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Biology 12 Unit 4

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on 6 February 2018

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Transcript of Biology 12 Unit 4

Enzymes & Digestive System
How do Enzymes Work?
• molecules are so small and the reaction so fast, we’ve never clearly seen how enzymes work
• 2 models used to explain how enzymes work:

1. Lock and Key Theory

• E and S meet during the reaction, and
fit together perfectly
from the very beginning, like a lock and key
Liver
Τhe liver is a critically important organ in
digestion & homeostasis.
Part 1 Enzymes
METABOLISM AND METABOLIC PATHWAYS

METABOLISM:
sum of all the chemical reactions occurring in a cell

METABOLIC PATHWAYS
A series of reactions that proceed in an orderly, step by step manner
One reaction leads to the next
Highly structured
Controlled by
enzymes
Each step within the metabolic pathway requires a
SPECIFIC
enzyme
5. The Presence of INHIBITORS
o Inhibitors: molecules that bind to the enzyme in some way to
prevent or reduce the rate
of substrate binding to enzyme
o Several ways in which inhibition can work:

a) Competitive Inhibition

• a molecule that looks like the substrate can compete for space at the
active site
(the place where the substrate binds to enzyme)
slows down the reaction rate - the inhibitor binding to E can be
REVERSIBLE or IRREVERSIBLE.
• Obviously, the more inhibitors are added, the lower the rate of reaction and
less product
is going to be made.
Biology 12 Unit 4
Hormonal Control of Digestive Gland Secretion
Hormones
: substances that are produced by one set of cells and affect a different set of cells

transported in blood
pancreas also has an endocrine function: produces the hormones
INSULIN and GLUCAGON
Human Nutrition:
Main Classes of Nutrients
1) Carbohydrates
2) Lipids
3) Fats
4) Vitamins & Minerals
Small Intestine: The Food Processor
• Up to this point, only some digestion has taken place (
starch in the mouth and protein in the stomach
). Most of
digestion and absorption
of most nutrients occur in the small intestine.
• Divided into three zones: the
DUODENUM, JEJUNUM, and ILIUM
.
Reasons why metabolic pathways exist:

• It is not possible in biological systems to have a single reaction that could produce complex molecules from simple reactants.
(e.g. 6CO2 + 6H2O ⇒ C6H12O6 + 6O2 would never happen in a cell in one step)
• Many
intermediate steps
are needed

• One pathway can lead to several others, intermediate products of one pathway can be
starting reactant
for another
pathway

• When you have more than one step, it means that there are more places where the overall reaction can be
controlled
ENZYMES: Biological Catalysts

ENZYME
: a protein that can speed up a chemical reaction without being consumed.

What are Enzymes made of?

1) A protein part called an
APOENZYME
that gives it its specificity (i.e. exactly what reaction it will catalyze)
2) A non-protein group called a
COENZYME
• Enzymes are the sites of chemical reactions,
but aren’t used up in the reaction or permanently changed by the reaction
(hold reactant molecules together long enough for them to react)


SUBSTRATE
: the reactant(s) in an enzyme’s reaction
• the equation for an enzyme-catalyzed reaction is always:
E + S ES E + P

“ES” =
ENZYME-SUBSTRATE COMPLEX
ACTIVE SITE
: the place where the substrates actually bind on the enzyme.
2. Induced Fit Theory
• we now believe that the enzyme actually
CHANGES SHAPE
slightly when it binds the substrates(get a better tighter grip)

• after reaction takes place, the ES complex separates, and the enzyme re-assumes its original shape (
now free to catalyze another reaction
)
How does an Enzyme Work?
• It
LOWERS
the
ACTIVATION ENERGY
required for the reaction to proceed
• Activation Energy: the energy that must be supplied to cause molecules to
react with one another.


