The process by which cells harvest the energy stored in food Cellular respiration The process by which cells harvest the energy stored in food

SAVING FOR A Rainy Day

Feel the Burn

How do living organisms fuel their actions How do living organisms fuel their actions? Cellular respiration: the big picture

ATP Adenine Ribose 3 Phosphate groups

ATP ATP Adenosine diphosphate (ADP) + Phosphate Energy Adenosine diphosphate (ADP) + Phosphate Adenosine triphosphate (ATP) Partially charged battery Fully

Fermentation (without oxygen) Chemical Pathways Section 9-1 Glucose Glycolysis Krebs cycle Electron transport Fermentation (without oxygen) Alcohol or lactic acid

Cellular Respiration: The Big Picture Energy (ATP) C6H12O6 + 6O2 →↑→ 6CO2 + 6H2O Glucose + 6 Oxygen →↑→ 6 Carbon dioxide + 6 Water

Cellular Respiration: The big picture All living organisms extract energy from the chemical bonds of molecules (which can be considered “food”) through a process called cellular respiration. To generate energy, fuels such as glucose and other carbohydrates, proteins, and fats are broken down in three steps: (1) glycolysis, (2) the Krebs cycle, and (3) the electron transport chain. Figure 4-34 The steps of cellular respiration: from glucose to usable energy. 9

Cellular Respiration: The Big Picture Section 9-1 Mitochondrion Electrons carried in NADH Electrons carried in NADH and FADH2 Pyruvic acid Glucose Electron Transport Chain Krebs Cycle Glycolysis Mitochondrion Cytoplasm

Four-Step Process Glycolysis (terribly inefficient) occurs in the cytoplasm; the Krebs cycle and the electron transport chain occur in the mitochondria. Acetyl-CoA production is a necessary preparatory step to the Krebs cycle. Figure 4-34 The steps of cellular respiration: from glucose to usable energy. 11

Electron Transport Chain Cellular Respiration Section 9-2 Glucose (C6H1206) + Oxygen (02) 1 Glycolysis 2 Acetyl CoA 3 Krebs Cycle 4 Electron Transport Chain Carbon Dioxide (CO2) + Water (H2O)

Cellular Respiration Requires (1) fuel and (2) oxygen.

Cellular Respiration

In Humans… our cells can extract some of the energy stored in the bonds of the food molecules Energy Bonds Molecules

Aerobic Respiration – the video

Glycolysis is the universal energy-releasing pathway splitting (lysis) of sugar (glyco) All organisms single-celled organisms

Glycolysis is the universal energy-releasing pathway

Glycolysis Three of the ten steps yield energy High-energy electrons are transferred to NADH Net result: pyruvate ATP molecules NADH molecules

Glycolysis Glucose (6C) is broken down into 2 PGAL (Phosphoglyceraldehyde – 3 Carbon molecules) Cost: 2 ATP

Glycolysis 2 PGAL (3C) are converted to 2 pyruvates Result: 4 ATP, 2 NADH net ATP production = 2 ATP

How Glycolysis Works Animation

Glycolysis: The Movie

Glycolysis is very inefficient Pyruvate can be further metabolized to yield more energy Answer: 4 25

The mitochondrion

The “bag-within-a-bag” Material inside the mitochondrion can lie in one of two spaces: (1) the intermembrane space, which is outside of the inner bag, or (2) the mitochondrial matrix, which is inside the inner bag. With two distinct regions separated by a membrane, the mitochondrion can create higher concentrations of molecules in one area or the other. And because a concentration gradient is a form of potential energy—molecules move from the high concentration area to the low concentration area the way water rushes down a hill—once a gradient is created, the energy released as the gradient equalizes itself can be used to do work. In the electron transport chain, this energy is used to build the energy-rich molecule ATP. Figure 4-32 “A bag within a bag.” 27

