Module 2 Unit 2 Flashcards
What is aerobic respiration?*
- in which oxygen is consumed as a reactant along with the organic fuel
- The cells of most eukaryotic and many prokaryotic organisms can carry out aerobic respiration
- aerobic respiration is in principle similar to the combustion of gasoline in an automobile engine after oxygen is mixed with the fuel (hydrocarbons)
What is anaerobic respiration?*
– Some prokaryotes use substances other than oxygen as reactants in a similar process to aerobic respiration that harvests chemical energy without oxygen
What is cellular respiration?
- includes both aerobic and anaerobic processes
- is an exergonic process that transfers energy from the bonds in glucose to form ATP (negative ΔG so the products store less energy than the reactants so the reaction can occur spontaneously)
- produces up to 32 ATP molecules from each glucose molecule and captures only about 34% of the energy originally stored in glucose
C6H12O6 + 6O2 —> 6CO2 + 6H2O + energy (ATP + heat)
How do catabolic processes make energy (where is the energy stored)?
– The energy necessary for life is contained in the arrangement of electrons in chemical bonds in organic molecules
– When the carbon-hydrogen bonds of glucose are broken, electrons are ultimately transferred to oxygen
»Oxygen has a strong tendency to attract electrons.
»An electron loses potential energy when it “falls” to oxygen.
– thus the reaction between hydrogen and oxygen (as H falls closer to O) is what makes energy
What are redox reactions?
– a transfer of one or more electrons (e−)
from one reactant to another is called an oxidation-reduction reaction (or redox reaction)
– oxidation: loss of electrons from one substance
– reduction: addition of electrons to another substance (called reduction because it reduces the amount of positive charge of the atom)
– reducing agent: electron donor
– oxidizing agent: electron accepter
– Because an electron transfer requires both a donor and an acceptor, oxidation and reduction always go hand in hand
– Not all redox reactions involve the complete transfer of electrons from one substance to another; some change the degree of electron sharing in covalent bonds (ex. methane combustion)
How is cellular respiration a redox reaction?
– Glucose
» loses its hydrogen atoms and becomes oxidized to CO2
– Oxygen
» gains hydrogen atoms and
» becomes reduced to H2O
– the energy state of the electron changes as hydrogen (with its electron) is transferred from glucose to oxygen; the oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available for ATP synthesis
What is NAD+/NADH and how do they work?
- As is often the case in oxidation reactions, each electron travels with a proton (thus, as a hydrogen atom); The hydrogen atoms are not transferred directly to oxygen, but instead are usually passed first to an electron carrier, a coenzyme called nicotinamide adenine dinucleotide, a derivative of the vitamin niacin; it can cycle easily between its oxidized form, NAD+, and its reduced form, NADH. As an electron acceptor, NAD+ functions as an oxidizing agent during respiration
- Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate (ex. glucose), thereby oxidizing it. The enzyme delivers the 2 electrons along with 1 proton to its coenzyme, NAD+, forming NADH. The other proton is released as a hydrogen ion (H+) into the surrounding solution
- By receiving 2 negatively charged electrons but only 1 positively charged proton, the nicotinamide portion of NAD+ has its charge neutralized when NAD+ is reduced to NADH
- Electrons lose very little of their potential energy when they are transferred from glucose to NAD+. Each NADH molecule formed during respiration represents stored energy that can be tapped to make ATP when the electrons complete their “fall” down an energy gradient from NADH to oxygen
What are electron-transport chains?
- respiration uses an electron transport chain to break the fall of electrons to oxygen into several energy-releasing steps
- There are other electron “carrier” molecules that function like NAD+; These electron carriers collectively are called the electron transport chain
- Instead of this energy being released and wasted in a single explosive step, electrons cascade down the chain from one carrier molecule to the next in a series of redox reactions, losing a small amount of energy with each step until they finally reach oxygen
- Each “downhill” carrier is more electronegative than its “uphill” neighbour, with oxygen at the bottom of the chain
- Putanotherway, oxygen pulls electrons down the chain in an energy-yielding tumble analogous to gravity pullingobjects downhill
SUMMARY
glucose→NADH→electrontransportchain→oxygen
How do you harvest energy from glucose?
1) Glycolysis
2) Pyruvate oxidation and the citric acid cycle
3) Oxidative phosphorylation
What is substrate level phosphorylation?
ATP is formed in glycolysis by substrate-level phosphorylation during which
» an enzyme transfers a phosphate group from a substrate molecule to ADP and…
» ATP is formed.
What is glycolysis?
