Cellular Respiration Harvesting Chemical Energy Chapter 9

Chemical Energy & Food How much energy is in food? A lot – 1 gram of sugar glucose, when burned in the presence of oxygen, releases 3811 calories of heat energy A calorie is the amount of energy required to raise the temperature of 1 gram of water 1 degree Celcius

Chemical Pathways - Overview

Overview of Cellular Respiration Cellular respiration is the process that releases energy by breaking down food molecules in the presence of oxygen Organisms that respire: animals, fungi, plants (during night hours) Cellular respiration requires a food molecule (such as glucose), oxygen, and it gives off carbon dioxide Because is occurs in the PRESENCE of OXYGEN, cellular respiration is known as AEROBIC respiration

Overview of Cellular Respiration Glycolysis takes place in cytoplasm of cell. The Krebs Cycle and Electron Transport chain take place inside the mitochondria. Glycolysis takes place in cytoplasm of cell. The Krebs Cycle and Electron Transport chain take place inside the mitochondria.

Chemical Formula - Cellular Respiration The chemical formula for cellular respiration is: 6O2 + C6H12O6 → 6 CO2 + 6 H2O + Energy oxygen + glucose → carbon dioxide + water + energy The reactants of cellular respiration are: oxygen (O2) & glucose (C6H12O6) The products of cellular respiration are: carbon dioxide (CO2) and water (H2O) The 3 main stages of cellular respiration are: Glycolysis Krebs Cycle (Citric Acid Cycle) Electron Transport http://www.sumanasinc.com/webcontent/animations/content/cellularrespiration.html

IT ALL HAPPENS HERE - Mitochondria

Energy Yielding Pathways - Catabolic Cellular respiration and fermentation are energy-yielding pathways (catabolic). To understand what happens when a catabolic pathway decomposes glucose, you must first understand: Redox Reactions: a chemical reaction involving the transfer of one or more electrons from one reactant to another. OXIDATION: loss of electrons from a substance REDUCTION: addition of electrons to a substance

Redox Reactions As electrons from redox reactions are transferred from one substance to another, energy is transferred as well. RECALL THAT ELECTRONS HAVE ENERGY! Oxidized substances LOSE energy Reduced substances GAIN energy

Fate of Catabolic Pathways The fate of glucose in the body can take one of two catabolic pathways: CELLULAR RESPIRATION a.k.a. aerobic respiration absolutely requires oxygen occurs in mitochondria FERMENTATION occurs without oxygen less efficient than cell respiration (makes less ATP)

Visual Overview of Cellular Respiration

Visual Overview of Cellular Respiration Krebs Cycle

Substrate Level Phosphorylation Some ATP created during cell respiration is made by direct enzymatic transfer of a phosphate group from a substrate to ADP.

Oxidative Level Phosphorylation ATP synthesis can be powered by the flow of H+ back across mitochondrial membrane (chemiosmosis)

Glycolysis http://www. sumanasinc The first set of reactions in cellular respiration is glycolysis Glycolysis is the process in which 1 molecule of glucose is broken in half, producing 2 molecules of pyruvic acid. Glycolysis occurs in the cytoplasm of the cell. Glycolysis produces a total of 4 ATP, but requires 2 ATP in the beginning to get the pyruvic acid through the membrane of the mitochondria to enter the next phase. Therefore, the NET ATP YIELD of glycolysis is 2 ATP!

Glycolysis – Visual Overview Glycolysis is the 1st stage of cellular respiration. During glycolysis, glucose is broken down into 2 molecules of pyruvic acid. Glycolysis means “splitting of sugar”. During glycolysis, a six-carbon sugar is split into 2 three-carbon sugars. These smaller sugars are then oxidized (lose electrons) and their remaining atoms rearranged to form 2 molecules of pyruvate. This is a 10-step process, each catalyzed by a specific enzyme…with an energy input and energy payoff phase (Figure 9.8 page 161). Glycolysis is the 1st stage of cellular respiration. During glycolysis, glucose is broken down into 2 molecules of pyruvic acid. Glycolysis means “splitting of sugar”…during glycolysis, a six-carbon sugar is split into 2 three-carbon sugars. These smaller sugars are then oxidized and their remaining atoms rearranged to form 2 molecules of pyruvate. This is a 10-step process, each catalyzed by a specific enzyme…with an energy input and energy payoff phase…see page 161 figure 9.8.

10 Steps of Glycolysis

Glycolysis “NEED TO KNOW” http://highered. mcgraw-hill The first step is the phosphorylation of glucose (glucose molecule gains 2 phosphates) – this ACTIVATES the glucose to split. The second step is the splitting of glucose – breaking it down into (2) 3-carbon molecules called pyruvic acid. 2 ATPs are needed to produce four ATPs (energy investment and energy payoff phases). A second product in glycolysis is 2 NADH, which results from the transfer of H+ to the hydrogen carrier NAD+. Occurs in the cytoplasm Net of 2 ATPs produced 2 pyruvic acids formed 2 NADH produced

FIRST - Convert Pyruvate into Acetyl CoA The conversion of pyruvate to acetyl CoA is the junction between glycolysis (step 1) and the Krebs cycle (step 2). If oxygen is present, Pyruvate (3 C each) from glycolysis enters the mitochondrion. Using Coenzyme A, each pyruvate is converted into a molecule of Acetyl CoA (2 C each). What happened to the other carbon from each molecule of pyruvate? CO2 released! NAD+ is reduced to form NADH

The Krebs Cycle (Citric Acid Cycle) In the presence of oxygen, the pyruvic acid produced during glycolysis passes to the second stage of cellular respiration: the Krebs Cycle. During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions. Every time you exhale, you expel the CO2 produced by the Krebs cycle.

