Bio 9

  • Living cells require energy from outside sources

  • Energy flows into an ecosystem as sunlight and leaves as heat

  • Photosynthesis generates O2 and organic molecules which are used in cellular respiration

  • Cells use chemical energy stores in organic molecules to regenerate ATP, which powers work

  • In respiration, electron transfer is key

  • 4 key pathways in aerobic cellular respiration

    • Glycolysis

    • Pyruvate oxidation

    • Citric acid cycle

    • Oxidative phosphorylation

  • Formula for Respiration

    • C6H12)6 → 6CO2 + 6H20 + ATP + heat

  • Principle of Redox

    • oxidation : substance loses electrons

    • Reduction: substance gains electrons

  • During cellular respiration, the fuel (such as glucose) is oxidized and O2 is reduced\

    • Product → ATP and heat

  • Electron Transport Chain

    • Electrons from organic compounds are usually first transferred to NAD+

      • As an electron acceptor, NAD+ functions as an oxidising agent during cellular respiration

      • Each NADH (reduced form of NAD+) represents stored energy that is tapped to synthesize ATP

  • Cellular respiration allows for a controlled release of energy for ATP synthesis by using electron transport chain

    • Control and storage of energy

  • Stages of cellular respiration:

    • 1. Glycolysis

    • 2. Pyruvate oxidation and citric acid cycle

    • 3. Oxidative phosphorylation and chemiosmosis

  • Glycolysis:

    • Harvests chemical energy by oxidizing glucose to 2 molecules of pyruvate

    • Occurs in cytoplasm

    • 2 phases

      • Energy investment phase

      • Energy payoff phase

    • Oxygen independent

  • Recognize starting and finishing materials; recognize two phases; describe which organisms perform is and where it occurs in the cell

  • After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules

  • Citric Acid Cycle-The Krebs cycle

    • Completes the breakdown of pyruvate into CO2

  • Steps in the citric acid cycle

    • 8 steps eac catalyzed by a specific enzyme

    • Acetyl group of acetyl CoA joins cycle by combining with oxaloacetate, forming citrate

    • Next 7 steps decompose the citrate back to oxaloacetate, making the process a cycle

    • The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain

    • 1 molecule of ATP for each turn of the cycle

    • Know the products of the cycle; it occurs in mitochondria; how much of what is made per turn

  • During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

    • Following glycolysis and the cycle, NADH and FADH2 account for most of the energy extracted from the food

    • These 2 electron carriers donate electrons to the ETC, which powers ATP synthesis via oxidative phosphorylation

  • The Pathway of Electron Transport

    • In Cristae of mitochondria

    • Most of chains components are proteins

      • Multiprotein complexes

      • Cytochromes

    • Carriers alternate reduced and oxidized

    • Electrons drop in free energy as they go down the chain

      • Breaks the large free-energy drop from food to )2 into smaller steps that release energy in manageable amounts

    • Finally passed to )2, forming H2O

      • ETC generates no ATP directly

  • Chemiosmosis: the energy-coupling mechanism

    • Electron transfer in the ETC causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space

    • H+ then moves back across the membrane, passing through the proton pump, ATP synthase

      • Uses the exergonic flow of H+ to drive phosphorylation of ATP

    • This is an example of chemiosmosis, the use of energy in an H+ to drive cellular work

  • Proton Pump: ATP Synthase

    • Energy stored in an H+ gradient across a membrane couples the redox reactions of the ETC to ATP synthesis

    • The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work

  • Fermentation and Anaerobic respiration enable cells to produce ATP without the use of oxygen

    • Most cellular respiration requires O2 to produce ATP

    • Without O2, the ETC will cease to operate

  • Anaerobic Respiration

    • Uses an ETC with a final electron acceptor other than O2

    • Much less ATP

    • Fermentation uses substrate level phosphorylation instead of an ETC to produce ATP

  • Types of Fermentation

    • Consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis

    • Two common types

      • Alcohol fermentation: beer, wine, bread

      • Lactic acid fermentation: muscle, cheese

  • Alcohol Fermentation

    • Pyruvate is converted to ethanol in two steps, with the first releasing CO2

    • AF by yeast is used in brewing, winemaking, and baking

  • Lactic Acid fermentation

    • Pyruvate is reduced by NADH< forming lactate as an end product, with no release of CO2

    • LAF by some fungi and bacteria can be used to make cheese and yogurt

    • Human muscle cells use lactic acid to generate ATP

      • White skeletal muscle produces lactate from pyruvate

      • Red muscle oxidizes lactate to pyruvate

      • Fast, less energy efficient production of ATP

      • Quick but low production of ATP

  • Cori Cycle

    • Lactate is converted back into glucose in the liver

  • Anaerobic and Aerobic Respiration vs. Fermentation

    • All use glycolysis to oxidize glucose and harvest chemical energy of food

    • In all three, NAD+ is the oxidizing agent (gets reduced into NADH) that accepts electrons during glycolysis

    • Different final electron acceptors:

      • Organic molecule such as pyruvate in fermentation

  • What to do with pyruvate?

    • Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2

    • Yeast and many bacteria are facultative anaerobes

      • Use fermentation or cellular respiration

  • The versatility of Catabolism

    • Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration

    • Glycolysis accepts a wide range of carbohydrates

    • Proteins must be digested into amino acids; amino groups are removed

      • The rest of the molecule can feed into glycolysis or the citric acid cycle

  • Fats

    • Digested to glycerol and fatty acids

    • Fatty acids are broken down by beta oxidation and yield acetyl CoA

    • An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate

  • Biosynthesis

    • Body uses small molecules to build other substances

    • Small molecules come directly from food, glycolysis, or the citric acid cycle

  • Regulation of Cellular Respiration via Feedback Mechanisms

    • Feedback inhibition is the most common mechanism for control

      • If ATP concentration begins to drop, respiration speeds up

      • When there is plenty of ATP, respiration slows down

    • Control of catabolism is based mainly on regulation the activity of enzymes at strategic points in the catabolic pathway

  • See summary of glycolysis, pyruvate oxidation, and citric acid cycle slides

  • Maximum number of ATP a cell can make with 1 molecule of glucose: 32