Digestion: big molecules into small ones
Digestion is hydrolysis: water is used to break the bonds between monomers. Each group of food has its own enzymes.
Carbohydrates
- Amylase (made by salivary glands and pancreas) hydrolyses glycosidic bonds in starch to make maltose.
- Membrane-bound disaccharidases sit on the microvilli of ileum cells. Maltase splits maltose into glucose, sucrase splits sucrose into glucose and fructose, and lactase splits lactose into glucose and galactose.
Proteins
- Endopeptidases hydrolyse peptide bonds inside the chain, making many shorter pieces (more ends).
- Exopeptidases remove single amino acids from the ends of chains.
- Membrane-bound dipeptidases split dipeptides into two amino acids at the cell surface.
Lipids
- Lipase (from the pancreas) hydrolyses ester bonds in triglycerides, making monoglycerides and fatty acids.
- Bile salts (from the liver) emulsify fat: one big droplet becomes many small droplets, so lipase has a much larger surface to work on. Bile salts are not enzymes.
Absorption of glucose and amino acids: co-transport
Absorption happens mainly in the ileum. Villi and microvilli give a huge surface area, the wall is one cell thick and blood flow keeps a steep gradient.
- The sodium–potassium pump on the blood side of the cell uses ATP to move Na⁺ out of the cell into the blood. This keeps Na⁺ low inside the cell.
- Na⁺ now diffuses into the cell from the lumen through a co-transporter protein. The protein only works if it also carries a glucose (or amino acid) molecule with the Na⁺.
- Glucose builds up inside the cell and moves into the blood by facilitated diffusion.
This is indirect active transport: ATP is used by the pump, not by the co-transporter itself. Amino acids are absorbed in the same way with their own co-transporters.
Micelles and the absorption of lipids
Micelles are tiny balls made of bile salts with monoglycerides and fatty acids inside. They keep these water-hating molecules moving through the watery lumen to the cell surface.
- Micelles reach the microvilli and release monoglycerides and fatty acids.
- Because these are lipid-soluble, they diffuse straight through the phospholipid bilayer.
- Inside the cell, in the smooth endoplasmic reticulum, they are joined back into triglycerides.
- In the Golgi apparatus the triglycerides are packed with cholesterol and proteins into chylomicrons (lipoprotein balls).
- Chylomicrons leave by exocytosis and enter the lacteal, a lymph vessel in the villus, then reach the blood later.
Exam focus and how to answer
Common questions: name the enzyme, the bond and the product; explain why endopeptidases speed up exopeptidases; describe co-transport in steps; explain the role of micelles; describe how a lipid goes from lumen to lacteal. Always name the bond (glycosidic, peptide, ester), the place (lumen or membrane) and the product.
Key formulas and definitions
- Starch + water —amylase→ maltose
- Maltose + water —maltase→ glucose + glucose
- Sucrose —sucrase→ glucose + fructose; lactose —lactase→ glucose + galactose
- Protein —endopeptidase→ polypeptides —exopeptidase→ amino acids + dipeptides —dipeptidase→ amino acids
- Triglyceride + water —lipase→ monoglyceride + 2 fatty acids
- Key terms: hydrolysis, emulsification, micelle, co-transport, chylomicron, lacteal
Worked examples
1. A protein chain has 60 amino acids. Explain why adding an endopeptidase first makes exopeptidase work faster.
Exopeptidases can only act on the ends of chains. One chain has 2 ends. If an endopeptidase cuts it in 5 places, there are 6 chains and 12 ends. More ends mean more places for exopeptidase to act at the same time, so amino acids are released faster.
2. A drug blocks the Na⁺–K⁺ pump in ileum cells. Explain why glucose absorption falls.
The pump keeps Na⁺ low inside the cell. Without it, Na⁺ builds up inside and the Na⁺ gradient from lumen to cell disappears. The co-transporter only moves glucose when Na⁺ moves down its gradient, so glucose is no longer taken in.
3. Describe the route of a fatty acid from the lumen to the lymph.
Lipase releases it from a triglyceride → it joins a micelle with bile salts → the micelle carries it to the microvilli → it diffuses through the membrane → in the smooth ER it is joined back into a triglyceride → the Golgi packs it into a chylomicron → exocytosis → lacteal.
4. A fat droplet of radius 10 µm is emulsified into droplets of radius 1 µm. By how many times does the surface area increase?
Volume stays the same. Number of small droplets = (10/1)³ = 1000. Area of one big = 4π(10)² = 400π. Total small = 1000 × 4π(1)² = 4000π. Area increases 4000π ÷ 400π = 10 times. More surface means faster lipase action.
Common mistakes
- Saying bile salts digest fat. They only emulsify it; lipase does the hydrolysis.
- Saying the co-transporter uses ATP. ATP is used by the Na⁺–K⁺ pump on the blood side; co-transport is indirect.
- Mixing up endo- and exopeptidases. Endo = inside the chain; exo = at the ends.
- Saying lipids enter the blood capillary directly. They leave the cell as chylomicrons into the lacteal (lymph).