Folding increases the surface area of the basal cell domain, allowing for more transport proteins and channels to be present. This abundance of transport proteins is particularly important in cells that participate in active transport of molecules. Mitochondria are typically concentrated at this basal site to provide the energy requirements for active transport.