SPEC 3.3.3 - DIGESTION & ABSORPTION
Digestion & Absorption
Enzymes, bile salts, co-transport and the role of micelles in lipid absorption
Overview of Digestion
Large insoluble biological molecules are hydrolysed into smaller soluble molecules that can be absorbed across cell membranes into the blood.
Definition
Hydrolysis: The breaking of a covalent bond using water, catalysed by enzymes. In digestion: glycosidic bonds (carbohydrates), peptide bonds (proteins), and ester bonds (lipids) are all hydrolysed.
Food GroupProducts of DigestionMain Absorption Site
Carbohydrates (starch)Monosaccharides (glucose)Ileum
ProteinsAmino acidsIleum
Lipids (triglycerides)Fatty acids + monoglyceridesIleum
Key Principle
Molecules must be small enough to cross the ileum epithelium. Hydrolysis reduces large polymers to monomers that can be transported across cell membranes.
Carbohydrate Digestion
Carbohydrates are digested in two stages: luminal digestion by amylase, then membrane digestion by disaccharidases on the brush border.
EnzymeSubstrateProductsLocation
Salivary amylaseStarchMaltoseMouth
Pancreatic amylaseStarchMaltoseDuodenum
Maltase (membrane-bound)MaltoseGlucose + GlucoseIleum brush border
Sucrase (membrane-bound)SucroseGlucose + FructoseIleum brush border
Lactase (membrane-bound)LactoseGlucose + GalactoseIleum brush border
Examiner Alert
Amylase produces MALTOSE, not glucose. Glucose is only released when membrane-bound disaccharidases (maltase etc.) act at the brush border. Never write "amylase produces glucose."
Protein Digestion
Proteins are digested by three enzyme classes acting in sequence: endopeptidases → exopeptidases → membrane-bound dipeptidases.
1
Endopeptidases (e.g. pepsin in stomach; trypsin, chymotrypsin in duodenum) — hydrolyse peptide bonds within the polypeptide chain → shorter polypeptide fragments with more free terminal ends.
2
Exopeptidases — hydrolyse peptide bonds at the ends of polypeptide chains, releasing individual amino acids from the terminals.
3
Membrane-bound dipeptidases on the ileum brush border — hydrolyse dipeptides into individual amino acids. Acting at the cell surface means amino acids are released directly at the point of absorption.
Why This Order?
Endopeptidases create many short fragments with multiple free terminal ends → exopeptidases have far more substrate sites → digestion is much faster than with exopeptidases alone.
Lipid Digestion & Bile Salts
Triglycerides are hydrolysed by lipase (pancreatic) into fatty acids and monoglycerides. Bile salts act first to make this efficient.
Bile Salts — Emulsification
Bile salts are amphipathic (hydrophilic head, hydrophobic tail)
Hydrophobic tails insert into fat globules
Large globules break into tiny droplets
Massively increases surface area for lipase
This is a physical process — no bonds broken
Lipase — Chemical Digestion
Acts at the fat droplet surface
Hydrolyses ester bonds in triglycerides
Products: fatty acids + monoglycerides
These then form micelles with bile salts
Examiner Alert
Bile salts EMULSIFY — lipase DIGESTS. Bile salts do not hydrolyse any bonds. "Bile digests fat" scores zero marks.
Micelles & Lipid Absorption
Products of fat digestion are packaged into micelles — tiny water-soluble structures that carry lipid products to the epithelial surface.
1
Micelle formation: Fatty acids and monoglycerides associate with bile salts → micelles (hydrophilic exterior, hydrophobic core). This allows transport through the aqueous lumen.
2
Diffusion into epithelial cell: Micelles reach the epithelial surface and release fatty acids + monoglycerides, which diffuse directly across the phospholipid bilayer (small, non-polar molecules).
3
Re-synthesis: Inside the smooth ER (SER), fatty acids and monoglycerides are re-synthesised into triglycerides. These are packaged with protein into chylomicrons in the Golgi apparatus.
4
Exocytosis into lacteals: Chylomicrons are too large to enter blood capillaries — secreted by exocytosis into lacteals (lymphatic capillaries in villi) → lymph → thoracic duct → blood.
Common Error
Lipids enter lacteals — NOT blood capillaries. Glucose and amino acids go to blood capillaries; fats go to lacteals.
Emulsification vs. Micelles
The 2024 AQA Examiner Report flagged that students frequently confuse these two separate processes.
Emulsification
Stage: Before lipase acts
What happens: Bile salts break large fat globules into small fat droplets. The fat is still triglyceride — no chemical bonds are broken.
Purpose: Increases surface area for lipase action.
Micelle Formation
Stage: After lipase digestion
What happens: Fatty acids + monoglycerides associate with bile salts to form micelles.
Purpose: Transports digestion products through the aqueous lumen to the epithelial surface.
2024 Examiner Report
"There is a common misconception that emulsification occurs to form micelles." Emulsification is physical (before digestion); micelle formation is after digestion. These are distinct stages.
Co-transport: Glucose & Amino Acids
Glucose and amino acids are absorbed by co-transport — a mechanism driven by a sodium ion electrochemical gradient.
