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AP®︎/College Environmental science
Course: AP®︎/College Environmental science > Unit 1
Lesson 3: Matter and energy flowPrimary productivity in ecosystems
Advanced Placement Environmental Science, AP Environmental science, AP Enviro Sci, APES, Environmental, Science
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- My question is, hiw do we release water ?(2 votes)
- the most common steps in water treatment used by nearly every utility company(1 vote)
- Does the net productivity correlated with an ecological impact of the land use change?(2 votes)
- yes the correlated net productivity has a huge ecological impact on the land use(1 vote)
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.