Figure 1 represents a plasma membrane and molecules present on either side.

Part A: Describe the biological need for cells to be surrounded by a membrane that is selectively permeable for different materials.

Part B: Explain how the model shows selective permeability of the membrane to specific ions.

Part C: Describe the characteristics of the phospholipid bilayer that permit small hydrophobic lipid molecules to pass directly

Part D: Based on the model, explain whether the molecules shown crossing the membrane require energy to do so.

A. Cell membrane is the outer layer of the cell that plays a role as the barrier between the internal and outer environment of the cell by regulating by allowing particles or substances to enter or exit the cell. It helps in maintaining the internal environment of the cell by allowing certain molecules to enter and stop other molecules.

B. Phospholipids bilayer is the part of the plasma membrane that makes it selective permeable. The plasma membrane does not allow ions with charges on the side of the molecules directly sodium, potassium, and many other ions are examples of this. Ions can enter or exit the cell with the help of the transporters or complexes.

C. There is hydrophilic-head and hydrophobic-tail present in the phospholipid bilayer in the structure of the cell membrane which makes it selectively permeable.

D. For active transport of molecules, the movement of molecules is always against the gradient which means movement from lower concentration to a region of higher concentration that requires energy (ATP).