ESFUNO - BCT - Biomembranas (segunda parte)

Protein Composition and Membrane Structure
📌 The protein content of biological membranes is variable: erythrocyte membranes contain approximately 50% protein, while myelin membranes have a significantly lower percentage.
🧬 Proteins interact with the lipid bilayer in various forms: single-pass alpha helices, multi-pass transmembrane proteins (such as receptors), or beta barrels that create aqueous channels.
🧪 Membrane proteins are classified as integral (extracted only with detergents) or peripheral (extracted by changing pH or ionic strength).

Cellular Functions of Membrane Proteins
⚙️ Membrane proteins serve multiple roles including acting as transporters/channels, enzymes (e.g., ATP synthase), and receptors for cell signaling like the insulin receptor.
📍 Glycosylation occurs in the endoplasmic reticulum and Golgi apparatus, exclusively on the non-cytosolic side of the membrane, contributing to the formation of the glycocalyx.
🛡️ Specialized proteins form occluding junctions that establish cell polarity, effectively acting as barriers that prevent the free diffusion of molecules between membrane domains.

Transport Mechanisms and Membrane Permeability
🌬️ Membranes exhibit selective permeability: small non-polar gases ($O₂$, $CO₂$, $N₂$) diffuse freely, while large polar molecules (glucose, amino acids) and ions require specific protein channels or carriers.
⚖️ Passive transport moves molecules along the electrochemical gradient, whereas active transport requires energy (from ATP hydrolysis or coupling to another gradient) to move substances against the gradient.
📦 Vesicular transport involves three stages: budding, transport along the cytoskeleton, and fusion with the target organelle, allowing the movement of both internal contents and membrane components.

Endocytosis and Exocytosis
📥 Pinocytosis allows for the non-selective intake of extracellular fluid, while receptor-mediated endocytosis provides a highly selective mechanism for internalizing specific molecules.
🦠 Phagocytosis is a specialized process used by cells like macrophages to engulf large particles, such as bacteria or dead cell debris, using actin-dependent pseudopods.
🚀 Exocytosis functions as the reverse process, facilitating the release of materials from the interior of the cell to the external environment.

Key Points & Insights
➡️ Protein mobility is not always random; it is often restricted by connections to the cytoskeleton, the extracellular matrix, or specific boundary proteins.
➡️ Cell polarity is critical for physiological function, particularly in tissues like the intestinal epithelium, where specific transporters must be localized to distinct domains to facilitate metabolite uptake and release.
➡️ The fluidity of membranes demonstrated by cell fusion experiments confirms that proteins can diffuse freely, though their movement is biologically regulated to maintain structural and functional integrity.

📸 Video summarized with SummaryTube.com on Apr 07, 2026, 03:34 UTC