Download Biology 12 Digestive System The digestive process can be divided

Biology 12
Digestive System
The digestive process can be divided into 4 phases:
1. ingestion - includes swallowing and peristalsis
2. digestion - the physical (by teeth) and chemical (by enzymes) breakdown of food
3. into nutrients (unit molecules) small enough to diffuse into cells
4. absorption - diffusion and active transport of nutrients into blood
5. defecation - removal of undigested material
Summary of chemical breakdown of food by hydrolytic enzymes (Protein enzymes).
Digestive System Anatomy
Mouth
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Food is ingested and the teeth begin mechanical breakdown of food into smaller pieces.
tongue pushes food against hard palate (roof of mouth) further physically break food to smaller pieces
Salivary glands secrete saliva
saliva contains:
1. mucus to make the food slippery for swallowing
2. hydrolytic enzyme salivary amylase - begins the chemical breakdown of starch into maltose
(disaccharide) making the food taste sweet and therefore taste better.
The tongue pushes food to pharynx which starts the swallowing reflex, food is called a bolus.
swallowing reflex:
1. As bolus is swallowed, soft palette pushes upward to close the nasal cavity.
2. Uvula, often mistaken for the tonsils, is a small flap that projects down into the pharynx and acts
to “kick” food away from the nasal cavity.
3. Epiglottis closes entrance to trachea (windpipe) so that food does not enter here.
Esophagus
• Food enters the esophagus a muscular tube which leads to the stomach. Wave-like contractions (called
peristalsis) of the smooth muscle which surrounds the esophagus pushes the food towards the stomach.
• At entrance to the stomach the cardiac sphincter (a ring of muscle), prevents food from leaving the
stomach back into the esophagus. Heartburn occurs when acid from the stomach escapes through the
cardiac sphincter into the unprotected esophagus.
Stomach
• food enters the stomach the gastric glands of the stomach secrete gastric juice
• Gastric juice contains:
1. hydrochloric acid (HCl)
HCl creates acidic environment of stomach (pH = 3), acidic environment kills bacteria in food
but its main function is to convert the inactive enzyme pepsinogen into the active enzyme
pepsin.
2. mucus coats the stomach wall protecting the cells from HCl
3. pepsinogen – an inactive enzyme
• pepsinogen (an inactive enzyme) + HCl = pepsin
• Pepsin begins breakdown of protein into peptides.
• An ulcer is caused by the digestion of proteins in the stomach (gastric) cell membranes,
effectively “eating” a hole in the stomach lining (caused mainly by bacteria, not excess
stomach acid)
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Food leaving stomach called acid chyme - travels out of stomach through pyloric sphincter to S.I.
Small intestine (S.I.)
• Duodenum - first part of small intestine, where the rest of food is chemically broken down by enzymes and
the absorption of nutrients into the blood begins.
• As acid chyme (containing fats and partially digested protein) enters the small intestine, the liver increases
production of bile and causes the release of bile from the gall bladder where bile is stored.
• Bile enters the duodenum (through a duct (tube)) where it emulsifies fat, causing fat to be broken down into
smaller droplets. Emulsification is defined as the process of causing fat to disperse in water.
Emulsification increases the surface area of fat (many smaller droplets), therefore increasing the surface
area for fat enzymes (lipase) to work.
• Pancreas secretes pancreatic juice which enters the duodenum through the pancreatic duct.
• Pancreatic juice contains:
Sodium bicarbonate (NaHCO3)
Amylase (pancreatic)
Lipase
Trypsin
An easy way to remember the components of pancreatic juice is to remember the acronym SALT
SALT =
• Sodium bicarbonate neutralizes acid chyme to a pH of 8 (slightly basic), provides an appropriate
environment (pH) for the pancreatic enzymes, amylase, lipase, and trypsin.
• Pancreatic amylase continues breakdown of starch to maltose (started in the mouth).
• Lipase breaks down fats to fatty acids and glycerol (unit molecules) which is helped by the emulsification
of fat into smaller droplets by bile. (Bile is not an enzyme)
• Trypsin continues the breakdown protein into peptides which was started in the stomach.
