Photosynthesis [replacing the lesson: Photosynthesis]

What do you do when you’re hungry, or feel you lack energy? You might rummage through the kitchen looking for a snack, or prepare yourself a meal. Food is essential not just to get energy, but to live! This is true not only for humans, but for animals and even plants. But plants can’t eat like we, or animals, do.

So, how do plants survive? Where do they get their nutrients and energy from? Here is a tomato plant. It has roots inside the soil, many green leaves, and a stem connecting them. When the soil is watered, the roots take up water, which is then transported to other parts of the plant, including the leaves.

Another important ingredient a plant needs to make its food is carbon dioxide. The plant takes up carbon dioxide from its environment. Carbon dioxide enters the plant from the air, through tiny holes in the leaves. Now that carbon dioxide and water are inside the leaf, let’s zoom in and see the cells the leaf is made up of. These cells are full of tiny green structures!

These are chloroplasts. You can think of them as tiny kitchens where plants prepare their food. Chloroplasts are green because of molecules called chlorophyll. Chlorophyll is what gives leaves their distinct green colour, but these molecules have an even more important role. They are responsible for capturing and absorbing energy from the sun.

So, now we have water, carbon dioxide, and energy from the sun, all inside the chloroplasts. What happens next? Energy from the sun drives a chemical reaction between carbon dioxide and water. This chemical reaction produces glucose and oxygen — complex compounds that sustain life. We can write this chemical equation as six carbon dioxide molecules plus six water molecules, which react in the presence of light, to produce one glucose molecule and six oxygen molecules.

Through this reaction called photosynthesis, the plant has just produced its own food — glucose! Glucose is the starting molecule that plants use to make other materials they need to live, such as starch, cellulose, fats, oils and amino acids. And much of the oxygen produced in the reaction is released into the plant’s surroundings and can be used by all other organisms on Earth to breathe! Plants are actually not the only organisms that can carry out photosynthesis. A certain type of aquatic bacteria — cyanobacteria — can photosynthesise too!

Even though cyanobacteria don’t have leaves or roots like plants do, they do contain chlorophyll. This is why they can also carry out photosynthesis and produce their own food. So photosynthesis uses light from the sun, to transform carbon dioxide and water into glucose and oxygen. Glucose can then be used to make other materials, such as starch, cellulose, even fats, oils and amino acids. These are all essential molecules that plants need to live, and that can be passed on to other organisms through the food chain.

And the oxygen produced ends up in the water or the air, sustaining all life on Earth. This makes photosynthesis an absolutely essential process for life.