Enzymes do this by bringing the substrate molecules together and
holding them long enough
for the reaction to take place.
FACTORS AFFECTING ENZYME ACTIVITY
• enzymes are proteins, so are affected by the same sorts of things that affect proteins
• since the shape of enzymes determines
the shape of the active site anything
that changes the shape of an enzyme will affect the enzymes function
• some factors are:
1.
pH
: most human enzymes prefer pH’s of 6 - 8 (some exceptions: pepsin in the stomach - pH ~ 2, trypsin in the small intestine - pH ~ 8)

o If the pH is too low or too high compared to the optimum pH, the enzyme
DENATURES
(a denatured protein is one that has lost its
normal configuration
, and therefore its ability to form an
enzyme-substrate
complex).
2. TEMPERATURE
o if the temperature is below the optimum temperature of 37 C, the rate of reaction will be
slower
o the lower the temp. drops, the lower the
rate of reaction
o very low temps don’t normally denature the enzyme
3. Concentrations of SUBSTRATES
o If the concentration (abbr. = “[ ]”) of substrate increases, amount of
product increases

o The rate of product formation will usually increase too
o However, after a certain [ ], the rate won’t increase anymore, as all the enzymes are “
saturated
” with substrates and can’t work
any faster
o if the [ ] of substrate decreases, the rate of product formation will generally
decrease as well
4. Concentration of ENZYMES
o Limits the overall rate of reaction

o Providing there is adequate substrate (and there are typically millions more substrate molecules than enzyme molecules), the more enzyme you add,
the more product you get
, and the less enzyme you have, the
less product
you get.
b) NON-COMPETITIVE INHIBITION
• inhibitor binds to
another place
on enzyme (not the active site)
• inhibitor may look
completely different
from the substrate
• when the inhibitor binds, it causes the enzyme to change shape at the active site so S cannot bind.
• binding may be
reversible or non-reversible
Examples of Inhibition
o Reversible inhibition is often used as a normal way of slowing down metabolic pathways (e.g. an intermediate or final product may be a reversible inhibitor of another enzyme in the pathway).

o Inhibitors can also be chemicals introduced into a system from the
outside
, and can act as
medicines or poisons
. e.g. penicillin is a medicine that kills bacteria. It works by binding irreversibly to the enzyme that makes bacterial cell walls.
ATP - The Molecule of Energy
o Cells use Adenosine Triphosphate when they require energy. ATP is used for all reactions requiring energy, like
synthesis, muscle contraction, active transport
, etc.

o ATP is a nucleotide composed of the base adenine and the sugar ribose, plus three phosphate groups. ATP has two high-energy P bonds (phosphate bonds). When they break,
energy is released
.
• ATP breaks down to ADP (adenosine diphosphate) and inorganic phosphate (P or “Pi”) when it releases energy.
• If energy is added to the system, the reverse action will occur.

ATP ADP + P + Energy
ADP + P + Energy ATP

• ATP, ADP and Pi are not destroyed during energy transfers. They are constantly being
formed and broken down.
THE DIGESTIVE SYSTEM
• DIGESTION:
The process of breaking down ingested food, both mechanically and chemically
, into molecules small enough to move through epithelial cells and into the internal environment of the body.

ABSORPTION
: The passage of digested nutrients from the small intestine into the
blood or lymph
, which then distributes them through the body.

ELIMINATION
: the expulsion of indigestible residues from the body.
DIGESTION
• During digestion, proteins are broken down into amino acids, carbohydrates into glucose, fats to glycerol and fatty acids, and nucleic acids to nucleotides.
MOUTH
• Your mouth is where digestion (both chemical and mechanical) begins.
• The mouth receives food, chews it up (
mechanical
), moistens it, and starts to digest any starch (
chemical
) in the food.
Structure
• The mouth is divided into an anterior
hard palate
(contains several bones) and a posterior
soft palate
, which is composed of muscle tissue. That thing that hangs down in the back of your throat people think is their tonsils is really the
uvula
, and is the end part of
soft palate
. (Tonsils lie on the sides of the throat).
• A normal adult mouth has
32 teeth
. The purpose of teeth is to chew food into pieces that can be swallowed easily.
• There are three sets of
SALIVARY GLANDS
that produce SALIVA:
1.
parotid
(below ears)
2.
sublingual
(below tongue)
3.
submandibular
(under lower jaw).
• Saliva contains the enzyme
salivary amylase
which allows starch digestion to begin in the mouth, even before the food is swallowed.
• Once food has been chewed, it is called a
bolus
.
• Food is then passed through the back of the mouth when you swallow. The first region that it enters is called the
1) The
SOFT PALATE MOVES BACK
to cover openings to nose (nasopharyngeal openings).
2)
TRACHEA
(WINDPIPE) MOVES UP under a flap of tissue called the
epiglottis
, blocking its opening. When food goes down the "wrong way" it goes into the trachea, and is then
coughed back up
.
• The food then has one route to go: down the
ESOPHAGUS
.
• Esophagus: a long muscular tube that extends from
pharynx to stomach
. Made of several types of tissue.
• Food moves down the esophagus through
PERISTALSIS
(rhythmical contractions of the esophageal muscles).
• Food bolus reaches the end of the esophagus, passing through the
cardiac sphincter
, and entering the stomach.
Stomach
o It is a thick-walled, J-shaped organ that lies
on left side of the body beneath the diaphragm.