• At this stage the only food to be completely broken down are fats.
Intestinal Juice: produced by cells lining the small intestine
• Includes enzymes needed to finish the breakdown of protein and carbohydrates. These include a group of
enzymes called peptidases which breakdown peptides into amino acids.
• Also, a group of enzymes that breakdown carbohydrates into glucose are found here. The most notable of
these is maltase which breaks down maltose into glucose. Lactase is also found here and it breaks down
lactose to glucose. Someone who is lactose intolerant cannot produce this protein hormone called lactase.
• Also includes nucleases to break down nucleic acids to nucleotides
Villi
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Nutrients are absorbed through the villi, the tiny folds in the surface of small intestine
Villi - increase surface area for absorption.
Fatty acids and glycerol diffuse into the lacteal, part of the lymphatic system which eventually empties
into the blood stream. This prevents the tiny capillaries in the villi from becoming clogged.
Glucose and amino acids (as well as vitamins and minerals like calcium and potassium) diffuse into the
capillaries (blood vessels) found inside the villi.
Glucose and amino acids can also be actively transported into the blood from the small intestine; of course
this requires ATP (energy) and allows transport from low to high concentration (against the concentration
gradient). This ensures that the blood absorbs as much glucose as possible.
Active transport is accomplished by carrier proteins which act like pumps
Colon
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The remaining undigested material travel into the colon or large intestine.
Here water and salt are absorbed and the feces is compacted and temporarily stored.
Helpful bacteria called E. coli are found in the colon where they breakdown some of the undigested
material and produce vitamins and amino acids for our use.
Finally the undigested material is removed in a process called defecation. Defecation is the removal of
undigested material which has never entered the blood; this is technically different from excretion or the
removal of waste products from the blood by the kidneys.
The Liver: The Gatekeeper - 6 major functions
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Produce bile which is then stored in the gall bladder
Destroys old red blood cells and converts hemoglobin breakdown products which become part of bile
Stores glucose as glycogen after eating and breaks down glycogen to glucose between eating.
This maintains blood glucose levels at about 0.1%.
Detoxifies blood by removing and metabolizing poisonous substances like alcohol.
Produces blood proteins like some blood clotting proteins
Produces urea - deaminates amino acids - removal of amine group forming NH3 which is toxic.
NH3 is converted to urea, a less toxic substance, by the liver. The rest of the amino acid can be
converted to glucose to meet emergency energy needs.
Enzyme Chart - Summarize major enzymes used in the digestive process
Enzyme
Salivary
amylase
Pepsin
Produced
Salivary glands
Works here
Mouth
Substrate
Starch
Product
Maltose
Preferred pH
7 or 8
Gastric glands
Stomach
Protein
Peptides
2 or 3
Pancreatic
amylase
Lipase
Pancreas
Small intestine
Starch
Maltose
8
Pancreas
Small intestine
Lipids
8
Trypsin
Pancreas
Small intestine
Protein
Fatty acids +
glycerol
Peptides
Peptidases
Intestinal glands
Small intestine
Peptides
Amino acids
8
Maltase
Intestinal glands
Small intestine
Maltose
Glucose
8
Nucleases
Intestinal glands
Small intestine
Nucleic acids
Nucleotides
8
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The homeostasis of blood glucose (sugar) levels- Insulin and Glucagon Hormones
Insulin:
1. After eating and digesting
carbohydrates, blood glucose levels
rise.
2. This causes the pancreas to secrete
insulin. Insulin (a hormone =
chemical messenger) is secreted into
blood vessels and travels throughout
the bloodstream.
3. Insulin attaches to receptors on
various cells of the body (like liver
and muscle cells). Insulin causes
these cells to take in glucose. Inside
these cells glucose can be used to
produce ATP or stored as glycogen.
4. These actions of glucose cause the
blood glucose levels to drop. This
drop in blood glucose inhibits (shuts
off) further insulin secretion. This is
an example of negative feedback.
Glucagon:
5. Between meals, blood glucose levels
drop.
