Respiration is the process of converting nutrients into usable energy.
Several different mechanisms exist in the bacterial world. In this lesson, we will examine the role of oxygen in bacterial aerobic respiration.
Metabolic Building Blocks
Humans consume food for energy and use oxygen for respiration in a process referred to as metabolism. Believe it or not, bacterial cells carry out almost exactly the same processes. Understanding bacterial metabolism can be difficult.
There are many pathways and enzymes, and it is not always clear how they link together. In this lesson, we will use a simple way to visualize some metabolic pathways. So stick with me, and by the end, you should have a good understanding of some basic pathways and how they fit together in a bacterial cell.One of the easiest ways to visualize the connections between metabolic pathways is to imagine the various pathways and processes as individual blocks. Let’s use these to build a metabolic model of a bacterial cell growing by aerobic respiration. Aerobic respiration is the process in which a compound is oxidized, using oxygen as the terminal electron acceptor. That definition probably doesn’t make much sense yet, but by the end of the lesson, it will come together.
Okay, let’s start building. Our cell will use glucose as an energy source under oxygen conditions.
The first metabolic block we will choose is glycolysis. The glucose is taken up by the cell and broken down via glycolysis. If you dissect the term ‘glycolysis,’ ‘glyco-‘ means sugar, and ‘-lysis’ means to split. So, very simply, glycolysis uses enzymes to split glucose molecules into pyruvate. During the breakdown of glucose, electrons are liberated and used to form NADH, or nicotinamide-adenine dinucleotide, an important electron carrier in the cell.
Let’s take a quick timeout. In biological systems, energy is captured and conserved using oxidation-reduction reactions, or redox reactions for short. Basically, redox reactions involve the moving around of electrons between molecules, with ‘oxidation‘ referring to the removal of electrons and ‘reduction‘ referring to the addition of electrons.
So, to relate that to glycolysis, the glucose is oxidized, meaning the electrons are stripped from it, breaking it down into pyruvate.The pyruvate formed by glycolysis serves as the connection between the glycolysis block and our next block, the citric acid cycle.
The Citric Acid Cycle
The citric acid cycle is a series of enzymes used to break down pyruvate all the way to carbon dioxide. This cycle oxidizes the pyruvate, capturing its electrons and depositing them on NAD to form NADH. The NADH formed during glycolysis and the citric acid cycle connects to the next block in our pathway, the electron transport chain.
The Electron Transport Chain
The electron transport chain is a series of enzymes, proteins and molecules embedded in the cell membrane that pass along electrons.
The components act as a sort of conveyor belt for electrons. The first enzyme complex is a dehydrogenase that removes the electrons from NADH and passes them to a molecule called quinone. The quinone molecules pass them on to a series of proteins called cytochromes that then transport them to the final multi-cytochrome enzyme in the chain, called the terminal oxidase. The terminal oxidase is an enzyme complex that transfers the electrons to oxygen, generating water and terminating the electron transfer process. The most important part is that as the conveyor belt passes along the electrons, energy can be captured to simultaneously pump protons out of the cell.
This results in a gradient of protons across the cell membrane, high on the outside and low on the inside. This proton gradient feeds the next block in our metabolic model, the ATP synthase.Okay, let’s use one more timeout. The best analogy for the proton gradient is the electrical current you use to power the appliances in your house. That current is simply the flow of charged particles (electrons) from a source (the outlet) to a refrigerator (the appliance).
The proton gradient is referred to as the proton motive force and, like the current you access through your outlets, results in the flow of charged particles – in this case, protons. The proton motive force is an energized state across the membrane due to the proton gradient. Just like the electricity in our homes, this energized state can be measured in volts.
The ATP Synthase
The ATP synthase utilizes the proton motive force to generate ATP. ATP, or adenosine triphosphate, is the energy currency of the cell. It is a high-energy molecule that is used to build all the cell components and structures. The ATP synthase is located in the cell membrane and is able to open up a channel that allows the protons to flow down the gradient.
The energy provided by the flow of protons results in a force that spins the ATP synthase. That spin energy is used to catalyze the addition of a phosphate group to ADP (or adenosine diphosphate) to generate ATP. This process of ATP synthesis is referred to as oxidative phosphorylation, or the generation of ATP using a proton motive force generated by an electron transport chain.
Now we have completed our metabolic model cell. This cell is using glucose as a source of energy and oxygen as an electron acceptor to generate ATP.
Now, we have done a great job building a model cell, but rarely is a cell growing on pure glucose.
Out there in the real world, there is a vast array of potential growth substrates, and we would have many additional blocks to choose from to build a functioning bacterial cell. Some of these blocks would completely replace one of the blocks we used today. Other blocks would fit neatly into our existing pathway. And still other blocks would replace almost every block in our original model.
Aerobic respiration is the process in which a compound is oxidized using oxygen as the terminal electron acceptor.
In the beginning, this may have seemed complicated, but now we can use our model cell to better understand that definition. At the start of the process, glucose is oxidized. Through a series of steps and pathways, the electrons that originated from glucose end up in the electron transport chain. The electron transport chain conveyor belt passes the electrons along while simultaneously pumping protons out of the cell. At the end of the chain, oxygen is reduced to water, and the electron’s work is done.
Importantly, the whole goal is to generate the proton gradient that results in a proton motive force that powers the ATP synthase to generate energy for the cell – a process termed oxidative phosphorylation.
After you’ve completed this lesson, you will be able to:
- Recite the process of aerobic respiration
- Provide an example of a metabolic building block
- Emphasize the importance of NADH during glycolysis
- Know the way in which the proton motive force is created and its importance
- Identify the role of ATP synthase