Hormones can be divided into three classes. lipid-derived, amino acid-derived, and peptide (peptide and proteins) hormones. Cholesterol is a fat and fats are lipids. They fall into the protein and lipid category.
During exercise there is an increase in physical activity and muscle cells respire more than they do when the body is at rest. The heart rate increases during exercise. The rate and depth of breathing increases - this makes sure that more oxygen is absorbed into the blood, and more carbon dioxide is removed from it.
The amount of available resources in the ecosystem must change at a predictable rate.
The answer is , because one cell divides to form two identical cells.
