Anatomy and Physiology Biology 2402 Chapter-25 Metabolism and Energetics

Metabolic Reactions Metabolism is the sum of all chemical reactions that occur in the body. It can be subdivided into two groups of reactions: Anabolism: Anabolic reactions require energy: here monomers (small molecules) join to form polymers (macromolecules). Catabolism: catabolic reactions release energy: macromolecules are broken down to monomers. Metabolism balances anabolism and catabolism: the energy released by catabolic reactions is transferred to anabolic reactions. The energy generated by the metabolic break down of carbohydrates, lipids, proteins is used to produce ATP (Adenosine TriPhosphate) through oxidation-reduction reactions

Anabolic and Catabolic Reactions

Oxidation-Reduction Reactions Oxidation-Reduction Reactions are also known as Redox reactions. These reactions are coupled: they occur simultaneously between two molecules: one molecule become oxidized, the other becomes reduced. Oxidation is the removal of electrons from a molecule. Reduction is the gain of electrons by a molecule. Redox reactions are transfer reactions: electrons are transferred from one molecule to the other.

Coenzymes Are molecules that can accept, carry and transfer electrons. Two of the most common electron carriers also known as coenzymes are NAD and FAD: NAD (nicotinamide adenine dinucleotide) The reduced form of NAD is NADH + H+ The oxydized form of NAD is NAD+ FAD (flavin adenine dinucleotide) The reduce form of FAD is FADH2 The oxidized form is FAD

ATP ATP is the energy currency of the cell. It is generated by phosphorylation (addition of a phosphate) of ADP (Adenosine DiPhosphate): ADP + Pi  ATP ATP can be generated by 3 mechanisms: Substrate level phosphorylation: a high energy phosphate group is transferred from a phosphorylated compound (substrate) to ADP. This occurs in the cytosol of cells. Oxidative phosphorylation: ATP synthase uses the proton motive force generated by an electron transport chain to transfer one phosphate group to ADP. This occurs in the innermitochondrial membrane. Photophosphorylation: this occurs during photosynthesis in plants, or in photosynthetic bacteria.

Functions of Organic Compounds Perform structural maintenance and repairs Support growth Produce secretions Store nutrient reserves Types of Organic Compounds Carbohydrates- energy source, Glycogen- storage form of carbohydrates. Triglycerides (fat) – energy source Proteins – most abundant organic compounds in our body.

Figure 25-2 Nutrient Use in Cellular Metabolism Structural, functional, and storage components Triglycerides Glycogen Proteins Organic compounds that can be absorbed by cells are distributed to cells throughout the body by the bloodstream. Nutrient pool Fatty acids Glucose Amino acids Three-carbon chains Two-carbon chains MITOCHONDRIA Citric acid cycle Electron transport system Coenzymes KEY  Catabolic pathway  Anabolic pathway 8

Carbohydrate Metabolism: Glucose Largely based on glucose metabolism although other carbohydrates are involved. Glucose is a monosaccharide, a 6 carbon sugar (hexose) from the aldose family. In the body it is mainly generated by the digestion of starch and glycogen. After absorption by secondary active transport in the small intestine, glucose enters the blood and it is routed with other nutrients to the liver. In the liver (and in skeletal muscle) it is stored as glycogen, a polysaccharide. When cells need glucose, glycogen is broken down.

Glucose Catabolism Aerobic Respiration Cells generate ATP by cellular respiration. Aerobic respiration is the complete oxidation of glucose by glycolysis, the Krebs cycle, and the electron transport chain (ETC) and oxidative phosphorylation. The complete degradation of glucose by cellular respiration can be written: C6H12O6 + 6O2  6CO2 + 6H2O + Energy

Glycolysis Break down of glucose into pyruvic acid in ten enzymatically catalyzed steps. Occurs in the cytosol with or without oxygen One can roughly breakdown glycolysis into 2 phases: The energy investment phase (first five reactions) The energy payoff phase (last five reactions) In the end it produces 2 pyruvates, 2 ATP, and 2 NADH + 2H+.

Pyruvic acid is a 3 carbon molecule produced by catabolism of glucose. During metabolism it can be used in two ways: When oxygen concentration is high pyruvic acid is transformed into AcetylCoA in the matrix of mitochondria and enters the Krebs cycle (aerobic pathway) When oxygen concentration is low, pyruvic acid is reduced to lactic acid by NADH + H+. This is called lactic acid fermentation. This process oxidizes NAD into NAD+ which allows glycolysis to continue, and lactic acid diffuses into the blood. Lactic acid is converted back into pyruvic acid in the liver.

Anaerobic Respiration Anaerobic respiration does not require oxygen. Glycolysis occurs, and may be followed by lactic acid fermentation, which is a way to continue glycolysis without the Krebs Cycle or the ETC.

Krebs Cycle The Krebs cycle which is also called the citric acid cycle, or the tricarboxylic acid (TCA) cycle is a group of 8 metabolic reactions that occur in the matrix of mitochondria.

Electron Transport Chain The ETC is a group of membrane bound electron carriers arranged in a redox cascade in the inner mitochondrial membrane. These membrane bound carriers receive electrons from NADH + H+ and FADH2.

ATP synthesis is linked to redox reactions and H+ ions pumping Chemiosmosis ATP synthesis is linked to redox reactions and H+ ions pumping

Summary of Cellular Respiration C6H12O6 + 6O2  + 6CO2 + 6H2O + 36 or 38ATP During aerobic respiration, 36 ATPs can be generated from one molecule of glucose. Two ATPs come from glycolysis. Two come from substrate-level phosphorylation in the Krebs cycle.

