Chapter 9 cellular respiration PDF

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Chapter 9: Cellular Respiration and Fermentation Cellular Respiration ● ●

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Breakdown of carbohydrates, lipids, and proteins Release of energy to do the work of the cell o The energy released is used to add a phosphate group to ADP to make ATP o ATP is used by the cell to do work Cells generally contain enough ATP to sustain from 30 seconds to a few minutes of activity o ATP is unstable o Most cells are making it all the time Cells obtain glucose t make ATP o Plants produce glucose during photosynthesis o Other organisms obtain glucose from food Organisms store glucose as glycogen or starch

What happens when glucose is oxidized? ● ● ●

When glucose is oxidized to carbon dioxide by burning, some energy is released as heat and light Oxygen atoms are reduced to form water In cells, glucose is oxidized through a long series of carefully controlled redox reaction o The released free energy is used to synthesize ATP o These reactions comprise cellular respiration

Cellular respiration stages ● ● ● ●

Glycolysis (cytoplasm)- glucose is partially broken down and a modest amount of energy is released. Fatty acids and amino acids may also be broken down by different pathways Pyruvate processing/ Acetyl-CoA synthesis (cytoplasm)- Pyruvate, produced from the breakdown of glucose in glycolysis, is oxidized to acetyl-CoA and CO2 Citric acid cycle (mitochondria)- Acetyl-CoA is broken down, releasing more CO2, a modest amount of energy, and electron carriers Electron transport chain and oxidative phosphorylation (mitochondria)- the electron carriers from stage 1-3 release high-energy electron to the electrons whose energy is used to set up a transport proton gradient to produce ___.

Carbohydrate catabolism ●

Carbohydrate and lipids have high potential energy because the electrons shared in bonds are far from the nuclei of the atoms in the bond

Glycolysis is a sequence of 10 reactions ● ●

Glycolysis is a series of 10 chemical reactions that occur in the cytosol Three key points o Glycolysis starts by using two ATP in the energy investment phase (reactions 1-5) o During the energy payoff phase (reactions 6-10), NADH is made and ATP is produced by substrate-level phosphorylation

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The net yield is NADH, two ATP, and two pyruvate

Substrate-level phosphorylation ●

During cellular respiration, the cell can produce ATP in two ways: o During substrate level phosphorylation, a phosphorylated organic molecule transfers a phosphate group to ADP to produce ATP; however only a small mount of ATP is generated this way. Substrate phosphorylation occurs during stages 1 (glycolysis) and 3 (the citric acid cycle) of cellular respiration ▪

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Glycolysis- this process is anaerobic, as oxygen is not consumed in the process

o Most ATP is produced by oxidative phosphorylation in stage 4 of cellular respiration Phase 1- preparatory phase, with consumption of 2 ATP o The phosphorylation of glucose traps the molecule inside the cell and destabilizes it so that it is ready for phase 2 Phase 2- Cleavage Phase o 2 glyceraldehyde 3-phosphate is formed Phase 3-payoff phase with production of 4 ATP and 2 NADH

At the end of glycolysis ● ●

4 ATP used during phase 1= 2 ATP (net gain) 2 NADH

How is Glycolysis regulated? ●

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Glycolysis is regulated by feedback inhibition o High levels of ATP (a product of glycolysis) inhibit the third enzyme: phosphofructokinase Phosphofructokinase has two binding sites for ATP: o When ATP binds to the active site, the enzyme catalyzes the third step in glycolysis- F6P to F1, 6BP o When ATP levels are high, it binds to a regulatory site and inhibits the enzyme

Processing Pyruvate to Acetyl COA ● ● ●

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Pyruvate produced during glycolysis is transported into mitochondria Mitochondria have both inner and outer membranes Cristae are extensions of the inner membrane o Layers of sac-like structures o Fill the interior of the mitochondria o Are connected to the inner membrane by short tubes The mitochondrial matrix is inside the inner membrane In the presence of oxygen, pyruvate can be broken down to release more energy Pyruvate processing takes place inside an enormous enzyme complex called pyruvate dehydrogenase o Located in the mitochondrial matrix in eukaryotes o Located in the cytosol in prokaryotes

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Pyruvate undergoes a series of reactions: o One of its carbons is oxidized to CO2 o NADH is produced o The remaining two-carbon unit is attached to coenzyme A, producing acetyl CoA During these reactions o Pyruvate, NAD+, and CoA go in o CO2, NADH, and acetyl CoA come out Pyruvate processing is also regulated by feedback inhibition o When products of glycolysis and pyruvate processing are abundant o Pyruvate dehydrogenase is phosphorylated o Changes shape and is inhibited

The Citric Acid cycle: Oxidizing Acetyl CoA to CO2 ● ●

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In the citric acid cycle, each acetyl CoA from pyruvate processing is oxidized into two CO2 The citric acid cycle is located in the o Mitochondrial matrix of eukaryotes o Cytosol in prokaryotes The reactions are organized in cycle o Starts by moving the acetyl group from acetyl CoA to oxaloacetate to form citrate o At the end, oxaloacetate is regenerated Some of the potential energy released I used to o Reduce three NAD+ to NADH o Reduce one FAD to FADH2 o Phosphorylate ADP (or GDP) to form ATP (or GTP) The cycle turns twice for each glucose molecule, since two pyruvate are produced by glycolysis During this stage of cellular respiration, the fuel molecules are complete oxidized The chemical energy in the bonds of acetyl-CoA is transferred to ATP by substrate level phosphorylation and to the electron carriers NADH and FADH2 Acetyl-CoA is completely oxidized o Resulting in ▪

