Download Digestion Secretions

SBI 3U
Digestion Package
Food contains the nutrients we need but not in a form that our bodies can use directly. The food we eat must be broken
down into nutrients (monomers) that can then be absorbed into the blood stream and carried to the cells of the body.
This process is called digestion.
There are four main steps in digestion: ingestion (the taking in of nutrients), digestion (the physical and chemical
breakdown of complex food molecules into smaller molecules), absorption (the transfer of digested nutrients from the
digestive system to the bloodstream), and egestion (the removal of waste food materials from the body).
More complex animals digest food along a digestive tract that has two openings—one for food intake and another for
waste elimination. The digestive tract is organized into specialized regions that enable the breakdown and absorption of
food as food moves along the tract in one direction. This design is often referred to as a complete digestive system.
Often referred to as the gastrointestinal tract (GI tract), the human digestive tract is approximately 7 m to 9 m long,
depending on the age and size of the individual.
Anus, Esophagus, gall bladder, pancreas, pharynx, liver, small intestine, large intestine, pancreas, rectum,
Mouth (oral cavity), salivary glands, stomach
Various specialized locations along the gastrointestinal tract work to break food down
physically and chemically. In humans, digestion begins in the mouth or Oral (Buccal) Cavity.
Food is broken down into smaller pieces by the teeth (physical digestion). Teeth in the front of your mouth, called
incisors and canines, are specialized for grabbing and cutting your food. Behind these teeth are premolars and molars—
broad flattened teeth specialized for grinding and crushing food. The type of teeth an animal has is directly related to its
diet. For example, herbivores have many molars for chewing plant matter, whereas carnivores have canine teeth that
allow them to grab and kill prey.
The sight, smell, or presence of food in the triggers the salivary glands to secrete a watery fluid called saliva. Saliva
contains enzymes— chemicals that increase the rate of chemical reactions in living things. One enzyme found in saliva is
amylase, which breaks down starch into smaller disaccharides (chemical digestion). Saliva also dissolves food particles,
making it possible for you to taste food. Saliva contains mucus, a protective secretion that acts as a lubricant and aids in
swallowing. On average, you produce 0.75 L to 1.5 L of saliva per day. Most of this saliva is water, which moistens the
food into a ball, or bolus, so that it can be swallowed.
Once food has been chewed and mixed with saliva in
the mouth, the tongue pushes it to the back of the
mouth where it is swallowed. As the food is pushed
into the pharynx, the soft palate is raised to prevent
food from entering the nasal passages. At the same
time, the larynx is raised against a flap of soft tissue
called the epiglottis. This covers the entrance to the
trachea to prevent food from entering the lungs. (If you
place your finger on the outside of your neck over your
larynx as you swallow, you will feel the larynx rise.)
This process of taking food into the body through the
mouth (by swallowing) is called ingestion (bolus moves through the pharynx, over the epiglottis, into the esophagus).
Once swallowed, food moves from the mouth (pharynx) to the stomach by way of the
esophagus a long, muscular tube (diameter 2 cm, length 25 cm). The food stretches the walls
of the esophagus, activating the smooth muscles to undergo rhythmic, wave-like
contractions called peristalsis. Peristalsis ensures the movement of food down the esophagus
and into the stomach. It takes about 8s for food to travel down the esophagus into the
stomach. Although gravity helps, it is peristalsis that ensures the movement of food down
through the esophagus to the stomach and through the entire digestive system. If you were
to stand on your head, you would still be able to swallow liquids!
The Structure of the Stomach
The stomach, normally the size of a fist, can expand in size to store up to 2 L of food.
Proteins are partly digested in the stomach and continue digesting in the small intestine. Lipids and carbohydrates are
not digested in the stomach. The movement of food into and out of the stomach is controlled by circular muscles called
sphincters. There are more than 50 sphincters in the human body, and several of these are in the digestive tract. The
gastroesophageal sphincter is located where the esophagus joins the stomach. When relaxed, the gastroesophageal
sphincter allows food to enter the stomach. When contracted, it prevents food from moving back into the esophagus. The
surface of the stomach looks wrinkled, these folds are called rugae and allow for expansion.