Glycogenesis & Glycogenolysis Glycogenesis is the synthesis of glycogen from glucose in the liver and skeletal muscle. It is regulated by insulin, and occurs when the blood glucose levels are high. Insulin stimulate liver and mucle cells to absorb glucose and store it as glycogen. Glycogenolysis is the break down of glycogen to release glucose. It is regulated by glucagon and epinephrine, and occurs in response to low glucose blood levels. Liver and muscle cells are stimulated by these two hormones to break down glycogen to release glucose. Only liver cells are able to release glucose to the blood.

Gluconeogenesis Gluconeogenesis is the generation of glucose from non carbohydrate molecules like pyruvic acid, lactic acid, glycerol, and glucogenic amino acids. It is stimulated by cortisol and glucagon and occurs primarily in the liver. It can also occur in the kidney.

Lipids are transported under 4 different forms of lipoproteins: Lipid Metabolism Lipids are transported under 4 different forms of lipoproteins: Chylomicrons: transport triglycerides from intestinal epithelial cells to lacteals (special lymph capillaries in villi). Once in circulation they travel to adipose tissue where triglycerides are stored. (2% protein) Very low-density lipoproteins (VLDLs): carry triglycerides synthesized in the liver to adipose tissue. They are converted to LDLs. (10% protein) Low-density lipoproteins (LDLs): transport cholesterol to body cells via blood circulation. They are also called “bad cholesterol” because in high amounts they may deposit cholesterol around blood vessels. (25% protein) High-density lipoproteins (HDLs): they remove excess cholesterol from cells and send to the liver for elimination. “Good cholesterol” (40% protein)

Lipolysis and Other Lipids Functions Adipose triglycerides are broken down and released as free fatty acids. Free fatty acids are taken up by cells and broken down by beta-oxidation into acetylCoA which can enter the citric acid cycle or can be converted to ketone bodies. Other lipids are used as structural molecules or to synthesize essential molecules. Examples include phospholipids of plasma membranes lipoproteins that transport cholesterol thromboplastin for blood clotting myelin sheaths to speed up nerve conduction cholesterol used to synthesize bile salts and steroid hormones.

Lipid Metabolism Lipid Synthesis (Lipogenesis) Can use almost any organic substrate Because lipids, amino acids, and carbohydrates can be converted to acetyl-CoA Essential fatty acids Cannot be produced by the body, must be consumed Unsaturated 18-carbon fatty acid from plants Linoleic acid Linolenic acid

Lipolysis and Lipogenesis

Protein Metabolism During digestion proteins are broken down into amino acids that are quickly asorbed. Like monosaccharides they are absorbed by villi capillaries and sent to the liver via the hepatic portal system. From the liver they enter general circulation to reach body cells by active transport. Amino acids are used to synthesize proteins or as energy source. Amino acids are not stored in the body.

Amino Acids Catabolism Amino acids must be converted to vaious substances to enter the Krebs cycle for oxidation. Amino acid conversion occurs in the liver. Amino acids can be: Deaminated Decarboxylated Dehydrogenated Deamination removes the amino group and creates nitrogenous wastes such as urea.

Essential Amino Acids Essential amino acids are molecules that we cannot synthesize, or that we synthesize in low amount. Humans cannot synthesize leucine, isoleucine, lysine,methionine, phenylalanine, threonine, tryptophan, and valine. Arginine and histidine are not synthesized in sufficient amounts.

Metabolic Adaptations Absorptive state The period immediately after eating when nutrients are absorbed through intestinal wall into circulatory and lymphatic systems about 4 hours after each meal

Metabolic Pathways during the Absorptive State

Metabolic Adaptations Postabsorptive state Occurs late in morning, afternoon, night after absorptive state concluded Blood glucose levels are maintained by conversion of other molecules to glucose

Metabolic Pathways during the Postabsorptive State

Metabolic Adaptations

Fasting and Starvation Fasting means going without food for many hours or a few days. Starvation implies weeks or months of food deprivation or inadequate food intake. Catabolism of stored triglycerides and structural proteins can provide energy for several weeks. The amount of adipose tissue determines the lifespan possible without food. During fasting and starvation, nervous tissue and red blood cells continue to use glucose for ATP production.

Total amount of energy produced and used by body per unit of time Metabolic Rate Total amount of energy produced and used by body per unit of time Estimated by amount of oxygen used per minute Components Basal metabolic rate Energy used at rest, 60% of metabolic rate Thermic effect of food Energy used to digest and absorb food, 10% Muscular activity Energy used for muscle contraction, 30%

Body Temperature Regulation A balance between heat gain and loss Heat is produced through metabolism Heat is exchanged through radiation, conduction, convection, evaporation The greater the temperature difference between body and environment, the greater the rate of heat exchange Regulated by a “set point” in the hypothalamus calle hypothalamic thermostat.

Hypothalmic Thermostat Several negative feedback loops work to raise body temperature when it is too low or it raises too high. Heat conservation mechanisms Vasoconstriction sympathetic stimulation skeletal muscle contraction (shivering) thyroid hormone production

Nutrition Kilocalories Nutrition Balanced Diet Malnutrition Is the absorption of nutrients from food The body’s requirement for each nutrient varies Balanced Diet Contains all nutrients needed for homeostasis Malnutrition Diet deficit in nutrients. Kilocalories Measure of energy supplied by food and released through metabolism

My Pyramid

Table 25-2-1 Minerals and Mineral Reserves 39

Table 25-3 The Fat-Soluble Vitamins 40

Table 25-4 The Water-Soluble Vitamins 41