Actually, a pint of water weighs 1.04375 pounds. So, although this phrase does not provide the EXACT weight of each liquid, it does provide a very simple rule for determining approximately how much liquids weigh. In the case of water, there are eight (8) pints of water in a gallon. Using this rule, a gallon (eight pints) would weigh approximately eight (8) pounds, which is very, very close to a gallon of water's exact weight of 8.35 pounds which is found in the more detailed and scientifically correct answers to this question.
In the simple example of the oxygen cycle shown above, you can see how oxygen is used and cycled by plants and animals. Plants are the main creators of oxygen in the atmosphere through the process of photosynthesis. Here the tree uses sunlight and carbon dioxide to produce energy and releases oxygen. The giraffe breathes in the oxygen and then breathes out carbon dioxide. The plant can then use this carbon dioxide and the cycle is complete.
A: Plants release oxygen as a by-product of photosynthesis, a process in which carbon dioxide is converted into sugar. The sugar is then used as an energy source for the plant. When oxygen is produced through photosynthesis, it is released by the plant into the atmosphere. The sugars produced by photosynthesis are utilized by combining them with oxygen through a process called oxidation. Photosynthesis chemically converts sunlight into energy after being collected by special cells in their leaves known as chloroplasts.