prompter: How do humans get the energy out of the food they eat?

assistant: Humans extract energy from food by breaking the chemical bonds in macronutrients (proteins, fats, and carbohydrates) during digestion. Carbohydrates are particularly important fuel molecules, and the human body breaks it down into carbon dioxide and water, releasing energy in the process. The energy is shuttled around the body in carrier molecules such as ATP and NADH. Food molecules are broken down in three stages to produce ATP:

  - **Stage 1:**  Digestion, which occurs either in the intestine outside cells, or in a specialized organelle within cells, the lysosome, breaks down large food molecules into simpler ones. Proteins --> Amino Acids, Carbohydrates --> Simple Sugars, Fats -> Glycerol.
  - **Stage 2:**  Glucose is broken down into an even simpler molecule, pyruvate, creating ATP and NADH. Pyruvate is then turned into acetyl CoA which is another energy carrying molecule. Fats can also be turned into acetyl CoA.
  - **Stage 3:**  Acetyl CoA is transported to the mitochondria, an organelle which is specialized for energy production. There, acetyl CoA is oxidized into CO2 and large amounts of NADH are produced. Finally, NADH is consumed in the mitochondria using molecular oxygen (O2) to make ATP. This final step releases the most of energy during metabolism.

In short, people get energy from food by 1) digesting complex nutrients to simpler ones, 2) breaking down simple molecules into even simpler forms, and 3) and then using the molecules from stage 2 to make NADH and ATP, the energy currency of the living things.

prompter: Ok, this explains how the body turns fats and carbohydrates into energy. Is it also possible for the body to get energy by digesting proteins?

assistant: Yes - great question! Protein are broken down into amino acids during the digestion in **Stage 1**. Some amino acids, like tryptophan, tyrosine, and phenylalanine, can be converted to into acetyl-CoA like fats and carbohydrates in **Stage 2**. From there on, the ATP and NADH production in **Stage 3** is identical to that of the fats and sugars.

assistant: Yes, the body can also get energy from digesting proteins. During digestion, proteins are broken down into their constituent amino acids. These amino acids can then be used by the body to make new proteins or they can be used as a source of energy. However, using proteins as an energy source is not the most efficient way for the body to produce energy, as breaking down proteins requires more energy than breaking down carbohydrates or fats. In addition, using proteins for energy can lead to the breakdown of muscle tissue, which is not desirable. Therefore, the body typically uses carbohydrates and fats as its primary sources of energy, while proteins are reserved for building and repairing tissues.

assistant: It is possible for proteins to be turned into energy, but it is not the body's preferred energy source. Carbohydrates and fats are used first. Only a small amount of proteins are directly turned into energy. However, proteins prevent fatigue by helping the body repair tissue.

prompter: How do proteins repair tissue? Also, I heard fat cells do this as well. Something about swimming to injuries in flies. Does this also happen in humans?

assistant: Our bodies digest the food we eat by mixing it with fluids (acids and enzymes) in the stomach. When the stomach digests food, the carbohydrate (sugars and starches) in the food breaks down into another type of sugar, called glucose.

The stomach and small intestines absorb the glucose and then release it into the bloodstream. Once in the bloodstream, glucose can be used immediately for energy or stored in our bodies, to be used later.

prompter: How do the feces we excrete differ from the food we eat, e.g. what exactly do we take out of the food and what can our body not use and therefor is removed through feces and urine. How do these waste products compare in weight to the food we consume?

assistant: Humans get energy from the food they eat through a process called cellular respiration. During cellular respiration, glucose and other molecules from the food we eat are broken down in the presence of oxygen, producing energy in the form of ATP (adenosine triphosphate) and releasing carbon dioxide and water as waste products.

The process of cellular respiration occurs in the mitochondria, which are organelles found in most cells in the body. The breakdown of glucose in the presence of oxygen occurs in three stages: glycolysis, the Krebs cycle, and the electron transport chain.

During glycolysis, glucose is broken down into two molecules of pyruvate, producing a small amount of ATP and NADH. The pyruvate then enters the mitochondria, where it is further broken down during the Krebs cycle, producing more ATP and NADH. Finally, the NADH is used in the electron transport chain, where it generates a large amount of ATP and releases water as a waste product.

Overall, the process of cellular respiration allows humans to extract energy from the food they eat and use it for various bodily functions, such as movement, growth, and maintaining body temperature.

prompter: Ok, now can you teach me how the cells get glucose?

assistant: Sure. The glucose mainly comes from carbohydrates in the food, which usually exists as sugars and starches, and they need to pass through the digestion process to be used by cells. Digestion of starches into glucose begins in the mouth with enzymes in the saliva, turning some of them into maltose. But the major part takes place in the small intestine by the action of specific enzymes from the pancreas, such as α-amylase and α-glucosidase. This is why chewing bread without sugars for longer still brings a sweet flavor. Other sugars such as disaccharides sucrose, lactose, and maltose are also broken down into single units by specific enzymes: maltase, sucrase, and lactase respectively.

The glucose is then absorbed in the intestine into blood stream and brought around the body. It can enter cells through transporter proteins embedded in the membrane. Not all cells will use the glucose they get immediately. For example, muscle cells can convert the glucose into glycogen, which can be quickly reconverted into glucose when needed, and fat cells can convert it to fat for long-term storage. A large portion of the absorbed glucose is also stored in the liver as glycogen to be released again in following hours, maintaining a stable blood glucose level.