UNIT 2
Digestion and Nutrition
Human Digestive System Function


The main function of the digestive system is
to break down large macromolecules
(proteins, fats and starch) into smaller
molecules (amino acids, fatty acids and
glucose) that can be absorbed into the
circulatory system for distribution around
your body.
This breakdown occurs because of a various
mechanical and chemical processes. We need
to digest food in order for us to get the
energy (ATP) that is in it, into a useful form.
Introduction to Digestion

The pathway that food takes as it moves
through the body during digestion is
known as the digestive tract or the
alimentary canal.
Five Stages of the Digestive Process
1.
Motility – movement of food through the
digestive system.
2.
Secretion – release of digestive juices in
response to a specific stimulus.
3.
Digestion – the physical and chemical
breakdown of food into small particles.
4.
Absorption – passage of the molecules into
the bloodstream.
5.
Elimination – removal of undigested food
and waste.
Structures in the Human
Digestive System

Food enters the alimentary canal through
the mouth opening; the chamber that it
enters is called the oral cavity.

The tongue helps to move the food
around in the oral cavity, pushing the food
between the teeth, which chew the food.
The tearing and biting of food breaks it
into smaller bits before it is swallowed.
This is a very important step in human
digestion, known as mechanical
digestion.
*Often referred to as mastication*

The food is also lubricated in the mouth
by the saliva, which is produced by three
sets of salivary glands. These glands
send their secretion, saliva, through
small ducts into the oral cavity. Not only
does saliva lubricate the food, it also
begins to chemically digest
carbohydrates in the food you have eaten.



There are 3 pairs of salivary glands that secrete
their fluids into the mouth:
The Parotid Gland is located just in front of and
slightly below the level of the opening of the ear.
This is the largest of the glands which secretes salts,
an enzyme (salivary amylase) and a watery serous
fluid into the mouth through the parotid duct.
The Submaxillary Gland is located below the
parotid gland near the angle of the lower jaw. It
produces some serous fluid and some mucous and
delivers them to the mouth through the
submaxillary duct.

The Sublingual Gland is located under the tongue
and secretes mostly thick stringy mucus, salts and
salivary amylase. These substances are emptied into the
mouth through the submaxillary duct.

Since the salivary glands deliver their secretions through
ducts, they are called exocrine glands.

Saliva flows from the salivary glands at all times, keeping
your mouth moist. However, the sight or smell of good
food may cause your mouth to "water". Of the
approximate 1L of saliva produced every day, 99% of
that is water, with the remainder being mucus and
enzymes. This water moistens and dissolves particles of
food while mucous helps make chewed food smooth
and easy to swallow.
Submaxillary
The Pharynx

The pharynx opens to two tubes that
descend down into the neck.
1.
Trachea, or windpipe, connects the
pharynx to the lungs and is found in
front of the esophagus.
2.
Esophagus is the digestive tube that
connects the pharynx to the stomach.
Uvula

a small mass of tissue that hangs down in
the back of the mouth, and moves up to
close off the openings to the nasal
cavity when food is swallowed into the
esophagus.
Soft Palate
the roof of the pharynx in the back of the
mouth
 this soft tissue functions in closing off the
openings to the nose and ears as food is
swallowed.

Epiglottis
a thin, flaplike piece of cartilage that
closes over the trachea opening,
preventing food that is swallowed from
entering the trachea
 lies above the esophagus and diverts food
into that tube
 part of the larynx or voicebox

The Esophagus

Food that has been chewed and
lubricated is ready to be swallowed. The
bolus (ball of food) is swallowed and
enters the esophagus or food tube,
which lies behind the trachea and runs
down the centre of your neck. The
esophagus ends when it joins the
stomach.

Food moves down your esophagus,
pushed through by the rings of smooth
muscle found in the walls of the
esophagus. These muscles involuntarily
contract and relax in waves as they push
the food along. This rhythmic contraction
is called peristalsis and it continues
through the entire length of the
digestive tract.
*involuntary - controlled subconsciously
- will happen without you
having to think about it.
Peristalsis Videos

Peristalsis Throughout the Body

NOT FOR THE FAINT OF HEART
Review of Swallowing
The Stomach
Cardiac
sphincter
Cardiac sphincter muscle

Close off the opening to the stomach and
serve to control the movement of
swallowed material into the stomach.

