How Cells Break Down Molecules for Energy

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Large cells in our food are broken down by the digestive system, and converted into energy through cellular respiration. The food provides both energy and the building blocks to create new cells and repair the body. Learn about how we break down molecules into energy in this process.

Digesting Molecules

We eat food every day of our lives. It's one of the constants that make up the rhythm of each day: breakfast, lunch, and dinner. But what happens to that food after we eat it? And why is it so important?

Our bodies are constantly building and rebuilding cells. Old cells die, to be replaced with new ones. Those cells are also always carrying out various processes. Our heart has to beat, our brain has to send electrical impulses, our bodies have to move, and without these things we wouldn't be able to function properly. But all of these functions require energy, and your cells require building blocks from which to make them. We get both energy and those building blocks from food.

Digestive system diagram
Digestive system image

Digestion is the process where the large molecules in the food that we eat are broken down into smaller ones that we can use for energy or as building blocks. This is done in the digestive system by enzymes found in saliva, in stomach acid, in the small intestine, and in the large intestine.

Amylase: one enzyme found in saliva
salivary amylase

These enzymes break down particular types of molecules into smaller ones. Proteins are broken down into amino acids, carbohydrates are broken down into sugars, lipids (otherwise known as fats) are broken down into fatty acids, and DNA is broken down into nucleotides. The energy that we get from this digestive process comes from the bonds that are broken inside each molecule - breaking bonds releases energy. These broken down molecules are then passed into the blood and move to the part of the body where they are needed.

Here are some details about enzymes involved in digestion:

Enzyme Produced In Site of Release pH Level
Carbohydrate Digestion
Salivary amylase Salivary glands Mouth Neutral
Pancreatic amylase Pancreas Small intestine Basic
Maltase Small intestine Small intestine Basic
Protein Digestion
Pepsin Gastric glands Stomach Acidic
Trypsin Pancreas Small intestine Basic
Peptidases Small intestine Small intestine Basic
Nucleic Acid Digestion
Nuclease Pancreas Small intestine Basic
Nucleosidases Pancreas Small intestine Basic
Fat Digestion
Lipase Pancreas Small intestine Basic

Cellular Respiration

The products of digestion are needed by the body in a more general sense, but they are also needed inside cells. One of the most important processes that happens inside cells is the use of simple sugars like glucose for energy. Although glucose is a much smaller molecule than most of the molecules in the food you eat, the cell breaks it down once more.

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