Answer:
Oxygen is a colourless, odourless, tasteless gas essential to living organisms, being taken up by animals, which convert it to carbon dioxide; plants, in turn, utilize carbon dioxide as a source of carbon and return the oxygen to the atmosphere.
Answer: Single cell organisms dont need a respiratory system because oxygen and carbon-dioxe diffuses between their body tissues and the enviorment.
Well it happens every six months, one in summer in June and winter December. And if i'm wrong then i understand :) <span />
