if a process converts something into something else, then that process is a source of that something else & photosynthesis means green plants convert from carbon dioxide and water and solar energy into carbohydrates and food and oxygen for themselves -> systems is a sink of space photosynthesis is a source of oxygen  systems is a sink of space & plants perform photosynthesis -> plants are a source of oxygen through photosynthesis plants are a source of oxygen through photosynthesis  plants are a source of oxygen through photosynthesis & a sink of something starling that something -> plants produce oxygen through photosynthesis plants produce oxygen through photosynthesis  animal wreck space through places & if something is given off of something , then something is the product of something -> plants give off oxygen through photosynthesis