Carbohydrates in the Plasma Membrane

Chapter 1 : Cellular Biology Introduction to Pathophysiology Carbohydrates in the plasma membrane ● Short chains of sugars mostly bound to membrane proteins and lipids ● Proteoglycans: long polysaccharide chains attached to membrane proteins ● Glycocalyx: carbohydrate coating on the outside of the plasma membrane ● Functions include protection against mechanical damage, facilitating cell mobility, and cell-to-cell recognition Proteostasis in the cell ● Proteostasis is the dynamic balance of protein synthesis, folding, and degradation ● Involves ribosomes, chaperones, and protein breakdown systems (lysosomes and the ubiquitin-proteasome system) ● Important for maintaining protein homeostasis under various conditions ● Misfunction or failure of proteostasis network is associated with human diseases Cellular Receptors ● Cellular receptors are protein molecules on the plasma membrane, in the cytoplasm, or in the nucleus ● They recognize and bind with specific smaller molecules called ligands ● Ligand binding activates or inhibits the receptor's associated signaling or biochemical pathway ● Recognition and binding depend on the chemical configuration of the receptor and its ligand Types of Cellular Receptors ● Plasma membrane receptors protrude from or are exposed at the external surface of the membrane ● Membrane receptors determine which ligands a cell will bind with and how it will respond ● Membrane receptors can be found for hormones, neurotransmitters, antigens, drugs, and other molecules ● Receptors for different drugs can be found on the plasma membrane, in the cytoplasm, and in the nucleus Cell-to-Cell Adhesions ● Cells are connected to each other through cell adhesions ● Cell adhesions help maintain the integrity of the cell membrane ● Cells rely on the integrity of the cell membrane for their survival Plasma membranes and cell adhesion ● Plasma membranes serve as the outer boundaries of all cells ● Plasma membranes allow cells to be held together in cell-to-cell adhesions, forming tissues and organs ● Cells can be linked by cell adhesion molecules (CAMs), extracellular matrix (ECM), and specialized cell junctions Cell adhesion molecules (CAMs) ● CAMs are cell surface proteins that bind cells to adjacent cells and components of the ECM ● Four types of CAMs include: integrins, cadherins, selectins, and immunoglobulin superfamily CAMs (IgSF CAMs) ● Integrins regulate cell-ECM interactions with collagen, cadherins are calcium-dependent glycoproteins, selectins bind carbohydrates, and IgSF CAMs bind integrins and other IgSF CAMs Extracellular matrix (ECM) and basement membrane ● ECM is an intricate meshwork of fibrous proteins and complex carbohydrates ● Basement membrane (BM) is a specialized type of ECM ● BM is thin, tough, and flexible, and surrounds cells like epithelial cells, muscle cells, fat cells, and Schwann cells ● ECM provides a pathway for diffusion of nutrients, wastes, and substances between blood and tissue cells Components of the extracellular matrix ● Fibrous structural proteins: collagen and elastin ● Adhesive glycoproteins: fibronectin ● Proteoglycans and hyaluronic acid Basement Membranes (Basal Laminae) ● Basal laminae surround certain cells like skeletal cells and underlie epithelia ● They occur between two cell sheets, such as the kidney glomerulus ● Collagen forms cable-like fibers or sheets that provide tensile strength ● Elastin is a rubber-like protein fiber that allows tissues to stretch and recoil ● Fibronectin promotes cell adhesion and anchorage ● All these macromolecules occur in intercellular junctions and cell surfaces ● The extracellular matrix (ECM) is secreted by fibroblasts and connects cells to form tissues and organs ● Connective tissues can be hard and dense (bone), flexible (tendons), resilient and shock-absorbing (cartilage), or soft and transparent (similar to the substance in the eye) ● The matrix helps regulate important functions like cell growth and differentiation Cell Junctions ● Cell junctions are specialized plasma membrane regions that connect cells to neighboring cells ● They are classified based on their function: tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions ● Tight junctions hold cells together and form a tight seal ● Adherens junctions, desmosomes, and hemidesmosomes provide strong mechanical attachments ● Gap junctions allow for chemical communication and the transfer of ions and small molecules between cells ● Tight junctions also help maintain apico-basal polarity of epithelial cells Cell Junctions in Epithelium ● Cell junctions make the epithelium leak-proof and mediate mechanical attachment between cells ● Types of cell junctions include tight junctions, belt desmosomes, desmosomes, and gap junctions ● Tight junctions act as barriers to diffusion and prevent the movement of substances between cells ● Desmosomes provide structural stability and unite cells either in bands or with button-like points of contact ● Gap junctions are clusters of communicating tunnels that allow small ions and molecules to pass between cells Role of Gap Junctions ● Gap junctions coordinate the activities of adjacent cells and are important for functions like synchronizing heart muscle cell contractions and spreading action potentials in neural tissues ● Gap junctions can exist independently or associated with junctional complexes ● They are composed of connexon proteins with four transmembrane domains ● Factors like voltage, pH, calcium concentration, and protein phosphorylation regulate gap junction intercellular communication ● Gating controls the permeability of junctional complexes, which can be affected by cytoplasmic calcium levels Cellular Communication and Signal Transduction ● Cells communicate to maintain homeostasis, regulate growth and division, oversee development, and coordinate functions ● Cells use signal molecules to communicate ● Communication can occur through plasma membrane-bound signaling molecules (receptors) in direct physical contact ● Small hydrophobic signaling molecules can pass through gap junctions from one cell to another ● Remote signaling occurs when cells use secreted molecules to communicate with target cells