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6.3: Function and Applications of Carboxylic Acids

Difficulty Level: At Grade Created by: CK-12
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Lactic Acid and Exercise

One commonly known carboxylic acid is lactic acid, the substance that builds up in muscles during anaerobic exercise. Whenever we move we use adenosine triphosphate, or ATP, for energy. ATP is created from glucose in a process called aerobic respiration that occurs in every one of our cells. Only approximately 85 grams of ATP can be stored at one time. During exercise, all the stored ATP is quickly used up, and the body has to produce more.

Runners build up lactic acid in their muscles during intense training.

One product of aerobic respiration is the hydrogen ion, H+. This product is easily eliminated because when two hydrogen ions combine with inhaled oxygen, H2O is formed. This is why you start to breathe harder when you exercise harder; more oxygen is needed as you produce more ATP. This website can explain the process in greater detail:


However, everyone reaches a point where they cannot breathe in enough oxygen to produce the necessary amount of ATP. As the hydrogen ion builds up in the cell, a chemical called pyruvic acid (CH3COCO2H) will take the H+ and become lactic acid. The lactic acid dissociates into lactate and H+ before exiting into the blood stream. During this process, if the concentration of hydrogen ions is high enough to bring the pH down to 6.5 or lower, the acidity begins to stimulate the nerve endings in the muscles and creates the perception of pain.

This pain hinders athletic performance, which is why runners often train to prolong the amount of time they can run before experiencing pain. The amount of time an athlete can exercise before reaching this point is called the lactic, or anaerobic, threshold. These training workouts are almost always longer, even-paced runs. For example, in one common workout, athletes add one minute to their fastest mile time and try to keep that pace up for three miles. For more anaerobic threshold workouts, check out this site:


Usually it takes one hour for the body to eliminate all of the lactate from the muscles into the bloodstream, although this process will be sped up if a proper cool-down is included after intense exercise. The light exercise in the cool down causes heavier breathing, which allows the body to obtain more oxygen. The oxygen can bond to the hydrogen ions, decreasing the acidity. The average amount of lactic acid in the bloodstream is 1-2 millimoles/liter blood. However, after reaching the anaerobic threshold the average amount of lactic acid is usually 2-4 milimoles/liter blood.

Salicylic Acid in Skin Care Products

Carboxylic acids are not only found in the body but in commercial products. Scientists have discovered that the carboxylic acid beta hydroxyl acid, or BHA, can be used to create acne medications. It is therefore used frequently in cleansers, liquid foundations, moisturizers, anti-aging hydrating creams, eye gels, and sun screens. The type of BHA that is used most commonly in skin care products is salicylic acid.

Face care products often contain salicylic acid

Acne is formed when substances such as dead skin cells or bacteria clog pores. BHAs like salicylic acid are able to exfoliate dead skin cells, which speeds up the natural body process of shedding the outer layer of dead skin. This helps to prevent dead skin cells from clogging pores and to smooth rough skin. In addition, BHA is soluble in fat, which means it can dissolve in oils. Bacteria and dead skin cells have fat-based membranes, which allow them to stick to keratin and sebum, oils produced by the pores. The fat solubility of BHA allows it to dissolve into the oils and helps prevent bacteria and dead skin cells from sticking there, thus preventing acne. Therefore, salicylic acid is used in many successful skincare products.

Acetylsalicylic Acid in Aspirin

Closely related to salicylic acid is acetylsalicylic acid, the chemical contained in aspirins. Both chemicals can be extracted from willow bark.

In the body, cylooxygenase-2 is an enzyme produced by damaged cells. The enzyme in turn produces prostaglandin, which sends a message to the brain that causes the sensation of pain. It also causes inflammation around the damaged area. When acetylsalicylic acid enters the bloodstream after an aspirin pill is consumed, it binds to clyooxygenase-2 and prevents cylooyxgenase-2 from producing prostaglandin. This diminishes the sensation of pain and reduces inflammation.

Aspirin pills contain acetylsalicylic acid.

Acetylsalicylic acid also interferes with thromboxane production. Thromboxane is the chemical that causes platelets to stick, forming a blood clot. Blood clots are often associated with heart attacks, so for people suffering from cardiovascular diseases, this is considered a positive side effect of aspirin.

Finally, acetylsalicylic acid affects the hypothalamus, or the region of brain that regulates hormones. The hypothalamus is like a control center for all body functions. Aspirin is able to reduce a fever because it causes the hypothalamus to signal a reverse temperature change to the interleukins, which are signaling molecules produced by the body’s cells. For more information, visit:


The Many Uses of Citric Acid

Another example of carboxylic acids used for industrial purposes is citric acid. Citric acid is exceptionally acidic; because it is so acidic, citric acid has a sour taste and is often used to add flavor to sour candies. You may notice that some sour hard candies are covered in a white powder. That powder is citric acid.

