Chapter 9: Cellular Respiration

Chemical Energy and Food Food = chemical building blocks living things need to grow and reproduce Broken down gradually Capture little bits of energy at key steps Use energy stored in chemical bonds of foods to produce ATP which powers the activities of the cell

Overview of Cellular Respiration: It is the process that releases energy by breaking down food molecules in the presence of oxygen 6O2 + C6H12O6  6CO2 + 6H2O + energy (ATP) oxygen + glucose  carbon dioxide + water + energy (ATP) A cell needs to trap those little pieces of energy to make ATP Takes 3 main steps Glycolysis Krebs Cycle (Citric Acid Cycle) Electron Transport System

Step 1: Glycolysis = “sugar breaking” Glucose  Pyruvate (pyruvic acid) The beginning of the energy pathway in respiration Takes place in the CYTOPLASM One molecule of glucose is broken in half, producing 2 molecules of pyruvic acid (a 3-carbon compound) Uses 2 ATP molecules to get the process started In the end 4 molecules will have been produced Net gain = 2 molecules of ATP (a relatively small amount)

Step 1: Glycolysis Glucose  Pyruvate Produces 2 molecules of pyruvate 2 molecules of ATP Advantages Happens quickly – can produce 1000s of ATPs in a few milliseconds Does not require oxygen If oxygen is present leads to 2 other pathways that release lots of energy = aerobic respiration If no oxygen, a different pathway is followed = anaerobic respiration

Aerobic Respiration (oxygen present)

Step 2: Krebs Cycle (Citric Acid Cycle) Occurs in the mitochondria Pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions 1st step – citric acid production Pyruvic acid passes through the mitochondrial membrane into the matrix Pyruvic acid – CO2 = Acetyl CoA Acetyl CoA + 4-C compound = Citric Acid

Step 2: Krebs Cycle (Citric Acid Cycle) Cha-Ching –O- Meter ATP NADH FADH2 2nd step – stepwise energy extraction, release of CO2 at each step 4-C molecule replenished at end, cycle can continue

Step 3: Electron Transport Chain Cha-Ching –O- Meter ATP NADH FADH2 Occurs in the MITCHONDRIAL MEMBRANE High energy electrons arrive in NADH and FADH2 from Krebs and Gylcolysis Movement of electrons down the ETC moves H into the mitochondrial membrane, establishes a concentration gradient Hydrogen moves back out through ATP synthase, spinning the turbine and creating ATP (massive amounts of ATP!!!) Requires O2; oxygen is the final electron acceptor at the end of the ETC Approximately 32-34 ATPs are generated for every glucose

Overview of Cellular Respiration It is the process that releases energy by breaking down food molecules in the presence of oxygen 6O2 + C6H12O6 6CO2 + 6H2O + Energy oxygen + glucose carbon dioxide + water + energy A cell needs to trap those little pieces of energy to make ATP Takes 3 main steps – glycolysis, krebs cycle, electron transport system

The beginning of the energy pathway in respiration Step 1: Glycolysis The beginning of the energy pathway in respiration The process in which one molecule of glucose is broken in half, producing 2 molecules of pyruvic acid (a 3-carbon compound) Uses 2 ATP molecules to get the process started In the end 4 molecules will have been produced Net gain = 2 molecules of ATP (a relatively small amount) If oxygen is present leads to 2 other pathways that release lots of energy = aerobic respiration If no oxygen, a different pathway is followed = anaerobic respiration

Aerobic Respiration – oxygen present

Step 2: Krebs Cycle/Citric Acid Cycle 90% of the chemical energy available in glucose is still unused at the end of glycolysis It is locked in the pyruvic acid Pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions 1st step – citric acid production 2nd step – energy extraction

Step 3: Electron Transport Chain Uses the high energy electrons from the Krebs cycle to convert ADP into ATP Don’t worry about all the steps Produces 34 more ATP molecules per glucose

Summary of ATP totals

Why do we need all this ATP? Cells need energy to work Primary fuel is ATP (adenosine triphosphate) ATP = nitrogen containing compound (adenine), 5-carbon sugar (ribose), and 3 phosphate groups ADP = same as ATP but with 2 phosphate groups The 3rd phosphate is the key to how cells store energy – when the cell has extra energy available it adds a phosphate to make 3 and when it needs energy it can take a phosphate away Releasing Energy from ATP Energy is released from ATP when ATP is changed into ADP + phosphate ATP = compressed spring (unstable) ADP = relaxed spring (stable) Using Biochemical Energy Active transport Powers movement of organelles within the cell ATP and Glucose Cells only contain a small store of ATP because it is not efficient Sugar can store up to 90 times the energy than ATP Cells use the energy in carbohydrates to generate ATP form ADP as needed

Anaerobic Respiration – no oxygen present Alcoholic Fermentation Used by yeasts and a few other microorganisms Pyruvic acid + NADH alcohol + CO2 + NAD+ Cause bread dough to rise When the yeast runs out of oxygen , it begins to ferment, giving off carbon dioxide Makes the bread rise by forming small air pockets in the bread Alcohol evaporates when the bread is baked Lactic Acid Fermentation If pyruvic acid accumulates it can be converted to lactic acid Pyruvic acid + NADH lactic acid + NAD+ Lactic acid is produced in your muscles when you exercise when the body can’t supply enough oxygen to the tissues Your muscles start to produce ATP by lactic acid fermentation and lactic acid builds up in your muscles causing a painful burn

Photosynthesis vs. Respiration Almost opposite reactions Photosynthesis deposits energy Respiration withdraws energy The products of photosynthesis are the reactants for respiration Respiration takes place in all eukaryotes and some prokaryotes Photosynthesis takes place in plants, algae, and some bacteria