Photosynthesis and Cellular Respiration: Cellular Respiration
Unit Objective I can compare the processes of photosynthesis and cellular respiration in terms of energy flow, reactants, and products.
During this unit, we will answer the following very important questions:
#1: What is photosynthesis?
#2: How does the process of photosynthesis result in the storage of energy required by cells?
#3: What is cellular respiration?
#4: How are the processes of photosynthesis and cellular respiration related?
We will continue our study of this unit by focusing on cellular respiration. To do this, we will learn how to summarize how glucose is broken down during the first stage of cellular respiration describe how ATP is synthesized during the second stage of cellular respiration identify the role of fermentation during the second stage of cellular respiration evaluate the importance of oxygen in aerobic respiration
What do YOU know about fermentation?
Fermentation on the cellular level is one method by which cells synthesize ATP in the absence of oxygen. Fermentation can indeed cause foul odors.
On the other hand, food scientists use fermentation to produce cheese, yogurt, bread, wine, and a host of other products to which you are exposed daily (and some of which you yourselves ingest).
Additionally, if you have ever exercised to the point of soreness and cramping, you may have forced your muscle cells to undergo fermentation.
Most of the food you eat contains useable energy.
Much of this energy occurs in the form of proteins, carbohydrates, and lipids.
Unfortunately, this energy cannot be used in the form in which it occurs in food cells cannot absorb particles of hamburger and pizza.
Before it becomes useable, this energy must be converted into ATP. As in most organisms, your cells must convert the energy present in food into ATP through a process called cellular respiration.
Cellular respiration is the process by which cells produce energy from carbohydrates, forming water and carbon dioxide as byproducts.
The oxygen in the air you breathe makes the production of ATP more efficient, although it is possible to synthesize ATP in the absence of oxygen.
Metabolic processes in which oxygen is required to synthesize ATP are aerobic processes, whereas metabolic processes that do not require oxygen to synthesize energy are anaerobic processes.
Aerobic respiration produces most of the ATP synthesized by cells. The intermediate products of aerobic respiration form the organic compounds required to assist in constructing and maintaining cells.
The process by which glucose (C 6 H 12 O 6 ) molecules present in food materials are broken down during cellular respiration can be summarized in the following chemical equation: enzymes C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + energy glucose oxygen gas carbon dioxide gas water ATP
Cellular respiration occurs in two stages: Stage 1: the conversion of glucose molecules present in food materials to pyruvate (the ionized form of the organic molecule pyruvic acid), occurring in the cytoplasm of a cell and producing a small amount of ATP and NADPH Stage 2: the use of pyruvate ions and NADPH, in the presence of oxygen, to synthesize large amounts of ATP in the mitochondria of eukaryotic cells and the cell membrane of prokaryotic cells
Stage One: Glycolysis The primary fuel that drives cellular respiration is the glucose obtained from the breakdown (in your digestive system) of carbohydrates such as starch and sucrose (table sugar).
If too few carbohydrates are consumed, other molecules (such as fats) will be broken down to synthesize ATP.
In fact, one gram of lipid contains more stored energy than two grams of carbohydrate. Proteins and nucleic acids can also be used to make ATP, but only as a last resort by the body, as these are normally used for the construction of cell components.
During the first stage of cellular respiration, glucose is broken down in the cytoplasm of cells during a process called glycolysis.
Glycolysis is an enzyme-assisted anaerobic process in which one six-carbon molecule of glucose is broken down into two three-carbon pyruvate ions containing some of the energy originally stored in glucose. As such, glycolysis is an example of one of the numerous metabolic pathways that occur within living cells. As will be seen in the following slides, the energy stored in glucose molecules then passed on to pyruvate ions is gradually in a series of enzyme-assisted chemical reactions.
As glucose is broken down, some of the hydrogen ions in glucose are transferred to an electron acceptor called NAD +, forming an electron carrier called nicotinamide adenine dinucleotide (NADH), a coenzyme found in all living cells, which transports electrons to other organic compounds involved in cellular respiration.
A coenzyme is an organic chemical required by many enzymes for the enzymes to complete their specific chemical reactions.
Without NADH (one molecules of NADH = 2.5 ATP), cellular respiration would not occur. When NADH donates electrons to other compounds, NAD + is recycled to accept more electrons.
QUESTION Why is it necessary for six-carbon glucose molecule to be broken down into two threecarbon pyruvate ions in the cytoplasm of a cell before continuing with cellular respiration?
Glycolysis is a complex process, and occurs as follows: Step 1: Assisted by the enzyme hexokinase, a phosphate group from a donor ATP molecule is attached to a glucose molecule at the sixth carbon in the chain, producing an ion of glucose-6-phosphate.
