Download Learning Objectives Week 8 – Digestion and Absorption (Part1) 1

Learning Objectives Week 8 – Digestion and Absorption (Part1)
1) Summarize chemical properties of amino acids, proteins, and carbohydrates contained in the diet.
Amino acids are chemical molecules that consist of carbon, nitrogen, hydrogen, oxygen and sometimes
sulfur. These molecules can form proteins through condensation reactions, where many amino acids
bond together while expelling water. Amino acids all contain amine and carboxyl functional groups. Each
amino acid contains a side chain group that differentiates it from the rest. There are 20 different amino
acids found in proteins which can display a vast range of chemical properties. Amino acids can be
separated into 7 different categories based on their chemical and structural properties:
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Acidic Amino Acids and Amides – polar molecules, negatively charged at physiological pH
Aliphatic Amino Acids – non-polar and hydrophobic
Aromatic Amino Acids – relatively non-polar
Basic Amino Acids – polar, positively charged and very hydrophilic
Cyclic Amino Acid – only one in the group (Proline), contains cyclic side chain, non-polar
Hydroxl Amino Acids – polar, uncharged and hydrophilic
Sulfur Containing Amino Acids – non-polar and hydrophobic, contains sulfur (obviously)
Proteins are essentially large molecules of amino acids bonded together. The chemical properties of
each protein will therefore be dependent on the amino acids that constitute it. Considering that,
proteins can have a large range of chemical properties just like amino acids. Overall, proteins can be
considered quite chemically stable and require enzymes to react/break down.
Carbohydrates can come in either simple or very complex forms. They are essentially groups of simple
sugars (saccharaides) bonded together. They can be broken down from their more complex forms but
the more complex the carbohydrate, the more energy and enzymes are required to break them down.
All carbohydrates are polar and soluble in water.
2. Describe briefly the enzymatic digestion of protein, lipid and carbohydrates. Include the role(s) of
saliva, gastric juices, pancreatic juices and bile in digestion and absorption.
Proteins are usually large and very complex molecules. Digestion of these proteins will break them down
into much smaller molecules which our bodies can use. This digestion begins in the stomach when
hydrochloric acid in the stomach “activates” pepsin, an enzyme capable of breaking down proteins, from
pepsinogen. Pepsin will break down large protein molecules into two products called peptone and
proteose. Both of these products are intermediates to amino acids and form from the result of a
decomposition reaction called hydrolysis with the assistance of the pepsin enzyme. Peptone and
proteose are basically molecules made from a small number of amino acids whereas proteins are much
bigger molecules that are formed from large numbers of amino acids.
Following their extraction from proteins in the stomach, peptones and proteose are brought into the
duodenum where they encounter another array of digestive juices. The pancreatic enzyme trypsin, is
introduced to the digestive bolus here and is responsible for the further breakdown of peptone and
proteose. Trypsin, like pepsin, will break down its reactants through hydrolysis but this time, peptone
and proteose will be broken down to their most simple form, the amino acid. The amino acid is the end
point of this process and will be absorbed in the lumen of the small intestines for use.
Pepsin (stomach)
Trypsin (Duodenum)
Proteins ----breakdown-- Peptones and Proteose ---breakdown------- Amino Acids (absorbable)
Lipids (fats) also have to undergo an enzyme-assisted breakdown before they can be absorbed. Lipids
will form large suspended collections in water because they are insoluble. This will cause any attempt to
break them down from a molecular level (necessary for digestion) very difficult. As a result, in order to
break them down efficiently, they must be emulsified. This is a process where an agent, the emulsifier,
will adhere to non-polar ends of the water-insoluble substance, reducing the substances’ hydrophobic
tendencies. The emulsion of lipids will not change the lipid at the molecular level but instead it will
reduce its “clumping”, limit collections of lipids to much smaller sizes and it will the lipids to be
distributed more evenly throughout the water (as opposed to collecting together as large units). This
emulsion of lipids happens in the duodenum and is instigated by a digestive enzyme known as bile. Bile
is created in the liver, stored in the gall-bladder and enters the duodenum from the bile duct. In the
duodenum bile emulsifies lipids, separating the large clusters into smaller units which will be much
easier to break down.
Once the lipids have been emulsified, they are then subject to a breakdown reaction from a pancreatic
enzyme called lipase. Lipase is secreted into the duodenum where it is allowed to break down the
emulsified lipid down into fatty acid and glycerol molecules. These fatty acids and glycerol molecules can
then be absorbed by the intestines.
Bile (duodenum)
Lipase (duodenum)
Lipids ---emulsification-- Emulsified Lipids ----breakdown----- Fatty Acids and Glycerol (absorbable)
Carbohydrates, like lipids and proteins, can be very complex structures that require breakdown before
they can be absorbed. Some forms of carbohydrates like cellulose (plant fiber) are too complex for the
human digestive system to break down. Breakdown of carbohydrates begins in the mouth where an
enzyme known as amylase begins to break down more complex carbohydrates into smaller ones. This is
a process that does not change the basic structure of a carbohydrate, but instead it will separate large
groups of them into much smaller clusters.
The stomach and the gastric juices it produces also have an important role in carbohydrate digestion.
The gastric juices provide a further ability to break down carbohydrates into much simpler carbohydrate
while, at the same time, the stomach churns and processes the carbohydrates (along with all the other
food) into a bolus called chime. The chime will then be moved to the duodenum where the majority of
the carbohydrate digestion occurs. In the duodenum, an enzyme called pancreatic amylase which aids to
break down the carbohydrates into maltose, lactose and sucrose. When these products move further
into the small intestine, they encounter three specific enzymes; maltase, lactase and sucrase which will
break each of the corresponding simple carbohydrates down even more into very small and easily
absorbable products.
Amylase (mouth)
Gastric Juices (stomach)
Carbohydrates -----breakdown- Smaller Carbohydrates --------breakdown------- even smaller carbs…
Pancreatic Amylase (duodenum)
Maltase, Lactase and Sucrase (SI)
… ---------breakdown------------------ Maltose, Lactose and Sucrose -----------breakdown------------- …
… - simple sugars (absorbable)
3. Distinguish between the mechanisms for absorption of amino acids, fats and carbohydrates.
Amino Acids are primarily absorbed in the intestines. Once broken down from proteins, they are small
enough to pass through the intestinal lining and are absorbed by the villi of the intestine. They transfer
directly to the bloodstream where they associate either with red blood cells or move freely in blood
plasma. Through the bloodstream they are distributed throughout all parts of the body.
Carbohydrates are absorbed in a similar manner to amino acids with one distinct difference. First of all,
they are broken down and absorbed through the intestinal lining by the villi into the bloodstream.
Unlike amino acids however, carbohydrates (as simple sugars) will pass through the liver as their first
port of call. The liver will be in charge of determining whether the simple sugars are stored or
distributed throughout the body.
Lipids are absorbed differently than amino acids and carbohydrates. While they also undergo a
breakdown reaction, once they are absorbed by the villi of the intestines they will progress through to
the lymphatic system. From here they will eventually make their way to the bloodstream where they
will be distributed throughout the body.