Mammalian recording of CH.5

Overview of Membrane Transport

• Membrane transport is critical for cellular function and involves various processes that allow substances to enter or exit cells.

Active Transport

• Active transport requires energy, typically derived from ATP (adenosine triphosphate). The gamma phosphate of ATP carries the energy necessary for this process.

• Key players in this process include motor proteins such as kinesin and dynein, which facilitate vesicular transport, specifically for processes like exocytosis and endocytosis.

Endocytosis and Exocytosis

• Endocytosis is the process of taking substances into the cell, while exocytosis is the process of expelling substances from the cell.

  • Phagocytosis: This is a form of endocytosis performed by phagocytes (e.g., neutrophils and macrophages) that engulfs larger particles.

  • Pinocytosis: The process of engulfing smaller particles or liquid.

  • Receptor-mediated endocytosis: Involves receptors binding to specific ligands, which triggers the formation of vesicles to bring those ligands into the cell.

Protein-Mediated Transport

• Transport across membranes can be facilitated by proteins in two main categories: facilitated diffusion and active transport.

  • Facilitated Diffusion: This passive transport process requires no energy and involves the movement of molecules along their concentration gradient (from high to low concentration) via specific transport proteins. Examples include sodium-glucose transporters where sodium is transported down its gradient while glucose is transported against its gradient.

  • Ion Channels: Proteins that allow ions to cross the membrane. Notable examples include potassium leak channels and aquaporins that facilitate water transport.

Simple Diffusion

• Simple diffusion moves substances directly through the lipid bilayer from an area of high concentration to low concentration.

• Factors influencing diffusion include temperature, molecular size, surface area of the membrane, and concentration gradient.

  • The rate of diffusion is inversely related to the molecular size; smaller molecules diffuse faster than larger molecules.

Key Concepts of Membrane Stability

• The lipid bilayer, composed of phospholipids, is impermeable to most water-soluble materials, requiring specific channels or transporters for passage.

• Diffusion dynamics: Molecules move spontaneously due to kinetic energy, and this motion persists until equilibrium is achieved in the concentration.

• Fick's Laws of Diffusion: A framework that quantifies diffusion across membranes based on solubility, surface area, and concentration gradients.

Types of Transport Proteins

• Structural Proteins: Maintain cell shape and organization.

• Enzymatic Proteins: Catalyze chemical reactions at cell membranes.

• Receptor Proteins: Receive chemical signals from outside the cell (e.g., G-protein coupled receptors such as rhodopsin).

• Transport Proteins: Include both channel proteins (which often remain open or are gated) and carrier proteins that change shape to move substances across membranes.

Active Transport Mechanisms

• Primary Active Transport: Directly uses ATP to transport ions against their concentration gradient, such as the sodium-potassium pump.

• Secondary Active Transport: Uses the energy stored in the gradient of one molecule to transport another molecule. An example is the sodium-glucose transporter.

Electrochemical Gradient

• Cells maintain an electrochemical gradient, primarily involving sodium and potassium ions, contributing to the resting membrane potential. This potential is essential for the generation of action potentials in neurons and muscle cells.

• The compartmentalization of charges is due to the selective permeability of the membrane and the action of ion pumps.

Membrane Potential and Action Potential

• Resting Membrane Potential: The state of a neuron when it is not sending signals, usually around -70 mV.

• Depolarization and Hyperpolarization: Changes in the membrane potential relative to resting level, with depolarization bringing the potential closer to zero and hyperpolarization moving it further away from zero.

Summary of Cellular Transport and Its Importance

• Understanding membrane transport mechanisms is vital for explaining many physiological processes, including nutrient uptake, waste removal, and cell signaling. Membrane integrity, selective permeability, and the dynamics of ion movement are central to cellular metabolism and function.