Working from the inside out.
 Mucosa - extensively folded layer that secretes gastric juice (mixture of digestive enzymes, acid, and mucus). The
epithelial cells in the mucosa divide rapidly to heal any damage. In fact, the entire stomach lining is replaced about
every three days!
 Submucosa- layer of connective tissue that contains networks of nerves and blood vessels.
 Muscularis, or muscle layer - consists of smooth muscles. These muscles contract frequently, churning and mixing the
food with gastric juices to produce a semi-liquid material called chyme.
 Serosa - smooth, outermost layer of the stomach that holds the stomach in place and secretes a lubricating fluid that
eliminates friction between organs.
Chemical Digestion in the Stomach
The process of digestion is a carefully controlled and coordinated process that involves enzymes, hormones, and nerves.
The nerves in the submucosa detect when food is present and initiate the release of a hormone called gastrin. Gastrin is
released into the bloodstream and transported to gastric cells in the stomach, where it stimulates the release of gastric
juice. Each gastric gland secretes only a tiny amount of gastric juice, but since there are millions of glands, the total
amount of gastric juice produced is up to 2 L per day. Gastric juice is mostly made up of mucus, but also contains acid
and digestive enzymes. The mucus, a slippery secretion, coats and protects the lining of the stomach from acid and
digestive enzymes. The hydrochloric acid present in gastric juice is released by parietal cells and is very strong; it
normally ranges from pH 2.0 to pH 3.0. In
comparison, lemon juice is pH 2.4 and battery
acid is pH 1.0. The acidic gastric juice kills many
harmful micro-organisms that are ingested with
food. It stops the action of amylase but provides
the necessary pH for the activation of other
digestive enzymes, such as pepsinogen. The
hydrochloric acid in gastric juice converts
pepsinogen (released by chief cells) to its active
form, pepsin, which begins the breakdown of
proteins into separate amino acids. Secreting the
inactive enzyme pepsinogen is a safety
mechanism that prevents damage to the
stomach tissue. If gastric glands were to make
pepsin instead of pepsinogen, the stomach
would digest itself.
Digestion in the small and Large Intestines
Carbohydrates, proteins, and lipids are digested in the small intestine with the help of hormones and enzymes. About 3 h
to 5 h are required to process the contents of a meal in the small intestine, where nutrients from digested food are
absorbed into the body.
The Structure of the Small Intestine
Most digestion and absorption of nutrients takes place in the small
intestine. The small intestine is a long tube that is only about 2.5 cm
in diameter. Although it is narrow and takes up very little space, this
section of the GI tract can be up to 7m in length. In comparison, the
large intestine can be up to 7.6 cm in diameter but is only about 1.5
m in length. Lipids and carbohydrates, as well as any remaining
proteins, are digested in the small intestine.
The small intestine is made up of three sections: the duodenum (first 25 cm to 30 cm of the small intestine and is where
most enzymes are added and digestion in the small intestine begins), the jejunum (digestion continues and some
nutrients are absorbed), and the ileum (last section, majority of nutrients is absorbed).
The inner surface of the
small intestine is adapted to
provide the maximum
surface area for efficient
nutrient absorption. The
inner layer of the small
intestine is folded into
ridges and has many small
finger-like projections
called villi (singular: villus)
which produce an
estimated tenfold increase
in surface area. To further
increase the surface area
available for absorption, each of the epithelial cells that make up the villi has even smaller, microscopic projections of
the cell membrane called microvilli (singular: microvillus). The combined effect of the villi and microvilli is estimated to
increase the surface area by a factor of 500. Within each villus is a network of tiny blood vessels called capillaries. All
nutrients, except digested fats, enter the bloodstream through the capillaries. Digested fats are transported through
small vessels called lacteals. The digested fats are transported into the lymphatic system, and from there into the
bloodstream.
Chemical Digestion in the Small Intestine
The pyloric sphincter controls the passage of food from the stomach into
the small intestine. When the food in the stomach has been mixed with
gastric juice and the proteins are partially digested, the pyloric sphincter
periodically relaxes to release small portions of chyme into the
duodenum. This slow and steady release of chyme into the small intestine
prevents overloading and allows time for thorough digestion. Most of the
enzymes required for digestion are added in the duodenum. This digestion requires input from the pancreas, liver, and
gall bladder.
