The skin functions as part of the system because it provides a physical barrier that prevents microorganisms from entering the body.

immune system

Explanation:

Immune system protects us from pathogen and harmful organisms that enter our body. Skin is the largest organ in our body and covers every inch of us thus it prevents pathogens from directly entering our organs or body acting as a part of immune system.

option a is correct i.e.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.

an active site is a region that is present on an enzyme where the reactants or the substrate molecules we can say bind and experience a chemical reaction. the substrate is basically a reactant whose concentration is changing and that is converting into a product after binding at the active site of an enzyme