Cellular Membrane and Extracellular Matrix: Structure and Function
Cellular Membrane: Structure and Function
The cellular membrane is a structure common to all cells of living beings. It surrounds a portion of cytoplasm (cytoplasmic membrane). The same membranous structure also delimits most internal cell structures. Biological membranes (plasma membrane or internal membranes of the cell) are called unitary membranes.
Structure
A clear strip (osmiophobic), bordered by two dark stripes (osmiophilic).
Composition
- Membrane Lipids: Phospholipids, cholesterol, and glycolipids.
- Membrane Proteins: Peripheral or extrinsic, or integral membrane proteins.
- Carbohydrates: Bound to proteins or lipids.
Role of the Membrane
- Transports substances (maintenance of cell structures and cell growth).
- Plays a role in cell movement and facilitates the penetration and excretion of substances.
- Participates in the exchange of information or physicochemical stimuli between cells.
Transportation of Molecules Across the Membrane
Small Molecules
- Passive Transport:
- Facilitated diffusion
- Simple diffusion
- Osmosis
- Active Transport
Macromolecules
- Exocytosis
- Endocytosis:
- Pinocytosis
- Phagocytosis
Specializations of Cell Membranes
- Microvilli: Extensions of the cell membrane that are intended to increase the exchange surface.
- Invaginations: They appear, for example, in epithelial cells that line the convoluted tubule of nephrons.
- Interdigitations: Membranous folds between neighboring cells that strengthen their union.
- Myelin Sheaths: Folds of the membrane of Schwann cells that cover the axons of some neurons.
Cell Junctions
- Adherens Junctions: Desmosomes; cells are joined mechanically.
- Tight Junctions: Tight junctions or septate junctions of invertebrates, which impede the passage of molecules through the membrane.
- Communicating Junctions: Gap junctions or chemical synapses, allowing the passage of small molecules.
Secretion Membranes
Glycocalyx
This is a glycocalyx, a secretion membrane of animal cells, which is continuously updated.
Functions:
- Membrane protection.
- Absorbent function.
- Intervention in enzymatic processes.
- Influence on permeability.
- Intervention in cell adhesion phenomena.
- Intervention in cell recognition (surface antigens).
- Carbohydrates can also play a role in stabilizing the folded structure of a glycoprotein.
- A glycoprotein’s carbohydrate can lead to its destination within the same membrane.
Cell Wall
Composition and Structure
It involves the intervention of the Golgi apparatus (synthesis of components). The formation process of the wall includes:
- Middle lamella
- Primary cell wall
- Secondary cell wall
Properties
- Low porosity.
- Resistance to intracellular pressure (turgor).
Features
- Allows the exchange of substances between the exterior and interior of the cell.
- Gives the specific shape of the plant cell and restricts cell mobility.
- Protects the cell from the external environment.
- Provides functions for the adult cell: plant coating, maintenance, driving substances, etc.
- Allows communication between cells through plasmodesmata or holes in the secondary cell wall.
Modifications of the Secondary Cell Wall
- Lignification
- Suberization
- Cutinization
- Mineralization
Composition and Structure of the Extracellular Matrix
A stable macromolecular complex network, in contact with the glycocalyx, which occupies the intercellular spaces (sometimes very apparent, such as in connective tissue) in animals (especially vertebrates). Formed by a series of fibers within a very hydrated gel. Its fundamental components are organized into:
- Protein Fibers: Collagen and elastin.
- Gelatinous Ground Substance: Glycosaminoglycans and proteoglycans.
- Adhesion Proteins: Heterogeneous (laminin and fibronectin).
Functions
- Maintains linked cells of tissues and tissues of organs.
- Fills intercellular spaces, giving consistency to tissues and organs, and compartmentalizes them.
- Influences cell morphology, polarity, and other cellular characteristics.
- Facilitates the transport of substances and influences cell metabolism.
- Directs cell growth and tissue regeneration, allowing cell movement.
- Stores various substances such as hormones.