The Role of the Pancreas in Digestion
The pancreas is a long, flat gland nestled between the stomach and
the duodenum. The pancreas has a dual role. It secretes enzymes
that are critical to the digestive process, and it also secretes
hormones that regulate the absorption and storage of glucose from
the blood. This pancreatic juice contains amylase (also found in
saliva), lipase, trypsin, chymotrypsin, and sodium bicarbonate
which raises the pH levels. The digestion of carbohydrates that
began in the mouth continues in the duodenum via amylase, which
continues the digestion of starch that was started in the mouth
(saliva). When fat-rich chyme enters the duodenum, a hormone
called cholecystokinin (CCK) is secreted by special cells in the
mucosa of the duodenum and released into the bloodstream. This hormone signals the pancreas to secrete a variety of
substances, including ones that control the pH of the intestine and enzymes that are needed for lipid, carbohydrate, and
protein digestion. These secretions enter the duodenum through the pancreatic duct. CCK also signals the stomach to
slow down the speed of digestion so that the small intestine can effectively digest the fats.
The chyme that enters the small intestine has a low pH of about 2.5. When the acidic chyme enters the small intestine, a
chemical called prosecretin that is present in the epithelial cells of the small intestine is converted into its active form,
secretin. Secretin is a hormone that stimulates the liver to make more bile and encourages the pancreas to secrete lipid
and protein enzymes. However, its primary function is to stimulate the pancreas to release bicarbonate ions (HCO3-) to
neutralize the acidic chyme and raise the pH from about pH 2.5 to pH 9.0. Pepsin is inactivated in the basic pH conditions
since it requires an acid environment to be activated. Thus, secretin protects the small intestine from stomach acids.
Since pepsin is active only in acidic conditions, the action of pepsin is discontinued in the small intestine. However,
protein digestion is carried on by other enzymes. The pancreas releases trypsinogen, which is an inactive form of a
protein-digesting enzyme called trypsin. The trypsinogen travels from the pancreas to the duodenum. Once it reaches
the duodenum, an enzyme called enterokinase converts it into active trypsin. Trypsin continues the work begun by
pepsin in the stomach, further breaking down any
partially digested proteins that remain. Other proteindigesting enzymes help break the short protein chains
into single amino acid molecules.
Lipids are also digested in the small intestine. Fats that
enter the duodenum are subjected to the action of
lipases, a group of enzymes secreted by the pancreas that break down lipids. Lipases break the lipid chains into shorter
chains and into individual fatty acid molecules. However, fats in chyme are present as large globules. Lipases cannot
penetrate beyond the surface of the fat globules, so for lipases to efficiently digest lipids, the liver and its secretions must
become involved.
The Liver and Gall Bladder
The liver is the largest internal organ of the body, located just underneath the diaphragm. The liver performs a number of
important functions. Its digestive function is producing and secreting bile to emulsify fats, breaking them into tiny
droplets called micelles. Once the large fat globules are broken down into micelles, the lipases have a much greater
surface area on which to act, and the rate of lipid digestion increases. Bile is continuously produced in the liver, but it is
stored in the gall bladder until food enters the duodenum. Lipids entering the duodenum stimulate the gall bladder to
contract, which causes bile to be squeezed out from the gall bladder into the duodenum through the bile duct.
All blood travelling through the capillary beds of the intestines goes directly to the liver before returning to the heart.
The liver begins the removal and breakdown of toxins, such as alcohol, that have been absorbed by the digestive system.
The liver is also involved in producing and storing various nutrients including glycogen and fat-soluble vitamins.
Absorption in the Small Intestine
Once proteins, carbohydrates, and lipids are broken down, they are absorbed in the jejunum and ileum. Vitamins,
minerals, and water are also absorbed in the small intestine. The structure of the small intestine plays a very important
role in absorption. Both the villi and microvilli increase the surface area for absorption of nutrients. Nutrients pass
through these cells, into the bloodstream, where they are then transported by capillaries to the tissues of the body.
