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By the end of this section, you will be able to:
  • Describe how the body digests proteins
  • Explain how the urea cycle prevents toxic concentrations of nitrogen
  • Differentiate between glucogenic and ketogenic amino acids
  • Explain how protein can be used for energy

Much of the body is made of protein, and these proteins take on a myriad of forms. They represent cell signaling receptors, signaling molecules, structural members, enzymes, intracellular trafficking components, extracellular matrix scaffolds, ion pumps, ion channels, oxygen and CO 2 transporters (hemoglobin). That is not even the complete list! There is protein in bones (collagen), muscles, and tendons; the hemoglobin that transports oxygen; and enzymes that catalyze all biochemical reactions. Protein is also used for growth and repair. Amid all these necessary functions, proteins also hold the potential to serve as a metabolic fuel source. Proteins are not stored for later use, so excess proteins must be converted into glucose or triglycerides, and used to supply energy or build energy reserves. Although the body can synthesize proteins from amino acids, food is an important source of those amino acids, especially because humans cannot synthesize all of the 20 amino acids used to build proteins.

The digestion of proteins begins in the stomach. When protein-rich foods enter the stomach, they are greeted by a mixture of the enzyme pepsin    and hydrochloric acid (HCl; 0.5 percent). The latter produces an environmental pH of 1.5–3.5 that denatures proteins within food. Pepsin cuts proteins into smaller polypeptides and their constituent amino acids. When the food-gastric juice mixture (chyme) enters the small intestine, the pancreas releases sodium bicarbonate    to neutralize the HCl. This helps to protect the lining of the intestine. The small intestine also releases digestive hormones, including secretin    and CCK, which stimulate digestive processes to break down the proteins further. Secretin also stimulates the pancreas to release sodium bicarbonate. The pancreas releases most of the digestive enzymes, including the proteases trypsin, chymotrypsin, and elastase    , which aid protein digestion. Together, all of these enzymes break complex proteins into smaller individual amino acids ( [link] ), which are then transported across the intestinal mucosa to be used to create new proteins, or to be converted into fats or acetyl CoA and used in the Krebs cycle.

Digestive enzymes and hormones

The left panel shows the main organs of the digestive system, and the right panel shows a magnified view of the intestine. Text callouts indicate the different protein digesting enzymes produced in different organs.
Enzymes in the stomach and small intestine break down proteins into amino acids. HCl in the stomach aids in proteolysis, and hormones secreted by intestinal cells direct the digestive processes.

In order to avoid breaking down the proteins that make up the pancreas and small intestine, pancreatic enzymes are released as inactive proenzymes    that are only activated in the small intestine. In the pancreas, vesicles store trypsin    and chymotrypsin    as trypsinogen    and chymotrypsinogen    . Once released into the small intestine, an enzyme found in the wall of the small intestine, called enterokinase    , binds to trypsinogen and converts it into its active form, trypsin. Trypsin then binds to chymotrypsinogen to convert it into the active chymotrypsin. Trypsin and chymotrypsin break down large proteins into smaller peptides, a process called proteolysis    . These smaller peptides are catabolized into their constituent amino acids, which are transported across the apical surface of the intestinal mucosa in a process that is mediated by sodium-amino acid transporters. These transporters bind sodium and then bind the amino acid to transport it across the membrane. At the basal surface of the mucosal cells, the sodium and amino acid are released. The sodium can be reused in the transporter, whereas the amino acids are transferred into the bloodstream to be transported to the liver and cells throughout the body for protein synthesis.

Questions & Answers

what is oesophagus?
Bhavani Reply
oesophagus also known as food pip
ABDULLAH
What is the meaning of dissection
Beryl Reply
what's the meaning of polar?
Jackson Reply
Sign of anaemia or whitish color or Hgb luck
Kassahn
Are you talking of Polar anaemia or what, because the word polar doesnt have one meaning
Kingsley
what are the three types of adrenocorticoids hormones
Lubega Reply
glucocorticoids mineralocorticoid and catecholamines
Kartik
what is polar and non-polar
Jackson Reply
polar unequal share of electron while non polar is equal share
Quran
why are phosphate molecules negatively charged?
Jackson Reply
Bocz of unpaired elections
Javid
because of unpaired electrons
ABDULLAH
what are amphipathic molecules?
Jackson Reply
Amphipathic molecules are molecules with both polar and non polar regions
Mohammed
How does the male organ develop
MADUBULA Reply
Hw does the male organ develop
MADUBULA
Review your questions madam
Aliyu
what is anaphylaxis?
Rugut Reply
it is an imagency condition resulting from abnormal and immediate alagic response to a substance to which the body has become intensively sensitized
Lubega
different between drug and medicine
ado Reply
drugs have no medical application (cocaine, heroin, crystal meth). medicine have medical purpose (fentanyl, albuterol, aspirin, ect ect)
Jordan
medicine is a substance or preparation used in treating disease,drug is chemical compound medicine are drugs but all drugs are not medicines
Wafa
assalam o alaikum
Sidra
what happen to ECF and ICF regarding to OEDEMA
Zwanga Reply
what are the smooth muscles of the heart
Sintung Reply
stomach
Sidra
identify external features of kidney
saba Reply
kidney weight on males?
saba
and female ?
saba
Sle full name & treatment
Samim
Syplymic lupw erythematous
Samim
blood supply to spleen ?
saba
g..
saba
ya 135 g females
saba
130 on male
saba
130g on male
saba
g samim
saba
no 130grms on male
saba
mbbs
saba
good saba khan
Shahab
Tom kya krti ho st?
saba
your name?
saba
ok
saba
tum kya krti ho? muskan
saba
ok
saba
good muskan
Shahab
ok bye I m studying
saba
Define cranial nerves with oder
Javid
O-olfactory O-optic O-occumulator T-trochlear T-trigemenal A-abducent F-facial A-auditory G-glossopharyngeal V-vagus A-acessory
Aniee
opd mean?
Shahab
out patient department
mahesh
thnxx
Shahab
Ty Aniee singh
Javid
Ap sb us book s parhte h
Areeej
Read KD Tripathi book of Pharmacology.
mayank
Hello
mayank
CT Scan means
Sintung
what's health?
Sintung
hello
Philip
hi
Sintung
CT means - computerized tomography
Vivek
connective tissue
Dee
they link some body organs
Sintung
Difference between drug and medicine
Javid
hiii muskan
saba
what is tomography?
Gideon
tomography ☝
shahid
yes
Gideon
how hemolytic anemia cause due to gas gangrene?
Huma Reply
what's gangrene?
Sidra
why retro abdominal region called or named "flank "?
Huma Reply

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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