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Cellular Metabolism

Chapter 4

Glycolysis

 Glycolysis – the first stage in cellular

respiration.

 A series of enzyme catalyzed reactions.  Glucose converted to pyruvic acid.  Small number of ATPs made (2 per glucose molecule), but it is possible in the absence of oxygen.  All living organisms use glycolysis.

Glycolysis

 Uphill portion primes the fuel with phosphates.  Uses 2 ATPs  Fuel is cleaved into 3-C sugars which undergo oxidation.  NAD+^ accepts e - s & 1 H+^ to produce NADH  NADH serves as a carrier to move high energy e-s to the final electron transport chain.  Downhill portion produces 2 ATPs per 3-C sugar (4 total).  Net production of 2 ATPs per glucose molecule.

Glycolysis

 Summary of the enzymatically catalyzed

reactions in glycolysis:

Glucose + 2ADP + 2Pi + 2 NAD +^ 2 Pyruvic acid + 2 NADH + 2ATP

Harvesting Electrons form

Chemical Bonds

 When oxygen is available, a second

oxidative stage of cellular respiration

takes place.

 First step – oxidize the 3-carbon pyruvate in the mitochondria forming Acetyl-CoA.  Next, Acetyl-CoA is oxidized in the Krebs cycle.

Producing Acetyl-CoA

 The 3-carbon pyruvate loses a carbon producing an acetyl group.  Electrons are transferred to NAD +^ forming NADH.  The acetyl group combines with CoA forming Acetyl-CoA.  Ready for use in Krebs cycle.

The Krebs Cycle

 The Krebs cycle is the next stage in oxidative respiration and takes place in the mitochondria.  Acetyl-CoA joins cycle, binding to a 4-carbon molecule to form a 6-carbon molecule.  2 carbons removed as CO 2 , their electrons donated to NAD +^ , 4-carbon molecules left.  2 NADH produced.  More electrons are extracted and the original 4- carbon material is regenerated.  1 ATP, 1 NADH, and 1 FADH 2 produced.

The Krebs Cycle

 Each glucose provides 2 pyruvates,

therefore 2 turns of the Krebs cycle.

 Glucose is completely consumed during

cellular respiration.

The Krebs Cycle

Acetyl unit + 3 NAD +^ + FAD + ADP + P i

2 CO 2 + 3 NADH + FADH 2 + ATP

Using Electrons to Make ATP

 NADH & FADH 2 contain energized electrons.  NADH molecules carry their electrons to the inner mitochondrial membrane where they transfer electrons to a series of membrane bound proteins – the electron transport chain.

Building an Electrochemical

Gradient

 In eukaryotes, aerobic metabolism takes

place in the mitochondria in virtually all

cells.

 The Krebs cycle occurs in the matrix , or

internal compartment of the

mitochondrion.

 Protons (H +^ ) are pumped out of the

matrix into the intermembrane space.

Producing ATP- Chemiosmosis

 A strong gradient with many protons outside the matrix and few inside is set up.  Protons are driven back into the matrix.  They must pass through special channels that will drive synthesis of ATP.  Oxidative phosphorylation

Electron Transport Review

Review of Cellular Respiration

 1 ATP generated for each proton pump

activated by the electron transport chain.

 NADH activates 3 pumps.  FADH 2 activates 2 pumps.

 The 2 NADH produced during glycolysis

must be transported across the

mitochondrial membrane using 2 ATP.

 Net ATP production = 4

Glucose + 2 ATP + 36 ADP + 36 Pi + 6 O 2 6CO 2 + 2 ADP + 36 ATP + 6 H Docsity.com 2 O

Fermentation

 In the absence of oxygen, the end-

product of glycolysis, pyruvate , is used

in fermentation.

 During glycolysis, all the NAD+^ becomes saturated with electrons (NADH). When this happens, glycolysis will stop.  2 NADH and 2 ATP produced.  Pyruvate is used as the electron acceptor resetting the NAD+^ for use in glycolysis.

Fermentation – 2 Types

 Animals add extracted electrons to pyruvate forming lactate.  Reversible when oxygen becomes available.  Muscle fatigue  Yeasts, single-celled fungi, produce ethanol.  Present in wine & beer.  Alcoholic fermentation

Metabolism of Lipids

 Triglycerides are broken down into glycerol and 3 fatty acid chains.  Glycerol enters glycolysis.  Fatty acids are oxidized and 2-C molecules break off as acetyl-CoA.  Oxidation of one 18-C stearic acid will net 146 ATP.  Oxidation of three glucose (18 Cs) nets 108 ATP.  Glycerol nets 22 ATP, so 1 triglyceride nets 462 ATP.

Metabolism of Proteins

 Proteins digested in the gut into amino acids which are then absorbed into blood and extracellular fluid.  Excess proteins can serve as fuel like carbohydrates and fats.  Nitrogen is removed producing carbon skeletons and ammonia.  Carbon skeletons oxidized.

Metabolism of Proteins

 Ammonia is highly toxic, but soluble.  Can be excreted by aquatic organisms as ammonia.  Terrestrial organisms must detoxify it first.

Regulating Cellular Respiration

 Rate of cellular respiration slows down

when your cells have enough ATP.

 Enzymes that are important early in the

process have an allosteric (regulating)

site that will bind to ATP.

 When lots of ATP is present, it will bind

to this site, changing the shape of the

enzyme, halting cellular respiration.

Regulating Cellular Respiration

 Enzyme activity is controlled by presence or absence of metabolites that cause conformational changes in enzymes.  Improves or decreases effectiveness as catalyst.