Biological Membranes - A. N. Ogurtsov 2012

Structure and Functions of Biomembranes
Cellular Membrane Structures
Plasma Membranes

All Cells possess an outer membrane that maintains a stable intracellular environment regardless of fluctuations in the surrounding extracellular medium.

While the cells of higher animal Tissues lack a rigid Cell wall, The Plasma Membrane in many of them is further surrounded by an outer coat, or glycocalyx, composed of Polysaccharides. The plasma membrane consists primarily of Polar Lipids and Proteins, remaining impermeable to many low-molecular-weight compounds, including various ions.

However, specialized carrier proteins known as channel formers can selectively increase membrane permeability to substances required by The Cell (Facilitated Diffusion). Through these channels, molecules and ions move across the membrane down their concentration gradient. In addition to this mechanism, there are so-called protein pumps capable of transporting substances across the membrane against a concentration gradient by utilizing energy derived from ATP Hydrolysis (Active Transport).

In model systems, substances are transported across Artificial Membranes along concentration gradients via passive diffusion until thermodynamic equilibrium is reached (passive transport).

In a living cell, the plasma membrane continuously mediates the Transport of substances consumed during the synthesis of key cellular components, thereby establishing a concentration gradient of these substances between the intracellular and extracellular environments.

The plasma membrane not only regulates the influx and efflux of substances across the cell boundary but also facilitates the exchange of "information" and energy between the cell and its environment. Introduction/36.html">Biological Membranes contain numerous receptors whose activation leads to elevated intracellular concentrations of second messengers that regulate cellular METABOLISM.

Biological membranes play a vital role in osmotic and electrical energy transductions. The concentration differences of ions across the inner and outer environments generate an electrochemical gradient, the energy of which can be converted into Other forms of energy: chemical (ATP synthesis), mechanical (ciliary and flagellar movement), and osmotic.

Beyond these universal Functions, the plasma membrane performs additional critical roles in Multicellular Organisms. Very few cells of multicellular plants and animals exist as independent entities; instead, groups of related specialized cells typically aggregate to form tissues. In animal cells, specialized Regions of the plasma membrane contain specific proteins and Glycolipids that establish Intercellular junctions, which both reinforce tissues and facilitate metabolite exchange between cells.

Owing to specific recognition sites or alterations in membrane conformation at specific sites of cell interaction, the outer surface of plasma membranes in multicellular organisms plays a crucial role in establishing intercellular contacts.

Certain plasma Membrane Proteins anchor the cell to Components of the Extracellular matrix, a complex meshwork of Fibrous proteins and polysaccharides that provides the foundational support for most Epithelial Tissues and small glands.

Other plasma membrane proteins serve as attachment sites for numerous Cytoskeleton filaments which, traversing the Cytosol, impart the necessary shape and mechanical strength to the cell.

The plasma membranes of many Eukaryotic Cell types contain receptor proteins that, upon binding to various signaling molecules (such as Hormones, growth factors, and Neurotransmitters), Relay extracellular signals into the interior of the cell, thereby governing developmental processes and cellular function.

Finally, peripheral cytosolic proteins associated with the membrane function either as Enzymes, chemical signal transducers, or structural proteins that stabilize the membrane.

Specialized membranes surrounding each intracellular organelle likewise contain unique sets of proteins, similar to the plasma membrane, which are essential for the proper function of each specific organelle.

Due to pronounced cellular specialization, many membranes exhibit highly distinctive structures. For example, myelin serves as an insulating sheath surrounding Nerve Cells axons and consists of flattened Schwann cells layered upon one another. Myelin is readily isolated from other subcellular structures obtained from tissue homogenates through sucrose density-gradient centrifugation. Myelin plasma membranes lack any enzymatic or receptor activity, with their dominant protein component being a structural protein known as myelin basic protein.

The plasma membranes of nerve and Muscle cells—the so-called excitable membranes—are capable of transmitting electrical impulses owing to the presence of ion permeability channels.

Membrane channels open in response to Changes in the Membrane Potential. During the Propagation of Excitation from one region of a nerve cell to another, the permeability first increases for Na⁺ ions entering the cell, and subsequently for K⁺ ions exiting the cell. This coordinated ion flux generates the Action Potential.

Thus, the processes occurring within The cell membrane that determine its functional purpose are strictly directional (vectorial), a property enabled by the intricate Molecular Organization of biological membranes.