What photosynthesis is

You met photosynthesis in grade 10, along with its equation. The definition below is the same process stated more exactly, and the three things it adds are what this chapter is about.

Photosynthesis

The food synthesis process that takes place inside cells containing chlorophyll, using the energy contained in sunlight, with carbon dioxide and water as raw materials, is called photosynthesis.

Figure to be added

Figure 2.2 — a whole plant drawn with the inputs and outputs of photosynthesis marked on it: light energy arriving from the sun at the top left, water labelled H2O being absorbed by the roots, carbon dioxide labelled CO2 entering the leaf through the stomata, a note inside the leaf that the chloroplasts absorb light and produce glucose, and oxygen labelled O2 and glucose leaving at the top right.

What this adds to the grade-10 definition

Grade 10 said photosynthesis is the process of producing food with the help of chlorophyll. True, and enough at the time. The version above names three things that one left out:

Where the energy comes from — the energy contained in sunlight. Not the light itself; the energy in it.
What the raw materials are — carbon dioxide and water, and nothing else.
Where it happens — inside cells containing chlorophyll. Not in plants; in cells, and only those cells.

The third one is doing real work

Defining photosynthesis by the cell rather than by the organism settles several questions at once. It explains why a root does not photosynthesise while a green stem does, why the white parts of a variegated leaf produce nothing — an experiment later in this chapter — and why organisms that are not plants at all can photosynthesise, so long as their cells carry chlorophyll.

Chlorophyll-containing cells, not green plants

Photosynthesis happens inside cells that contain chlorophyll, using sunlight energy with carbon dioxide and water as raw materials. Every part of that sentence is tested by an experiment in this chapter.