Digestion and Absorption in the Mammalian Gut
Explain how villi support absorption and how an ATP-powered pump maintains the sodium gradient used for glucose co-transport. Connect villus damage in coeliac disease with reduced nutrient absorption.
What a learner can do afterwards
- Explain the structure of a villus in terms of the quantities in Fick's law
- Describe glucose absorption as sodium-linked co-transport and say what supplies the gradient
- Account for the symptoms of a named absorption disorder from the mechanism that fails
1 · Read
Ingestion takes food in. Digestion breaks large food molecules into smaller products. Absorption moves nutrients across the gut lining into the body. Elimination removes unabsorbed material. Most chemical digestion and nutrient absorption occur in the small intestine. The large intestine absorbs water and salts from the remaining contents. Absorbed nutrients can later supply respiration; digestion itself does not release energy into ATP.
Villi are finger-like tissue projections of the small-intestinal lining. Microvilli are smaller membrane projections on its lining cells. Both increase surface area. Each villus has a one-cell-thick epithelium, blood capillaries and a lymph vessel called a lacteal. A short path supports diffusion. Lining cells also contain transport proteins and digestive enzymes, including enzymes on the microvilli.
For diffusion, Fick's law gives rate proportional to surface area × concentration difference ÷ diffusion distance. Hold other factors steady when comparing rates. If area triples and distance doubles, rate becomes 3 ÷ 2, or 1.5 times the original. Villi add area, their thin epithelium shortens the path, and blood flow can maintain a concentration difference. Fick's law alone does not explain carrier-driven uptake.
Glucose enters a lining cell from the gut lumen, the space containing digested food. A sodium-glucose co-transporter moves sodium and glucose into the cell together. Sodium moves down its electrochemical gradient, supplying energy for glucose to move against its own concentration gradient. This is secondary active transport. The movement described here is entry into the lining cell, not direct entry into blood.
On the cell's blood-facing side, the sodium-potassium pump uses ATP to move sodium out and potassium in. Keeping sodium low inside helps maintain its inward gradient. Glucose leaves through a separate carrier by facilitated diffusion, down its concentration gradient towards the blood. The pump uses ATP directly; the glucose co-transporter uses the sodium gradient. Glucose exit does not itself pump sodium out.
In coeliac disease, gluten triggers an immune response that damages the small-intestinal lining and can flatten villi. Reduced surface area contributes to poor nutrient absorption. Small nutrients may still be present in the gut, yet less enters the body. This can cause weight loss and nutrient deficiencies. Damage can also affect enzymes on lining cells. Absorption, digestion and cellular respiration are different processes.
Villi support absorption. An ATP-powered pump maintains the sodium gradient for glucose entry into lining cells. Villus damage can reduce nutrient uptake and cause deficiencies.
This lesson, question bank and flashcards were written for this dataset.
These readings and flashcards were written for this dataset.
More to read
A villus at two scales
A villus is a projection of intestinal tissue. An epithelium covers its surface. Individual absorbing cells have much smaller membrane projections called microvilli. Blood capillaries lie within a villus, not inside each microvillus.
These projections provide more absorbing area within the intestine. The epithelium is one cell thick, keeping the crossing distance short. Blood flow removes absorbed solutes and can maintain concentration differences for diffusion.
The cells also do chemical and transport work. Some enzymes on the microvilli complete digestion. Membrane proteins provide routes for substances that cannot cross the lipid membrane freely. Surface area alone therefore does not explain how every nutrient is absorbed.
A villus also contains a lacteal, which is a lymph vessel. Much absorbed dietary fat is packaged into particles that enter lymph before reaching blood. Glucose and amino acids enter blood capillaries. Different nutrients can take different transport routes.
Following glucose through a lining cell
Absorbing glucose involves two sides of an epithelial cell. One membrane faces the gut lumen. The other faces tissue fluid and blood capillaries.
At the lumen-facing membrane, a sodium-glucose co-transporter brings both substances into the cell. Sodium moves down its electrochemical gradient. Coupling the movements allows glucose to enter against its own concentration gradient.
The sodium gradient depends on another protein. In the blood-facing membrane, a sodium-potassium pump uses ATP to move sodium out and potassium in. This helps keep sodium low inside the cell. The pump uses ATP directly; the co-transporter draws on the sodium gradient.
Glucose leaves by a separate carrier through facilitated diffusion, down its concentration gradient towards blood. The glucose exit carrier does not export sodium. Comparing only glucose concentration in gut fluid with glucose concentration in blood leaves out the intermediate cell and its two different membrane crossings.
Diffusion and damage to the absorbing surface
For diffusion, increasing surface area or concentration difference increases the rate when other conditions stay fixed. Increasing diffusion distance decreases it. For example, four times the area and twice the distance give twice the original rate.
Villi and microvilli provide a large area. A thin epithelium reduces the distance to cross. These features help exchange, but the diffusion relationship alone cannot explain glucose entry against its gradient. That requires coupled transport and an energy source.
Coeliac disease can damage this absorbing surface. Gluten triggers an immune response in affected people, damaging the intestinal lining and sometimes flattening villi. Reduced surface area contributes to malabsorption. Small nutrient molecules may be available in the gut but enter the body in smaller amounts. Possible consequences include weight loss and nutrient deficiencies.
Damage may also affect enzymes on lining cells. It is therefore inaccurate to say that digestion always remains unchanged. Digestion produces small molecules, absorption moves nutrients into the body, and respiration later transfers energy to ATP.
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