Final answer:
Carbohydrates are the food molecules most directly related to the use of sun energy by animals, as they are products of photosynthesis in plants. Lipids provide the largest amount of free energy per gram, but carbohydrates like glucose are quickly metabolized for energy. Animals depend on the digestion of these nutrients to release the energy required for cellular functions.
Step-by-step explanation:
The food molecule, most directly related to the use of sun energy and used by animals for energy, which is obtained from plants, is carbohydrates. Plants convert sunlight into chemical energy through a process called photosynthesis, primarily producing glucose. Animals then consume plants or other organisms that have eaten plants to obtain glucose for energy. Glucose serves as the primary fuel for cellular respiration, where it is metabolized to produce ATP (adenosine triphosphate), the energy currency of cells.
Free energy is obtained from the metabolism of nutrients such as carbohydrates, lipids, proteins, and nucleic acids, but among these, lipids provide the largest amount of free energy per gram due to their dense caloric content. However, carbohydrates like glucose are more directly involved in quick energy release since they are more readily metabolized.
During digestion, food is broken down into smaller molecules that can be absorbed and utilized by the body. The breakdown products of carbohydrates and lipids are used in metabolic processes to convert them to ATP, thereby providing energy to the organism. In the absence of food, the body can still survive for a period by metabolizing stored macromolecules for energy.