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Energy in a Cell Chapter 9 Why do our cells need energy? Active transport Cell division Movement of cilia and flagella The production and storage of proteins Contracting skeletal, cardiac and smooth muscles Messages sent from the brain Adenosine Triphosphate (ATP) Structure: adenosine molecule with 3 phosphates attached The energy is stored in the bonds between each phosphate; – Each phosphate has the same charge – When the phosphates bond, a HUGE amount of energy is required to hold them together since they do not attract. How do cells tap into the energy stored in the bonds between the phosphates of the ATP molecule? When ATP loses its last phosphate, the energy stored between the 2nd and 3rd phosphate is released. This is the source of energy for the cell. Adenosine Diphosphate (ADP) is then formed. Eventually, another phosphate will bond to ADP forming ATP which can be used for energy again. Sometimes cells will use ADP for energy by breaking the second phosphate off of the molecule. The energy stored between the 1st and 2nd phosphate is release. This, however, is not efficient since there is not as much energy stored between these phosphates. Refer to Figure 9.2 (page 229) Where does this ATP come from? The sun is the source of all energy! Chlorophyll, a pigment found in the chloroplasts of plant cells, captures sunlight energy which drives photosynthesis. Heterotrophs do not contain chlorophyll, therefore, these organisms must rely on green plants to transform sunlight energy into chemical energy (ATP). Photosynthesis Light-Dependent Reactions: – After capturing sunlight energy, chlorophyll passes energy down an electron transport chain. – Half of the energy is used to split water releasing oxygen and forming NADPH – Half of the energy is used to form ATP – NADPH and ATP are needed for lightindependent reactions. Light-Independent Reactions: – During the Calvin cycle, which takes place in the stroma of chloroplasts, the carbon in CO2 is used as well as the ATP and NADPH from the light-dependent reactions to form carbohydrates through a series of reactions (page 235). – It takes a total of 6 rounds to form 1 sugar molecule. Photosynthesis 6CO2 + 6H2O C6H12O6 + 6O2 The sunlight energy is now chemical energy trapped in the carbohydrate, glucose! So, how do cells release the energy in glucose? Cellular Respiration Glycolysis – A series of chemical reactions in the cytoplasm of the cell that breaks down glucose into 2 molecules of pyruvic acid. – 4 molecules of ATP are produced but 2 are used from earlier reactions for glycolysis to occur; therefore, the net gain of ATP is only 2 ATP molecules. – NADH and H+ are also formed which will be used in the citric acid cycle. Following glycolysis, the pyruvic acid molecules move to the mitochondria of the cell. Within the mitochondria of the cell, the pyruvic acid formed during glycolysis is converted into acetyl-CoA – During this process, a molecule of CO2 is released. – NADH + H+ are formed for use in the citric acid cycle. Cellular Respiration The Citric Acid Cycle – A molecule of acetyl-CoA is broken down, forming ATP and CO2 through a series of reactions. – NADH + H+ and FADH are also formed for use in the final step, the electron transport chain. The Electron Transport Chain – NADH and FADH2 pass energized electrons from protein to protein within the inner membrane of the mitochondria. – Oxygen in the chain combines with H+ to form water. – 32 molecules of ATP are produced making this process very efficient. Cellular Respiration C6H12O6 + 6O2 6H2O + 6CO2 +36ATP What if oxygen is not present? Lactic Acid Fermentation – Glycolysis occurs – Instead of water and carbon dioxide, lactic acid is produced. – Only 2 molecules of ATP are produced Alcoholic Fermentation – Glycolysis occurs – Instead of water, alcohol is produced. – Only 2 molecules of ATP are produced – This process is done by yeast cells. Since CO2 is produced, yeast is used to make dough rise. Compare Photosynthesis and Respiration