Download lesson i - MisterSyracuse.com

LESSON I:
INTRODUCTION TO EXCRETION
To Address NYS Standard:
5 (Living things maintain a dynamic equilibrium which sustains life)
Behavioural Objective:
The students will be able to understand the purpose of excretion, what is excreted, and in
what form it is gotten rid of.
Explanation of Lesson Plan:
This lesson will give students the basic vocabulary necessary to properly discuss the
concept of excretion. We will begin with very simple, single-celled organisms, and work
our way up to the human urinary system.
Hook: (3 minutes)
Students will brainstorm a list of things that they themselves need to get rid of from their
body. They will write them on paper, and stick them to the wall. They will then surmise
WHERE the wastes came from, and how they are gotten rid of.
Test of Prior Learning: (5 minutes)
1. Why is it important to get rid of wastes?
2. How do you think humans get rid of wastes?
New Learning: (30 minutes)
1. All organisms do some form of cellular respiration. Therefore, they all create at least
some kinds of wastes. You’ve come up with a list of some of these wastes.
2. So therefore, every cell that is alive makes wastes. But what would happen if it didn’t
get rid of them? It would drown in its own waste.
3. Every organism, and every cell in every organism, must have a way to get rid of its
wastes.
4. The primary waste that will concern us in our discussions are the nitrogenous wastes.
5. There are many ways to get nitrogenous wastes, which are wastes that….contain
nitrogen. You get them from breaking down molecules that contain nitrogen, such as
DNA, RNA, or proteins.
6. One way is ammonia, NH3. This is highly toxic, and no good.
7. Ammonia can be converted into something less toxic, though, called urea. It is a
liquid, and can be stored for longer periods of time. If you filled your bladder with
ammonia, you’d be NG. You can, however, hold urea for an extended period of time.
8. The last form that can be made is uric acid. Uric acid is very concentrated urea, and
often is thick enough to be excreted as a paste (think of bird droppings).
9. Let us first consider a single-celled organism, such as an amoeba or paramecium. It
makes nitrogenous wastes, carbon dioxide, heat, and other wastes. It can just shoot it
right out through its cell membrane. After all, it lives in water, what worry does it have?
10. It can excrete its nitrogenous wastes as ammonia, which is poisonous, but just wafts
away into the water.
11. What other organisms do you think use ammonia as the way to get rid of wastes?
12. Fish do, as do many aquatic organisms.
13. More complex organisms can’t use ammonia, because they either don’t live in water
(terrestrial), or they have too many cells to just let it all out.
14. They use urea, then.
15. Earthworms use a system of nephridia, which are essentially like small kidneys, that
concentrate wastes and excrete them out pores on the side of the worm. They do this
with a concentration gradient of salt! Keep this in mind!
16. Insects and arachnids use a similar structure that concentrates ammonia and converts
it into uric acid. It is called a Malpighian tubule.
17. Uric acid in this case is used because it is more concentrated. Therefore, less water
is lost. This is always good for terrestrial organisms.
18. The main reason why some organisms use ammonia, urea, or uric acid, is based on
trying to avoid water loss. If you live in water, you really don’t have to worry about it.
But moving on to land, we do have to worry. This can be traced in a phylogenic tree.
Test of New Learning: (5 minutes)
1. Why do you think a concentration gradient of salt is so important?
2. Why are Malpighian tubules good for insects, but not so much for humans?
Assignment: