Chapter 7

Pathways of Cellular Respiration

Glucose breakdown releases energy

Cellular respiration is a redox reaction that requires C6H12O6 and O2

• Oxidation– Removal of H+

• Reduction– Addition of H+

• Glucose is oxidized to CO2

• Oxygen is reduced to H2O• Oxidation of glucose to CO2 releases energy, which is

then used for ATP production

• NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are coenzmes that aid in glucose oxidation

• (NAD+) + (2e-) + (H+) NADH

• (FAD) + (2e-) + (2H+) FADH2

• The high energy electrons in NADH and FADH2 are taken to the electron transport chain within a mitochondrion

• O2 picks up electrons at the end of the chain, then takes on H+ to become H2O

• NAD+ and FAD return to pick up more electrons

Cellular respiration involves the cytoplasm and mitochondria

4 stages of cellular respiration• Glycolysis

– In cytoplasm– No oxygen required

(anaerobic)• Preparatory reaction

– In mitochondria matrix– Requires oxygen (aerobic)

• Krebs cycle– In mitochondria matrix– Requires oxygen (aerobic)

• Electron transport chain (ETC)– In mitochondria cristae– Requires oxygen (aerobic)

In aerobic phases, oxygen is the final acceptor of electrons

A: intermembrane space

B: matrix

C: cristae

Oxidation of glucose

• Glycolysis– Glucose is broken into 2 molecules of pyruvate– NADH produced– Net 2 ATP produced

• Preparatory reaction– Pyruvate oxidized (loses H+) releasing CO2

– NADH produced– End product – 2, 2 carbon acetyle groups

• Krebs cycle (citric acid cycle)– Oxidation of glucose products– NADH and FADH2 result– CO2 released– 2 ATP produced per glucose molecule

Production of most ATP

• Electron transport chain– NADH and FADH2 bring high energy electrons

– As electrons go down chain, energy is released and captured

– O2 accepts electrons at the end of the chain, which then combines with H+ to form H2O

The chemical energy of glucose becomes the chemical energy of ATP

Glycolysis: Glucose breakdown begins

• Occurs in cytoplasm

• No oxygen required

Energy investment steps

• 2 ATP breaks glucose into 2 G3P molecules

Energy harvesting steps

• G3P oxidized (H+ removed)

• H+ combines with NAD+ to create NADH– 2 high energy electrons on NADH

• P attaches to the oxidized G3P

• P is added to ADP to create ATP

• ATP synthesis occurs again

• 2 pyruvate and 2 ATP are final products

The preparatory reaction occurs before the Krebs cycle

• The preparatory reactions– In mitochondria matrix– Pyruvate is oxidized (H+ removed) creating an acetyl group– H+ taken up by NAD+ to form NADH– Acetyl group combines with CoA and goes to the Krebs cycle– NADH goes to electron transport chain

– CO2 leaves the body

The Krebs cycle generates much NADH

• In matrix• Acetyle group removed from CoA• Acetyl group joins a 4-carbon group to create citrate• Citrate oxidized (H+ removed)

– NADH created– CO2 created

• 2 ATP formed• FAD oxidized to form FADH2

• 4 CO2, 6 NADH, 2 FADH2, and 2 ATP are final products• FADH2 and NADH goes to electron transport chains

The electron transport chain captures energy for ATP production• In mitochondria cristae

• NADH and FADH2 release electrons and H+– Electrons energy electron transport chain– NAD+ and FAD result

• Electrons release energy as they go down chain, which is used to make ATP

• Electrons combine with O2 and H+ to form H2O

• 3 ATP produced per NADH (30 ATP total)• 2 ATP produced per FADH2 (4 ATP total)

The ATP synthase complex produces ATP

• Hydrogen pumped to intermembrane space from NADH and FADH2 creating a concentration gradient

• Hydrogen then moves down the gradient into matrix via H+ channel protein

• ATP synthase enzyme is attached to H+ channel protein and produces ATP as H+ passes through protein

• ATP leaves mitochondria to go to where it is needed

The ATP payoff can be calculated• In the cytoplasm

– Glycolysis• 2 ATP

• In the mitochondria– Preparatory reaction

• No ATP produced– Krebs cycle

• 2 ATP– Electron transport chain

• 32 or 34 ATP• 3 ATP per NADH• 2 ATP per FADH2

• In many animals NADH formed in glycolysis cannot cross inner mitochondrial membrane so 1 ATP per NADH is used to move them across

Glucose breakdown results in 36 or 38 ATP39% of available energy in glucose is transferred to ATP,

the rest of the energy is lost as heat

http://media.pearsoncmg.com/bc/bc_0media_bio/bioflix/bioflix.htm?cc5respiration

Fermentation is inefficient

When oxygen is in short supply, the cell switches to fermentation

• 2 ATP produced per glucose molecule

• In animals: pyruvate accepts electrons and is reduced to lactate

• In other organisms: alcohol and CO2 are produces

• Benefits versus drawbacks of fermentation– Can occur without oxygen– Provide quick bursts of energy (important for

muscle cells)– Products are toxic to cells

• Yeast produces alcohol, but it kills them (wine)• Lactate builds up in muscle cells, which changes

the cell pH and causes the “burn”

– The liver can convert lactate back to pyruvate so cellular respiration can produce the remaining ATP

Metabolic pathways cross at particular substrates

Organic molecules can be broken down and synthesized as needed

• Catabolism – breaking down of molecules– Fats are a glycerol (can enter glycolysis) and 3 fatty

acids (can convert to acetyl CoA and energy the Krebs cycle)

– Carbon skeleton of amino acids can enter glycolysis, be converted to acetyle CoA, or enter the Krebs cycle directly

• Anabolism – building up of molecules– G3P from glycolysis can be converted to glycerol,

acetyl groups from the preparatory reactions can be joined to form fatty acids, fat synthesis follows, which can lead to weight gain

Essential Amino Acids

• Plants are able to produce all the amino acids they need

• Animals can only produce 11 of 20 essential amino acids because they lack some enzymes for synthesis of the other 9 amino acids. These amino acids are required in the diet of animals or protein deficiency will result.