Introduction to Lipids

Study Notes

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

Triglycerides (Triacylglycerols):
- Composed of glycerol and three fatty acids; primary energy storage in animals and plants; stored in adipose tissue.
Phospholipids:
- Structure includes glycerol, two fatty acids, and a phosphate group; critical components of cell membranes, contributing to membrane fluidity and barrier function.
Steroids:
- Composed of four fused carbon rings; includes cholesterol, vital for steroid hormone production (e.g., estrogen, testosterone) and membrane stabilization.
Glycolipids:
- Consist of glycerol, fatty acids, and carbohydrates; involved in cell recognition and communication within membranes.
Waxes:
- Made of long-chain fatty acids and alcohols; serve as protective barriers in plants and animals (e.g., plant cuticles, earwax).
Sphingolipids:
- Built on a sphingosine backbone with one fatty acid; significant in signaling and cell recognition, particularly in nerve and immune cells.

Energy Storage: Primarily served by triglycerides in adipose tissue.
Structural Components: Phospholipids and cholesterol are fundamental to cell membrane architecture.
Signaling Molecules: Lipids such as steroid hormones regulate physiological functions.
Insulation and Protection: Lipids provide cushioning for organs and thermal insulation.

Include triglycerides, phospholipids, and cholesterol, crucial for energy, structure, and hormone synthesis.

Digestion:
- Begins in the mouth with lingual lipase; gastric lipase continues in the stomach; pancreatic lipase acts in the small intestine with bile salts to hydrolyze triglycerides into monoglycerides and free fatty acids.
Absorption:
- Digested lipids form micelles for absorption by intestinal mucosa; monoglycerides and fatty acids are reassembled into triglycerides and packaged into chylomicrons for transport.

Chylomicrons: Transport dietary lipids from intestines to tissues; hydrolyzed by lipoprotein lipase (LPL) for energy or storage.
VLDL: Transports endogenous triglycerides from the liver to tissues; also processed by LPL.
IDL: Intermediate form of VLDL; converts to LDL or taken up by the liver.
LDL: Carries cholesterol to tissues, often termed "bad cholesterol" due to atherosclerosis risk.
HDL: Collects excess cholesterol from tissues, transporting it back to the liver; known as "good cholesterol" for reducing cardiovascular risk.

Synthesis:
- Occurs mainly in liver and adipose tissues; initiated by acetyl-CoA through the acetyl-CoA carboxylase (ACC) enzyme.
β-Oxidation:
- Takes place in mitochondria; fatty acids are activated to fatty acyl-CoA and broken down into acetyl-CoA, NADH, and FADH2.

Synthesis: Involves glycerol-3-phosphate from glucose or glycerol; fatty acids are esterified to form triacylglycerols, catalyzed by diacylglycerol acyltransferase (DGAT).

Location: Primarily in adrenal glands and gonads.
Process: Cholesterol converts to pregnenolone and then to various steroid hormones, including glucocorticoids (e.g., cortisol) and sex steroids (e.g., estrogen).

Dyslipidemia: Elevated LDL, low HDL levels linked to increased cardiovascular disease risk.
Obesity: Excess triglyceride accumulation in adipose tissue, a key factor in metabolic syndrome and type 2 diabetes.
Fatty Liver Disease: Lipid accumulation in liver can lead to NAFLD or alcoholic liver disease.

Lipid metabolism interacts with carbohydrate and protein metabolism, maintaining energy balance during fasting and feeding.
Hormonal influences include insulin for lipogenesis and glucagon/epinephrine for lipolysis.

Hypercholesterolemia: Elevated cholesterol levels increasing cardiovascular risk; managed through lifestyle and statin medications.
Familial Hypercholesterolemia: Genetic disorder causing high LDL, early atherosclerosis development.

Adrenal Insufficiency: Low adrenal steroid production, leading to fatigue and weight loss.
Cushing’s Syndrome: Excess cortisol; symptoms include obesity, hypertension, diabetes.