ESFUNO - BCT - Biomembranas (segunda parte)

Membrane Structure and Function

Overview of Membrane Composition

• The second part of the class discusses biological membranes, which are composed of lipids, proteins, and carbohydrates. Proteins play a crucial role in providing specific functions to these membranes.

• The protein composition varies across different types of membranes; for instance, the plasma membrane of centrosomes contains about 50% protein, while myelin-forming cell membranes have significantly less.

Protein Integration in Membranes

• Proteins can integrate into the plasma membrane in various ways: some traverse it once (single-pass), typically forming an alpha helix with hydrophobic amino acids interacting with the membrane.

• Other proteins may span the membrane multiple times (multi-pass), such as receptors or those mediating cell-cell adhesion. These often exhibit complex structures that facilitate their function.

Types of Membrane Proteins

• Some proteins adopt a beta-barrel structure where hydrophobic groups contact the membrane while hydrophilic groups form an aqueous channel inside. This configuration allows for ion transport or acts as transporters.

• Certain proteins attach to one side of the membrane without fully traversing it, utilizing hydrophobic domains for insertion. Others bind indirectly through lipid interactions.

Peripheral vs Integral Membrane Proteins

• Peripheral proteins do not directly attach to the membrane but associate weakly with integral proteins via non-covalent bonds. They can be found on either cytosolic or extracellular sides.

• Integral membrane proteins require harsh conditions like detergents for separation from membranes due to their strong attachment, whereas peripheral proteins can be easily detached by altering pH or ionic strength.

Functions of Membrane Proteins

• Membrane proteins serve diverse functions including forming channels/transporters that facilitate molecular movement across membranes and participating in cellular recognition processes (e.g., insulin receptor).

• Receptors like insulin span the membrane with distinct extracellular and intracellular domains that mediate signal transduction upon ligand binding.

Enzymatic Activity and Structural Roles

• Some integral proteins act as enzymes catalyzing biochemical reactions within the membrane environment; ATP synthase is highlighted as an example.

• Other roles include anchoring cytoskeletal elements and facilitating connections between cells and extracellular matrices, contributing to tissue structure and integrity.

Cellular Polarity and Glycosylation

• Specific junctional proteins maintain cellular polarity by regulating substance passage between adjacent cells, influencing domain composition on either side.

• Many membrane proteins undergo glycosylation—addition of sugars—which occurs primarily on the cytosolic side during synthesis in organelles like endoplasmic reticulum and Golgi apparatus.

This structured overview captures key insights from the transcript regarding biological membranes' composition, protein integration methods, types of membrane-associated proteins, their functions, enzymatic activities, structural roles, cellular polarity maintenance mechanisms, and glycosylation processes.

Membrane Proteins and Their Functions

Importance of Membrane Proteins

• Membrane proteins are crucial as they form hydrophilic channels in the plasma membrane, which is essential for cellular functions.

• An experiment demonstrated that mouse cells with a specific membrane protein marked with two labels fused with human cells marked with fluorescein, allowing observation of protein distribution.

Protein Diffusion in Hybrid Cells

• Initially, mouse proteins localized to their original regions; however, over time, these proteins diffused evenly throughout the hybrid cell, indicating free diffusion across membranes.

• The movement of membrane proteins can be restricted by cytoskeletal attachments or interactions with extracellular matrix proteins.

Cell Polarity and Transport Mechanisms

• Certain membrane proteins act as barriers that prevent the passage of molecules between different domains of a cell, critical for maintaining polarity.

• In intestinal epithelial cells, specific transporters on one side facilitate metabolite entry while others on the opposite side enable metabolite release.

Types of Membrane Proteins

• Various types of membrane-binding proteins mediate cell-cell junctions; tight junctions prevent protein passage between domains while transmembrane proteins form anchoring connections.

• Some transmembrane proteins create channels that allow small molecules to pass between adjacent cells, facilitating biological synchronization.

Selective Permeability and Transport Processes

• Biological membranes exhibit selective permeability, allowing certain compounds like small gases (O2, CO2) to diffuse freely while restricting larger polar molecules (e.g., amino acids).

• The rapid diffusion of gases through membranes is vital for cellular respiration processes.

Mechanisms Influencing Molecular Transport

• Polar but uncharged molecules such as water and ethanol can cross membranes more easily than charged ions or large polar molecules which require specific transport mechanisms.

• Factors influencing transport include molecular size, polarity, charge, and electrochemical gradients affecting concentration differences across membranes.

Types of Transport Across Membranes

• Different transport methods exist: passive transport occurs along electrochemical gradients without energy input; active transport requires energy to move substances against gradients.

• Facilitated diffusion involves carrier or channel-mediated movement for substances unable to diffuse freely due to size or charge constraints.

This structured summary captures key insights from the transcript regarding membrane proteins' roles in cellular function and transport mechanisms.

Mechanisms of Cellular Transport

Energy-Driven Transport Processes

• The energy released from ATP hydrolysis is coupled with the energy required for transporting solutes against their gradient, a process utilized by pumps powered by ATP.

• Other types of pumps, such as those driven by light energy, are not found in animals but are present in certain bacteria.

Vesicular Transport System

• The endoplasmic reticulum (ER) and Golgi apparatus work together in a vesicular transport system that includes endosomes and lysosomes.

• Three key processes are involved in vesicular transport:

• Vesicle Formation: Creation of vesicles from organelles.

• Transport: Movement along the cytoskeleton.

• Fusion: Merging with target organelles to release contents.

Exocytosis and Endocytosis

• Exocytosis refers to the process where vesicles release their contents outside the cell, while endocytosis involves cells taking in external compounds through vesicles.

Types of Endocytosis

• There are various forms of endocytosis:

• Pinocytosis: Non-selective uptake of extracellular molecules via vesicles; occurs continuously in most cells.

• Receptor-Mediated Endocytosis: A selective process where specific molecules bind to receptors on the membrane before being internalized into vesicles.

• This mechanism ensures that only certain molecules are captured based on receptor binding.

Specialized Forms of Endocytosis

• Phagocytosis is a specialized form mediated by pseudopodia that engulfs large particles like bacteria or dead cells; it occurs primarily in specialized cells such as macrophages.