The Molecules of Photosynthesis & Respiration

Water H2OCarbon dioxide CO2

Photosynthesis

Plants, algae, and other living things containing chlorophyll can capture the energy of Sunlight and use it to tear apart and rearrange these two molecules (water, H2O + carbon dioxide CO2), in a process called "photosynthesis" (literally means "making with light").

After rearrangement, two new molecules are formed (using the very same atoms present originally: 3 O's (oxygen), one C (carbon), and 2 H's (hydrogen). One of the new molecules contains carbon, and H and O in the proportions of 2 H's to every O (like water, or "hydrate"), so it's called "carbohydrate". Examples are various types of sugar, starch, cellulose (in wood), and (shown below) formaldehyde, the simplest example.

As a chemical equation: H2O + CO2 + light energy -> CH2O + O2 (with the energy stored in the sugar)

Carbohydrate CH2ODiatomic oxygen O2
Some of the energy captured from the Sun is stored in the carbohydrate, making it a type of fuel. Like other fuels e.g. methane, natural gas in a kitchen stove, the energy can be released again by burning. In living things, this burning process is called respiration.


Respiration

 Living things need energy to live (and all the processes associated with living: growing, digesting, moving, ...). Where does this energy come from? It's stored in FOOD - a type of fuel, adsorbed by eating and digestion. How is the energy gotten out of the food/fuel? By burning it. Oxygen needed is obtained by breathing (air for land creatures, or from oxygen dissolved in water for water creatures). The oxygen and food combine in the cells of the body just as CH4 burns in a stove flame. The rearrangement of atoms is exactly the reverse of photosynthesis - CH2O + O2 -> H2O + CO2.

Practice these concepts in class with a hands-on activity, and observe them in action in mini-aquariums.
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