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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

what is math number
Tric Reply
x-2y+3z=-3 2x-y+z=7 -x+3y-z=6
Sidiki Reply
Need help solving this problem (2/7)^-2
Simone Reply
what is the coefficient of -4×
Mehri Reply
the operation * is x * y =x + y/ 1+(x × y) show if the operation is commutative if x × y is not equal to -1
Alfred Reply
An investment account was opened with an initial deposit of $9,600 and earns 7.4% interest, compounded continuously. How much will the account be worth after 15 years?
Kala Reply
lim x to infinity e^1-e^-1/log(1+x)
given eccentricity and a point find the equiation
Moses Reply
12, 17, 22.... 25th term
Alexandra Reply
12, 17, 22.... 25th term
College algebra is really hard?
Shirleen Reply
Absolutely, for me. My problems with math started in First grade...involving a nun Sister Anastasia, bad vision, talking & getting expelled from Catholic school. When it comes to math I just can't focus and all I can hear is our family silverware banging and clanging on the pink Formica table.
I'm 13 and I understand it great
I am 1 year old but I can do it! 1+1=2 proof very hard for me though.
Not really they are just easy concepts which can be understood if you have great basics. I am 14 I understood them easily.
hi vedant can u help me with some assignments
find the 15th term of the geometric sequince whose first is 18 and last term of 387
Jerwin Reply
I know this work
The given of f(x=x-2. then what is the value of this f(3) 5f(x+1)
virgelyn Reply
hmm well what is the answer
If f(x) = x-2 then, f(3) when 5f(x+1) 5((3-2)+1) 5(1+1) 5(2) 10
how do they get the third part x = (32)5/4
kinnecy Reply
make 5/4 into a mixed number, make that a decimal, and then multiply 32 by the decimal 5/4 turns out to be
can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
okay, so you have 6 raised to the power of 2. what is that part of your answer
I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
I'm not sure why it wrote it the other way
I got X =-6
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
so you not have an equal sign anywhere in the original equation?
is it a question of log
I rally confuse this number And equations too I need exactly help
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Commplementary angles
Idrissa Reply
im all ears I need to learn
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salut. from Algeria
A soccer field is a rectangle 130 meters wide and 110 meters long. The coach asks players to run from one corner to the other corner diagonally across. What is that distance, to the nearest tenths place.
Kimberly Reply
Jeannette has $5 and $10 bills in her wallet. The number of fives is three more than six times the number of tens. Let t represent the number of tens. Write an expression for the number of fives.
August Reply
What is the expressiin for seven less than four times the number of nickels
Leonardo Reply
How do i figure this problem out.
how do you translate this in Algebraic Expressions
linda Reply
why surface tension is zero at critical temperature
I think if critical temperature denote high temperature then a liquid stats boils that time the water stats to evaporate so some moles of h2o to up and due to high temp the bonding break they have low density so it can be a reason
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
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Source:  OpenStax, Principles of biology. OpenStax CNX. Aug 09, 2016 Download for free at http://legacy.cnx.org/content/col11569/1.25
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