Keeps partially digested food and stomach
acid from working their way back up the
esophagus, called reflux or better known
as heartburn.
What Happens in the Stomach?
Three layers of muscle in the wall of the
stomach contract and relax to
mechanically break down the stomach's
contents.
 The bolus is churned and mixed with
stomach secretions.
 These secretions and the stomach's
mechanical churning of the food act to
further soften it and break it up into
smaller pieces, taking on a soup-like
consistency.

Chyme
term used for food after the stomach has
digested it.
 Rhymes with time (the h is silent)
 About 9L of chyme enters your small
intestine every day.

Hydrochloric Acid
This powerful acid protects us from
harmful bacteria-what we may ingest with
foods is fried by this strong acid.
 HCl also helps keep the three pounds of
bacteria found in the colon from moving
up into the small intestine.
 If this happens, "bad" bacteria and yeast
can establish a foothold in the intestine,
resulting in less than optimum absorption
of nutrients and inflamed intestines.


HCL is paramount in our digestive process. It
begins the digestion of protein and
stimulates the pancreas to produce digestive
enzymes. When we don’t digest foods well,
there may be three results:
We don't get the nutrition we need
2. Our food passes through our bodies and into
our bloodstream too quickly and becomes
seen as invaders (allergens) in our body.
3. Food lingers and becomes a form of "bad"
bacteria, which can make us very sick.
Cheeseburger in HCl
1.
Pepsin

It is not an enzyme but a forerunner of
the enzyme pepsin. Once the pH level of
the stomach reaches a level of 2 or lower,
pepsinogen is converted into pepsin,
an active enzyme which begins the
breakdown of proteins.
Mucous

The stomach is lined with mucous that
coats the cells lining the stomach and
protects them from the digestive action
of the enzyme pepsin.

Unfortunately these protective measures
do not always work, and the stomach
starts to digest itself. The result is a peptic
ulcer.
The stomach is primarily an organ of
digestion.
The stomach is NOT an organ of
absorption.
The Small Intestine
The small intestine lies between the
stomach and the large intestine.
 In adult humans, the small intestine is 5-6
metres in length, has a surface area of a
500-600m long tube and takes up a
significant portion of the abdominal cavity.
 The main processes of digestion and
absorption are accomplished by the
small intestine.

Pyloric Sphincter
Located between the stomach and the
small intestine
 Controls the movement of the chyme
into the small intestine
 The stomach pushes chyme into it by
peristalsis.

Digestion in the Small Intestines
The digestion that occurs is primarily
chemical, most of which occurs in the
duodenum.
 The enzymes that act chemically on
food to digest it come from the small
intestine and the pancreas.
 The pancreatic duct connects the
pancreas with the small intestine and
effectively carries pancreatic juice

The small intestine also possesses
intestine glands that produce its own
secretions; intestinal juice.
 Peristalsis continues in the small
intestine, churning and pushing the
partially digested food through until it
reaches the large intestine.

The Importance of Bile

In the small intestine, large lipid molecules
(fats) are physically broken down or
emulsified by bile (this is a mechanical
process).

Droplets of lipids are formed that are
more easily digested.

Bile is a product of the liver and is stored
temporarily in the gall bladder.

The bile duct carries bile into the
duodenum of the small intestine where it
acts to emulsify fats, breaking them up
into tiny droplets.
Villi

Facilitates the absorption that occurs in
the small intestine

Tiny finger-like projections that line the
walls of the small intestine.

Capillaries uptake those food molecules
and they are transported to other
structures in the body via the blood.

The primary types of food molecules that
are absorbed at this point are derived
from the digestion of carbohydrates,
proteins, and lipids.
The Large Intestine

Once the digestion of food in the small
intestine is essentially finished, the
remaining material passes into the large
intestine or colon.

It extends from the end of the small
intestine to the anus and is 1.5 metres
long. The large intestine is shorter than
the small intestine but is larger in
diameter.

There are three portions of the large
intestine. These three sections create an
upside-down U-shape and surround the
small intestine.
Direction of food

What is left of the food after digestion is
moved through the large intestine with
the help of peristalsis. In the large
intestine, water and essential minerals
are reabsorbed into the bloodstream.
Fibre, bacteria and other undigested
products are then excreted as feces
through the anus by peristalsis.
Appendix
A small pouch located at the point where
the small intestine meets the large
intestine.
 This organ apparently serves no function
in humans unless it is infected.
 An infected appendix could rupture and
release the infection into the body cavity
which can be fatal.