1
Na⁺/K⁺ ATPase on the basolateral membrane uses ATP to actively pump Na⁺ out of the epithelial cell → creates a low [Na⁺] inside the cell.
2
Na⁺ diffuses into the cell down its electrochemical gradient via a sodium-glucose co-transporter on the apical membrane. Glucose (or amino acid) is carried in simultaneously against its own concentration gradient.
3
Glucose accumulates inside the cell → moves out by facilitated diffusion via carrier proteins on the basolateral membrane → into blood capillary → hepatic portal vein → liver.
Energy & Co-transport
This is co-transport — a form of active transport powered indirectly by ATP. Na⁺/K⁺ ATPase uses ATP to create the Na⁺ gradient; the gradient then drives co-transport. AQA mark schemes accept "active transport" for this mechanism.
Co-transport: Key Points
FeatureDetail
Driving forceNa⁺ electrochemical gradient (created by Na⁺/K⁺ ATPase)
ATP used directly?No — ATP powers Na⁺/K⁺ ATPase only; co-transport itself is driven by the Na⁺ gradient
Apical membraneSodium-glucose co-transporter — carries Na⁺ and glucose simultaneously into cell
Basolateral membraneCarrier proteins — facilitated diffusion of glucose into blood capillary
Amino acidsSame mechanism — Na⁺ co-transport in at apical membrane, facilitated diffusion out at basolateral
DestinationBlood capillaries → hepatic portal vein → liver
2024 Examiner Report
State carrier proteins (not channel proteins) for facilitated diffusion of glucose at the basolateral membrane. "Transport proteins" alone is insufficient.
Comparing Absorption Routes
MoleculeInto epithelial cellOut of cellDestination
GlucoseCo-transport with Na⁺Facilitated diffusion (carrier proteins)Blood capillary
Amino acidsCo-transport with Na⁺Facilitated diffusionBlood capillary
Fatty acids & monoglyceridesSimple diffusion (non-polar)Exocytosis as chylomicronsLacteal (lymph)
Route Summary
Glucose and amino acids → blood capillaries → hepatic portal vein → liver.
Lipids → lacteals → lymph → thoracic duct → blood (bypassing liver initially).
Practical: Visking Tubing Model
Visking tubing (partially permeable membrane) is used to model absorption in the ileum. It is permeable to small molecules but not large ones.
Method
Fill Visking tubing with starch + glucose solution
Suspend in distilled water bath
Test water at intervals with Benedict's reagent and iodine solution
Benedict's positive = glucose diffused through
Iodine negative = starch did NOT cross
Interpretation
Glucose (small) crosses the membrane — models monosaccharide absorption
Starch (large polymer) cannot cross — models how undigested molecules are not absorbed
Extension: add amylase inside the tubing — starch hydrolysed to glucose, which then diffuses out; iodine stays negative, Benedict's turns positive
Practice Question
Which enzyme hydrolyses peptide bonds at the ends of polypeptide chains?
A. EndopeptidaseB. ExopeptidaseC. DipeptidaseD. Amylase
B — Exopeptidase.
terminal ends
Practice Question
A student claims bile salts digest triglycerides. Which statement correctly explains why this is wrong?
A. Bile salts hydrolyse ester bonds in triglyceridesB. Bile salts convert triglycerides directly into chylomicronsC. Bile salts emulsify fat (a physical process) but lipase performs chemical hydrolysis of ester bondsD. Bile salts are only active in the stomach, not the duodenum
physical
chemically
Practice Question
A drug blocks Na⁺/K⁺ ATPase pumps in ileum epithelial cells. Which is the most likely direct consequence?
A. Fatty acids would not be absorbed into lactealsB. Glucose absorption via co-transport would decreaseC. Lipase activity in the duodenum would decreaseD. Amylase would fail to hydrolyse starch
Practice Question
After a fatty meal, fatty acids and monoglycerides enter ileum epithelial cells. Which sequence correctly describes what happens next?
A. Exocytosis → chylomicrons → blood capillary → hepatic portal veinB. Re-synthesis to triglycerides (smooth ER) → chylomicrons (Golgi) → exocytosis → lactealC. Co-transport with Na⁺ → blood capillary → hepatic portal veinD. Diffusion directly into blood capillary as individual fatty acids
smooth ER (SER)
lacteals
Lesson Summary
Carbohydrate Digestion
Amylase → starch → maltose (not glucose)
Membrane-bound disaccharidases → monosaccharides
Protein Digestion
Endopeptidases → cut internal bonds
Exopeptidases → remove terminal amino acids
Dipeptidases (membrane-bound) → final step at brush border
Lipid Digestion & Absorption
Bile salts: emulsify (physical — no bonds broken)
Lipase: hydrolyses ester bonds in triglycerides
Micelles → diffusion into epithelial cell
Smooth ER → chylomicrons (Golgi) → lacteals
Co-transport
Na⁺/K⁺ ATPase creates Na⁺ gradient (uses ATP)
Na⁺ + glucose co-transported into cell together
Glucose exits by facilitated diffusion via carrier proteins
Glucose & amino acids → blood capillaries → liver
Next: Overview of Digestion