A distinct boundary between the cell and its surroundings is found by microscopy, which is named the plasma membrane or cell membrane. It separates the cell from its surroundings. Lipids and proteins are the main components of cell membranes with a few sugars. Lipids account for about 50%, proteins for about 40%, and sugars for the rest. The fluid mosaic model is the current widely accepted model that describes the structure and behavior of the cell membrane. The phospholipid bilayer is the basic scaffold where protein molecules are embedded. It is not static, but similar to liquid crystal: not only can it flow like a liquid, but the basic membrane structure remains unchanged during the movement. This mobility is the basis for material exchange and information transfer in biomembrane.
Most of the lipids are amphoteric phospholipids that form a bilayer as the membrane backbone: their heads with hydrophilic phosphoric groups face the aqueous solution and hydrophobic fatty acid tails pointing toward the interior of the bilayer. The weak hydrophobic interactions and bent tails that result from double bonds make phospholipids are not very tightly arranged. So the cell membrane is not stationary, it can flow like a liquid. The amount of unsaturated fatty acids and their length play a key role in their fluidity. The more double bonds in fatty acids and the shorter fatty acid chains, the greater membrane fluidity, and vice versa. Temperature also has a significant effect on the membrane movement: if the temperature is too low, membrane will solidify and become immobile, and vice versa.
Steroids regulate membrane stability and fluidity
If there are no other molecules in the membrane, the lipid bilayer is an ordered repeating structure, which tends to become crystalline at low temperatures. steroids can be inserted into the membrane to separate adjacent lipids, disrupting the repetitive structure and reducing their ability to pack tightly together. This results in a membrane that flows even at low temperatures. The hydrogen bonding and hydrophobic interactions of steroids and lipids help enhance the stability and elasticity of cell membranes, allowing them to better resist stresses and changes. When the temperature is too high, these interactions hinder the free movement of molecules and prevent the membrane from becoming a disordered liquid state.
However, if the level of steroid is too high or too low, it will lead to instability. High levels of steroid can lead to excessive rigid of the membrane, making it stiff and difficult to transport substances. Low levels of steroid, on the other hand, can lead to excessive fluidity, making membrane loose and vulnerable to damage. Therefore, moderate steroid levels are essential to maintain the stability and function of cell membranes.
Proteins embedded in plasma membrane
The cell membrane is rich in proteins that perform a variety of functions. These proteins are divided into integral protein and peripheral proteins. Integral protein: Some are embedded in membrane with their tips exposed, while others are large proteins that span both sides of the membrane; their hydrophobic parts are located in the membrane center, while the hydrophilic parts are exposed on the surface. Peripheral proteins: they are not embedded in the membrane and are are bound to the protein hydrophilic part or lipid by ionic or other weaker bonds like appendage; these proteins detach from the cell membrane when solution temperature increases or ion concentration changes.
Protein movement in the plane of the membrane
Some proteins are attached to the cytoskeleton to keep in a stationary manner; others can move within the cell membrane in a number of ways, including lateral diffusion, rotational diffusion, and transverse diffusion (also known as flip-flop). The movement of proteins within the cell membrane is influenced by a variety of factors, including temperature, the size and shape of the protein, the lipid composition of the membrane, and the presence of other proteins or molecules within the membrane.
Lateral diffusion refers to the movement of proteins within the plane of the membrane, from one area of the membrane to another. This type of movement is relatively fast, and is the most common way the proteins move within the membrane.
Rotational diffusion refers to the rotation of a protein around its long axis within the membrane. This type of movement is relatively slow, and is thought to play a role.
Transverse diffusion, or flip-flop, refers to the movement of a protein from one side of the membrane to another. This type of movement is relatively rare, and requires the protein to pass through the hydrophobic interior of the membrane.
Membrane carbohydrates are generally short and branching chains. Most are covalently linked to proteins in the membrane to form glycoproteins. Others are covalently linked to lipids, which are called glycolipids. The wide variety of membrane sugar present in the extracellular plasma membrane plays a key role in cell-cell recognition. They vary from species to species, from individual to individual of the same species, and even from cell to cell of the same individual. For example, differences in glycoproteins give rise to four blood types, A, B, AB and O.