Topic 3.7 Cell Respiration Topic 8.1 Cell Respiration

Cell Respiration Who does it? What is it? Where does it occur? All living things (including plants!) What is it? Carbohydrates and O2 are used to make ATP (energy). CO2 and H20 are waste products. The opposite of photosynthesis. Involves three steps: glycolysis, kreb’s cycle, and electron transport chain. Where does it occur? The cytoplasm and the mitochondria of the cell

Mitochondria

Cell Respiration C6H12O6 + 6O2 6CO2 + 6H20 + ATP Glucose+ oxygen carbon dioxide + water + energy

Redox Reactions Redox-reaction, or an oxidation-reduction reaction, is the movement of electrons from one molecule to another. Because an electron transfer requires both a donor and acceptor, oxidation and reduction always go together. Cellular respiration is an example of a redox-reaction “fall” of electrons, with energy released in small amounts that can be stored in ATP

Redox Reactions Oxidation Reduction The loss of electrons from one substance Glucose loses electrons (in H atoms) and becomes oxidized Reduction The addition of elections to another substance O2 gains electrons (in H atoms) and becomes reduced

Cell Respiration occurs in three main stages Glycolysis Occurs in the cytoplasm; glucose is broken down to two pyruvate molecules; provides energy for ETC The citric acid cycle (Kreb’s cycle) Takes place in the matrix of the mitochondria; further breaks down pyruvate to carbon dioxide; provides energy for ETC Oxidative phosphorylation (Electron Transport Chain) Takes place in the cristae of the mitochondria. Also known as chemiosmosis; NADH and FADH2 made in glycolysis and Kreb’s shuttle electrons and H+ to make ATP.

Glycolysis Means “splitting sugar” Begins with a single molecule of glucose (6-C) and concludes with two molecules of another organic compound, called pyruvate (3-C). A net gain of 2 NADH molecules and 2 ATP molecules ATP can be used by cell immediately; NADH must pass down the ETC in mitochondria Substrate-level phophorylation occurs An enzyme transfers a phosphate group from a substrate molecule directly to ADP, forming ATP

Glycolysis 9 Steps (Figure 6.7C) Steps 1-3: A sequence of three chemical reactions converts glucose to a molecule of fructose using 2 ATP. Step 4: Fructose splits into two G3P molecules Step 5: G3P gets oxidized and NAD+ is reduced to NADH Steps 6-9: specific enzymes make four molecules of ATP by substrate-level phosphorylation. Water gets produced as a by-product

Glycolysis 2 ATP produced account only for 5% of the energy that a cell can harvest from a glucose molecule. 2 NADH account for another 16%, but there stored energy is not available for use in the absence of O2.

Pyruvate chemical “grooming” As pyruvate forms at the end of glycolysis, it is transported from the cytoplasm into the mitochondria Pyruvate does not enter the Kreb’s Cycle as itself. It undergoes major chemical “grooming”

Pyruvate chemical “grooming” A large, multienzyme complex catalyzes three reactions: A carbon atom is removed from pyruvate and released in CO2 The two-carbon compound remaining is oxidized while a molecule of NAD+ is reduced to NADH A compound called coenzyme A, derived from a B vitamin, joins with the two-carbon group to form a molecule called acetyl coenzyme A: Abbreviated acetyl CoA, is a high-energy fuel molecule for the Kreb’s Cycle For each molecule of glucose that enters glycolysis, two molecules of acetyl CoA are produced and enter the Kreb’s cycle.

Pyruvate chemical “grooming”

Kreb’s Cycle Overview: Called Krebs in honor of Hans Krebs, German-British researcher who worked out much of this cyclic phase of cellular respiration in the 1930s. Only the two-carbon acetyl part of the acetyl CoA molecule actually participates in the citric acid cycle. Coenzyme A helps the acetyl group enter the cycle and then splits off and is recycled. Occurs in the matrix of the mitochondria Compared with glycolysis, Kreb’s Cycle pays big energy dividends to the cell This makes 1 ATP, 4 NADH and 1 FADH2, per acetyl coA (double that for each glucose molecule) Releases CO2 as waste is aerobic (requires oxygen)

Kreb’s Cycle Details of the citric acid cycle: Figure 6.9B: Step 1 Acetyl coA is stripped via enzymes: coA is recycle and the remaining acetyl (2-C) is combined with oxaloacetate already present in the mitochondria forming citrate (6-C) Step 2 and 3 Redox reactions take place stripping hydrogen atoms from organic intermediates producing NADH molecules and dispose of 2-C that came from oxaloacetate, which are released as CO2. Substrate-level phos. of ADP occurs to form ATP. A 4-C molecule called succinate forms. Step 4 and 5 Oxaloacetate gets regenerated from maltate, and FAD and NAD+ are reduced to FADH2 and NADH, respectively. Oxaloacetate is ready for another turn of the cycle by accepting another acetyl group

Kreb’s Cycle

Electron Transport Chain Involves oxidative phosphorylation A clear illustration of structure fitting function: the spatial arrangement of electron carriers built into a membrane makes it possible for the mitochondrion to use the chemical energy released by redox reactions to create an H+ gradient and then use the energy stored in the gradient to drive ATP synthesis Chemiosmosis also occurs The potential energy of the concentration gradient is used to make ATP.

Electron Transport Chain Built into the inner membrane of the mitochondrion, or in the cristae folds, providing space for thousands of copies of the electron transport chain and many ATP synthase complexes With all these ATP-making “machines,” a mitochondrion can produce many ATP molecules simultaneously.

