Topic 8.2 Cell Respiration

8.2 (U1) Cell respiration involves the oxidation and reduction of compounds.

• As organic molecules are broken down during catabolic reactions, a transfer of electrons occurs and this transfer releases energy stored in the organic molecules. This energy can then be used by the cell to synthesize ATP.

• When the transfer of electrons occurs, if there is a loss of electrons from a substance it is called an oxidation reaction and if there is an addition of electrons to another substance it is called a reduction reaction.

Oxidations & Reductions

• There are many oxidation and reduction reactions involved in cell respiration.

• The chart shows a comparison between the two reactions.

• Remember OIL RIG!

• OIL – Oxidation Is Loss (of electrons)

• RIG – Reduction Is Gain (of electrons)

Oxidation Reaction Reduction Reaction

Loss of electrons Gain of electrons

Gain of oxygen Loss of oxygen

Loss of hydrogen Gain of hydrogen

Results in many C-O bonds

Results in many C-H bonds

Results in a compound with lower potential energy

Results in a compound with higher potential energy

Redox Reactions

• Oxidations and reductions always occur together and are referred to as redox reactions. In chemical reactions when one compound or element loses an electron another compound or element gains the electron.

• These redox reactions are very important in the flow of energy through living things because each time an electron is passed from one molecule to the next they carry energy with them.

Hydrogen Carriers• When hydrogen atoms are removed from

substrates during cell respiration they are accepted by hydrogen carriers. In cell respiration the most commonly used hydrogen carrier is NAD+ (nicotinamide adenine dinucleotide).

• When two hydrogen atoms are removed from a substrate during cell respiration, the NAD+ accepts the proton from one atom and the electrons of both hydrogen atoms. This means that there is one H+ atom left over.

• NAD+ + 2H + 2e- → NADH + H+

8.2 (U2) Phosphorylation of molecules makes them less stable.

• When a molecule (i.e., ADP) has a phosphate molecule attached this is phosphorylation.

• The phosphorylation of a molecule makes it unstable which means it is more likely to react.

• When ATP is hydrolzyed into ADP and P it is an exergonic reaction (releases energy). That energy can be used to fuel cell metabolism.

• Most reactions that occur inside the cell are endergonic.

Glycolysis

• 8.2 (U3) In glycolysis, glucose is converted to pyruvate in the cytoplasm.

Glycolysis• Glycolysis is the first stage of cell respiration when

glucose is the substrate being oxidized. The word glycolysis means “sugar splitting” and it occurs in the cytoplasm and is catalysed by enzymes.

• During glycolysis, a hexose sugar (usually glucose) is partially oxidized which produces a small yield of ATP and pyruvate. This partial oxidation can occur with or without the presence of oxygen so glycolysis can occur in both anaerobic and aerobic cell respiration.

• There are four main stages in the process of glycolysis; phosphorylation, lysis, oxidation, and ATP formation.

4 Steps In Glycolysis

• Phosphorylation- a phosphate molecule is attached to the 6 carbon sugar.

• Lysis – the glucose -6 –phosphate is split into two molecules of triose phosphate.

• Oxidation – The triose phosphate is oxidized (hydrogen atoms removed). During the oxidation enough energy is released to make 2 ATP and the product is glycerate-3-phosphate.

8.2 (U4) Glycolysis gives a small net gain of ATP without the use of oxygen.

•The last step in glycolysis is the formation of ATP. The phosphate groups are removed by an enzyme and passed to ADP to form ATP. •This results in the formation of four molecules of ATP, two molecules of NADH and two molecules of pyruvate.•If you remember one thing about glycolysis, remember that it yields 2 pyruvate, 2 ATP, and 2 NADH

8.2 (U5) In aerobic cell respiration pyruvate is decarboxylated and oxidized.

• After glycolysis is complete the end product is two 3-carbon pyruvate molecules, NADH and ATP. As long as oxygen is present the pyruvate can now be used in the Krebs cycle, the second stage of cell respiration.

• Before the entering the Krebs cycle the pyruvate will have carbon removed in the form of carbon dioxide (decarboxylation) and the oxidation occurs when the hydrogen atoms are removed are accepted by NAD+.

8.2 (U6) In the link reaction pyruvate is converted into acetyl coenzyme A.

• When the oxidative decarboxylation of pyruvate is complete the product is an acetyl group which is then accepted by CoA.

• This is often referred to as a link reaction due to the fact that it links glycolysis to the Krebs cycle

Coenzyme A

• The Krebs cycle uses acetyl groups (CH3CO) as substrates. Coenzyme A (CoA) is a carrier that accepts acetyl groups that are produced through metabolism and brings them in for use during the Krebs cycle.

• Acetyl CoA can now enter the Krebs cycle

• 8.2 (U7) In the Krebs cycle, the oxidation of acetyl groups is coupled to the reduction of hydrogen carriers, liberating carbon dioxide.

Krebs Cycle

• The Krebs cycle involves two more decarboxylations and four more oxidations.

• Acetyl CoA joins with a C4 compound (oxaloacetate) to form citric acid (C6 ) which is involved in a series of reactions which will convert it into C5 and then back into C4 (oxaloacetate) and ATP will be produced in the process.

• In the first reaction during the Krebs cycle, the acetyl CoA (a two carbon acetyl fragment) combines with a four carbon compound oxaloacetate and is changed into a six carbon compound (C6) called citrate.

Decarboxylation

• Two of the reactions involve carbon dioxide being removed (decarboxylation). The waste product CO2 is removed and excreted along with the CO2 from the link reaction.

