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The plasma membrane is characterized by its fluid and semi-permeable structure, which is fundamental to its function in living cells. This fluidity is primarily due to the phospholipid bilayer, where the phospholipids can move laterally within the layer, allowing for flexibility and the ability to adapt to changing conditions. The semi-permeable nature of the plasma membrane means that it selectively allows certain substances to pass through while restricting others, thereby maintaining homeostasis within the cell.

This selective permeability is critical for various cellular processes, including the transport of nutrients, waste products, and signaling molecules. The presence of embedded proteins and cholesterol further contributes to this property, with proteins facilitating the transport of materials and cholesterol helping to maintain membrane stability and fluidity. Overall, the fluid and semi-permeable characteristics of the plasma membrane enable it to perform essential functions in cell communication, nutrient exchange, and protection against harmful substances.