Carbohydrate Digestion and Absorption: Process & End Products

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  • 0:07 Carbohydrates
  • 1:35 Brush Border Enzymes
  • 2:07 Absorption
  • 3:04 Lesson Summary
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Lesson Transcript
Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Expert Contributor
Brenda Grewe

Brenda has 25 years of experience teaching college level introductory biology and genetics. She earned her PhD in Genetics from Indiana University.

Carbohydrates that you eat are broken down to monosaccharides by enzymes in your digestive tract. In this lesson, you will learn about these digestive enzymes and how monosaccharides are absorbed out of the digestive tract.


Carbohydrates are nutrients that provide your body with energy. But, before carbohydrates can fuel your morning run, they must be broken down into their basic units, called monosaccharides, and absorbed out of your digestive tract and into your bloodstream. In this lesson, you will learn about the enzymes that break down carbohydrates and how this important nutrient is absorbed.

Carbohydrate digestion begins in the mouth with salivary amylase.
Carbohydrate Digestion Begins in Mouth

We previously learned that digestion of carbohydrates, and in particular starches, begins in the mouth with the action of salivary amylase. This enzyme catalyzes, or speeds along, the hydrolysis of the starch molecule. You may recall that hydrolysis is how nutrients that you eat are broken down, and it involves splitting bonds with water.

Even though carbohydrate digestion begins in your mouth, very few of us chew our food long enough for salivary amylase to have a significant effect on the carbohydrates that we eat. So, we swallow the carbohydrate somewhat intact. When you swallow the food mass, some of the salivary amylase travels along with it, and you would think that the enzyme would keep working to break down the carbohydrate. However, the enzyme is inactivated in the stomach because the environment of the stomach is too acidic.

Digestion of the carbohydrate does not resume until the food mass reaches the first part of the small intestine that we call the duodenum. There, the carbohydrate meets pancreatic amylase, which is similar to salivary amylase and continues the breakdown of the carbohydrate.

Brush Border Enzymes

Brush border enzymes in the small intestine complete the digestion of carbohydrates.
Brush Border Enzymes

Any remaining sugars are acted upon by brush border enzymes. Brush border enzymes are special enzymes found on the microvilli of the small intestine that complete digestion. We previously learned that microvilli are tiny, hair-like projections that increase the surface area of the small intestine and therefore increase nutrient absorption. Because there are so many microvilli, the epithelial cells appear to be fuzzy, like the bristles of a paint brush, leading some anatomists to refer to them as the brush border, hence the name.

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Additional Activities

Practice Scenarios

  1. On your way to your morning Nutrition class, you stop at the campus coffee shop to grab a pastry and a coffee. You order the coffee with three sugars. You read about carbohydrate digestion the night before and begin to imagine what is happening to the carbohydrates in your pastry and coffee as they enter and pass through your digestive system. Describe the processing steps for the carbohydrates in your breakfast. Include locations of each step and the enzyme(s) involved. The pastry contains a combination of starch (a polysaccharide) and sucrose (a disaccharide). The sugar in your coffee is pure sucrose.
  2. One afternoon a week, you visit an elderly woman at the local retirement community. On your current visit, you arrive and she appears to be napping. When you get closer you realize that she is barely breathing and is unresponsive when you see if you can wake her. You immediately call 911. When the EMTs arrive, one of them places an oxygen mask over your friend's mouth and nose and starts an I.V. of electrolytes and glucose. Explain why glucose is included in the I.V. rather than some other carbohydrate. Consider what is needed at the cellular level.
  3. Crohn's disease causes chronic inflammation and irritation of the digestive tract. Most commonly, it affects tissues of the small intestine. Symptoms include abdominal pain, diarrhea, anemia, weight loss, and fatigue. A recent study examined tissue samples from the small intestine of patients with Crohn's disease. Researchers found that the microvilli were of decreased length and had ultrastructural defects compared with intestinal tissue from individuals without Crohn's disease. How might the altered length of the microvilli affect carbohydrate digestion and absorption?


  1. Your answer should include the role and location of action of salivary amylase, pancreatic amylase and brush border enzymes.
  2. Glucose can directly enter the cellular process of glycolysis, and in the presence of oxygen, be fully oxidized in cellular respiration to release maximal energy for ATP production. ATP is the energy source for all cell activities, and without it cells die. Other carbohydrates would require more processing before they can enter glycolysis or later steps of cellular respiration.
  3. The microvilli of the small intestine are the location of digestive enzymes and nutrient absorption. These numerous hairlike structures greatly increase the membrane surface area for absorption of glucose into the bloodstream. If the microvilli are of reduced length, the surface area for absorption of glucose and other nutrients will be reduced. The altered structure may further impair absorption, leading to inability to fully meet the body's energy needs.

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