In cells, are proteins that are needed to lower the amount of energy required to start chemical reactions.
a) acids
b) enzymes
c) products
d) substrates

In cells, enzymes are proteins that are needed to lower the amount of energy required to start chemical reactions. Enzyme by definition is a chemical substance in plants and animals that helps in causing natural process. Lactase, for example, is an enzyme that hydrolyzes galactosides and occurs in intestines of young mammals and in yeasts.

both anaerobic and aerobic respiration use glucose (or other carbohydrates), proteins and lipids to produce atp and nadh. cell uses those energy molecules, obtained from food and   breaks them down during the chemical reactions of respiration. in anaerobic respiration, nadh is regenerated to nad+ by enzymes lactate dehydrogenase (lactate is produced, e.g. in muscle cells) or by alcohol dehydrogenase where ethanol is produced (alcohol fermentation), while in aerobic respiration, electrons from nadh are deposited in the mitochondria to regenerate nad+ (electron transport chain. anaerobic respiration produces 2 atp per molecule of glucose compared to aerobic respiration where about 38 atp molecules are produced.

cell regulation is conducted by special proteins.