The Total of All Chemical Reactions in an Organism

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, we will learn about metabolism in plants and animals. We will learn what types of reactions each participate in and the net energy each creates.


The sun shines on a tree, and it grows taller and bigger creating oxygen for us to breathe. We eat a hamburger and, get energy, and build muscles. All of these reactions are part of metabolism. Metabolism are the results from all of the chemical reactions that occur in an organism. Each organism is different in terms of what they take in and what is produced, but in general metabolism in every organism will include anabolism (building up larger, complex molecules) and catabolism (breaking down molecules into simpler compounds).

The two main groups of organisms are plants and animals. Other subsets such as bacteria take certain aspects of each type of reactions for their specific needs. The main difference between the reactions that plants and animals use is that plants create their own food while animals need to intake that food.

Total Reactions in Plants

Plants use a set of reactions called photosynthesis to store energy, in other words to create their food. Photosynthesis has two parts, called the light and dark phases. The light phase is when plants take in the sunlight and using water they change the energy in sunlight into useable energy, called ATP. This process releases oxygen.

This energy can then be used in the dark phase, or the Calvin cycle in combination with carbon dioxide to create sugar. This sugar isn't typical table sugar that you'd have in your pantry. Sugar here can include complex sugars such as cellulose which can't be digested by the human body which is why it is referred to as fiber. This sugar is stored in all parts of the plant. It can act as a structure for the plant, energy for reproduction, or future energy for the plant itself.

Photosynthesis reaction

When energy is needed (perhaps to grow), the plants undergo respiration where they break down the stored sugar. The respiration process that plants undergo is very similar to what animals use. The main difference is that they don't breathe in oxygen; instead, it simply passes through the membrane passively.

For plants, overall, more photosynthesis occurs than respiration. This is why plants continue to grow, they are putting the sugars that they made into growth and not much of that energy is needed to survive. When you look at the overall equations in plants, we see that it makes a complete circle. Carbon dioxide, water, and energy go in and create sugar and oxygen. Then sugar and oxygen react to produce carbon dioxide, water, and energy.

Total Reactions in Animals

Animals cannot create their own food; they need to get it from another source. Either from plants or other animals. Once this food has been consumed; they can either put it into structure, storage, or energy. If the food is put into structure or storage, these are anabolic processes, while creating energy is a catabolic process.

Animals store fat in triglycerides, sugars in glycogen, and protein in muscles. Even if an animal eats sugar, fat, or protein in the storage form that it, needs it still needs first to be broken down in order to be absorbed through the intestines. If the body has enough current energy this food will then be sent to storage or to build up structures.

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