Strategies for writing an effective introduction. Start by thinking about the question (or questions) you are trying to answer. Your entire essay will be a response to this question, and your introduction is the first step toward that end.
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.
Photosynthesis from plants use carbon dioxide, sunlight, and water and a byproduct is oxygen. Before humans existed, the earth existed for millions of years creating an enormous supply of oxygen. Organisms and plants created a balance of oxygen and CO2. The Earth's net supply of oxygen comes from phytoplankton. Plants produce oxygen but use up what they make by producing carbon dioxide at night and when they are burned or decompose, so you see, the only thing that keeps us breathing is the tiny phytoplankton in the sea.