Biol 12000 biology 1b life processes – animal physiology

Life 21 - Aerobic respiration – Raven & Johnson Chapter 9 (parts)
Objectives
1: Describe the overall action of the Krebs cycle in generating ATP, NADH and 2: Understand the generation of ATP from NADH and FADH2 by the electron 3: Explain why NADH produced by glycolysis and by the Krebs cycle, and FADH2, differ in the number of ATP they produce 4: Compare the energy efficiency of aerobic respiration of glucose with that of
The Krebs cycle (Fig. 9.13, and simplified form)

Carbon from carbohydrates, fats and proteins all ends up as the 2-C acetyl group
of acetyl-CoA. This is oxidised to synthesise ATP
The oxidation occurs in a series of 9 reactions forming a cycle (compare the
linear sequence of glycolysis)
Krebs cycle – Sir Hans Krebs (discoverer in the 1930s), or
Citric acid cycle – first molecule in the cycle is citric acid or citrate. Organic acids
(citric) exist as anions (citrate) at pH 7 in the cell
The Krebs cycle occurs in the matrix (contents) of the mitochondria – “power
plants” of the cell
Reaction (1) combines the 4-C oxaloacetate with the 2-C acetyl group to form the
6-C citrate and release coenzyme A
The whole cycle involves removal of 2 CO2 to return to the 4-C oxaloacetate, in a
series of oxidations
Reaction (4) is the first oxidation, with CO2 removed and NAD+ reduced to
NADH. This gives the 5-C α ketoglutarate
Reaction (5) is the second oxidation, with another CO2 removed and reduction of
another NAD+ to NADH
The molecule also combines with coenzyme A to give the 4-C succinyl-CoA, with
a high-energy bond
This bond is broken in reaction (6) to form guanosine triphosphate (GTP) from
guanosine diphosphate (GDP) and Pi. This is similar to ATP/ADP
The ~P is transferred to ATP in a substrate-level phosphorylation
Reaction (7) is the third oxidation, with flavin adenine dinucleotide (FAD) reduced
to FADH2

Similar to NAD+/NADH, but has a smaller ∆G. This oxidation does not yield
enough energy to reduce NAD+
Reaction (9) is the fourth oxidation, NAD+ is reduced to NADH, and oxaloacetate
is restored
Oxaloacetate combines with acetyl-CoA to start another turn of the Krebs cycle The net result of glycolysis and the Krebs cycle is that the 6-C glucose has been converted to six CO2 (and water) The Krebs cycle also generates 2 ATP from each glucose molecule by substrate-level phosphorylation, the same yield as glycolysis More important, catabolism has harvested many electrons as reduced electron carriers. A total of 10 NADH and 2 FADH2 from each glucose molecule 6 NADH and 2 FADH2 from the Krebs cycle (2 turns per glucose) 2 NADH from glycolysis 2 NADH in formation of acetyl-CoA (2 molecules per glucose) Energy is harvested from these carriers as electrons move along the electron transport chain Energy moves with the electrons. The change in free energy at each stage depends on the change in position of the valency (bond) electron relative to the atomic nuclei Carbon and hydrogen atoms have low electronegativity; their nuclei attract the electrons in a covalent bond weakly An electron in a C-H bond is shared equally (on average a median position) between the two nuclei. This bond has high energy. Energy decreases as the bond electron moves closer to the nucleus Oxygen atoms have high electronegativity and attract bond electrons strongly. An electron in a C-O or H-O bond is close to the oxygen; the bond has low energy Electrons in NADH have high energy. If they were donated directly to oxygen, the energy released would be large and most would be lost as heat Instead, they pass along the chain to carriers of increasing electronegativity. Energy is released in a series of small steps (Fig. 9.14) Some of these steps are just large enough to synthesise ATP, so the loss of energy to heat is small At the end the electrons are donated to oxygen, the final electron acceptor, where they combine with protons (H+) to form water The chain is a series of molecules (mostly proteins) embedded in the inner mitochondrial membranes of eukaryotes and the plasma membrane of prokaryotes The surface area is increased by folding into cristae, especially in metabolically active tissues (Fig. 9.16) The molecules are in geometrically ordered assemblies. A liver cell has 1000 mitochondria, each with 15000 assemblies NADH and FADH2 each have a pair of high-energy electrons, which move along the chain
1: NADH dehydrogenase + NADH ⇒ pumps out H+
2: Ubiquinone (coenzyme Q, carrier) + FADH2
3: bc1 complex (cytochrome) ⇒ pumps out H+
4: cytochrome c (carrier)
5: cytochrome oxidase ⇒ pumps out H+
6: oxygen
Cytochromes have haem groups (with iron) similar to haemoglobin, red colour.
Iron changes between Fe3+ and Fe2+ as electrons move along, in a series of
redox changes. Fe3+ is the oxidised and Fe2+ is the reduced state (with electron)
Unlike haemoglobin which always has Fe2+, and combines with molecular
oxygen, rather than being chemically oxidised. Oxyhaemoglobin has molecular
O2, deoxyhaemoglobin does not; both have Fe2+
Haemoglobin with Fe3+ (methaemoglobin) has been poisoned, non-functional The last step, cytochrome oxidase, donates the electrons to oxygen. Cyanide inhibits this therefore is quickly lethal, stops aerobic respiration Each pair of electrons from NADH that move along the chain pump 3 protons (H+) from the matrix through the inner mitochondrial membrane The proteins are excited by the electrons and change shape, moving H+ through the membrane, in one direction only; out The pair of electrons from FADH2 enter the chain at step 2, bypassing the first pump, and so only move out 2 H+

Pumping out H+ leaves the matrix slightly negatively charged
H+ tend to move back through the inner membrane, following both electrical
(inside negative) and chemical concentration (inside low H+) gradients

H+ re-enter the matrix through protein channels in the inner membrane, as the
membrane itself is relatively impermeable to ions (Fig. 9.17)
Channels are ATP synthase enzymes, form ATP in the matrix as the H+ pass
through. Moving subunit structures – smallest rotary engines in nature
The formation of ATP is thus driven by a diffusion process similar to osmosis,
known as chemiosmosis …
Or oxidative phosphorylation, ATP synthesis by electron transport, not by
phosphorylated chemical intermediates (substrate-level phosphorylation)
One ATP is generated by each proton pumped out of the matrix. (Fig. 9.19)
NADH ⇒ 3 ATP and FADH2 ⇒ 2 ATP (from step 2)

But 2 of the NADH are produced by glycolysis in the cytoplasm, and these yield
only 2 ATP each, as it costs energy to move them into the matrix
In theory, glycolysis and oxidation of glucose would give 36 ATP
But the membrane is slightly leaky to protons and some of them avoid the ATP
synthase channels
Also some H+ are used to transport pyruvate into the matrix to form acetyl-CoA.
The actual yield is about 30 ATP per glucose molecule
Aerobic respiration is much more efficient than glycolysis alone
About 32% of the energy available from glucose is harvested as ATP, compared
to 2% for glycolysis (and to a car engine at about 25%)
This efficiency puts a natural limit on the length of food chains, of 3 or 4 stages,
as most of the energy is still lost as heat at each trophic level
Anaerobic respiration, using other inorganic molecules to accept electrons, does
not use an electron transport chain
The process is inefficient, e.g. sulphate oxidation,
SO4 ⇒ H2S in sulphur bacteria, gives 6 ATP per molecule of glucose, a yield

Source: http://www.ahailey.f9.co.uk/note/life21.pdf