2 ATP

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6 NADH

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2 FADH2

How is the citric acid cycle regulated? o The citric acid cycle can be turned off at multiple points ▪

Via several different mechanisms of feedback inhibition

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Reaction rates are high when ATP and NADH are scarce

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Reaction rates are low when ATP or NADH are abundant

What happens to the NADH and FADH2? o For each molecule of glucose that is oxidized, the cell produces

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6 CO2-disposed of when you exhale

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4 ATP-directly used as fuel

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10 NADH

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2 FADH2

Most of glucose’s original energy is contained in the electrons transferred to NADH and FADH2 The electrons (and protons) are ultimately transferred to form water Two questions remain: ▪

What happens to the energy that is released as electrons are transferred?

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How is this energy used to make ATP?

Electron Transport Chain ●

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The molecules that oxidize NADH and RADH2 are called the electron transport chain o Most are proteins that are easily oxidized o One is a lipid-soluble, non-protein ubiquinone or coenzyme Q or “Q” o They have different ability to accept electrons, called their redox potential o Some accept only electrons; other accept electrons plus protons Organization of the electron transport chain o The ETC is organized into four protein complexes ▪ Called complexes I-IV ▪ Cytochrome c transfers electrons between complexes o At the end of the ETC ▪ Low energy electrons are passed to oxygen, along with protons ▪ Water is formed Electron transport Chain 1 o Electrons enter ETC via either complex I or II depending on whether they enter HADN or FADH2 o Electrons are passed from electron donors to acceptors until they reach the final electron acceptor, oxygen. When oxygen accepts the electron, it is reduced to water Electron transport chain 2 o Electrons must be transported between the four complexes of the ETC. Coenzyme Q or ubiquinone accepts electrons from both complexes I and II. In doing so, it is Electron transport chain 3 o Energy is released as the electrons are passed from the high-energy-electron carriers NADH and FADH2 to the final low-energy acceptor oxygen. Some of this energy is used to reduce the next carrier in the chain, but in complexes I, II, and IV it is also used to __, and the end result is ____ The flow of energy in cellular respiration o The energy of glucose is released slowly in a series of reactions and captured in chemical form

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Some energy is released by substrate level phosphorylation, and some is generated through redox reactions that transfer energy to electron carriers to NADH and FADH2 These carriers donate electron to electron transport chain. That energy is used to pump protons across the inner membrane of the mitochondria The energy of the electron carriers is this transformed into energy stored in a proton electrochemical gradient. ATO synthase then converts the energy of the proton gradient to rotational energy, which drives the synthesis of ATP

Aerobic versus anaerobic respiration ●

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All eukaryotes and many prokaryotes o Use oxygen as the final electron acceptor for the ETC o This is called aerobic respiration Some prokaryotes o Especially those in oxygen-poor environments o Use other electron acceptors o This is called anaerobic respiration

Fermentation ●

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What happens when there is no electron acceptor? o The electrons have no place to go o The ETC stops NADH builds up and there is no NAD+ available to accept electrons o Glycolysis, pyruvate processing, and the citric acid cycle stops o The situation is life threatening o NAD+ must be regenerated Fermentation is a metabolic pathway that regenerates NAD+ from NADH o electrons from NADH are transferred to pyruvate o serves as an emergency backup Glycolysis can continue to produce ATP by substrate-level phosphorylation in the absence of oxygen Many different fermentation pathways exist o When our muscle cells cannot get enough oxygen, they convert to lactic acid fermentation ▪ Pyruvate produced by glycolysis accepts electrons from NADH ▪ Lactate and NAD+ are produced o As muscle cells get more oxygen, lactate can be converted back to pyruvate o Some yeast cells can perform alcohol fermentation ▪ Pyruvate is converted to acetaldehyde and CO2 ▪ Acetaldehyde accepts electrons from NADH ▪ Ethanol and NAD+ are produced o Cells that perform other types of fermentation are used to make soy sauces, tofu, yogurt, cheese, etc. o Prokaryotes that reply on fermentation are present in our intestines Glucose storage

Glycogen is stored in muscle and liver cells. when stored in muscle cells, it is used to provide ATP for muscle contraction. The liver stores glycogen for the whole body, releasing it when it is needed elsewhere o Glucose molecules at the end of glycogen chains can be cleaved one at a time in the form of glucose 1-phosphate, which is then converted into glucose 6-phosphate, the intermediate in glycolysis o Ex: fructose receives a phosphate group to form either fructose 6-phosphate (which can enter glycolysis at step 3) or fructose 1-phosphate (which can be converted into glyceraldehyde 3-phosphate in the liver and enter glycolysis at step 6). Catabolic pathways break down a variety of molecules o For ATP production, cells ▪ First use carbohydrates ▪ Then fats ▪ And finally proteins o Proteins, carbohydrate, and fats can all furnish substrates for cellular respiration o Fats are broken down into ▪ Glycerol, which enter glycolysis ▪ Fatty acids, which are converted to acetyl CoA through a process called B-oxidation, which enter the citric acid cycle o Proteins are broken down into amino acids ▪ Amino groups are removed and excreted as waste ▪ The remaining carbon compounds are converted pyruvate, acetyl CoA, or other intermediates ▪ Used in glycolysis and the citric acid cycle o

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