6.1 - Digestive System

Uploaded by luizgmello on 10.10.2012

Topic 6.1 Digestion
Hey everyone and welcome to topic 6.1, Digestion. This is one of the more straightforward topics
in Human Health and Physiology because there are so many links to your previous study of
biomolecules and the activity of enzymes. It would be interesting to revisit Enzymes
(3.6), Carbohydrates, Lipids and Proteins (3.2) and Membrane Transport (2.4) before
watching this video.
The purpose of the digestive system is to break down large molecules into smaller molecules
that our body can then re-assemble and use in whatever way it wants. Why do they need
to be broken down in the first place? Because the molecules in the food we eat are typically
very large to be absorbed and transported in our blood stream.
I like the metaphor where the food we eat are houses, and the bricks that form that
house are the molecules. What our body does is break down a house and then recycle the
bricks to form an entirely different house.
Our body has two distinct types of digestion to do this. The chewing and grinding of our
teeth and the churning of our stomach perform mechanical digestion. To speed up the process
of digestion, enzymes accelerate the reactions taking place through enzymatic (enzyme-assisted)
or chemical digestion.
You remember from topic 3.2 that there are 4 types of biomolecules: carbohydrates, lipids,
proteins and nucleic acids. All of these molecules are acquired through out diet and are too
large to be absorbed by the digestive system, and as such, must be broken down.
Read these two questions and attempt to answer them. I will discuss the answers in this video.
If you’d like to try them later, advance the video until you see a different slide.
If you’d like to try them now, pause the video now.
Carbohydrates are acquired in plants through photosynthesis. All other molecules must be
acquired from the soil. One of the most extreme adaptations for xerophytes, are insect-eating
plants, which obtain protein from insects and then convert it into simple carbohydrates
to compensate for their low photosynthetic rates.
Nitrogen-fixating bacteria are great at making nitrogen available to plants so that they
can build their own nucleic acids. In general, fungus, bacteria and invertebrates (informally
known as the “FBI” in Biology), will degrade these large molecules, breaking them down
into smaller molecules that are more easily absorbed by plants roots.
Here’s another question. If you’d like to try it later, advance the video until the
slide changes. If you’d like to try it now, pause the video now.
Molecules are going to go from the inside of the digestive system into the cells of
our body. To do this, it will have to cross cell membranes, which it can do in one of
three ways: diffusion, facilitated diffusion or active transport.
Digestion in humans is called ‘enzyme-assisted’ because, well, it’s assisted by enzymes.
These speed up chemical reactions and make sure that molecules from the food you eat
are available to the body as quickly as possible. One of the major advantages of enzyme-assisted
digestion is the fact that the body can now regulate the activity of these enzymes. One
clear example is right before you’re going to eat and your salivary glands begin producing
saliva (which contain enzymes to digest starch). If you’re not eating, the production of
saliva is constant relatively low.
How do enzymes speed up chemical reactions? The short answer is that they lower the activation
energy of the reaction. Without enzymes, a much higher amount of free energy must be
available for the reaction to take place. Enzymes speed up a reaction by making it require
less energy.
The activation energy is reached by using surrounding heat energy in the body. An alternative
to using enzymes would be for the body to increase its temperature. Why doesn’t it
do it, though? Think about it! If you’d like to try this later, advance the video
until the slide changes. If you’d like to try it now, pause the video now.
The first benefit is the ability to produce only the enzymes for the reactions that need
to happen. Enzymes are made through protein synthesis and regulated by genes in the cell
nucleus. Environmental factors (such as the sight of food) would activate these genes,
producing the appropriate enzyme only when it is needed.
A major disadvantage of using a higher body temperature is the energy cost. Maintaining
a body temperature of 36.5oC is already costly in terms of energy, and increasing that temperature
would mean that we would have to eat much more.
This chart shows three groups of enzymes: amylase (digest carbohydrates), protease (digest
proteins) and lipase (digest lipids). Notice the similarities and differences between enzymes.
Notice that only Pepsin requires a low pH, which explains the acidity of the stomach,
the source of Pepsin. The substrate is what the enzyme binds with, and the products are
the resulting molecules of the activity of the enzyme. Pause the video for a minute and
take it all in.
You also have to be able to draw the human digestive system. There’s a schematic diagram
on Campbell page 884. I’ve seen a multiple choice question on Paper 1 a few years back
that referred to a similar diagram, plus it’s simpler to understand. However, when asked
to draw the digestive system, you’re expected to draw something more like on Allott page
47. Use that to generate a drawing in your drawing booklet and label the items in this
slide. Also, label the sources of pepsin, salivary amylase and lipase and annotate these
labels with pH, substrate and products of the enzyme activity.
The organs in the digestive system include the stomach, responsible for temporarily storing
food, chemically digesting proteins and killing pathogens due to its low pH. The small intestine
continues digestion through enzyme activity (including lipases and pancreatic amylase)
while the large intestine will re-absorb water and minerals (including through active transport)
to minimize nutrient waste.
To distinguish between absorption and assimilation is very important when you are communicating
information about the digestive system. Absorption refers to the passage of nutrients from the
digestive system to the blood stream. Once a molecule crosses a cell membrane in the
large intestine into the blood stream, it is said that the molecule has been absorbed.
Once a molecule is absorbed, it will travel in the blood stream until its destination:
another cell. Once there, this molecule will likely be used a building block for a larger
molecule (e.g.: an amino acid into a protein). This process of using an absorbed molecule
to build a larger molecule within the body is called assimilation.
The digestive system is yet another beautiful example of the form of a structure being driven
by its function. The villi are finger-like structures that cover the surface of the small
intestine – which you remember is responsible for absorption. The villi create a higher
surface area than if the wall of the small intestine were flat.
If you magnify an image to see the surface of an individual villus, you would find that
it has microvilli, further increasing the surface area and the ability of the small
intestine to absorb nutrients. Another characteristic of the structure of
the villus directly related to its function is the fact that it contains a single cell
layer. If it needs to absorb nutrients, the layer of cells it needs to cross needs to
be quite thin, that’s why it’s just one cell thick.
Always keep in mind that the small intestine is absorbing nutrients through the cell membranes
of this single-cell epithelium. This means membrane transport (topic 2.4) is taking place.
As such, these cells are rich in membrane protein channels (for facilitated diffusion)
and mitochondria (to provide energy for active transport).
Once across the membrane of the epithelium, these are then going to the blood stream so
that they can be distributed to the rest of the body. That’s why the structure of the
villus includes capillaries (small blood vessels) to collect nutrients and taken them away.
Because the small intestine also absorbs lipids, which are not soluble in the water-based blood,
a lacteal, which is part of the lymphatic system, will take away any hydrophobic molecules.
This last slide has a lot of vocabulary and concepts that are linked to this diagram.
It may be a good idea to draw it out in your notes and annotate it this information. If
you’d like to do it using a computer, you’ll find this image next to the link to this video
on our course page. Another good idea for this and other topics is to generate a glossary
where key terms are define.
And this concludes topic 6.1. Please bring questions for discussion and I’ll see you
guys in class.