1. Why is the plasma membrane called a "Fluid-Mosaic Model"? 2. Name two functions of the plasma membrane. 3. What factors contribute to the fluidity of the plasma membrane?

1. The plasma membrane is called a "Fluid-Mosaic Model" because it describes the structure of the membrane as a dynamic and flexible layer composed of various components. The "fluid" aspect refers to the ability of the lipid bilayer to move and flow, allowing proteins and lipids to move laterally within the layer. The "mosaic" aspect refers to the diverse array of proteins, cholesterol, and carbohydrates that are embedded or associated with the lipid bilayer, creating a patchwork appearance. This model emphasizes that the membrane is not a static structure but rather a complex and adaptable one. 2. Two functions of the plasma membrane are: - **Selective Permeability**: The plasma membrane regulates the entry and exit of substances, allowing essential nutrients to enter the cell while keeping harmful substances out. This selective permeability is crucial for maintaining homeostasis within the cell. - **Cell Communication**: The plasma membrane contains various receptors and proteins that facilitate communication between the cell and its environment. These receptors can detect signals from other cells or external stimuli, allowing the cell to respond appropriately. 3. Factors that contribute to the fluidity of the plasma membrane include: - **Lipid Composition**: The types of phospholipids present in the membrane can affect fluidity. Unsaturated fatty acids, which have one or more double bonds, create kinks in the fatty acid tails, preventing tight packing and increasing fluidity. In contrast, saturated fatty acids pack more tightly, reducing fluidity. - **Cholesterol Content**: Cholesterol molecules are interspersed within the phospholipid bilayer. At moderate temperatures, cholesterol helps to stabilize the membrane and maintain fluidity by preventing the fatty acid chains from packing too closely together. At low temperatures, cholesterol can prevent the membrane from becoming too rigid. - **Temperature**: The temperature of the environment can also influence membrane fluidity. Higher temperatures increase the kinetic energy of the molecules, leading to greater fluidity, while lower temperatures can decrease fluidity as the molecules move less freely.