Passive Transport
Passive transport is the movement of materials (3
ways) across a cell membrane without the use of
energy from the cell.
(Simple) Diffusion - Due to their constant, random
motion, particles in a gas or liquid state will spread out
until all the particles are evenly distributed Diffusion
will follow the concentration gradient, with molecules
moving from an area of higher concentration to an
area of lower concentration
Osmosis - the diffusion of water molecules across a
selectively permeable membrane, from an area of
higher concentration (of water molecules) to an area of lower concentration.
Facilitated diffusion- diffusion of molecules across a membrane via transport proteins. The transport proteins are
embedded in the cell membrane and help, or facilitate, diffusion. Transport proteins physically bind to molecules on one
side of the membrane and release them on the other side. Each transport protein has a unique size and shape that allows
only certain substances to pass through. Transport proteins act as gateways that increase the rate of diffusion.
Active Transport
Materials are moved across a cell membrane, from an area of lower concentration to an area of higher concentration,
against the concentration gradient using energy provided by the cell. As in facilitated diffusion, special transport
proteins embedded in the cell membrane actively move materials through the membrane. Unlike facilitated diffusion,
this movement across the cell membrane requires energy from the cell. Active transport is used to transport molecules
that are too large to diffuse through the cell membrane on their own, as well as to transport molecules (or ions) that
have a strong and uneven electrical charge that prevents diffusion across the membrane.
Absorption of nutrients in Capillary networks
Whether transported by diffusion, osmosis, facilitated diffusion, or active transport, all nutrients make their way
through the mucosa of the small intestine and into the capillary networks in the villi (out of the GI tract and into the
circulatory system). These capillary networks carry nutrients through the bloodstream to the rest of the body. The
lacteals, which are small vessels of the lymphatic system, carry dietary fats through the lymphatic system, eventually
reaching the bloodstream. Nutrients are transported from the bloodstream into the body cells by means of passive or
active transport.
The Structure and Function of the Large Intestine
The large intestine is approximately 1.5 m in length but is two
to three times larger in diameter than the small intestine,
about 7.6 cm. The large intestine consists of the cecum, colon,
rectum, and anus (external opening).
The small intestine does not simply continue on and become
the large intestine. The small intestine joins the large intestine
a few centimetres from the end. The short upper end of the
large intestine creates a blind pouch called the cecum, a sort
of dead end that receives the processed material from the
small intestine. A small finger-like projection from the cecum
is called the appendix. The appendix does not serve any
digestive function in modern humans but may have had a
digestive function in its evolutionary past.
The colon is the longest part of the large intestine and has four segments: the ascending colon, the transverse colon, the
descending colon, and the sigmoid colon. The rectum is the last 20 cm of the large intestine. The rectum holds the waste
products of digestion until they are eliminated through the external opening, the anus. Digestion is complete and most of
the nutrients have been absorbed by the time the digested material reaches the large intestine. However, there is still a
significant quantity of undigested and indigestible material, such as cellulose (fibre), that cannot be broken down by
humans. As this matter passes through the colon, most of the water is re-absorbed through the process of osmosis.
Approximately 20 L of fluids pass through the large intestine daily, and most of this is absorbed back into the body. This
fluid comes from the water ingested in the diet and from saliva, mucus, gastric juices, and other digestive fluids that are
secreted as food moves through the GI tract. Vitamins B, K (which are a by-product of bacteria activity in the large
intestine), sodium (Na+), and chloride (Cl–) ions are also absorbed. It may take 4 h to 72 h for the undigested material to
pass through the large intestine, depending on the types and volume of food eaten.
There are more than 500 species of bacteria that normally inhabit the large intestine. Some of these species of bacteria
are important partners in human nutrition. The most common species of bacteria in the human large intestine is
Escherichia coli, or E. coli. These bacteria exist in the intestine in a mutually beneficial relationship. The bacteria live in a
suitable environment and have access to a plentiful food supply. In return, they produce essential substances such as
vitamin K and some B vitamins. Another by-product of bacterial action is gas—a mixture of carbon dioxide, methane,
and hydrogen sulphide.
