Đề 10 – Bài tập, đề thi trắc nghiệm online Hóa sinh lipid

Amphipathic molecules have both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions. Phospholipids have a polar, hydrophilic head (phosphate group) and nonpolar, hydrophobic tails (fatty acids), allowing them to form bilayers in water.

Bile salts, synthesized in the liver and released into the small intestine, act as detergents to emulsify dietary fats. Emulsification breaks down large fat globules into smaller droplets, increasing the surface area for pancreatic lipase to act. Bile salts do not hydrolyze triacylglycerols, transport fatty acids into cells, or synthesize cholesterol.

Malonyl-CoA, the product of the committed step in fatty acid synthesis, inhibits carnitine acyltransferase I (CAT-1), the enzyme that transports fatty acids into the mitochondria for beta-oxidation. This reciprocal regulation ensures that fatty acid synthesis and breakdown are not simultaneously active, preventing futile cycles. High malonyl-CoA, indicating active fatty acid synthesis, will inhibit beta-oxidation.

Unsaturated fatty acids contain double bonds that introduce kinks in the fatty acid chains. These kinks prevent tight packing of phospholipids in the membrane, increasing the space between molecules and thus increasing membrane fluidity. Saturated fatty acids, lacking double bonds, allow for tighter packing and decrease fluidity.

White adipose tissue is primarily for long-term energy storage as triacylglycerols, insulation, and hormone secretion. Brown adipose tissue`s primary function is heat generation (thermogenesis). It contains a protein called uncoupling protein 1 (UCP1), which uncouples mitochondrial respiration, producing heat instead of ATP.

Cortisol, a steroid hormone derived from cholesterol, plays a significant role in regulating blood glucose levels by promoting gluconeogenesis and glycogenolysis in the liver. While insulin and glucagon are also crucial in glucose regulation, they are peptide hormones. Prostaglandins are eicosanoids involved in inflammation and pain, not primary regulators of blood glucose.

Phospholipase C (PLC) is a key enzyme in cell signaling pathways. It hydrolyzes phosphatidylinositol bisphosphate (PIP2), a membrane phospholipid, to generate two important second messengers: inositol trisphosphate (IP3) and diacylglycerol (DAG). These second messengers activate downstream signaling cascades. PLC is not involved in triacylglycerol hydrolysis, phospholipid synthesis, or transport between organelles.

LDL (Low-Density Lipoprotein) is often referred to as `bad cholesterol` because it carries cholesterol from the liver to tissues. High levels of LDL can lead to cholesterol deposition in arteries, forming plaques. HDL (`good cholesterol`) removes cholesterol from tissues and carries it back to the liver.

Acyl Carrier Protein (ACP) is a crucial component of the fatty acid synthase complex. It acts as a molecular `tether` to which the growing fatty acid chain is attached during the synthesis process, carrying the intermediates between different enzymatic sites within the complex. It`s not involved in mitochondrial transport, beta-oxidation activation, or ACC regulation.

Triacylglycerols (fats and oils), phospholipids, and steroids (like cholesterol) are major classes of lipids. Proteins are another major class of biomolecules, distinct from lipids, carbohydrates, and nucleic acids.

Ketogenesis is increased during fasting, starvation, or in uncontrolled diabetes when glucose availability is low. In these conditions, the body shifts to using fats as an energy source. Increased fatty acid oxidation leads to excess acetyl-CoA, which is then converted to ketone bodies in the liver. Ketogenesis is suppressed in a well-fed state or with a high carbohydrate diet when glucose is readily available.

Phospholipids, cholesterol, and sphingolipids are major components of animal cell membranes, contributing to membrane structure and fluidity. Triacylglycerols are primarily storage lipids found in adipose tissue and are not a significant component of cell membranes.

Chylomicrons are lipoprotein particles that transport dietary lipids (triacylglycerols, cholesterol, etc.) from the intestine to the bloodstream and eventually to tissues, including the liver. They are not involved in transporting lipids from the liver to the intestine. The other statements are correct regarding lipid digestion and absorption.

Cholesterol is not a primary energy source or found in plant cell walls or RNA. It is a precursor for steroid hormones like testosterone and estrogen and modulates membrane fluidity in animal cells.

