CHAPTER 7 10/16/2012 How cells release Chemical Energy 1
7.1 OVERVIEW OF CARBOHYDRATE BREAKDOWN PATHWAYS Organisms stay alive by taking in energy. Plants and all other photosynthetic autotrophs get energy from the sun. Hetrotrophs get energy by eating plants and one another. Regardless of its source, the energy must be in a form that can drive thousands of diverse lifesustaining reactions. Energy that becomes converted into chemical bond energy of adenosine tri-phosphate- ATP serves that function. 2
COMPARISION OF THE MAIN PATHWAYS 10/16/2012 The first metabolic pathways were operating billions of years before earth have an oxygen-rich atmosphere (anaerobic). Fermentation pathways is an example of anaerobic pathway that produce ATP. Fig.7.2 However, the cells of nearly all species extract energy efficiently from glucose by way of aerobic respiration, an oxygen-dependent pathway. Each breath you take provides your actively respiring cells with a fresh supply of oxygen. 3
In all cells, all of the main energy-releasing pathways start with the same reactions in the cytoplasm. During the initial reactions, glycolysis, enzymes cleave and rearrange a glucose molecule into two molecules of pyruvate, an organic compound that has a three-carbon backbone. 4
OVERVIEW OF AEROBIC RESPIRATION Of all energy-releasing pathways, aerobic respiration gets the most ATP for each glucose molecule. Whereas anaerobic routes have a net yield of two ATP. Aerobic respiration typically yields thirty six or more ATP. If you were a bacterium, you would not require much ATP. Being far larger, more complex, and highly active, you depend on the aerobic pathway s high yield. When a molecule of glucose is used as the starting material, aerobic respiration can be summarized this way: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + 36 ATP Glucose Oxygen Carbon Dioxide Water Energy 5
OVERVIEW OF AEROBIC RESPIRATION/CONT. 6
7.2 GLYCOLYSIS-GLUCOSE BREAKDOWN STARTS First Stage: Glycolysis 10/16/2012 Any of several six-carbon sugars can be broken down in glycolysis. Each molecule of glucose, has six carbon, twelve hydrogen, and six oxygen atoms. During glycolysis, this one molecule is partly broken down to two molecules of pyruvate, a three-carbon compound: Glucose P-glucose 2 pyruvate 7
GLYCOLYSIS The initial steps of glycolysis are energy-requiring, and that energy is delivered by ATP. One ATP molecule activates glucose by transferring a phosphate group to it. Then another ATP transfers a phosphate group to the intermediate that forms. Thus, it takes an energy investment of two ATP to start glycolysis (Figure 7.4).
Glycolysis 10/16/2012 9
7.3 SECOND STAGE OF AEROBIC RESPIRATION ACETYL-CoA FORMATION Start with Figure 7.5, which shows the structure of a typical mitochondrian.
SECOND STAGE OF AEROBIC RESPIRATION Figure 7.5 zooms in on part of the interior where the second-stage reactions occur. At the start of these reactions, enzyme action strips one carbon atom from each pyruvate and attaches it to oxygen, forming CO 2. Each two-carbon fragment combines with a coenzyme (designated A) and forms acetyl-coa, a type of cofactor that can get the Krebs cycle going. The initial breakdown of each pyruvat also yields one NADH (Figure 7.5).
