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Primary productivity in ecosystems

Advanced Placement Environmental Science, AP Environmental science, AP Enviro Sci, APES, Environmental, Science ENG-1.A.1 ENG-1.A.2 ENG-1.A.3 ENG-1.A.4 . Created by Khan Academy.

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Video transcript

- [Instructor] In this video, we're going to talk about energy, and in particular, we're gonna talk about the energy of life, the energy that I need to live and all of us need to live, the energy you need to think, the energy I'm using to make this video right now. And some of you might already guess where this energy is coming from, the surface of our planet is constantly being bombarded with light energy from the Sun, and you might know that there is certain organisms on our planet that are capable of taking that light energy and then storing it as chemical energy. And those things, and there's many types, but the ones that we see most often in our day-to-day life, are plants. And so let's imagine a plant here. And what it's doing is, it's using that light energy in conjunction with water, typically from the soil, that maybe it's getting through its roots, and carbon dioxide in the air. And it's using that light energy to actually stick, or you could say fix, the carbon to construct itself. And in its own tissue, it's storing that energy, and then if it were to break down that tissue, it can release that energy in various forms. Now, as it does this, you might also be familiar that these photosynthesizers, or these primary producers, or these autotrophs, they're also releasing molecular oxygen. Now, if we were to describe this in chemical terms, or chemistry terms, we would describe this process of photosynthesis as taking carbon dioxide from the air, in conjunction with water from the soil, and what's, I guess you could say, fueling all of this is light energy, usually from the Sun, and what that is yielding is the tissue of the plant that is actually storing that energy is chemical energy in an organic form, and the primary way that this is done is through glucose, which is C6H12O6. I know what you're thinking, "All plants don't taste sweet." Well, if you take chains of sugars and put them together, you get carbohydrates, and you adapt 'em a little bit, you get things like starches, and that's what most of the plant tissue is. So some variations of this, or variations of this linked together, but this is where the energy is stored, so energy stored, in the actual plant tissue, and then of course, it releases that molecular oxygen. And this is the process of photosynthesis. And even if you look at the word photosynthesis, and what the parts of it mean, photo is referring to light, and synthesis is referring to putting something together, synthesizing something. So photosynthesis, you're using light to put together these, essentially, fix the carbon together, to store energy. Now, you might say, "All right, that's nice. "I'm storing the energy this way, "how do I actually use the energy?" And that's something that all of us are doing, all living systems have to do, and that process is respiration. And you could already guess what at least the chemical reaction for respiration will look like, you're going to start with our stored energy, our glucose C6H12O6, in the presence of oxygen. And since we're respiring all the time, this is why we need to breathe oxygen, and this is going to yield carbon dioxide, and that's why we exhale more carbon dioxide than we inhale, it's also going to release water, and it's going to release, and this is the whole point of it, cellular energy. And in other videos that you'll see in a biology class will talk about how this form of stored energy gets converted to other forms and then how that's used by the various machinery and cells to actually live, to reproduce, to move, in many cases. Now, an interesting question is, how do you measure how much photosynthesis is going on, how much primary productivity is going on? Well, one way to think about it is, find an ecosystem and take a certain area of the surface of that ecosystem, and it could be a terrestrial ecosystem, on land, it could be a marine ecosystem, and then say, for this area, in a given period of time, oftentimes, a year, how much stuff is growing? So this is the stuff that grows, and obviously it would seem that the more stuff that is growing, that the more photosynthesis that is taking place. And the way that they measure how much, how much, is growing, you could either measure it in terms of grams of biomass, so grams of biomass, and biomass is just a fancy way of saying that the mass of biological stuff that's just growing on this area, and usually they'll take the water out so they get a consistent measurement, or you can convert this to calories, and it's usually measured in thousands of calories, kilocalories. And when you see calories on a packaging food label, what most of us think of calories, those are actually kilocalories when we think about it in scientific terms. And I know what you're thinking, you're like, "Wait, mass and kilocalories, "calories, that's just a form of energy." Well, those two things you can go between, because usually a certain type of biomass, a gram of a certain type of biomass, will have a certain amount of energy stored in it, not energy that necessarily all animals could use or that we could use, but it does have energy in it. Now, when we talk about this primary productivity, you might already be thinking about, "Well, don't the plants need to use some of the energy "that they are producing themselves to live?" And my answer to you is of course they need it. In fact, that's probably the most important reason why they need to photosynthesize, is because they need to do respiration in order for them to grow and metabolize and live and reproduce. And so when you see how much has been produced in a given area in given year, you're actually seeing the net primary productivity. This is the, you could think about it, how much photosynthesis they did minus how much respiration they did. So if you think of how much photosynthesis they did as gross primary productivity, so that's the total amount of photosynthesis, and then you subtract out the amount of energy, chemical energy or cellular energy they needed for respiration, that would then give you the net primary productivity. And as I mentioned, just to make things a little bit tangible, if you took a very productive ecosystem, let's say something like a rainforest that I have here in the background, a very productive ecosystem like this, if you were to take, on average, a square meter of this, it produces in a year about 2,000 grams of biomass. So here, we would say that the net primary productivity of this rainforest that you see in the background here would be approximately 2,000 grams per square meter per year, and if you wanted to think about this in terms of kilocalories, you just have to say, well, if each gram of biomass is how many kilocalories, and it depends on the type of biomass, but let's say that we have four kilocalories per gram of biomass. So then we could also say that this net primary productivity is equal to 2,000 grams per square meter per year times four kilocalories per gram, the grams cancel out, and then you multiply four times 2,000, that's going to be 8,000 kilocalories, kilocalories per square meter per year. That would be the net primary productivity, because that's after the plants have been doing respiration. Now, how would you figure out gross primary productivity? Well, you're not going to be able to do it directly, but you can figure that out by figuring out the rate of respiration. If you took some plants in that ecosystem, and then you put them in a dark room with no light, and then you if you saw how much oxygen they are absorbing, or they're having to use, then that gives you a sense of how much respiration they are doing. And there's ways that you could look at the ratios of the oxygens and the carbons to figure out exactly how much respiration is going on. And then if you know the net primary productivity and the rate of respiration, then you could figure out the gross primary productivity. But I will leave you there, because these are really useful measurements. Well, one, it's really useful to think about where all of the energy that allows us to live comes from, but it's also useful for ecologists to think about how productive a system is, or what's making it more productive, or less productive. And as we'll see, these numbers here, these are for sure on the high end of net primary productivity. If we were in a desert type of ecosystem, this number might be in the low hundreds and not in the 8,000 range.