Final answer:
The plasma membrane is a selectively permeable barrier that manages substance entry and exit from the cell, maintains cellular structure, and facilitates communication and recognition, which are essential for immune response and development.
Step-by-step explanation:
The plasma membrane, often referred to as the cell membrane, is a critical part of a cell, providing a barrier between the internal cytoplasm and the external environment. Acting as the gatekeeper of the cell, the plasma membrane's selective permeability allows it to regulate what substances can enter and leave the cell. This regulation of traffic is essential for maintaining the cell's integrity and controlling its interaction with its surroundings.
The plasma membrane is also responsible for the cell's shape, enabling certain cells, like red blood cells and white blood cells, to alter their form to pass through narrow spaces like capillaries. It supports cellular communication and recognition, a critical aspect for the immune response and developmental processes. Proteins like receptors on the plasma membrane play roles in transmitting signals and interacting with hormones, while in some cases, pathogens such as viruses can exploit these receptors to enter the cells.
The composition of the plasma membrane includes lipids and proteins, which are arranged flexibly to fulfill its functions—the concept of the fluid mosaic model proposed by S.J. Singer and Garth L. Nicolson in 1972. This model explains the dynamic nature of the plasma membrane, which is continually in flux, adapting to environmental conditions and cellular demands.