Name: protein breakdown and amino acid use
Uploaded: 2026-08-26T08:20:16.635Z
Duration: 1 min 31 s
Channel: JoVE

K12 Standards

Alabama Course of Study Science

9-12

Human Anatomy and Physiology

from-molecules-to-organisms-structures-and-processes

From Molecules to Organisms: Structures and Processes

9-12.HA.1.10

Protein Digestion

Protein digestion begins in the stomach. Hydrochloric acid unfolds dietary proteins by disrupting their three-dimensional structure. This unfolding exposes the peptide bonds within the polypeptide chain.The stomach enzyme pepsin then breaks these peptide bonds, forming shorter peptide fragments.As peptide-rich acidic chyme enters the duodenum, bicarbonate released by the pancreas neutralizes the acid. At the same time, pancreatic enzymes, including trypsin and chymotrypsin, continue breaking longer peptides into short peptides.Brush-border peptidases on the epithelial surface of the small intestine then break these short peptides into amino acids, dipeptides, and tripeptides. Intestinal epithelial cells then absorb these digestion products. Inside the cells, cytosolic peptidases break most dipeptides and tripeptides into individual amino acids.These amino acids move across the intestinal epithelium and into the bloodstream. The body uses them to build new proteins, including enzymes, hormones, and structural components. The liver can also metabolize amino acids for energy when needed.

protein digestion

Protein digestion begins in the stomach, where the highly acidic environment can easily disrupt protein structure by exposing the peptide bonds of polypeptide chains. After polypeptide chains are broken into individual amino acids by a series of digestive enzymes, the amino acids are transported to the liver via the bloodstream to produce energy.

<h4>Pepsin</h4>

Pepsin is a protease, or protein-digesting enzyme, that is produced in the stomach and is one of the main digestive enzymes in the human digestive system. Working in conjunction with chymotrypsin and trypsin released in the small intestine, pepsin severs the links between specific types of amino acids to form shorter polypeptide chains. Other enzymes, called peptidases, then split off one amino acid at a time from the ends of these polypeptide chains. The small intestine can easily absorb the resulting amino acids.

<h4>The Liver and Protein Metabolism</h4>

The liver plays an essential role in the metabolism of proteins. Liver cells alter digested amino acids from the small intestine so that they can be used to produce energy or make carbohydrates and fats. A byproduct of this process is a toxic substance called ammonia, which the liver then converts into a much less toxic substance called urea. Urea is then released into the blood and transported to the kidneys, which excrete urea out of the body through urine.

Protein digestion begins in the stomach, where the highly acidic environment can easily disrupt protein structure by exposing the peptide bonds of ...

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Biology

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Nutrition and Digestion

KEY TERMS AND CONCEPTS

protein breakdown and amino acid use

Protein digestion starts in the stomach, where strong acid helps unfold protein structure and expose peptide bonds in polypeptide chains. Digestive enzymes then begin breaking those chains into smaller pieces that can be processed by the body.

Pepsin is a protease, which means it is a protein-digesting enzyme. It is made in the stomach and works with chymotrypsin and trypsin, enzymes released in the small intestine. These enzymes cut specific links between amino acids and form shorter polypeptide chains.

Next, peptidases remove one amino acid at a time from the ends of those chains. The small intestine absorbs the resulting amino acids easily. They then travel through the bloodstream to the liver.

The liver uses these digested amino acids for protein metabolism. Liver cells can change them so they can be used to make energy, carbohydrates, or fats. During this process, ammonia is produced. The liver converts ammonia into urea, a much less toxic substance. Urea enters the blood, goes to the kidneys, and leaves the body in urine.

Protein Breakdown and Amino Acid Use

overview of human digestion

Saliva, Stomach Acid, and Gut Health

The human digestive system is monogastric, which means the stomach has one chamber for digestion. Several other animals also have monogastric digestive systems, including pigs, horses, dogs, and birds. This chapter focuses on the human digestive system. Saliva is a watery fluid released by the salivary glands into the mouth. It is about 99.5% water and also contains electrolytes, mucus, white blood cells, epithelial cells, enzymes, and antimicrobial agents. The enzymes in saliva begin...

anatomy of the intestines

Small and Large Intestine Functions

The small and large intestines complete digestion and absorb nutrients, water, and electrolytes. They are two structurally distinct organs with different shapes and jobs. Together, they move food material from the stomach toward elimination. The small intestine is about 7 meters long and has an inner diameter of only 2.5 cm. Most nutrient absorption happens here, so its inner lining is highly folded and covered with villi, which are finger-like projections. Each villus contains hundreds of...

accessory organs

Digestive Glands and Enzymes

Digestive glands and accessory organs help break down food even though food does not pass through them. They add enzymes and other fluids to the digestive tract to support digestion. The salivary glands release saliva, a fluid that contains water, mucus, and amylase. Amylase is a digestive enzyme that starts breaking down starches and other carbohydrates before food reaches the stomach. The liver, gallbladder, and pancreas also support digestion. They send their secretions into the duodenum,...

carbohydrate digestion

Carbohydrate Breakdown in the Digestive Tract

Carbohydrate digestion breaks food carbohydrates into saccharides, or sugars, that the body can use for energy. It begins in the mouth during mastication, which means chewing. The masticated carbohydrates then pass through the stomach without being broken down further. Digestion starts again in the duodenum, the first part of the small intestine. There, pancreatic alpha-amylase and brush border enzymes on the microvilli help convert complex carbohydrates into monosaccharides. These simple...

