Cell Biology: Comprehensive Cellular Junctions and Internal Systems

Cellular Unions and Plasma Membrane Specialization

Specialized regions of the plasma membrane centered in specific proteins are known as cellular unions (or cell junctions). Transmembrane unions are especially important in epithelial tissues.

Classification by Structure

• Zonulae: Completely surround the cell.
• Maculae: Spot-like unions.

Classification by Function

• Occluding Junctions: Their primary function is to close the intercellular space between cells, preventing molecules from passing between them.
• Anchoring Junctions: Connect the cytoskeleton filament of one cell to another or to the extracellular matrix, increasing resistance against strong mechanical tensions. Types include zonulae adherens, maculae adherens (desmosomes), and hemidesmosomes.
• Communicating Junctions: Gap junctions that allow direct communication between the cytoplasm of adjacent cells through ions and small molecules.

Cell Communication and Signaling Systems

Cellular survival requires the ability to respond to external stimuli, a capability known as irritability. Stimuli are transmitted by signaling molecules produced by one cell and recognized by specific receptors present on the target cell, which convert extracellular signals into intracellular signals using transduction systems.

Signaling Molecules

• Hormones: Endocrine signaling.
• Local Chemical Mediators: Paracrine signaling.
• Neurotransmitters: Neuronal signaling.

Receptors and Signal Transduction

Receptors are distributed across the plasma membrane of target cells. Transmembrane proteins contain two regions: an extracellular region that recognizes a given signal molecule and a cytosolic region responsible for signal transduction. The activation of the junction region initiates a cytosolic signaling pathway, prompting a cellular response.

Internal Membrane Systems in Eukaryotic Cells

The most important feature of eukaryotic cells is the subdivision of the cytoplasm into internal differentiated compartments surrounded by membranes with a specific lipid and protein composition, which confers distinct structural and functional properties.

Endoplasmic Reticulum

The endoplasmic reticulum is a system of membranes forming a network of ramified and flattened sacs and tubules that interconnect and delimit a continuous space, continuous with the nuclear envelope.

Rough Endoplasmic Reticulum (RER)

Composed of flattened sacs with attached ribosomes. Its functions include protein synthesis, glycosylation, and the acquisition of the definitive folding structure of glycosylated proteins.

Smooth Endoplasmic Reticulum (REL)

Composed of tubules lacking ribosomes. Functions include the synthesis of glycerophospholipids and cholesterol, communication with the Golgi apparatus, accumulation of calcium ions ($Ca^{2+}$), and detoxification of liposoluble toxic substances.

Golgi Apparatus

Constituted by several dictyosomes (formed by 4 to 7 sacs) connected to each other and localized near the nucleus. Each dictyosome features three distinct regions: the cis face, the medial region, and the trans face. Proteins enter through the cis face, move through the medial region via transport vesicles, and reach the trans face, where they are sorted and given their final destination.

Secretion Pathways

• Constitutive Secretion: Materials that serve the plasma membrane and renew the extracellular matrix components are synthesized and continuously secreted by all cells.
• Regulated Secretion: Secretion products such as hormones and neurotransmitters are released to the exterior only in response to specific stimuli.

Functions of the Golgi Apparatus

• Continuation of protein glycosylation initiated in the RER.
• Acts as a direct center for molecular traffic in the cell.
• Synthesizes glycosphingolipids, cellulose, and contributes to the formation of phragmoplasts and the acrosome.

Lysosomes

Membranous vesicles originating from the Golgi apparatus that contain hydrolytic enzymes utilized for intracellular digestion. Lysosomes are highly heterogeneous organelles capable of digesting various materials.

Digestive Processes

• Primary Lysosomes: Homogeneous and electron-dense, awaiting participation in digestive processes.
• Secondary Lysosomes: Formed when primary lysosomes fuse with material to be digested, eventually becoming waste bodies (residual bodies).

Lysosomes act as the "stomach" of the cell through intracellular digestion mechanisms:

• Heterophagy: Digestion of nutrients acquired via endocytosis (pinocytosis or phagocytosis). In phagocytosis, the engulfed material forms a phagosome that merges with a primary lysosome to become a phagolysosome.
• Autophagy: Digestion of materials originating from inside the cell. It begins when an organelle destined for destruction is surrounded by endoplasmic reticulum-derived membranes to form an autophagosome, which then merges with a primary lysosome.