Step 2: Assisted by the enzyme phosphoglucose isomerase, the hydrogen atoms attached to carbon atom 2 are relocated to carbon atom1, forming fructose-6-phosphate.
Step 3: Assisted by the enzyme phosphofructokinase, a second phosphate group is attached to the fructose-6-phosphate ion at carbon 1, forming fructose-1, 6-biphosphate.
Step 4: Assisted by the enzyme fructose biphosphate aldolase, the fructose-1, 6- biphosphate is split into two three-carbon ions, dihydroxyacetone phosphate and glyceraldehyde-3-posphate.
Step 5: Assisted by the enzyme triose phosphate isomerase, the structure of dihydroxyacetone is altered to form a second glyceraldehyde-3- phosphate ion.
Step 6: Assisted by the enzyme glyceraldehyde- 3-phosphate dehydrogenase, another phosphate group is attached to each of the glyceraldehyde- 3-phosphate ions to form 2 ions of 1,3- biphosphoglycerate, while at the same time liberating four hydrogen ions, two of which combine with NAD + ions to form NADH.
Step 7: Assisted by the enzyme phosphoglycerate kinase, the phosphate group at carbon 1 on each ion is added to an ADP molecule to form one molecule of ATP and produce two ions of 3-phosphoglycerate.
Step 8: Assisted by the enzyme phosphoglycerate mutase, the phosphate group at carbon 3 on each ion is moved to carbon 2, forming two ions of 2-phosphoglycerate.
Step 9: Assisted by the enzyme enolase, one molecule of water is removed from each ion of 2-phosphoglycerate to form two ions of phosphoenolpyruvate (PEP).
Step 10: Assisted by the enzyme pyruvate kinase, the remaining phosphate group on each ion is stripped away by an ADP molecule to form two ATP molecules and two pyruvate ions.
The process of glycolysis consumes 2 ATP molecules, but produces four ATP molecules, yielding a net gain of two ATP molecules. Glycolysis is followed by another set of reactions that uses the energy temporarily stored in NADH and ATP molecules to synthesize even more ATP.
According to fossil records, prokaryotes were present on Earth 3.5 billion years ago, although oxygen was not abundant in Earth s atmosphere until approximately 2.5 billion years ago. For this reason, it is thought that early life on Earth probably used glycolysis to synthesize ATP long before oxygen was present in Earth s atmosphere.
Because glycolysis is an anaerobic metabolic pathway occurring in all cells, glycolysis most likely originated in early cells.
Stage One: Production of ATP In the presence of oxygen, the pyruvate ions produced in the cell cytoplasm during glycolysis enter the cell mitochondria and are converted into two-carbon compounds, yielding CO 2, NADH, and two-carbon acetyl groups (CH 3 CO).
These acetyl groups, in turn, are attached to coenzyme A molecules, forming 2 molecules of acetyl-coa, a molecule that participates in many biochemical reactions in protein, carbohydrate, and lipid metabolism and the main function of which is to deliver its acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for energy production.
The acetyl-coa then enters the Krebs cycle (also known as the citric acid cycle (CAC) or the tricarboxylic acid cycle (TAC)), a series of enzyme-assisted chemical reactions used by all aerobic organisms to release CO 2 and stored chemical energy in the form of ATP through the oxidation of acetyl-coa derived from carbohydrates, lipids, and proteins.
The Krebs cycle is named for Germanborn British physician, biochemist, and Nobel laureate Sir Hans Adolf Krebs (1900-1981), who first described the metabolic pathway in 1937, and who was awarded the Nobel Prize in Physiology or Medicine for his discovery in 1953.
Krebs Cycle, Step 1 Assisted by the enzymes citrate synthase and Coenzyme A, four-carbon oxaloacetate ions already present in the mitochondria combine chemically with incoming acetyl-coa molecules and one molecule of water to form a citrate ion.
Krebs Cycle, Step 2 Assisted by the enzyme citrate aconitase, carbon 3 exchanges a hydroxyl (OH - ) group for a hydrogen ion (H + ) with carbon 4, forming an isocitrate ion.
Krebs Cycle, Step 3 Assisted by the enzyme isocitrate dehydrogenase, one hydrogen ion is lost to an NAD + to form one NADH molecule. A CO 2 molecule is lost as well, and a five-carbon α- ketoglutarate ion is formed as a result.
Krebs Cycle, Step 4 Assisted by the enzymes α-ketoglutarate dehydrogenase and Coenzyme A, two more hydrogen ions are lost, one of them to an NAD + to form a second NADH molecule. Another CO 2 molecule is lost as well, forming a fourcarbon succinyl-coa ion.
