Biochemistry of Respiration

Oxidative breakdown of organic molecules to store energy as ATP

Animals and plants respire; FAD and NAD are coenzymes

Aerobic respiration

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

Complete oxidation of an organic substrate to CO2 and H2O using free O2

Production of CO2, NADH + H+ and FADH + H+, 38ATP

1) Glycolysis → cytoplasm

Glucose enters cell by facilitated diffusion

ATP activates glucose to produce 2 unstable compounds

Substrate-level phosphorylation produces 4ATP

Net yield of 2ATP and 2reducedNAD per glucose molecule

2) Link reaction → matrix of mitochondria

Pyruvate enters matrix of mitochondrion for further -

Net yield of 2reducedNADH per glucose

 3) Krebs cycle → matrix of mitochondria

Citrate is gradually broken down to re-form oxaloacetate

Substrate-level phosphorylation forms 2ATP

Removal of hydrogen from respiratory substrate

Net yield of 2ATP, 2reducedFADH, 6reducedNADH per glucose

4) Electron Transport Chain ETC → inner membrane/cristae of mitochondria

Reduced coenzymes arrive at ETC

Split into coenzyme + 2H+ + 2e- by hydrogen carriers

2e- are transferred to electron carriers (cytochrome)

Pass down ETC by redox reaction and release energy as they go

Energy produces ATP by oxidative phosphorylation

Final electron acceptor 1/2O2 is reduced by 2H+ and 2e- to produce H2O

Net yield of 34ATP (30NADH, 4FADH) per glucose

//Cytochromes are iron-containing proteins → cytochrome a3 also contains copper and is irreversibly damaged by cyanide

IMG 5-14-8

Anaerobic respiration (fermentation)

Substrate-level phosphorylation: 2ADP + 2Pi → 2ATP directly by enzymes in glycolysis

No O2 to accept electrons from NADH + H+ → no Krebs cycle or ETC

NADH + H+ reduces (gives off H+ ions to) pyruvate to produce

Lactate C3 in animal cells → can be re-oxidised

Ethanol C2 in yeast cells → irreversible, CO2(g) lost

Regenerates NAD

NAD can be re-used to oxidise more RS/allows glycolysis to continue

Can still form ATP/release energy when O2 is in short supply

Role of ATP

Adenosine (ribose + adenine) triphosphate (3 phosphate groups)

Produced by adding Pi to ADP → phosphorylation

Breaks down to ADP (adenosine diphosphate) and Pi (inorganic phosphate ion) by hydrolysis

ATP is useful as an immediate energy source/carrier because

Energy release only involves a single reaction

Energy released in small quantities

Easily moved around inside cells, but cannot pass through cell membranes

Light-dependent reaction cannot be the only source of ATP

"Photosynthesis cannot produce ATP in the dark

Need more ATP than can be produced in photosynthesis

Cannot be produced in plant cells lacking chlorophyll

ATP cannot be transported"1

Central molecule in metabolism (ATP hydrolysis)

Muscle contraction → changes of position of myosin head relative to actin

Protein synthesis → ATP "loads" amino acids onto tRNA

Active transport → driven by phosphorylation of membrane-bound proteins

Calvin Cycle → cyclic reduction of CO2 to TP

Nitrogen fixation → involves ATP-driven reduction of molecular nitrogen

ATP in liver is used for active transport / phagocytosis / synthesise of glucose, protein, DNA, RNA, lipid, cholesterol / urea in glycolysis / bile production / cell division

Brown fat

White fat insulates the body and reduces heat loss

Brown fat cells in mitochondrial membrane produce heat

Mitochondria in other tissue / chemiosmosis

H+ ions pass back from space between two mitochondrial membranes into matrix

Through pores which are associated with the enzyme ATP synthetase

Energy from the ETC will be used to produce ATP

Mitochondria in brown fat

H+ ions flow back through channels not associated with ATP synthetase

Energy produces heat instead of ATP

Found in chest, larger arteries for heat distribution round the body or in hibernating mammals