Cellular Transport and Membrane Specialization
Cellular Transport
The plasma membrane allows selective permeability. This is achieved because the lipid bilayer acts as a barrier. Transport:
Passive Transport
Passive transport does not require energy. It is the process where molecules move spontaneously down a concentration gradient, from an area of higher concentration to an area of lower concentration across the membrane.
- Simple Diffusion: Direct passage through the lipid bilayer.
- Facilitated Diffusion: Transport through membrane proteins.
- Channel proteins
- Transporters (carriers)
- Specific ligand-gated ion channels: Channel opening occurs when a ligand binds to a receptor, allowing ion diffusion.
- Voltage-dependent ion channels
Active Transport
Active transport is performed against a concentration gradient and requires energy consumption. Proteins involved are called pumps. It maintains internal concentrations, such as the higher external Na+ and K+ concentration. This is achieved by pumping 3 Na+ ions out of the cell, coupled with the hydrolysis of ATP.
Functions:
- Maintaining osmotic-electric balance.
- Allows glucose and amino acid transport driven by the Na+ gradient.
Macromolecule Transport
Includes the incorporation of substances into the cell, as well as secretion. Both processes occur through the formation of vesicles.
Endocytosis
The process by which macromolecules are engulfed by invagination of a part of the membrane, forming intracellular vesicles.
A special type of pinocytosis is receptor-mediated endocytosis, where the macromolecule-receptor complex is incorporated into endocytic vesicles. This confers specificity and increases efficiency. These specified regions are coated internally by a protein, clathrin. Ingested materials, either by phagocytosis or pinocytosis, end up in lysosomes.
Exocytosis
The process opposite to endocytosis. Intracellular vesicles fuse with the plasma membrane, and their contents are released into the extracellular medium.
Plasma Membrane Specializations
Microvilli
Digitiform prolongations. They are abundant in cells of the small intestine epithelium, whose function requires a maximum absorption surface.
Cell Junctions
Specialized regions of the plasma membrane through which important connections are established, especially in epithelia.
Occluding Junctions (Zonula Occludens/Tight Junctions)
Used to seal epithelial cells lining cavities, forming permeability barriers.
Anchoring Junctions
Present in areas more resistant to strong mechanical stress.
- Adherens Junctions (Zonula Adherens): Link cytoskeletal filaments between neighboring cells.
- Desmosomes: Anchor intermediate filaments of the cytoskeleton of adjacent cells.
- Hemidesmosomes: Anchor intermediate filaments of the cytoskeleton of a cell to the extracellular matrix.
Gap Junctions
Points of direct communication between the cytoplasm of adjacent cells, through which ions and molecules pass. These channels are called connexons.
Cell Glycocalyx (Cell Coat)
The peripheral area of the membrane surface, rich in carbohydrates. Function: Cell recognition; these are identity markers.
Cell Communication
Signaling molecules are recognized by specific receptors present in the target cell, which converts the extracellular signal into an intracellular signal. Receptors are usually transmembrane proteins. The signaling molecule (1st messenger) is responsible for the modification that causes the cellular response, which consists of the release of intracellular signaling molecules (2nd messengers). Then, the 2nd messenger diffuses into the interior of the cell and produces its response.
Secretory Membranes
Chemicals produced by the cell that are deposited on the outside surface of the membrane.
Extracellular Matrix
A secretion product that acts as a liaison and fills intracellular spaces.