Introduction to Lipids

Study Notes

Lipids are hydrophobic or amphiphilic molecules, soluble in nonpolar solvents and generally insoluble in water.
Key functions include energy storage, cell membrane structure, and signaling.
Major categories: triglycerides, phospholipids, steroids, glycolipids, waxes, and sphingolipids.

Composed of a glycerol backbone and three fatty acid chains.
Serve as the primary energy storage form in animals and plants, stored in adipose tissue.

Characterized by four fused carbon rings.
Cholesterol is the main steroid, serving as a precursor for steroid hormones (like estrogen and testosterone), bile acids, and vitamin D.

Based on a sphingosine backbone with one fatty acid and various polar head groups.
Play a role in signaling and cell recognition, especially in nerve and immune cells.

Energy Storage: Triglycerides are long-term energy reserves.
Structural Components: Phospholipids and cholesterol are vital for cell membrane integrity.
Signaling: Steroid hormones regulate physiological functions.
Insulation and Protection: Adipose tissue offers thermal insulation and cushioning.

Include triglycerides, phospholipids, and cholesterol essential for energy, cellular structures, and hormone synthesis.

Mouth: Lingual lipase begins triglyceride digestion.
Stomach: Gastric lipase assists in digestion.
Small Intestine: Bile acids emulsify fats; pancreatic lipase hydrolyzes triglycerides into monoglycerides and free fatty acids.
Absorption: Digested lipids form micelles for intestinal absorption; reassembled into triglycerides within enterocytes and packaged into chylomicrons.

Chylomicrons: Transport dietary lipids from the intestine; broken down by lipoprotein lipase (LPL) to release fatty acids for energy.
VLDL: Transports endogenous triglycerides from the liver; also hydrolyzed by LPL.
IDL: Intermediate formed from VLDL after triglyceride removal.

Lipid metabolism interacts with carbohydrate and protein metabolism, influencing energy homeostasis.
Regulated during fasting and feeding based on energy balance.

Fatty Acid Synthesis: Occurs in the cytoplasm using acetyl-CoA; initiated by acetyl-CoA carboxylase, extended by fatty acid synthase.
Fatty Acid Oxidation: Takes place in mitochondria via β-oxidation, converting fatty acids into acetyl-CoA.

Synthesis: Begins with glycerol-3-phosphate; fatty acids esterified to form triacylglycerols, catalyzed by DGAT.
Breakdown: Lipolysis in adipose tissue yields glycerol and free fatty acids for energy use.

Production: Occurs in liver mitochondria during fasting; involves conversion of fatty acids into ketone bodies.
Utilization: Ketone bodies transported to tissues, oxidized back to acetyl-CoA for energy in muscle and brain.

Insulin: Promotes fatty acid and triacylglycerol synthesis; inhibits lipolysis.
Glucagon and Epinephrine: Stimulate lipolysis and ketogenesis.
AMPK: Activated during energy stress, inhibits fatty acid synthesis while promoting β-oxidation.

Metabolic Disorders: Conditions like type 2 diabetes can disrupt lipid metabolism, affecting fatty acid and triglyceride levels.
Ketosis: Normal during fasting; excessive ketosis can cause ketoacidosis, especially in diabetics.
Fatty Liver Disease: Caused by lipid accumulation in the liver due to insulin resistance or high alcohol intake.

Cholesterol Synthesis: Occurs in the liver; starts with the conversion of acetyl-CoA to mevalonate, regulated by HMG-CoA reductase.
Cholesterol Transport: Dietary cholesterol is absorbed in the intestine into chylomicrons; LDL and HDL transport cholesterol to and from tissues.
Lipoprotein Classes: Chylomicrons transport dietary lipids; VLDL, IDL, and LDL transport endogenous lipids and cholesterol, with implications for cardiovascular health.