Our planet is lively in many ways. Everything on Earth is part of one system, so Gas Exchange could be discussed under Cycles, Interliving, or under Balance. Everything really is connected to everything else.
Every living organism must exchange gases to stay alive. We each take in a gas (breathing) we need from the atmosphere, use it to make food or transform food, and release the leftover part of the gas, which the living cells have chemically changed into a different gas.
Animals take in oxygen gas, transfer it to their blood, and carry it to every cell, where it is used to transform food into energy, where carbon dioxide gas is created as a by-product or leftover. This is called respiration. This carbon dioxide is released into the air.
Plants and algae breathe in carbon dioxide and carry it to every green cell, where it is used to transform sunlight into food. This is called photosynthesis. In this process, oxygen gas is created as a by-product or leftover, and is released into the air.
Plants must have carbon dioxide to live. Animals must have oxygen to live. Both plants and animals give each other their by-products. We all live on leftovers. The diagram below show the circle of gas exchange between photosynthetic life and animal life.
Gas Exchange is a system of Giving and Receiving
Plants give oxygen to animals (including you), and animals give carbon dioxide to plants.
Gas Exchange is also a system of Receiving.
Plants receive carbon dioxide from animals, and animals receive oxygen from plants.
So Gas Exchange is really a Gift Exchange, a life and death Gift Exchange. We can't live without each other.
Some Details of Gas Exchange for Animals
Not only flowering land plants give off oxygen. All the billions of green algae in the oceans do as well--and all the cyanobacteria in lichens of the forest, and in the cryptobiotic crusts of deserts. All photosynthesis creates free oxygen as a by-product.
Every life has to breathe, or respire. This is a chemical process within cells Here are the basic steps of respiration:
Plant cells respire in roughly similar fashion, but by using a little of the oxygen in the sap that it liberates during photosynthesis.