Matter inDigestionAfter being digested in the tract, its decomposition products enter the blood andlymph。Food is digested in the digestive tract and finally decomposed intoglucose，amino acidAnd other substances that can be absorbed by the human body.
process
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The process by which the body absorbs nutrients from the environment into the body.Unicellular animalTake nutrients directly from the living environment;Multicellular animalIn the digestive tube (cavity), the digestive products of various foods, water, salt and other substances pass throughdigestive tractepithelial cellsInto the blood andlymphAndvertebrateRenal tubuleThe substances in are retransported to the blood, which belongs to absorption.There are many modes of absorption, but they are all designed to supply the bodyNutritionAnd keep the bodyinternal environmentIs constant.
Absorption: The process by which a substance absorbs other material objects or energy.Gas is sucked by liquid or solid, or liquid is sucked by solid.In the process of absorption, one substance absorbs another substance into the body to blend or combine with it.For example, sulfuric acid or lime absorbs water, blood absorbs nutrition, felt, mineral wool, soft fibreboard, expanded perlite and other materials can absorb noise, and chemical wood pulp or cotton pulp is used to make loose paper blotting paper, which is used to absorb ink, gas or liquid solids, and has a porous structure.When the radiation of sound wave, light wave and electromagnetic wave is projected onto the surface of the medium, part of it is reflected by the surface, and part of it is absorbed and transformed into other forms of energy. When the energy propagates in a certain direction in the medium, it is gradually absorbed by the medium with the depth of incidence.
mechanism
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Unicellular organismandHigher animalThe absorption process of nutrients is the process of material molecules passing throughcell membraneEnter the cell, or pass through the cell membrane on the other side from the cell to leave the cell and enterTissue fluidOr blood.along withBiologyThe special absorption mechanism of the specificity of different substances plays a more important role.Mammaliansmall intestineTaking absorption as an example, the general mechanism of absorption can be summarized as simple diffusion, facilitated diffusion and active transport.
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Simple diffusion(free diffusion).That is, the molecules of the substance enter the region with low concentration from the region with high concentration.The cell membrane is located in theIntracellular fluidA layer of lipid membrane between the membrane and extracellular fluid. Therefore, only the substance molecules that can dissolve in lipid can diffuse from the high concentration side of the membrane to the low concentration side (also called dispersion).The absorption process of simple diffusion does not consume energy, and the material molecules move according to the concentration gradient or potential gradient.Simple diffusion is not an important way for the small intestine to absorb nutrients.
Facilitating diffusion(Assisting proliferation).With the help of specific protein molecules (carriers) in the cell membrane, material molecules pass through the diffusion process of the cell membrane. Like simple diffusion, this facilitated diffusion also passes through the membrane from the side with high concentration to the side with low concentration.Some non fat soluble substances are absorbed in this way.No consumption is required for facilitating diffusionMetabolic energy。
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Active transshipment(Active transportation).A substance that needs to consume the energy of cell metabolism and can be transported through the membrane in reverse electrochemical gradient.For example, in the small intestineglucoseandamino acidIt is transported against the concentration difference in an active way. ④Endocytosis.The original way of food intake in phylogeny.It surrounds food particles or protrudes through the invagination of cell membranePseudopodiaEntanglement of food particles into cells.The small intestine affects some macromolecular substances and material clumps, such as intact proteinstriglyceride, can be absorbed by endocytosis.
The absorption capacity of different parts of the digestive tube is very different, which is mainly related to the organizational structure of each part of the digestive tube, the length of food stay in this part, the degree of food decomposition and other factors.Under normal circumstances,oral cavityThe stomach can only absorb a small amount of water, inorganic salts and alcohol.The small intestine absorbs glucose, amino acid, glycerinfatty acidMost of the water, inorganic salts and vitamins, most of the nutrients have been absorbed in the small intestine, and the contents of the small intestine enterlarge intestineIt does not contain much absorbable material.The large intestine mainly absorbs water, inorganic salts and some vitamins.
absorb
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Absorption of sugarSugar is almost absorbed in the small intestine in the form of monosaccharide.The absorption of glucose and galactose is fast, and it can reverse the concentration gradient, which is a typical active transport mode.The absorption of some sugars, such as sorbose, xylose and arabinose, is a simple diffusion process, while fructose is between the two.Active absorption of glucose requires the presence of Na+. In the absence of Na+, glucose is absorbed in a way that facilitates diffusion.It is generally believed that the mechanism of active absorption of glucose lies in the coupling of glucose and Na+transport phase.Their relationship is that the carrier on the brush border membrane of small intestinal epithelial cells combines with Na+, and then combines with glucose to form a "Na+- glucose carrier" complex, that is, the carrier can only combine with glucose when it carries Na+.The "Na+- glucose carrier" complex migrates from the intestinal cavity surface of the membrane to the intracellular fluid depending on the electrochemical gradient of Na+. The intracellular high concentration glucose enters the subcellular space from the bottom of the cell, and then enters the blood (Figure 2).
Protein absorptionThe protein eaten is almost completelydigestive enzymeIt can be absorbed only after hydrolysis to amino acids.The absorption of amino acids is also active transport, and its mechanism is similar to that of glucose.
Fat absorptionFat or triglyceride is decomposed into two molecules of free fatty acid (FFA) and one molecule of ethyl glycerol by pancreatic lipase in the intestinal cavity.The two decomposition products are fat soluble.Before absorption, they must form water-soluble micelles with bile acid before passing through the water layer covering the surface of small intestine epithelium.After reaching the absorption surface, such lipid micelles separate FFA and monoglyceride, which can diffuse into cells through the cell membrane.Fatty acids and monoglycerides re synthesize triglycerides in the smooth endoplasmic reticulum of cells.With non-polar substances such as triglycerides and cholesterol esters as the core and apolipoproteins and phospholipids with greater polarity as the shell, chyle particles are formed.The size of chyle particles is quite different, ranging from 750 to 5000 angstroms.When the synthesis of apolipoprotein and phospholipid is blocked, the volume of chyle particles increases.Chyle particles inGolgiThe organ is packaged as a secretory vesicle. The vesicle moves to the lateral membrane of the cell, and is fused with the lateral membrane. It is released into the paracellular space through the exocytosis, passes through the basement membrane and the proper membrane, and finally enters the lymphatic vessels. Until then, the whole process of fat absorption is completed.
