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Aerobic Respiration
The overall reaction for aerobic respiration is essentially photosynthesis in reverse.
Basically, the process involves oxidizing sugar carbons to CO2. Once again, an electron transport
system is used to create a hydrogen ion (proton) gradient that is used to make ATP. The final
depository for the electrons is O2, which is reduced to water.
Aerobic respiration is strongly identified with the mitochondria, however, the starting reactions
are in the cytosol. If oxygen is available, the mitochondria get involved with respiration, and the
internal structure of the mitochondria is critical to the process.
Mitochondria
Two membranes: outer membrane and inner membrane
Small volume area: intermembrane space
Large volume area: matrix
The carbon oxidation takes place in two sets of reactions, glycolysis in the cytoplasm and the
Krebs cycle in the matrix of the mitochondria. Both glycolysis and the Krebs cycle occur in
steps. Stepwise oxidation is important because:
1. dissipate energy that is released as heat
2. generate intermediates ==> steps to start making amino acids, N-bases, other sugars for cell
wall and nucleic acids, fatty acids, chlorophyll, anthocyanins, hormones, alkaloids, essential oils,
etc.
Glycolysis
Glycolysis oxidizes glucose or fructose (C6 sugars) to pyruvate (C3 acid). Some of the energy
released during the oxidation is captured as ATP. The electrons released by the oxidation are
held by NADH. Pyruvate is an organic acid. If the carbon is organic, it is at least partly reduced.
Therefore, there are still calories available in pyruvate.
The Krebs cycle
The Krebs cycle finishes the carbon oxidation process. All of the carbons in pyruvate are
oxidized to CO2. It is a cyclic process, with the pyruvate carbons getting attached to an existing
cycle molecule prior to oxidation. Once the pyruvate carbons are completely oxidized, the cycle
molecule needs to be regenerated. A little bit of ATP is made during the Krebs cycle, but mostly
it is an oxidation process. Most of the electrons are held by NADH.
Electron Transport Chain (ETC)
The electrons from NADH (and other electron holders) are passed to a system of electron
carriers in the inner mitochondrial membrane, eventually reaching O2 which is then reduced to
H2O. As the electrons pass through the carriers, protons (H+) are moved from the matrix to the
intermembrane space. This establishes a proton gradient. When the protons pass through a
channel protein back to the matrix, ATP is synthesized. How much ATP gets made? It depends
on who is doing the counting. Most plants that oxidize one glucose molecule completely to CO2
will get between 30 and 38 ATP from glycolysis, Krebs cycle, and ETC. The majority of that
ATP (85-90%) comes from the ETC. The high end of ATP yield (35-38 ATP) represents about
40% of the energy that was available in a molecule of glucose. The remaining energy is given off
as heat.
Some plants deliberately have an ATP yield in the mid-teens. Their electrons take an alternative
route in the ETC and don’t build as large as proton gradient by the time they get to O2. Because
the ATP yield is down, these plants release more heat. These plants are called thermogenic.
They can raise the temperature in their microenvironment as much as 10oC. This temperature
increase volatilizes scents to attract pollinators more efficiently.
Anaerobic respiration
Plants are able to respire anaerobically – without oxygen – for a short time. Glycolysis still
occurs, but any reactions involving the mitochondria are not available. Without oxygen and the
ETC, where can NADH dump off its electrons? Enter fermentation: a set of reactions that let
NADH unload electrons so glycolysis can continue. Most plants use alcohol (ethanol)
fermentation. The most likely anaerobic scenario for plants ==> roots in water-logged soil.
Some plants are specialized to have their roots always in water, like anchored aquatic plants and
emerged plants (water lilies, cattails, etc.) The internal anatomy of these plants has large air
spaces that form passage ways to get O2 to the roots. Hydrophytes
Review
Aerobic Respiration
Mitochondria structure: outer membrane, inner membrane, intermembrane space, matrix
carbon oxidation: glycolysis in the cytosol and the Krebs cycle in the mitochondrial matrix. Be
able to briefly state what happens as a result of these two processes.
Why is stepwise oxidation of sugar important?
What is the importance of generating carbon intermediates during glycolysis and the Krebs
cycle?
Electron Transport Chain (ETC)
Be able to briefly state what happens as a result of this process.
thermogenic plants
Anaerobic Respiration
From a cellular point of view, what is the function of fermentation reactions?