BIO 426 Lecture Notes: Nutrition/Metabolism PDF

BIO 426 Lecture Notes: Nutrition/Metabolism (Chapter 26) As mentioned previously, one of the main reasons that individuals need food is as a source of energy for all cells. Cells require energy for all their activities (except diffusion and osmosis). They produce the energy for their activities in...

BIO 426 Lecture Notes: Nutrition/Metabolism (Chapter 26) As mentioned previously, one of the main reasons that individuals need food is as a source of energy for all cells. Cells require energy for all their activities (except diffusion and osmosis). They produce the energy for their activities in the form of adenosine triphosphate (ATP). The building blocks for making ATP are adenosine monophosphate (AMP) and phosphate groups (p.67). These building blocks come from food sources (AMP is a nucleotide derived from eating RNA, and phosphates are plentiful in a variety of foods). Cells can keep adenosine diphosphate (ADP) for periods of time because it is relatively stable. However, to make ATP from ADP, they must have energy to attach the third phosphate. This third phosphate is attached to ADP with a high-energy bond: A-P-P + P + energy ----------- A-P-P---P The source of the energy needed to make the high-energy bond that attaches the third phosphate onto ADP is the process called cellular respiration. During this process, glucose (most often), in the presence of oxygen, is broken down in cells to form carbon dioxide and water. C6H12O6 + 6O2 ------------- 6CO2 + 6H2O + energy left over to make 36ATP Some of the energy left over is lost as heat energy. Although the heat energy helps keep the body warm, it cannot be used by cells to do work. However, there is enough energy left over from the breakdown of glucose to make 36ATP molecules. These ATP molecules can be used anywhere in the cell that work needs to be done. To do work, the high-energy bond holding the third phosphate of ATP molecule must be broken by the enzyme ATPase to release the energy. A-P-P---P (in the presence of ATPase) ------------ A-P-P + P + energy released for work ATP molecules produced during cellular respiration allow all cells to do the work of staying alive (maintaining homeostasis, replacement and repair, protein synthesis, active transport, mitosis etc.) plus all the tasks each differentiated cell must do in its special role in the body. The stages of cellular respiration (CR) are descried below (p. 986– 992) Glycolysis (p. 987): Glucose is broken down into two 3-carbon molecules called pyruvic acid or pyruvate. This is a multistep process that takes place in the cytoplasm. Oxygen is not needed for this stage and it is considered anaerobic. There is a net gain of only 2ATPs. 4 of the 12 hydrogens from glucose are removed. The 2 pyruvic acids and the 4 hydrogens are further processed in the mitochondria in the next stages. The pyruvic acids are processed during the Krebs Cycle (KC) in the central matrix of the mitochondria, while all hydrogens are processed during the Electron Transport Chain (ETC) along the inner membranes of the mitochondria called cristae. Krebs Cycle (p. 989): The first 3-carbon pyruvic acid is converted to a 2-carbon molecule called acetyl coenzyme A. During this process, a CO2 molecule is formed and a single hydrogen is removed. The acetyl-coA is then broken down in a multistep process that produces two additional CO 2 molecules and removes 3 more hydrogens (for a total of 3 CO 2 molecules and 4 hydrogens removed from this first pyruvic acid). The second pyruvic acid is processed in the same way producing 3 more CO 2 molecules and 4 more hydrogens. The 8 hydrogens removed during the Krebs cycle and the 4 removed during glycolysis results in all 12 hydrogens from the original glucose being transported to the cristae of the mitochondria where the ETC takes place. Most of the hydrogens are brought to the cristae by hydrogen acceptors (carriers) called NADH and FADH2. There is a net gain during the Krebs cycle of only 2ATPs. Electron Transport Chain (p. 990-991): This stage is the source of the remaining 32ATPs that are produced during CR. Each of the 12 hydrogens (and their electrons) are brought to the ETC by NADH and FADH 2. As they are passed along to acceptors (carriers) situated on the cristae, many of which are called cytochromes, energy is released with each transfer from one acceptor to the next. The energy that is released during the passing of hydrogens (and their electrons) is eventually used to make 32ATPs in a process called chemiosmosis (chemiosmosis involves more details than needed for BIO 426). When an acceptor (carrier) takes a hydrogen (and its electron), that acceptor is described as being reduced. Then, when it passes the hydrogen (and its electron) to the next acceptor, that is described as being oxidized. Therefore, the entire process of passing all 12 hydrogens (and their electrons) down the ETC is sometimes referred to as an oxidation-reduction chain or “redox” chain. The final hydrogen acceptor along the ETC is oxygen. Each oxygen has the capacity to take 2 hydrogens at a time to produce a water molecule (H2O). Thus, 6 oxygens are needed to take the 12 hydrogens, forming 6H2O molecules as products of the ETC. Cellular respiration must take place every second in a cell’s life in order to provide ATP molecules for the work of staying alive. This is true whether the body is in the absorptive state (food is being absorbed during mealtime and there is usually a surplus), or in the post- absorptive state (resources are in short supply and glucose levels are dropping)..p.997-998. Absorptive State The absorptive state is considered to include the 4 hours during/after mealtime and is a time of surplus when, most importantly, glucose levels are too high. The hormone, insulin, is primarily responsible for regulating the body’s responses to this surplus. Carbohydrates: Glucose molecules are absorbed into the superior mesenteric capillaries and delivered to all cells for CR and carbohydrate synthesis. The liver can convert any monosaccharide to glucose. Excess glucose molecules are delivered to the liver and to skeletal muscles for conversion to the polysaccharide, glycogen (glycogenesis). Still excess glucose molecules are delivered to adipose tissues for conversion to fat. (lipogenesis). Lipids: Glycerol and fatty acids are absorbed into the cells of the intestinal mucosa, where they are recombined into triglycerides and covered with a protein coating to make them more water- soluble. These small units, called chylomicrons, are absorbed into the lacteals and travel through the lymphatic system to be returned to the blood. Some of the chylomicrons remain in the blood while others are delivered to adipose tissue and converted to fat (lipogenesis). Proteins: Amino acids are absorbed into the superior mesenteric capillaries and delivered to all cells for protein synthesis. Excess amino acids are delivered to the liver where they are deaminated (removal of the amino group) while the remainder of the amino acid molecule is converted to fat (lipogenesis). After deamination of the amino acid, the nitrogen waste product ammonia (NH3) is formed. Because NH3 is highly toxic, the liver converts most to urea before releasing both into the blood for removal by the kidney. Nucleic Acids: Nucleotides of RNA and DNA are absorbed into the superior mesenteric capillaries and delivered to all cells for synthesis and repair of RNA and DNA (and ATP). Post-Absorptive State The post-absorptive state begins after the 4 hours during/after mealtime when shortages of glucose begin to occur. The goal is to raise glucose levels so that CR can continue and to provide other molecules that can be used as “substitutes” for glucose in CR. Since the continuation of CR is critical for survival, many hormones from numerous glands help the body to respond appropriately. These hormones include glucagon (pancreas), glucocorticoids like cortisol (adrenal cortex), growth hormone (anterior pituitary gland), thyroxine or T4 and triiodothyronine or T3 (thyroid gland) and epinephrine (adrenal medulla). Carbohydrates: Glycogen in the liver is broken down to glucose and released into the blood for all cells to use. Glycogen in skeletal muscles is also broken down to glucose, but only for those muscle cells to use. In both cases, this is referred to as glycogenolysis. Mobilization of fats: Adipose cells break down fats into glycerol and fatty acids and release them into the blood. Chylomicrons