 Why do these plants in my balcony need sunlight? Why is this apple rotting? And why does my head eat so much? To answer this seemingly random questions, we need to figure out the modes of nutrition. Basically, what are the different ways of getting food? And that's what we'll do in this video. So, based on how different living things get their food, we can classify them into two categories. Producers and consumers. So producers, as the name suggests, they are the ones who can produce their own food. Most examples involve the green trees and plants and all the green stuff around you and some bacteria as well. And how do they make their own food? Well, they do that by using the energy available from their surrounding. For example, plants and all the green things can use the energy from the sunlight to make their own food. And you may have heard of this process called photosynthesis, which we'll talk about in more detail in future videos. But that is the reason why I need to expose these plants to sunlight because that's how they make their own food. And you and I are not producers because we can't make our own food like that. So what are we? Well, if you're not a producer, you are a consumer. Consumers, as the name suggests, are the ones that consume others, that eat others for food. You and I are consumers, my friend. And most animals and insects that you are familiar with and even most bacteria, they're all consumers. Either we eat vegetables and fruits or we eat other animals like chicken or fish or maybe eggs. So anyways, producers make their own food. Consumers eat others. Now, before we proceed, let me tell you their biological names. In biology, everything has a technical name. So the technical name or the biological name of producers is autotrophs. The word auto means self and trophs means nourish. So this means they can self-nourish. They can make their own food. They can feed themselves, self-nourish. On the other hand, consumers, their biological name is heterotrophs. The word hetero means others, trophs means again nourish. So like the name says, they can't self-nourish. They rely on others to nourish themselves. You and I and donkeys are heterotrophs, insects and most bacteria's heterotrophs. Now, guess what? Based on how heterotrophs eat, we can further classify them into three categories. Holozoic, saprotropes and parasites. Again, let's look at each one in detail. Holozoic are consumers which digest their food inside of their body. You are a holozoic. I am also a holozoic. Most animals and insects, we are all holozoics because we eat and then the food gets digested inside our body. And we are called so because the word holo means whole. In the sense, we eat whole food. Okay, what does that mean? Take an example of an apple. It doesn't say that you have to eat the whole apple. But even if you take a slice of an apple, then that apple has some constituents, right? Like it has, for example, sugars. Then it has some proteins, maybe it also has some vitamins and so many other constituent. These dots just represent constituents, okay? Now when you eat this slice of apple, you take in all the constituents inside your mouth, right? That's the meaning of whole. That's the meaning of holo. You're eating this whole thing. And then once it goes into your body, then your digestive system breaks it into its constituent particles. Right? That's the whole idea behind holozoic. These are the animals that digest their food inside their body. Now in order to be a holozoic, you need to have a pretty good digestive system so that you can break down whatever you eat inside your body. But what if you don't have a good digestive system? Then what happens? Well, then you become a saprotrope. Saprotropes are the ones that don't have a good digestive system and as a result, they digest the food outside of their body and then eat it. And as we will see, it's these guys who are responsible for rotting and decaying things and they include most bacteria and mushrooms. Well, mushrooms is a very specific example. In general, these are fungus, right? So bacteria and fungi, fungi is plural, fungus is singular. So bacteria and fungi are the saprotropes. They digest the food outside of their body. And they're there everywhere, okay? I mean, if you take that piece of apple and just leave it outside, don't eat it. Then pretty soon you will find these bacteria and fungi will start attacking it. So let's say these bacteria start attacking it. Now since they cannot digest the food inside their body because they don't have a very good digestive system, what they do is they try to digest the food outside. How do they do that? They spit all the enzymes or the acids needed to break down the food. Right? So as they spit these enzymes and acid, chemical reactions take place and the food gets digested then and there. And so this apple starts getting broken into its constituents. And then once it's broken, then these bacteria will start absorbing these constituents. Okay, but why does it rot and start smelling? Well, that's because whenever digestion is happening, whenever chemical reactions are happening, some by-product is created, right? And it's these by-products that start making things rot. And that's what makes the whole thing smell as well. And because of that more insects get attracted and now you can't eat this. And this sounds very disgusting, right? But