Digestion and absorption

In a nutshell

Your food is made of the same large polymers as your own body: polysaccharides, proteins and lipids. They are far too big to cross a cell membrane, so they cannot be absorbed as they are.

Digestion solves this by hydrolysing large biological molecules into small, soluble ones that can be absorbed across cell membranes. Those products are then taken up by the cells lining the ileum and passed into the blood or lymph.

This note covers the enzymes that digest each food group, and the three absorption mechanisms AQA tests: co-transport of monosaccharides, co-transport of amino acids, and the role of micelles for lipids.

Assumed knowledge: Carbohydrates, Lipids, Proteins, Transport across cell membranes.

Core content

Digestion is hydrolysis

Every food group is digested by the same chemical trick: a hydrolysis reaction that uses a water molecule to break a bond.

• Carbohydrates: glycosidic bonds are hydrolysed.
• Proteins: peptide bonds are hydrolysed.
• Lipids (triglycerides): ester bonds are hydrolysed.

The rule that earns marks: say "hydrolyses the [named] bond", and name the specific enzyme. "Breaks down" and "an enzyme digests it" are not enough.

Carbohydrate digestion

Starch is digested in two stages, by two enzymes:

Membrane-bound disaccharidases are enzymes fixed in the cell-surface membrane of the epithelial cells lining the ileum. They hydrolyse a disaccharide into its monosaccharides right at the surface where the products are absorbed:

• Maltase: maltose ⟶ glucose + glucose
• Sucrase: sucrose ⟶ glucose + fructose
• Lactase: lactose ⟶ glucose + galactose

Lipid digestion: bile salts and lipase

Lipids are digested in the small intestine, and there is an extra step first because triglycerides are insoluble and clump into large globules.

Bile salts (made in the liver, stored in the gall bladder) emulsify the lipid: they break large lipid globules into many tiny droplets.

• This gives the lipid a much larger surface area, so lipase can work faster.
• Bile salts are not enzymes. They emulsify the lipid; they do not hydrolyse it.

Lipase (from the pancreas) then hydrolyses triglycerides into monoglycerides and fatty acids (a monoglyceride is glycerol with one fatty acid still attached).

You can see the emulsifying effect in a required-practical graph. As lipase releases fatty acids, the pH of the mixture falls. With bile salts the pH falls faster and further, because the larger surface area speeds up hydrolysis:

Protein digestion

Proteins are digested by three classes of enzyme working in sequence. The key idea is that the first enzyme creates many more ends for the second to attack.

• Endopeptidases hydrolyse peptide bonds in the middle of a polypeptide, cutting it into shorter chains. This creates more ends (a larger surface area of terminal bonds) for the exopeptidases.
• Exopeptidases hydrolyse peptide bonds at the ends of chains, removing single amino acids or dipeptides.
• Membrane-bound dipeptidases, fixed in the cell-surface membrane of the ileum epithelial cells, hydrolyse dipeptides into individual amino acids.

The ileum is adapted for absorption

Absorption happens across the epithelial cells lining the ileum. Their adaptations are a common exam target, so learn them as causes and effects:

• Villi and, on each epithelial cell, microvilli: give a large surface area for absorption. (The microvilli are the cell adaptation; villi are the wall adaptation.)
• A wall just one epithelial cell thick: a short diffusion pathway.
• Many mitochondria: release energy as ATP for active transport.
• Many carrier and co-transport proteins in the membrane, plus the membrane-bound digestive enzymes above.

Absorption of monosaccharides and amino acids: co-transport

Glucose and amino acids are absorbed by co-transport with sodium ions. The concentration of glucose in the lumen can fall below that in the cell, so simple diffusion alone cannot absorb it all: the cell couples glucose to the sodium gradient instead.

The mechanism is the same for glucose and for amino acids:

1. The sodium-potassium pump actively transports Na⁺ out of the epithelial cell into the blood, using ATP.
2. This keeps the Na⁺ concentration low inside the cell, so there is a Na⁺ concentration gradient from the lumen into the cell.
3. Na⁺ diffuses back in from the lumen through a co-transport protein, and as it does it carries a glucose molecule (or an amino acid) in with it, even against that solute's concentration gradient.
4. The glucose or amino acid then moves from the cell into the blood by facilitated diffusion.

Still don't get it? · why the sodium gradient pulls glucose in

Think of a revolving door that only turns when two people push through it together: one going where they want to go anyway, and one going the "wrong" way, dragged along by the first.

Sodium ions are the willing person. The sodium-potassium pump has spent ATP pumping Na⁺ out of the cell, so Na⁺ is desperate to rush back in down its concentration gradient. Glucose is the person being dragged. On its own, glucose has no reason to enter (there may already be more glucose inside than in the lumen), so it cannot move by diffusion.

The co-transport protein is the revolving door: it only lets Na⁺ back in if a glucose molecule comes through at the same time. So the energy stored in the Na⁺ gradient is what drags glucose in against its own gradient.

Two things students must get right for the marks: the ATP is used by the sodium-potassium pump (active transport), not by the co-transport step itself, and once inside, glucose leaves for the blood by facilitated diffusion.

Absorption of lipids: the role of micelles

The products of lipid digestion, monoglycerides and fatty acids, are carried to the epithelial cells inside micelles.