Rectum
Located at the end of the large intestine.
 Primarily acts to store the feces before it
is excreted by muscular contractions.
 The anus is the opening of the rectum to
the outside of the body.
 The movement of feces to the exterior is
controlled voluntarily by the external
anal sphincter.
Digestion

Chemical Digestion

As we have seen, the organs in the
digestive system secrete many different
substances that play a role in the chemical
digestion of food.
Chemical Digestion
Aids in preparing food particles for
absorption.
 Takes place through the action of
enzymes.
 Large food molecules are chemically
changed into smaller molecules

The types of nutrients that are digested
through mechanical and chemical action are
carbohydrates, proteins, and lipids.
Carbohydrates
Made up of carbon, hydrogen, and oxygen
 Quick sources of energy and include both
simple molecules (sugars) and complex
molecules (starches)
 Common food sources are breads, pasta,
and fruit

Proteins
Large complex molecules that contain
carbon, hydrogen, oxygen, nitrogen,
phosphorus, and sulphur.
 Serve as the structural framework of
most living tissue and also serve as
enzymes, but are not usually used as an
energy source.
 Made up of smaller molecules called
amino acids.
 Common food sources include meat, eggs,
and nuts.

Lipids
Make up fats, oils, and waxes.
 Contain carbon, hydrogen and oxygen.
 Capable of storing great amounts of
energy in their chemical bonds.
 Common food sources include margarine,
cooking oil, and animal fat found in meat.

Enzymes
Biological catalyst that can speed up
chemical reactions without being altered
in the process.
 Allows for the reaction to occur at a
faster rate.

Digestive Enzymes

Enzymes are very specific; they only act on
certain molecules called a substrate. Each
type of enzyme is very specific in terms of
which substrate it will act on. The names
for enzymes start with the name of the
substrate involved and end in "-ase". For
example, an enzyme that acts on a protein
molecule is a protease. An enzyme that
acts on a carbohydrate molecule is a
carbohydrase. And an enzyme that acts
on a lipid molecule is a lipase.

An analogy that is often used to
understand enzyme action is the “lock
and key" concept in which the substrate
acts as the lock and the enzyme is the
key.
LOCK
KEY
The key is
unchanged but the
lock has been
changed, “opened”.
Enzymes
Animation of an Enzyme
Chemical Digestion in the Human
Digestive Tract
Type of
Location Secretion
Enzyme
Mouth
Saliva
Stomach
Gastric
Juice
Action
Begins digestion of
Amylase
starch to simple sugars.
Begins digestion of
Protease
proteins.
Location
Secretion
Pancreatic
Juice
Small
Intestine
Intestinal
Juice
Type of Enzyme
Action
Amylase
Completes digestion of
starch to simple sugars.
Protease
Continues digestion of
proteins.
Lipase
Completes digestion of
lipids to fatty acids and
glycerol with the help of
bile.
Protease
Completes digestion of
proteins to amino acids.
Carbohydrase
Completes digestion of
carbohydrates to simple
sugars.

In addition to carbohydrates, proteins, and
lipids, nucleic acids are also nutrient
molecules that are broken down in the
digestive system. When nucleic acids are
broken down chemically, the results are
nucleotides, which contain nitrogen,
phosphorus, and sugars.
Once nutrient molecules are broken down
chemically and mechanically, they are
absorbed into the circulatory system.
Most absorption occurs in the small
intestine except vitamins, minerals, and
water are absorbed in the large intestine.
Factors that Affect Enzyme Action
pH
The pH of a solution refers to how acidic
or basic it is.
 An enzyme that is structurally changed as
a result of pH is said to be denatured;
which leads to a loss of enzymatic activity.

The protease in the stomach, pepsin, is
activated by this acidic environment and
becomes deactivated when it enters the
basic environment of the small intestine
 Less acidic environment of the small
intestine allows the enzymes produced by
the small intestine and pancreas to be
more effective in digestion.

Surface Area
When a food particle is mechanically
broken down by chewing it into a number
of smaller particles, the total surface area
of exposed particles increases.
 An increased surface area allows more
enzymes to act on the food particles.

Coenzymes
Small organic molecules that are not
proteins that help to facilitate the
enzyme-substrate connection and are
often produced from various vitamins
 They are located at the active site of the
enzyme molecule and facilitate the "lock
and key" action of the enzyme and
substrate molecules.
 They are not themselves affected by the
action of the enzyme on the substrate
molecule.

Inhibitors
Compete with substrate molecules for
the active sites of the enzymes.
 Binds to the enzyme, rendering the
enzyme ineffective on the substrate
molecule that it was designed to act on.

