It is a protective covering that acts as a barrier between the internal and external environment of a cell (in animals).
In-plant cells, the membrane encapsulates the protoplasm.
This organelle is also known as the plasma membrane. The characteristic of this organelle is that it only allows the passage of certain substances.
Most of the investigations carried out to study the cell membrane structure use red blood cells (RBC) since the absence of internal membranes and nuclei in RBC results in isolation is carried out quite quickly.
The cell membrane is also known as the plasma membrane or plasmalemma; The cell membrane is one of the vital parts of a cell that encloses the internal organelles. This membrane separates the inside of a cell from the outside environment.
A cell membrane physically separates the contents of the cell from the outside environment, but in plants, fungi, and some bacteria, a cell wall surrounds this membrane. However, the cell wall acts as solid mechanical support only.
The actual function of the cell membrane is the same in both cases and is not significantly altered by the mere presence of a cell wall.
Structure and composition
The cell membrane comprises two layers of phospholipids, and each phospholipid molecule has a head and a pair of tails. The head region is hydrophilic (attraction toward water molecules), and the tail ends are hydrophobic (it stays away from water molecules).
Both phospholipid layers are arranged so that the head regions form the outer and inner surface of this membrane, and the tail ends approach the center of the cell membrane.
The cell membrane houses different types of protein molecules, which are built into the phospholipid layer. Most of these protein molecules, as well as phospholipids, are capable of lateral movement.
Membrane proteins can be classified into three main subdivisions:
- Whole proteins.
- Anchored to lipids.
The integrals span the entire width of the cell membrane, while the peripherals are found on its inner or outer surfaces. Those in the third category are anchored to the membrane with the help of lipid molecules.
While some of these protein molecules provide structural support to the membrane, others bind to the cytoskeleton suspended in the cytoplasm. There are specific proteins responsible for the transport of ions and molecules across the cell membrane.
Some of these proteins have other functions, such as:
- Cell-to-cell communication.
- Enzymatic activities.
The main components of the cell membrane are phospholipids and proteins. However, it has cholesterol molecules that make the membrane rigid and flexible.
They also make it difficult for water-soluble substances to pass through the membrane. On the cell membrane’s outer surface, glycolipids and glycoproteins are found. They are nothing more than lipids and protein molecules attached to short-chain carbohydrates.
All of these components work together to carry out the cell membrane functions.
Cell membrane functions
Anchor the cytoskeleton
A cell membrane functions as an enclosure for the internal organelles and protects them. This function is vital in animal cells, which lack a cell wall. This membrane anchors the cytoskeleton (a cellular “skeleton” made of protein and contained in the cytoplasm) and shapes the cell.
The cytoskeleton microfilaments are attached to specific proteins in the cell membrane, especially the integral ones. It has also been suggested that these microfilaments hold proteins in place since the latter tend to move.
The cell membrane is responsible for the transport of molecules and ions in and out of the cell. The semi-permeable membrane allows specific molecules to move freely through it.
Most of the small hydrophobic molecules (without affinity for water) pass-through this membrane freely. Some of the small hydrophilic molecules can also be successful. But others have to be transported across the membrane.
Molecules’ movements across the membrane may or may not require the use of cellular energy. These movements through the plasma membrane can be divided into three types: passive, active, and bulk transport.
Passive transport does not require cells to expend energy. It can occur in simple diffusion, facilitated diffusion, or osmosis.
Simple diffusion refers to the movement of molecules through a membrane, from an area of higher concentration to one of lower concentration. Such activity continues until both sides have a uniform concentration.
Certain factors affect the rate of passive or simple diffusion. A faster movement may be noticed in the case of a significant concentration difference (between the inner and outer sides) or if the temperature is high.
The smaller the molecules, the faster they can pass through the membrane. In the case of osmosis also, the water (solvent) moves through a semi-permeable membrane, in case one side of the membrane has a higher concentration of solute (solute is a substance dissolved in a liquid solvent) than the other.
In this case, the movement of the solvent molecules is from a less concentrated solution to a more focused part. Osmosis is also a form of passive diffusion.
A type of passive transport facilitates diffusion. In facilitated diffusion, molecules pass through channels in specific transport proteins. Even this type of diffusion does not require any energy.
However, if the molecules being transported are too large or the intended motion is against the gradient (from low to high concentration), energy must be expended.
In such cases, the molecules/ions are identified by specific proteins before they are transported across the plasma membrane by another set of proteins that obtain energy from ATP, and this is called active transport.
In this type of transport, proteins are selective and specific. These transport proteins have openings at one end, through which molecules or ions enter and bind to functional groups within the protein.
Mass transport is often done with the help of vesicles. The transport of materials out of cells is called exocytosis.
If the transport is from the outside to the inside, the process is called endocytosis, which can be of three types:
- Mediated by receptors.
In endocytosis, the plasma membrane creates a slight depression (pseudopodium), in which the materials to be transported gather to form a cyst. The vesicle moves to the inner surface of the cell membrane and then fuses with the Golgi apparatus.
Phagocytosis is the transport of solids; pinocytosis refers to the movement of liquids transported within the vesicles.
Receptor-mediated endocytosis is a complex way membrane receptor proteins bind to materials to be transported. Only specific molecules/ions can be transported through this method.
In exocytosis, the vesicles move to the inner surface of the plasma membrane, pass through it and open to the outside so that the contents are released outside the cell.
The ruptured vesicles fuse with the plasma membrane. In addition to transporting materials out of the cell, exocytosis is also helpful in restoring the plasma membrane. Vesicles for exocytosis are formed from the endoplasmic reticulum or the Golgi complex.
These vesicles full of materials to be expelled are transported from the internal regions to the periphery with the help of the cytoskeleton.
Cell transport is one of the vital functions of the plasma membrane. In addition to supporting the cytoskeleton and transporting molecules and ions, cell membranes also have other functions.
Interaction with other cells
The membrane is also responsible for attaching the cell to the extracellular matrix (non-living material found outside of cells) so that cells can clump together to form tissues.
Communication with other cells
Protein molecules in the cell membrane receive signals from other cells or the external environment and convert the signals into messages that are passed to organelles within the cell.