Lipids are fats that are either absorbed from food or synthesized by the liver. Triglycerides (TGs) and cholesterol contribute most to disease, although all lipids are physiologically important. Cholesterol is a ubiquitous constituent of cell membranes, steroids, bile acids, and signaling molecules. Triglycerides primarily store energy in adipocytes and muscle cells. Lipoproteins are hydrophilic, spherical structures that possess surface proteins (apoproteins, or apolipoproteins) that are cofactors and ligands for lipid-processing enzymes (see table Major Apoproteins and Enzymes Important to Lipid Metabolism). All lipids are hydrophobic and mostly insoluble in blood, so they require transport within lipoproteins. Lipoproteins are classified by size and density (defined as the ratio of lipid to protein) and are important because high levels of low-density lipoproteins (LDL) and low levels of high-density lipoproteins (HDL) are major risk factors for atherosclerotic heart disease. Dyslipidemia is elevation of plasma cholesterol and/or triglycerides, or a low HDL cholesterol level that contributes to the development of atherosclerosis.

1. Lipid Metabolism

2. Lipid Digestion and absorption:
Lipids present in the diet include:
Triacylglycerol ,
Cholesterol,
Phospholipids,
Essential fatty acids and
Lipid soluble vitamins.

3. The insolubility in water is a
problem in digestion and
absorption.
This could be solved by bile
secretion

5. Fatty acid synthesis

6. Fatty acids are synthesized in the
cytoplasm of cells in many tissues
including:
liver , kidney, brain , lung , mammary
gland and adipose tissue.
Fatty acids synthesis

7. Fatty acids are synthesized from
Acetyl-CoA obtained from
carbohydrates to produce palmitic
fatty acid (16 carbon) .
Then all other fatty acids are made
by modification of palmtic acid.

8. Insulin stimulates fatty acid synthesis
by:
Induction of genes of the enzymes
responsible for fatty acid synthesis.

9. Fatty acid oxidation

10. Fatty acids act as source of energy in
liver , skeletal muscles and heart
during periods of fasting
Fatty acid oxidation

11. Palmitic acid needs seven cycles of β
oxidation which gives rise to 8 acetyl CoA
, NADH and FADH2
Every acetyl-CoA when oxidized
in TCA cycle gives ATP

12. NADH and FADH2 enter TCA cycle to give
ATP when oxidized in electron transport chain
This oxidation occurs in mitochondria and
gives a large number of ATP .
The net energy produced from palmitic acid
oxidation is 129 ATP.

14. Ketogenesis Ketolysis
Acetoacetic Acid
Β- Hydroxybutyric
acid
Acetone
in the liver mitochondria
from acetyl coA
Ketone bodies synthesized
in the liver are transported
to other tissues ( skeletal
muscles , heart , kidney
and
brain ) .
Ketone bodies are oxidized
in TCA cycle in the
mitochondria of these
tissues to produce energy.
Metabolism of Ketone Bodies

15. Normally their serum concentration is less
than 0.1 mg / dl .
Ketosis
Their level in blood increases resulting in
ketonemia and their excretion in urine Ketonuria
This condition is called Ketosis. This occurs in :
1- Uncontrolled diabetes mellitus
2- Starvation

16. Lipogenesis And Lipolysis

17. Lipogenesis :
esterification of three fatty acids with the
alcohol glycerol to form triacylglycerol ( tri
= 3 , acyl = fatty acid ) TAG.

18. Lipolysis :
hydrolysis of TAG into glycerol and three fatty
acids during fasting to release fatty acids for
energy production.
Insulin stimulates lipogenesis and inhibits
lipolysis.
Anti- insulin hormones:
stimulate lipolysis by activating hormone
sensitive lipase

19. During the well fed state lipogenesis is
activated
During fasting state lipolysis is activated

21. Plasma lipoproteins include :
1 – Chylomicrones
2- Very low density lipoproteins ( VLDL)
3- Intermediate density lipoproteins (IDL)
4-Low density lipoproteins (LDL)
5- High density lipoproteins (HDL)
Lipid transport & Lipoproteins

22. Classification of lipoproteins

23. low density lipoproteins
(LDL): carries
cholesterol from liver
to tissues (Bad
Lipoprotein

High density lipoproteins
(HDL) : carries
cholesterol from tissues
to liver to be excreted
in bile (Good
Lipoprotein)

24. Bad versus Good Cholesterol

25. What is Fatty Liver?
Accumulation of fat (TAG) in the liver.
It is not a disease ,as it does not damage
the liver,
but is a condition that can be identified by
taking a sample of liver tissue (liver
biopsy) and examining it under a
microscope .

26. What causes Fatty Liver?
Fat accumulates in the liver usually in the following
conditions
1 -Heavy use of alcohol
2 -Obesity and high carbohydrate diet
3 -Diabetes mellitus
4-Drugs such as corticosteroids