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Why is emulsification important for digestion of lipids? Pancreatic juices contain enzymes called lipases (enzymes that break down lipids). If the lipid in the chyme aggregates into large globules, very little surface area of the lipids is available for the lipases to act on, leaving lipid digestion incomplete. By forming an emulsion, bile salts increase the available surface area of the lipids many fold. The pancreatic lipases can then act on the lipids more efficiently and digest them, as detailed in [link] . Lipases break down the lipids into fatty acids and glycerides. These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules and become coated with proteins. These large spheres are 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 in the subclavian vein.

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.

Vitamins

Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins are absorbed in the same manner as lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins. Water-soluble vitamins can be directly absorbed into the bloodstream from the intestine.

This website has an overview of the digestion of protein, fat, and carbohydrates.

Art connection

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.

Which of the following statements about digestive processes is true?

  1. Amylase, maltase, and lactase in the mouth digest carbohydrates.
  2. Trypsin and lipase in the stomach digest protein.
  3. Bile emulsifies lipids in the small intestine.
  4. No food is absorbed until the small intestine.

Elimination

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 colon is 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. As the 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 the rectum.

Common problems with elimination

Diarrhea and constipation are some of the most common health concerns that affect digestion. Constipation is a condition where the feces are hardened because of excess water removal in the colon. In contrast, if enough water is not removed from the feces, it results in diarrhea. Many bacteria, including the ones that cause cholera, affect the proteins involved in water reabsorption in the colon and result in excessive diarrhea.

Emesis

Emesis, or vomiting, is elimination of food by forceful expulsion through the mouth. It is often in response to an irritant that affects the digestive tract, including but not limited to viruses, bacteria, emotions, sights, and food poisoning. This forceful expulsion of the food is due to the strong contractions produced by the stomach muscles. The process of emesis is regulated by the medulla.

Section summary

Digestion begins with ingestion, where the food is taken in the mouth. Digestion and absorption take place in a series of steps with special enzymes playing important roles in digesting carbohydrates, proteins, and lipids. Elimination describes removal of undigested food contents and waste products from the body. While most absorption occurs in the small intestines, the large intestine is responsible for the final removal of water that remains after the absorptive process of the small intestines. The cells that line the large intestine absorb some vitamins as well as any leftover salts and water. The large intestine (colon) is also where feces is formed.

Art connections

[link] Which of the following statements about digestive processes is true?

  1. Amylase, maltase and lactase in the mouth digest carbohydrates.
  2. Trypsin and lipase in the stomach digest protein.
  3. Bile emulsifies lipids in the small intestine.
  4. No food is absorbed until the small intestine.

[link] C

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Questions & Answers

what does DNA mean
innocent Reply
what is a zygote
Victor Reply
zygote is developed or fertilized egg cell
Lekan
what is the difference between cell wall and cell membrane
Ruhiyatu Reply
cell wall is found in plant while cell membrane is found in animal cell
Lekan
how cell I form
Elijah Reply
what difference between animal cell and plant cell
Lazarus Reply
what is animal call
Rita
Animal cell does not have a cell wall but plant cell have a cell wall
Ruhiyatu
plant have cell Wall well animal have not cell wall
Yula
what is fertilization?
Muhamed Reply
Fertilization is the fusion of male sex cell and female sex cell to form a zygote.
Ruhiyatu
What kind of nutrients is composed of plants
Annie Reply
phosphorus,potassium,nitrogen
Kritika
what is hormones
Igwe Reply
hormon is the chemical messanger
Sneha
Genes can make someone dull?
Taperah
21ecological instrument and their diagrams
Ayomide Reply
cell biology
Muhamed
I am sorry
onuoha
for wat
ezeadaugo
no reason
KP
nothing but speculate
KP
Sorry without reason is nothing but speculate
Leilah
what are the kidney disease
Immaculate Reply
kidney stones
Gracelyne
it is a disease that affects the kidney
Miriam
what are some lung diseases
Gracelyne
kidney cancer
Victor
what is micro-organism
Jackson Reply
what is the hypothesis
Jackson
hypothesis is a proposed explanation for a phenomenon
Miriam
hypothesis is raw materials
KP
what is biology
KP
biology is the study of living things and their interaction with their environment
Miriam
what is zyogot?
Muhamed
what is fertilization?
Muhamed
zygote is an unfertilized eggs
KP
fertilization refers to the fusion of a sperm and ovum
KP
organs that can't be seen physically
Victor
what does mean stigma
Amira Reply
the stigma receives pollen grains during pollination
Ruhiyatu
what is the full of the MOST dangerous disease in the world where one stops sleeping and just dies :Hint ; FFI
God Reply
fatal familial insomnia which affects the thalamus
Miriam
there are other dangerous diseases like CAD i.e coronary artery disease
Miriam
what is matter
Thomas Reply
it is any thing that has weight and occupies space
Anye
matter is any substances that occupies spaces and has mass
Jackson
describe photosynthesis
Mavis Reply
What is equilibrium
Mavis
What is equilibrium
Mavis
like corporal intern balance right?
FRANCISCA
on my own understanding is just a balanced state
Stanley
photosynthesis is the process by which plants and other organisms convert light energy to chemical energy
Miriam
photosynthesis is describe as the process by which green plants use energy from the sunlight to manufacture their own food by combining carbon dioxide and water.
Ruhiyatu

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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