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Human Physiology - Digestion and absorption

Grade 12IBBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The contraction of circular and longitudinal muscle of the small intestine, known as peristalsis, mixes the food with enzymes and moves it along the gut.

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The pancreas secretes enzymes (amylase, lipase, and endopeptidase) into the lumen of the small intestine to catalyze hydrolysis reactions.

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Villi increase the surface area of the intestinal epithelium to optimize absorption. Villi absorb monomers formed by digestion as well as mineral ions (Na+Na^+, Ca2+Ca^{2+}, Cl−Cl^-) and vitamins.

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Different methods of membrane transport are required to absorb different nutrients: Glucose is absorbed via co-transport with Na+Na^+ ions, while fatty acids are absorbed by simple diffusion or facilitated diffusion via fatty acid transporters.

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Starch digestion involves the breakdown of amylose and amylopectin. Amylase breaks α\alpha-1,4 bonds to produce maltose and maltotriose, while dextrinase breaks α\alpha-1,6 bonds in amylopectin.

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The hepatic portal vein carries blood containing absorbed nutrients from the small intestine to the liver for processing before it enters the general circulation.

📐Formulae

Starch+nH2O→AmylaseMaltose(C12H22O11)\text{Starch} + nH_2O \xrightarrow{\text{Amylase}} \text{Maltose} (C_{12}H_{22}O_{11})

Triglyceride+3H2O→LipaseGlycerol+3Fatty Acids\text{Triglyceride} + 3H_2O \xrightarrow{\text{Lipase}} \text{Glycerol} + 3 \text{Fatty Acids}

Lactose+H2O→LactaseGlucose(C6H12O6)+Galactose(C6H12O6)\text{Lactose} + H_2O \xrightarrow{\text{Lactase}} \text{Glucose} (C_6H_{12}O_6) + \text{Galactose} (C_6H_{12}O_6)

Surface Area to Volume Ratio=SAV\text{Surface Area to Volume Ratio} = \frac{SA}{V}

💡Examples

Problem 1:

Explain how the concentration gradient of Na+Na^+ is maintained to allow for the secondary active transport of glucose into the villus epithelium.

Solution:

The Na+/K+Na^+/K^+ pump on the basolateral membrane of the epithelial cell actively pumps 3Na+3Na^+ ions out of the cell into the interstitial fluid and 2K+2K^+ ions into the cell using ATPATP.

Explanation:

By keeping the intracellular [Na+][Na^+] low, a concentration gradient is established between the lumen of the small intestine and the cytoplasm. Glucose can then be moved against its own gradient via a Na+Na^+-glucose co-transporter protein (SGLT1) on the apical membrane.

Problem 2:

Calculate the magnification of a micrograph of a villus if the actual length of the villus is 0.5 mm0.5\text{ mm} and its length in the diagram is 5.0 cm5.0\text{ cm}.

Solution:

Magnification=Size of ImageActual Size=50 mm0.5 mm=100×\text{Magnification} = \frac{\text{Size of Image}}{\text{Actual Size}} = \frac{50\text{ mm}}{0.5\text{ mm}} = 100\times

Explanation:

First, convert all units to the same scale (5.0 cm=50 mm5.0\text{ cm} = 50\text{ mm}). Then, divide the image size by the actual size to find the magnification factor.

Digestion and absorption - Revision Notes & Key Diagrams | IB Grade 12 Biology