Final answer:
Membranes define specific boundaries to control what enters and exits the cell, maintain homeostasis, and enable cellular recognition necessary for development and immune response. The plasma membrane is selectively permeable, flexible, and has organizational markers for cellular interactions.
Step-by-step explanation:
Membranes define specific boundaries to maintain homeostasis within a cell, to allow selective permeability, and to facilitate cellular recognition. The plasma membrane is essential for determining what enters and exits the cell, as some substances are excluded, and others are taken in, all in a controlled manner. This selectivity is crucial for the cell's survival and proper functioning.
The flexibility of the plasma membrane is vital for certain cells like red and white blood cells to change shape when navigating through narrow capillaries. Additionally, the plasma membrane carries markers that are important for cells to recognize one another, crucial for tissue and organ formation during development and for the immune response to distinguish between 'self' and 'non-self'.
Characteristics like fluidity and the ability to change shape are essential for various functions. For instance, the membrane's dual nature as a 'liquid crystal' allows it to be partially rigid for organization and somewhat fluid for movement and shape alteration.