It's easy to think that plants are boring. They don't seem to do anything! They stay in one place and grow so slowly that we can't see them move. They don't hunt, hide, fly, build homes, communicate, or do any of the other things that fascinate us about other living creatures. But plants have a secret! Inside that calm exterior they are busily working at a complex process that fuels the whole planet: photosynthesis.
Photosynthesis comes from Greek and means 'putting together with light.' While we humans are trying hard these days to harness the sun's energy to power our homes and vehicles, every green leaf in the world is making the most of solar energy to 'put together' food from water and carbon dioxide. The carbohydrates they make in this process forms the foundation of the food chain - plants (and some photosynthetic bacteria & algae) are the only 'producers' of food; all other living things are 'consumers,' feeding directly or indirectly on the food produced in photosynthesis. But that's not all - photosynthesis is also the main source of oxygen that most living creatures need in order to breathe.
So how does photosynthesis work?
Photosynthesis primarily happens in green leaves. Leaves are ideal for photosynthesis because they are usually broad and flat, giving plenty of surface area for light to be absorbed. They are also thin, which means diffusion of gases such as carbon dioxide can happen quickly. Leaf cells are full of organelles called chloroplasts, which contain chlorophyll, a pigment that absorbs light. (Chlorophyll absorbs all the red and blue wavelengths of light, but it reflects green wavelengths, making the leaf look green.) Leaves cannot perform photosynthesis without chlorophyll. Some plants have variegated leaves, with patterns of white and green. In these plants only the green parts of the leaf can photosynthesize, because the white parts have no chlorophyll.
A leaf has all its chloroplasts ready and waiting - what else does it need for photosynthesis?
- Carbon Dioxide - This gas enters through pores called stomata located on the underside of the leaf. The stomata can close at night when no photosynthesis is taking place, or during the heat of the day when the plant is in danger of too much water evaporating from its leaves.
- Water - this is absorbed by the roots and sent up to the leaves through the xylem part of the plant's vascular tissue.
- Sunlight - the sun provides the energy that makes the process run!
When these three elements are present, the following chemical reaction takes place (Light is in brackets because though it is necessary to power the reaction, it isn't actually one of the reacting substances):
- carbon dioxide + water + [light energy] → oxygen + glucose
(The chemical equation looks like this: 6CO2 + 6H2O + [light energy] → 6O2 + C6H12O6)
The oxygen is released through the stomata into the air, giving us what we need to breathe. The plant usually makes more glucose than it needs immediately, so the extra is stored as a more complex sugar or as starch until the plant needs it for growth or for food when it is too dark to perform photosynthesis. (One of the ways to test if photosynthesis has occurred is to test for the presence of starch.)
In order to use the food they have made, plant cells must perform cellular respiration. Interestingly, respiration is almost exactly the opposite of photosynthesis. The cell uses oxygen and glucose to create water, carbon dioxide, and energy. (Our cells do this too, which is why we breathe in oxygen but breathe out carbon dioxide.) Respiration happens all the time, not just in the daylight. You may be wondering how plants produce oxygen for us to breathe when they have to use it themselves for cellular respiration. Well, the rate of photosynthesis is usually faster than respiration, so a plant produces more oxygen than it needs for itself. It also produces more sugar than it needs right away, which is how it has some left over to store. (Many times this storage becomes food for us - potatoes are made of extra stored starch, for example!)
So leaves might not seem like much, but really they are one of the foundations of life. Without them performing photosynthesis, you wouldn't have oxygen to breathe or food to eat...you wouldn't be here, in fact! Next time you look at a leaf, think of the amazing, complex process going on in its microscopic cells, helping keep you alive.
Want to watch it happen? Check out this photosynthesis science project!