Week 8

Aerobic cellular respiration continued

and Membrane structure

Review

• Glycolysis– products include:

• 2 molecules of pyruvate

• 2 ATPs via substrate level phosphorylation

• 2 NADH (reduced NAD+)

– NAD+ is required to keep glycolysis going. How is NAD+ regenerated from NADH?

– What is the fate of pyruvate?

Kreb’s cycle

• Syn. Tricarboxylic acid cycle (TCA) and Citric acid cycle

• Occurs in matrix of mitochondria

• Requires:– pyruvate– oxaloacetate from Kreb’s cycle– indirectly requires oxygen

Kreb’s cycle

• Pyruvate enters mitochondria• Pyruvate + CoASH --> acetyl-CoA + NADH + CO2

– enzyme: pyruvate dehydrogenase complex– hydrolysis of citrate-CoA intermediate releases

lots of energy that is used to drive the above reaction to the right.

– See panel 4-2

Kreb’s cycle

• See panel 4-2

• Key points:– you need to be able to account for the number

of carbons in each intermediate.

• Important intermediate compounds – citrate -ketoglutarate– oxaloacetate

Kreb’s cycle

• Summary of products starting with acetyl CoA– 2 CO2 released - complete oxidation of acetate

– 1 ATP (GTP)– 3 NADH

– 1 FADH2

• What is the fate of NADH and FADH2?

So, What is the fate of NADH and FADH2?

• Like glycolysis, without NAD+ and FAD, the Kreb’s cycle would shut down. They are required cofactors for enzyme activity.

• NADH and FADH2 pass their high energy electrons to the electron transport chain embedded in the inner membrane of the mitochondrion.

Electron transport chain• The electrons move down the electron transport

chain (see figure 13-9) and generate a proton motive force.

• Oxygen, indirectly required by the Kreb’s cycle, is the terminal electron acceptor of the electron transport chain.

• The proton motive force is the driving force behind oxidative phosphorylation via ATP synthase.

Yield of ATP

• ATP yields:– Aerobic cellular respiration (glycolysis, TCA

cycle, and electron transport chain activities) yields approximately 30 ATP from glucose

– glycolysis alone yields only 2 ATPs per glucose

Anaerobic conditions

• Exercise physiology– strenuous exercise causes muscles to be

depleted of oxygen. What happens?– Lactic acid builds up -burning sensation in

muscle. Why?– Because the oxygen level has dropped, NADH

and FADH2 are not being oxidized back to NAD+ and FAD.

Anaerobic cellular physiology

• Oxygen is depleted through normal cellular respiration during strenuous exercise.

• To oxidize NADH to NAD+ needed for glycolysis, muscles use pyruvate as a terminal electron acceptor in what is called lactic acid fermentation.

• Pyruvate + NADH --> lactic acid + NAD+

• NAD+ can return to function in glycolysis.

Alcohol fermentation

• Yeast can grow either aerobically using aerobic respiration or anaerobically using alcohol fermentation.

• Under anaerobic conditions, pyruvate is oxidized to acetaldehyde + CO2

• Acetaldehyde + NADH --> ethanol + NAD+

• NAD+ can function in glycolysis again.

Comparison of aerobic respiration and alcohol

fermentation

• Alcohol fermentation yields ATP via substrate level phosphorylation only during glycolysis. ONLY 2 ATPs per glucose yielded

• Aerobic respiration yields ATP via both substrate level phosphorylation and oxidative phosphorylation. Up to 30+ ATPs yielded per glucose!

Anabolic pathways

• A number of intermediates of both glycolysis and Kreb’s cycle are used as substrates for synthesis of nucleotides, amino acids, lipids etc. See Figure 4-18.

Poisons of respiration

• Rotenone:– blocks NADH dehydrogenase (complex I).

NADH cannot transfer its high energy electrons to the electron transport chain in the mitochondria. Therefore the cells cannot generate a proton motive force and no ATP can be made by oxidative phosphorylation. The cells sense death!

Poisons cont• Dinitrophenol - used as a diet supplement in the

1960s– Dinitrophenol is called an uncoupler of oxidative

phosphorylation. It makes the inner membrane of mitochondria permeable to protons and diffuses the proton gradient. Electrons move through the electron transport chain and try to make a proton gradient. But the membrane is permeable to protons which disrupts the proton motive force and very little ATP is made by ATP synthase. Instead the energy of the electrons moving down the electron transport chain is released as heat.

Poisons cont

• Arsenate– chemically similar to inorganic phosphate.– Competes where inorganic phosphate is used to

make ATP via substrate level phosphorylation, e.g., glycolysis and Kreb’s cycle

Poisons cont

• Cyanide– interferes with complex III which is the complex that

reduces oxygen to water.

– If this is blocked, than the electron transport chain would be saturated with electrons. Therefore NADH and FADH2 would not be able to be oxidized to NAD+ and FAD respectively. Then glycolysis and Kreb’s cycle would be negatively effected.

Hibernating bears

• Bears have lots of BAT- brown adipose tissue (fat cells rich with mitochondria)

• During hibernation the fat of these cells is metabolized to yield heat. – How? Uncouple ATP synthesis from proton

motive force. Use the energy of the electrons to generate heat instead of ATP

Membranes

Membranes

• Function:– selective barrier between cytoplasm and its

contents and outside the cell. – Composed of phospholipid, other lipid material

and proteins

Phospholipids

• Composed of a phosphate head, glycerol, and two fatty acids. (see Fig. 11-6)

• Amphipathic molecule– means that there is a hydrophilic and

hydrophobic region - the phosphate head is hydrophilic and the fatty acid tails are hydrophobic

Liposomes

• See Fig. 11-13

• Formed from phospholipids

• Lipid bilayer with an aqueous interior and exterior environments.

• Energetically favorable structure due to the amphipathic nature of the phospholipids.

membranes

• Plasma membrane– circa 50 nm thick lipid bilayer– separates inside from outside environment of a

cell– Bacteria have a single plasma membrane– Eucaryotes have a plasma membrane and

numerous internal membranes and membrane bound organelles.

Lipid bilayer

• See Fig. 11-4

• Types of lipids– See Fig. 11-7– phospholipids– sterols– glycolipids

Lipid bilayer

• Fluid bilayer

• lipids float freely in their plane of the bilayer– rarely flip to otherside of bilayer

– enzyme called flipase carries out flips.

• Fluidity function of composition of fatty acids in phospholipids.– Short, unsaturated fatty acids increase fluidity

– long, saturated fatty acids decrease fluidity

Saturated vs unsaturated fatty acids

CH3 - CH2 - CH2 - CH2 - CH2 - CH2 - COOH = saturated fatty acidvs

CH3 - CH2 - CH2 - CH = CH - CH2 - COOH = unsaturated fatty acid

Adaptation of membrane phospholipids

• As the temperature goes up, some yeast and bacteria can reduce the number of double bonds and increase the length of the fatty acids of their phospholipids.

• Yeast can also include cholesterol in their membranes to reduce the fluidity as the temperature goes up.

Lipid bilayer

• See Fig. 11-17

• Asymetrical in composition of phospholipids– e.g., glycolipids found on non-cytosolic side of

membrane– inositol phospholipids found on cytosolic side

of membrane