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THE DIGESTIVE SYSTEM
• as heterotrophs we need to take in our
food from the outside
• once inside the cell, the nutrients are used
to make energy
• the main function of the digestive system
is to break down food into nutrients, which
are then absorbed and transported by the
circulatory system throughout the body
All Cells Require Nutrients for:
•
1. growth
•
2. maintenance
•
3. repair
Stages of Digestion
The Four Stages of Digestion
1. Ingestion – taking in nutrients, aka eating
2. Digestion – breaking down complex
molecules into smaller ones
3. Absorption – taking up of digested
molecules
4. Egestion – removal of wastes
1. INGESTION and DIGESTION:
the mouth
• Food enters mouth
–
–
–
–
it is broken up by teeth:
incisors cut
canines and bicuspids pierce and tear
molars crush and grind
• action of chewing stimulates salivary
glands which secrete saliva
• saliva: a mixture of water, mucus and
enzymes (ex. amylase: breaks down
starches into simpler carbohydrates)
• * let a soda cracker dissolve on your
tongue and see if you can taste the
difference
• Food and saliva form a bolus that is
pushed to the pharynx (dual purpose: air
or food)
– flap-like epiglottis directs food down
esophagus
– bolus moves down esophagus through
rhythmic muscle contractions (peristalsis)
until it reaches the cardiac sphincter of the
stomach
1. Physical /Mechanical Digestion - biting,
tearing, crushing, grinding, and mixing food
(teeth, tongue)
2. Chemical Digestion - the enzyme ‘salivary
amylase’ begins the digestion of starch
(Amylase) as saliva is mixed with the food.
Amylase
STARCH (AMYLOSE) ------------------ MALTOSE
3. Propulsion - swallowing
Down the Esophagus…
• Location
–found beneath the diaphragm toward the left
side of abdomen
• Description
–thick walled, J-shaped , muscular but flexible
bag
–it has deep folds that stretch out as it fills with
food (1 Litre)
2. DIGESTION
a) DIGESTION: the stomach
• - The stomach is the site of food storage
(1.5 L capacity) and initial protein digestion
• - To enter and to exit the stomach the food
must pass through sphincters: constrictor
muscles that surround a tube-like structure
• - Cardiac sphincter relaxes and lets bolus
fall in
Function: (Be able to give an example of
what these terms mean.)
• PHYSICAL DIGESTION
• CHEMICAL DIGESTION
• ABSORPTION
• PHYSICAL DIGESTION:
–the stomach can expand to store large
amounts of food
–it churns and mixes the food
CHEMICAL DIGESTION
• Contains gastric juice (secreted by
stomach lining): a mixture of hydrochloric
acid, enzymes that work well at low pH
(ex. pepsin: begins to break down proteins
into polypeptides), and mucus
• HCl breaks down fibers, including bacteria
• Mucus protects stomach lining from HCl (if
mucus layer is destoroyed, ex. Heliobacter
pylori we get stomach ulcers as the lining
is digested)
Absorption
• Some absorption starts here: water,
alcohol, some medications (ex. aspirin)
Function Continued
• The mixture of partially digested food,
water, and gastric juices that results is
called chyme
• Chyme passes out of stomach through
pyloric sphincter and into small intestine
From the Stomach into the Small
Intestine
b) DIGESTION: the small
intestine and pancreas
• Measures up to 7 m in length, only 2.5 cm
in diameter with the surface area of a
tennis court
• it is formed of three parts: duodenum,
jejunum and ileum
• Pyloric sphincter empties chyme into
duodenum, the first 25 cm of the small
intestine
• The major organ of chemical digestion
and nutrient absorption
• Secretions from the pancreas and liver
enter the duodenum through a duct
• (Tomorrow’s Lesson)
3. ABSORPTION
• a) ABSORPTION: small intestine
• Fingerlike villi increase the surface
area of the small intestine, and
absorb the glucose, amino acids,
fatty acids and glycerol
• Once absorbed the nutrients enter
the bloodstream and are distributed
throughout the body
Villi and Microvilli
On to the Large Intestine
• b) ABSORPTION: LARGE INTESTINE (colon)
• Water is absorbed
• Harmless bacteria live in large intestine and
produce vit K and B which are also absorbed
4. EGESTION: Rectum and Anus
• Note: FIBRE aids the function of
the colon. It helps keep enough
water in the feces and
stimulates peristalsis promoting
“regularity”
• People who don’t eat enough
cellulose are at risk of colon
cancer
• The entire process takes 24 36 hours
• The complete digestion of CHO, lipids and
partly digested proteins take place???
• The duodenum!!!
• Where does the majority of water absorption
occur?
• The Large Intestine
8Ac
Mouth
Gullet
Stomach
Teeth grind food, saliva added.
Small intestine
Large intestine
Rectum
Anus
Absorbs all nutrients.
Muscles squeeze food down.
Food mixed with strong acid.
Water is absorbed.
