<< Chapter < Page Chapter >> Page >

The next step of carbohydrate digestion takes place in the duodenum. The chyme from the stomach enters the duodenum and mixes with the digestive secretions from the pancreas, liver, and gallbladder. Pancreatic juices also contain an amylase enzyme, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases, which are also present in cells lining the small intestine. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (or “table sugar”) into glucose and fructose, and lactase breaks down lactose (or “milk sugar”) into glucose and galactose. The monosaccharides (e.g., glucose and fructose) thus produced are absorbed by the intestinal cells and transported into the bloodstream. The steps in carbohydrate digestion are summarized in [link] and [link] .

Pathways for the breakdown of starch and glycogen, sucrose, and lactose are shown. Starch and glycogen, which are both polysaccharides, are broken down into the disaccharide maltose. Maltose is then broken down into the monosaccharaide glucose. Sucrose, a disaccharide, is broken down by sucrose into the monosaccharides glucose and fructose. Lactose, also a disaccharide, is broken down by lactase into glucose and galactose.
Digestion of carbohydrates is performed by several enzymes. Starch and glycogen are broken down into glucose by amylase and maltase. Sucrose (table sugar) and lactose (milk sugar) are broken down by sucrase and lactase, respectively.
Digestion of Carbohydrates
Enzyme Produced By Site of Action Substrate Acting On End Products
Salivary amylase Salivary glands Mouth Polysaccharides (Starch) Disaccharides (maltose), oligosaccharides
Pancreatic amylase Pancreas Small intestine Polysaccharides (starch) Disaccharides (maltose), monosaccharides
Oligosaccharidases Lining of the intestine; brush border membrane Small intestine Disaccharides Monosaccharides (e.g., glucose, fructose, galactose)


A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids. In the duodenum, other enzymes—trypsin, elastase, and chymotrypsin—act on the peptides reducing them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides). Specifically, carboxypeptidase, dipeptidase, and aminopeptidase play important roles in reducing the peptides to free amino acids. The amino acids are absorbed into the bloodstream through the small intestines. The steps in protein digestion are summarized in [link] and [link] .

Protein digestion begins in the stomach, where pepsin breaks proteins down into fragments, called peptides. Further digestion occurs in the small intestine, where a variety of enzymes break peptides down into smaller peptides, and then into individual amino acids. Several of the protein-digesting enzymes found in the small intestine are secreted from the pancreas. Amino acids are absorbed from the small intestine into the blood stream. The liver regulates the distribution of amino acids to the rest of the body. A small amount of dietary protein is lost in the feces.
Protein digestion is a multistep process that begins in the stomach and continues through the intestines.
Digestion of Protein
Enzyme Produced By Site of Action Substrate Acting On End Products
Pepsin Stomach chief cells Stomach Proteins Peptides
  • Trypsin
  • Elastase Chymotrypsin
Pancreas Small intestine Proteins Peptides
Carboxypeptidase Pancreas Small intestine Peptides Amino acids and peptides
  • Aminopeptidase
  • Dipeptidase
Lining of intestine Small intestine Peptides Amino acids


The bulk of lipid digestion occurs in the small intestine, via the action of pancreatic lipase. When chyme enters the duodenum, it triggers a hormonal response resulting in the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides, by emulsification. Emulsification is a physical process in which large lipid globules are dispersed into several small lipid globules. Lipids are hydrophobic substances: in the presence of water, they will aggregate to form large globules to minimize exposure to water. These small globules have a larger surface-to-volume ratio and thus an increased surface area for the lipases to interact with. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile salts emulsify large lipid globules into small lipid globules.

By forming an emulsion, bile salts increase the available surface area of the lipid particles significantly. The pancreatic lipases can then act on the lipids more efficiently and digest them, as detailed in [link] . Lipases break down the dietary triglycerides into fatty acids and monoglycerides (one fatty acid attached to a glycerol molecule). These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. Lipase products (fatty acids and monoglycerides) pass through the intestinal cells where they are reassembled into triglycerides, and then are combined with proteins to form large fatty complexes called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface. The surface is also composed of the hydrophilic phosphate "heads" of phospholipids. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood via the thoracic duct on their way to the liver.

Illustration shows a row of absorptive epithelial cells that line the intestinal lumen. Hair-like microvilli project into the lumen. On the other side of the epithelial cells are capillaries and lymphatic vessels. In the intestinal lumen, lipids are emulsified by the bile. Lipases break down fats, also known as triglycerides, into fatty acids and monoglycerides. Fats are made up of three fatty acids attached to a 3-carbon glycerol backbone. In monoglycerides, two of the fatty acids are removed. The emulsified lipids form small, spherical particles called micelles that are absorbed by the epithelial cells. Inside the epithelial cells the fatty acids and monoglyerides are reassembled into triglycerides. The triglycerides aggregate with cholesterol, proteins, and phospholipids to form spherical chylomicrons. The chylomicrons are moved into a lymph capillary, which transports them to the rest of the body.
Lipids are digested and absorbed in the small intestine.

Summary of digestion

Steps in mechanical and chemical digestion are shown. Digestion begins in the mouth, where chewing and swallowing mechanically breaks down food into smaller particles, and enzymes chemically digest carbohydrates. In the stomach, mechanical digestion includes peristaltic mixing and propulsion. Chemical digestion of proteins occurs, and lipid-soluble substances such as aspirin are absorbed. In the small intestine, mechanical digestion occurs through mixing and propulsion, primarily by segmentation. Chemical digestion of carbohydrates, lipids, proteins and nucleic acid occurs. Peptides, amino acids, glucose, fructose, lipids, water, vitamins, and minerals are absorbed into the bloodstream. In the large intestine, mechanical digestion occurs through segmental mixing and mass movement. No chemical digestion occurs except for digestion by bacteria. Water, ions, vitamins, minerals, and small organic molecules produced by bacteria are absorbed into the bloodstream.
Mechanical and chemical digestion of food takes place in many steps, beginning in the mouth and ending in the rectum.


The final step in digestion is the elimination of undigested food content and waste products. The undigested food material enters the colon, where most of the water is reabsorbed. Recall that the colonis also home to the microflora called “intestinal flora” that aid in the digestion process. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. Asthe rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of therectum.

Questions & Answers

Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Principles of biology. OpenStax CNX. Aug 09, 2016 Download for free at http://legacy.cnx.org/content/col11569/1.25
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Principles of biology' conversation and receive update notifications?