CoachA cell is autotrophic if it builds its own organic molecules from mineral matter alone — carbon dioxide, water, mineral salts — using an external source of energy. Plant cells with chloroplasts, algae and some bacteria are autotrophs, and their energy source is light: this is photosynthesis. A cell is heterotrophic if it must take in organic molecules made by other organisms, both as building material and as a source of energy: animal cells, fungi, most bacteria, and the non-green cells of plants (roots, for instance) are heterotrophs.
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Example 4.6 (Euglena, both at once)
The single-celled Euglena has chloroplasts and swims with a flagellum. In the light, in mineral water, it grows: autotroph. In the dark it survives only if organic molecules are added to the water, which it absorbs: heterotroph. Kept long in the dark it loses its chloroplasts; brought back to light with a few days’ patience it makes them again. Its metabolism depends on its genes, which allow both options, and on its environment, which selects one.
Example 4.12 (The alga’s true rate)
In the Chlorella figure the oxygen rises by in 5 minutes of light (slope ) and falls by in 5 minutes of dark (slope ). Respiration consumes per minute in the light as well, so photosynthesis produces per minute: six times what respiration uses. The alga is a net producer of oxygen and organic matter, which is why it grows.