What is cell membrane ?
All living organisms are composed of cells. Cell membrane is one of the most essentials parts of a cell. It forms the outer covering of cells. The primary function of cell membrane is to manage the inflow and outflow of substances from the living cells. It is also responsible for a number of other cell functions. A cell membrane is similar in appearance to other membranes present in the cell, such as those membranes that cover the cell nucleus, etc.
Structure of cell membrane
All membranes, including the cell membrane consist of proteins, phospholipids and carbohydrates that are structurally arranged.
The proteins can be present on both sides of the phospholipid bilayer (known as integral proteins) or can be seen on the surface (known as peripheral proteins). They can rapidly move around cell membrane and smash into each other, but they can never jump from one side to another. Proteins are composed of hydrophobic amino acids that work with the fatty chains within the membrane, and hydrophilic amino acids that work with water present outside the membrane. A majority of cell membrane properties are determined by proteins as they comprise of nearly half of cell membrane mass.
Cell mobility and cell shape are maintained by proteins present on the inside cell membrane surface. They may connect to the cytoskeleton or may be enzymes that facilitate activities in the cytoplasm
Integral proteins perform the function of carrying items through the membrane.
Proteins present on the outside cell membrane surface may perform the function of receptors by providing bonding locations for chemicals and hormones to combine, which in turn cause reactions in the cell. They may also act as enzymes or carry out cell recognition and signaling activities.
The phospholipids are assembled in a bilayer, wherein the non-polar, hydrophobic fatty acid tails face one another in the center of the bilayer, and the polar, hydrophilic phosphate heads are facing outwards. The two layers are isolated from each other due to the restrictive properties of the hydrophobic layer that allows the entry of only the smallest molecules. Various types of membranes have phospholipids that contain different fatty acids. This affects the flexibility and strength of the cell membrane.
The carbohydrates are present on the outer plane of every eukaryotic cell membrane and are joined to the proteins in the cell membrane; or to phospholipids on rare occasions. Proteins attached to carbohydrates are known as glycoproteins, whereas phospholipids attached to carbohydrates are called glycolipids. Carbohydrates are tiny polysaccharides that consist of many different monosaccharides and create a cell coat on the outer part of cell membrane. They are responsible for cell recognition and protection.
What are the functions of the cell membrane ?
The basic function of the cell membrane is to act as an impediment for the entry and exit of most substances from the cell. This function allows reactions to take place inside the cell without any interference.
But there is a need for substances to enter and leave the cells, as is the case with the complete body, which also requires items to enter and leave the body. There are five ways, through which matter can move across the cell membrane. They are:
- Lipid Diffusion
- Passive Transport
- Active Transport
Certain substances like lipid-soluble molecules (steroids) and tiny molecules like water and oxygen can diffuse through the cell membrane lipid bilayer. The cell membrane does not act as a barrier for such matter. Lipid diffusion does not require any energy and the cell cannot control this process.
In passive transport, matter is transferred through the cell membrane via a protein molecule. The substance can get transported into a cell, only if has that particular transport protein. Passive transport does not require energy and like osmosis, matter can only shift down their concentration gradient
Two types of transport proteins are:
- Channel Proteins create water-filled holes in the cell membrane to allow charged particles to pass through. Such open channels can be closed by the cell, and hence the passage of ions is controlled by it.
- Carrier Proteins consist of a binding region for a specific solute. They continuously change between two states to facilitate the passage through the cell membrane. The matter will bond on the high concentration site and release at a lower concentration site.
It is similar to lip diffusion, but since the water content in the body is so large, diffusion of water has its own name, i.e. osmosis. The cell and the cell membrane consist of a number of substances that increase the concentration of matter in the cell. Water dilutes this concentration level. Osmosis or transfer of water molecules automatically takes place from an area where the concentration is less (dilute) to an area (cell, cell membrane) which is more concentrated.
In active transport, matter is transported across cell membranes via a protein pump molecule. Such a protein bonds with a particular substances, alters shape and releases it on the other side of the cell membrane. Such proteins are very specific and are differ for each type of matter molecule. Active transport can transfer substances above their concentration gradient.
Vesicles are used to transport large molecules to and from the cells. In this, process the large molecules are pinched by cell membrane and then enclose them within a vesicle. The molecule is then broken down into smaller molecules and the above processes are then employed for their entry or exit from within the cell.
Cell membrane diagram