Plants produce oxygen and animals and humans use oxygen. about 21 percent of earth’s atmosphere is oxygen, enough for all living creatures. for many years, plants and animals have kept a bargain. animals breathe the oxygen produced by plants. at the same time, plants use the carbon dioxide that animals exhale. they mix it with water and sunlight to produce a sugar called glucose. as it turns out, glucose is the basic energy source for both plants and animals. the waste produced by this process is oxygen. the bargain between plants and animals has been in the right balance for many millions of years. if we humans are smart enough not to upset that balance too much, all life on earth will benefit from it for millions of years to come. plants are able to use water, carbon dioxide, and sunlight to produce glucose plus oxygen. what's the effect of this for animals? what might be a cause that would break the bargain between plants and animals?