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By the end of this section, you will be able to:
  • Explain how energy can be derived from fat
  • Explain the purpose and process of ketogenesis
  • Describe the process of ketone body oxidation
  • Explain the purpose and the process of lipogenesis

Fats (or triglycerides) within the body are ingested as food or synthesized by adipocytes or hepatocytes from carbohydrate precursors ( [link] ). Lipid metabolism entails the oxidation of fatty acids to either generate energy or synthesize new lipids from smaller constituent molecules. Lipid metabolism is associated with carbohydrate metabolism, as products of glucose (such as acetyl CoA) can be converted into lipids.

Triglyceride broken down into a monoglyceride

The top image shows the chemical formula for a triglyceride, and the bottom panel shows the formula for a monoglyceride.
A triglyceride molecule (a) breaks down into a monoglyceride (b).

Lipid metabolism begins in the intestine where ingested triglycerides    are broken down into smaller chain fatty acids and subsequently into monoglyceride molecules    (see [link] b ) by pancreatic lipases    , enzymes that break down fats after they are emulsified by bile salts    . When food reaches the small intestine in the form of chyme, a digestive hormone called cholecystokinin (CCK)    is released by intestinal cells in the intestinal mucosa. CCK stimulates the release of pancreatic lipase from the pancreas and stimulates the contraction of the gallbladder to release stored bile salts into the intestine. CCK also travels to the brain, where it can act as a hunger suppressant.

Together, the pancreatic lipases and bile salts break down triglycerides into free fatty acids. These fatty acids can be transported across the intestinal membrane. However, once they cross the membrane, they are recombined to again form triglyceride molecules. Within the intestinal cells, these triglycerides are packaged along with cholesterol molecules in phospholipid vesicles called chylomicrons    ( [link] ). The chylomicrons enable fats and cholesterol to move within the aqueous environment of your lymphatic and circulatory systems. Chylomicrons leave the enterocytes by exocytosis and enter the lymphatic system via lacteals in the villi of the intestine. From the lymphatic system, the chylomicrons are transported to the circulatory system. Once in the circulation, they can either go to the liver or be stored in fat cells (adipocytes) that comprise adipose (fat) tissue found throughout the body.

Chylomicrons

This figure shows a chylomicron containing triglycerides and cholesterol molecules as well as other lipids.
Chylomicrons contain triglycerides, cholesterol molecules, and other apolipoproteins (protein molecules). They function to carry these water-insoluble molecules from the intestine, through the lymphatic system, and into the bloodstream, which carries the lipids to adipose tissue for storage.

Lipolysis

To obtain energy from fat, triglycerides must first be broken down by hydrolysis into their two principal components, fatty acids and glycerol. This process, called lipolysis    , takes place in the cytoplasm. The resulting fatty acids are oxidized by β-oxidation into acetyl CoA, which is used by the Krebs cycle. The glycerol that is released from triglycerides after lipolysis directly enters the glycolysis pathway as DHAP. Because one triglyceride molecule yields three fatty acid molecules with as much as 16 or more carbons in each one, fat molecules yield more energy than carbohydrates and are an important source of energy for the human body. Triglycerides yield more than twice the energy per unit mass when compared to carbohydrates and proteins. Therefore, when glucose levels are low, triglycerides can be converted into acetyl CoA molecules and used to generate ATP through aerobic respiration.

Questions & Answers

Why is important to check if the blood if compatible before transfusion ?
Kelvin Reply
to avoid cases of intravenous clupping in blood which can be fatal....bcos blood is incopartable
John
Thank you.good answer
Kelvin
what are the clinical significant of thyroid gland
Jennifer Reply
what is a metabolism?
Kheth Reply
Metabolism is the sum total of all the chemical processes in the body. It is divided into an anabolic(building up) and catabolic(breaking down) metabolism)
Jonas
Oh ok
King
sir what is constructive and destructive metabolism
Soul
is anyone
Soul
Describe the system s that maintain the internal environment of a human body
Nora Reply
an organism is a living being that had a cellular structure and that can independently perform all physiological functions needed for life.
Nwecho Reply
a tissue is a group of similar cells that work together to perform a particular function
Nwecho
chemical level, cellular level, tissue level, organs level, organ systems, organisms
Nwecho Reply
they're reproduction but also produces hormones dus they're endocrine system
Nwecho
cells are smallest independent functional unit of a living organism
Nwecho
an organ is an anatomically distinct structure of the body composed of two or more tissue
Nwecho
an organ system is a group of organs that work together to perform a major function.
Nwecho
Structural Organization of the human body.
Tammy
What is a cavity
Isaac Reply
Permanently damaged areas in teeth that develop into tiny holes
MASLAH
what is meant by epithelial tissue
Zahid
What is the difference between regional anatomy and systematic anatomy?
Andrew Reply
Regional anatomy is the study of the interrelationships of all of the structures in a specific body region, such as the abdomen. ... In contrast, systemic anatomy is the study of the structures that make up a discrete body system—that is, a group of structures that work together to perform a unique
Tammy
Pls is that all
Petra
regional anatomy studies structures that contribute to specific body region example the thoracic region while systematic anatomy studies structures that contribute to specific body systems example respiratory system
Nwecho
so meaning of dissect
Mary Reply
it is the dismembering or the cutting of living organism to study the anatomical structure of it body
Kwasi
what are lymph nodes
Memory Reply
what is the best book that shows unit 1 about cell unit 2 about tissue unit 3 about embroyology
Abraham Reply
2
Hellen
to understand structure of body able to understand function of system. to understand how to build human body and function
Dereje Reply
hi
Neela
to determine body structures
Petra Reply
to know the use of each and every part of our body
Petra
to understand how our body structures work to support our lives
Petra
Is this a question?
Tammy
So what was the question?
Thamie
All the body work together to make the whole organism life possible
Kwasi
what is the importance of conversation
Hellen
To be able to identify the parts of the body and their functions
Hodasi
ok
Hodasi
Ideas are shared during conversation also informations are given
Hodasi
it reduces cost
Kwasi
what is carbon dioxide respiratory system why?
Prachi Reply
Carbon dioxide (CO2) is involved with the chemical process of cellular respiration. It is produced as a cellular waste product during the decomposition reactions and serves an important role in maintaining the appropriate acid-base balance in the body.
Tammy
thank you
Prachi
You're welcome!
Tammy
i have not under stood human heart
Simbwa Reply
means?
Dilshan
why a man became wild at the time of sexual intercourse. reason?
Soul

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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