Electron Transport Chain Figure 6.10: Path of electron flow from the shuttle molecules NADH and FADH2 to O2, the final electron acceptor. Each oxygen atom (1/2 O2) accepts two electrons from the chain and picks up two hydrogen ions from the surrounding solution to form H2O, one of the final products of cellular respiration. Most of the carrier molecules reside in the three main protein complexes, while two mobile carriers transport electrons between the complexes.

Electron Transport Chain Figure 6.10 (continued): All of the carriers bind and release electrons in redox reactions, passing electrons down the “energy staircase.” Protein complexes shown in the diagram use the energy released from the electron transfers to actively transport H+ across the membrane, from where they are less concentrated to where they are more concentrated. Hydrogen ions are transported from the matrix of the mitochondrion (its innermost compartment) into the mitochondrion’s intermembrane space.

Electron Transport Chain Figure 6.10 (continued): The resulting H+ gradient stores potential energy, similar to a dam storing energy by holding back elevated water. Dams can be harnessed to generate electricity when the water is allowed to rush downhill, turning giant wheels called turbines. Similarly, ATP synthases built into the inner mitochondrial membrane act like minature turbines. H+ can only cross through ATP synthases bc they are not permeable to the membrane. Hydrogen ions rush back “downhill” through an ATP synthase, spinning a component of the complex, just as water turns the turbine in a dam. Rotation activates catalytic sites in the synthase that attach phosphate groups to ADP molecules to generate ATP.

Electron Transport Chain Why is this process called oxidative phosphorylation? The energy derived from the oxidation-reduction reactions of the electron transport chain that transfer electrons from organic molecules to oxygen is used to phosphorylate ADP. By chemosmosis, the exergonic reactions of electron transport produce an H+ gradient that drives the endergonic synthesis of ATP.

Cell Respiration Summary TOTAL= 38 ATP (theoretical) Glycolysis Occurs in cytoplasm 2 ATP 2 NADH 2 H20 get released 2 pyruvate Kreb’s Cycle (including pyruvate grooming) 8 NADH 2 FADH2 6 CO2 get released Electron Transport Chain H20 gets released 10 NADH get converted to 3ATP= 30 ATP 2 FADH2 get converted to 2 ATP= 4 ATP

Poisons Some poisons block the electron transport chain. Rotenone Often used to kill pest insects and fish. binds tightly with the electron carrier molecules in the first protein complex, preventing electrons from passing to the next carrier molecule. Literally starves an organism’s cells of energy bc it blocks the ETC near its start thus preventing ATP synthesis.

Poisons Cyanide and Carbon Monoxide Bind with an electron carrier in the third protein complex Block the passage of electrons to oxygen Similar to turning off a faucet; electrons cease to flow through the “pipe” Result is the same as that or rotenone: no H+ gradient is generate and no ATP is made. ***refer to page 99 in your book for other examples***

Fermentation-Anaerobic Respiration Glycolysis is the metabolic pathway that generates ATP during fermentation. No O2 is required; it generates a net gain of 2 ATP while oxidizing glucose to two molecules of pyruvate and reducing NAD+ to NADH. Significantly less ATP is generated, but it is enough to keep your muscles contracting for a short while when the need for ATP outpaces the delivery of O2 via the blood stream Many microorganisms supply all their energy needs with the 2 ATP yield of glycolysis.

Fermentation-Anaerobic Respiration Strict Anaerobes require anaerobic conditions and are poisoned by oxygen Facultative Anaerobes can make ATP either by fermentation or by oxidative phosphorylation, depending on whether O2 is available.

Fermentation-Anaerobic Respiration Fermentation provides an anaerobic step that recycles NADH back to NAD+; essential to harvest food energy by glycolysis. Two types of fermentation: Lactic acid Alcohol

Fermentation-Anaerobic Respiration Lactic acid fermentation Figure 6.13A NADH is oxidized to NAD+ as pyruvate is reduced to lactate (the ionized form of lactic acid) Lactate builds up in muscle cells during strenuous exercise is carried in the blood to the liver, where it is converted back to pyruvate Dairy industry use this to with bacteria to make cheese and yogurt

Fermentation-Anaerobic Respiration Alcohol fermentation Figure 6.13A Used in brewing, winemaking, and baking Used by yeasts and bacteria (facultative anaerobes) Recycle their NADH to NAD+ while converting pyruvate to CO2 and ethanol (ethyl alcohol). CO2 provides bubbles in beer and champagne, and bread dough to rise Ethanol is toxic to organisms that produce it; must release it to their surroundings

Fuels for cell respiration Free glucose molecules are not common in our diet We obtain most of our calories as fats, proteins, sucrose, and other disaccharide sugars, and starch

Fuels for cell respiration Carbohydrates (polysaccharides and starch) Figure 6.14 Enzymes in our digestive tract hydrolyze starch to glucose; glycogen can be hydrolyzed to glucose to serve as fuel between meals.

Fuels for cell respiration Proteins: First must be digested to their constituent amino acids Typically, cell will use most of the amino acids to make its own proteins, but enzymes will convert excess a.a. to intermediates of glycolysis or the Kreb’s cycle, and their energy is harvested by cell respiration. Amino groups unused are disposed in urine.

Fuels for cell respiration Fats: Excellent cellular fuel bc they contain many hydrogen atoms and thus many energy-rich electrons Cell first hydrolyzes fats to glycerol and fatty acids It converts glycerol to G3P; fatty acids are broken into 2-carbon fragments that enter the Kreb’s as acetyl coA A gram of fat yields more than twice as much ATP as a gram of starch. Because so many calories are in each gram of fat, a person must expend a large amount of energy to burn fat stored in the body.

Fuels for cell respiration Food is also used as the raw materials a cell uses for biosyntheis, to make its own molecules for repair and growth…not just for ATP! To make cells, tissues, and organisms: Amino acids proteins Fatty acids and glycerols fats Sugars carbohydrates