Temperature
Most reactions in the human body that
involve enzyme activity occur within a
fairly small temperature range — usually
from about 30°C to 40°C .
 Temperatures outside of this range can
cause denaturation of the enzyme.

Enzymes are very important in the human
digestive system. Their production and
accumulation in the body are carefully
controlled so that homeostasis is
maintained.
The Liver
Almost all the blood circulating from the
intestines to the heart passes through the
liver.
 Everything you eat that gets into the
bloodstream passes through your liver.
 The liver then either stores nutrients or
breaks them down even more.
 The liver transforms nutrients into
proteins, fats, and cholesterol and stores
vitamins (A, D, K, and B12), minerals, and
carbohydrates.

The liver also plays the role of a filtering
system. Toxic substances, including alcohol,
are transformed into less harmful
substances.
The Role of the Liver in
Homeostasis

Recall: Glucose is necessary for cells to
produce ATP, the molecule that stores
energy. The amount of ATP that the body
needs at any one time changes; therefore,
the body needs to be able to store
glucose when it is not needed, but release
glucose when it is needed.

Two hormones responsible for controlling
the concentration of glucose in the blood
are insulin and glucagon, which are
produced in the pancreas.

The liver also plays an important role in
blood glucose control. It is here that
excess glucose is stored in the form of
glycogen.

When you eat a meal, blood glucose
levels start to rise. When they reach a
certain concentration, receptors in the
pancreas stimulate the production of
insulin. This hormone reaches the liver,
which then converts blood glucose into
glycogen. Blood glucose levels drop and
return to a normal level.

If blood glucose levels drop below a
certain level, receptors in the pancreas
stimulate the production of glucagon.
This hormone reaches the liver, which
then converts glycogen into glucose.
Glucose is released into the bloodstream
and blood glucose levels rise until they
return to a normal level.
Nutrients

There are six basic types of nutrients
used by the human body.
◦ carbohydrates, lipids, proteins, vitamins,
minerals, and water

Some of the uses that your body makes
of nutrients are functional and others are
structural in nature.
Carbohydrates
Used by the cells of the body to produce
energy.
 The human body cannot produce its own
carbohydrates and must rely on eating
other organisms to acquire them.
 Primary source of energy for humans,
supplying 50 to 80% of the total energy
required.
 Provide short-term energy to cells
through the process of cellular
respiration.

Monosaccharides
Simple single sugars
 Most common is glucose which provides
energy for cellular respiration and the
production of ATP and fructose which is
found in fruit

Glucose
Disaccharides
Double sugars
 Made by joining together simple sugars
with chemical bonds.
 Most common is sucrose (white table
sugar), lactose (dairy products)

Sucrose
Polysaccharides
Complex sugars made of many simple
sugars joined by chemical bonds.
 Several hundred glucose molecules join
together to form starches which are the
primary form of storage carbohydrate
used by plants.
 Cellulose, which is found in the cell wall
of plants, is the fibre we need in our diets;
which our digestive system requires to
function correctly.

Simple Carbohydrates
Usually sweet-tasting like cookies, candy,
pop, pastries, fruits, and juices.
 Good sources of carbohydrates, but do
not always contain vitamins or
minerals.
 Considered “empty-carbs”

Complex Carbohydrates
Starchy carbohydrates take longer to be
digested than simple carbohydrates and
therefore provide a more long-term
source of energy for the body.
 Complex carbs, are found in breads, pasta,
rice, and potatoes and most vegetables
like corn, tomatoes, carrots, and lettuce

Lipids (Fats)
are only used for energy when the supply
of carbohydrates in the bloodstream is low
 difficult to break down
 stored source of excess energy for the
body
 about twice as energetic as one gram of
carbohydrate

important in helping the body absorb
vitamins effectively
 key structural components of cell
membranes, a form of insulation for the
body and serve as protection for delicate
vital organs
 the raw material from which hormones
are produced
 an important component in the
transmission of nerve impulses in the
nervous system

Cholesterol
Fat molecule that occurs naturally in
the body
 Produced by the liver and is transported
by the blood to cells
 Found in all animal products
 Very important component of cell
membranes.
 The body processes cholesterol in the
diet in different ways, depending on what
type of food contained the cholesterol
that was ingested.