– occurs in cytosol and begins the degradation process by enzymatically breaking glucose into two molecules of a compound called pyruvate
– along the way, two molecules of NAD+ are reduced to two molecules of NADH, and
a net of two molecules of ATP (by substrate-level phosphorylation) is produced
– glycolysis can be divided into two phases: the energy investment phase and the energy payoff phase. During the energy investment phase, the cell actually spends ATP. ATP is used to put the glucose in an unstable state so that you can get the energy pay off phase. This investment is repaid with interest during the energy payoff phase, when ATP is produced by substrate-level phosphorylation and 2 NAD+ molecules are reduced to 2 NADH molecules by electrons released from the oxidation of glucose. The 2 NADH then go to the electron transport chain
–glycolysis: glucose is put in an unstable state by ATP to split the glucose into two 3 carbon compounds called pyruvate (which is less stable than glucose), and two ATP is made from substrate level phosphorylation
glucose –> 2 pyruvate + H2O
4 ATP formed - 2 ATP used –> 2 ATP
2 NAD + 4 e- 4 H+ –> 2 NADH + 2 H+
What is pyruvate oxidation?
–before entering the citric acid cycle, pyruvate must be oxidized first
» a carboxyl group is removed and given off as CO2,
» the two-carbon compound remaining is oxidized forming acetate, while a molecule of NAD+ is reduced to NADH which goes to the ETC
» coenzyme A joins with the two- carbon group to form acetyl coenzyme A, abbreviated as acetyl CoA, and
» acetyl CoA enters the citric acid cycle.
2 pyruvate —> 2 CO2 + 2 NADH + 2 acetyle CoA
What is the citric acid cycle?
— During the citric acid cycle the two- carbon group of acetyl CoA is added to a four-carbon compound, forming citrate,
– citrate is degraded back to the four-carbon compound oxaloacetate at the end of the cycle,
» two CO2 are released, and
» 1 ATP
» 3 NADH
» 1 FADH2 are produced.
– The cycle generates one ATP per turn by substrate-level phosphorylation, but most of the chemical energy is transferred to NAD+ and a related electron carrier, the coenzyme FAD, during the redox reactions. The reduced coenzymes, NADH and FADH2, shuttle their cargo of high-energy electrons into the electron transport chain.
– The cycle has eight steps, each catalyzed by a specific enzyme
– Recall that each glucose gave rise to 2 Acetyl CoAs that enter the cycle.
– Thus, after two turns of the citric acid cycle, the overall yield per glucose molecule is
» 2 ATP,
» 6 NADH, and
» 2 FADH2
» 4 CO2
What is oxidative phosphorylation?
– The electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion; Most components of the chain 4 are proteins. Tightly bound to these proteins are prosthetic groups, nonprotein components essential for the catalytic functions of certain enzymes.
– During electron transport along the chain, electron carriers alternate between reduced and oxidized states as they accept and then donate electrons. Each component of the chain becomes reduced when it accepts electrons from its “uphill” neighbour and oxidized as it passes electrons to its “downhill” (more electronegative) neighbour.
– Electrons from NADH and FADH2 (produced from glycolysis/pyruvate oxidation/citric acid cycle) travel down the electron transport chain to O2.
– Oxygen picks up H+ to form water.
– As these electrons move downhill (from one member of the chain to the next), they release a little energy
– Energy released by these redox reactions is used to pump H+ from the mitochondrial matrix into the intermembrane space (outside the mitochondria in cytosol)
» This generates a proton concentration gradient (proton motive force) which drives chemiosmosis (chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work)
– In chemiosmosis, the H+ diffuses back across the inner membrane through ATP synthase complexes, driving the synthesis of ATP
– thus, the ETC does not directly produce energy (ATP), it sets up a condition where it allows for the production of ATP by chemiosmosis
– oxygen is needed to pick up the electron because the proteins forming the ETC need to pick up the electron from its “uphill” neighbour, which also needs to pick up the electrons from its “uphill” neighbour. If oxygen did not pick up the electron, then NADH and FADH2 have nothing to pass their electrons to (this is why we breathe oxygen)
– You get about 26-28 ATP formed at the end, but these numbers also take into account the slight energetic cost of moving the ATP formed in the mitochondrion out into the cytosol
– a single molecule of NADH generates enough proton-motive force for the synthesis of 2.5 ATP; Depending on the kind of shuttle in a particular cell type, the electrons are passed either to NAD+
or to FAD in the mitochondrial matrix; If the electrons are passed to FAD, as in brain cells, only about 1.5 ATP can result from each NADH that was originally generated in the cytosol. If the electrons are passed to mitochondrial NAD+, as in liver cells and heart cells, the yield is about 2.5 ATP per NADH.
How does anaerobic cellular respiration work?
– Such organisms use a electron
transport chain to generate a proton
motive force that drives chemiosmosis, however
» Oxygen is not the final electron acceptor (hence ‘anaerobic’)
» A different electronegative molecule is used instead – hence anaerobic respiration
– e.g. Sulphate-reducing bacteria use sulphate ion at the end of their chain.
Hydrogen sulphide, rather than water is generated as a byproduct
