cellular respiration biology 1

Cellular respiration and fermentation are catabolic (energy yielding) pathways Redox reactions release energy when electrons move closer to electronegative atoms Electrons ‘fall’ from organic molecules to oxygen, stepwise, via NAD + and an electron transport chain Cellular respiration consists of –Glycolysis –Krebs Cycle –Electron transport chain Fermentation - an anaerobic alternative

Fermentation and Respiration Fermentation is an anaerobic ATP producing catabolic pathway Cellular respiration is an aerobic catabolic pathway, where O2 acts as the final electron acceptor –Summarized as: C 6 H 12 O 6 + 6O 2 6H 2 O + 6CO 2 + energy –Energy from respiration is used to recycle ADP to ATP

Respiration as a redox reaction Oxidation = partial or complete loss of electrons Reduction = partial or complete gain of electrons Redox reaction = shunt of electrons from one reactant to another. e.g., in respiration, –O 2 (oxidizing agent) receives electrons from sugar (oxidized) –Sugar (reducing agent) donates electrons to O 2 (reduced) –Movement of electrons to more electronegative state causes loss of potential energy, and therefore release of energy

In respiration, hydrogen is transferred to oxygen, and carbon is oxidized C 6 H 12 O 6 + 6O 2 6H 2 O + 6CO 2 + energy –Carbohydrates and fats are excellent energy stores because they are rich in C-H bonds –Respiration does not occur in one explosion - its done stepwise so that energy can be harnessed at each step –1 mole of glucose = 2870 kJ of energy –Catabolic pathway of respiration is aided by enzymes that lower the activation energies of the reactions oxidation reduction

How energy is harnessed in respiration Remember that energy from respiration comes from electrons falling from a high potential energy to a lower potential energy This fall is performed stepwise Electrons are not passed directly to O2, but are picked up by an electron acceptor, NAD + (nicotinamide adenine dinucleotide), which acts as an interim oxidizing agent –NAD + is aided by dehydrogenases that remove a pair of hydrogen atoms –2 electrons and one proton go to NAD + (becomes NADH) –Remaining proton ‘floats’ Purpose of first two stages of respiration is to produce NADH, which goes to an Electron Transport Chain, which is the main source of ATP production in cellular respiration

Respiration Stage 1: Glycolysis Converts 1 molecule of glucose (hexose sugar) to 2 molecules of pyruvate (triose sugar) in 10 steps Requires initial investment of 2 ATP(energy investment phase) Yields 4ATP (net gain = 2 ATP), and 2 NADH (energy yield phase) Conversion is through series of substrate- level phosphorylations and enzymes Occurs in the cytoplasm

A summary of Glycolysis C 6 H 12 O 6 2 C 3 H 4 O 3 (pyruvate) + 2 NAD NADH + 2 H ADP + 2 P i + 2 ATP + 2 H 2 O

Respiration Stage 2: The Krebs Cycle Completes the energy yielding oxidation of pyruvate Occurs in mitochondrion –Translocation across mitochondrial membrane by multienzyme complex. This results in (per molecule of glucose) Release of 1 CO 2 Reduction of 1 NAD + to NADH Attachment of coenzyme A –Forms Acetyl Coenzyme A

Acetyl Coenzyme A enters into Krebs cycle (in mitochondrial matrix), where remaining acetyl groups are oxidized The Krebs cycle is an energy mill that produces (per molecule of pyruvate) –2 CO 2 –3 NADH –1 FADH –1 ATP Regenerates CoA Two turns of Krebs cycle required to oxidize 1 molecule of glucose

Head count so far... Per molecule of glucose:

The electron transport chain (ETC) All ATP produce so far by substrate-level phosporylation (not much!) A majority of ATP production is via oxidative phosphorylation in the ETC Analogy: the ETC is like a salmon ladder operating in reverse. Each step represents a level of potential energy. Electrons ‘fall’ down the ladder to reach their lowest potential state (ie bound to O2. Each ‘fall’, releasing some potential energy, is used to convert ADP TO ATP

The ladder starts with NADH donating its electrons to the first ‘rung’ - an electron carrier Each successive rung is an electron carrier of increasing electronegative potential. Electron carriers include: –Flavoproteins –Iron-sulfur proteins –Cytochromes FADH donates its electrons further down the ladder

How does the ETC harness energy - chemiosmosis The ETC generates a proton gradient: released potential energy is used to pump a proton (H + ) across the inner membrane of a mitochondrion into its intermembrane space H + can’t leak back across membrane - it has to pass through a specific gate - a protein (enzyme) called ATP-synthase ATP-synthase uses proton gradient to convert ADP to ATP

The final count (per molecule of glucose)

Fermentation Glycolysis oxidizes glucose to pyruvate using NAD +, not oxygen Alcohol fermentation: glucose is reduced to ethanol Lactic Acid fermentation: glucose is reduced to lactate Organisms may be obligate aerobes, obligate anaerobes, or facultative aerobes

The control of respiration Catabolic pathways are controlled by regulating enzymes at key points Key step is 3rd stage of glycolysis, catalyzed by phosphofructokinase –Sensitive to ratio of ATP:ADP –Citrate (produced in Krebs) and ATP are allosteric inhibitors of Phosphofructokinase –Other allosteric enzymes that control the rate of cellular respiration