Biology, Answering the Big Questions of Life/Photosynthesis
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What is photosynthesis? 
Photosynthesis is making sugar using the energy of light.
What is light? 
Light is a form of energy. The form of light that plants capture, as well as the light that you and I can see, is called visible light. The white light that shines on Earth from the sun is actually composed of light of many different colors. You can see these colors by using a piece of glass called a prism that can split the light into different colors or wavelengths of light.
The rainbow of light created by a prism is called a spectrum and the colors of light are always in the same order: Red, Orange, Yellow, Green, Blue, Indigo and Violet. It also spreads into colors that we can't see becoming infrared off the edge of the red side of the spectrum, and ultraviolet off of the edge of the violet side of the spectrum.
In fact, the spectrum of light (called the electromagnetic spectrum) is very large going from radio waves as big as planets to very powerful light called gamma rays whose wavelengths can be the size of subatomic particles.
Photosynthesis captures the energy from visible light but it doesn't use all of the colors of light equally. It mostly uses light from the blue and red parts of the spectrum.
Why are plants green? 
The simple answer is that plants are green because they have green Chloroplasts (organelles that carry out photosynthesis).
But why are Chloroplasts green? Chloroplasts are green because they contain the green pigment chlorophyll in their thylakoid membranes. Chlorophyll is a pigment that absorbs red and blue light.
Then why is chlorophyll green? 
To understand why chlorophyll is green, we must learn about pigments. A pigment is something that absorbs light. We said that chlorophyll is a pigment that absorbs blue and red light, so then why is it green?
That's because pigments absorb some wavelengths of light and reflect others. A yellow pigment is one that absorbs all of the wavelengths of light except yellow. The yellow color is reflected into your eyes, and that is why it looks yellow. The reason a red shirt is red is because the red pigments in the shirt absorb all of the light except red. That reflected light is the color that the shirt appears. If it reflected all light, then it would be white.
So plants are green because chlorophyll reflects green light. And chlorophyll is found in all plants because it is the molecule that absorbs the light that is used to make sugar.
Why do plants change color in the autumn? 
Although plants are green, in the fall they can turn other colors such as yellow, red, and orange. This is because plants also contain other pigments such as carotenoids (yellow orange) and anthocyanins (mostly red). These other accessory pigments sit in the leaf along with the chlorophyll and they are the reason that leaves are many different shades of green.
The green pigment covers them up for the most part in the summer, but in the fall, when the plants are beginning to go dormant in anticipation of the winter, the plant cells stop making chlorophyll, and the leaves lose their green color. Without it, we can see the colors of the other pigments in the leaves.
How do plants capture light and use it? 
Photosynthesis is made up of two sets of reactions. The light reactions and the light-independent reactions (formerly the dark reactions). The light reactions capture light energy and use it to make high energy chemicals that are used to power the light-independent reactions which make the sugar.
Want more detail? 
- Light and Dark reactions,
- Accessory pigments
- Excited states
- C4 plants