Membrane proteins contribute significantly to the functioning of the immune system. For instance, major histocompatibility complex (MHC) proteins, which are integral membrane proteins, play a crucial role in immune response. MHC proteins present antigenic peptides on the surface of cells, allowing immune cells to recognize and respond to foreign substances (antigens). Additionally, receptor proteins on the membranes of immune cells, such as T-cell receptors and B-cell receptors, are essential for recognizing specific antigens. These interactions initiate immune responses, including the activation of T-cells and B-cells and the production of antibodies. Furthermore, membrane proteins facilitate cell-to-cell communication in the immune system, allowing coordination and amplification of the immune response.

Yes, the Fluid Mosaic Model can explain the permeability of the membrane to water and ions. While the phospholipid bilayer is impermeable to most polar molecules and ions, specific proteins embedded in the membrane facilitate their transport. Aquaporins are integral membrane proteins that form channels specifically for water molecules, allowing for rapid and controlled water transport across the membrane. Similarly, ion channels and transporters are specialized proteins that regulate the passage of ions like sodium, potassium, and calcium. These channels can be gated, opening or closing in response to various stimuli, ensuring controlled ion flow in and out of the cell, vital for processes like nerve impulse transmission and muscle contraction.

Glycolipids are lipids with a carbohydrate attached, and they play a significant role in the structure and function of cell membranes. They are located on the outer layer of the phospholipid bilayer, contributing to the asymmetry of the membrane. The carbohydrate portion of glycolipids extends into the extracellular environment, where it plays a role in cell recognition and signalling. These molecules are involved in interactions between cells and their surroundings, including cell-to-cell recognition and adhesion. They also contribute to the stability of the membrane and can be involved in immune responses, acting as markers for cellular identification. Glycolipids, therefore, are integral to the membrane's role as a barrier and a mediator in cellular communication.

The 'mosaic' aspect of the Fluid Mosaic Model refers to the diverse array of proteins interspersed within the phospholipid bilayer of cell membranes. This mosaic of proteins is crucial for various cellular functions. These proteins include integral and peripheral proteins, each with specific roles like transport, enzyme activity, and cell signalling. The arrangement of these proteins is not fixed; they float within the fluid lipid bilayer, allowing the cell to dynamically adjust to changes in its environment. This distribution of proteins contributes to the membrane's functionality, including selective permeability, signal transduction, and cell-to-cell communication. Thus, the 'mosaic' nature illustrates the diversity and adaptability of the membrane in response to cellular needs.

Changes in the membrane's lipid composition can significantly affect its functions. The types of phospholipids, the degree of saturation of fatty acids, and the cholesterol content can alter the membrane's fluidity and, consequently, its behaviour. For example, a higher proportion of unsaturated fatty acids in the phospholipids increases fluidity, which can affect membrane protein functions, transport processes, and cell signalling. Conversely, a higher saturation level or increased cholesterol content can decrease fluidity, potentially hindering processes like endocytosis and the movement of membrane-bound proteins. Therefore, the lipid composition of the membrane is a critical determinant of its physical properties and its ability to perform various cellular functions effectively.