Digestion and absorption in the human gut

Digestion as hydrolysis

Digestion is the hydrolysis of large biological molecules into small, soluble ones that can be absorbed across a cell membrane. Every reaction below is a hydrolysis, the reverse of the condensation reactions that formed the molecules: a water molecule is added across a bond and the bond breaks. Starch to maltose, maltose to glucose, protein to amino acids, triglyceride to monoglyceride and fatty acids. All of it is hydrolysis, all of it enzyme-catalysed, and all of it happening outside your cells in a tube that runs through you rather than in you.

One precision worth carrying: not everything digested is a polymer. Starch and proteins are. A triglyceride is a large molecule built by condensation but not a polymer, because it is not a chain of repeating monomers, and a disaccharide is two units rather than many. 'Large biological molecules' is the safe phrase.

Alongside it runs physical digestion, which breaks nothing chemical and matters enormously. Teeth cut and grind, the stomach churns, bile salts split fat droplets. Each increases the surface area available to the enzymes, and since those enzymes work on the outside of solid or oily material rather than on molecules free in solution, area sets the rate.

Describe the drawing

A schematic gut down the left of the figure, from the mouth through a coral stomach, an amber duodenum and a coiled ileum to the colon, with four cards beside it in the same colours. Each card names a compartment, gives the pH it holds, names the enzymes working there and says what that compartment achieves: mouth at pH 6.8 with salivary amylase, stomach at pH 1.5 to 2.0 with pepsin, duodenum at pH 7 to 8 with pancreatic enzymes and bile salts, and ileum at pH 7 to 8 with the membrane-bound maltase and dipeptidases.

The stomach holds pH 1.5 to 2.0. The duodenum holds pH 7 to 8. A difference of six pH units is a factor of a million in hydrogen ion concentration, and the gut manages it across a sphincter a few centimetres long, using hydrogencarbonate ions secreted in pancreatic juice and in bile to neutralise the acid arriving from the stomach. That neutralisation is essential. Pancreatic amylase, trypsin and lipase all have optima between about pH 7 and 8 and would be denatured at pH 2, so the pancreas has to fix the pH of its own workplace before its enzymes are of any use.

Two consequences follow. Salivary amylase stops working in the stomach, so starch digestion pauses and resumes in the duodenum with a different enzyme doing the same job. And the acid is doing chemistry although it is not an enzyme: at pH 2 enough of the ionic and hydrogen bonds stabilising a protein's tertiary structure are disrupted (which ones depends on the protein) that the chain unfolds, and peptide bonds buried in the tertiary structure become reachable. Acid denaturation is what makes pepsin efficient.

Digestion

The hydrolysis of large biological molecules into small soluble molecules that can be absorbed across cell membranes.

Physical digestion

The mechanical breakdown of food into smaller pieces, increasing the surface area available to digestive enzymes without hydrolysing anything.

Absorption

The movement of the products of digestion from the lumen of the gut, across the epithelium, into the blood or the lymph.

Exam-board note. AQA sets digestion and absorption out as a section of its own and names the enzymes, the micelles and the co-transport. Cambridge 9700 has no digestion topic, and OCR A does not teach mammalian digestion as a topic either; on those courses the hydrolysis and the membrane transport are examinable and the anatomy is context. Check the current specification for the qualification being taken.

Carbohydrases, and where the last bond is cut

Starch is a polymer of alpha glucose held by glycosidic bonds, in two forms: amylose, an unbranched helix joined only by 1,4 bonds, and amylopectin, which carries a 1,6 bond at each branch point. Amylase hydrolyses the 1,4 bonds and cannot touch the 1,6 ones, so its product is maltose plus short branched fragments needing a separate enzyme.

Amylase appears twice: salivary amylase starts work in the mouth, where food stays for seconds, and pancreatic amylase is secreted into the duodenum and does most of the actual work. Both hydrolyse starch to maltose at a similar, near-neutral optimum pH, and they differ in where they are made and where they work.

The last bond is cut somewhere unexpected. The disaccharidases, maltase, sucrase and lactase, are not released into the lumen at all. They are built into the cell surface membrane of the epithelial cells lining the ileum, with their active sites facing the gut contents. Maltase hydrolyses maltose to two glucose molecules, sucrase gives glucose and fructose, lactase gives glucose and galactose. Being membrane-bound means each monosaccharide is released a few nanometres from the transport protein that will carry it in, so it is absorbed before it can diffuse away or be taken by gut bacteria.