o It can stretch to hold about half a gallon (~2 liters) of solids and/or liquids in an average adult.
o It consists of three layers of
muscle
that contract to churn and mix its contents
o The mucus lining of the stomach contains inner
GASTRIC GLANDS
which produce
GASTRIC JUICE
. Gastric juice contains
PEPSINOGEN and HCl
(hydrochloric acid). When the two combine, pepsinogen forms
PEPSIN, a HYDROLYTIC ENZYME
that breaks down proteins into smaller chains of amino acids called peptides.
o HCl gives stomach a pH of ~3. Highly corrosive. This
kills bacteria in food and helps break it down
.
o Why doesn’t the stomach digest itself? This is because its inner wall is protected by a thick layer of
MUCUS
secreted by mucosal cells.
o After 2-6 hours (depending on the type of food), the food has been turned into a semi-liquid food mass called
CHYME
, and the stomach empties into the first part of the small intestine (called the duodenum). This emptying is controlled by the
PYLORIC SPHINCTER
at the bottom of the stomach.
• It is about 7 meters long (~23 feet), compared to 1.5 m (5 feet) for large intestine.
• The first 25 cm of small intestine is called the DUODENUM. The duodenum plays a major role in digestion. It is here that
SECRETIONS SENT FROM THE LIVER AND PANCREAS
to break down fat , carbs and protein are received.
• The liver produces
BILE
, an emulsifier not an enzyme, which is sent to the duodenum via a duct from the GALL BLADDER (where bile is stored).
• Bile is a thick green liquid (it gets its green colour from
byproducts of hemoglobin breakdown
(another function of the liver).
• Bile contains emulsifying agents called
BILE SALTS which break FAT into FAT DROPLETS
.

PANCREAS
sends pancreatic juice into duodenum through the same duct as the bile travels through.
• The juice contains enzymes and sodium bicarbonate (NaHCO3)
• NaHCO3 makes the juice
highly alkaline
(pH ~ 8.5). It neutralizes the acid chyme and make the small intestine
pH basic
• Pancreatic juice contains hydrolytic enzymes including pancreatic amylase (digests
starch to maltose
), trypsin (digests
protein to peptides
), and lipase (digests
fat droplets to glycerol & fatty acids
)
• The LIVER, GALL BLADDER, AND PANCREAS are required for digestion but
NEVER see food
.
The walls of the duodenum and small intestine are lined with millions of GLANDS that produce juices containing enzymes that finish the digestion of
protein and starch
.
Secretions from the intestinal glands contain digestive enzymes:
Large Intestine
o Consists of
COLON and RECTUM
(the rectum is thelast 20 cm of the colon). Opening of rectum is called
ANUS
.
o Τhe colon has 3 parts(
ascending, transverse, and descending
)
MAIN FUNCTIONS OF LARGE INTESTINE
REABSORPTION OF WATER
from indigestible food matter (feces)
Absorption of certain vitamins
 Feces also contain bile pigments, heavy metals, and billions of E. coli. While there is no question that they are parasites, they provide a valuable service for us. These bacteria break down some indigestible food and in the process produce some
vitamins that are in turn absorbed by the colon.

