Đề 12 – Bài tập, đề thi trắc nghiệm online Hóa sinh lipid

Glucagon, along with epinephrine and cortisol, stimulates lipolysis. These hormones activate hormone-sensitive lipase in adipose tissue, leading to the release of fatty acids and glycerol into the bloodstream for energy use. Insulin, conversely, inhibits lipolysis and promotes fat storage.

HDL (High-Density Lipoprotein) is often referred to as `good cholesterol` because it participates in reverse cholesterol transport. HDL picks up excess cholesterol from peripheral tissues and returns it to the liver for excretion or recycling, reducing the risk of atherosclerosis.

After a high-fat meal, digestion and absorption of dietary fats are immediately stimulated. CCK is released to stimulate bile and pancreatic enzyme secretion. Pancreatic lipase activity increases to digest triglycerides. Chylomicrons, which transport dietary fats, will increase in the lymph. However, fatty acid synthesis (lipogenesis) is typically suppressed after a high-fat meal, as the body is focused on processing and storing the ingested fat, not synthesizing more.

Ceramide is composed of a sphingosine molecule linked to a fatty acid via an amide bond. It is a central molecule in sphingolipid metabolism and serves as a precursor for more complex sphingolipids like sphingomyelin and cerebrosides, as well as playing a role in cell signaling.

The hydrophobic region of a phospholipid consists of the long, nonpolar hydrocarbon chains of the fatty acid tails. The phosphate group and glycerol backbone form the hydrophilic head.

Sphingolipids are built upon a sphingosine backbone, an amino alcohol, unlike glycerophospholipids which are based on a glycerol backbone. Ceramide is a key intermediate in sphingolipid synthesis and consists of sphingosine linked to a fatty acid.

Triglycerides (triacylglycerols) are the primary storage form of lipids in animals. They are highly energy-dense and are stored in adipose tissue, providing a long-term energy reserve.

LDL (Low-Density Lipoprotein) is the main carrier of cholesterol in the blood and delivers it to cells. Elevated LDL levels are associated with increased risk of atherosclerosis, hence the term `bad cholesterol`.

Lipid rafts are specialized membrane microdomains enriched in cholesterol and sphingolipids. These ordered regions are involved in organizing membrane proteins and lipids, playing roles in cell signaling, membrane trafficking, and protein sorting.

Trans fatty acids, often formed during industrial hydrogenation of vegetable oils, have been shown to negatively impact cholesterol levels. They raise LDL (`bad`) cholesterol and lower HDL (`good`) cholesterol, significantly increasing the risk of cardiovascular diseases.

Saturated fatty acids lack carbon-carbon double bonds, allowing them to pack tightly together, resulting in a solid state at room temperature. Unsaturated fatty acids contain one or more double bonds, causing kinks in the chain and making them liquid at room temperature.

Beta-oxidation of fatty acids takes place in the mitochondria. This process sequentially removes two-carbon units from fatty acids, generating acetyl-CoA which enters the citric acid cycle for further energy production.

PPARs are a family of nuclear receptors that function as transcription factors. They regulate the expression of genes involved in various aspects of lipid metabolism, including fatty acid oxidation, lipid storage, and lipoprotein metabolism.

Bile salts, synthesized in the liver and stored in the gallbladder, act as detergents to emulsify dietary fats in the small intestine. This process breaks down large fat globules into smaller droplets, increasing the surface area available for pancreatic lipase to digest triglycerides.

Carnitine acts as a carrier molecule to transport long-chain fatty acids across the inner mitochondrial membrane, enabling them to undergo beta-oxidation within the mitochondria. This transport system is essential for fatty acid breakdown and energy production.

White adipose tissue primarily functions in energy storage as triglycerides. Brown adipose tissue, on the other hand, is specialized for heat generation (thermogenesis). It contains a protein called uncoupling protein 1 (UCP1) in mitochondria, which uncouples oxidative phosphorylation, dissipating energy as heat rather than ATP.

Waxes are esters formed between long-chain fatty acids and long-chain alcohols. They are highly hydrophobic and serve protective functions in organisms, such as waterproofing in plants and animals.

While lipids play roles in energy storage, membrane structure, and signaling, catalysis of biochemical reactions is primarily the function of proteins (enzymes), not lipids.

Vitamins A, D, E, and K are fat-soluble vitamins, meaning they are absorbed along with dietary fats and can be stored in the body`s fat tissues. Vitamin C is a water-soluble vitamin, and excess amounts are typically excreted in urine.

Cholesterol is the precursor for all steroid hormones, including cortisol, aldosterone, testosterone, estrogen, and progesterone. These hormones are synthesized from cholesterol through a series of enzymatic reactions.

Linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid) are essential fatty acids for humans. Humans lack the enzymes to introduce double bonds beyond the delta-9 carbon in fatty acids, so these must be obtained from the diet.

Acetyl-CoA is the key building block for fatty acid synthesis. It is carboxylated to malonyl-CoA, which then participates in chain elongation steps to build fatty acids.

Phospholipases are enzymes that hydrolyze phospholipids. They play crucial roles in lipid metabolism by degrading phospholipids and also in cell signaling by releasing signaling molecules like arachidonic acid (precursor for eicosanoids) and diacylglycerol (DAG) from phospholipids.

Pancreatic lipase, secreted by the pancreas into the small intestine, is the major enzyme responsible for hydrolyzing triglycerides into monoglycerides and free fatty acids, facilitating their absorption.

Arachidonic acid, a polyunsaturated omega-6 fatty acid, is the main precursor for many eicosanoids, including prostaglandins, thromboxanes, and leukotrienes. These signaling molecules are involved in inflammation, pain, and blood clotting.

Acetyl-CoA carboxylase (ACC) catalyzes the carboxylation of acetyl-CoA to malonyl-CoA, the first committed step in fatty acid synthesis. ACC is a highly regulated enzyme, controlling the rate of fatty acid biosynthesis.

Triglycerides, also known as triacylglycerols, are formed by the esterification of three fatty acids to a single glycerol molecule. This structure is ideal for energy storage.

The `omega` nomenclature refers to the position of the first double bond from the methyl (omega) end of the fatty acid chain. Omega-3 fatty acids have their first double bond at the 3rd carbon, and omega-6 fatty acids at the 6th carbon from the omega end. Both can be unsaturated and found in both plant and animal sources, and both include essential and non-essential members within their families.

Peroxisomes are organelles essential for the beta-oxidation of very-long-chain fatty acids (VLCFAs). In Zellweger syndrome, peroxisomes are dysfunctional or absent, leading to the accumulation of VLCFAs in tissues, causing severe neurological and other health problems. Mitochondrial beta-oxidation handles shorter chain fatty acids, but peroxisomes are critical for VLCFAs.

Cholesterol acts as a membrane buffer, modulating membrane fluidity. It prevents membranes from becoming too rigid at low temperatures and too fluid at high temperatures, maintaining optimal membrane function.