Cellular Respiration

What is Cellular Respiration? The process of converting food energy into ATP energy C 6 H 12 O O 2 → 6 CO H 2 O + 36 ATP

Why are both Photosynthesis and Cell Respiration important to Ecosystems? Light is the ultimate source of energy for all ecosystems Chemicals cycle and Energy flows Photosynthesis and cellular respiration are opposite reactions

Why do plants need both chloroplasts and mitochondria? Chloroplasts use energy from the sun to make glucose Mitochondria convert glucose to ATP—the energy currency of the cell

Definitions: Oxidation-Reduction Definitions: Oxidation-Reduction = transfer of electrons from electron donor to electron acceptor Example: A:H + B = A + B:H donor acceptor donor electron donor (reducing agent, reductant) is itself oxidized acceptor electron acceptor (oxidizing agent, oxidant) is itself reduced

-- both oxidation and reduction must occur simultaneously --one compound donates electrons to (reduces) a second compound; the second accepts electrons from (oxidizes) the first. Redox Redox/oxidation-reduction -- electrons always move from compounds with lower reduction potential to compounds with higher reduction potential ( more positive).

Biological redox Biological redox = Two half-reactions A:H A Reductant  Oxidant + e - B B:H Oxidant + e -  Reductant (acceptor) (donor) Standard reduction potential, E° -- measure of the tendency of oxidant to gain electrons, to become reduced, a potential energy.

Biological redox Biological redox = Two half-reactions A:H A Reductant  Oxidant + e - B B:H Oxidant + e -  Reductant (acceptor) (donor) Standard reduction potential, E° -- measure of the tendency of oxidant to gain electrons, to become reduced, a potential energy.

*********************************************************** So, the more negative the reduction potential is, the easier a reductant can reduce an oxidant and The more positive the reductive potential is, the easier an oxidant can oxidize a reductant The difference in reduction potential must be important ************************************************************

Reduction Potential Difference =  Eº   Eº = E°  (acceptor) - E  (donor)  measured in volts.  The more positive the reduction potential difference is, the easier the redox reaction  Work can be derived from the transfer of electrons and the ETS can be used to synthesize ATP.

 The reduction potential can be related to free energy change by:  Gº = -n F  Eº where n = # electrons transferred = 1,2,3 F = 96.5 kJ/volt, called the Faraday constant

********************************************************************************* Table of Standard Reduction Potentials Oxidant + e -  reductant -- e.g., Lehninger, 5th ed., p. 511 Note:  oxidants can oxidize every compound with less positive voltage -- (below it in the Table)  reductants can reduce every compound with a less negative voltage -- (above it in the Table) *****************************************************************************************

-- Electrons can move through a chain of donors and acceptors -- In the electron transport chain, electrons flow down a gradient. -- Electrons move from a carrier with low reduction potential (high tendency to donate electrons) toward carriers with higher reduction potential (high tendency to accept electrons).

-- The overall voltage drop from NADH E  = -(-0.32 V) to O Eº = V is  Eº = 1.14 V

-- This corresponds to a large free energy change of  G  = - n F  E  = -220 kJ/mole (n =2) -- Since ATP requires 30.5 kJ/mole to form from ADP, more than enough energy is available to synthesize 3 ATPs from the oxidation of NADH.

What is ATP? Adenosine Triphosphate –5-Carbon sugar (Ribose) – Nitrogenous base (Adenine) –3 Phosphate groups Energy currency of the cell The chemical bonds that link the phosphate groups together are high energy bonds When a phosphate group is removed to form ADP and P, small packets of energy are released

How is ATP used? As ATP is broken down, it gives off usable energy to power chemical work and gives off some nonusable energy as heat. Synthesizing molecules for growth and reproduction Transport work – active transport, endocytosis, and exocytosis Mechanical work – muscle contraction, cilia and flagella movement, organelle movement

Why use ATP energy and not energy from glucose? Breaking down glucose yields too much energy for cellular reactions and most of the energy would be wasted as heat. 1 Glucose = 686 kcal 1 ATP = 7.3 kcal 1 Glucose → 36 ATP How efficient are cells at converting glucose into ATP? –38% of the energy from glucose yields ATP, therefore 62% wasted as heat.

