Bacteria: Cell Walls & Respiration

Instructor: Amanda Robb
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.

Bacteria structure
bacteria

Structure of the Cell Wall

The cell wall is an important protective structure for bacteria. Think of it like a wall around a castle. It keeps the bacteria safe from unwanted intruders, including your immune system. The cell wall also plays an important role in attaching bacteria to surfaces, protecting it from harsh environmental conditions, and in making energy for the cell.

Bacteria cell walls are made of a thick layer of a molecule called peptidoglycan, a combination of sugars and proteins linked together. Some bacteria have one thick layer, and others have two thin layers with space in between. Other bacteria have a waxy substance in their cell wall, like mycobacteria, the bacteria that causes tuberculosis.

Cell wall structure in bacteria
cell wall structure

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

Respiration is part of how cells make energy. Bacteria have two ways of making energy:

  1. aerobic respiration which does involve oxygen, and
  2. anaerobic respiration, which does not involve oxygen.

Aerobic respiration makes more energy and involves the cell wall, so we'll start there.

Glycolysis

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.

Glycolysis in bacteria
glycolysis

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.

Citric acid cycle in bacteria
citric acid cycle

Oxidative Phosphorylation

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.

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