Faeces are formed and stored.
Faeces are egested.
Your Work
1) Define the following words…
Digestion
Absorption
Egestion
2) What is the function of saliva?
3) How does chewing assist in digestion of food?
4) Use your lesson notes and text book to answer the
following questions…
Page 216 # 1, 5
5) How is the small intestine structured to allow
maximum absorption?
6) Why should we ensure that we eat fiber?
Accessory Organs
• Although digestion in the small intestine is aided by
digestive juices (enzymes) secreted by its own cells,
most of the chemical digestion (breakdown) of the
macromolecules, carbohydrates, lipids and protein,
occurs as a result of secretions produced by the
liver and the pancreas that empty into the small
intestine to act on these molecules.
• The pancreas, liver and gallbladder are referred to as
accessory digestive organs as foods
(macromolecules) do NOT enter any of these organs.
• The pancreas, liver and gallbladder are both organs
and glands
Liver
• Has four functions: synthesis and breakdown,
detoxification, storage
1. Synthesis: produces bile (made of bile salts), which
breaks down fats, bile is stored and concentrated in
gallbladder until needed
- bile also functions as an emulsifier, to allow the fats to
travel through the digestive system and be well brokendown
- if too much bile /too concentrated bile is produced the
bile stored in the gallbladder can start to crystallize
forming
gallstones which then block the bile duct
2. Breakdown: removes the highly toxic
nitrogen group from amino acids to form
urea (component of urine)
• Breaks down blood cells (stores products in
gallbladder for removal – makes feces
brown)
3. Detoxification: works to remove toxins
from the body
- removes alcohol and other chemicals
through detoxification
4. Storage: stores carbohydrates and
vitamins
• Jaundice
• yellow pigmentation in the skin when bile
pigment (bilirubin) is high in the blood
stream
• caused by a malfunctioning liver
• Cirrhosis
• chronic disease of the liver
• liver becomes fatty and eventually forms
scar tissue
www.thestencil.com/archives/2006/06/index.html
Pancreas
• Produces sodium
bicarbonate, which raises
the pH of chyme from 2.5 to
9.0
• Secretes digestive
enzymes, ex. lipase (breaks
down fat) and trypsin
(breaks down proteins)
Small Intestine
• Secretes maltase (maltose to glucose) and
peptidase (complete protein breakdown)
Break down of the Macromolecules
(AMYLASE)
CHO’s ------digested to------ MONOSACCHARIDES
(starch)
(glucose)
(PEPTIDASE)
PROTEIN -------digested to--------- AMINO ACIDS
(BILE)
(LIPASE)
FATS ---emulsified----digested to --- GLYCEROL &
FATTY ACIDS
8A Where it all happens
What happens where? Follow the food
down through the body.
In your mouth, teeth chop up food into smaller pieces.
Muscles in the (esophagus) squeeze the chewed food from
your mouth to your stomach
Stomach produces acid and enzymes to break down food.
Intestines Absorbs all nutrients and water
Rectum: Faeces are formed and stored
Faeces (poo) is stored in the rectum and pushed out of the
anus.
Homeostasis
• The body’s ability to adjust to the
fluctuating internal and external
environment
DIGESTION AND HOMEOSTASIS
• The nervous and hormonal systems act on
digestion before we even eat: seeing,
smelling and tasting food stimulates
gastric secretions
• Swallowing stimulates production of
gastric juices, ex. the hormone gastrin
stimulate gastric juice release before food
gets to stomach
Speed of Digestion
• The speed of digestion also varies with food type
- large meal activates stronger stomach
muscle contractions and faster emptying
- fatty meal: digestion slows down in
response to a hormone from the small
intestine (enterogasterone) to allow more
time for fat digestion (we feel full longer after
a fatty meal)
Regulating Blood Glucose
• - special receptors located in the body sense blood sugar, body T,
oxygen levels, etc.
- when homeostasis is disrupted this sends a message to the brain
to correct it
• ex. blood sugar levels:
•
- constant concentration is crucial to our well-being, if fall below
a certain value our brains shut down (coma)
•
- when we eat the blood sugar (glucose) goes up
•
- our body releases insulin (a hormone that activates cell
surface proteins) from the pancreas to bring the glucose inside
the cells
•
- liver cells collect any extra glucose and convert it to glycogen
for storage
•
- decrease in blood sugar levels activates the pancreas to
release glucagon, a hormone that converts glycogen in the liver
back to glucose and releases it into the bloodstream
food
insulin
release
(pancreas)
cells take up excess
glucose, liver makes
glycogen
high
glucose
normal
glucose
normal
glucose
low glucose
glycogen
release
as glucose
(liver)
glucagon
release
(pancrea
s)
You work…
• Complete the worksheet by labeling the
diagram and describing the function of
each organ.
• What function does bile play in digestion?
• What does the pancreas secrete that
protects the duodenum from stomach
acids (chyme)?
• Page 228 # 1, 2
• Page 238 # 2