Cellular Respiration is a Redox Reaction C 6 H 12 O O 2 → 6 CO H 2 O Oxidation is the loss of electrons or H + Reduction is the gain of electrons or H + Glucose is oxidized when electrons and H + are passed to coenzymes NAD + and FAD before reducing or passing them to oxygen. Glucose is oxidized by a series of smaller steps so that smaller packets of energy are released to make ATP, rather than one large explosion of energy. (Oxidation ) (Reduction )

Cell Respiration can be divided into 4 Parts: 1) Glycolysis 2) Oxidation of Pyruvate / Transition Reaction 3) The Krebs Cycle 4) The Electron Transport Chain and Chemiosmotic Phosphorylation Chemiosmotic Phosphorylation

Where do the 4 parts of Cellular Respiration take place? Glycolysis: –Cytosol Oxidation of Pyruvate: –Matrix The Krebs Cycled: –Matrix Electron Transport Chain and Cheimiosmotic Phosphorylation: –Cristae

Parts of the Mitochondria

Anaerobic Respiration (no oxygen required, cytoplasm) 1.Glycolysis (substrate level) Glucose  4 ATP (Net 2 ATP) 2 ATP2 NADH 2 Pyruvate Aerobic Respiration (oxygen required, mitochondria) 2. Oxidation of Pyruvate 2 Pyruvate  2 CO 2 2 NADH 2 Acetyl CoA 1.Krebs Cycle (substrate level) 2 Acetyl CoA  4 CO 2 2 ATP 6 NADH 2 FADH 2 1.Electron Transport Chain (chemiosmotic ) 10 NADH  32 ATP 2 FADH 2 6 H 2 O 6 O 2 Total: 36 ATP produced

ATP is made in two ways: 1) Substrate Level Phosphorylation (glycolysis & Krebs cycle) 2) Chemiosmotic Phosphorylation (electron transport chain) Substrate-Level Phosphorylation: Energy and phosphate are transferred to ADP using an enzyme, to form ATP. Phosphate comes from one of the intermediate molecules produced from the breakdown of glucose.

Glycolysis Glucose (C 6 ) is split to make 2 Pyruvates (C 3 ) –1 st : ATP energy used to phosphorylate glucose (stored energy) –2 nd : phosphorylated glucose broken down into two C 3 sugar phosphates –3 rd : the sugar phosphates are oxidized to yield electrons and H + ions which are donated to 2 NAD + → 2 NADH (stored electron and hydrogen for the Electron Transport Chain) –4 th : The energy from oxidation is used to make 4 ATP molecules (net 2 ATP) This is substrate level phosphorylation because an enzyme transfers phosphate to ADP making ATP Glycolysis produces very little ATP energy, most energy is still stored in Pyruvate molecules. Glucose  2 Pyruvate 2 ATP4 ATP (Net 2 ATP) 2 NADH

Oxidation of Pyruvate /Transition Reaction When Oxygen is present, 2 Pyruvates go to the matrix where they are converted into 2 Acetyl CoA (C 2 ). Multienzyme complex: –1 st: each Pyruvate releases CO 2 to form Acetate. –2 nd: Acetate is oxidized and gives electrons and H + ions to 2 NAD + → 2 NADH. –3 rd Acetate is combined with Coenzyme A to produce 2 Acetyl CoA molecules. 2 NADH’s carry electrons and hydrogens to the Electron Transport Chain. 2 Pyruvate  2 CO 2 2 NADH 2 Acetyl CoA

The Krebs Cycle / Citric Acid Cycle 8 Enzymatic Steps in Matrix of Mitochondria: Break down and Oxidize each Acetyl CoA (2-C’s) to release 2 CO 2 and yield electrons and H + ions to 3 NAD FAD → 3 NADH + FADH 2. This yields energy to produce ATP by substrate level phosphorylation. The first step of the Krebs cycle combines Oxaloacetate (4 C’s) with Acetyl CoA to form Citric Acid, then the remaining 7 steps ultimately recycle oxalacetate. Two Turns of the Krebs Cycle are required to break down both Acetyl Coenzyme A molecules. The Krebs cycle produces some chemical energy in the form of ATP but most of the chemical energy is in the form of NADH and FADH 2 which then go on to the Electron Transport Chain. 2 Acetyl CoA  4 CO 2 2 ATP 6 NADH 2 FADH 2

