In 1972, the theory was put forward that a partially permeable membrane surrounds the cell and performs a number of vital tasks, and the structure and function of cell membranes are significant issues regarding the proper functioning of all cells in the body. Cell theory became widespread in the 17th century, along with the invention of the microscope. It became known that plant and animal tissues are composed of cells, but due to the low resolution of the device, it was impossible to see any barriers around the animal cell. In the 20th century, the chemical nature of the membrane was studied in more detail, it was found that lipids are its basis.
Structure and function of cell membranes
The cell membrane surrounds the cytoplasm of living cells, physically separating the intracellular components from the external environment. Fungi, bacteria, and plants also have cell walls that provide protection and prevent the passage of large molecules. Cell membranes also play a role information of the cytoskeleton and attachment to the extracellular matrix of other vital particles. This is necessary in order to hold them together, forming the tissues and organs of the body. Structural features of the cell membrane include permeability. The main function is protection. The membrane consists of a phospholipid layer with embedded proteins. This part is involved in processes such as cell adhesion, ion conduction, and signaling systems and serves as an attachment surface for several extracellular structures, including the wall, glycocalyx, and internal cytoskeleton. The membrane also maintains the potential of the cell by acting as a selective filter. It is selectively permeable to ions and organic molecules and controls the movement of particles.
Biological mechanisms involving the cell membrane
1. Passive diffusion: Some substances (small molecules, ions), such as carbon dioxide (CO2) and oxygen (O2), can diffuse through the plasma membrane. The shell acts as a barrier to certain molecules and ions that can be concentrated on either side.
2. Transmembrane channel and transporter protein: Nutrients such as glucose or amino acids must enter the cell, and some metabolic products must leave.
3. Endocytosis is the process by which molecules are taken up. A slight deformation (invagination) is created in the plasma membrane, in which the substance to be transported is swallowed. It requiresenergy and thus is a form of active transport.
4. Exocytosis: occurs in various cells to remove undigested residues of substances brought by endocytosis in order to secrete substances such as hormones and enzymes and transport the substance completely through the cellular barrier.
Molecular structure
Cell membrane is a biological membrane, consisting mainly of phospholipids and separating the contents of the entire cell from the external environment. The formation process occurs spontaneously under normal conditions. In order to understand this process and correctly describe the structure and functions of cell membranes, as well as properties, it is necessary to assess the nature of phospholipid structures, which are characterized by structural polarization. When phospholipids in the aquatic environment of the cytoplasm reach a critical concentration, they combine into micelles, which are more stable in the aquatic environment.
Membrane properties
- Stability. This means that after the formation of the membrane is unlikely to collapse.
- Strength. The lipid membrane is reliable enough to prevent the passage of a polar substance; both dissolved substances (ions, glucose, amino acids) and much larger molecules (proteins) cannot pass through the formed border.
- Dynamic character. This is perhaps the most important property when considering the structure of the cell. The cell membrane canbe subjected to various deformations, can be folded and bent without collapsing. Under special circumstances, such as the fusion of vesicles or budding, it can be broken, but only temporarily. At room temperature, its lipid constituents are in constant, chaotic motion, forming a stable fluid boundary.
Liquid Mosaic Model
Speaking about the structure and functions of cell membranes, it is important to note that in the modern view, the membrane as a liquid mosaic model was considered in 1972 by scientists Singer and Nicholson. Their theory reflects three main features of the membrane structure. Integral membrane proteins provide a mosaic template for the membrane, and they are capable of lateral in-plane movement due to the variable nature of lipid organization. Transmembrane proteins are also potentially mobile. An important feature of the membrane structure is its asymmetry. What is the structure of a cell? Cell membrane, nucleus, proteins and so on. The cell is the basic unit of life, and all organisms are made up of one or more cells, each with a natural barrier separating it from its environment. This outer border of the cell is also called the plasma membrane. It is made up of four different types of molecules: phospholipids, cholesterol, proteins, and carbohydrates. The liquid mosaic model describes the structure of the cell membrane as follows: flexible and elastic, similar in consistency to vegetable oil, so that everythingthe individual molecules simply float in the liquid medium, and they are all capable of moving sideways within that shell. A mosaic is something that contains many different details. In the plasma membrane, it is represented by phospholipids, cholesterol molecules, proteins and carbohydrates.