Carnitine is essential for transporting long-chain fatty acids across the inner mitochondrial membrane for beta-oxidation. Carnitine deficiency leads to impaired fatty acid transport, reduced energy production from fats, and potentially accumulation of fatty acids in the cytoplasm. It does not increase fatty acid synthesis or enhance beta-oxidation. Ketone body accumulation is usually associated with impaired glucose metabolism, not carnitine deficiency directly.

HMG-CoA reductase is the enzyme that catalyzes the rate-limiting step in cholesterol synthesis. Statin drugs are competitive inhibitors of HMG-CoA reductase, thus reducing cholesterol synthesis in the liver. This is the primary mechanism by which statins lower blood cholesterol levels. It does not directly increase fatty acid or triacylglycerol synthesis, nor does it significantly affect ketone body synthesis.

Trans fatty acids are unsaturated fatty acids with trans double bonds, often created during food processing (partial hydrogenation). They have been shown to have detrimental effects on lipid profiles, raising LDL (`bad`) cholesterol and lowering HDL (`good`) cholesterol levels, thereby increasing the risk of cardiovascular diseases. They are not metabolized like saturated fats without health consequences, nor are they beneficial or rapidly eliminated without accumulation.

Insulin, released in response to high blood glucose, promotes energy storage. It stimulates fatty acid synthesis (lipogenesis) and storage as triacylglycerols, primarily in adipose tissue and liver. Insulin inhibits fatty acid breakdown (beta-oxidation) and promotes glucose utilization as the primary energy source.

Lipid rafts are membrane microdomains enriched in cholesterol and sphingolipids, which contribute to their increased rigidity and distinct properties compared to the surrounding membrane. They are not depleted in cholesterol, nor are they primarily composed of glycerophospholipids or enriched in polyunsaturated fatty acids.

Beta-oxidation, the process of breaking down fatty acids to generate acetyl-CoA for the citric acid cycle and ATP production, takes place primarily in the mitochondria in eukaryotic cells.

Hormone-sensitive lipase is activated by hormones like epinephrine and glucagon to hydrolyze stored triacylglycerols in adipose tissue, making fatty acids available for energy use. Lipoprotein lipase acts on lipoproteins in the bloodstream, pancreatic lipase digests dietary fats in the intestine, and phospholipase A2 hydrolyzes phospholipids.

Acetyl-CoA carboxylase (ACC) catalyzes the carboxylation of acetyl-CoA to malonyl-CoA, which is the committed and rate-limiting step in fatty acid synthesis. Fatty acid synthase is a multi-enzyme complex that carries out the subsequent steps of fatty acid chain elongation. Thiolase and enoyl-CoA hydratase are enzymes involved in fatty acid beta-oxidation.

Linoleic acid (omega-6) and alpha-linolenic acid (omega-3) are essential fatty acids for humans. Palmitic acid, oleic acid, and stearic acid are non-essential fatty acids that can be synthesized by the body.

Sphingolipids are characterized by having a sphingosine or dihydrosphingosine backbone, whereas glycerophospholipids are based on a glycerol backbone. Both contain fatty acid chains and can have phosphate groups, but the backbone structure is the key difference.

The `omega` designation refers to the position of the first double bond from the methyl (omega) end of the fatty acid chain. Omega-3 fatty acids have the first double bond at the 3rd carbon, while omega-6 fatty acids have it at the 6th carbon from the omega end. Both can be unsaturated and essential, and the number of double bonds can vary within each group.

Lipid storage diseases, a type of lysosomal storage disorder, are caused by genetic deficiencies in specific lysosomal enzymes required for the breakdown of complex lipids. This leads to the accumulation of undigested lipids within lysosomes, causing cellular dysfunction and various clinical symptoms. They are characterized by lipid accumulation, not increased breakdown, enhanced transport, or decreased synthesis.

Lipids serve as energy storage, membrane components, and signaling molecules. However, catalysis of biochemical reactions is primarily the function of proteins (enzymes), not lipids.

Saturated fatty acids lack double bonds, allowing for tighter packing of molecules, resulting in a solid state at room temperature. Unsaturated fatty acids contain double bonds, disrupting packing and making them liquid at room temperature.

Waxes are esters formed between long-chain fatty acids and long-chain alcohols. Triacylglycerols are esters of fatty acids with glycerol. This difference in alcohol component is the key structural distinction. Both are hydrophobic, but triacylglycerols are primarily for energy storage, and waxes often have protective functions.

Eicosanoids are potent local hormones involved in inflammation, pain, fever, and blood clotting. They are not for energy storage, membrane structure, or dietary fat digestion.