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Cellular Respiration

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Zachary McAllister

on 24 September 2014

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Transcript of Cellular Respiration

Cellular Respiration
Chemical Energy & Food!
When a person is hungry, they typically do not feel like going for a 5-mile jog or participating in an aerobics class. Why? Because the foods we consume serve as a source for raw materials needed by the cells in our body. Most of all, food serves as a source of energy.
Chemical Energy and Food!
There is quite a lot of energy stored in food, and the amount varies from one food type to another. For example, one gram of the sugar glucose, when burned in the presence of oxygen, releases 3811 calories of heat energy. A calorie is the amount of energy needed to raise the temperature of 1 gram of water 1 Celsius. The Calories (capital "C") you see on a food label is actually a kilocalorie or the equivalent to 1,000 calories.
Chemical Energy and Food!
On the previous slide, we learned how much energy can be generated when cells "burn" glucose, but cells don't really "burn" glucose. Cells gradually release the energy from glucose and other food compounds. This pathway begins with a process called glycoslysis.
Cellular Respiration
In the presence of xygen, glycolysis is followed by the Krebs cycle and the electron transport chain. Glycolysis, the Krebs cycle, and the electron transport chain make up a process called cellular respiration. Cellular respiration is the process that releases energy by breaking down glucose and other food molecules in the presence of oxygen.
Glycolysis
The first set of reactions is glycolysis. Glycolysis is the process in which one molecule of glucose is broken in half, producing 2 molecules of pyruvic acid, a 3-carbon compound. Glycolysis comes from the Greek words glukus and lysis.
Glycolysis and ATP
Even though ATP is an energy-releasing process, the cell needs to put a little energy to get things going. At the beginning, 2 molecules of ATP are used up, but they can be thought of as an investment that will pay 100% interest. When glycolysis is complete, 4 ATP molecules have been produced. This gives the cell a net gain of 2 ATP; an excellent investment.
Glycolysis and NADH
One of the reactions of Glycolysis removes 4 high-energy electrons and passes them to an electron carrier called NAD+. Just like NADP+ in photosynthesis, NAD+ accepts a pair of high-energy electrons and becomes the molecule NADH. The molecule NADH holds the electrons until they can be transferred to other molecules. When a cell generates large amounts of ATP from glycolysis, it runs into a problem. In only a few seconds, all of the NAD+ will pick up electrons and the process will stall until more NAD+ in available.
Glycolysis and Oxygen
As we have just learned, glycolysis will release energy in the the forms of ATP and NADH. This, however, only occurs when oxygen is present. If oxygen is not present, glycolysis will continue, but by a different pathway. This pathway leads to fermentation.
O2
No O2
Glycolysis Hwy
Glycolysis and Oxygen
When oxygen is not present, glycolysis is followed by a different pathway. The combined process of this pathway and glycolysis is called fermentation. Fermentation releases energy from food molecules by producing ATP in the absence of oxygen. Because fermentation does not require oxygen, it is said to be anaerobic.
Fermentation!!!
During fermentation, cells convert NADH to NAD+ by passing high-energy electrons back to pyruvic acid. This action converts NADH back into the electron carrier NAD+, allowing glycolysis to continue producing a steady supply of ATP. The two main types of fermentation are alcoholic fermentation and lactic acid fermentation.
Alcoholic Fermentation
Yeasts and a few other microorganisms use alcoholic fermentation, forming ethyl alcohol and carbon dioxide as wastes. Alcoholic fermentation causes bread to rise. When yeast in the dough runs out of oxygen, it begins to ferment, giving off bubbles of carbon dioxide that form the air spaces you see in a slice of bread.
Alcoholic Fermentation
Yeast and other organisms that can carry out both fermentation and cellular respiration, are called facultative anaerobes. Human muscle cells also behave as facultative anaerobes. Baker's yeast, Saccharomyces cerevisiae, is the most common organism used to leaven bread. Yeast multiplies best between temperatures of about 27 C and 43 C. It is dormant below 10 C, and dies at temps above 49 C.
Lactic Acid Fermentation
In many cells, the pyruvic acid that accumulates as a result of glycolysis can be converted to lactic acid. Lactic acid is produced in your muscles during rapid exercise when the body cannot produce all of the ATP that is required. The buildup of lactic acid causes a painful, burning sensation. This is why muscles may feel sore after only a few seconds of exertion.
Lactic Acid Fermentation
Unicellular organisms also produce lactic acid as a waste product during fermentation. For example, prokaryotes are used in the production of a wide variety of foods and beverages, such as cheese, yogurt, buttermilk, and sour cream. Pickles, sauerkraut, and kimchi are also produced using lactic fermentation.
After Glycolysis
