The following points highlight the top five functions of plasma membrane. The functions are: 1. Providing a Selectively Permeable Barrier 2. Transporting Solutes 3. Transporting Macromolecules 4. Responding to External Signals 5. Intercellular Interaction 6. Energy Transduction.
Plasma Membrane: Function # 1.
Providing a Selectively Permeable Barrier:
Plasma membrane prevents the free interchange of materials from one side to the other, and it ensures that the appropriate substances are allowed into the cytoplasm from the external space and the inappropriate substances are kept out.
Plasma Membrane: Function # 2.
In all types of cells, there exists a difference in ionic concentration with extracellular medium. The plasma membrane contains the machinery for physically transporting substances from one side of the membrane to another. Transport across the membrane may be active or passive and occur via the phospholipid bilayer or by the help of specific integral membrane proteins, called permeases or transport proteins.
(i) Passive Transport:
It is a type of diffusion in which an ion or molecule crossing a membrane moves down its electrochemical or concentration gradient.
It is of following types:
The process by which the water molecules pass through a semipermeable membrane from a region of higher water concentration to the region of lower water concentration is known as osmosis.
(b) Simple Diffusion:
In simple diffusion, transport across the membrane takes place without the help of any permease and occurs only in the direction of concentration gradient. During simple diffusion, a small molecule in aqueous solution dissolves into the phospholipid bilayer, crosses it and then dissolves into the aqueous solution on the opposite side. There is little specificity to the process.
(c) Facilitated Diffusion:
In this special type of passive transport, ions or molecules cross the membrane rapidly with the help of permeases in the membrane. It occurs only according to concentration gradient and is very specific, i.e., each facilitated diffusion protein transports only a single type of ion or molecule.
Currently, it is believed that transport proteins form the channels through the membrane that permit certain ions or molecules to pass across the membrane. For example, Ca++ channels occur in axonal membranes for the entrance of Ca++ in the cell and glucose permease in the mammalian RBC facilitates the diffusion of glucose into the cell.
(ii) Active Transport:
In this process, ions or molecules move across the membrane against the concentration gradient using metabolic energy. It is done with specific transport proteins, called pumps. For example, Na+-K+-ATPase – it is an ion pump or cation exchange pump which is driven by energy of one ATP molecule to export three Na+ outside the cell in exchange of the import of two K+ inside the cell.
Plasma Membrane: Function # 3.
Cells routinely import and export large molecules across the plasma membrane through different processes like:
Macromolecules such as proteins, lipids or carbohydrates are secreted out from the cell by exocytosis.
The process of ingestion of large-sized solid substances (e.g., bacteria, food, parts of broken cells etc.) by the cell is known as phagocytosis.
In endocytosis, small regions of the plasma membrane fold inwards or invaginates, until it has formed new intracellular membrane limited vesicles.
In eukaryotes two types of endocytosis can occur:
(a) Pinocytosis which is the nonspecific uptake of small droplets of extra-cellular fluid by endocytosis vesicles or
(b) Receptor-mediated endocytosis which is a specific endocytosis, in which a receptor on the surface of the plasma membrane “recognizes” an extracellular macromolecule and binds with it.
Unlike pinocytosis, which is a constitutive process that occurs continuously, the phagocytosis is a triggered process. However, both phagocytosis and pinocytosis are active mechanisms in the sense that the cell requires energy for their operation.
Plasma Membrane: Function # 4.
Responding to External Signals:
The plasma membrane plays a critical role in the response of a cell to external stimuli, a process known as signal transduction. Membrane possesses receptors that combine with specific molecules or ligands having a complimentary structure.
The interaction of a membrane receptor with the ligand may cause the membrane to generate a new signal that stimulates or inhibits internal activities. For example, signals generated at the plasma membrane may tell a cell to manufacture more glycogen, to prepare for cell division etc.
Plasma Membrane: Function # 5.
Being situated at the outer edge of living cell, the plasma membrane mediates the interactions that occur between the cells of a multicellular organism. The plasma membrane allows cells to recognize one another, to exchange materials and information.
Plasma Membrane: Function # 6.
Membranes are intimately involved in energy transduction, a process, by which one type of energy is converted to another type. For example, during photosynthesis energy in sunlight is absorbed by membrane bound pigments and converted into chemical energy contained in carbohydrates.