Plasma membrane

The plasma membrane is the dynamic boundary that encloses each cell, maintaining its internal environment and mediating interactions with the surroundings. Its core consists of phospholipids, forming a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

Embedded steroids, notably cholesterol, enhance membrane fluidity, while small amounts of waxes can provide structural stability. According to the fluid mosaic model, proteins float within this lipid framework, acting as channels, transporters, and surface markers.

Membrane dynamics

Membrane dynamics involve processes like endocytosis and exocytosis:

Endocytosis: the membrane invaginates to internalize particles or liquids in an envelope

• Phagocytosis- the process by which cells engulf large particles such as bacteria or cellular debris. During this process, the plasma membrane extends around the target, often aided by a coating of clathrin, to form a vesicle that internalizes the material. This vesicle then fuses with a lysosome, creating an endosome where the engulfed substance is degraded and its nutrients extracted. Once digestion is complete, the endosome can fuse with the plasma membrane again to release any residual contents into the extracellular space.
• Pinocytosis- cell drinking", involves uptake of extracellular fluid and dissolved molecules into small vesicles that typically do not merge with lysosomes. A specialized form of pinocytosis, potocytosis, utilizes the protein caveolin to form distinct, receptor-rich invaginations called caveolae, which transport molecules across the cell by transcytosis or deliver them to specific organelles, such as the endoplasmic reticulum, for further processing.
• Receptor-mediated endocytosis- a highly selective form of endocytosis where specific cell surface receptors bind targeted molecules. These receptors, often associated with clathrin coats, trigger the formation of vesicles that internalize the bound substances. This process is critical for removing compounds like low-density lipoprotein (LDL) from the bloodstream. When receptor-mediated endocytosis fails—as seen in familial hypercholesterolemia—LDL accumulates, leading to dangerously high cholesterol levels. Moreover, some pathogens and toxins can exploit this pathway by mimicking natural ligands, thereby gaining entry into the cell.

Exocytosis: releases materials from inside the cell to the extracellular environment. In this process, waste or secretory substances are first packaged into a vesicle, which then fuses with the inner surface of the plasma membrane, causing its membrane to merge and open toward the outside, discharging the contents into the extracellular space. Exocytosis is responsible for the secretion of extracellular matrix proteins and the release of neurotransmitters into the synaptic cleft via synaptic vesicles.

Cells can adjust their shape for chemotaxis, often guided by the cytoskeleton, and permit small nonpolar molecules to diffuse directly, with ions requiring membrane channels or pumps.

Membrane transport

Transport across the membrane depends on thermodynamic considerations: mixing charged ions with the hydrophobic bilayer is unfavorable, so assistance is needed. Osmosis allows water to diffuse freely and can generate colligative properties affecting osmotic pressure, where excessive pressure risks cell lysis.

Passive transport (facilitated diffusion) proceeds down a concentration gradient without ATP, whereas active transport (like the sodium-potassium pump) needs ATP to move solutes against gradients, thus maintaining a negative membrane potential.

Membrane receptors and cell signaling
The membrane also contains membrane receptors that initiate cell signaling pathways by producing second messengers, which alter intracellular processes. Signaling types include contact signaling, chemical signaling, and electrical signaling, exemplified by neurotransmitter release in neurons or action potentials in muscle cells.

Intercellular junctions
In tissue organization, intercellular junctions play a key role:

• Gap junctions permit direct exchange of small molecules between adjacent cells
• Tight junctions form barriers that prevent leakage
• Desmosomes anchor cells together by linking their cytoskeletons