Cellular Respiration
KEY CONCEPTS
▪ Catabolic pathways yield energy by oxidizing organic
fuels
▪ Glycolysis harvests chemical energy by oxidizing
glucose to pyruvate
▪ After pyruvate is oxidized, the citric acid cycle
completes the energy-yielding oxidation of organic
molecules
▪ During oxidative phosphorylation, chemiosmosis
couples electron transport to ATP synthesis
▪ Fermentation and anaerobic respiration enable cells to
produce ATP without the use of oxygen
▪ Glycolysis and the citric acid cycle connect to many
other metabolic pathways
▪ Catabolic pathways yield energy by oxidizing organic fuels
Metabolic pathways that release stored energy by breaking down complex molecules are called catabolic
pathways
Transfer of electrons plays a major role in these pathways
• Catabolic Pathways and Production of ATP
→ Organic compounds possess potential energy as a result of the arrangement of electrons in the bonds
between their atoms
→ Through the activity of enzymes, a cell systematically degrades complex organic molecules that are rich
in potential energy to simpler waste products that have less energy.
→ Fermentation ~ is a partial degradation of sugars or other organic fuel that occurs without the use of
oxygen
→ 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.
→ Some prokaryotes use substances other than oxygen as reactants in a similar process that harvests
chemical energy without oxygen ~ anaerobic respiration
→ Food provides the fuel for respiration, and the exhaust is carbon dioxide and water
→ Carbohydrates, fats, and protein molecules from food can all be processed and consumed as fuel
→ In animal diets, a major source of carbohydrates is starch, a storage polysaccharide that can be broken
down into glucose (𝐶6 𝐻12 𝑂6 ) subunits
→ breakdown of glucose is exergonic, having a free-energy change of -686 kcal (2,870 kJ) per mole of
glucose decomposed (∆G = -686 kcal/mol). Recall that a negative ∆G (∆G < 0)
→ indicates that the products of the chemical process store less energy than the reactants and that the
reaction can happen spontaneously
→ Catabolic pathways do not directly move flagella, pump solutes across membranes, polymerize
monomers, or perform other cellular work
→ Catabolism is linked to work by a chemical drive shaft—ATP
TJW NOTES
,• Redox Reactions: Oxidation and Reduction
→ The relocation of electrons releases energy stored in organic molecules, and this energy ultimately is
used to synthesize ATP.
→ The Principle of Redox
⤷ In many chemical reactions, there is a transfer of one or more electrons (e-) from one reactant to
another.
⤷ These electron transfers are called oxidation-reduction reactions
⤷ Oxidation~ the loss of electrons from one substance
⤷ Reduction~ the addition of electrons to another substance
⤷ In the generalized reaction, substance Xe-, the electron donor, is called the reducing agent~ it reduces
Y, which accepts the donated electron.
⤷ Substance Y, the electron acceptor, is the oxidizing agent~ it oxidizes Xe- by removing its electron.
⤷ 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
⤷ Energy must be added to pull an electron away from an atom, just as energy is required to push a
ball uphill.
⤷ The more electronegative the atom (the stronger its pull on electrons), the more energy is required
to take an electron away from it.
⤷ An electron loses potential energy when it shifts from a less electronegative atom toward a more
electronegative one, just as a ball loses potential energy when it rolls downhill.
• Oxidation of Organic Fuel Molecules During
Cellular Respiration
→ In general, organic molecules that have an
abundance of hydrogen are excellent fuels
because their bonds are a source of “hilltop”
electrons, whose energy may be released as
these electrons “fall” down an energy gradient
during their transfer to oxygen.
→ The summary equation for respiration indicates
that hydrogen is transferred from glucose to
oxygen
→ But the important point, not visible in the
→ The main energy-yielding foods: carbohydrates summary equation, is that the energy state of
and fats, are reservoirs of electrons associated the electron changes as hydrogen (with its
with hydrogen, often in the form of C—H bond electron)is transferred to oxygen
→ Only the barrier of activation energy holds back → In respiration, the oxidation of glucose transfers
the flood of electrons to a lower energy state electrons to a lower energy state, liberating
→ Without this barrier, a food substance like energy that becomes available for ATP
glucose would combine almost instantaneously synthesis
with 𝑂2 → we see fuels with multiple C—H bonds
oxidized into products with multiple C—O
TJW NOTES bonds
, • Stepwise Energy Harvest via NAD+and the Electron Transport Chain
→ glucose is broken down in a series of steps, each one catalyzed by an enzyme
→ At key steps, electrons are stripped from the glucose
→ 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
→ This coenzyme is well suited as an electron carrier because 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
How does 𝑁𝐴𝐷+ trap electrons from glucose and the other organic molecules in food?
⤷ Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 electrons and 2 protons) from
the substrate 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
→ NAD+ is the most versatile electron acceptor in cellular respiration and functions in several of the redox
steps during the breakdown of glucose
→ 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.
→ This energy can be tapped to make ATP when the electrons complete their “fall” in a series of steps
down an energy gradient from NADH to oxygen.
How do electrons that are extracted from glucose and stored as potential energy in NADH finally reach
oxygen?
⤷ Mix 𝐻2 and 𝑂2 , provide a spark for activation energy, and the gases combine explosively
⤷ The explosion represents a release of energy as the electrons of hydrogen “fall” closer to the
electronegative oxygen atoms
⤷ Cellular respiration also brings hydrogen and oxygen together to form water but there are two important
differences:
▪ In cellular respiration, the hydrogen that reacts with oxygen is derived from organic molecules
rather than H2.
▪ Second, instead of occurring in one explosive reaction, respiration uses an electron transport
chain to break the fall of electrons to oxygen into several energy-releasing steps
TJW NOTES
