- Chemoorganotrophy is a term used to denote the oxidation of organic chemicals to yield energy.
- In other words, an organic chemical serves as the initial electron donor.
- The process can be performed in the presence or absence of oxygen, depending upon what is available to a cell and whether or not they have the enzymes to deal with toxic oxygen by-products.
- To start, let us focus on the catabolism of organic compounds when it occurs in the presence of oxygen. In other words, oxygen is being used as the final electron acceptor.
- When the process utilizes glycolysis and the tricarboxylic acid (TCA) cycle to completely oxidize an organic compound down to CO2, it is known as aerobic respiration.
- This generates the most ATP for a cell, given the large amount of distance between the initial electron donor (glucose) and the final electron acceptor (oxygen), as well as the large number of electrons that glucose has to donate.
ORGANIC ENERGY SOURCES
- In Chemoorganotrophy, energy is derived from the oxidation of an organic compound.
- There are many different organic compounds available to a cell, such as proteins, polysaccharides, and lipids.
- But cellular pathways are arranged in such a way to increase metabolic efficiency.
- Thus, the cell funnels reactions into a few common pathways.
- By convention, glucose is used as the starting molecule to describe each process.
- Glycolysis is a nearly universal pathway for the catabolism of glucose to pyruvate.
- The pathway is divided into two parts: part I, which focuses on modifications to the 6-carbon sugar glucose, and part I1, where the 6-carbon compound is split into two 3-carbon molecules, yielding a bifurcated pathway.
- Part I actually requires energy in the form of 2 molecules of ATP, in order to phosphorylate or activate the sugar.
- Part II is the energy conserving phase of the reaction, where 4 molecules of ATP are generated by substrate-
- level phosphorylation, where a high-energy molecule directly transfers a Pi to ADP.
- The net yield of energy from glycolysis is 2 molecules of ATP for every molecule of glucose.
- In addition, 2 molecules of the carrier NAD+ are reduced, forming NADH.
- In aerobic respiration, these electrons will ultimately be transferred by NADH to an electron transport chain, allowing the cell to capture more energy.
- Lastly, 2 molecules of the 3-carbon compound pyruvate are produced, which can be further oxidized to capture more energy for the cell.
TRICARBOXYLIC ACID (TCA) CYCLE
- The tricarboxylic acid (TCA) cycle picks up at the end of glycolysis, in order to fully oxidize each molecule of pyruvate down to 3 molecules of CO2, as occurs in aerobic respiration.
- It begins with a type of connecting reaction before the molecules can enter the cycle proper.
- The connecting reaction reduces 1 molecule of NAD+ to NADH for every molecule of pyruvate, in the process of making citrate.
- The citrate enters the actual cycle part of the process, undergoing a series of oxidations that yield many different products.
- It’s many of them important precursor metabolites for other pathways.
- As electrons are released, carriers are reduced, yielding 3 molecules of NADH and 1 molecule of FADH2 for every molecule of pyruvate.
- In addition, 1 molecule of GTP (which can be thought of as an ATP-equivalent molecule) is generated by substrate level phosphorylation.
- Taking into account that there were two molecules of pyruvate generated from glycolysis.
- the net yield of the TCA cycle and its connecting reaction are: 2 molecules of GTP, 8 molecules of NADH, and 2 molecules of FADH2.
- But where does the ATP come from? So far we only have the net yield of 2 molecules from glycolysis and the 2 molecules of ATP-equivalents (i.e. GTP) from the TCA cycle.
- This is where the electron transport chain comes into play.
- The synthesis of ATP from electron transport generated from oxidizing a chemical energy source is known oxidative phosphorylation.
PROTON MOTIVE FORCE
- Protons that are not accepted by electron carriers migrate outward, to line the outer part of the membrane.
- For bacteria and archaea, this means lining the cell membrane and explains the importance for the negative charge of the cell.
- As the positively charged protons accumulate, a concentration gradient of protons develops.
- This results in the cytoplasm of the cell being more alkaline and more negative, leading to both a chemical and electrical potential difference.
- This proton motive force (PMF) can be used to do work for the cell, such as in the rotation of the bacterial flagellum or the uptake of nutrients.
- The PMF can also be used to synthesize ATP, with the help of an enzyme known as ATP synthase (or ATPase).
- This large enzyme has two components, one that spans the membrane and one that sticks into the cytoplasm and synthesizes the ATP.
- Protons are driven through the membrane-spanning component, generating torque that drives the rotation of the cytoplasmic portion.
- When the cytoplasmic component returns to its original configuration it binds Pi to ADP, generating a molecule of ATP.