Hello, Bio400 people, this is Miss Boghossian, and this is our first
screencast of the year. This is going to be about cycles in biology. So, here
are some tips to keep in mind about watching a screencast: usually i'm
going to use screencast instead of lecture for this class, and I'm going to
use them to deliver the really dense material. Even though the screencasts
are about 10 minutes long, usually it's going to take you about 30 minutes of
work because you're going to be taking notes, stopping and starting, and making
sure that you record things about the screencast. You're going to want to keep
all of your notes in the same place and you're gonna want to bring those to
class with you. It's up to you whether you want to do it in a notebook or
whether you want to do it on your iPad, but you're gonna need to bring those to
class with you. And, the screencasts are also really great to rewatch. When you
don't understand something the first time, you can back up the screencast and
rewatch that segment, and you can also rewatch all of your screencasts from the
unit to help you review when it comes time to take a test.
Ok, so now onto cycles in biology. So, there are quite a few of them and some
of the most commonly discussed ones are the carbon cycle, the oxygen cycle, the
nitrogen cycle, the phosphorus cycle and the water cycle. Something to remember
about cycles s that the overall amount of substance doesn't increase or decrease
you're simply changing its form or changing its position. Here's a
classic example that you probably learned about in middle school, which is
the water cycle. Now, because this is a cycle, I could start anywhere, but let's
start up here with the cloud. So, we have precipitation, so the cloud drops water
as rain or snow, the water then either becomes runoff into the ocean or in
low-lying areas it goes into the ground and becomes groundwater. Then we have
evaporation, when the water gets really warm, and condensation into a cloud and
the cycle starts all over again. But we're not using or losing
any more water than we started out with. So when we talk about any of these
cycles in this list, we're not going to be losing any nitrogen at the end of the
cycle and we're not going to be losing any phosphorus. We're not
losing any of the other biomolecules. So, there are a few different atoms that
you're probably familiar with from chemistry that are going to be really
crucial in biology, and those are going to be carbon, hydrogen, oxygen, nitrogen,
phosphorus, and sulfur, and so all of those different atoms are going to
combine in different ways to form large molecules which we call biomolecules;
carbohydrates, lipids, proteins and nucleic acids. A lot of these molecules
also form nutrients, so we see a lot of nitrogen in the atmosphere. We also see
carbon being recycled from one form to another and we're going to talk about
that in this screencast. So, to start off our discussion of cycles it's important
to remember that organisms are influencers on their environment and
they are also influenced by their environment.
Here's what i mean by that: If I look at the really easy example of a tree,
it's going to be influenced by its environment in a few ways. It's going
to take in water from the ground, it's going to take in carbon dioxide from the
environment around it, and it's going to absorb photons of light from the sun. The
tree also influences and its environment in turn because it provides things like
oxygen, which is a byproduct of photosynthesis, it also does things like
provide habitat for other organisms, it provides shade, and it also might even
provide a building area for other organisms like vines.
So know that the organisms don't exist in isolation, they all depend on one another.
We're going to take a look at that in a little bit more detail now. We're going
to segue into something called the nutrient cycle. Now, when I refer to
nutrients, I'm talking about some of those biomolecules that we discussed
earlier, especially something like nitrogen or phosphorus.
No matter how healthy the tree is, ultimately it's going to either die or
fall over in some kind of storm and wind up on the forest floor. Because this tree
used to be alive and is composed of organic matter, it's full of tons and
tons of nutrients which are now trapped inside the dead tree. Given some time
however this tree is ultimately going to become a habitat for things like lichen
and moss, and other things like fungi. These organisms are collectively known
as decomposers and they will break down the old trunk of the tree and start
releasing the nutrients back into the environment. Soil or dirt is made up of
many different ingredients one of the most important is called hummus, which is
decomposed organic matter, and ultimately the organic matter from the dead tree
will become part of the soil in the general area. The soil will be full of
many of the nutrients that were originally held inside the tree when it
was alive. That soil will become extremely nutritious and will allow more
organisms to grow and will also nourish the plants that are still in the area
that are alive. So, in this way we've gone from a living tree, to a dead tree, to a
dead tree with fungus growing on it, to soil, and then returned all the nutrients
back to the environment. We haven't lost any of them.
This is a pretty straightforward nutrient cycle, but we're going to zoom
out a little bit and think about one that's little bit more realistic. So, when
you were younger you probably learned about the food chain in middle or
elementary school, and it starts off with creatures called producers. Depending on
whether you're looking at a land or a marine environment, these are going to
include plants and/or algae but the thing that they have in common is that
they're all photosynthetic organisms that get their energy from the sun. The
next step is going to be primary consumers, and these are going to be herbivores;
creatures that live on plants are on algae. So on land this might be
something like a mouse or a rabbit, and in the ocean you might have something like
zooplankton or "animal plankton".
Secondary consumers are going to prey upon primary consumers and so indirectly
they also depend on the presence of producers as well.
Finally at the so-called top of the food chain, you have something called tertiary
consumers, and they depend on the presence of all of the other organisms
that are below them in the food pyramid. We tend to think of the nutrients as
flowing from producers, to primary, to secondary, to tertiary consumers and then to
decomposers. This is accurate; the decomposers will break down the organic
matter from each of these different kinds of organisms, and then will return
the nutrients to the soil which allows the nutrients to re-enter the cycle at
the producer stage. However the cycle can also be slightly more complex because
decomposers can also break down dead producers, dead primary consumers, and
dead secondary consumers to allow their nutrients to re-enter the nutrient cycle.
We should also think about the energy flow that occurs in most ecosystems. With
very few exceptions, such as at deep sea ocean vents, the energy in most
ecosystems comes originally from the sun. The sun emits particles called photons
of light which allow producers to undergo a process called photosynthesis,
where they use energy from the Sun as well as a couple of different kinds of
biomolecules in order to generate their own food. When primary, secondary and
tertiary consumers eat organisms from the level below them, we have energy flow
that flows from the lower organisms to the higher organisms.
However it's not a perfect system; you don't have nearly as many tertiary
consumers as you have primary consumers. Here's why.
In biology, there isn't a perfect transfer of energy from one organism to
another. It follows something called the ten percent rule. If, for instance, you
have a hundred thousand units of producers, whether that's a hundred
thousand kilograms of them a hundred thousand calories worth of producers, or
just a hundred thousand organisms, it can only support ten percent as many primary consumers. So,
in our example, 100,000 organisms of producers could only support about
10,000 rabbits. 10,000 rabbits who only support about a 1000 secondary consumers.
And about 1,000 secondary consumers can support roughly 100 tertiary consumers.
So, now our question has to become what happens to all of that extra energy
that's not accounted for here? Most of the energy that's lost between one
trophic levels, like producers right here and the next level up like primary
consumers, is lost because we have to run the animals' metabolic processes. The rest
of it is actually lost as heat. A lot of these are endotherms, and so they
release a lot of heat and are not very efficient; that's where a lot of that
extra energy goes. That pretty much wraps it up! See you all in class.