6. This causes the pancreas to secrete
glucagon. Glucagon (a hormone =
chemical messenger) is secreted into
blood vessels and travels throughout
the bloodstream.
7. Glucagon causes the liver and
muscles to convert glycogen back to
glucose and release this glucose into
the blood stream.
8. These actions cause blood glucose to rise. This rise in blood glucose inhibits (shuts off) further glucagon
secretion. This is an example of negative feedback.
Insulin and Glucagon Summary
Liver glycogen
Blood glucose
Glucose uptake
Skeletal muscle glycogen
Metabolic Actions of Insulin and Glucagon
Insulin
Glucagon
Stimulates synthesis of triglycerides
Stimulates release of free fatty acids
from free fatty acids; inhibits release of
from triglycerides. (decreases fat
free fatty acids from triglycerides.
storage)
(increases fat storage)
Increases glycogen synthesis; increases
Stimulates breakdown of glycogen so
glucose uptake and storage.
that glucose can be released into blood.
Stimulates glucose uptake and storage
No receptors, no effect.
of glucose as glycogen.
Amino acid uptake
Protein synthesis
Stimulates amino acid uptake by cells
and is necessary for protein synthesis.
Action
Fatty acid use
Triglyceride production
No receptors, no effect.
So How Does Insulin Increase Body Fat? – Insulin is a storage hormone.
Insulin Action:
1. Insulin attaches to receptor sites on the cell membrane.
2. This signals the glucose channels to open and glucose can diffuse into the cell.
3. Glucose can be used to produce ATP to provide energy for cellular functions.
4. Some glucose can be stored as glycogen. This glycogen can be reconverted back to glucose if the cell is
low on glucose and needs ATP.
5. Extra glucose is converted to fat (triglycerides). This fat can be reconverted back to glucose like substances
and used to produce ATP. If glucose is always present, then this fat stays stored in the cell. Adipose cells
fill with fat, especially in the abdominal region.
6. If the insulin receptors are not functioning (and/or insulin is not present), then there is no signal to open
glucose channels. Without glucose entering cells, ATP production is severely limited. Fats become the next
energy source for ATP but with limited fat storage the cell will turn to amino acids (proteins) to supply
emergency glucose.
7. Insulin also promotes the uptake of amino acids by muscles cells, therefore, assisting in muscle protein
synthesis (muscle growth)
So How Does glucagon Decrease Body Fat?
Glucagon Action:
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When there are a lot of fatty acids (FA) in the blood, these fatty acids diffuse into fat cells (adipose cells)
and are turned into triglycerides (TG). Therefore, fat cells grow in size, especially abdominal fat cells.
Between meals, glucagon hormone is secreted from the pancreas. Glucagon attaches to receptor sites on fat
cells
Through some complex biochemical events (cascade), ATP is used to phosphorylate (add phosphate to)
HSL (hormone sensitive lipase). This activates HSL.
Activated HSL will hydrolyze (digest) stored triglycerides into free fatty acids.
These free fatty acids diffuse out of the cell and can be used to produce ATP in other cells (like muscle
cells). Adipose (fat cells) shrink in size meaning less body (abdominal) fat.
Turning down insulin and increasing glucagon tips.
1.
Eat low glycemic carbohydrates: Low glycemic carbohydrates results in slower absorption of glucose and
lower insulin release. Therefore, less fat storage. This link is a good place to start your research:
http://www.eatrightontario.ca/en/Articles/Carbohydrate/Getting-to-know-the-Glycemic-Index.aspx
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Eat carbohydrates with protein and fat: This also slows the absorption of glucose and results in lower
insulin levels after each meal. Therefore, less fat storage.
Eat more whole foods like fruit and vegetables rather than processed carbohydrates (including juices and so
called energy drinks).
Avoid fat free foods which contain high fructose corn syrup. High fructose corn syrup results in insulin
spike and increased fat storage.
Change your exercise routine to include body weight circuits that help build muscle. Reduce long slow
cardio exercises.
Understand the concept of biochemical individuality – what works for others might not work for you.
Keep researching and asking questions – if it was easy then everyone would do it!!