are broken down in the blood by plasma enzymes into glycerol and fatty acids. Some glycerol is converted to glucose by the liver (gluconeogenesis) and released into the blood for all cells to use in CR. Some glycerol can be absorbed directly by cells and converted to pyruvic acid for CR. Fatty acids are converted to ketones (keto-acids) by the liver and released into the blood. Most cells can absorb the ketones and convert them to acetyl-coA for CR. The process of converting fatty acids to acetyl-coA is called beta-oxidation. Proteins: Amino acids are again deaminated by the liver to form NH 3 and urea. However, the remainder of each amino acid molecule is converted to glucose (gluconeogenesis) for all cells to use in CR. Vitamins and Minerals The vitamins and minerals that are provided in our foods all have important roles to play in the body’s metabolic reactions. Tables on p.984,985 list food sources for each but not functions. Vitamins: Fat-Soluble: A: maintains epithelia; needed for synthesis of visual pigments. D: normal bone growth (absorption of calcium and phosphorus). E: antioxidant. K: synthesis of clotting proteins. Water-Soluble: B1 (thiamine): coenzyme leading to the Krebs cycle. B2 (riboflavin): part of FAD. Niacin: part of NAD. B6 (pyridoxine) coenzyme in amino acid and lipid metabolism, protein synthesis, antibody synthesis and nucleic acid synthesis. Folacin (folic acid): coenzyme in amino acid and nucleic acid synthesis and metabolism, blood cell production. B12 (cobalamin) coenzyme in nucleic acid metabolism and synthesis, metabolism of carbohydrates and fats, RBC production. Biotin: coenzyme leading to the Krebs cycle, metabolism of amino acids and fatty acids, synthesis of nucleic acids. Pantothenic acid: part of acetyl-coA. C (ascorbic acid): coenzyme for metabolism of amino acids, production of collagen, promotes Fe absorption, synthesis of steroid hormones from cholesterol, antioxidant. Minerals: Na (sodium): nerve and muscle function. K (potassium): nerve and muscle function. Cl (chlorine): pH balance, water balance. Ca (calcium): nerve, muscle, hemostasis, skeletal function. P (phosphorus): ATP, creatine phosphate, nucleotides, skeletal functions. Mg (magnesium): membrane functions, cofactor for enzymes, bone formation. S (sulfur) component of hormones, vitamins and many proteins. Fe (iron): hemoglobin, myoglobin, cytochromes. Zn (zinc): cofactor for more than 70 enzymes, carbonic anhydrase, growth, taste. Cu (copper): hemoglobin synthesis, cytochromes, melanin. Mn (manganese): cofactor for enzymes, hemoglobin synthesis, urea formation, activation of ATPase. F (fluorine): bones, teeth. Co (cobalt): part of vitamin B12, erythropoiesis. Basal Metabolic Rate (BMR): p.998,999. BMR refers to the amount of energy required to maintain the body’s functions/per unit of time. Since every activity requires a different amount of energy, it must be measured under basal conditions (awake, at rest, room To, no exercise for 2-3 hrs. and no food for 12 hrs.), so that norms can be established and comparisons can be made based on an individual’s age, size and sex. Cell respiration is the process by which energy is obtained for the body to use, and since the O 2 we breathe is primarily used for CR, O2 consumption/min. provides a way of measuring how much CR takes place each minute under basal conditions. The unit of energy used in BMR is a unit of heat energy called the Calorie (calorie vs. Calorie). Relationship between O2 and Calories: When 1L of O2 is breathed to oxidize an average mixture of food (carbohydrates, lipids and proteins), 4.825 Calories are consumed. Under basal conditions, approximately 14L of O2 are breathed/hr. and approximately 70 Calories/hr. are consumed (14L x approximately 5 Calories). Therefore, to maintain the body for a day (24 hrs.) one needs approximately 1680 Calories (24 hrs. x 70 Calories) just to provide enough energy to carry out basic functions (basal conditions). Temperature Regulation: p.1000-1002 The body’s source of heat is cellular respiration. As discussed earlier in the semester: To raise body To when it is too low: we shiver and constrict superficial blood vessels. To lower body To when it is too high: we sweat and dilate superficial blood vessels.