Phospholipids
Phospholipids make up the basic structure of the cell membrane. These molecules have two distinct ends: a head and a tail. The head end contains a phosphate group and is hydrophilic. This means that it is attracted to water molecules. The tail is made up of hydrogen and carbon atoms called fatty acid chains. These chains are hydrophobic, they do not like to mix with water molecules. This process is similar to what happens when you pour vegetable oil into water, that is, it does not dissolve in it. The structural features of the cell membrane are associated with the so-called lipid bilayer, which consists of phospholipids. Hydrophilic phosphate heads are always located where there is water in the form of intracellular and extracellular fluid. The hydrophobic tails of phospholipids in the membrane are organized in such a way that they keep them away from water.
Cholesterol, proteins and carbohydrates
When people hear the word "cholesterol", people usually think it's bad. However, cholesterol is actually a very important component of cell membranes. Its molecules consist of four rings of hydrogen and carbon atoms. They are hydrophobic and occur among the hydrophobic tails in the lipid bilayer. Their importance lies inmaintaining consistency, they reinforce the membranes, preventing crossover. Cholesterol molecules also keep the phospholipid tails from coming into contact and hardening. This guarantees fluidity and flexibility. Membrane proteins act as enzymes to speed up chemical reactions, act as receptors for specific molecules, or transport substances across the cell membrane.
Carbohydrates, or saccharides, are found only on the extracellular side of the cell membrane. Together they form the glycocalyx. It provides cushioning and protection to the plasma membrane. Based on the structure and type of carbohydrates in the glycocalyx, the body can recognize cells and determine whether they should be there or not.
Membrane proteins
The structure of the cell membrane of an animal cell cannot be imagined without such a significant component as protein. Despite this, they can be significantly inferior in size to another important component - lipids. There are three main membrane proteins.
- Integral. They completely cover the bi-layer, cytoplasm and extracellular environment. They perform a transport and signaling function.
- Peripheral. Proteins are attached to the membrane by electrostatic or hydrogen bonds at their cytoplasmic or extracellular surfaces. They are involved primarily as a means of attachment for integral proteins.
- Transmembrane. They perform enzymatic and signaling functions, and also modulate the basic structure of the lipid bi-layer of the membrane.
Functions of biological membranes
The hydrophobic effect, which regulates the behavior of hydrocarbons in water, controls structures formed by membrane lipids and membrane proteins. Many properties of membranes are conferred by carriers of lipid bilayers, which form the basic structure for all biological membranes. Integral membrane proteins are partially hidden in the lipid bilayer. Transmembrane proteins have a specialized organization of amino acids in their primary sequence.
Peripheral membrane proteins are very similar to soluble proteins, but they are also membrane bound. Specialized cell membranes have specialized cell functions. How do the structure and functions of cell membranes affect the body? The functionality of the whole organism depends on how biological membranes are arranged. From intracellular organelles, extracellular and intercellular interactions of membranes, the structures necessary for the organization and performance of biological functions are created. Many structural and functional features are common to bacteria, eukaryotic cells, and enveloped viruses. All biological membranes are built on a lipid bilayer, which determines the presence of a number of common characteristics. Membrane proteins have many specific functions.
- Controlling. Plasma membranes of cells define the boundaries of the interaction of the cell with the environment.
- Transport. The intracellular membranes of cells are divided into several functional blocks with differentinternal composition, each of which is supported by the necessary transport function in combination with control permeability.
- Signal transduction. Membrane fusion provides a mechanism for intracellular vesicular notification and preventing various kinds of viruses from freely entering the cell.
Meaning and conclusions
The structure of the outer cell membrane affects the entire body. It plays an important role in protecting integrity by allowing only selected substances to penetrate. It is also a good base for anchoring the cytoskeleton and cell wall, which helps in maintaining the shape of the cell. Lipids make up about 50% of the membrane mass of most cells, although this varies depending on the type of membrane. The structure of the outer cell membrane of mammals is more complex, it contains four main phospholipids. An important property of lipid bilayers is that they behave like a two-dimensional fluid in which individual molecules can freely rotate and move laterally. Such fluidity is an important property of membranes, which is determined depending on temperature and lipid composition. Due to the hydrocarbon ring structure, cholesterol plays a role in determining membrane fluidity. The selective permeability of biological membranes to small molecules allows the cell to control and maintain its internal structure.
Considering the structure of the cell (cell membrane, nucleus, and so on), we can conclude thatthat the body is a self-regulatory system that cannot harm itself without outside help and will always look for ways to restore, protect and properly function each cell.
