Primary productivity in ecosystems| Matter and Energy Flow| AP Environmental Science| Khan Academy

In this video, we're going to talk about energy, and in particular, we're going to 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.

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. You might know that there are certain organisms on our planet that are capable of taking that light energy and then storing it as chemical energy. There are many types, but the ones that we see most often in our day-to-day life are plants.

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. It's using that light energy to actually stick, or you could say fix, the carbon to construct itself. 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 as chemical energy in an organic form. 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 if you adapt them a little bit, you get things like starches, and that's what most of the plant tissue is.

So some variations of this are linked together, but this is where the energy is stored: energy stored in the actual plant tissue. Then, of course, it releases that molecular oxygen, and this is the process of photosynthesis. 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, essentially fixing 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.

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. Since we're respiring all the time, this is why we need to breathe oxygen. 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. In other videos that you'll see in a biology class, we'll talk about how this form of stored energy gets converted to other forms, and then how that's used by the various machinery in cells to actually live, to reproduce, and 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 to find an ecosystem and take a certain area of the surface of that ecosystem. It could be a terrestrial ecosystem on land; it could be a marine ecosystem.

Then say for this area, in a given period of time, often times a year, how much stuff is growing. So this is the stuff that grows, and obviously, or it would seem that the more stuff that is growing, the more photosynthesis that is taking place. The way that they measure how much is growing, you can either measure it in terms of grams of biomass.

So grams of biomass, and biomass is just a fancy way of saying the mass of biological stuff that's just growing in this area. 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. When you see calories on a packaging food label, what most of us think of as calories, those are actually kilocalories when we think about it in scientific terms.

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.

So when you see how much has been produced in a given area in a given year, you're actually seeing the net primary productivity. You could think about it as 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 chemical energy, or cellular energy, they needed for respiration, that would then give you the net primary productivity.

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 2000 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 2000 grams per square meter per year. If you wanted to think about this in terms of kilocalories, you just have to say, well, each gram of biomass is how many kilocalories? It depends on the type of biomass, but let's say that we have four kilocalories per gram of biomass.

Then we could also say that this net primary productivity is equal to 2000 grams per square meter per year times 4 kilocalories per gram. The grams cancel out, and then you multiply 4 times 2000. That's going to be 8000 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 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.

There are ways that you can look at the ratios of the oxygens and the carbons to figure out exactly how much respiration is going on. 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. 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 ecosystem, this number might be in the low hundreds and not in the 8000 range.