Oxidation

• Four reactions involve the removal of hydrogen, and this process is known as oxidation. In three of the oxidations the hydrogen is accepted by NAD+ and FAD (flavine adenine dinucleotide) accepts it in the other.

• The oxidations result in a release of energy, most of which is stored by carriers when they accept the hydrogen. Later this energy will be released by the electron transport chain and used to produce ATP.

Oxidation in The Krebs Cycle

Substrate Level Phosphorylation• Substrate level

phosphorylation results in the production of ATP.

• In the Krebs cycle, ADP is phosphorylated into ATP.

• The Krebs cycle yields 2 ATP

Krebs Cycle Summary

• The Krebs cycle must run twice for every glucose molecule that enters cell respiration because each glucose molecule produces two pyruvate molecules.

• One turn of the Krebs cycle yields the following:

– 2 molecules of CO2

– 3 molecules of NADH + H+

– 1 molecule of FADH2

– 1 molecule of ATP

8.2 (S1) Analysis of diagrams of the pathways of aerobic respiration to deduce where decarboxylation and oxidation

reactions occur.  

• You need to be able to identify where decarboxylations and oxidation reactions occur in cell reapiration.

• Refer to diagrams given in class.

8.2 (U8) Energy released by oxidation reactions is carried to the cristae of the inner mitochondrial

membrane by reduced NAD and FAD.

• During aerobic respiration as the reactants are being oxidized, NAD+ and FAD are both being reduced, aborbing energy and carrying it to cristae for the final stage of cell respiration.

• Reduced NAD is produced in both glycolysis and the Krebs cycle whereas FADH is reduced only during the Krebs cycle.

8.2 (U8) Energy released by oxidation reactions is carried to the cristae of the inner mitochondrial

membrane by reduced NAD and FAD.

• The final stage in cell respiration is oxidative phosphorylation.

• It involves the electron transport chain and chemiosmosis and together they account for the majority of ATP produced during cell respiration.

8.2 (U9) Transfer of electrons between carriers in the electron transport chain is coupled to proton pumping.

• The electron transport chain is a chain of electron carriers, mostly proteins, embedded in the inner membrane of the mitochondria.

• The highly folded inner membranes known as cristae allow for thousands of transport chains, each capable of carrying out the final stage of cell respiration.

Electron Transport Chain

• The first carrier in the chain is provided with two electrons from NADH that are a result of oxidation reactions earlier in cell respiration during glycolysis and the citric acid cycle. FADH provides electrons to the chain as well but at a lower energy level

• As the electrons pass along the chain of carriers they give up energy each time they are passed from one carrier to another in a series of oxidation and reduction reactions.

8.2 (U10) In chemiosmosis protons diffuse through ATP synthase to generate ATP.

• The chemiosmotic theory is based on the idea that ATP synthesis is linked to the electron transport chain and the movement of protons. The energy released by oxidation is trapped and used by the enzyme ATP synthase to make ATP.

• Basically, the free energy released during the transport of electrons is used to move protons. As NADH + H+ and FADH2 are oxidized they release energy and this energy is used to pump or force protons against the concentration gradient from the matrix in the mitochondria to a compartment between the inner and outer membrane of the mitochondria.

Chemiosmosis• There is now a store of

potential energy in the form of a concentration gradient of protons building up.

• The protons will make their way back into the matrix through chemiosmotic channels in the synthase molecules in the membrane.

• The energy released as the protons flow down the gradient back to the matrix is used by ATP synthase to phosphorylate ADP into ATP

8.2 (U11) Oxygen is needed to bind with the free protons to form water to maintain the hydrogen gradient.

• Oxygen is the final electron acceptor in the electron transport chain. At the same time it is accepting electrons, oxygen is forming covalent bonds with hydrogen ions to form water.

• All of this takes place in the matrix of the mitochondria on the surface of the inner membrane and it is the only time oxygen is used in cell respiration.

8.2 (U12) The structure of the mitochondrion is adapted to the function it performs.

• The mitochondrion is considered an excellent example of an organelle that is structurally built based on its function.

• Mitochondria have a smooth outer membrane and a highly folded inner membrane containing cristae. The cristae increase the surface area inside the mitochondria for the electron transport chain.

Structure of the Mitochondria

Outer Mitochondrial Membrane

The membrane keeps the contents of the mitochondria separate from the rest of the cell. This creates a special compartment for the biochemical reactions of aerobic respiration.

Matrix Internal cytosol-like area where enzymes for the link reaction and Krebs cycle are found.

Cristae Membrane surrounded, tubular regions which increase the surface area for oxidative phosphorylation to take place

Inner Mitochondrial Membrane

Site of oxidative phosphorylation and contains the carriers of the electron transport chain. Also contains ATP synthase which is used in chemiosmosis.

Space between inner and outer

membranes

Hydrogen ion (protons) reservoir; the high concentration of protons is needed for chemiosmosis. Small volume of space which allows for rapid build up of proton gradient.

8.2 (S2) Annotation of a diagram to indicate the adaptations of a mitochondrion to its function.

• See page 387 in text. Transfer this drawing to your sketch book

8.2 (A1) Electron tomography used to produce images of active mitochondria.

• Technique for obtaining detailed 3D structures of sub-cellular macro-molecular objects (extension of electron microscopy)

• The outer membrane and cristae are shown in purple and yellow, respectively.

http://www.sci.sdsu.edu/TFrey/MitoMovies/CrisMitoMovie.htm

http://ncmir.ucsd.edu/research/highlights/2010_Electron_Tomography.shtm

• Read page 388 in text