In this lesson we’ll review what bacteria are and the structure of the cell wall. Then we’ll learn the purpose of respiration in the cell and explain in detail how the cell wall allows for respiration in bacteria.
What Are Bacteria?
As it gets colder outside, more and more of us succumb to infection.
Your sickness might start with a tingling in the throat, or fatigue. As you walk around you’re a living incubator. The pathogen mutiplies inside of you and a war rages on undetected.Often times, this invader is a bacteria. Bacteria are prokaryotes, single-celled organisms with a very simple structure. They contain a goopy material called cytoplasm and a few protein factories.
All cells are enclosed by a thin barrier on the outside that only lets certain things in and out of the cell, called the plasma membrane, kind of like a sock on your foot. Outside of that, bacteria have a thick outer covering called the cell wall, kind of like a shoe over your sock. Let’s look more at the structure and function of the cell wall in bacteria.
All of these walls exist around the plasma membrane. In between the cell wall and the plasma membrane is a small amount of space called the periplasm. This space is responsible for certain types of respiration in bacteria.
Respiration is part of how cells make energy. Bacteria have two ways of making energy:
- aerobic respiration which does involve oxygen, and
- anaerobic respiration, which does not involve oxygen.
Aerobic respiration makes more energy and involves the cell wall, so we’ll start there.
The first step in aerobic respiration is glycolysis, which occurs in the cytoplasm. In glycolysis, glucose in the cell is broken down. In the process, electrons from the original glucose molecule are released and collected by electron carrier molecules called NAD+, which, once they have the electron, are made into NADH. This step makes a little energy, called ATP, but not much.
The broken down glucose molecule that results is called pyruvate, which is needed for the next step.
Citric Acid Cycle
The next step is the citric acid cycle, where pyruvate goes through a series of chemical reactions and is converted to other molecules. This happens in the mitochondria of eukaryotic cells, but bacteria don’t have mitochondria, so we’re still in the cytoplasm. This process releases more electrons, and electron carrier molecules NAD+ and FADH rush in to grab them, which turns them into NADH and FADH2. This step also makes a little ATP (but not a lot) and releases carbon dioxide.
After the citric acid cycle, the cell gathers all its electron carriers to dump their electrons off at the plasma membrane in a process called oxidative phosphorylation, the last step of respiration. This area of the plasma membrane is called the mesosome and has lots of folds to give the cell more space to do respiration. The electron carriers unload their electrons one at a time into a chain of proteins, called the electron transport chain.
Each protein in the chain likes electrons more than the protein before it, so the electrons keep moving towards the next protein. As each protein gets the electrons, they pump hydrogen ions into the periplasm (the space between the membrane and the cell wall). The cell wall acts as an outer barrier, allowing the hydrogen ions to build up.
All the electrons eventually end up at the end of the electron chain. The last molecule to take the electrons is oxygen. Oxygen loves electrons more than any of the proteins in the chain, so all the electrons end up at the oxygen molecules. Oxygen grabs electrons as well as hydrogen ions from inside the cell and makes water.Now, back to our periplasm. At this point, lots of hydrogen ions are collecting in the periplasm.
This creates a chemical gradient, where there are more hydrogen ions on one side than the other. Everything in the whole world wants to move from where there is more to where there is less until there is a balance. So these hydrogen ions are itching for a way to get in or out of the cell. In the plasma membrane there is a special enzyme called ATP synthase.
This protein allows hydrogen ions to flow back into the cell, and harnesses the energy released to make a ton of ATP.
In our cells, oxidative phosphorylation occurs in an organelle called the mitochondria. Bacteria don’t have this organelle, so they have to use the cell wall instead.
The cell wall is important, because without it, bacteria would be stuck using fermentation, a process occurring in the cytoplasm that only makes four ATP per glucose, which is much less lucrative than the 36-38 ATP using cellular respiration!
The bacterial cell wall is a thick outer structure made of peptidoglycan (a combination of sugars and proteins) that protects the cell and is important for making energy. Bacteria do aerobic respiration using oxygen.
- The first step, glycolysis, occurs in the cytoplasm and makes a few ATP and NADH, an electron carrier.
- Next, the citric acid cycle makes a few more ATP, carbon dioxide and more NADH and FADH2 in the cytoplasm.
- Next, the electron carriers move to the mesosomes of the plasma membrane and the cell wall. Proteins in the electron transport chain move hydrogen ions into the periplasm, the space between the cell wall and the plasma membrane.
- Finally, the hydrogen ions flow back into the cell through ATP synthase to make ATP.