Krebs Cycle, Step 5 Assisted by the enzyme succinyl-coa synthetase, a phosphate group is joined to guanosine diphosphate and a coenzyme A molecule is released from succinyl-coa to form a four-carbon succinate ion and guanosine triphosphate (GTP). Like ATP, GTP has a role as a source of energy and an activator of substances in metabolic reactions, although more specific in its function than ATP.
Krebs Cycle, Step 6 Assisted by the enzyme succinate dehydrogenase, two more hydrogen ions are released to a flavin adenine dinucleotide (FAD) molecule, a cofactor involved in numerous enzymatic reactions during metabolism, forming FADH 2 and a four-carbon fumarate ion.
Krebs Cycle, Step 7 Assisted by the enzyme fumarase, a water molecule is added to fumarate to form a fourcarbon malate ion.
Krebs Cycle, Step 8 Assisted by the enzyme malate dehydrogenase, two hydrogen ions are lost, one of them to NAD + to form NADH. Oxaloacetate is also reformed, allowing another molecule of acetyl- CoA to renew the cycle.
After one repetition of the cycle has been completed NADH and FADH 2 now contain most of the energy that was previously stored in the glucose and pyruvate that were metabolized.
The products of one completion of the cycle are: one molecule of GTP three molecules of NADH one molecule of FADH 2 two molecules of CO 2
Remember, however, that two acetyl-coa molecules are produced from each glucose molecule entering glycolysis. Thus, two repetitions of the cell cycle are required to completely process each glucose molecule. At the end of these two cycles, then, two molecules of GTP six molecules of NADH two molecule of FADH 2 four molecules of CO 2 are produced.
But, wait.there S MORE!
When the Krebs cycle has been completed, some of the products of the Krebs cycle enter an electron transport chain. This electron transport chain is located on the inner membranes of a cell s mitochondria, and is responsible for most of the ATP synthesized during cellular respiration. The process by which an electron transport chain synthesizes ATP is called oxidative phosphorylation, the metabolic pathway by which cells use enzymes to oxidize nutrients, thereby releasing energy to form ATP.
During aerobic respiration, electrons are donated by the NADH and FADH 2 synthesized during the Krebs cycle to proteins embedded in the inner membrane of the mitochondria, in similar fashion as happens during photosynthesis.
The energy contained in these electrons is used to pump hydrogen ions out of the inner mitochondrial compartment and into the outer mitochondrial compartment, where they accumulate, producing a concentration gradient across the inner mitochondrial membrane.
QUESTION What type of transport pump hydrogen ions out of the inner mitochondrial compartment and into the outer mitochondrial compartment? Hmmmm.
Hydrogen ions diffuse back into the inner mitochondrial compartment through a specialized membrane carrier protein (ATP synthase) that adds a phosphate group to an ADP molecule, producing ATP.
QUESTION What type of transport involves the diffusion of hydrogen ions back into the inner mitochondrial compartment?
QUESTION Why are all of the folds in the membranes of the mitochondria so important?
QUESTION What is the role of oxygen in in the electron transport chain?
At the end of the electron transport chain, hydrogen ions and spent (de-energized) electrons combine with oxygen (O 2 ) molecules to form molecules of water. All told, oxidative phosphorylation produces water molecules (with the help of the oxygen you breathe in) up to 34 ATP (thanks to the hydrogen ion gradient) NAD + ions and FAD molecules (recycled to glycolysis and the Krebs cycle)
The human body uses approximately 1 million (1 000 000) ATP molecules per cell per second to stay alive and function properly. There are in excess of 100 trillion (100 000 000 000 000) cells in the average human body. This means that your body consumes approximately 100 000 000 000 000 000 000 ATP molecules every second to keep you alive and functioning properly!
When there is no oxygen to metabolize, the electron transport chain does not function because there is no oxygen to act as the final electron acceptor. The high-energy electrons being carried by NADH and FADH 2 cannot be offloaded to the nonfunctioning electron transport chain, meaning that NAD + and FAD cannot be recycled for glycolysis and the Krebs cycle.
Not to worry, though. When there is no oxygen to metabolize, NAD + is recycled in another way.
Under anaerobic conditions, electrons carried by NADH are transferred to pyruvate ions during glycolysis, recycling the NAD + required to continue synthesizing ATP through glycolysis.
The recycling of NAD + using a hydrogen acceptor (instead of oxygen) is known as fermentation.
One of the important types of fermentation is lactic acid fermentation, characteristic of some prokaryotes and fungi. Indeed, these organisms are used industrially in the production of foods such as yogurt and some cheeses.
Lactic acid fermentation is an anaerobic metabolic process by which glucose and other six-carbon sugars are converted into cellular energy and the metabolite lactate.
Lactic acid fermentation begins with glycolysis, just as in aerobic cellular respiration. Instead of moving on to the Krebs cycle, however, the two pyruvate molecules generated during glycolysis are converted into three-carbon lactate, the ionized form of the organic acid lactic acid.