Lactase differs from the others in its expression. Most mammals switch the gene off after weaning, and most adults worldwide make little of it; a change in the DNA controlling that gene keeps it on into adulthood, and is common in northern Europe and in several East African herding populations. Where lactase is absent, lactose reaches the large intestine undigested, lowers the water potential of the gut contents and is fermented by bacteria, which accounts for the watery faeces and the bloating of lactose intolerance, with no allergy involved.

Proteases and the order in which they act

Protein digestion uses three classes of enzyme, and the order in which they act follows from what each does. An endopeptidase hydrolyses peptide bonds in the interior of a polypeptide. An exopeptidase removes amino acids one at a time from an end. A dipeptidase splits the last two-residue fragment into single amino acids.

Describe the drawing

Three rows showing one polypeptide taken apart in stages. The top row is a chain of twelve joined circles with three coral cut marks between internal residues. The middle row is the four three-residue fragments those cuts produce, with an amber arrow pointing at each of the eight exposed ends. The bottom row is four dipeptides, each beside the single amino acid an exopeptidase has freed, waiting to be split at the membrane.

An exopeptidase can only work where there is a free end. A chain of three hundred amino acids has two of them, so exopeptidases on their own would have very few places to work. Endopeptidases multiply the ends: each internal cut adds two more places for the exopeptidases to start. That is a rate argument of exactly the same shape as the surface-area argument for bile.

Endopeptidase

An enzyme that hydrolyses peptide bonds within a polypeptide chain, producing shorter chains and so more ends. Pepsin, trypsin and chymotrypsin.

Exopeptidase

An enzyme that hydrolyses the peptide bond at an end of a chain, removing a single amino acid. Carboxypeptidases work from the end carrying the free carboxyl group, aminopeptidases from the amine end.

Dipeptidase

An enzyme, bound to the membrane of the ileum epithelium, that hydrolyses a dipeptide into two amino acids.

The named examples matter. Pepsin is an endopeptidase secreted by the stomach with an optimum near pH 2, and it is the only digestive enzyme named here that works in acid conditions. Trypsin and chymotrypsin are endopeptidases from the pancreas, working near pH 8. The carboxypeptidases are pancreatic as well, while the aminopeptidases and dipeptidases sit in the membrane alongside the disaccharidases.

A gland that makes protein-digesting enzymes must avoid hydrolysing its own proteins. Pepsin is secreted as inactive pepsinogen and converted by the stomach acid once safely out of the cell; trypsin is secreted as trypsinogen and activated by an enzyme anchored in the duodenal wall, after which trypsin activates the rest. The pancreas therefore never contains working protease. When that fails, it digests itself, which is what acute pancreatitis is.

Bile salts, lipase and the micelle

Lipids arrive in the duodenum as droplets, because they are insoluble in the watery gut contents and coalesce. Pancreatic lipase is water-soluble and can only reach the ester bonds at a droplet's surface, so the rate of lipid digestion is set by the total surface area of droplets rather than by how much fat is present.

Bile salts solve that. They are amphipathic: one face is hydrophilic and one hydrophobic, so they coat a lipid surface with their hydrophobic face inwards and leave a hydrophilic outside. Droplets so coated repel each other instead of merging, and the churning of the gut can break them into much smaller ones that stay broken. That is emulsification, and its whole effect is geometric.

Describe the drawing

Lipid absorption animated in four steps. First one large amber lipid droplet is shown beside a cluster of thirteen small ones, with the note that emulsification raises the surface area a thousandfold for droplets a thousandth of the diameter. Second, coral lipase molecules ring the surface of a droplet and an arrow leads to monoglyceride and fatty acids. Third, three micelles, each an amber core of lipid products inside a ring of cyan bile salts, sit above a strip of microvilli, with an arrow under each showing the products passing into the cell while the micelle itself stays outside. Fourth, inside the cell, triglyceride is re-formed, packaged into a larger chylomicron and passed out into a lacteal.

