PLANT BIOPHYSICS - Y. I. Posudin - 2004

I. PHYSICAL PROPERTIES OF PLANT CELLS AND TISSUES

2. MOLECULAR PROCESSES IN CELLS

2.1. BIOLOGICAL MEMBRANES

Membranes are highly organized cellular structures that separate The Cell from its external environment and participate in The formation of numerous intracellular Organelles. Membranes perform vital Functions, ensuring the integrity and activity of the cell. One of their key purposes is to form isolated, closed structures, or compartments. Furthermore, membranes are characterized by selective permeability, acting as selective barriers for both The Cell as a whole and its internal structures. The membrane surrounding the cell is termed the surface membrane (cell membrane, Plasma Membrane, Plasmalemma). This membrane forms a selective barrier that maintains the Homeostasis of the cell's various physicochemical properties; it mediates the Transport of substances and energy between the cell and its environment, as well as Intercellular Communication processes. Intracellular membranes compartmentalize specific organelles within the cell, such as the Cell Nucleus, METABOLISM/14.html">Chloroplasts, Golgi apparatus, Endoplasmic reticulum, and Mitochondria.

Membranes are composed primarily of Lipids, Proteins, and CARBOHYDRATES. According to modern concepts, Membrane Structure is described by the fluid-mosaic model, in which the membrane consists of a lipid bilayer. Membrane Lipids are phospholipid molecules, each having a polar hydrophilic HEAD and a non-polar hydrophobic tail. Globular proteins are embedded within The Lipid Bilayer. Proteins that span the bilayer or even cross it completely are called integral proteins. By spanning the bilayer, integral proteins form channels through which ions can pass. Peripheral proteins are located On the surface of the lipid layer. It is believed that Membrane Proteins are capable of lateral movement within the plane of the membrane. Lipids are also capable of lateral diffusion and flip-flop between the leaflets.

Short-chain carbohydrates are located on the outer surface of the membrane and play a crucial role in intercellular interactions and cell surface modification. Carbohydrates are attached to proteins and lipids, forming Glycoproteins and Glycolipids, respectively.