Formation of biomolecules | High school biology | Khan Academy

• [Sal] So all organisms need food to survive. Now, for some of you, this might be pretty obvious. You realize what might happen to your body if you don't get food. You might realize that you need that food for both energy and you need that to actually build up your actual body. So you need it for matter as well.

But some of you might be thinking, all right, I have a mouth. I understand where the food goes. I also understand what's left over when I'm done with the food. And so I must have extracted some energy and matter from it, but you just said all organisms, Sal, and I'm looking outside of a window and I see a tree and a tree does not have a mouth. A tree is an organism. How does it get its food?

And the answer you might already realize is that the tree can make its own food through photosynthesis. We've seen this in other videos. You have carbon dioxide in the air and you have water. And the presence of energy in the form of sunlight, through the process of photosynthesis, would produce glucose and molecular oxygen as a byproduct.

And you can count the various carbons that are in this dark gray color. Oxygen's in this red color and hydrogen's here. And you can see that everything all adds up. You have one, two, three, four, five, six carbons here in the six carbon dioxide molecules. And then you could see in this glucose, you have one, two, three, four, five, six carbons. I encourage you to pause the video and make sure that the oxygens are all accounted for between the glucose molecule and this molecular oxygen, and that the hydrogens are all accounted for.

And so, a plant can produce its own food, and then when they need it, they can metabolize that food through a process of respiration. It's really important to realize that respiration does not just occur in your and my bodies. Even organisms like plants need to break down the food that they produced if they wanna use it for energy.

Now, the question you might have is, where is the energy here? Just as it takes energy to rearrange these atoms and molecules into glucose, under the right conditions, through a metabolic pathway, you can go the other way around. And chemically, that will release energy, which even a plant which isn't running around, it isn't doing jumping jacks, it isn't talking. It needs energy just to be alive.

All living things need energy in order to exist, in order to maintain their cells, in order to reproduce. So many of you all have probably heard the term carbohydrate when we're thinking about food or when we're thinking about an energy context. And it's important to note that this glucose molecule here is an example of a carbohydrate. It's not the only example of it, but one question might be, why is it even called a carbohydrate?

Well, when you break down the different parts, it seems like it would involve carbon. Right over here, you have the carbo part. And it seems like it's somehow involving water: carbohydrate. And that's because early chemists, they didn't actually understand the structure of a carbohydrate the way that we do now. All they saw was the ratio between the carbons, the hydrogens and the oxygens. That for every carbon.

So let's say there are N carbons, there's going to be twice as many hydrogens, and N oxygens as well. So one thing, one way to think about it, for every single carbon, you have an H2O. So every carbon, you have a water. So that's why early chemists, when they saw this ratio, they called it carbohydrates.

Glucose is a very simple carbohydrate, but you can make up chains of things like glucose to build up more complex carbohydrates. Now it's important to realize that these types of molecules aren't just used for energy. They can also be used for matter. The more that you study biochemistry, you're going to see a lot of different molecules that are made up of these building blocks of carbons, oxygens, and hydrogens.

And sometimes you might even recognize structures that look a little bit like glucose, or look like things that have been put together from some of these basic building blocks. For example, this molecule right over here is known as thymidine monophosphate, and it's a fancy name. But you can look at the building blocks here. The monophosphate, you have a phosphate group right over here. Thymidine comes from this part of the molecule.

It's sometimes known as a nitrogenous base because it has nitrogen that you see in blue over here. And then right over here, connecting the pieces, you have a five carbon sugar: Ribose. Now glucose is a six carbon sugar, Ribose is a five carbon sugar, but there are metabolic pathways where you can go from five carbon sugars to six carbon sugars, and back and forth.

And what's interesting about things like thymidine monophosphate is it is a building block for something that is very, very, very important: DNA. Thymidine monophosphate is a nucleotide with a nitrogenous base thymine. You put a bunch of these nucleotides together, not all of them have a nitrogenous base of thymine here, but they form this double helix structure that we study in a lot of depth in many, many other videos.

And you might already realize that DNA is the molecular basis of inheritance. We could not be who we are without these types of molecules. Now, what's also interesting is how do these different constituent molecules rearrange themselves, even in the presence of energy, to make other molecules? Or to get energy, how do they re-rearrange themselves to release that energy?

And all of these metabolic pathways are facilitated by what are known as enzymes. And just to give an example of an enzyme this big thing here is commonly known as the Rubisco enzyme. Now you don't have to know its name at this point in your careers, but this is one of the enzymes in the metabolic pathways that's able to take carbon dioxide and attach it to another molecule that eventually can get us to forming a glucose molecule.

And what happens here is the various constituents attach to different parts of these enzymes. And these enzymes change their shape as they attach to certain things and they can jam things together. They can synthesize other molecules or they can even help to break them apart.

And it all comes full circle because the enzymes themselves, these are proteins. These are made up of amino acids, which themselves are made up of a lot of these building block molecules that contain your carbons, your oxygens, and your hydrogens in them.

I'll let you go now. But the important thing is to realize is that we have whole universes occurring in our cells, and that all of these molecules and biological systems are connected in different ways. And you have a whole series of metabolic pathways that are facilitated by enzymes that take one set of things and step-by-step, put them together or break them apart in order to do all of the different biological functions that we know are necessary for life.