Protein digestion starts in the stomach, where strong acid helps unfold protein structure and expose peptide bonds in polypeptide chains. Digestive enzymes then begin breaking those chains into smaller pieces that can be processed by the body. Pepsin is a protease, which means it is a protein-digesting enzyme. It is made in the stomach and works with chymotrypsin and trypsin, enzymes released in the small intestine. These enzymes cut specific links between amino acids and form shorter...

lipid digestion

Lipases and Bile in Fat Breakdown

Lipases and bile help the body break down lipids, which are large molecules that do not mix well with water. Because most digestive enzymes are water-based, lipids need special steps before the body can use them. Lingual lipase is an enzyme made by acinar cells in the sublingual gland. It is found in saliva, but it has only a small role in the mouth. Its best pH, or acidity level, is 3.5-6.0, so it becomes active after chewed food reaches the acidic stomach. Gastric lipase is another acidic...

neural regulation

Brain and Gut Control of Digestion

Brain and gut control digestion through a cephalic phase and the enteric nervous system. The cephalic phase begins when the brain sees or smells food. It sends impulses through cranial nerves to the salivary glands and stomach so the body is ready to eat. This first response is a conditioned, or learned, reaction to familiar foods. Appetite also changes the response. People may make more saliva and hear more stomach rumbling when they think about apple pie than broccoli. The hypothalamus and...

hormonal regulation

Digestive Hormones and Stomach Control

Digestive hormones control when the stomach makes acid and when food moves into the small intestine. They work with the neuroendocrine system, which links nerve signals and hormone release during digestion. In the stomach, proteins are detected by sensory neurons in the enteric nervous system. This stimulates the pyloric gland to release gastrin. Gastrin then helps trigger histamine release, and together they start hydrochloric acid production. The acid helps break food down into chyme. As the...

key terms and concepts

Follow protein digestion from the stomach to the liver, including pepsin, peptidases, amino acid absorption, and urea formation for excretion.

Protein Breakdown and Amino Acid Use | JoVE K12

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Florida Department of Education

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Structure and Function

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structural-organization-of-the-human-body-an-overview

Structural Organization of the Human Body: An Overview

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a. Construct an explanation of the relationships between the integumentary system and other organ systems, including the body's mechanisms for maintaining homeostasis. Examples: Explain how changes in vessel diameter and evaporation of water over the skin contribute to thermoregulation (cardiovascular). Explain how sweat is used to excrete nitrogenous waste from the blood (cardiovascular). Explain how the skin and mucous membranes create a barrier against infection (immune system).

9-12.HA.1.2.a

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3. Develop and use a model to illustrate how the structures of the skeletal system contribute to its function. Examples: Create a diagram or 3D model to illustrate bone shape, joint types, and bones in the appendicular and axial skeletons. Use models to illustrate the role of the skeletal system in movements around joints, support of a body structure, and protection of soft tissues.

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a. Obtain, evaluate, and communicate information describing the growth and development of the skeletal system.

9-12.HA.1.3.a

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9-12.HA.1.3.b

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4. Develop and build a three-dimensional model to illustrate the structures of the muscular system, including muscle locations, origins, and insertions, and explain their roles in movement and support. Example: Design and build a model of a lever to illustrate the relationship between bones and muscles and explain their roles in body movement and support.

9-12.HA.1.4

Microscopic Anatomy of Skeletal Muscles

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The Sarcomere

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Excitation-Contraction Coupling in Skeletal Muscles

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a. Model the cellular physiology of skeletal muscle, including how the cell functions in muscle contraction and relaxation. Example: Use straws and paper to create a model of a sarcomere to illustrate how the fibers move during contraction and relaxation.

9-12.HA.1.4.a

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b. Obtain, evaluate, and communicate information to explain muscle fatigue and tone in terms of muscle cell physiology.

9-12.HA.1.4.b

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5. Obtain, evaluate, and communicate information explaining the relationship between the structures and functions of the central nervous system and the peripheral nervous system.

9-12.HA.1.5

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Neurons communicate by firing action potentials&#8212;the electrochemical signal that is propagated along the axon. The signal results in the release of ...

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9-12.HA.1.5.d

Protein Breakdown and Amino Acid Use

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