Cell membranes or plasma membranes are the boundaries that separate cells from other cells or from objects around them. The structure of the cell membrane consists of a thin layer about 8 nm thick.
1 nm = 0.000000001 M
The function of cell membranes is to control the traffic of substances that enter and exit the cell.
Like other biological membranes, the plasma membrane also has selective permeable properties. Permeable selective properties are the ability of the membrane to select some substances that can pass easily and other substances cannot pass through it.
Membranes limit a solution that has a different composition from the surrounding solution, but can still absorb nutrients and waste disposal.
The structure of the cell membrane consists of a phospholipid, hydrophilic, and hydrophobic layer. You can see the structural picture in the picture below:
The structure of the cell membrane consists of a phospholipid, hydrophilic, and hydrophobic layer
The ability of cells to differentiate a chemical is a fundamental ability that is needed for life, and the plasma membrane is a part of the cell that has this ability to be effective.
Function of cell membranes
The cell membrane functions as a semipermeable barrier that allows small molecules to enter the cell. The results of electron microscope observations on cell membranes indicate that the cell membrane is a lipid bilayer . (referred to as the fluid-mosaic model ).The main constituent molecule is phospholipid , which consists of a polar ( hydrophilic ) head and two nonpolar ( hydrophobic ) heads. These phospholipids are composed of nonpolar parts forming a hydrophobic area flanked by the area of the membrane which is on the inside and outside of the membrane.
Hydrophilic = likes of water
Hydro phobic = expel water
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Structure of cell membranes
The structure of the cell membrane is composed of fat and protein in which each component is bound by a non-covalent bond. In addition to fat and protein, the structure of the cell membrane also consists of carbohydrates.The ratio between fat and protein varies depending on the type of cellular membrane. For example between the plasma membrane and the endoplasmic reticulum. The types of prokaryotic and eukaryotic organisms also have different structural ratios. The mitochondrial membrane has a high protein / fat ratio compared to the plasma membrane in red blood cells.
Lipids on the membrane are composed of phospholipids (phosphate-compounded fats). Phospholipids are the most abundant lipids in most membranes. Phospholipids play a role in forming membranes in accordance with their molecular structure.
Phospholipid is an amphiphatic molecule which means that this molecule has both hydrophilic and hydrophobic regions.
Most membranes contain phosphate. Phosphate molecules are hydrophilic (can bind water) while fat molecules are hydrophobic (cannot bind water).
Another fat component is cholesterol wherein certain animals can reach 50% of the fat molecules contained in the plasma membrane. Cholesterol is not present as a large membrane of plant plasma and bacteria.
Lipids contained in the membrane can be extracted with chloroform, ether and benzene. By using thin layer chromatography and gas chromatography, the lipid composition of the cell membrane can be determined. Lipids that are always found are phospholipids, sphingolipids, glycolipids and sterols. Cholesterol is the most lipid that makes up the cell membrane.
Carbohydrate
Carbohydrates in the membrane act as constituents of cells and to differentiate the types of cells around them. Carbohydrates also play an important role as selector of cells that make up various tissues and organs in animal embryos.Cell recognition is also the basis for rejection of foreign cells (rejection of transplanted or transplanted organs) by the immune system.
Carbohydrates in membranes are usually short branched chains composed of less than 15 sugar units, some of which bind covalently to lipids, forming a molecule called glycolipids. But most carbohydrates bind covalently to proteins, forming glycoproteins.
Protein
Protein membranes are composed of glycoproteins or proteins that are compounds with carbohydrates. Depending on the type of cell and certain organelles in the cell, the membrane has 12 to more than 50 different types of proteins. This protein is not randomly arranged but each location and orientation is arranged in a certain position on the lipid bilayer.Proteins in the membrane are asymmetrical on the outside of the membrane and inside of the membrane, aka arranged in different positions. This position allows the outer membrane to interact with extracellular ligands such as hormones and growth factors, while the inside can interact with cytoplasmic molecules such as protein G or protein kinase. There are two main layers of protein membranes.
Integral protein
Integral proteins are proteins that mix into lipid bilayers. This protein can penetrate the membrane so that it has a domain on the extra cellular and cytoplasmic side of the membrane. Integral proteins are generally transmembrane proteins, with a hydrophobic area that extends throughout the interior of the hydrophobic membrane.The hydrophobic area of integral proteins consists of one or more ranges of nonpolar amino acids, which usually coils into helix a on the hydrophilic end of this molecule is exposed to aqueous solubility on both sides of the membrane.
Peripheral protein
Peripheral protein is not found in lipid bilayers at all. All of them are found on the outside of the lipid bilayer, both on the extracellular and cytoplasmic adjoining surfaces and are related to the membrane through non-covalent bonds. This protein is a member that is loosely bonded to the surface of the membrane, often also in the integral part of the protein that is left exposed. Proteins in the membrane determine most of the specific functions of the membrane.Lipid anchor protein
The cell structure consists of many parts such as carbohydrates, glycoproteins, proteins, cholesterol and others Lipid anchor proteins are found outside the lipid bilayer but bind covalently to the fat molecules found in lipid bilayers.Plasma membrane protein has a very broad function, among others, as a protein carrier (carrier) compounds through cell membranes, recipients of hormonal signals and forward the signal to the cell itself or other cells. Plasma membrane protein also functions as a binding component of the cytoskeleton with extracellular compounds.
Outer surface proteins provide individual cell characteristics and protein types can change according to cell differentiation. Proteins in many cell membranes also function as enzymes, especially those found in the mitochondrial membrane, endoplasmic reticulum and chloroplasts. For example, plasma membrane phospholipid compounds are synthesized by enzymes found in the membrane of the endoplasmic reticulum.
Cell membrane proteins have the ability to move, so they can move. The displacement takes place laterally by diffusion. But not all pseudo proteins can move. Several types of integral proteins are retained in the membrane by woven protein molecules that are just below the inner surface of the plasma membrane. This webbing is related to the cytoskeletal or cell skeleton.
The physico-chemical structure of the cell membrane protein is less known, given that its shape varies greatly. Based on microscopic studies and freeze fracture techniques it is known that proteins in cell membranes are globular shaped.
Compartment
The plasma membrane divides the protoplasm into several compartments (spaces). Cell membranes wrap around the entire protoplasm. The membrane of the nucleus separates the nucleoplasm from the stoplasma.In addition the plasma membrane divides the cytoplasm into several compartments called organelles. The presence of this limiting membrane is very important because it allows the activities of each compartment to take place without interference from other compatments but can still work together.
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