Final answer:
The 'leaky' structure allowing filtration of plasma and solutes is the plasma membrane, which is selectively permeable to control substance movement into and out of the cell. Therefore, the correct answer is 1) Cell membrane.
Step-by-step explanation:
The structure designed to be "leaky" to allow some plasma and solutes to be filtered is the plasma membrane. The plasma membrane, also known as the cell membrane, is composed of a phospholipid bilayer embedded with proteins that controls the entry and exit of substances. It maintains the cell's environment by being selectively permeable, which means it allows some substances to pass while restricting others.
The plasma membrane's semi-permeable nature facilitates the movement of organic molecules, ions, and water, while it may actively transport others or prevent the passage of certain substances to maintain homeostasis within the cell. Passive transport such as diffusion or osmosis and active transport mechanisms are key ways substances move across the plasma membrane. Therefore, the correct answer is 1) Cell membrane.