I. Objectives: Students will be able to:
Compare and contrast extracellular and intracellular digestion.
Describe how swallowing works by using an online animation.
II. Procedure
A. Anticipatory Set
So…let’s talk about something gross….that’s always fun, isn’t it? Anyone in
here ever vomited? What does it look like? (chunky, liquidy, strange colors,
etc.) Okay…so…does it look the same as when you put it in your body?
(No…depends on how long ago you’d eaten) What causes the food to look so
different from when you put it in your mouth? (you chewed it, stomach acid)
Your body has started to digest it, right?
B. Lecture:
Digest. Most of you have heard that word before. It is the process by which your
body breaks down food. Since all organisms take in food (or make their own in some cases) in some way,
all organisms digest food, even plants and fungi, but all digest a bit differently. Digestion is the way
organisms get at the energy and raw materials that are stored in food. The organism takes the large
molecules it ingests, or eats, and then breaks it up into its smaller subunits. This means that proteins get
broken into amino acids, polysaccharides get broken into monosaccharides, and lipids get broken down into
fatty acids and glycerol.
For you and for most animals, digestion is extracellular. This means that it occurs outside the cells
of your body. This seems odd to you I’m sure because your digestive system is within your body.
Consider, though that your digestive tract is open to the environment on both ends. Consider also, that your
digestive system is actually a cavity lined with cells. This is a specialized place where digestion occurs.
Most organisms with digestive tracts have a one-way tube system. There is a mouth, where food enters,
and an anus, where waste leaves. Some organisms, like jellyfish and coral polyps have only one opening
where food goes in and wastes leave.
Digestion in an extracellular system has two parts: physical and chemical. Physical digestion
simply means breaking down food into smaller pieces. How do you do this? You chew your food.
Animals that cannot chew such as birds have specialized organs called gizzards that grind food. Physically
breaking up the food increases the surface area available for chemical digestion. Have you ever eaten
something without chewing? What happens? Usually you get a stomachache. The reason for this is that is
that your stomach wants to move the food along, but it is often too big to do so and it is difficult for your
stomach to break down. Chemical digestion involves the enzymes we talked about a few days ago. The
enzymes cause the catabolic reactions of breaking chemical bonds to happen much faster than on their own.
These reactions break large macromolecules, which are? (proteins, lipids, carbohydrates,) into their smaller
subunits: amino acids, fatty acids, and monosaccharides.
Before you can digest something, you must chew and swallow it. Chewing will start physical
digestion by breaking the food into much smaller pieces. Also, chewing will mix saliva into your food as
well. Your saliva has three purposes: to lubricate your food to enable you to swallow it, to add water for
enzymes to work, and to add an enzyme: salivary amylase, that begins starch digestion. After you are
finished chewing, you then swallow. Let’s go online to explore how swallowing works.
Go to: http://hopkins-gi.org/multimedia/database/intro_250_Swallow.swf
Click on the animations about swallowing. Answer the following questions:
1. What are the parts of your anatomy associated with chewing?
2. What is the bite of food called after it is chewed?
3. How does your food move into your stomach?
4. The uvula is the part of your palate that hangs in the back of your throat that moves up when
you swallow. Why do you think this is important?
5. Why do you think the epiglottis is so important?
With a partner, explain how humans swallow in your own words. Prepare this for class tomorrow, as we
will be presenting it.
III. Closure and Assessment:
The two types of digestion are intracellular, where digestion occurs inside the cells, and
extracellular, where digestion occurs outside the cells. Extracellular digestion is made up of two processes,
physical digestion, where food is broken up into smaller pieces and chemical digestion, where food is
broken up into its monomers. Physical and chemical digestion begin in the mouth with chewing and the
mixing of saliva. Afterwards, we swallow. Tomorrow we will be presenting how we swallow.
IV. Assessment: Swallowing presentations