Diagram of a citric acid molecule.

Because citric acid is non-toxic and acidic, it is an ideal preservative. Bacteria can only live and grow in certain pHs. The presence of citric acid causes the pH to drop to a point where it is difficult for bacteria to survive, allowing food to stay fresh for longer. Citric acid is abundant in most citrus fruits, like lemons and limes, making it an easy preservative to use at home. Sprinkling lemon juice on a fruit salad will prevent the fruit from turning brown.

Carboxylic Acids and Nutrition

It is a common misconception that consuming fats will result in body fat. However, carboxylic acids make up a series of fatty acids that are extremely good for your health. The omega-6 and omega-3 fatty acids are considered to be “essential” because they are required for good health yet cannot be produced by the body.

Another name for omega-6 is linoleic acid. It helps to maintain cell membranes and to control nutrient use and metabolism. If a meal is consumed without unsaturated fats, all the glucose and other nutrients will rush into the bloodstream without being absorbed. When unsaturated fats are consumed, they slow down digestion, which gives the body more time to absorb all the energy and nutrients from a meal.

Linoleic acid is also necessary to produce prostaglandins in a cell. Prostaglandins are produced by an enzyme called clyooxygenase-2 in cells and are responsible for creating the sensation of pain. Although feeling pain sounds like a bad thing, pain is necessary to help your body respond correctly to external stimuli. For example, if you touch a very hot object, the burning sensation tells your body to release the object.

Diagram of eicosapentaenoic acid (EPA), a type of omega-3.

Diagram of doxosahexaenoic acid (DHA), a type of omega-3.

Omega-3 is another extremely important fatty acid. There are two major types, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). They are both called “omega-3” because, in both compounds, the first double bond occurs after the third carbon. DHA is a 22-carbon chain with six double bonds, and EPA is a 20-carbon chain with five double bonds. Though both of these have many positive health effects, it is preferable to have higher levels of DHA in a supplement. Removing two carbon atoms from the DHA chain requires less energy than adding two carbon atoms to the EPA chain, making DHA easier to convert. Also, the brain uses DHA more frequently than EPA.

The most beneficial aspect of omega-3 is its ability to reduce the risk of heart disease. The American Heart Association (AHA) recommends consuming fish a minimum of two times every week because of the high omega-3 content in fish. According to the Maryland University Medical Center, consuming this much omega-3 reduces the risk of having a stroke by 50%.

Consuming omega-3 also leads to increased elasticity of the arteries. People with less flexible arteries are at risk for systolic hypertension, or high blood pressure when the heart contracts, which increases the risk for coronary diseases. This theory has been proven by multiple studies. In one study, 38 middle-aged men and women consumed an EPA or DHA supplement for 7 weeks. The EPA group saw artery elasticity increase by 36%, and the DHA group saw artery elasticity increase by 27%. In comparison, the placebo group, or control group, noticed no effect. Sudden cardiac deaths have also been shown to occur less frequently among people with a diet high in EPA/DHA.

Omega-3 has many other positive effects. It is contained in nerve endings in the brain and is important for cognitive and behavioral function. In fact, studies show that those who are deficient in omega-3 are at risk for vision and nerve problems. Additionally, a clinical study of 100 boys tested the effects of DHA on behavioral issues. Those with lower DHA levels had more behavioral issues, such as tantrums and sleep problems. Researchers are now testing the positive effects DHA has for ADHD patients. Taking a DHA supplement has also been shown to help patients alleviate some of the symptoms of depression or bipolar.

Omega-3 also decreases the effects of osteoporosis, as studies have indicated that omega-3 increases levels of calcium in the body. It can also help reduce the severity of rheumatoid arthritis, an autoimmune disease involving inflammation in the joints, because it decreases inflammation. Some of the other benefits of omega-3 consumption include improved eyesight, decreased menstrual pain, decreased severity of symptoms of Crohn’s disease (also known as Inflammatory Bowel Disease or IBD), reduced symptoms of asthma, and reduced risk of colon, prostate and breast cancer.

Salmon is a good source of omega-3.

Some symptoms of omega-3 deficiency include fatigue, poor memory, dry skin, heart problems, mood swings or depression, and poor circulation. Taking a DHA or EPA supplement or consuming more fish (salmon, tuna, and halibut), seafood (algae and krill), plants, and nut oils can help alleviate some of these symptoms. It is also important to balance your intake of omega-3 and omega-6 because omega-3 reduces inflammation in the body while omega-6 increases inflammation.

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