An organism needs a source of energy for its movement and maintenance of the body. Carbon compounds like starch, cellulose, glucose, sucrose and fats found in the environment fill that requirement. We generally call them food.
Some organism like plant and blue-green algae can trap the energy of sunlight by converting carbon dioxide into sugars. So they need carbon dioxide and water.
Organism process carbon compounds through various reactions called cellular respiration and convert foods to energy. These reactions require oxygen, so organisms also need oxygen from outside.
Muscle growth needs proteins. Plant prepares protein molecules from amino acids by themselves. The organism also makes some amino acid while they obtain some from an outside source like plant and its products, other bodies. We call it non-essential amino acids and essential amino acids respectively.
Apart from these some minerals and vitamins act as co-factor in various processes including respiration and photosynthesis.
Next: What processes would you consider essential for maintaining life?
See also: Read what criteria do we use to decide whether something is alive?
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