In this edition of HTS, we’ll be looking into a process called photosynthesis. This is one of those big words you might have heard in a quiz or from Spongebob.

We can start to explain photosynthesis by picking apart the word. The photo- part means light, as in the stuff you see. The -synthesis part basically means making things, and the things plants make are the same substances that we rely on for food. So this process isn’t just important for plants: it ultimately provides all the energy that animals need to stay alive. How’s that for important?

There are a wide array of plant species growing in New Zealand. Some have pretty flowers, some have nasty thorns, but they all have green parts. I’m talking about leaves. Granted, in some trees the green parts die and fall off once a year, but that doesn’t take away from their function. They may not smell as nice as flowers, but they’re the interesting part of a plant if you want to know about photosynthesis.

Very recently, a report was released saying that there are three trillion trees on Earth. That’s almost 500 times the number of humans! And trees are just one group of the gigantic plant kingdom. The point is, there are quite a lot of leaves on our planet. This trick that plants can do must be very successful, if there are so many of them in existence.

Photosynthesis turns light, carbon dioxide and water into oxygen.

Photosynthesis turns light, carbon dioxide and water into oxygen. (Pic credit: Masterman535 )

In high school science classes, you will probably see a diagram like this at some point. It’s an equation that shows what plants suck in and what they make in photosynthesis. They all need water, as we know, and I have outlined some of the oddities of water in a previous edition of HTS. They also take in a lot of carbon dioxide, the infamous greenhouse gas. This is why plants are sometimes called “carbon sinks”: they absorb carbon from the air and store it until they die. They also produce products which we need to live.

The products are oxygen and glucose. Oxygen is the gas that you collect when you breathe. It keeps your brain going and your muscles flexing. Glucose is a ring-shaped molecule that makes up sugar. That’s also very significant, because sugar molecules are the backbone of pretty much everything we eat. They are where we get our energy from.

Now to the key ingredient, light from the sun is needed for this reaction to take place. You see, light is a type of energy, which I mentioned during this discussion of Pop-Tart chemistry. It’s actually very hard for the plant to turn water and carbon dioxide into oxygen and sugar. Why?

In any system, whether it be a leaf, a set of playground swings or a burning Pop-Tart, there is always energy being moved around. As a general rule, the system becomes happier the less energy it has. In other words, if you let the total energy go down, the system moves to a more stable form. Molecules are systems too, and it turns out that the molecules in water and carbon dioxide are very stable on Earth.

That’s a problem, because stable molecules don’t like being broken up and made into new substances. Extra energy is needed to convince them to become oxygen and glucose. Sunlight is what gives that extra energy to the plants, so they literally live to catch sunlight. If a plant has plenty of water in its roots and carbon dioxide in its pores, it just needs big leaves for the sun to shine on.

Chlorophyll in action.

Chlorophyll in action.

Next question, why are leaves green? They contain a green chemical called chlorophyll. It’s a pigment, like you’d find in paint, but is perfectly adapted for capturing light energy. Chlorophyll is able to absorb almost all the colours of light that come from the sun. The only colour it doesn’t absorb is greenish, so that colour gets re-directed into your eyes and the whole leaf looks green.

Scientists are still working hard to understand how photosynthesis works on a really deep level. New discoveries are made all the time, but new questions always pop up. The diagram above turns out to have two steps in it. There is an array of funky molecules called enzymes that drive the reaction along. Even people who teach quantum mechanics are getting involved, as if the process weren’t complicated enough already.

So there is a lot more to learn and, if you’re really keen, studying botany or biochemistry is a great path to understanding how plants work. It’s no exaggeration to say they live by eating the sun, but they do so much for us too. Without plants, we would have nothing to eat and there would be no oxygen on Earth for us to breathe. There would also be no pretty flowers to admire on a warm summer day.