Concepts

ATP: Adenosine triphosphate, the main energy currency of the cell, generated from the metabolism of carbohydrates, fats, and proteins.

Hydrophobic: Lacking affinity for water; tending to repel and not absorb water. Lipids are hydrophobic.

Triglyceride: A glycerol-based lipid made when three fatty acids link with a glycerol backbone by ester bonds. It is the most predominant lipid form in nature and the body fat in animals.

Ester bond (in lipids): The chemical bond that links the fatty acids to the glycerol backbone in lipids like triglycerides.

Saturated fatty acid: A fatty acid having all single bonds between carbon atoms in its hydrocarbon chain.

Unsaturated fatty acid: A fatty acid having at least one double bond between carbon atoms in its hydrocarbon chain. Can be monounsaturated (one double bond) or polyunsaturated (more than one double bond).

Omega-3 fatty acid: An unsaturated fatty acid where the first double bond from the methyl (omega) end is located at the third carbon. Linolenic acid (ALA), EPA, and DHA are examples.

Omega-6 fatty acid: An unsaturated fatty acid where the first double bond from the methyl (omega) end is located at the sixth carbon. Linoleic acid and arachidonic acid are examples.

Chylomicron: A large lipoprotein that transports triglycerides absorbed from the small intestine to other tissues and organs.

Lipoprotein lipase (LPL): An enzyme secreted by blood capillaries that breaks down triglycerides in chylomicrons, releasing fatty acids to the tissues or organs for uptake.

Lipolysis: The breakdown of triglycerides (body fat) into glycerol and fatty acids. In the rumen, it is the hydrolysis of ester bonds in dietary lipids by microbial lipases, releasing fatty acids with free carboxyl ends.

Beta-oxidation: The breakdown of fatty acids into acetyl-CoA in the mitochondria to subsequently produce ATP. Occurs in the fasted state.

Fed state: The metabolic state characterized by high blood glucose concentration, typically right after a meal. Insulin is the dominant hormone in this state.

Fasted state: The metabolic state characterized by low blood glucose concentration, occurring during fasting when the diet doesn't provide enough glucose. Glucagon is the dominant hormone in this state.

Insulin: A hormone secreted by the pancreas in response to high blood glucose, promoting anabolic processes like glycogenesis, lipogenesis, and protein synthesis.

Glucagon: A hormone secreted by the pancreas in response to low blood glucose, promoting catabolic processes like glycogenolysis and gluconeogenesis.

Glycogenesis: The synthesis of glycogen from glucose, primarily in the liver and skeletal muscle, for future glucose storage. Stimulated by insulin.

Glycogenolysis: The breakdown of glycogen into glucose, releasing glucose into the bloodstream. Stimulated by glucagon.

Gluconeogenesis: The synthesis of new glucose from non-carbohydrate precursors such as propionate, amino acids, and glycerol, primarily in the liver. Important when dietary glucose is limited, like in ruminants and carnivores.

Acetyl-CoA: A central two-carbon molecule in metabolism, formed from the breakdown of carbohydrates, fats, and proteins. It enters the Krebs cycle for ATP production and serves as a building block for fatty acid synthesis.

Gross Energy: The total energy present in the diet, representing the energy in the carbon-hydrogen bonds of organic matter (carbohydrates, fat, and protein).

Digestible Energy: The gross energy minus the energy lost in feces.

Metabolizable Energy: The digestible energy minus the energy losses in urine and gases.

Net Energy: The metabolizable energy minus the heat produced by the animal. The energy available for maintenance, growth, activity, and reproduction.

Volatile Fatty Acids (VFAs): Short-chain fatty acids (acetate, propionate, butyrate) produced by the fermentation of carbohydrates by rumen microorganisms. They are a major energy source for ruminants.

Biohydrogenation (in the rumen): The process by which rumen microorganisms convert unsaturated fatty acids into more saturated fatty acids. It involves isomerization and hydrogenation.

Isomerization (in rumen biohydrogenation): The first step in biohydrogenation where some of the cis double bonds in unsaturated fatty acids are converted to trans double bonds.

Hydrogenation (in rumen biohydrogenation): The second step in biohydrogenation where hydrogen is added to the double bonds of fatty acids, leading to saturation.

Pre-gastric esterase (lingual lipase): A lipase secreted before the stomach (in the oral cavity of young ruminants) that breaks down ester bonds in lipids.

ATP citrate lyase: An enzyme that catalyzes the conversion of citrate to acetyl-CoA in the cytoplasm, important for de novo fatty acid synthesis from glucose in non-ruminants. Ruminants have limited expression of this enzyme.

Hexokinase: An enzyme that catalyzes the phosphorylation of glucose to glucose-6-phosphate, trapping glucose inside the cell and preventing it from returning to the blood.

Protein synthesis: The process of building proteins from amino acids, taking place in ribosomes. Requires amino acids and ATP.

Deamination: The removal of the amino group (NH2) from an amino acid. This is the first step in utilizing amino acids for gluconeogenesis or ATP production.

Urea cycle: A series of biochemical reactions in the liver that convert toxic ammonia (NH3), produced from amino acid deamination, into urea, which is then excreted in urine.

Anabolism: The branch of metabolism involving the synthesis of complex molecules from simpler ones, requiring energy input (e.g., protein synthesis, glycogenesis, lipogenesis). Insulin is a major anabolic hormone.

Catabolism: The branch of metabolism involving the breakdown of complex molecules into simpler ones, releasing energy (e.g., glycogenolysis, lipolysis, beta-oxidation). Glucagon is a major catabolic hormone.

Glycolysis: The metabolic pathway that converts glucose into pyruvate, occurring in the cytoplasm, as the first step of cellular respiration. Produces a small amount of ATP.

Krebs cycle: Also known as the citric acid cycle, a series of biochemical reactions in the mitochondrial matrix that further oxidizes acetyl-CoA, generating ATP, NADH, and FADH2.

Electron transport chain: The final stage of aerobic cellular respiration, located in the inner mitochondrial membrane, where electrons from NADH and FADH2 are passed along a series of protein complexes, driving the synthesis of a large amount of ATP. Requires oxygen.

Lactose synthesis: The production of lactose (milk sugar) from glucose and galactose in the mammary gland.

Emulsification (of fat): The process by which large fat globules are broken down into smaller droplets, increasing the surface area for enzymatic digestion by lipases. Bile salts and phospholipids facilitate this process in the small intestine.

Mixed micelles: Structures formed in the small intestine lumen from monoglycerides, fatty acids, bile salts, phospholipids, and cholesterol. They help transport the hydrophobic lipid digestion products across the unstirred water layer to the enterocyte membrane for absorption.

Enterocyte: An absorptive cell lining the wall of the small intestine.

Glycerol-3-phosphate: An activated form of glycerol that serves as the backbone for triglyceride synthesis. Can be produced from glucose via glycolysis.

Fatty acyl-CoA (activated fatty acid): A fatty acid molecule linked to coenzyme A, an activated form required for both triglyceride synthesis and beta-oxidation. This activation step requires ATP.