Is light is necessary for photosynthesis?
Light is a condition necessary for photosynthesis, as seen in the photosynthesis equation.
Light provides the energy for photolysis of the water molecule in the light-dependent stage of photosynthesis.
How can we demonstrate that light is a necessary condition for photosynthesis? This classic photosynthesis experiment has two versions which involve the use of a monochromatic green leaf or a white variegated leaf.
Using a green leaf
A completely (monochromatic) green leaf is used In one version of the photosynthesis experiment to demonstrate that light is necessary for photosynthesis. Portions of that leaf are obscured from sunlight by fitting a leaf of the plant to Gangong's Light Screen (also known as Ganong's Photoscreen. Alternatively, parts of a leaf may be obscurred by sandwiching it using black paper, foil or cardboard stencil or any light-proof material. Cutting in a hole into the light-proof material in the shape of a star - or any other shape - mimics Gangong's Light Screen.
The entire plant is then de-starched. When the the test for the presence of starch in a leaf is conducted, the color change to blue-black - which is a positive result for the starch test - corresponds with the portions of the leaf exposed to sunlight.
Procedure - using a green leaf
Areas of the leaf not exposed to sunlight - no colour change
Areas exposed to sunlight - blue-black colour change
You Tube Video showing the photosynthesis experiment - To demonstrate that light is necessary for photosynthesis.
Using a white variegated leaf
Variegated leaves are leaves with multiple or a variety of colors. The colors indicate an absence of the green pigment chlorophyll in that part of the leaf ( white variegation) or a lesser production chlorophyll (light green, orange, yellow, pink yellow and purple). When chlorophyll is present in adequate quantities it masks the other pigments such as the carotenoids (orange), xanthophylls (yellow) and anthocyanins ( pink, red and purple).
White variegated leaves are used in the experiment to demonstrate that light is necessary for photosynthesis. The plant is de-starched, placed in sunlight and the leaves tested for the presence of starch.
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