credit: modification of work by Mariana Ruiz Villareal Chapter 7 General Biology I BSC 2010 Cellular Respiration credit: modification of work by Mariana Ruiz Villareal

Cellular Respiration is (sort of) the Reverse of Photosynthesis Photosynthesis stores light energy in the form of chemical bonds. It produces food. Cellular respiration extracts energy from chemical bonds. It breaks down food.

- Electrons and Energy Redox is two simultaneous events. One compound is reduced (an electron is added). Another compound is oxidized (an electron is removed). It represents a very small transfer of energy. The extra electron adds energy to the reduced compound. The oxidized compound loses energy. Sometimes the electron is transferred as part of complete hydrogen atom.

Electron Carriers - Certain molecules in cells carry electrons from one molecule to another. They can be easily reduced or oxidized. An important electron carrier in cells is nicotinamide adenine dinucleotide (NAD). NAD+ is the oxidized form, NADH is the reduced form.

More on Electron Carriers FAD (FAD+, FADH2) is also an electron carrier. Like NAD, it is important in respiration. NADP is like NAD with an extra phosphate group. It is important in photosynthesis.

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ATP is the Cell’s Energy Currency Adenosine triphosphate is an unstable molecule (a nucleotide) that is good for short-term energy storage. Download for free at http://cnx.org/content/col11448/latest/ ATP has 3 phosphate groups that can be removed by hydrolysis to form ADP or AMP. Negative charges on the phosphate groups naturally repel each other, requiring energy to bond them together and releasing energy when these bonds are broken.

Phosphorylation When a phosphate group is added to a molecule, we say it has been “phosphorylated”. Molecules can also be “dephosphorylated”(a phosphate group is removed). Dephosphorylation of ATP yields ADP. Phosphorylation of ADP yields ATP.

Download for free at http://cnx.org/content/col11448/latest/ A Phosphate from ATP Can Be Used to Phosphorylate Other Molecules, Changing Their Function Download for free at http://cnx.org/content/col11448/latest/

Enzymes Transfer Phosphate Groups During Varied Types of Phosphorylation to Make ATP Substrate-level phosphorylation A phosphate group is removed from an intermediate reactant in a pathway, and free energy of the reaction is used to add the third phosphate to an ADP, producing ATP. Oxidative phosphorylation A phosphate group is added to ADP during a chemiosmosis process, producing ATP. Most ATP from cellular respiration is produced this way. Photophosphorylation Light energy is used to add the phosphate, as seen in photosynthesis.

Steps for Production of ATP by Cellular Respiration Glycolysis, the breakdown of glucose. We begin by studying glucose, but other molecules can yield energy for use in the cellular respiration pathway. Pyruvate Oxidation The Citric Acid Cycle Oxidative Phosphorylation

Glycolysis Occurs in the Cytoplasm Energy (activation energy) is required to get the first part of glycolysis started by splitting glucose into two smaller molecules. 2 ATPs are used In the second part of glycolysis, ATP is produced. 4 ATPs are produced. 2 NADHs (reduced electron carriers) are produced. 2 pyruvate molecules remain.

Download for free at http://cnx.org/content/col11448/latest/ Glycolysis, Part 1 Download for free at http://cnx.org/content/col11448/latest/ The first half of glycolysis uses two ATP molecules in the phosphorylation of glucose, which is then split into two three-carbon molecules (G3Ps).

Download for free at http://cnx.org/content/col11448/latest/ Glycolysis, Part 2 Download for free at http://cnx.org/content/col11448/latest/ The second half of glycolysis involves phosphorylation without ATP investment (step 6) and produces 2 NADH and 4 ATP molecules per glucose.

Download for free at http://cnx.org/content/col11448/latest/ Pyruvate Oxidation At the end of glycolysis, 2 pyruvate molecules remain. In order to continue to the Citric Acid Cycle, the pyruvates must be converted to acetyl CoA. During this process, the pathway enters the mitochondrial matrix. In the process, carbon dioxide is released and one molecule of NADH is formed. Download for free at http://cnx.org/content/col11448/latest/

The Citric Acid Cycle (TCA Cycle, Krebs Cycle) The acetyl group from acetyl CoA is attached to oxaloacetate to form 6-carbon citrate. Subsequent steps release 2 CO2 molecules, 3 NADH and one FADH2, and one GTP/ATP (made by substrate-level phosphorylation). At the end of the cycle, oxaloacetate remains to accept another acetyl CoA. credit: modification of work by “Yikrazuul”/Wikimedia Commons

Oxidative Phosphorylation This process makes most of the ATP. There are 2 main parts of the process: The electron transport chain embedded in the inner membrane of the mitochondria This is where the oxygen is required. Chemiosmosis across the inner membrane of the mitochondrion The actual phosphorylation event (to make ATP) occurs in the matrix of the mitochondrion.

The Electron Transport Chain The chain is a series of electron transporters that shuttles electrons from NADH and FADH2 to molecular oxygen. In the process, protons are pumped from the matrix to the intermembrane space, and oxygen is reduced to form water.

Download for free at http://cnx.org/content/col11448/latest/ Chemiosmosis Download for free at http://cnx.org/content/col11448/latest/ The energy from electron flow is used to pump protons (H+) into the intermembrane space. The crowded protons diffuse across the inner membrane, through ATP synthase, into the matrix. ATP is made (by oxidative phosphorylation).

credit: modification of work by Klaus Hoffmeier ATP Synthase ATP synthase is a complex molecular machine that uses a proton (H+) gradient to form ATP from ADP and inorganic phosphate (Pi). credit: modification of work by Klaus Hoffmeier

Metabolism Without Oxygen Some organisms lack the enzymes to use oxygen lack the enzymes to detoxify the by-products of oxygen- based metabolism are poisoned by oxygen Anaerobic cellular respiration is used by some, such as the methanogens and sulfate-reducing bacteria. Others use fermentation.

Fermentation Some organisms make ATP only during glycolysis. It is critical to not run out of NAD+ for glycolysis to continue. Fermentation is dedicated to turning NADH from glycolysis back into NAD+. (recall that glycolysis requires NAD+) During fermentation, waste products are produced, but really, the organism just wants its NAD+ back so it can continue doing glycolysis. Two types of fermentation are lactic acid fermentation and alcohol fermentation.

Lactic Acid Fermentation Lactic acid fermentation is used by some micro-organisms. It also occurs (short-term) in our muscle cells that have run out of oxygen. The waste product is lactic acid, which is used for production of yogurt. Download for free at http://cnx.org/content/col11448/latest/

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Other Molecules in Metabolic Pathways We can get food energy not just from glucose, but also from lipids, proteins, and other types of carbohydrates. These molecules participate in pathways that connect eventually with glycolysis and/or the citric acid cycle. In addition, intermediates from cellular respiration can be shunted to other pathways for anabolism. Download for free at http://cnx.org/content/col11448/latest/

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