Comprehensive Summary of DIGESTIVE ENZYMES
o The breakdown of food (fats, carbohydrates, proteins) into molecules small enough to be absorbed requires the action of specific enzymes.
o Each enzyme has specific site where it works, and a specific pH range in which it can operate.
o All are hydrolytic enzymes that catalyze a reaction of the substrate with water.
SALIVARY GLANDS
Salivary Amylase
STOMACH
Pepsin
PANCREASE
Pancreatic Amylase
Lipase
Trypsin
Deoxyribonuclease
Ribonuclease
SMALL INTESTINE
Peptidase
Tripeptidase
Dipeptidase
Maltase
Lactase
The small intestine is specialized for both digestion and absorption.
DIGESTION
Receives enzymes from
pancreas and intestinal glands
Receives
bile from the liver
Alkali pH
8.5 due to bicarbonate ions
released from pancreatic juice
Peristalsis
Very long
Hormones control digestion
(CKK, gastrin, secretion)
ABSORPTION
Large surface area (
folding, villi, microvilli
)
Villi have capillary network giving a large blood supply because it is connected to
hepatic portal vein
(liver)
Villi contains lacteals which are part of the
lymphatic system
Small Intestine walls are
very thin
o ABSORPTION: takes place across the wall of each villus (can happen
passively or actively
)
o Nutrient can now enter the blood or the lymphatic system (depending what it is)
o Fatty acids & glycerol are absorbed across the villi, & recombined into fat molecules in the epithelial cells of the villus
 fats then move into the
LACTEAL
of each villus and enter the
LYMPHATIC SYSTEM
 fats will eventually enter the blood stream at the
subclavian vein
(shoulder).
o Glucose & amino acids enter the blood through the capillary network.
o The blood vessel from the villi in the small intestine merges to form the
HEPATIC PORTAL VEIN
which leads to the liver.
1) Keeps blood concentrations of nutrients, hormones etc. constant (e.g. converts
glucose to glycogen

after meals
and then
glycogen to glucose between meals
to keep blood glucose levels constant).

2)
Interconversions of nutrients
(e.g. carbohydrates to fats, amino acids to carbohydrates and fats).
3) Removes toxins from the blood (
detoxifies
). Removal of unwanted particulate matter from the blood through the
mediation of macrophages
.

4)
Production of Bile
. Up to 1.5 liters of bile per day!

5) Destroys old
red blood cells
.
6)
Production of urea
. (deamination of amino acids and excretion of resulting ammonia as urea, uric acid, etc.)

7)
Manufacture of plasma proteins
such as fibrinogen and albumin.
8) Manufacture of cholesterol.

9)Storage of iron.

10) Storage of vitamins.
Insulin
: is released from pancreas when
glucose levels are high
(released directly into blood and travels to target cells throughout body)

tells liver to “
store glucose as glycogen
” (ie. glucose in blood is taken up by cells)
lowers blood [glucose]
Glucagon
: opposite to insulin

 is released from pancreas when glucose levels are low
 tells liver to “
release glucose from glycogen


raises blood glucose levels
People who don’t produce insulin or enough insulin, or who lack insulin receptors on target cells, will suffer from diabetes.

HORMONES control secretion of specific digestive juices
Hormones
Gastrin

Secretin

CCK
(cholescystokinin)

GIP (gastric
inhibitory peptide)
1.Carbohydrates
o
Primary source of energy
o Diet should consist primarily of
complex carbohydrates
(not refined sugars)
o Carbohydrates are digested eventually to glucose, which is stored by liver as
glycogen
o
Glucose is only fuel brain will use

2.Fats
Most fats can be made by liver.
o Fats in food are mostly found in animal products (meat and dairy). These are especially high in saturated fats (saturated fats tend to be solid at room temp).
o High fat and protein diets are number one cause of death in North America (heart disease, strokes, hypertension, many forms of cancer, many other disorders and diseases).
o You should get ~15% of your calories from fat. Most Americans and Canadians get between 40 and 60% of their calories from fat!
o Fats are high in calories (> twice as many per gram (9.1) as carbohydrates or protein (4.4.))
3.Proteins
o Protein is necessary for tissues, metabolism, enzymes etc.
 It is NOT an energy food
o Of the twenty types of amino acids, 8 cannot be manufactured by humans --
- called essential amino acids.
o Protein deficiency is the most common form of malnutrition in poorer
countries. The swollen abdomen of starving children is caused by edema due to the lack of plasma proteins in the blood.
o Most North Americans eat more than 2 to 3 times the amount of protein they
need.
4. Vitamins & Minerals
• Vitamins are organic compounds that the body can't produce but must be present in the diet (though they are only required in very small amounts). Lack of any one vitamin can cause serious health disorders.
• Vitamin D: deficiency leads to rickets(bowing of legs). Manufactured naturally by skin upon exposure to sun.
o Vitamin C:
deficiency leads to scurvy
o Riboflavin: deficiency causes fissures of lips (cheilosis)
o Niacin: deficiency causes dermatitis of areas of skin exposed to light (called pellagra)
o Many vitamins are
coenzymes
. e.g. Niacin: coenzyme of NAD. Riboflavin: coenzyme of FAD.
o Best source of vitamins is
fresh fruits and vegetables
in a balanced diet.
o Certain
MINERALS
are also needed by the body.
1)
MACRONUTRIENTS
: gram amounts needed daily. Na, Mg, P, Cl, Ca.
• e.g. Calcium makes up structural component of important tissues (e.g. bone, cartilage), and is also a necessary ion for the transmission of nerve impulses across synapses and the initiation of muscle contraction.
2)
MICRONUTRIENTS
: (“trace elements”). Minute amounts (micrograms) needed. Very specific. e.g. Fe (for hemoglobin), Iodine(for the hormone thyroxin).
Catabolism
refers to the breaking down of molecules. The term
anabolism
refers to the building up of molecules, or
synthesis
.