At the end of glycolysis, about 90% of the chemical energy stored in glucose is still unused, locked in the high-energy electrons of pyruvic acid. To extract that energy, the cell must have oxygen. Oxygen is a requirement for the final pathway of cellular respiration. Because the final pathway requires oxygen, it is said to be an aerobic process. Since your cells need oxygen, you must continue breathing in order to respire.
O
2
Krebs Cycle
In the presence of oxygen, pyruvic acid produced in glycolysis passes to the second stage of cellular respiration, called the Kreb cycle. During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions. Because citric acid is the first compound formed in this series of reactions, the Krebs cycle is also called the citric acid cycle.
I torment kids, even from beyond the grave!!
Krebs Cycle
The Krebs cycle begins when pyruvic acid from glycolysis enters the mitochondrion. One carbon atom is removed from the pyruvic acid to form CO2, which will eventually be released. The other two carbon atoms will join with a compound (coenzyme A) to become acetyl-CoA. Acetyl-CoA then adds the two carbon atoms to a 4-carbon molecule, producing a 6-carbon molecule called citric acid.
Krebs Cycle
As the cycle continues, citric acid is broken down into a 4-carbon molecule, more CO2 is released, and electrons are transferred to energy carriers. The 6-carbon molecule citric acid has one atom removed, and then another, releasing 2 molecules of CO2 and leaving a 4-carbon molecule. This 4-carbon molecule is now ready to accept another 2-carbon acetyl group, which starts the cycle over again.
Products of Krebs Cycle
The products of the Krebs cycle are CO2, ATP, and Electron carriers. The CO2 is expelled each time you exhale and the ATP is used primarily to power cellular functions. The electrons carriers, however, can be used to generate huge amounts of ATP in the presence of oxygen.
Electron Transport
NADH and FADH carry high-energy electrons and pass them to the electron transport chain. At the end of the electron chain is an enzyme that combines these electrons with hydrogen ions and oxygen to form water. Oxygen serves as the final electron acceptor of the transport chain.
Electron Transport
Each time 2 high-energy electrons transport down the chain, their energy is used to transport hydrogen ions across the membrane. This causes a build up of positively charged ions on one side of the intermembrane (positive side) and a negative charge in the areas where the ions were removed. This creates a charge differential that will serve in the next step.
Electron Transport
The inner membranes of the mitochondria contain protein spheres called ATP synthase. The charge differential causes the hydrogen ions to pass through ATP synthase, which in turn causes the protein to spin. Each time it rotates, enzymes grab an ADP, attaches a phosphate, and forms a high-energy ATP.
The Totals
Glycolysis only produces 2 ATP per molecule of glucose. However, in the presence of oxygen, everything changes. The Krebs cycle and electron transport enable the cell to produce roughly 36 ATP molecules per glucose molecule.
Efficiency
Efficiency in machines is defined as energy input divided by energy output. So, how efficient is cellular respiration? The cell will be able to get about 38% of the energy stored in glucose when it has formed the final 36 ATP's. 38% may not seem efficient, but the typical automobile is less efficient in its use of gasoline. "What about the 62%?" you ask. It is released as heat, which is why your body feels hot after vigorous exercise.
Short-term Energy Use
When the starter's pistol goes off, the runners begin their rapid sprint. What happens when your body needs a lot of energy in a hurry? Cells normally contain small amounts of ATP from glycolysis and cellular resp. The runners' muscles only have enough ATP to power them through the first few seconds. After about 50 meters, the muscles have used all the available ATP and begin lactic acid fermentation, which can power them through the next 60-90 seconds.
Long-term Energy Use
What if you need to run a marathon? For exercise longer than 90 seconds, cellular respiration is a must. It is the only way to continue making a steady supply of ATP. Cellular respiration releases more energy than fermentation, but it does so more slowly. This is why marathon runners don't sprint the entire race, rather, they maintain a pace that is acceptable for their ATP production rate.
Long-term Energy Use
Your body stores energy in muscles and other tissues in the form of glycogen. These stores of glycogen are usually enough to last for 15-20 minutes of exercise. After that, your body begins to break down stored molecules, including fats, for energy. This is why aerobic forms of exercise are running, dancing, and swimming are beneficial in weight control.
Photosynthesis vs Cellular Resp.
The energy flows in photosynthesis and cellular respiration take place in opposite directions. In fact, photosynthesis can be viewed as a deposit in an account (energy deposit), while cellular respiration can be viewed as a withdrawal from the account.
Photosynthesis vs Cellular Resp.
Function

Location

Reactants

Products

Equation
Energy Capture
Energy Release
Chloroplasts
Mitochondria
CO and H O
2
2
C H O and O
12
6
6
2
C H O and O
6
12
6
2

CO and H O
2
2
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