Imagine an atom of carbon floating around in the atmosphere just above a farm. That is one atom of C in the air – but packaged in the gas called carbon dioxide or CO
2. (If it was an atom of carbon it would be either diamond or graphite powder and would be lying on the ground.)
The molecule of carbon dioxide gets captured by a nearby blade of grass and gets turned into a useful protein or carbohydrate by photosynthesis, and helps build the blade of grass. (There is a chance it will be exhaled as CO
2 by the process of respiration but let's ignore that.) Then along comes Daisy the Cow who eats the blade of grass and starts digesting it in her rumen, and in other parts of her intestinal tract. Some of these atoms of carbon will be stripped off the oxygen and ingested into the cow, turned into protein, and finally turn up on your dinner plate as steak or whatever.