The basic chemical equation describing lactic acid fermentation is lactic acid bacteria C 6 H 12 O 6 2 CH 3 CH(OH)COOH glucose lactic acid
During vigorous exercise, pyruvate in muscle tissue is converted to lactate when the demand for oxygen by the muscles is greater than the oxygen available in the muscle tissue i.e., when the muscles are working so hard, they consume oxygen faster than the body can provide it.
Sometimes, if one continues to work past the point at which oxygen is available, fermentation in the muscle tissue will enable glycolysis to continue producing ATP in very small amounts as long as there is adequate glucose in the body to do so.
In this situation, glycolysis extracts the required ATP from glycogen (a multi-branched polysaccharide of glucose that functions as a form of energy storage in animals, fungi, and bacteria) in the muscle tissue itself, converting the glycogen to pyruvate, which is then fermented into lactic acid.
Excess lactate is removed from muscle tissue in the bloodstream. If so much lactate builds up in muscle tissue that it cannot be removed fast enough (i.e., if one exercises so strenuously that they produce large amounts of lactate in a short period of time in the absence of sufficient oxygen and glucose reserves), severe cramping and muscle soreness can occur.
The more lactate that accumulates in the muscle tissue, the more fatigued muscles become, and less able the muscles are to generate the force to exercise.
Additionally, constant intensive exercise may cause a substance called myoglobin to build up in muscle tissue.
Myoglobin is an iron- and oxygen-bonding protein found in the muscle tissue of vertebrates in general and in almost all mammals.
Myoglobin is related to hemoglobin, the ironand oxygen-bonding protein located in red blood cells.
In humans, myoglobin is only found in the bloodstream after muscle injury (like the type of muscle injury that occurs during intensive, prolonged exercise).
It is important, following strenuous exercise, to allow the body to rest and the muscle tissue to heal.
Not allowing the body to do so can cause myoglobin to build up in the muscle tissue. Myoglobin is released from muscle tissue into the bloodstream.
The continued presence of myoglobin in the bloodstream is abnormal. Although myoglobin can be filtered by the kidneys, it is toxic to epithelial tissue in the kidneys, and cause acute kidney injury.
The ability to perform continuous exercise is limited by the body s supply of glycogen. Thus, physical endurance will increase if glycogen stored in the muscle tissue is spared during exercise.
Trained athletes like Michael Jordan obtain a relatively-significant amount of their energy from aerobic respiration. As a result, their muscle glycogen reserve is depleted more slowly than that in untrained individuals.
Indeed, the greater the level of physical training, the higher the proportion of energy the body derives from aerobic respiration.
Endurance-trained athletes generally have more muscle mass than do untrained people. It is the high aerobic capacity of endurance-trained athletes (rather than their greater muscle mass), however, that allows these athletes to exercise longer before lactic acid production and glycogen depletion cause muscle fatigue.
Cyanide is a fast-acting poison that blocks the action of the electron transport chain. The most potent substances containing the cyanide ion are hydrogen cyanide gas,
and cyanide salts, produced during gold or other metal extractions, electroplating, and industrial metal cleaning.
Cyanide enters the body by absorption through the lungs, skin, or gastrointestinal tract. It is highly toxic, and symptoms of poisoning appear soon after exposure. Ingestion of as little as 3 g (⅑the mass of a paper clip) can be almost immediately fatal.
Many persons visit health clubs to exercise or work out. These health clubs employ fitness trainers to direct exercise programs for groups or custom-tailor programs for individuals.
Another important type of fermentation is alcoholic fermentation.
Unlike lactic acid fermentation which converts three-carbon pyruvate ions to three-carbon lactate ions, in alcoholic fermentation, threecarbon pyruvate ions are broken down to form two-carbon ethanol molecules, releasing carbon dioxide in the process.
The basic chemical equation describing alcoholic fermentation is C 6 H 12 O 6 2CO 2 + 2C 2 H 5 OH glucose carbon dioxide ethanol
During alcoholic fermentation, the pyruvate ions formed during glycolysis are broken down into a two-carbon compound, 2-acetaldehyde, releasing carbon dioxide in the process.
Electrons are then transferred from two molecules of NADH to the 2-acetaldehyde, forming ethanol and two NAD+ ions.
As in the case of lactic acid fermentation, NAD + is recycled and glycolysis continues to produce ATP.
Alcoholic fermentation by yeast, a fungus, has been used in the preparation of many foods and beverages for thousands of years. Wine and beer, for example, contain ethanol produced during alcoholic fermentation by yeast.
Carbon dioxide released by yeast causes the rising of bread dough and the carbonation of some alcoholic beverages, such as beer.