Cellular Biology

How Photosynthesis Works



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Photosynthesis is an amazing process carried out in amazing organisms - plants. Plants are fascinating as a whole and incredibly complex in ther processes. They use the primary energy source for our planet, the sun, and transfer this energy up the food chain to feed other organisms.

Without photosynthesis to capture the sun's energy, all other life would cease to exist. How do they do it? Well, put simply , the plants take molecules of water and carbon dioxide and combine them to form sucrose. From sucrose they can create, by enzyme reactions and the incorporation of minerals and nutrients, all other materials like lipids, fats and other substances.

The formula is 6CO2 (Carbon dioxide) plus 6H20 (water) gives (in the presence of light) C6H12O6 (glucose) plus 6O2 (oxygen)

So, not only do plants transform molecules of carbon dioxide and water into glucose but they release oxygen for our respiration (and theirs) into the air as a bi-product. Plants, of course, use some of this oxygen in respiration but they absorb perhaps 5 times more Co2 than they release in respiration so the net result is an increase in oxygen in the air.

Photosynthesis actually occurs in two phases - the light dependent reaction during which the molecules of water and carbon dioxide are taken in through the root hairs and stomata (pores on the leaf) respectively. In specialized cell organelles called chloroplasts and under the catalytic effect of the green pigment chlorophyll,whose molecules become excited as the energy from the sun hits them, the water molecule is split. The special molecule called ATP (Adenine tri phosphate) is created.

During the light independent phase the ATP is synthesized during several chemical processes and the carbon dioxide is split. The carbon and hydrogen atoms are combined, releasing the oxygen. This process results in carbon being stored in the form of glucose, within the tissues of the plant.

This is carbon fixation and plants are great carbon sinks as they fix large amounts of carbon in their tissues and remove it from the atmosphere when they fossilize. We, of course are now releasing far too much by burning the fossil fuels - releasing carbon stores and thus increasing the carbon dioxide content of the air again.

Back to photosynthesis. So, you can see it occurs in two phases. The light independent phase needs the products of the light phase to complete the process. Therefore, the whole process relies on light to take place.

The processes take place in the chloroplast which has two types of tissue, the grana and stroma. The light phase occurs in the grana and the dark in the stroma.

In green plants, chlorophyll gives them a green look because it absorbs all light wavelengths apart from green, which it reflects and we see. Chlorophyll actually exists intwo forms, chlorophyll a and chlorophyll b, each using slightly different light wave lengths. In some plants the reflected light is different and different wave lengths are used so you get brown,red or orange colored aquatic plants for example.

So, plants ensure that the energy from the sun is taken into the food chain. By a series of herbivore, predator - maybe us? the energy is transferred up the food chain to the top.

Plants - where would we be without them?

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