Membrane Transport

Endocytosis and Membrane Transport

Endocytosis is the process by which cells internalize substances from their surrounding environment. There are different types of endocytosis:
- Phagocytosis: Ingestion of large particles via the formation of pseudopodia that encircle the particle.
- Pinocytosis: Uptake of liquids or small molecules by invagination of the membrane to form vesicles.
- Receptor-mediated endocytosis: Selective uptake of molecules based on their interaction with specific receptors on the cell surface, involving coated pits that form coated vesicles.

Membranes delineate the internal environment of cells and organelles from their external surroundings, allowing selective permeability.
Passive Transport: Movement of molecules across the membrane without energy input. This can occur through:
- Simple Diffusion: Movement of small nonpolar molecules across the lipid bilayer (e.g. O2, CO2).
- Facilitated Diffusion: Utilization of transport proteins that assist in moving molecules down their concentration gradient.

Membrane fusion is the merging of two separate lipid bilayers to form a single bilayer, often involved in vesicle transport.
Budding is the process of forming vesicles from a membrane, important in exocytosis and receptor-mediated endocytosis.

Passive Transport: Gradient-driven flow of solutes (e.g., ions, water) without energy.
Active Transport: Requires energy input, moving solutes against their gradient, including:
- Primary Active Transport: Direct use of ATP to move molecules (e.g., Na+/K+ pump).
- Secondary Active Transport: Use of energy from the movement of another molecule (eg. symporters and antiporters).

Solute movement can be predicted by changes in free energy (ΔG). The signs of ΔG indicate whether a process is spontaneous (< 0) or non-spontaneous (> 0).
Equilibrium Condition: [o]1 = [o]2; no net movement when concentration is equal on both sides.
The influence of the electrochemical gradient is crucial, particularly for charged ions.

Ion channels selectively allow ions to pass through the membrane, crucial for neurotransmission and muscle contractions.
Potassium channels, for example, have a specific selectivity filter and rapid transport capabilities through a tetrameric structure.

Voltage-gated channels respond to depolarization, where specific residues within the channel confine and transport ions (e.g., S4 helix in K+ channels).

Primary Active Transport: ATP-driven (e.g., Na+/K+ ATPase).
Secondary Active Transport: Driven by ion gradients (e.g., sodium-glucose cotransporter).

ABC (ATP-Binding Cassette) transporters: Proteins that use ATP hydrolysis to transport various substances across membranes, contributing to drug resistance in cells (e.g., P-glycoprotein).

Passive Transport: Includes simple and facilitated diffusion, does not require energy.
Active Transport: Requires energy, can be primary (directly ATP-coupled) or secondary (coupled to ion gradients).

Transport proteins significantly increase the rate compared to simple diffusion due to saturation kinetics.
GLUT transporters: Specific for glucose, facilitating its entry into cells, notably in muscle and fat tissues under insulin regulation.

The membrane is a dynamic structure pivotal for cellular function, regulating the in/outflow of substances critical for cellular homeostasis. Understanding these transport mechanisms is essential for grasping fundamental biological processes and membrane physiology.