Photosynthesis and Cellular Respiration: Crash Course Botany #5

Plants grow. Okay, yeah, you probably knew that. In fact, you also probably know that plants grow because they make their own food from a combination of water, sunlight, and gases from the air. To those of us who have to grocery shop for food, photosynthesis seems almost like a magic trick. Woooow! But there's a lot of work going on behind the scenes. Inside many, many cells inside

every single leaf of every single plant on earth is a cranking, shunting, thundering workshop that never stops. These living workshops are consumed with photosynthesis and cellular respiration. Processes dedicated to making, breaking, and reshaping molecules with the singular purpose of keeping a plant alive. Though, we also have these processes to thank for our beautiful green planet and all of the creatures who call it home. In plants

may be structured perfectly for these intricate tasks, but they can't do it all on their own. This magic show includes audience participation. That's us. Hi, I'm Alexis, and this is Crash Course Botany. When you consider the amount of living stuff on this planet, plants really do come out on top. They make up about 80% of the world's biomass, or at the total weight of living organisms.

But where does all of that heft actually come from? You might initially think that soil plays a big part. After all, most plants grow in soil, and it's similar to the solid, heavy stuff that we use to make building materials. But the botanical world actually does something much more remarkable. The building blocks for all that biomass come predominantly from the air. In this biological magic act

plants pull microscopic carbon molecules from the atmosphere, which become the major materials for building plant bodies, and then, ala kazam. We have a plant kingdom to rule the world. Okay, it's not that simple. I'll break it down for you bit by bit. I recognize this first trick as the process of photosynthesis. The word comes from Greek, and basically means put together with light, which is a

fairly accurate description of what's going on. Nearly all of Earth's energy comes from the sun. So, in photosynthesis, plant cells combine a zap of that energy with carbon dioxide pulled from the air and water. The end products of this chemical reaction are the sugar glucose that plants use as food, and the byproduct, or unintended extra product, oxygen. There are two stages to the photosynthesis process, the

photo part, and the synthesis part. The photo part is more accurately known as the light dependent reactions. Because as the name suggests, they can only happen with the help of light energy. Here, tiny organelles called chloroplasts inside the cells of leaves take center stage. They contain the green pigment chlorophyll, which is able to capture the light energy that falls on. And then harness that energy to

do work in the plant. So, you can think of chloroplasts a bit like the original solar panels, taking energy from the sun and converting it to a usable form. With the help of light energy, water molecules are split apart, generating spare oxygen in the process. And to be clear, the goal of photosynthesis is to help plants thrive. Full stop. It's really not to make oxygen for

us humans to breathe. But on the other hand, we help plants accomplish photosynthesis too. Carbon dioxide is released into the air in a few different ways, including as an unintentional byproduct for us when we exhale. That CO2 is essential to the photo part of photosynthesis. So, we're not just volunteers ready to pick a card, any card. We're full, fledged magicians' assistance. Does this mean I get

a cane? Okay. So, with the energy trapped, water split, and some CO2. To absorb, it's time for the synthesis phase, a photosynthesis, more commonly known as the light independent reactions, which don't need any input from the sun. At this point, the energy that's already been captured is used, along with the split water to convert carbon dioxide into glucose, completing the reaction. Can we get some applause,

please? Oh, oh, plants don't usually disappear in a puff of smoke at the end. Although this final stage doesn't need to be light, it does need the products of the light dependent reactions, which don't last for very long inside plant cells. So photosynthesis really only happens during the day. In any case, the whole point of photosynthesis is to make food for the plant and the form

of glucose. And those small sugar molecules have several different potential fates inside the plant. Some of them are used as the building blocks for much larger cellulose molecules, which surround every cell in the plant and are the main construction materials for roots, stems, and leaves. And some are converted to other types of molecules like sucrose and starch that can save energy for later. The rest become

fuel for the other major function of the cellular workshop, cellular respiration. By the way, cellular respiration isn't unique to plants. Every living thing does it, including you and me. It's happening all the time, day and night, and in plants case, often at the same time as photosynthesis. Cellular respiration is the process organisms use to transfer the energy they get from food. And to the usable energy

we need to do everything we do, like hiking up a mountain, taking a test, or plotting elaborate plant-related fan fiction. Oh, oh, that's just me? The only difference between cellular respiration and animals, like us, and in plants is that we need to eat food to get our glucose fuel. Plants can make their own. And because plants can do that, they give us things to eat, from

plants themselves to the animals that eat them. If a worker photosynthesis converting energy from the sun to a form usable, aka, edible to us, there'd be no food tans at all. And we wouldn't be able to power our own cellular respiration process. So for plants and people alike, cellular respiration happens in organelles called the mitochondria, often referred to as the powerhouses of the cell. What that

