Membrane carbohydrates and glycocalyx formation
The cell membrane is not exclusively a lipid-protein structure. An important component is carbohydrates, the proportion of which can reach up to 10% of the total membrane composition. In the membrane, they are not present in a free state, but as part of complex compounds:
- Glycoproteins — carbohydrate chains tightly bound to membrane proteins.
- Glycolipids — carbohydrates linked to the lipid bilayer.
The main architectural feature of these structures lies in their localization. Carbohydrate components are always located exclusively on the outer surface of the membrane. These long, highly branched chains of carbohydrates intertwine to form a supramembrane complex — the glycocalyx. It is important to remember that the presence of a pronounced glycocalyx is a specific feature of animal cells.
Receptor function of carbohydrate chains
The branched carbohydrate chains of the glycocalyx act as specific molecular "antennas" on the cell surface. Their most important task is the receptor function, which includes:
- Intercellular recognition: due to the unique pattern of carbohydrate structures, cells are able to identify each other ("self-nonself").
- Contact formation: following successful recognition, cells can establish specific intercellular connections, which is critical for uniting cells into tissues.
Thus, the receptor apparatus of the glycocalyx allows cells to adequately respond to their environment and act as a single whole.
Immunological function and genetic individuality
The glycocalyx possesses extremely high immunogenicity. Carbohydrate chains form specific antigens on the cell surface, which is of paramount importance in clinical medicine and biology:
- Blood groups: glycocalyx structures (specifically on erythrocytes) determine blood group antigens. This dictates strict compatibility rules during blood transfusion.
- Tissue compatibility: carbohydrate complexes form histocompatibility antigens. These are precisely what is evaluated when selecting a donor for tissue and organ transplantation.
Together, this complex set of surface markers provides the unique genetic individuality of each individual cell and organism.
Submembrane complex: internal framework
While the glycocalyx is located outside the membrane, the submembrane complex is located on its inner, cytoplasmic side.
This complex is formed by protein filaments of two main types:
- Microtubules
- Microfilaments
Interweaving into a complex three-dimensional network, microtubules and microfilaments form the cytoskeleton. The cytoskeleton acts as an internal supporting framework that not only maintains cell shape but also provides mechanical stabilization of the cell membrane from the inside.
Briefly: answers to topical questions
What types of intercellular junctions are formed by cells after the stage of intercellular recognition?
Following the stage of intercellular recognition, long-term junctions are formed, which are functionally divided into four main groups:
- Simple junctions — simple intercellular junctions and interdigitations (finger-like infoldings).
- Anchoring junctions — desmosomes (strong spot junctions) and adhesion belts.
- Occluding junctions — tight junction (zona occludens), ensuring complete sealing of the space between cells.
- Communicating junctions — gap junctions (gap-junctions or nexuses) and synapses, serving for exchange and signal transmission.
What specialized proteins ensure the attachment of microfilaments to the cell membrane?
The attachment of microfilaments to the plasmalemma is ensured by special linking and anchoring proteins. Within the cortical network, microfilaments bind to the membrane via intermediate proteins, such as $\alpha$-actinin. During the formation of intercellular adhesion contacts, microfilaments attach to special anchoring proteins, which in turn bind to the intracellular portion of transmembrane proteins — cadherins.
To memorize the composition of the internal framework of the cell, use the phrase: "Membranes Require Fixation" — Microtubules and microFilaments.
Class and exam questions
Which cells are characterized by the presence of a glycocalyx?
The glycocalyx is characteristic exclusively of animal cells. In other cell types (plants, fungi), a rigid cell wall is located above the membrane.
What substances make up the glycocalyx?
It is formed by long branched carbohydrate chains linked to membrane proteins (glycoproteins) and lipids (glycolipids).
What is the role of the glycocalyx in blood transfusion?
The carbohydrate chains of the glycocalyx form surface blood group antigens. They determine the immunological compatibility of donor and recipient blood.
What forms the submembrane complex?
The submembrane complex consists of microtubules and microfilaments, which together form the cell cytoskeleton.
What else to study on this topic
- Biochemical structure of glycoproteins and glycolipids
- Mechanisms of intercellular recognition and types of cell junctions
- Major histocompatibility complex (MHC) antigens
- Molecular structure of microtubules and microfilaments
- AB0 blood group system and rhesus factor
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