
Metabolism
Metabolism is such a big word to explain a simple idea. We all need energy to survive. Whether we are plants, animals, or bacteria, we all need energy. Energy doesn't just float around in a form we can use to survive. We need to eat (mainly sugars) and digest food. That process of chemical digestion and its related reactions is called metabolism. Metabolism is the total of all the chemical reactions an organism needs to survive.
Building Up
First, you need to build up the molecules that store energy. We'll start with photosynthesis. It's no use explaining the breakdown of sugars without telling you how they were made:LIGHT (Energy) + CO2 + H2O --> C6H12O6 + O2
You will only find this reaction in plants and algae (maybe some bacteria). They take sunlight and combine carbon dioxide (CO2) and water (H2O). Then they create glucose (C6H12O6) and oxygen gas (O2). Chemists say that they are fixing the atmospheric carbon (C). Remember, plants put the energy in glucose. Glucose is in most of the foods you eat, and the oxygen you breathe comes from those plants. Even if you have a piece of meat, that animal was originally able to get its glucose from a plant. You need to understand just how important plants are to you and the rest of life on Earth.
Breaking Down
Respiration is a three-step process that includes glycolysis, the Krebs cycle, and a bunch of electrons being pushed around the membranes of mitochondria. Together they take that energy out of the sugar-related molecules. Glucose is combined with oxygen and releases usable energy, carbon dioxide, and water.C6H12O6 + O2--> Usable Energy (ATP) + CO2 + H2O

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Related Video...
Chalk Talk: Mitochondria (US-NSF Video)