Lipid metabolism
59 flashcards covering Lipid metabolism for the MCAT Biology & Biochemistry section.
Lipid metabolism refers to the processes by which the body handles fats and related molecules, such as triglycerides, phospholipids, and cholesterol. It involves breaking down lipids for energy through reactions like beta-oxidation, synthesizing new lipids for cell membranes and hormones, and storing excess energy as fat in adipose tissue. This topic is essential because lipids provide a major source of fuel, help regulate cell function, and play roles in insulation and signaling, making it a cornerstone of understanding human physiology and biochemistry.
On the MCAT, lipid metabolism appears in biology and biochemistry questions that test your knowledge of pathways, enzymes, and regulation, often integrated with other systems like carbohydrate metabolism. Expect multiple-choice questions on topics such as fatty acid breakdown, ketogenesis, or lipoprotein transport, with common traps including confusing anabolic and catabolic processes or overlooking the role of hormones like insulin. Focus on energy yields, key intermediates, and how defects lead to disorders like diabetes. Practice drawing out pathways to solidify your understanding.
Terms (59)
- 01
Triglyceride
A lipid molecule composed of three fatty acids attached to a glycerol backbone, serving as the main form of fat storage in the body and a major energy source.
- 02
Fatty acid
A long-chain hydrocarbon with a carboxylic acid group, which can be saturated or unsaturated and is a key building block for lipids like triglycerides and phospholipids.
- 03
Saturated fat
A type of fatty acid with no double bonds between carbon atoms, typically solid at room temperature and found in animal products, potentially raising cholesterol levels.
- 04
Unsaturated fat
A fatty acid with one or more double bonds in its chain, usually liquid at room temperature and found in plant oils, which can lower the risk of heart disease.
- 05
Phospholipid
A lipid with a hydrophilic head and hydrophobic tails, forming the bilayer of cell membranes and aiding in compartmentalization within cells.
- 06
Sterol
A type of lipid with a four-ring structure, including cholesterol, which is essential for membrane fluidity and as a precursor for hormones like steroids.
- 07
Cholesterol
A sterol molecule vital for cell membrane integrity, bile acid production, and hormone synthesis, but high levels can lead to plaque buildup in arteries.
- 08
Bile salt
A derivative of cholesterol produced in the liver and stored in the gallbladder, emulsifying fats in the intestine to facilitate their digestion and absorption.
- 09
Lipase
An enzyme that catalyzes the hydrolysis of triglycerides into fatty acids and glycerol, primarily in the pancreas and small intestine for fat breakdown.
- 10
Emulsification
The process by which bile salts break down large fat globules into smaller droplets in the small intestine, increasing the surface area for lipase action.
- 11
Micelle
A spherical structure formed by bile salts that solubilizes dietary fats and fat-soluble vitamins, allowing them to be absorbed in the small intestine.
- 12
Chylomicron
A lipoprotein assembled in intestinal cells to transport dietary triglycerides and cholesterol from the gut through the lymphatic system into the blood.
- 13
Very Low-Density Lipoprotein (VLDL)
A lipoprotein produced by the liver that carries triglycerides to tissues for energy or storage, becoming LDL after delivering its cargo.
- 14
Low-Density Lipoprotein (LDL)
A lipoprotein that delivers cholesterol to cells, and high levels can lead to atherosclerosis due to plaque formation in arteries.
- 15
High-Density Lipoprotein (HDL)
A lipoprotein that removes excess cholesterol from tissues and transports it to the liver for excretion, often called 'good cholesterol'.
- 16
Apolipoprotein
A protein component of lipoproteins that helps in their structure, recognition, and interaction with cell receptors for lipid transport.
- 17
Beta-oxidation
The mitochondrial process that breaks down fatty acids into acetyl-CoA units, producing energy through a series of four reactions repeated for each two-carbon segment.
- 18
Carnitine shuttle
A transport mechanism that moves long-chain fatty acids from the cytosol into the mitochondria for beta-oxidation by converting them to acyl-carnitine.
- 19
Acetyl-CoA
A two-carbon molecule produced from fatty acid breakdown or other pathways, entering the citric acid cycle to generate ATP or used in ketogenesis.
- 20
Ketogenesis
The liver process that converts acetyl-CoA from fatty acids into ketone bodies during fasting or low carbohydrate states to provide an alternative energy source.
- 21
Ketone bodies
Water-soluble molecules like acetoacetate and beta-hydroxybutyrate produced in the liver from acetyl-CoA, serving as fuel for tissues when glucose is scarce.
- 22
Lipogenesis
The biosynthetic pathway that converts excess carbohydrates or proteins into fatty acids and triglycerides in the liver and adipose tissue for storage.
- 23
Fatty acid synthase
A multi-enzyme complex that catalyzes the synthesis of fatty acids from acetyl-CoA and malonyl-CoA, requiring NADPH for reduction steps.
- 24
HMG-CoA reductase
The rate-limiting enzyme in cholesterol biosynthesis that converts HMG-CoA to mevalonate, inhibited by statins to lower cholesterol levels.
- 25
Mevalonate pathway
The metabolic route starting from acetyl-CoA that produces isoprenoids, including cholesterol, through intermediates like mevalonate and squalene.
- 26
Squalene
A 30-carbon hydrocarbon intermediate in cholesterol synthesis, formed from farnesyl pyrophosphate and cyclized to produce sterols.
