3. What is an enzyme and what is its role in a reaction?

An enzyme is a protein that serves as a catalyst in a reaction. Enzymes help to speed up the rate of the reaction by lowering the initial activation energy (the minimum amount of energy needed to undergo a reaction).

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.