Photosynthesis

Photosynthesis is the process used by plants to turn light energy from the Sun into chemical energy. Plants can then use this chemical energy to produce their own food and to grow. The plants absorb light, carbon dioxide (CO2), nutrients and water and turn these into oxygen and chemical energy. The chemical energy is normally stored in carbohydrates such as sugars.

Measuring photosynthesis on Bibb lettuce being grown hydroponically for study in the Space Life Sciences Lab. Credit: NASA

One of the fundamental laws of physics states that energy can’t be created or destroyed, only converted from one form to another. Photosynthesis is one of the methods used to convert energy from one form to another and a very important one at that. A carrot, for instance, uses photosynthesis during the growth phase to convert the light energy from the Sun into chemical energy and stores it in its root. When we pull and eat the carrot, our cells use the chemical energy within the carrot to power our bodies. For example, the muscles of our bodies, uses the chemical energy and converts it into kinetic energy (movement). Without photosynthesis, we wouldn’t have any food on Earth.

Another consequence of photosynthesis is that it produces the oxygen we breath. Forests, plants and even phytoplankton in the oceans are responsible for producing the oxygen we breath. They suck up vast quantities of carbon dioxide and pump back out oxygen in return. In addition to this when they die they act as a store for the carbon, taking it out of the atmosphere, unless someone takes the tree, burns it and releases it again!

The composition of Earth’s atmosphere has not been constant. During an event called the Great Oxygenation Event, the Earth’s atmosphere became oxygen rich. This allowed complex life to develop. It is thought the cyanobacteria and photosynthesis played a large role in this. Photosynthesis is not a very efficient process. Only about 3 to 6 % of the light energy is converted into chemical energy. The rest is dissipated mostly as heat.

A lot of people associate the pigment chlorophyll with photosynthesis. The role of chlorophyll is to help absorb the light from the Sun. Chlorophyll absorbs mostly red and blue light which is why we see leaves as green. Below is the formula for photosynthesis, it looks complicated but it isn’t. In simple terms, when sunlight is added to oxygen and water, glucose (a type of sugar) and oxygen are outputted.

6CO2 + 6H2O —> (Sunlight) —> C6H12O6 +6O2

Photosynthesis happens in the leaf of the plant and the part of the leaf where the gasses are exchanged are called the stoma and are located on the underside of the plant.

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