Introduction to Lipids

Study Notes

Lipids are hydrophobic or amphiphilic molecules, mainly soluble in nonpolar solvents but insoluble in water.
Essential functions include energy storage, cell membrane structure, and cell signaling.

Triglycerides (Triacylglycerols):
- Structure: Glycerol backbone + three fatty acids.
- Function: Primary energy storage form in animals and plants; stored as adipose tissue.
Phospholipids:
- Structure: Glycerol backbone + two fatty acids + phosphate group with polar head.
- Function: Key components of cell membranes, forming lipid bilayers; contributes to membrane fluidity.
Steroids:
- Structure: Four fused carbon rings.
- Function: Includes cholesterol, which is a precursor for steroid hormones, bile acids, and vitamin D; stabilizes cell membranes.
Glycolipids:
- Structure: Glycerol backbone + one or more fatty acids + carbohydrate group.
- Function: Present in cell membranes, aiding in cell recognition and communication.
Waxes:
- Structure: Long-chain fatty acids linked to long-chain alcohols.
- Function: Provide protective barriers in plants and animals.
Sphingolipids:
- Structure: Based on sphingosine with one fatty acid and various polar head groups.
- Function: Involved in signaling and recognition in nerve and immune cells.

Store kinetic energy as triglycerides.
Provide structural integrity to cell membranes via phospholipids and cholesterol.
Act as signaling molecules, regulating physiological processes.
Offer insulation and protection through adipose tissue and other lipid forms.

Comprised of triglycerides, phospholipids, and cholesterol, crucial for energy, membrane structure, and hormone synthesis.

Digestion:
- Initiated by lingual lipase in the mouth; gastric lipase in the stomach; and primarily by pancreatic lipase in the small intestine, aided by bile acids.
Absorption:
- Formation of micelles allows absorption in the intestinal mucosa; triglycerides are reassembled in enterocytes and then packaged into chylomicrons for transport into the bloodstream.

Chylomicrons:
- Transport dietary lipids from intestines to tissues; hydrolyzed by lipoprotein lipase (LPL) to release fatty acids.
Very Low-Density Lipoprotein (VLDL):
- Transports endogenous triglycerides from liver to tissues; also hydrolyzed by LPL.
Intermediate-Density Lipoprotein (IDL):
- Formed from VLDL post triglyceride removal.

Lipid metabolism is interconnected with carbohydrate and protein metabolism, influencing energy balance.

Fatty Acid Synthesis:
- Occurs in liver/adipose tissue; initiated from acetyl-CoA via acetyl-CoA carboxylase and the fatty acid synthase complex.
Fatty Acid Oxidation:
- Occurs in mitochondria; fatty acids are converted to acetyl-CoA through β-oxidation.

Synthesis:
- Glycerol-3-phosphate forms from glucose; fatty acids are added, catalyzed by diacylglycerol acyltransferase.
Breakdown:
- Hydrolysis by hormone-sensitive lipase releases fatty acids into the bloodstream for energy.

Production:
- Occurs in liver mitochondria during fasting or low carb; involves ketogenesis from fatty acids.
Utilization:
- Ketone bodies are converted back to acetyl-CoA in peripheral tissues for energy.

Insulin:
- Promotes fatty acid synthesis and triacylglycerol production; inhibits lipolysis.
Glucagon and Epinephrine:
- Stimulate lipolysis and ketogenesis during fasting.
AMPK Activation:
- Inhibits fatty acid synthesis and promotes β-oxidation during energy stress.

Metabolic Disorders: Conditions like type 2 diabetes and obesity can disrupt lipid metabolism.
Ketosis: Can occur during fasting; excessive ketosis risks ketoacidosis.
Fatty Liver Disease: Accumulation of lipids in the liver due to insulin resistance or high alcohol intake leading to non-alcoholic fatty liver disease (NAFLD).

Phospholipid Synthesis:
- Includes phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine, crucial for cell membranes and signaling.
Degradation of Phospholipids:
- Hydrolyzed by phospholipases (e.g., PLA2, PLC, PLD), participating in various signaling pathways.
Glycosphingolipid Synthesis:
- Involves sphingosine backbone with added sugar residues for cell recognition and membrane integrity.