Cellular respiration | Food and energy in organisms | Middle school biology | Khan Academy

So, let's say I was preparing to go on a long hike. Besides packing water, dressing for the weather, and mapping my route, I'd probably also want to eat a snack before leaving to make sure I have enough energy for the hike. We, as humans, need food to nourish our bodies. But how exactly does that food give us energy? How does something like an apple or a piece of chocolate get converted into something our bodies can use?

The answer to this question can be found in a process called cellular respiration, which is how organisms get their energy from food. In other words, cellular respiration helps make life possible. It's happening right now in your cells and in mine, in my little brother's pet hamster cells, and in the cells of the tree outside the window. It's providing all of us with the energy that we need to function. So, let's dive into this process to find out more about how it works.

Cellular respiration is a series of chemical reactions that happen in an organized way inside our cells. It's these chemical reactions that result in an overall release of energy that our bodies can use. Although cellular respiration involves many chemical reactions, this entire process for plants and animals can be summarized here by this equation. On the left side of this equation are the inputs: glucose, which is a type of sugar, and oxygen. Glucose comes from our food and oxygen comes from the air we breathe.

The right side of this equation shows the outputs: carbon dioxide and water. The arrow in the middle shows that during the process of cellular respiration, glucose and oxygen molecules are broken apart, and their atoms are rearranged to eventually form carbon dioxide and water. This whole process releases energy, similar to how firewood releases energy as it burns. Also, no atoms are lost during cellular respiration. You can see the same number of each type of atom on both sides of the arrow; for example, six carbon atoms on both sides. It's similar to how you could take apart a brick house and use those bricks to build something else, like a bridge. You're not throwing away any of the bricks, but instead, you're creating something new out of them.

So, we know that cellular respiration involves a series of chemical reactions, but where in the cell do these steps happen? To find out, let's walk through the process together. After eating, your food is broken down into small molecules during digestion. These small molecules, which include glucose, can enter your cells. Then cellular respiration can begin.

The first set of chemical reactions happen in the cytosol, which is the jelly-like fluid containing all of the cell's organelles, and they involve glucose as an input. The rest of the reactions happen in mitochondria, with oxygen as an input and carbon dioxide and water as outputs. Together, these chemical reactions result in an overall release of energy. It might be a little mind-boggling for you to think about how the food we eat provides the energy our bodies need to carry out all of life's processes, and I know it was for me. After all, it's not every day you take a bite of an apple and wonder how that apple's molecules are rearranged inside your body.

What's important to know, though, is that cellular respiration is the way our cells get usable energy. Through cellular respiration, you and I get the energy that we need to do the things we want to do, including accomplishing our goals. So, the next time you feel proud of yourself for finishing a big hike or acing your biology quiz, make sure to thank your mitochondria.