HUMAN MEDICAL BIOLOGY, ANATOMY, PHYSIOLOGY, AND PATHOLOGY - Y.I. Fedoniuk 2010

BIOLOGY

CHAPTER 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.3. MOLECULAR-GENETIC AND CELLULAR LEVELS OF ORGANIZATION OF LIFE

1.3.2. Structural, Chemical, and Functional Organization of Eukaryotic Cells

Cell/30.html">The Plasma membrane and Transport of substances Across It

The plasma membrane occupies a unique position among cellular membranes. Located at The surface of the Cytoplasm, it separates The Cell interior from the surrounding environment. This positioning brings it into direct contact with all substances acting upon the cell. The plasma membrane is extremely thin (10 nm) and can only be visualized using an Electron microscope. In animal Cells, the plasma membrane is covered by the glycocalyx and underlined by the cortical layer. The glycocalyx is a supramembrane layer in animal cells, 3–4 nm thick, formed by long, branched oligosaccharide molecules linked to Membrane Proteins and Lipids. The cortical layer consists of a zone of cytoplasm directly adjacent to the plasma membrane. It contains microfilaments and microtubules, which provide cytoskeletal and locomotory properties to the cell surface. Together, the glycocalyx, plasma membrane, and cortical layer form the cell coat (Plasmalemma) of an animal cell.

The Structural Organization of the plasma membrane follows the same principle as other cellular membranes, featuring a trilaminar Structure. The plasma membrane performs various Functions, the primary ones being barrier and transport functions. The plasma membrane contains pores and exhibits selective semipermeability. By selectively allowing certain substances to pass while blocking others, it regulates The chemical composition of the cell, thereby maintaining Homeostasis. The transport of substances across the plasma membrane is categorized into passive transport, Active Transport, and cytosis (exocytosis and endocytosis).

Passive transport occurs via osmosis (the Movement of Water) and diffusion. Diffusion is divided into two types: simple and facilitated. In simple diffusion, small hydrophilic molecules (such as CO₂, O₂, glycerol, and urea) pass through membrane pores along a concentration gradient (from a region of higher concentration to one of lower concentration) without Energy Expenditure. In Facilitated Diffusion, as in simple diffusion, substance molecules are always transported along a concentration gradient. This process also does not require energy, but it is mediated by carriers. These carriers can be receptor proteins that bind Hormones and Vitamins on the cell surface, and they are known as permeases.

Active transport is the movement of substances across membranes against a concentration gradient, mediated by carriers and utilizing energy in the form of ATP or electrochemical potentials generated by concentration differences of ions inside and outside the cell. A classic example of active transport is the sodium-potassium pump. This transmembrane protein utilizes ATP energy to transport Na⁺ ions out of the cell (where they are already abundant) and, conversely, K⁺ ions from the outside into the cell (where potassium is concentrated).

Cytosis refers to the Transport of Macromolecules or particles (such as proteins, membrane fragments, and foreign substances) across The cell membrane. There are two types of cytosis: exocytosis and endocytosis. If substances are transported into the cell, the process is called endocytosis; if they are moved out of the cell, it is exocytosis. Endocytosis is carried out via phagocytosis or pinocytosis. Phagocytosis is the process by which a cell captures and engulfs solid particles. Phagocytosis is widely distributed in nature. Through phagocytosis, Protozoa (such as amoebas) and certain lower invertebrates capture and digest food. In higher animals and humans, phagocytosis plays a protective role. Leukocytes use phagocytosis to engulf and destroy pathogenic microorganisms. The phenomenon of phagocytosis was discovered by the prominent Ukrainian scientist I.I. Mechnikov, the author of the cellular theory of Immunity (Nobel Prize, 1908). Pinocytosis is The process of capturing and engulfing droplets of fluid containing dissolved substances. Phagocytosis and pinocytosis are similar processes; in both cases, substances at the cell surface are enveloped by the plasma membrane and internalized as vesicles. Once inside the cell, pinocytotic or phagocytotic vesicles fuse with Lysosomes, where the engulfed substances are digested by hydrolytic Enzymes. Both phagocytosis and pinocytosis require energy expenditure. The plasma membrane also participates in the export of substances from the cell—such as hormones, Polysaccharides, proteins, and fat droplets (exocytosis). These substances are enclosed in membrane-bound vesicles and transported to the plasmalemma. The two membranes fuse, and the vesicle contents are released into the extracellular environment. Both exocytosis and endocytosis are associated with cytoplasmic streaming.

In addition to barrier and transport functions, the plasma membrane performs several other vital functions: 1) it forms Intercellular junctions, enabling cellular communication; 2) membrane receptor proteins perceive chemical and physical stimuli and transmit signals into the cell; 3) it participates in membrane-associated Digestion (in the epithelium of Small Intestine villi); 4) it generates electrochemical potentials across the membrane; 5) it mediates immunological reactions.

Alterations in plasma membrane properties underlie a wide range of pathological states. Membrane dysfunction can be either the cause or the consequence of pathological processes. The Development of diseases such as malignant tumors, atherosclerosis, radiation sickness, burn disease, and immune system disorders is closely associated with Changes in the structural properties of Membrane Lipids and receptor proteins. The Introduction of new pharmacological agents into clinical practice is carried out only after comprehensive testing of their interactions with cellular membranes.