The Digestive System: Mechanism for Nourishing the Body
1. Name the different types of transport mechanisms and describe each. How are
they the same? How are they different? What type of nutrients utilize which
mechanism and why?
Diffusion Facilitated Diffusion Active Transport Endocytosis
• small, lipid‐ soluble molecules/substrate
• diffuse through the membrane (no transport protein required)
• move down a concentration gradient
• linear kinetics (velocity vs. substrate concentration)
• some molecules proceed through a protein “pore” rather than the membrane
itself – still considered simple diffusion
Facilitated Diffusion (aka carrier‐ mediated):
• larger, more polar compounds transported
• requires a protein to mediate the process, thus…
• is a saturable process kinetically; i.e., a maximal rate of transport is achieved
regardless of substrate concentration. The only way to increase this maximum is
by having more transporters. • like simple diffusion, can only proceed down a
• same as above (saturable, polar molecules, protein‐ mediated, etc.) except…
• requires energy (ATP & Na)
• can transport against a gradient (this is the payoff from investing energy)
• this involves the cell wall engulfing a substance by surrounding it with the cell
2- What is required for the efficient digestion of carbohydrates? lipids? proteins?
Your discussion should include the required organs, regulatory peptides, enzymes,
and transport mechanisms.
CCK – secreted by the small intestine into the bloodstream, it stimulates the gallbladder
to release bile into the small intestine lumen, and stimulates the pancreas to release both
pancreatic enzymes and pancreatic juice into the small intestine lumen.
Secretin – same as CCK above except for the action on the gall bladder; secretin is
secreted by the small intestine and stimulates the pancreas to release enzymes and juice.
Gastrin – released by the stomach and acts on the stomach to release HCl; along with
HCl it aids in the activation of pepsinogen to pepsin.
GIP – secreted by the small intestine into the bloodstream, it inhibits the action of the
stomach (gastrin secretion, contraction, etc.).
Amylase – secreted in the mouth (saliva) and by the pancreas (part of pancreatic
enzymes) into the small intestine. Most of the amylase we need to digest carbohydrates
(i.e., polysaccharides) into smaller saccharides (olio‐ , di‐ , and mono‐ ) is pancreatic
Pepsinogen – secreted in this zymogen form in the stomach; activated to pepsin by HCl
and gastrin; pepsin is a protease, meaning it breaks down proteins into smaller peptides
and amino acids.
Lipase – same as amylase above in that it is found in both the mouth and the small
intestine via the pancreas; also like the mouth, most of the hydrolysis of lipids (i.e.,
triglycerides, TGs) into free fatty acids (FAs) and glycerol occurs in the small intestine.
Note: 90+% of the lipids you ingest are TGs; lipase does not convert lipids into TGs; for
humans, dietary lipids are TGs. Trypsin – one of many proteases that make up part of the
pancreatic enzymes which are secreted into the small intestine. Trypsin is released as
trypsinogen, which via the action of enteropeptidase is converted into trypsin. Being a
protease, trypsin catalyzes the hydrolysis of proteins into smaller peptides and amino
Dipeptidase – resides near the brush border (i.e., microvilli of enterocytes) and breaks
down peptides into 2 amino acids.
Disaccharidase – resides near the brush border (i.e., microvilli of enterocytes) and
breaks down disaccharides (e.g., sucrose, lactose, maltose) into 2 monosaccharides (e.g.,
glucose, fructose, galactose); thus, the specific disaccharidases we are talking about are
sucrase, lactase, and maltase.
Enteropeptidase – see trypsin above.
HCl – secreted by the stomach in response to gastrin; its primary function is to aid in
digestion, in part by denaturing proteins and activating pepsinogen to pepsin.
Bile – is a mixture of bile salts, phospholipids, and other components. Released by the
gallbladder (via CCK into the small intestine), its function is to emulsify lipids (i.e., help
mix them within the aqueous environment into smaller droplets) by forming micelles.
Then, because of the greater surface area of the lipid and the new aqueous environment,
lipase from the pancreas can more efficiently break down the lipid (TG into free FAs and
glycerol). Bile is not an enzyme – it breaks down lipids in the sense that it transforms
large lipid droplets into smaller ones as micelles, but it does not carry out the hydrolysis –
that is the function of lipase.
Pancreatic juice – composed primarily of water and bicarbonate; released by the
pancreas (via CCK and secretin) and in the small intestine it neutralizes the acidity of the
chyme due to the HCl in the stomach. Remember, digestive enzymes are proteins – thus,
HCl would denature them just like it does dietary proteins.
Saliva – secreted by the mouth to lubricate food and aid in swallowing. Also contains
amylase and lipase.
Transport of Nutrients
For nutrient transport, it depends on if it is water soluble (monosaccharides, amino acids),
or lipid soluble.
Water‐ soluble: need transporters to cross the lipid environment of cell membranes, but
don’t need transporters to travel in the aqueous environment of the circulation. For
digestion, once the water‐ soluble substance enters the enterocyte via a transporter, it
goes into the portal vein (using another transporter to cross that membrane) and is
delivered to the liver first. After the liver, it circulates to the rest of the body.
Lipid‐ soluble: don’t need transporters typically to cross membranes (can you think of
one exception? β‐ oxidation), but do need them in the circulation. Once in the enterocyte,
lipid‐ soluble substances are packaged in chylomicrons and released into the lymphatic
system, which merges with the general circulation near the heart. Thus, the entire body
sees these chylomicrons at the same time – the liver does not get the first shot at them.
Note: small FAs
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