The Preparatory Phase to the Krebs Cycle The two pyruvate molecules move into the mitochondria and then undergo three quick modifications that prepare them to be broken down in the Krebs cycle: Modification 1. Each pyruvate molecule passes some of its high-energy electrons to the energy-accepting molecule NAD+, building two molecules of NADH. Modification 2. Next, a carbon atom and two oxygen atoms are removed from each pyruvate molecule and released as carbon dioxide. The CO2 molecules diffuse out of the cell and, eventually, out of the organism. In humans, for example, these CO2 molecules pass into the bloodstream and are transported to the lungs, from which they are eventually exhaled. Modification 3. In the final step in the preparation for the Krebs cycle, a giant compound known as coenzyme A attaches itself to the remains of each pyruvate molecule, producing two molecules called acetyl-CoA. Each acetyl-CoA molecule is now ready to enter the Krebs cycle. Figure 4-30 Preparations of pyruvate. 28

The Conversion of Pyruvate to Acetyl Co-A for Entry Into the Krebs Cycle glycolysis (cytoplasm), pyruvic acid  interior of mitochondrion

The Conversion of Pyruvate to Acetyl Co-A for Entry Into the Kreb's Cycle 2 NADH are generated 2 CO2 are released

The Krebs Cycle extracts energy from sugar

There are eight separate steps in the Krebs cycle There are eight separate steps in the Krebs cycle. Three general outcomes are depicted here. Outcome 1. A new molecule is formed. Acetyl-CoA adds its two-carbon acetyl group to a molecule of the starting material of the Krebs cycle, a four-carbon molecule called oxaloacetate. This process creates a six-carbon molecule. Outcome 2. High-energy electron carriers (NADH) are made and carbon dioxide is exhaled. The six-carbon molecule then gives electrons to NAD+ to make the high-energy electron carrier NADH. The six-carbon molecule releases two carbon atoms along with four oxygen atoms to form two carbon dioxide molecules. This CO2 is carried by the bloodstream to the lungs from which it is exhaled into the atmosphere. Outcome 3. The starting material of the Krebs cycle is re-formed, ATP is generated, and more high-energy electron carriers are formed. After the CO2 is released, the four-carbon molecule that remains from the original pyruvate-oxaloacetate molecule formed in Outcome 1 is modified and rearranged to once again form oxaloacetate, the starting material of the Krebs cycle. In the process of this reorganization, one ATP molecule is generated and more electrons are passed to one familiar high-energy electron carrier, NADH, and a new one, FADH2. The formation of these high-energy electron carriers increases the energy yield of the Krebs cycle. One oxaloacetate is reformed, the cycle is ready to break down the second molecule of acetyl-CoA. Two turns of the cycle are necessary to completely dismantle our original molecule of glucose. Figure 4-31 Overview of the Krebs cycle. 32

The Kreb’s Cycle extracts energy from sugar 6 NADH 2 FADH2 2 ATP 4 CO2 (to atmosphere)

The Krebs Cycle extracts energy from sugar

The Kreb’s Cycle extracts energy from sugar Animation

Krebs: The Movie

Krebs: The Movie (Part 2)

the electron transport chain 2 key features of mitochondria

the electron transport chain 2 mitochondrial spaces  higher concentrations of molecules in one area or the other

The “bag-within-a-bag” Material inside the mitochondrion can lie in one of two spaces: (1) the intermembrane space, which is outside of the inner bag, or (2) the mitochondrial matrix, which is inside the inner bag. With two distinct regions separated by a membrane, the mitochondrion can create higher concentrations of molecules in one area or the other. And because a concentration gradient is a form of potential energy—molecules move from the high concentration area to the low concentration area the way water rushes down a hill—once a gradient is created, the energy released as the gradient equalizes itself can be used to do work. In the electron transport chain, this energy is used to build the energy-rich molecule ATP. Figure 4-32 “A bag within a bag.” 40

Follow the Electrons, as We Did in Photosynthesis Start with the electrons carried by NADH and FADH2. Over-the-counter NADH pills provide energy to sufferers of chronic fatigue syndrome. Why might this be? Figure 4-33 The big energy payoff #2) This proton concentration gradient represents a significant source of potential energy! 41

Proton Gradients and Potential Energy

Electron Transport: The Movie

Electron Transport: The Movie (Part 2)

Figure 4-34 The steps of cellular respiration: from glucose to usable energy.