Oakley-Shei actually means is that mitochondria acquire the energy needed to power cell's chemical reactions. They do this by breaking down raw fuel to make special molecules known as ATP. Come on Earth, there are two main ways that plants can generate these energy storing molecules. The first aerobic respiration is the most common. And the aerobic part of its name tells us that it involves oxygen. This kind

of respiration is a reversal of the photosynthesis reaction. Glucose and oxygen combine to release energy with water and carbon dioxide as waste products instead of the other way around. That released energy can be used to make stuff happen in the plant on a microscopic level. But that work translates into things we can observe, like flowers growing, fruit ripening, or leaves arching towards the sun. And it's

efficient. Through aerobic respiration, each molecule of glucose can make over 30 molecules of ATP. That's like the energy you get from a nice big practice. A warm bowl of oatmeal, some avocado toast, and half a grapefruit. The other way that plants respire is by burning their glucose in the fuel without oxygen around in a process known as anaerobic respiration. This is much less efficient than the

cells have yet another important application to us humans. They produce bio-mass that can be used as bio-fuel. For example, around the world more than 180 million tons of tomatoes are produced every year. In my ideal world, these would all go towards making delicious salads, pasta sauces, and ketchup. But in reality, not every tomato makes the grade. And to wonky for our supermarket shelves and many are

rejected as waste from various sauce-making processes. Enter environmental engineer Dr. Venkatramna, Gautam Shetty, and his team in South Dakota, USA. In 2016, Bay and their collaborators figured out how to give those poor, discarded tomatoes and new lease on life by using them to generate electricity. It works a bit like the lemon or potato batteries you might have made at school. An electrochemical device called a fuel

cell helps to break down the tomato waste and extract electrons making an electric current. It turns out that tomato pulp is an ideal fuel for this because it has loads of high energy sugars. Sugars that were originally created by photosynthesis. So not only are plants perfect for photosynthesis, but some plants like tomatoes, corn, and soybean seem to be perfect for generating electricity too. And we're already

in the middle of the night. We're only using the energy stored in biomass today. Every time we burn wood in an open fire or charcoal on a barbecue. That energy came from the sun, was made usable by photosynthesis, and then rather than eating it, we converted it into electricity to power our lives. In the US, up to 10% of the gas we put in our cars

contains a biofuel called ethanol, which comes from corn. And other fuels from plants are increasingly being used for heating and generating electricity. Biofuels are burned in the same way as fossil fuels. And they do release carbon dioxide into the air. The difference is that the carbon dioxide biofuels release was already in the air until very recently. When a living plant worked its photosynthesis magic to turn

the CO2 into glucose. By contrast, when we burn traditional fossil fuels, we're releasing carbon that's been locked up for millions of years. So the magic of burning biofuels is that there's practically no overall increase in the amount of carbon dioxide in the atmosphere, which is good news for anyone looking to slow down the pace of climate change. While there are pros and cons of any energy

source, it's helpful to have a variety of options to work with. And biofuels are just one of them thanks to the power of photosynthesis. So the next time you come across a plant, whether it's a mighty oak tree or a tiny daisy, spare a thought for the relentless chemical industry that's going on behind the scenes, photosynthesis and cellular respiration may seem as simple as some sleight

of hand. But those amazing tricks are the result of super efficient processes that plants are doing all the time. And we get to be a part of that. We may rely on plant power photosynthesis, but we get to contribute carbon dioxide when we exhale stinky garlic breath and plants make it something beautiful. Next time, we'll enroot the great tree of life to discover how plants evolved

into the incredible organisms we know today. Hey, before we go, let's branch out. What organisms make up the next largest percentage of Earth's biomass after plants? Look closely to find the answer in the comments because they're pointy tiny. Thanks for watching this episode of Crash Course Botany, which was filmed at the Damiar Farisawitch studio and made in partnership with PBS Digital Studios and Nature. If you

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