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Discussion Points: Cellular Respiration Cells, and therefore organisms, require energy to perform all of their different functions and activities. For example, we already know that energy is required for active transport. Cell replication is also an example of a process that requires a lot of energy in order to replicate DNA and organelles, double the cytoplasm, divide the nucleus and synthesize new membranes in order to form two daughter cells. Muscle contraction is another good example. Cell use chemical energy to carry out their activities. This energy is stored in the bonds that join atoms together in molecules. If a molecule is broken apart, the energy in these bonds is released. This is one very important function of macromolecules, glucose, fats, and protein have bonds that contain a lot of energy that can be released for the cell to use. ATP/ADP Cycle All living cells rely on one source of chemical energy to do everything, ATP (adenosine triphosphate). ATP has a high energy terminal phosphate bond that when broken releases energy. This energy is then used to carry out the cells processes. Once ATP has released its energy it becomes ADP (adenosine diphosphate). ADP can be recharged by the addition of a phosphate molecule making it ready to use again. The recharging process also requires some energy which is derived from the breakdown of glucose molecules from the food we eat. However, this requires much less energy than making a whole new ATP molecule. The constant transformation between ATP and ADP molecules is called the ATP/ADP cycle. Although glucose is the main energy source for energy for cells, other macromolecules such as fats and proteins can be used to release energy to make ATP. Providing Energy in Cells The first stage of the breakdown of glucose to produce ATP is called glycolysis. Glycolysis occurs in the cytosol of the cell and splits the glucose molecule into two pyruvate molecules plus two ATP. In reality glycolysis releases energy from glucose in a series of steps that involve a number of enzymes. In humans glycolysis provides energy for things that require short bursts of energy such as 100 m dashes or weight lifting because these activities happen so quickly that cells don’t get a chance to get oxygen. As such glycolysis is an anaerobic process meaning it does not require oxygen. The process that occurs next depends on whether oxygen is available or not: Aerobic Respiration: If oxygen is available, the pyruvate molecules are further broken down into carbon dioxide and water and an additional 36-38 ATP. This is called aerobic respiration (because it requires oxygen) and and it occurs in the mitochondria. The simplified formula for aerobic breakdown of glucose through cellular respiration is: Glucose + Oxygen C6H12O6 + 6O2 Carbon Dioxide + Water + Energy 6CO2 + 6H20 + 36-38 ATP The mitochondria are large organelle with two cell membranes. The inner membrane is folded over on itself many times. The folds of the inner membrane are called cristae which has large surface area for aerobic respiration to take place. Aerobic respiration actually involves two linked processes, called the Kreb’s cycle and the electron transport chain. The number of mitochondria in a cell is related to the cell’s energy requirements. As such you would expect cell that require a lot of energy, such as your heart muscle cells, to have thousands of mitochondria. A good example of your body performing aerobic respiration is how heavily you breath after going for a run. This is because you’re the cells in your muscles and are trying to get as much oxygen as they can to produce energy for the activity. Anaerobic respiration: if oxygen isn’t available, then anaerobic respiration continues to occur in the cytosol. The products of anaerobic respiration depend on what the organism is. In plants, and microorganisms such as yeasts and bacteria the pyruvate molecules are broken down into carbon dioxide and ethanol. This is otherwise known as fermentation and is how the alcohol is made in wine and beer. In most animals the pyruvate molecules are broken down into lactic acid. This can also be reffered to as homolactic fermentation. No ATP is produced from this stage of cellular respiration.