- Living cells require transfusions of energy from outside sources to perform their many
tasks—for example, assembling polymers, pumping substances across membranes,
moving, and reproducing.
- Energy flows into an ecosystem as sunlight and exits as heat; in contrast, the chemical
elements essential to life are recycled.
9.1 – Catabolic pathways yield energy by oxidizing organic fuels
- Compounds that can participate in exergonic reactions can act as fuels.
- One catabolic process,
, is a partial degradation of sugars or other organic
fuel that occurs without the use of oxygen.
- However, the most prevalent and efficient catabolic pathway is
which oxygen is consumed as a reactant along with the organic fuel.
- Some prokaryotes use substances other than oxygen as reactants in a similar process
that harvests chemical energy without oxygen; this process is called
- Technically, the term
includes both aerobic and anaerobic
processes. However, it originated as a synonym for aerobic respiration because of the
relationship of that process to organismal respiration, in which an animal breathes in
oxygen. Thus, cellular respiration is often used to refer to the aerobic process.
- The breakdown of glucose is exergonic.
- The transfer of electrons in a chemical reaction is known as oxidation-reduction
- In a redox reaction, the loss of electrons from one substance is called
, and the
addition of electrons to another substance is known as
Reducing agent –
The electron donor
Oxidizing agent –
The electron acceptor.
- Xe- (oxidized)+ Y (reduced) => X (reducing agent) + Ye- (oxidizing agent)
- Nicotinamide adenine dinucleotide, a coenzyme that cycles easily between
oxidized (NAD+) and reduced (NADH) states, thus acting as an electron carrier
electron transport chain
consists of a number of molecules, mostly proteins, built
into the inner membrane of the mitochondria of eukaryotic cells and the plasma
membrane of aerobically respiring prokaryotes.
- In summary, during cellular respiration, most electrons travel the following “downhill”
route: glucose => NADH => electron transport chain => oxygen.
which occurs in the cytosol, begins the degradation process by breaking
glucose into two molecules of a compound called pyruvate.
- In eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound called
acetyl CoA, which enters the
citric acid cycle.
- The energy released at each step of the chain is stored in a form the mitochondrion
(prokaryotic cell) can use to make ATP from ADP. This mode of ATP synthesis is called
because it is powered by the redox reactions of the electron
- Oxidative phosphorylation accounts for almost 90% of the ATP generated by