Pancreatic lipase hydrolyses two of the three ester bonds in a triglyceride, giving a monoglyceride and two fatty acids. Those products, together with bile salts, cholesterol and any fat-soluble vitamins present, assemble into micelles: aggregates a few nanometres across with the hydrophobic parts inside and the hydrophilic parts facing the water.

State clearly that a micelle is not itself absorbed. Micelles form and break up continuously, and they carry monoglycerides and fatty acids through the watery layer next to the epithelium and release them there, keeping the concentration of free products high right at the membrane. Because monoglycerides and fatty acids are non-polar and small, they then diffuse straight through the phospholipid bilayer: simple diffusion, no carrier, no ATP.

Inside the epithelial cell the products are rebuilt. The smooth endoplasmic reticulum re-forms triglycerides, and the Golgi apparatus wraps triglyceride and cholesterol in a coat of phospholipid and protein to make a chylomicron. Chylomicrons leave by exocytosis into the lacteal, the blind-ended lymph vessel at the centre of each villus, because they are far too large to cross into a blood capillary. The lymph rejoins the blood near the left shoulder, so absorbed fat reaches the circulation without passing through the liver first.

The ileum, and why glucose needs sodium

The ileum is built for one job. Its wall is folded, the folds carry villi about half a millimetre to a millimetre long, and each epithelial cell on a villus carries several thousand microvilli around a micrometre long. Careful measurements published in 2014 put the mucosal surface area of an adult small intestine at roughly 30 m², closer to the floor of a living room than to the tennis court of older textbooks, which would be about 260 m². Earlier estimates counted folds that are not there in a living, moving gut.

Surface area is only one of the adaptations, and a question asking how the ileum is adapted for absorption wants several. The epithelium is one cell thick, so the diffusion path is short. A dense capillary network carries absorbed products away, maintaining the gradient. Muscle in the wall and in each villus keeps the contents moving, so the fluid next to the membrane is not depleted. The cells are packed with mitochondria, because much of what happens next costs ATP. And the final digestive enzymes are built into the membrane itself.

Describe the drawing

One epithelial cell of the ileum, drawn with microvilli along its upper surface facing the gut lumen and the blood below it. Coral dots representing sodium ions crowd the lumen and are sparse inside the cell. A co-transporter protein sits in the upper membrane with an arrow carrying sodium and glucose in together, while a sodium-potassium pump and a separate glucose carrier sit in the lower membrane with arrows leading out to the blood. Three numbered cards beside the drawing give the order of events, starting with the pump.

Glucose in the gut is often at a lower concentration than glucose inside the epithelial cell, so diffusion alone would carry glucose out of the cell into the lumen, and a meal would be left half-absorbed. The solution is co-transport, and it works in three moves.

First, a sodium-potassium pump in the basal membrane, the face towards the blood, hydrolyses ATP and moves three sodium ions out of the cell for every two potassium ions in. The cell's internal sodium concentration falls well below the concentration in the gut lumen.

Second, a co-transporter protein in the apical membrane binds one glucose molecule and two sodium ions together. Sodium moves into the cell down the electrochemical gradient the pump created, and the protein will only let it through if glucose comes with it. Glucose is therefore dragged in against its own concentration gradient, using no ATP at this membrane at all.

Third, glucose leaves the far side of the cell into the blood by facilitated diffusion through a different carrier, down the gradient that its accumulation has created.

Co-transport

The transport of two substances across a membrane through the same carrier protein at the same time, one moving down its concentration gradient and providing the means for the other to move against its own.

Indirect active transport

Transport that is driven by a gradient which was itself established using ATP elsewhere in the cell. Glucose uptake in the ileum is the standard example.

Amino acids are absorbed by the same mechanism, with their own co-transporters. Fructose is not: it crosses the apical membrane by facilitated diffusion alone, with no sodium involved and no gradient available to concentrate it, which is why large intakes of fructose can overwhelm its carrier and pass on unabsorbed. Both routes are carrier-mediated, so both saturate; the difference is that the co-transporter can move glucose against its own gradient and the fructose carrier cannot.

Glucose is not directly actively transported into the epithelial cell. Its uptake moves glucose against its concentration gradient, so it is active, but only in the indirect, secondary sense defined above: no ATP is hydrolysed at the apical membrane, and a cell whose sodium-potassium pump is inhibited absorbs glucose for a short time and then stops, because the sodium gradient runs down. State where the ATP is spent.