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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 it 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?
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):
(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.
Watch It Happen: Photosynthesis Project
All these busy plants around us, producing what we need to live, and they don't look like they're doing anything. How can we tell if they are performing photosynthesis? One way is to see if they are giving off oxygen, the most important byproduct of photosynthesis. Of course, we can't usually see leaves producing oxygen, but watch what happens when you use an underwater plant!
Normally we can't see the oxygen produced by photosynthesis, but when it is produced underwater it appears as bubbles in the water. These float up through the funnel and displace the water in the test tube. Fire needs oxygen to burn, so when you insert the splint, the pure oxygen in the test tube causes it to glow brighter or produce a flame.
The rate of photosynthesis varies with several factors, including the intensity of sunlight and the temperature of the plant. (Other factors include the amount of water and the concentration of carbon dioxide (CO2) in the air.) You can design an experiment to test some of these variables: for example, will photosynthesis happen faster or slower if you put the elodea in warm water? Collect data by measuring how much oxygen is produced in a given amount of time when the elodea is submerged in warm water vs. cold water. You can either mark the level of oxygen on the glass with a wax pencil, or you can use a graduated cylinder instead of a test tube and measure more precisely in milliliters. Try to keep the other variables constant - it's best if you can run two jars simultaneously so you know they are getting the same intensity of sunlight. If you only have one jar, how else could you make sure the light intensity is constant? Could you use a light source other than the sun?
(This project is adapted from The Amateur Naturalist by Nick Baker.)
Phototropism Obstacle Course
Without light, a plant can't make its food by photosynthesis. Sunlight is so important to a plant that it will change the way it grows so that it points toward the light. This is called phototropism, from the Greek words for 'light' and 'turn.' You can see this with a houseplant: the leaves grow to point toward the window. If you turn it around it will eventually move to orient itself toward the window again. How strong is this attraction to sunlight? Can a plant grow around obstacles to find the light? Time to find out!
The bean plant grows towards the only source of light, the hole in the top of the box, even if it means growing around the cardboard obstacles you placed inside the box! The energy it needs to sprout and start growing was stored in the seed, but eventually that food will be all used up and the plant will need to make more through photosynthesis. The plant spends the energy from the seed trying to find light so it can survive.
(This project is adapted from The Amateur Naturalist by Nick Baker.)
Meat-Eaters: Some plants, like Venus flytraps or pitcher plants, actually capture insects and digest them for protein! These plants still perform photosynthesis to make their food, but they grow in soil that lacks some of the nutrients that they need, so they have to get those nutrients from insects.
Photosynthesis at Sea: Forests are in important source of oxygen, but did you know that at least half of the earth's oxygen comes from the oceans? Microorganisms like phytoplankton and cyanobacteria live on the water's surface and produce oxygen through photosynthesis.