The Electron Transport Chain NADH and FADH 2 produced earlier, go to the Electron Transport Chain. NADH and FADH 2 release electrons to carriers/proteins embedded in the membrane of the cristae. As the electrons are transferred, H + ions are pumped from the matrix to the intermembrane space up the concentration gradient. Electrons are passed along a series of 9 carriers until they are ultimately donated to an Oxygen molecule. ½ O electrons + 2 H + (from NADH and FADH 2 ) → H 2 O. 10 NADH  32 ATP 2 FADH 2 H 2 O Oxygen m

Sequence of Respiratory Electron Carriers Inhibitors in green in green

In-depth Summary of the Site Components

Complex 1 Has NADH binding site –NADH reductase activity NADH -  NAD + –NADH ---> FMN--->FeS---> ubiquinone –ubiquinone ---> ubiquinone H 2 –4 H + pumped/NADH

NAD + /NADH NADP + /NADPH Never covalently bound- freely diffusible Nicotinamide 

Reaction at Enzyme + reduced substrate + NAD(P) +  H + oxidized substrate + NAD(P)H + H + substrate relieved of two H atoms = hydride to NAD + & H + into solution Hydride - Hydride = H - = H: + = H + + 2e - two electrons transferred reduced

NADH Dehydrogenase transfers electrons from NADH to Co Q, an electron carrier Uses two bound cofactors to accomplish this.  flavin and iron-sulfur center

Complex II succinate ---FAD—ubiquinone –Contains coenzyme Q –FADH 2 binding site FAD reductase activity FADH 2 --  FAD

For NADH, one of two entry points into the electron transport chain: -- So the oxidation of one NADH results in the reduction of one Co Q -- Another important function of the enzyme will be mentioned later.

Succinate Dehydrogenase -- similar reaction can be written yielding Co QH 2 -- second entry into electron transport -- substrate is succinate -- FAD is reduced, not FMN

Flavin-linkedDehydrogenases Flavin mononucleotide = FMN Flavin adenine dinucleotide = FAD Riboflavin = ring + ribitol I soalloxazine ring ribitol

Reduced form = FADH 2 Transfer 2 H atoms with 2e - or 2H  Reaction AH 2 + E-FMN = A +E-FMNH 2

2H + +2e Coenzyme Q Coenzyme Q = Ubiquinone  a lipid in inner membrane  carries electrons  polyisoprene tail  moves freely within membrane

Complex III ubiquinone -  ubiquinone ox while cyt C gets reduced Also contains cytochromes b –proton pump 4H + Adds to gradient –8 H + / NADH –4 H + / FADH 2

Cytochromes - proteins in ETS  Carry electrons  Contain heme or heme-like group  Heme is based on porphyrins with iron in center, usually as Fe(II), and is tightly bound at sides, sometimes covalently  Contrast heme in cytochromes & hemoglobin

--For cytochrome heme, a) all 6 sites of Fe are filled (4 from porphyrin, 2 above and below from protein) = no molecule can approach b) carries electrons only: Fe(III) + e -  Fe(II) Note: Only one electron is transferred at a time.

COMPLEX III  COMPLEX III = b, an Fe-S and c 1. Cytochrome c  Cytochrome c is mobile. COMPLEX IV  COMPLEX IV = a+a 3 = cytochrome a-a 3 cytochrome a-a 3 = cytochrome c oxidase cytochrome c oxidase -- large protein. -- both a and a 3 contain heme A and Cu -- a 3 Cu binds to oxygen and donates electrons to oxygen cytochrome a 3 cytochrome a 3 - only component of ETS that can interact with O 2

Complex IV reduction of oxygen cytochrome oxidase cyt a+a3 red ---> oxidized state oxygen ---> water –2 H e - + ½ O 2 --  2 H 2 O –transfers e - one at a time to oxygen Pumps 2H + out Pumps 2H + out –Total of 10 H + / NADH –Total of 6 H + / FADH 2