Amino acid absorption
Full protein absorption is possible only after their preliminary hydrolysis into amino acids. This process starts in the stomach, where hydrochloric acid (HCl) causes denaturation of protein molecules, and pepsin enzymes begin their cleavage.
Intestinal epithelial cells (enterocytes) have pronounced polarity, so the transport mechanisms at different poles of the cell differ:
- Apical membrane (facing the intestinal lumen): the transfer of amino acids inside the enterocyte is carried out via active transport. This process requires the mandatory participation of specific carrier molecules (5 types are known) and ATP energy expenditure.
- Basal membrane (facing blood vessels): the exit of amino acids from the cell into the blood occurs via the mechanism of facilitated diffusion.
The intensity of the process is not constant. It is higher in young organisms and directly depends on the current level of protein metabolism in the body, the concentration of free amino acids in circulating blood, as well as neural and humoral signals.
Carbohydrate absorption
The intestinal epithelium lets through carbohydrates only in the form of monosaccharides. At the same time, their assimilation rate varies: hexoses (glucose and galactose) cross the membrane barrier significantly faster than pentoses.
For glucose and galactose, a mechanism of secondary active symport with Na⁺ ions operates on the apical membrane. This is a co-transport, thanks to which glucose actively accumulates inside the epithelial cell. It leaves the cell via the basal and lateral membranes using passive transport, moving into the intercellular fluid and blood along the concentration gradient.
Regulation of glucose absorption:
- Enhance: parasympathetic nervous system, certain amino acids, as well as hormones (insulin, thyroxine, glucocorticoids).
- Inhibit: histamine (has a minor effect). A significant and strong inhibition is caused by the hormone somatostatin, activation of the sympathetic nervous system, and tissue respiration inhibitors.
Physiology of fat absorption: luminal and cellular stages
On average, a person consumes 60–100 g of fats per day. The main difficulty of their assimilation lies in the fact that lipids are hydrophobic, while all metabolic processes of the gastrointestinal tract occur in an aqueous environment. The process is divided into three sequential phases.
Luminal stage (in the intestinal lumen) Dietary fats (triglycerides) are split into free fatty acids (FFAs) and monoglycerides (MGs) under the action of lipase enzymes. To prevent these hydrophobic products from clumping together, bile acid salts emulsify them, forming micelles. A micelle is a transport particle covered externally with phospholipids (hydrophilic part) and containing FFAs, MGs, cholesterol derivatives, and fat-soluble vitamins inside. Micelles deliver lipids through the unstirred water layer directly to the brush border of enterocytes.
Intracellular stage (in the enterocyte) Upon contact with the apical membrane of microvilli, the micelle disintegrates. Lipid components penetrate inside the cell via diffusion. Bile acids remain in the intestinal lumen to be later absorbed in the ileum via active transport. Inside the enterocyte, in the smooth endoplasmic reticulum, resynthesis of triglycerides occurs (MG $\rightarrow$ DG $\rightarrow$ TG). Next, phospholipids, cholesterol, and lipoproteins join them, forming the smallest fat particle in a protein shell — chylomicron.
Lipid transport into the circulation
The formed chylomicrons are packaged into vesicles and expelled through the basolateral membrane via exocytosis. Due to their large size, chylomicrons are unable to squeeze into blood capillaries. They penetrate into the central lymphatic vessel of the villus (contractions of the villi themselves mechanically assist this process).
The bulk of exogenous lipids is transported by lymph. Soon after a meal, the lymph begins to resemble milk, for which it is called "chyle". Through the thoracic duct, chylomicrons enter the systemic circulation, bypassing the liver. Alternative pathway: short-chain fatty acids can be absorbed directly into the blood capillary, bypassing the chylomicron packaging stage. Via the portal vein, they head straight to the liver bound to the protein albumin.
Regulation: the parasympathetic system, secretin, cholecystokinin-pancreozymin, as well as pituitary, thyroid, and adrenal cortex hormones stimulate fat absorption. The sympathetic nervous system, conversely, slows it down.
Briefly: answers to topical questions
Where and how does the resynthesis of triglycerides occur inside the enterocyte?
The resynthesis of triglycerides inside the enterocyte occurs in the smooth endoplasmic reticulum via the reverse assembly of fat molecules from hydrolysis products.
The mechanism of this intracellular stage includes the synthesis of the body's own fats:
- Basic chain of transformations — monoglycerides (MGs) are converted into diglycerides (DGs), and then into triglycerides (TGs).
- Biochemical reaction — $\beta$-monoacylglycerol interacts with two molecules of activated fatty acids (Acyl-CoA), resulting in the formation of triacylglycerol (TAG) and the release of two molecules of HS-CoA.
To easily remember the distribution of fat absorption pathways: "Large ones into the lymph, small ones into the blood." Resynthesized chylomicrons are too huge for capillaries and go into the chyle lymphatic vessel, while short-chain fatty acids slip directly into the bloodstream.
Class and exam questions
In what form are carbohydrates absorbed in the small intestine?
Carbohydrates can enter enterocytes exclusively in the form of monosaccharides (e.g., glucose, galactose, or fructose).
What happens to bile acids after the micelle disintegrates?
They do not enter the enterocyte together with fats, but remain in the intestinal cavity. Later, they undergo reabsorption in the ileum using active transport mechanisms.
What mechanism ensures the exit of amino acids from the enterocyte into the blood?
The transport of amino acids across the basal membrane of the cell into the bloodstream is carried out via the mechanism of facilitated diffusion.
Which part of the autonomic nervous system stimulates nutrient absorption?
The parasympathetic nervous system enhances the absorption of both glucose and fat hydrolysis products, whereas the sympathetic system exerts an inhibitory effect.
What else to study on this topic
- Types of amino acid transporters on the apical membrane
- Molecular mechanism of glucose and sodium ion symport
- Biochemical stages of triglyceride resynthesis in the endoplasmic reticulum
- Structure of chylomicrons and functions of apolipoproteins
- Mechanisms of action of somatostatin on enterocyte transport systems
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