Metabolism is the sum of catabolism and anabolism.
Exergonic reactions
are spontaneous and release energy, while
endergonic reactions
require an input of energy in order to occur.
ATP is a carrier of
energy between exergonic and endergonic reactions.
• Enzymes are Highly Specific
each enzyme speeds up only
one reaction
enzyme names usually end with the suffix “ase” (or sometimes “sin” e.g. trypsin, pepsin)
o increasing the temp towards the optimum will
increase
the rate of reaction and product formation (as it speeds up the rate at which substrates bump into enzymes)

o however, temps above the optimum will
slow the rate of reaction
and a temp that is too high (above about 45 °C) will
DENATURE
the enzyme
o HCN (hydrogen cyanide) is a lethal irreversible inhibitor of enzyme action in human.

o Lead (Pb++) and other
HEAVY METALS
(like mercury (Hg++) and cadmium) are non-competitive inhibitors that bind irreversibly to enzymes and make them denature.
METABOLIC RATE AND THE THYROID AND PARATHYROID GLANDS
Thyroid Gland
o The thyroid gland is a large gland
located in the back of the neck.
One of the products of the thyroid gland is that it produces a
hormone called calcitonin.
Calcium is a crucial molecule that serves various purposes in our bodies such as,
nerve conduction, muscle contraction, and blood clotting.
When the calcium levels in our blood rise,
the thyroid gland secretes calcitonin to allow calcium to be deposited into our bones
, until the calcium levels even out.
Parathyroid Glands
o These four small glands are located
behind the thyroid gland.
When our body does not have an adequate supply of calcium in the blood stream, the
parathyroid gland releases the Parathyroid hormone, (PTH).
This hormone promotes the release of calcium from the
bones
,
kidney’s and the intestinal tract.
Once the blood calcium levels are back to normal,
PTH is no longer secreted.
PHARYNX
, which is simply the region between mouth and esophagus where swallowing takes place.
• Swallowing is a
reflex action
(requires no conscious thought).
• To prevent food from going down your air passages, some clever manoeuvring is necessary.
peptidases digest
peptides to amino acids
and maltase digests maltose (a disaccharide)
to glucose
. Other enzymes made here digest other
disaccharides
(e.g.lactase digests
lactose,
the sugar in milk).
Digestive Disorders
Stomach Ulcer
is an open sore in the wall of the stomach caused by a gradual disintegration of the tissue.
typically there is a thick layer of mucus that protects the wall of the stomach.
when this layer is compromised due to a bacterial infection or overuse of the inflammatory response, an ulcer is formed and if left untreated it can become a perforated ulcer which can be life threatening.
Diarrhea
Most cases of diarrhea are acute, meaning they are sudden due to a bacterial or viral infection in the small intestine or the large intestine. Most commonly known as food poisoning.
Severe diarrhea can lead to dehydration, as well as disturb the hearts contraction due to an imbalance of salts in the blood.
Crohn's Disease
Some people are afflicted with chronic diarrhea.
Persistent inflammation of the intestine that results in recurrent bouts of cramping and bloody diarrhea is known as Crohn's disease.
Researcher's believe that it's caused by a misdirected immune response, in severe cases small, or large parts of the colon must be removed.
Pancreatitis
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