In 1972, a theory was put forward that a partially permeable membrane surrounds a cell and performs a number of vital tasks, and the structure and functions of cell membranes are significant questions regarding the proper functioning of all cells in the body. Cell theory was widely adopted in the 17th century, along with the invention of the microscope. It became known that plant and animal tissues consist 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 its basis is lipids.
The structure and functions of cell membranes
The cell membrane surrounds the cytoplasm of living cells, physically separating intracellular components from the 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 in the formation of the cytoskeleton and the attachment of other vital particles to the extracellular matrix. This is necessary in order to hold them together, forming 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 integrated proteins. This part is involved in processes such as cell adhesion, ionic conductivity and signaling systems and serves as the attachment surface for several extracellular structures, including the wall, glycocalyx and internal cytoskeleton. The membrane also retains 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 penetrate the plasma membrane by diffusion. The shell acts as a barrier to certain molecules and ions; they can concentrate on both sides.
2. Transmembrane protein of channels and transporters: nutrients, such as glucose or amino acids, must enter the cell, and some metabolic products must leave it.
3. Endocytosis is the process by which molecules are absorbed. A small deformation (invagination) is created in the plasma membrane in which the substance to be transported is swallowed. This requires energy and, therefore, is a form of active transport.
4. Exocytosis: occurs in various cells to remove undigested residues of substances brought by endocytosis, to secrete substances such as hormones and enzymes, and transport the substance completely through the cell barrier.
Molecular structure
The cell membrane is a biological membrane, consisting mainly of phospholipids and separating the content of the entire cell from the external environment. The process of education occurs spontaneously under normal conditions. To understand this process and correctly describe the structure and functions of cell membranes, as well as properties, it is necessary to evaluate the nature of phospholipid structures, for which structural polarization is characteristic. 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 formation, membrane breakdown is unlikely.
- Strength. The lipid shell is reliable enough to prevent the passage of the 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, if we consider the structure of the cell. The cell membrane can undergo various deformations, can be folded and bent and not collapse. Under special circumstances, for example, when vesicles merge or budding, it can be broken, but only for a while. At room temperature, its lipid constituents are in a constant, chaotic motion, forming a stable fluid boundary.
Liquid Mosaic Pattern
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 contribute to the mosaic pattern for the membrane, and they are capable of lateral movement in the plane due to the variable nature of the lipid organization. Transmembrane proteins are also potentially mobile. An important feature of the membrane structure is its asymmetry. What is the structure of the cell? Cell membrane, nucleus, proteins and so on. A cell is the basic unit of life, and all organisms consist of one or many cells, each of which has a natural barrier that separates it from the environment. This outer cell boundary is also called a plasma membrane. It consists of four different types of molecules: phospholipids, cholesterol, proteins and carbohydrates. The liquid mosaic model describes the structure of the cell membrane as follows: it is flexible and elastic, similar in texture to vegetable oil, so that all individual molecules simply float in a liquid medium, and they are all able to move sideways within this membrane. 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 form the basic structure of the cell membrane. These molecules have two different ends: the head and tail. The head end contains a phosphate group and is hydrophilic. This means that it is attracted to water molecules. The tail consists of hydrogen and carbon atoms, called chains of fatty acids. These chains are hydrophobic; they do not like to mix with water molecules. This process resembles 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
Hearing the word "cholesterol", people usually think it's bad. However, in fact, cholesterol is a very important component of cell membranes. Its molecules consist of four rings of hydrogen and carbon atoms. They are hydrophobic and are found among hydrophobic tails in the lipid bilayer. Their importance is to maintain consistency, they strengthen the membrane, preventing intersection. Cholesterol molecules also keep phospholipid tails from coming into contact and hardening. This guarantees fluidity and flexibility. Membrane proteins act as enzymes to accelerate chemical reactions, act as receptors for specific molecules, or transport substances across a cell membrane.
Carbohydrates, or saccharides, are found only on the extracellular side of the cell membrane. Together they form glycocalyx. It provides cushioning and protection of the plasma membrane. Based on the structure and type of carbohydrates in 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 types of basic 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 in their cytoplasmic or extracellular surfaces. They are involved mainly as an attachment tool for integral proteins.
- Transmembrane. They perform enzymatic and signaling functions, and also modulate the basic structure of the lipid bi-layer of the membrane.
The functions of biological membranes
The hydrophobic effect, which regulates the behavior of hydrocarbons in water, controls the structures formed by membrane lipids and membrane proteins. Many membrane properties are conferred by carriers of lipid bi-layers, 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 ones, but they are also attached to membranes. Specialized cell membranes have specialized cell functions. How do the structure and functions of cell membranes affect the body? On how the biological membranes are arranged, the functionality of the whole organism depends. From the intracellular organelles, extracellular and intercellular interactions of the membranes, the structures necessary for organizing and performing 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 leads to the presence of a number of common characteristics. Membrane proteins have many specific functions.
- Controlling. The plasma membranes of the cells determine the boundaries of the interaction of the cell with the environment.
- Transport. The intracellular cell membranes are divided into several functional blocks with different internal composition, each of which is supported by the necessary transport function in combination with control permeability.
- Signal Transduction. The fusion of membranes provides a mechanism for intracellular vesicular warning and preventing various kinds of viruses from freely entering the cell.
Value and Conclusions
The structure of the outer cell membrane affects the entire body. It plays an important role in protecting integrity, allowing only selected substances to enter. It is also a good base for attaching 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 indicator varies depending on the type of membrane. The structure of the outer cell membrane of mammals is more complex, it contains four basic phospholipids. An important property of lipid bi-layers is that they behave as two-dimensional fluids in which individual molecules can freely rotate and move in lateral directions. 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 the fluidity of membranes. The selective permeability of biological membranes for 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 that the body is a self-regulating system that without outside help cannot harm itself and will always look for ways to restore, protect and properly function each cell.