Review of Cellular Respiration Review Animation

Energy is obtained from a molecule of glucose in a stepwise fashion. Answer: 2 47

Plants have both chloroplasts and mitochondria. Answer: 3 48

The Fate of Pyruvate Yeast: pyruvic acid is decarboxylated and reduced by NADH to form a molecule of carbon dioxide and one of ethanol accounts for the bubbles and alcohol in, for examples, beer and champagne (alcoholic fermentation) process is energetically wasteful because so much of the free energy of glucose (~95%) remains in the alcohol (a good fuel!) Red blood cells and active muscles: pyruvic acid is reduced by NADH forming a molecule of lactic acid (lactic acid fermentation) process is energetically wasteful because so much free energy remains in the lactic acid molecule Mitochondria: pyruvic acid is oxidized completely to form CO2 & H2O (cellular respiration) ~ 40% of energy in original glucose molecule is trapped in molecules of ATP

Alternative Pathways to Energy Rapid, strenuous exertion  O2 deficiency

Alternative Pathways to Energy NAD+ /FAD+ halted back-up method for breaking down sugar lactic acid

Alternative Pathways to Energy Acquisition Animation: Lactic Acid Fermentation

Alternative Pathways to Energy Acquisition Yeast acetaldehyde produce alcohol only in the absence of oxygen

Alternative Pathways to Energy Acquisition Animation: Alcoholic Fermentation

With rapid, strenuous exertion, our bodies soon fall behind in delivering oxygen from the lungs to the bloodstream to the cells and finally to the mitochondria. Oxygen deficiency then limits the rate at which the mitochondria can break down fuel and produce ATP. This slowdown in ATP production occurs because the electron transport chain requires oxygen as the final acceptor of all the electrons that are generated during glycolysis and the Krebs cycle. If oxygen is in short supply, the electrons from NADH (and FADH2) have nowhere to go. Consequently, the regeneration of NAD+ (and FAD+) in the electron transport chain is halted, leaving no recipient for the high-energy electrons harvested from the breakdown of glucose and pyruvate, and the whole process of cellular respiration can grind to a stop. Organisms don’t let this interruption last long, though; most have a back-up method for breaking down sugar. Among animals, there is one willing acceptor for the NADH electrons in the absence of oxygen: pyruvate, the end product of glycolysis. When pyruvate accepts the electrons, it forms lactic acid. Figure 4-36 Energy production compared: with and without oxygen. 55

anaerobic respiration Answer: 5 56

Cells can run on protein and fat as well as on glucose 4-17. Eating a complete diet: cells can run on protein and fat as well as on glucose. Evolution has built humans and other organisms with the metabolic machinery that allows them to extract energy and other valuable chemicals from proteins, fats, and a variety of carbohydrates. For that reason, we are able to consume and efficiently utilize meals comprising various combinations of molecules: Sugars: In the case of dietary sugars, many are polysaccharides—multiple simple sugars linked together—rather than solely the simple sugar glucose. Before they can be broken down by cellular respiration, the polysaccharides must first be separated by enzymes into glucose or related simple sugars that can be broken down by cellular respiration. Lipids: Dietary lipids are broken down into their two constituent parts: a glycerol molecule and a fatty acid. The glycerol is chemically modified into one of the molecules produced during one of the ten steps of glycolysis. It then enters glycolysis at that step in the process and is broken down to yield energy. The fatty acids, meanwhile, are chemically modified into acetyl-CoA, at which point they enter the Krebs cycle. Proteins: Proteins are chains of amino acids. Upon consumption they are broken down chemically into their constituent amino acids. Once that is done, each amino acid is broken down into (1) an amino group that may be used in the production of tissue or excreted in the urine and (2) a carbon compound that is converted into one of the intermediate compounds in glycolysis or the Krebs cycle, allowing the energy stored in its chemical bonds to be harnessed. 57