- 27
Lipolysis
The breakdown of triglycerides in adipose tissue into free fatty acids and glycerol, triggered by hormones like epinephrine during fasting.
- 28
Hormone-sensitive lipase
An enzyme in adipose cells activated by hormones such as glucagon and epinephrine to hydrolyze triglycerides into fatty acids for energy use.
- 29
Adipose tissue
Specialized connective tissue that stores energy as triglycerides and releases fatty acids when needed, also producing hormones like leptin.
- 30
Glycerol
A three-carbon alcohol that forms the backbone of triglycerides and, after release from lipolysis, is converted to glucose via gluconeogenesis in the liver.
- 31
Acyl-CoA
An activated form of fatty acids that links to coenzyme A, serving as the substrate for beta-oxidation and other metabolic reactions.
- 32
Peroxisomes in fatty acid oxidation
Organelles that perform beta-oxidation on very long-chain fatty acids, producing hydrogen peroxide and shortening chains before mitochondrial processing.
- 33
Essential fatty acids
Fatty acids like linoleic and alpha-linolenic acid that the body cannot synthesize and must obtain from diet for functions like membrane formation.
- 34
Eicosanoids
Signaling molecules derived from arachidonic acid, including prostaglandins and leukotrienes, that regulate inflammation, blood flow, and immune responses.
- 35
Prostaglandins
Eicosanoids produced from arachidonic acid via cyclooxygenase, acting locally to mediate pain, fever, and smooth muscle contraction.
- 36
Leukotrienes
Eicosanoids synthesized from arachidonic acid through lipoxygenase, involved in allergic reactions and asthma by promoting inflammation.
- 37
Thromboxanes
Eicosanoids derived from arachidonic acid that promote platelet aggregation and vasoconstriction, playing a role in blood clotting.
- 38
Omega-3 fatty acids
Polyunsaturated fats with the first double bond at the third carbon, found in fish oils, that reduce inflammation and support heart health.
- 39
Digestion of lipids
The process beginning in the stomach and completing in the small intestine, where lipases break down triglycerides into monoglycerides and fatty acids.
- 40
Absorption of lipids
The uptake of digested fats into intestinal cells, packaged into chylomicrons, and transported via the lymphatic system to avoid the liver initially.
- 41
Lipoprotein lipase
An enzyme on capillary walls that hydrolyzes triglycerides in chylomicrons and VLDL, releasing fatty acids for uptake by tissues.
- 42
Reverse cholesterol transport
The process by which HDL collects cholesterol from peripheral tissues and delivers it to the liver for excretion or bile acid synthesis.
- 43
Statins
A class of drugs that inhibit HMG-CoA reductase to reduce cholesterol production in the liver, lowering LDL levels and preventing cardiovascular disease.
- 44
Atherosclerosis
The buildup of plaque from LDL cholesterol in artery walls, leading to narrowed blood vessels and increased risk of heart attacks.
- 45
Familial hypercholesterolemia
A genetic disorder with defective LDL receptors, causing high LDL cholesterol levels and early onset of cardiovascular disease.
- 46
Ketosis
A metabolic state where the body produces and uses ketone bodies for energy, occurring during prolonged fasting or low-carb diets.
- 47
ATP yield from fatty acid oxidation
The total ATP produced from beta-oxidation of a fatty acid, calculated by the number of acetyl-CoA units and NADH/FADH2 generated, minus activation costs.
- 48
Equation for beta-oxidation
The overall reaction for each cycle of beta-oxidation removes two carbons from a fatty acid, producing one acetyl-CoA, one NADH, and one FADH2.
- 49
Common trap: Confusing LDL and HDL
Students often mistake LDL as beneficial and HDL as harmful, but LDL delivers cholesterol to cells while HDL removes it, so high HDL is protective.
- 50
Strategy for remembering lipid transport
Associate lipoproteins by density: VLDL is triglyceride-rich and lightest, LDL is cholesterol-rich, and HDL is densest and removes cholesterol.
- 51
Example: Calculating ATP from palmitate
Palmitate, a 16-carbon fatty acid, yields 129 ATP through beta-oxidation: 8 acetyl-CoA (each 12 ATP), 7 FADH2 (each 2 ATP), and 7 NADH (each 3 ATP), minus 2 ATP for activation.
- 52
Sphingolipid
A lipid with a sphingosine backbone, found in cell membranes and involved in cell recognition and signaling, such as in myelin sheaths.
- 53
Glycolipid
A lipid with carbohydrate attachments, present in cell membranes to aid in cell-cell recognition and as antigens in blood types.
- 54
Isoprene units
Five-carbon building blocks used in the synthesis of terpenes and steroids, derived from the mevalonate pathway in cholesterol production.
- 55
Bile acid synthesis
The conversion of cholesterol in the liver to bile acids like cholic acid, which are then conjugated and secreted to aid in fat digestion.
- 56
Enterohepatic circulation
The recycling process where bile acids are secreted into the intestine, reabsorbed, and returned to the liver via the portal vein.
- 57
Phosphatidic acid pathway
A key route for synthesizing phospholipids and triglycerides, starting from glycerol-3-phosphate and fatty acyl-CoAs.
- 58
Brown adipose tissue
A type of fat tissue rich in mitochondria that generates heat through uncoupling proteins, important for thermoregulation in newborns and some adults.
- 59
Uncoupling proteins
Proteins in mitochondrial membranes that dissipate the proton gradient, producing heat instead of ATP, as seen in brown fat during thermogenesis.