what's interesting is a similar digestion process happening in our bellies as well. Even when we digest food, all this stuff is happening. But because it's inside our body, we don't smell it. But if you throw up, if you vomit, then you can smell that. I know it's pretty disgusting, sorry for that. But when you vomit, basically you are vomiting out digested food with all these by-products and you know even that smells, right? So we digest the food inside, but these saprotropes digest the food outside and that causes things to rot. And that's the same reason why this apple is rotting. It's being digested by these saprotropes. Another example is you may have seen if you keep bread outside for some time then they will start growing this fungus over here. The fungus are digesting it outside now. And your teeth also gets decayed for the same reason. There are a lot of bacteria in our mouth and when we eat food, the food gets stuck in the teeth and then they start getting digested by the saprotropes. And that's why I always brush my teeth twice every day. And just a small thing, okay? After learning this, I used to feel they are so annoying. I mean, if they were not there, I wouldn't have to brush my teeth. Things wouldn't rot at all. Won't that be awesome? No, because guess what? They also feed on the dead. They are the ones that make sure to decompose all the dead stuff, right? So if they were not there, then all the dead things like the dead animals and plants and everything would just pile up. You cannot imagine what earth would be. So thanks to them, they decompose all the dead stuff as well and recycle all the nutrients, make sure they go back into the nature which can be reabsorbed by the plants, reused to make more food. They are super essential for our environment. And the word sapro itself means rotting. And that's why they are called saprotropes because they eat on the dead and decay and make them rot. Okay, and finally, what if you can't digest food at all? Whether inside or outside, then what? Then you become a parasite. Parasites hardly digest any food. Then how do they get their nutrients? Well, they depend on a host, meaning another living organism. For example, this leech is a parasite. It gets stuck to my leg, let's say, and then it starts sucking my blood. Now my blood already contains digested food and so it just starts getting nutrients from my blood directly. You see, they don't have to digest anything. So in that case, since they are depending on me, I am their host. Similarly, I have to scratch my head so much because there are some parasites on my head. Lices are examples of parasites as well. They also suck on the blood from the scalp. Again, I am their host. So parasites can either live on the surface of their hosts like leeches and lices or they can live inside them as well. For example, tapeworms and roundworms live inside intestines. So even though you may be healthy, you have these inside your intestines. And guess why are they there? Well, because again, intestine is where most food is already digested and so they can just eat that. And so the word para kind of means alongside. In the sense, they're eating alongside its host. So whatever I eat, they go to these guys as well. Now one important thing about parasites is I should think they're trying to kill me. But no, they're not trying to kill me. They want me to be alive so that I can keep on feeding them for their entire life. So parasites don't try to kill their hosts but they end up harming their hosts. For example, when this leech starts sucking on my blood, they rupture the skin. Because of these lices, my head is getting itchy. Tapeworms and roundworms can cause my stomach to ache. So they are definitely not trying to kill us but they end up harming us. And again, I should think parasites are pretty useless and many scientists you should think as well. But recently we realized even they are pretty useful. For example, some scientists believe that the worms in our intestines actually help in boosting our immunity over there. Another example, some leeches are useful in removing infected blood from our body. And would you be surprised if I told you that 50% of all the species of living things on our planet are parasites? That's right, there are a lot of parasites on our planet. Almost every living thing has a parasite inside or on top of it. Anyways, that's pretty much it. So let's quickly look at what we learned. There are two basic kinds of living beings, autotrophs who generate the food themselves using the energy from the surrounding and the heterotrophs who can't produce their own food so they eat others. And based on how you digest the food, heterotrophs can be further classified as holozoic if you take the whole food inside and digest it inside your body. Most animals and insects come under this category and you need a pretty good digest system for that. If you don't have a great digest system, no worries, you become a safrotrope. Then you just digest the food outside of your body and then you make things rot. They are the decomposers, they are the ones who eat on the dead and the decaying animals. Most bacteria and fungi. And finally, if you can't even do that, then you just become a parasite. Just stick yourself to a host or live inside of them and then you get free digested food. They might harm you, but most of them have no intentions of killing you.