LDL (“bad” cholesterol)
Low-density lipoprotein
 Can accumulate on artery walls and
restrict blood flow.
 Liver maintains a "normal" level of
cholesterol by removing excess LDL from
the blood

HDL (“good” cholesterol)
High-density lipoprotein
 Lowers blood cholesterol by carrying
LDLs back to the liver which can break
down the LDL molecule
 The level of HDL in the blood can be
increased if you decrease the amount of
saturated fat in your diet and if you
increase your physical activity.
 HDL can reduce the risk of heart attack
and stroke

Comparison between Unsaturated
and Saturated Fats
Unsaturated Fats
o liquid at room temperature
o usually come from plants
o oils
o Example:
• olive oil
• sunflower oil
• corn oil
Saturated Fats
o solid at room temperature
o come from animals
o linked to heart disease
o more difficult for the body to break down
o can accumulate on the inside of blood
vessels, causing circulatory problems.
o Example: fat found in beef, pork, chicken,
and dairy products made from milk, such
as butter
It is recommended that no more than 30%
of your total energy intake should be in the
form of lipids, since lipid molecules store
about twice as much chemical energy as
carbohydrate molecules.
Proteins

Primary structural component of all
cells

Broken down by your digestive system
into amino acid molecules which are
then rebuilt into other proteins.

Muscle, cartilage, ligaments, skin, and hair
are all examples of protein molecules.

Smaller protein molecules also play vital
roles as hormones, antibodies, enzymes,
and hemoglobin.

Vary in size and shape which allows for
incredible diversity of functions
performed by proteins
Amino Acids
20 amino acids
 9 of them cannot be made by the human
body and are referred to as the essential
amino acids because they must be
eaten.
 All 20 "building blocks" allow the
synthesis of all of the many proteins that
your body manufactures.

Glycine
Food Sources of Proteins
Complete Proteins
o contain all of the essential amino acids
o beef, chicken, fish, eggs, milk, and many animal
products
Incomplete Proteins
o do not have all of the essential amino
acids
o vegetables, fruits, grains, seeds, and nuts
It is recommended that you consume 0.8 1.5 grams of protein per kilogram of body
weight. People who are on a vegetarian
diet need to pay special attention to the
foods they eat to make sure that they
acquire adequate amounts of all amino
acids.
Vitamins
Organic substances (must contain a
carbon chain) that are needed in very
small amounts
 Part of the structure of many enzymes
 If your diet is deficient in a certain
vitamin, the result could be an enzyme
deficiency that can in turn cause serious
health problems

Are chemicals that are necessary for
normal growth and metabolism.
 Some vitamins are required for the
chemical reactions that result in the
release of energy from carbohydrates, fats,
and proteins.


Fat-soluble vitamins (A, D, E, and K)
◦ are not as readily excreted from the body
◦ stored in the liver

Water-soluble vitamins (C, and the
B vitamins)
◦ can become depleted more quickly leading to
a deficiency
◦ must be included regularly in your diet
Minerals

Unlike vitamins, minerals are inorganic
— that is, they are not based on a carbon
chain. Most minerals are molecules of a
single element. Minerals do not provide
energy for the body, but they do play
very important roles in many cellular
reactions. For example, minerals are vital
in maintaining nervous system functions,
water balance, and skeletal health.
16 Essential Minerals
Sodium is found in table salt and helps
maintain proper blood pressure.
 Potassium is found in bananas and keeps
nerves and muscles working properly.
 Calcium is found in dairy products and is
needed to build strong bones.
 Iron is found in red meats and
wholegrain vegetables; it is very important
in the production of red blood cells.

Water
Water is an essential component of
cytoplasm — the intracellular fluid that
makes up all cells.
 Water is an important solvent because
most chemical reactions in the body
occur between molecules that are
dissolved in it.
 Water is also an important lubricant and
bathes body cells.


Water is a primary component of blood
and transports vital nutrients and
unneeded wastes throughout the body.

Water also serves an important role in
temperature regulation.
*without water, many life
functions would not occur*

Water is acquired in many ways. Drinking
water is the most obvious way that you
obtain water. However, many foods have a
high water content, such as fruit and
vegetables. Also, many cellular processes
involve the production of water. For these
reasons, it is suggested that you drink
about 8 cups of water or other noncaffeinated fluids per day to maintain
adequate hydration.
Dietary Decisions
Guidelines for Healthy Eating
1. Enjoy a variety of foods.
2. Emphasize whole grain cereals, breads, other
whole grain products, vegetables, and fruit.
3. Choose lower-fat dairy products, leaner meats,
and foods prepared with little or no fat.
4. Achieve and maintain a healthy body weight by
enjoying regular physical activity and healthy
eating.
5. Limit salt, alcohol, and caffeine.