7

Vital Harvest:Deriving Energy from Food

Relevant questions to be answered:1.How do cells convert food into

energy? Do they convert fats differently from carbohydrates? Do you get fat by eating fat or calories?

2.Why do we need to breathe? How does exercise affect metabolism?

3.What about energy supplements? What does it mean to have a fast or slow metabolism?

ATP is the most important energy storage molecule:

Potential energy from food breakdown is used to drive the endergonic synthesis of ATP (like recharging a battery).2.The charged ATP has energy that

can be released at any time by breaking off the third phosphate to do a variety of actions).

Electrons from food carry energy to make ATP

1.Electrons from glucose run downhill. Transferred by carriers, the electron drop powers uphill synthesis of ATP.

2.Electron transfers to molecules—redox reactions occur side by side:

a) One molecule is oxidized—loses electrons

b) Another molecule is reduced—gains those electrons (reduces charge).

Electrons from food carry energy to make ATP

3.Intermediate electron carriers serve to shuttle electrons through these reactions transferring energy as they go:

NAD+ (empty city cab) in redox reaction is an oxidizing agent (removes electrons causing a substance to be oxidized), accepts a hydrogen atom and one electron becomes NADH (full cab).

Intermediate electron carriers serve to shuttle electrons through these reactions transferring energy as they go: NADH can carry electrons (proceed down energy hill) on to another acceptor, thus being regenerated (empty cab).

NAD+ is made by cells from the vitamin niacin.

Enzymes coordinate all these transfers by bringing together the glucose derivatives with energy carrier molecules.

Overview of the three stages:

1. General reaction:

C6H12O6 +6O2 + ADP 6CO2 + 6H2O + ATP

2. Energy coupling: downhill breakdown of glucose releases electrons, carried along and used to transfer energy to drive uphill synthesis of ATP.

First Stage of Respiration:

Glycolysis—For eukaryotes this is the first stage. It begins breakdown of glucose, yielding little energy, but it does transfer electrons to NAD+. On the plus side, it doesn’t require oxygen and occurs in the cytoplasm, and some prokaryotes and single-celled eukaryotes have long used it as the sole source of energy.

Krebs cycle and electron transport chain—later evolved but generate larger quantities of energy; only problem is they occur only in mitochondria (only eukaryotes) and the electron transport chain requires oxygen.

Glycolysis: Steps in the process:1. Sugar in bloodstream enters cytoplasm, where this breakdown begins. Enzymes catalyze each reaction in metabolic pathway (first is hexokinase, which adds a phosphate from ATP called phosphorylation).

2. Although breaking sugar apart generates energy, it requires some activation energy (another ATP is used to attach another phosphate: -2ATP total).

Rearrangement eventually leads to splitting the molecule in half from one 6-carbon sugar into two 3-carbon sugars (pyruvic acid is end product).

Oxidation by 2NAD+ transfers electrons, and leads to the attachment of a high-energy phosphate to each sugar. Enough energy is generated by the eventual release of these four total phosphates in the next two steps to attach them to ADP to make ATP (+ 4ATP).

Glycolysis

Plus side—Very fast reactions cut glucose in half, generating small amount of energy (net 2 ATP), and electrons (2 NADH), but no oxygen was required.

Minus side—What is next redox reaction where electrons can be transferred to empty the cab (NADH) for more passengers (NAD+)? Not much ATP made for all the work.

Glycolysis

3.Sidebar: When Energy Harvesting Ends at Glycolysis

a) Bacteria and certain eukaryotes may only use glycolysis. Problem—How can they recycle the NAD+?

Solution—Alcoholic fermentation, yeast in absence of oxygen (bread and wine) must regenerate NAD+, so they dump electrons from NADH onto the acetaldehyde (converted from pyruvic acid and spewing off CO2), reducing it to ethanol, but regenerating the NAD+.

Solution—Lactate fermentation, in animals in absence of oxygen (muscle fatigue), pyruvate accepts electrons from NADH and regenerates NAD+, but is converted into lactic acid (muscle burn).

Pyruvate from glycolysis lactic acid

Energy and Exercise—Huge quantities of ATP are required to contract skeletal muscle, but ATP is generated in different ways depending on circumstances.

a) First burst of activity (6 seconds): cells have stockpile of ATP and phosphocreatine:

b) Look at Sidebar Figure 1—What is greatest source of energy aerobic or anaerobic at 30 seconds? 10 minutes? Why? Because, although aerobic respiration is slower, it is much more productive. The body moves to aerobic respiration if it can, using anaerobic only under extreme exertion when oxygen drops:

Krebs cycle sugar derivatives are oxidized to yield electrons in interior of inner membrane of mitochondria:

Each of the two pyruvic acid travels into the mitochondria, where they combine with coenzyme A to make acetyl CoA, one NADH and CO2 (breathe out):

Acetyl CoA combines with oxaloacetic acid to make citric acid, continues around through a series of reactions that finally yield oxaloacetic acid again (cycle).

Steps in the process: Figure 7.8b) During the cycle of reactions, as the acetyl

CoA is transformed, it is being oxidized by electron carrier molecules NAD+ and FAD.

c) Also, ATP and CO2 are produced.

Ledger: 8 NADH, 2 FADH2, and 2 ATP, acetyl CoA completely broken down into CO2.

Majority of electrons for next stage (electron transport chain)

Electron transport chain—Series of molecules in the mitochondrial inner membrane that are the destination of the electrons carried by NADH and FAHD2.

1. Steps in the process: a) NADH arrives, and it bumps the ETC’s first carrier,

which accepts the electrons, then passes them on along the chain (like a hot potato).

b) Movement of electrons at each transfer releases enough energy to power the movement of H+ ions from the inner compartment into the outer compartment (like heat of hot potato dissipating as it is passed). They are being pumped against their concentration gradient (uphill).

c) Hydrogen ions are allowed to flow downhill through an enzyme in the membrane called ATP synthase, like a water wheel spinning; as the ions pass, energy is used to transfer phosphate onto ADP to make ATP.

Greatest amount of ATP is made in this stage

(up to 32 ATP per glucose).

At the end of the ETC , who accepts the electron?

½ O2 + 2 electrons + 2 H+ = H2O

Other Foods, Other Respiratory Pathways Fats, proteins, and other sugars can also enter

pathway to be converted to energy, but not in exactly the same way:

Food eaten in excess of caloric demands can also be converted from amino acids, fatty acids, and sugars into proteins, fats, and carbohydrates for structure or storage (98 percent of energy reserves of animals are fats).

The 3 steps of

Aerobic Respiration:

1

2

3

8

Up to 36

Summary of Aerobic respiration

Glycolysis Krebs Cycle Electron Transport Chain

Produces:

Net gain 2 ATP

2NADH

Produces:

CO2

8NADH

2FADH2

2ATP

Produces:

Up to 32 ATP

From one molecule of glucose

The End