1. Digestion: Animals ingest food containing energy-rich molecules such as carbohydrates, proteins, and fats. The digestive system breaks down these complex molecules into simpler components that can be absorbed and utilized by the body.
2. Cellular Respiration: Cellular respiration is a set of metabolic reactions that occur within the mitochondria of cells. It involves the breakdown of glucose, a simple sugar obtained from carbohydrates, to produce adenosine triphosphate (ATP), the primary energy currency of cells.
a. Glycolysis: This initial phase of cellular respiration occurs in the cytoplasm. One molecule of glucose is broken down into two molecules of pyruvate, releasing a small amount of ATP and generating high-energy electron carriers, NADH (nicotinamide adenine dinucleotide) and FADH2 (flavin adenine dinucleotide).
b. Krebs Cycle (Citric Acid Cycle): The pyruvate molecules from glycolysis enter the mitochondria and undergo a series of chemical reactions known as the Krebs cycle. This cyclic process further breaks down the pyruvate molecules, releasing carbon dioxide and generating additional ATP, NADH, and FADH2.
c. Electron Transport Chain: The electron carriers (NADH and FADH2) generated in glycolysis and the Krebs cycle pass their electrons to the electron transport chain, a series of protein complexes located in the inner mitochondrial membrane. As the electrons move through the chain, their energy is used to pump hydrogen ions (H+) across the membrane, creating a gradient.
d. Oxidative Phosphorylation: The hydrogen ions pumped across the membrane during the electron transport chain flow back through a specific protein complex called ATP synthase. This movement of ions drives the synthesis of ATP from ADP (adenosine diphosphate) and inorganic phosphate (Pi). The energy released by the hydrogen ion gradient is captured and used to form the high-energy bonds of ATP.
3. ATP Utilization: The ATP molecules generated through cellular respiration are then utilized by various cellular processes and activities that require energy. ATP provides the necessary power for muscle contraction, nerve impulse transmission, synthesis of biological molecules, and many other cellular functions.
It's important to note that while carbohydrates are the primary source of energy for most animals, some species may have specialized adaptations to utilize other energy sources, such as fats or proteins. Additionally, there are variations in the metabolic pathways and energy production efficiency among different animal groups and species.