How Does It Work?

From Seed to Seed:
Plant Science for K-8 Educators

How Does It Work?

As we have said before, in order to begin photosynthesizing, the plant must absorb light from the sun. Light is a form of radiant energy and can be described in two ways. Light can be thought of as consisting of particles, called photons, or it can be thought of as consisting of waves. The wavelength of light is the distance between the crests of two successive waves.

Sunlight is made up of all the colors of light-shown by the colors in a rainbow-and each of these colors has a different wavelength. When light hits a surface, it can be reflected, transmitted, or absorbed. The color that we see when we look at an object is the color of the wavelength that is being reflected. For example, an object that appears white is reflecting all wavelengths, whereas an object that appears black is absorbing all wavelengths and reflecting none. It is logical to conclude, then, that a leaf appears green because it is reflecting green and absorbing all other colors.

The green pigment chlorophyll is contained in the cell's chloroplasts and is responsible for absorbing light from the sun. When chlorophyll is exposed to light, it is bombarded by photons. When an individual chlorophyll molecule absorbs a photon, the energy of that photon pushes the outermost electron-a negatively charged subatomic particle-to a new, larger orbit. When this happens, the molecule is said to be in an "excited," or energized, state. The molecule has acquired the energy of the photon.

This is only the very beginning of the process. The chlorophyll molecule does not remain in the excited state for very long-only for about one thousand-millionth of a second! But this is enough time to trigger a series of reactions in which this outer electron is transferred from one chlorophyll molecule to another in a process called electron transfer. Eventually, the electron reaches a collection center, known as the "reaction center." By doing this, the chlorophyll is, essentially, "harvesting" the sun's energy.

But what about the original chlorophyll molecule, now lacking an outer electron? Almost immediately upon losing the electron, the molecule gains another one-donated by water, one of the raw materials that must be present for photosynthesis to take place. The result is that water is split into its two components, hydrogen (H) and oxygen (O). Can you guess what happens to the oxygen? This is the oxygen, now in gaseous form, that is released into the atmosphere through the stomata. This completes the first phase of photosynthesis, called the light reaction. Light reactions happen in the part of the chloroplast known as the thylakoids (Gr. thylakos = sac + oidos = like).

The chemical energy acquired during the light reaction fuels the second phase of photosynthesis. This phase is called the dark reaction (or Calvin cycle) because light is not required for it to proceed. Dark reactions take place in the part of the chloroplast known as the stroma (Gr. stroma = anything spread out; not to be confused with the stomata!). In the dark reaction, carbon (C) and oxygen (O), from the carbon dioxide molecule, combine with the hydrogen (H) left over from the splitting of water in the light reaction, to form (in a series of complex reactions) glucose, a simple sugar.

The plant may then take these simple sugars and combine them into long chains, forming the large molecules of starch and cellulose. Plants store food in the form of starch, and cellulose is incorporated into the structure of cell walls.

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