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Photosynthesis and Light

Lesson Seven -Light

Transpiration and Photosynthesis
Plants require a constant supply of energy to grow and this energy comes from light. In nature, plants receive light from the sun. In a classroom, you may need to add artificial light so your plants have an adequate amount of light to grow.

There are various types of artificial lights that provide differing light spectrums. Before learning about these artificial lights, it is important to understand how plants use light in the growth process.

Transpiration and photosynthesis are the two major processes that are carried out by green plants that use energy from the sun. Both of these processes use large amounts of light energy but. only in photosynthesis is a significant amount of energy from light actually stored for future use. Light influences other processes such as flowering, seed germination, certain growth stages and pigment production but, in these cases, only very small amounts of energy from light are used.

During the transpiration process, plants draw in carbon dioxide from the air through their pores and water from their roots and give off oxygen and water vapor. Energy from the sun evaporates water from the plant cell walls. Although this results in a movement of water in the plant tissue (xylem). this energy is neither stored nor used to bring about vital reactions involved in the synthesis of foods, in assimilation, growth or reproduction.

In photosynthesis, which literally means "putting together (synthesis) by means of light (photo)," water is drawn up through the stem from the roots and into the leaf tissue where the chloroplasts, containing chlorophyll (a green pigment) can be found. There the water encounters carbon dioxide which entered the leaf from the air through minute breathing pores (stomata) located abundantly on the underside of the leaves. The stomata also permits the outflow of water vapor and oxygen. The light, carbon dioxide and water produce carbohydrates which are stored in the plant and later released as energy for other vital plant functions.

Energy stored as chemical energy in foods ( carbohydrates, fats, proteins) is continually released in living cells during the process of respiration. Basica1ly, photosynthesis stores energy and respiration releases it. enabling cells to perform the work of living. By releasing energy, respiration provides the energy needed for all other plant functions.
All animals ultimately depend on photosynthesis because it is the method by which all basic food is created.

Light Spectrums
White light, as it comes from the sun, is composed of waves of red light, through successively shorter waves to violet light. The band of colors that compose the visible spectrum of light (that which we can see) include, starting with the longest rays, red, orange, yellow, green, blue, indigo and violet. The visible spectrum represents only a part of the radiant energy that comes from the sun and only a part of the visible spectrum is effective in photosynthesis.

Wavelengths exist that we our unable to perceive with our eyes. Beyond the red rays are still longer rays called infrared and beyond the violet rays are even shorter rays called the ultraviolet.
The fact that chlorophyll is green to the eye is evidence that some of the blue and red wavelengths of white light are absorbed, leaving proportionally more green to be transmitted, reflected and seen.

Much of the red, blue, indigo and violet wavelengths are absorbed and used in photosynthesis while part of the red and most of the yellow, orange and green are barely used in photosynthesis.

Signs of Light Deficiencies:


plants will stretch and reach toward the light source


stem elongation


plant deformities


no fruit set

Artificial Lighting
If your hydroponic garden is in direct sunlight, the plants should receive adequate amounts of light and absorb the spectrums they need.

In a greenhouse setting, supplemental light is sometimes used to extend the hours of light a plant receives during low light conditions (cloudy weather or short days), and to extend the growing season of a plant. If you are growing in an area with some, but limited sunlight, such as a windowsill, supplemental lighting will be needed.

Any supplemental light is beneficial to increase plant growth and production. The higher the intensity and the broader the spectrum, the greater the benefit.

You can grow in a completely enclosed space with no natural light if you provide all artificial light but there are several drawbacks including the cost of the lights and the energy to run them is high, there may be a compromise of the plants needs if the artificial lighting does not provide the complete light spectrum the plant needs and artificial lighting will not exactly duplicate the spectrum of light the sun provides.