Egestion
The indigestible components of food, such as cellulose and other fibres, are important in the diet. They provide bulk and
help maintain a full feeling for a longer time. This can reduce overeating and help maintain a healthy weight. Fibre also
helps retain some water in the large intestine, which is important in egestion, the elimination of digestive wastes.
The absorption of water in the large intestine changes the liquid material in the colon into a soft solid called feces. If too
much water is absorbed back into the bloodstream, the feces become firmer and constipation may be the result. If too
little water is absorbed, watery feces, or diarrhea, may result, and this can lead to dehydration. The anus is surrounded
by two sphincter muscles. The internal anal sphincter is a smooth muscle and therefore under involuntary control. The
external anal sphincter is a skeletal muscle and under voluntary control. Feces are eliminated through the anus when
both sphincters are relaxed.
Review Questions –These questions can be completed in point form or sentence.
1.
2.
3.
4.
Compare and contrast monomers to polymers, give examples from carbohydrates, lipids, and proteins.
What are the 4 main steps in digestion
Compare and contrast physical and chemical digestion (provide examples of structures or compounds involved).
Food is consumed and broken down into smaller pieces in the _________________ cavity by ______________ digestion,
when combined with water found in _____________________secreted from the ______________________ glands it turns into
a ball or _________________________.
5. Humans are designed rather cleverly, the trachea (wind pipe) and esophagus (food pipe) run side by side, must
have been German engineering! Describe the fail safes in place that ensure your poutine from the café doesn’t
end up in your lungs.
6. You can drink and eat laying down (albeit a bit awkwardly). How is this accomplished without gravity? Explain.
7. What food types are digested in the stomach?
8. What characteristic of the stomach is responsible for it being able to hold 2 litres of content (think about a pop
bottle) if its only the size of our fist?
9. What is a sphincter? Roughly how many do we have in our body? Name and describe the location & function of
two attached to your stomach, and two in your external opening.
10. How many species of bacteria call our GI tract home? What type of relationship do we have with Escherichia
coli? Explain its role in digestion.
11. Digestion is dependent on our bodies ability to absorb nutrients as they are being broken down. Explain the
possible transport routes from the GI tract into the circulatory system (4 of them).
12. Biology is fundamentally based around the notion that structure determines function. How does the structure of
the small intestine make it ideally suited for nutrient absorption?
13. What is the main role or two of the following… Oral cavity, stomach, small intestine, large intestine, liver, gall
bladder, pancreas, sphincters.
14. Compare and contrast the large intestine to the small intestine in terms of function, segmentation, structure,
orientation and location.
15. What is the role of gastric juice? What is it composed of? What digestive enzymes does it contain?
16. What does fibre (dietary roughage) in the diet help do for the large intestine?
17. Describe the dual role the pancreas has.
18. Name any chemical or enzymes involved in the breakdown or digestion of lipids, carbs, and proteins, and what
the end product of those macronutrients will be.
19. Why is the pH level of our stomach, chime, and small intestine important? Describe some of the effects it has,
and what enzymes/secretions impact it.
20. You just sat down to eat a slice of Za, you splurged and got the meatlovers! Aside from taking in 1200 calories,
describe how your body will digest it
Digestion Secretions (By organ)
Organ
Mouth
Esophagus
Stomach
Liver
Gall bladder
Pancreas
Pancreas
Pancreas
Pancreas
Pancreas
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Large intestine
Secretion
Amylase & Mucus
Mucus
Mucus, HCl, Pepsinogen (pepsin), Rennin, & Gastrin
Bile
Bile
Amylase
Lipase
Insulin/glucagon
Trypsin/chymotrypsin
NaHCO3
Secretin
Peptidase
Maltase
Sucrase
CCK
Lactase
Mucus
Digestion Secretions
(Non-enzyme)
Secretion
Mucus
Bile
Released from
Action
Disrupt extracellular matrix in plants
and meats, kill bacteria, convert
pepsinogen to pepsin
NaHCO3
Stomach
Cholecystokinin Duodenum
(CCK)
Secretin
Small intestine
Insulin and
glucagon
Pancreas
Gastrin
Becomes pepsin (P)