THE KREBS CYCLE The two acetyl-coa molecules enter the krebs cycle separately. Each transfers its two-carbon acetyl group to four-carbon oxaloacetate. Incidentally, this cyclic pathway is also called the citric acid cycle, after the first intermediate that forms (citric acid, or citrate). 12
THE KREBS CYCLE
Abnormal structure, altered function: Pyruvate dehydrogenase speed the conversion of pyruvate to acetyl-coa. Mutation that change structure of the enzyme, usually destroy it function. When this enzyme is mutated, citrate cannot form during Krebs cycle. This correlated with sever disorders such as Alzheimer disease, Parkinson s disease and certain cancers. 14
7.4 AEROBIC RESPIRATION S BIG ENERGY PAYOFF ELECTRON TRANSFER PHOSPHORYLATION The third stage starts as coenzymes donate electrons to electron transfer chains that are located in the inner mitochondrial membrane (Figure 7.7). The flow of electrons through the chains drives the attachment of phosphate to ADP molecules. Hence the name electron transfer phosphorylation. When electrons flow through transfer chains, they give up energy bit by bit, in usable parcels, to substances that can briefly store it. 15
Electron transfer chain 16
The two NADH that formed in the cytoplasm (by glycolysis) can t reach the ATP-producing machinery directly. They give up their electrons and hydrogen to transport proteins, which shuttle them into the inner compartment. There, NAD+or FAD inside pick them up. Eight NADH and two FADH2 from the second stage are already inside. 17
SUMMING UP: THE ENERGY HARVEST
7.5 ANAEROBIC ENERGY-RELEASING PATHWAYS Fermentation Pathways Glycolysis is the first stage of fermentation, as it is in aerobic respiration. Here, too, pyruvate and NADH from, and the net energy yield is two ATP. But fermentation reactions cannot completely degrade glucose (to carbon dioxide and water). They produce no more ATP beyond the small yield from glycolysis. The final steps simply regenerate NAD+, the coenzyme that is essential for the breakdown reactions. The regeneration allows glycolysis reactions to continue production of small amounts of ATP in the absence of oxygen. 19
ALCOHO FERMENTATION In alcoholic fermentation, the three-carbon backbone of the two pyruvate molecules glycolysis is split. The reactions result in two molecules of acetaldehyde (an intermediate having a two carbon backbone), and two of carbon dioxide. Next, the acetaldehydes accept electrons and hydrogen from NADH, thus becoming an alcohol product called ethanol (Figure 7.9a). 20
ALCOHO FERMENTATION
FERMENTATION 22
LACTATE FERMENTATION In lactate fermentation, NADH gives up electrons and hydrogen to two pyruvate molecules from glycolysis. The transfer converts each pyruvate to lactate, a threecarbon compound (Figure 7.9b). You ve probably heard of lactic acid, the non-ionized form of this compound, but lactate is by far the most common form inside living cells, which is our focus here. Lactobacillus and some other bacteria use lactate fermentation. Lactate fermentation as well as aerobic respiration yields ATP for muscles that are partnered with bones. 23
7.6 THE TWITCHERS Skeletal muscles, which move bones, consist of cells fused as long fibers. The fibers differ in how they make ATP. Slow-twitch muscle fibers have many mitochondria and produce ATP by aerobic respiration. They dominate during prolonged activity, such as long runs. Slow-twitch fibers are red because they have an abundance of myoglobin., a pigment related to hemoglobin, myoglobin store oxygen in muscle tissue. Fast-twitch muscle fibers have few mitochondria and no myoglobin; they are pale. The ATP they make by lactate fermentation sustain only short bursts of activity, such as weight lifting. Fig. 7.11 24
ALTERNATIVE ENERGY SOURCES IN THE BODY THE FATE OF GLUCOSE AT MEALTIME AND IN BETWEEN MEALS What happens to glucose at mealtime? While you and all other mammals are eating, glucose and other small organic molecules are being absorbed across the gut lining, and your blood is transporting them through the body. The rising glucose concentration in blood prompts an organ, the Pancrease, to secret insulin. This hormone make cells take up glucose faster. 25
Cells trap the incoming glucose by converting it to glucose-6-phosphate. This is the first intermediate of glycolysis, formed by a phosphate group transfer from ATP. Phosphorylated glucose can t be transported out of the cell. When glucose intake exceeds cellular demands for energy, the body s ATP-producing machinery goes into high gear. Unless a cell is using ATP rapidly, the ATP concentration in cytoplasm rises, and glucose-6- phosphate is diverted into a biosynthesis pathway. 26
Disposition of Organic Compounds 27
THE FATE OF GLUCOSE AT MEALTIME AND IN BETWEEN MEALS
ENERGY FROM FATS How does a human body access its far reservoir? A fat molecule, recall, has a glycerol head and one, two, or three fatty acid tails. The body stores most fats as triglycerides, with three tails each. Triglycerides build up inside of the fat cells of adisope tissue. This tissue is strategically located under the skin of buttocks and other body region. When the blood glucose level falls, triglycerides are tapped as an energy alternative. Enzymes in fat cells cleave bonds between glycerol and fatty acids, which both enter the blood. Enzymes in the liver convert the glycerol to PGAL. 29
ENERGY FROM PROTEINS Some enzymes in your digestive system split dietary protein into their amino acid subunits, which are then absorbed into the bloodstream. Cells use amino acid subunits, which are then absorbed into the bloodstream. Cells use amino acids to build other proteins or nitrogencontaining compounds. 30
7.8 REFLECTIONS ON LIFE S UNITY 31