Chapter 12 Food as Fuel A Metabolic Overview

Transcription

1 Chapter 12 Food as Fuel A Metabolic Overview

2 Outline 12.1 How Metabolism Works 12.2 Metabolically Relevant Nucleotides 12.3 Digestion From Food Molecules to Hydrolysis Products 12.4 Glycolysis From Hydrolysis Products to Common Metabolites 12.5 The Citric Acid Cycle Central Processing 12.6 Electron Transport and Oxidative Phosphorylation 12.7 ATP Production 12.8 Other Fuel Choices

3 12.1 How Metabolism Works Animals get energy from the covalent bonds contained in carbohydrates, fats, and proteins. In the first stage of metabolism, biomolecules in food are digested into smaller units through hydrolysis reactions. Polysaccharides are hydrolyzed into monosaccharide units. Triglycerides are broken down to glycerol and fatty acids. Proteins are hydrolyzed into their amino acid units.

4 12.1 How Metabolism Works The molecules produced by the breakdown are absorbed through the intestinal wall into the bloodstream and transported to different tissues for use by the cells. In the cells, the hydrolysis products are broken down into a few common metabolites containing two or three carbons. Metabolites are chemical intermediates formed by enzyme-catalyzed reactions in the body.

5 12.1 How Metabolism Works As long as cells have oxygen and are producing energy, two-carbon acetyl groups can be broken down further to carbon dioxide through the citric acid cycle. This cycle works to produce the molecules ATP, nicotinamide adenine dinucleotide (NADH), and flavin adenine dinucleotide (FADH 2 ).

6 12.1 How Metabolism Works Chemical reactions that occur in living systems are biochemical reactions. Chemical reactions occur in a series called a metabolic pathway. The sugar molecule glucose (containing six carbons) is broken down to two molecules of pyruvate (three carbons each) through a series of chemical reactions referred to as glycolysis. Metabolism can be considered in two parts, catabolism and anabolism.

7 12.1 How Metabolism Works Catabolism refers to chemical reactions in which larger molecules are broken down into a few common metabolites. These reactions tend to be exergonic (-ΔG). Anabolism refers to chemical reactions in which metabolites combine to form larger molecules. These reactions tend to be endergonic (+ΔG). The energy released during catabolic reactions is captured in ATP and used to drive anabolic reactions.

8 12.1 How Metabolism Works

9 12.1 How Metabolism Works In animals, a cell membrane separates the materials inside the cell from the exterior aqueous environment. The nucleus contains DNA that controls cell replication and protein synthesis for the cell. The cytoplasm consists of all the material between the nucleus and the cell membrane. The cytosol is the fluid part of the cytoplasm. It is the aqueous solution of electrolytes and enzymes that catalyzes many of the cell s chemical reactions.

10 12.1 How Metabolism Works Within the cytoplasm are organelles. Ribosomes are the sites of protein synthesis. Mitochondria are the energy-producing factories of the cells. A mitochondrion consists of an outer membrane, an inner membrane, and an intermembrane matrix. Enzymes in the matrix and inner membrane catalyze the oxidation of carbohydrates, fats, and amino acids.

11 12.1 How Metabolism Works

12 12.1 How Metabolism Works

13 12.2 Metabolically Relevant Nucleotides Nucleotides act as energy exchangers and can also be coenzymes. All of these nucleotides have two forms: a high-energy form and a low-energy form. They consist of some basic components: the nucleoside adenosine, a phosphate, and a five-carbon sugar. Many of these molecules also have a vitamin within their structure.

14 12.2 Metabolically Relevant Nucleotides

15 12.2 Metabolically Relevant Nucleotides

16 12.2 Metabolically Relevant Nucleotides

17 12.2 Metabolically Relevant Nucleotides ATP is often referred to as the energy currency of the cell. ATP can undergo hydrolysis: during hydrolysis, energy is released as a product, so in this case, ATP is the high-energy form and ADP is the low-energy form. The energy given off during the hydrolysis of ATP can be coupled to drive a chemical reaction that requires energy.

18 12.2 Metabolically Relevant Nucleotides NADH/NAD + and FADH 2 /FAD Nicotinamide adenine dinucleotide (NAD + ) and flavin adenine dinucleotide (FAD) are energy-transferring compounds with a high-energy form that is reduced (hydrogen added) and a low-energy form that is oxidized (hydrogen removed). The abbreviations for these forms are NADH (reduced form) and NAD + (oxidized form) and FADH 2 (reduced form) and FAD (oxidized form). The active end of each molecule contains a vitamin component. Nicotinamide is derived from the vitamin niacin (B 3 ), and riboflavin (B 2 ) is found in FAD.

19 12.2 Metabolically Relevant Nucleotides Acetyl Coenzyme A and Coenzyme A Another important energy exchanger is coenzyme A (CoA). The two forms of this compound are acetyl coenzyme A (high energy) and coenzyme A (low energy). Energy is released from acetyl coenzyme A when the C S bond in the thioester functional group is hydrolyzed, producing an acetyl group and coenzyme A. CoA contains adenosine, three phosphates, and a pantothenic acid (vitamin B 5 )-derived portion.

20 12.3 Digestion From Food Molecules to Hydrolysis Products Carbohydrates Starch (amylose and amylopectin) begins to be digested in your mouth by alpha-amylase in saliva. This salivary amylase hydrolyzes some of the α-glycosidic bonds in the starch molecules, producing glucose, the disaccharide maltose, and oligosaccharides. Only monosaccharides are small enough to be transported into the bloodstream. To complete the digestion of starch, enzymes in the small intestine hydrolyze starch and disaccharides into monosaccharides. Cellulose cannot be digested because we lack the enzyme cellulase that hydrolyzes its β-glycosidic bonds.

21 12.3 Digestion From Food Molecules to Hydrolysis Products

22 12.3 Digestion From Food Molecules to Hydrolysis Products Fats Dietary fats are nonpolar molecules, so to assist in digestion, bile is excreted from the gall bladder into the stomach during digestion. Bile contains bile salts, which are amphipathic: they place their nonpolar face toward the dietary fats and their polar face toward the water, forming micelles. Breaking up larger nonpolar globules into smaller droplets (micelles) is called emulsification. The micelles move the dietary fats closer to the intestinal cell wall so cholesterol can be absorbed and triglycerides hydrolyzed. Once across the intestinal wall, free fatty acids and monoglycerides are reassembled as triglycerides while the cholesterol is linked to another free fatty acid forming a cholesterol ester. These are repackaged as a lipoprotein called a chylomicron. Chylomicrons transport triglycerides to the tissues, where they are used for energy production or stored.

23 12.3 Digestion From Food Molecules to Hydrolysis Products

24 12.3 Digestion From Food Molecules to Hydrolysis Products Proteins Protein digestion begins in the stomach, where proteins are denatured (unfolded) by the acidic digestive juices. Digestive enzymes like pepsin, trypsin, and chymotrypsin hydrolyze peptide bonds. Amino acids are absorbed into the bloodstream for delivery to the tissues.