Video Transcript
In this video, we will learn to recall the reactants and products of anaerobic respiration and compare this process to aerobic respiration. So let’s make like yeast with or without access to oxygen and get started. The process of cells breaking down nutrients to release energy is called cellular respiration. If cellular respiration relies on oxygen, it’s referred to as aerobic respiration. Organisms that thrive in the presence of oxygen are referred to as aerobic organisms. If cellular respiration occurs without the necessity for oxygen, it’s referred to as anaerobic respiration. An- is a prefix that means not.
Many types of microscopic organisms, including most of the bacteria that live in and on our bodies, carry out anaerobic respiration. Many of these organisms actually find oxygen to be toxic and can only live in oxygen-free or low-oxygen environments, like our digestive tracts, where many species of bacteria help us to digest our food. Organisms that cannot use oxygen for cellular respiration are referred to as anaerobic organisms. Sometimes aerobic cells find themselves in situations where oxygen is scarce. In these cases, they must rely on anaerobic processes to generate the cellular energy that they need. However, anaerobic processes in aerobic cells often generate toxic byproducts, and they’re much less efficient than aerobic respiration.
Another major difference is that aerobic respiration takes place within the mitochondria of the cell, while the anaerobic processes we’re about to describe take place within the cytoplasm. So let’s take a closer look at two examples of anaerobic processes that occur in aerobic organisms. Let’s start by recalling that aerobic respiration utilizes oxygen and glucose, which are converted into carbon dioxide and water. This exothermic chemical reaction releases energy, and that energy is stored in a molecule called ATP, which is used to power almost all cellular functions.
Let’s imagine that a busy biology student steps outside and immediately takes off running as fast as they can. At first, they will likely continue to breathe normally. Over time, their respiration rate will increase as their muscles demand more oxygen to produce more ATP to use as a source of energy. Eventually, they’ll be breathing as hard as they can, and their heart will be pumping as fast as it can. And the mitochondria within their muscle cells will be producing as much ATP as they can. Their body cannot take in any more oxygen, but they continue to run. Their muscle cells have to keep making ATP for energy somehow, so the cells begin to use anaerobic processes.
The anaerobic process utilized by your muscle cells is called lactic acid fermentation. Lactic acid fermentation converts one molecule of glucose into two molecules of lactic acid, also called lactate. This is the chemical equation for lactic acid fermentation. While aerobic respiration releases enough energy from one molecule of glucose to generate up to 38 molecules of ATP, lactic acid fermentation is a less efficient process, incompletely breaking down the glucose and only releasing enough energy to generate two molecules of ATP. Lactic acid is also toxic to yourselves in high concentrations. It’ll have to be broken down further later and safely removed.
Let’s return our attention to our runner and our graph. The longer they keep running after they’ve reached the maximal oxygen intake, the more lactic acid fermentation their cells will have to undergo and the more lactic acid concentration will increase. It’s important to note that once lactic acid fermentation starts, aerobic respiration does not stop. Aerobic respiration occurs in the mitochondria of your cells continuously. Lactic acid fermentation is like a backup plan for your cells for when they need more ATP but cannot get more oxygen. But they can’t keep this up forever.
Once our runner’s cells start lactic acid fermentation, they can’t keep running at this pace for much longer. Luckily for our biology student, if they continue to exercise regularly, their muscle cells will grow and produce more mitochondria. So they’ll be able to run for longer and generate cellular energy more efficiently.
Lactic acid fermentation has some other interesting uses. Did you know that one of the most common and ancient methods for preserving food involves lactic acid fermentation? There are some organisms that only rely on fermentation to generate cellular energy. An example of one such organism is a very common type of bacteria called Lactobacillus. Lactobacillus live in many locations on and within the human body, including the mouth, the skin, the kidneys, the stomach, and the intestines, where they help to protect our tissues from other dangerous pathogenic microorganisms.
These bacteria are also used in lactofermentation, a common type of pickling. Vegetables are placed into a container, and a high concentration of salt is added. The salt prevents the growth of other types of bacteria. The Lactobacillus, so common that they’re already present on the vegetables, are tolerant of the high salt concentration and continue to grow. They feed on the sugars and the vegetables, producing lactic acid as a result. The combination of salt and lactic acid not only provides the vegetables with a transformed flavor and texture; they prevent the growth of microorganisms that may spoil the food, thus preserving it.
Keep in mind that humans didn’t always have access to technology like refrigeration, which keeps our food fresh today. Lactic acid fermentation is also used in the production of cocoa, yogurt, many cheeses, and certain types of sour breads.
Next, let’s look at another type of fermentation. Yeast are a type of single-celled fungus. You’re probably familiar with yeast if you’ve ever baked bread. Yeast don’t do much with their time other than reproduce. But reproduction is a process that requires a significant amount of energy. Yeast, like humans, typically carry out aerobic respiration to generate this cellular energy. They break down glucose and oxygen, which is rearranged into carbon dioxide and water, releasing energy that’s stored in the ATP molecule.
When yeast are grown in an environment with enough glucose but not enough oxygen, they also switch to anaerobic processes to continue to generate cellular energy. In yeast, this anaerobic process is called alcoholic fermentation. During alcoholic fermentation, one molecule of glucose is converted into two molecules of ethyl alcohol and two molecules of carbon dioxide. This exothermic process releases enough energy to produce two molecules of ATP. Like lactic acid fermentation, alcoholic fermentation is less efficient than aerobic respiration. Less ATP is generated, and the glucose is incompletely broken down into ethyl alcohol, a substance that’s toxic to most living organisms. In fact, while certain beverages contain ethyl alcohol, it’s also commonly used as a disinfectant. It’s an effective chemical solvent, and it’s combustible, so it’s often used as fuel.
Let’s return our attention to our baker. In order to bake bread, first, yeast are usually provided with a supply of sugar. This causes them to begin to reproduce. Within the bread dough, yeast have limited access to oxygen. So they utilize the anaerobic process of alcoholic fermentation. This produces some ethyl alcohol and carbon dioxide gas. The carbon dioxide is trapped, creating bubbles in the stretchy dough, which over the course of a few hours causes the bread to rise. Then the bread is baked. The heat of the oven removes any traces of alcohol and locks the structure into place. This process gives bread the fluffy texture that so many people find delicious.
In this video, we learned the difference between aerobic and anaerobic methods for producing cellular energy. We were also introduced to the anaerobic processes, lactic acid fermentation and alcoholic fermentation.
