How does energy use differ between rest and activity in organisms?

Energy use in organisms is significantly higher during activity compared to rest due to increased metabolic processes.

During rest, organisms primarily use energy to maintain basic bodily functions, such as breathing, circulating blood, and regulating body temperature. This is known as the basal metabolic rate (BMR), which is the minimum amount of energy required to keep the body functioning while at rest. The energy used during this period is primarily for the synthesis of new molecules, maintenance of ion gradients across membranes, and other 'housekeeping' tasks. The primary source of this energy is the breakdown of glucose in a process called cellular respiration, which produces ATP (adenosine triphosphate), the main energy currency of the cell.

However, during periods of activity, the energy demands of the organism increase dramatically. This is because additional energy is required for muscle contraction, increased heart rate, and other physiological changes associated with activity. The body responds to this increased demand by accelerating the rate of cellular respiration, leading to a higher rate of ATP production. In addition to glucose, the body may also break down fats and, in some cases, proteins to meet this increased energy demand.

The body also has different strategies for meeting short-term and long-term energy demands. For short bursts of intense activity, such as sprinting, the body relies on stored ATP and creatine phosphate, which can be quickly converted into ATP. For longer periods of activity, the body must break down stored glucose and fats, a process that takes more time but can provide a steady supply of energy.

In summary, the energy use in organisms varies greatly between periods of rest and activity. While the energy demands at rest are relatively low and constant, the energy demands during activity can be many times higher and vary depending on the intensity and duration of the activity.