The correct answer is a, b and d. Explanation & Key Takeaways: • Enzymes are proteins that act as catalysts in biochemical reactions. They are highly selective and only speed up specific reactions. Hence statement a is correct. • The molecules that an enzyme works with are called substrates. The substrates bind to a region on the enzyme called the active site. • Enzymes work by combining with molecules to start a chemical reaction. They work best at certain pH levels and temperatures. • They play a vital role in the proper functioning of the digestive system, the nervous system, muscles, and more. • Enzymes are made up of amino acids. Enzymes are proteins that act as biological catalysts. Hence statement b is correct. • Enzymes catalyze reactions at high rates through their active sites. An enzyme’s active site is a pocket that the substrate fits into. • An amylase is an enzyme that catalyses the hydrolysis of starch (Latin amylum) into sugars. Amylase is present in the saliva of humans and some other mammals, where it begins the chemical process of digestion. Hence statement d is correct. • Enzyme reactions depend strongly on pH and temperature. The direct dependence of the enzyme becomes obvious, when the enzyme activity is measured at the respective pH or temperature, while preincubation for a distinct time yields the stability of the enzyme against these influences. Hence statement c is incorrect. • pH-optimum curve is characteristic for the pH behaviour, while the activity increases with temperature until denaturation of the enzymes counteracts the increase. • The temperature increase can be linearized in an Arrhenius diagram, the slope of which gives the activation energy.