NOW – Enter the Krebs Cycle http://www. sumanasinc For each turn of the 8-step cycle, 5 pairs of high energy electrons are captured by the carrier molecules NADH and FADH2. NADH and FADH2 are energy carriers! They carry electrons captured during the Krebs Cycle to the Electron Transport Chain – so we can make ATP!!!

Electron Transport Following the Krebs cycle, the electrons captured by NADH and FADH2 are passed to the electron transport chain: The electron transport chain uses the high-energy electrons from the Krebs cycle to convert ADP to ATP Every time 2 high energy electrons transport down the ETC, their energy is used to transport H+ across the inner membrane of the mitochondria…this creates a + charge on the inside of the membrane and a – charge in the matrix of the mitochondria As a result of this charge difference, H+ ions escape through channel proteins called ATP synthase causing it to rotate Each time it rotates, the enzyme ATP synthase grabs a low energy ADP and attaches a phosphate, forming high-energy ATP

Electron Transport Chain http://www. sumanasinc The electron transport chain uses the high-energy electrons from the Krebs cycle to convert ADP to ATP Every time 2 high energy electrons transport down the ETC, their energy is used to transport H+ across the inner membrane of the mitochondria…this creates a + charge on the inside of the membrane and a – charge in the matrix of the mitochondria As a result of this charge difference, H+ ions escape through channel proteins called ATP synthase causing it to rotate Each time it rotates, the enzyme ATP synthase grabs a low energy ADP and attaches a phosphate, forming high-energy ATP

The Energy Totals http://www. science. smith The complete breakdown of 1 glucose molecule through cellular respiration results in the production of 36 molecules of ATP

Overview…In the Presence of Oxygen

Respiration in the Absence of Oxygen When oxygen is NOT present, glycolysis is followed by a different pathway called fermentation. Fermentation releases energy from food molecules in the absence of oxygen Because fermentation does not require oxygen, it is said to be anaerobic The 2 main types of fermentation are: alcoholic fermentation lactic acid fermentation

No Oxygen Fermentation – aside from the original 2 ATP’s made during glycolysis, the only energy produced is that which is in the bonds of… ethyl alcohol -- C2H6O and lactic acid – C3H5O3

Lactic Acid Fermentation Lactic Acid Fermentation converts glucose into lactic acid. This type of fermentation occurs in human muscle cells during strenuous exercise when breathing cannot supply the cells with enough oxygen. In the absence of oxygen, a cell will use fermentation to produce ATP by substrate-level phosphorylation. Lactic Acid Fermentation converts glucose into lactic acid. This type of fermentation occurs in human muscle cells during strenuous exercise when breathing cannot supply the cells with enough oxygen.

Overview of Cellular Respiration A. Glycolysis – *takes place in cytosol *breaks glucose into 2 molecules of pyruvate *produces a net of 2 ATP’s B. Krebs cycle - *takes place in mitochondrial matrix *makes a derivative of pyruvate into carbon dioxide C. ETC and Oxidative Phosphorylation – *takes place in inner membrane of mitochondrion *accepts e-’s from A and B via NADH *at end, e-’s are combined with H+ and oxygen to form water *forms a net of 34 ATP’s D. Fermentation Pathways *takes place in cytosol *2 types: Alcoholic and Lactic Acid Fermentation *stores most of the energy in chemical bonds *no production of ATP

Electron Transport Chain (34 ATP) Glycolysis (2 ATP) No oxygen present (Anaerobic respiration) Oxygen present (Aerobic respiration) Fermentation pathways Krebs Cycle (2 ATP) (Citric Acid Cycle) 2 types: Alcoholic fermentation Lactic Acid fermentation *found in yeasts and a few other microorganisms Electron Transport Chain (34 ATP) *found in most heterotrophs *involves 2 more “shopping cart” molecules – NADH and FADH2

Figure 9.18 Pyruvate as a Key Juncture in Catabolism Glycolysis is common to fermentation AND respiration. The end product of glycolysis is pyruvate…represents a fork in the catabolic pathways of glucose oxidation. In a cell capable of both respiration and fermentation, pyruvate is committed to one of those two pathways, usually depending on the presence of oxygen. Glycolysis is common to fermentation AND respiration. The end product of glycolysis is pyruvate…represents a fork in the catabolic pathways of glucose oxidation. In a cell capable of both respiration and fermentation, pyruvate is committed to one of those two pathways, usually depending on the presence of oxygen.

Figure 9.6 An overview of cellular respiration (Layer 1) During glycolysis, each glucose molecule is broken down into two molecules of the compound pyruvate, The pyruvate crosses the double membrane of the mitochondrion to enter the matrix…

Figure 9.6 An overview of cellular respiration (Layer 2) The Krebs cycle decomposes pyruvate into Carbon Dioxide.

Figure 9.6 An overview of cellular respiration (Layer 3) NADH or FADH2 transfers electrons from molecules undergoing glycolysis and the Krebs cycle to the electron transport chains…which are built into the inner mitochondrial membrane. The ETC converts the chemical energy to a form used to drive oxidative phosphorylation.

Figure 9.12 A summary of the Krebs cycle Inputs & outputs as pyruvate is broken down to 3 molecules of CO2…including the molecule of CO2 released during pre-Krebs cycle conversion of pyruvate to acetyl CoA. Cycle generates 1 ATP per turn by substrate phosphorylation…but most of the chemical energy is transferred during the redox reactions to NAD+ and FAD. The reduced coenzymes, NADH and FADH2, shuttle their cargo of high-energy electrons to the electron transport chain, which uses the energy to synthesize ATP by oxidative phosphorylation.

Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis

Figure 9.16 Review: how each molecule of glucose yields many ATP molecules during cellular respiration: http://www.wadsworthmedia.com/biology/0495119814_starr/big_picture/ch07_bp.html