Notes of Nature: Photosynthesis

12 March 2012

Photosynthesis

While photosynthesis seems obvious now when we learn about it at school, it took many scientists, building up from each other's work, many years to research and figure out how plants intake food.

Discovery of photosynthesis
The first person to look into this question scientifically was Jan Baptista Van Helmont, who weighed a plant pot of soil and then weighed a willow tree. He looked after this tree for five years, at the end of which he weighed the plant pot of soil followed by the tree. The tree had added over 150lbs of weight, but the plant pot had only lost a few ounces of weight. Where had the tree gained its energy to grow? Helmont incorrectly concluded that the tree was using and keeping the water within its tissues for growth. What may seem obvious now, was back then a great leap forward. This research was published after his death in 1644.

In the 1770s, Joseph Preistly discovered that plants add oxygen to the air. Already known was the fact that flames needed air to burn, but used up this air in an enclosed space. Using part of a mint plant, Preistly found that the air could be renewed in the enclosed space.

Jan Ingenhousz, a Dutch doctor, was able to ascertain in 1779 that the green parts of plants renewed air. He found that non-green parts of the plants used oxygen. He then hypothesised that the green parts of the plant took carbon dioxide present in the air and separated this into carbon and oxygen, using the carbon and releasing oxygen.

Saussure, using evidence that simple sugars like the type created by plants contained carbon, hydrogen and oxygen. He repeated the original Helmont experiment. By keeping track of the carbon dioxide and the water the plant was exposed to, he hypothesised that the plant hydrogen from water and carbon from carbon dioxide.

The first theory of photosynthesis was presented by Julius Robert von Mayer in the 1840s. He theorised that plants took these elements and fused them into simple sugars. He concluded that plants used energy from sunlight to create the energy found in plant cells.

So what actually happens in photosynthesis?
While plants absorb water and nutrients from the soil, they make energy in the form of a sugar called glucose in the chlorophyll cells in the leaves using sunlight. The leaves take in carbon dioxide through pores in the leaves. The sunlight fuels the reaction between carbon dioxide and water. This converts the carbon dioxide and water into glucose and oxygen (as a by product). The plant stores the glucose and releases the oxygen. The oxygen is expelled through the same pores that the carbon dioxide was taken through. The chlorophyll colouration is what makes the plant green.

To see evidence of photosynthesis, I placed some Canadian Waterweed into a glass of water. The Gif below shows the small bubbles of oxygen that are released (bottom left) from the plant over a period of 3 minutes.