Photosynthesis in organisms | Matter and energy in organisms | Middle school biology | Khan Academy

Hey, I'm going to let you in on a little secret of mine. I love gardening! In fact, I have a huge garden with apples, blueberries, pumpkins, and tomatoes. I give my plants micronutrients and maybe some fertilizer, but I don't give them food in the same way that I would give my dog, who eats multiple times a day.

With plants, I just put them in the soil, water them regularly, and watch them grow. How do they do it? Well, they use a process called photosynthesis. Photosynthesis is a scientific term to describe how certain living things use energy from sunlight to live and grow. Many different kinds of plants, algae, and various single-celled organisms are able to carry out this amazing process.

Today, let's describe photosynthesis using tomatoes. So here you see a young tomato plant, similar to the ones I have in my own garden. Let's name it Planty. First, let's start off by looking at Planty's immediate surroundings. The soil Planty is growing in contains water, and the air surrounding Planty contains molecules of carbon dioxide. These two compounds are the starting materials or inputs that Planty needs in order to carry out photosynthesis.

Now, let's take a look at the weather forecast. Well, it looks like it's going to be a sunny day today! So when the sun shines on the earth, it sends out energy in the form of light. This energy is essential for Planty to be able to carry out photosynthesis.

Next, let's take a closer look at Planty, because Planty’s cells are hiding a secret of their own. Inside Planty’s cells are microscopic structures that help Planty carry out photosynthesis. These structures are called chloroplasts. Chloroplasts contain green pigment molecules called chlorophyll. It is actually within these molecules that the sun's energy is captured for use during photosynthesis.

Interestingly, chlorophyll is where leaves get their green color. To help you remember, think of chlorophyll like this: "chloro" means green, and "phil" means leaf. So when you put them together, you get "green leaf."

Okay, so now we know that carbon dioxide, water, and sunlight are required for photosynthesis to occur. We know that in plants like Planty, photosynthesis happens in the chloroplasts with the help of chlorophyll. Next, let's take a look at what happens during the process of photosynthesis itself.

When the sunlight reaches Planty's chloroplasts, its energy is used to rearrange the atoms in the carbon dioxide and water molecules through a series of chemical reactions. The outputs of these reactions are oxygen molecules and sugars. These sugars are vital to Planty's survival because they are used as a source of chemical energy that helps Planty live and grow. In other words, they're Planty's food.

Planty can do a few things with the sugars made during photosynthesis. It can break the sugars down to get usable energy right away, or it can store the sugar molecules for later use. If Planty stores the sugar molecules, it can then use them as a source of energy in the future. This means that Planty can grow even when the sun isn't shining.

Planty can also use the sugar molecules to build larger molecules such as cellulose that make up the structure of Planty itself. In this way, Planty can grow bigger and bigger with very little help from me, all thanks to the process of photosynthesis. Planty's other output, oxygen, is released into the atmosphere.

This is really cool because it provides organisms in the ecosystem, including us, with oxygen. So thanks, Planty, and other photosynthetic organisms for giving us the oxygen we need to breathe. And that, my friends, is photosynthesis.

So next time you're in a garden, I want you to think about this: all the plant structures you can see, such as stems, leaves, and even the fruits and vegetables that we eat, are made up mostly of the atoms that were once a part of the starting materials of photosynthesis. With the help of a little sunlight, the plants made their own food and grew bigger, taking CO2 out of the atmosphere and releasing oxygen back in for us to breathe.

And on that note, I think I should go outside and give my own garden some water. See you later!