• Micelles are tiny structures formed from bile salts, monoglycerides and fatty acids.
• They keep the monoglycerides and fatty acids soluble in the watery contents of the ileum and carry them to the epithelial cell membrane.
• At the membrane the micelle releases the monoglycerides and fatty acids. Because these are non-polar (lipid-soluble), they diffuse straight through the phospholipid bilayer into the cell (simple diffusion).

After absorption, the products are repackaged for transport in the body:

• In the smooth endoplasmic reticulum, monoglycerides and fatty acids are recombined into triglycerides.
• In the Golgi apparatus the triglycerides are combined with proteins to form chylomicrons.
• Chylomicrons are too large to diffuse out, so they leave the cell by exocytosis and enter the lacteals (lymphatic vessels), not the blood capillaries directly.

Still don't get it? · what micelles actually do (and do not do)

Imagine trying to deliver a drop of oil across a swimming pool. On its own the oil just floats and clumps; it never reaches the far wall. Now wrap the oil in a tiny soap bubble that mixes happily with water. The bubble ferries the oil across, touches the far wall, and lets the oil out right where you need it. The bubble itself does not go through the wall.

The micelle is that soap bubble. Bile salts, monoglycerides and fatty acids cluster into a micelle that mixes with the watery gut contents and shuttles the fatty acids and monoglycerides up to the epithelial cell membrane. There it breaks open and releases them.

The fatty acids and monoglycerides are lipid-soluble, so they slip straight through the phospholipid bilayer by simple diffusion. Two marks are lost every year here: it is the droplets (from emulsification) that increase the surface area, not the micelles, and the products enter by diffusion through the bilayer, not through a membrane protein.

Required practicals and skills (PS 1.1)

Two investigations are named in the specification for this subtopic:

• Effect of pH or bile salts on the rate of a digestive enzyme. For lipase you can follow the reaction by measuring pH against time: as fatty acids are produced the pH falls. Adding bile salts increases the rate (the graph above). A pH probe gives a more accurate, quantitative reading than a subjective colour change with an indicator.
• Visking tubing model of absorption. Visking (partially permeable) tubing acts as a model gut wall. Small molecules (e.g. glucose from hydrolysed starch) pass through into the surrounding water; large molecules (starch, protein) do not. You test the water outside with Benedict's (reducing sugar), iodine in potassium iodide (starch) and biuret (protein) to show which molecules were small enough to cross.

Worked examples

Model 5-mark answer: "Describe how glucose is absorbed from the lumen of the ileum into the blood."

Mark the number and order of points, and notice each is one creditable idea:

1. Sodium ions are actively transported out of the epithelial cell into the blood by the sodium-potassium pump (using ATP).
2. This maintains a lower concentration of Na⁺ inside the cell than in the lumen (a Na⁺ concentration gradient).
3. Na⁺ diffuses from the lumen into the cell through a co-transport protein, bringing glucose in with it (against the glucose concentration gradient).
4. Glucose then moves from the cell into the blood.
5. This final step is by facilitated diffusion (through a carrier protein).

The same five-point structure answers the amino-acid version: swap "glucose" for "amino acid" throughout.

Model 4-mark answer: "Describe how a protein is fully digested to amino acids."

1. Endopeptidases hydrolyse peptide bonds in the middle of the polypeptide, producing shorter chains.
2. This creates more ends for the exopeptidases to act on.
3. Exopeptidases hydrolyse peptide bonds at the ends, removing single amino acids or dipeptides.
4. Membrane-bound dipeptidases hydrolyse the remaining dipeptides into single amino acids.

Common exam mistakes

• Writing that amylase breaks starch straight into glucose. Amylase produces maltose; maltase then produces glucose. Missing the maltose step loses marks.
• Confusing maltose (the sugar) with maltase (the enzyme). Check every spelling: the enzyme ends in "-ase".
• Saying an enzyme "breaks down" its substrate instead of "hydrolyses", and forgetting to name the bond (glycosidic / peptide / ester). The bond and the word "hydrolysis" carry marks.
• Claiming bile salts digest lipids. Bile salts are not enzymes; they only emulsify the lipid. Lipase does the hydrolysis.
• Saying micelles increase the surface area. It is the droplets produced by emulsification that increase surface area for lipase; micelles transport the products to the membrane.
• Saying fatty acids are absorbed through a membrane protein / by active transport. They are lipid-soluble and diffuse directly through the phospholipid bilayer.
• Getting endopeptidase and exopeptidase the wrong way round: endo = internal bonds (middle), exo = bonds at the ends. Forgetting the "more ends for exopeptidases" point loses the explanation mark.
• On co-transport, being vague about where Na⁺ moves. State clearly: pumped out of the cell into the blood, then diffuses in from the lumen with glucose.
• Confusing co-transport with active transport. The sodium-potassium pump uses ATP (active transport); the co-transport step itself does not use ATP directly, it uses the Na⁺ gradient.
• For glucose entering the blood, only facilitated diffusion (or diffusion) is credited in that specific context, not active transport.
• Saying chylomicrons leave by diffusion or facilitated diffusion. They are too large; they leave by exocytosis, into a lacteal.
• Writing that mitochondria "make" or "produce" energy. Energy is released (or ATP is produced); energy cannot be made.

Key definitions

• Digestion: the hydrolysis of large, insoluble biological molecules into smaller, soluble molecules that can be absorbed across cell membranes.
• Hydrolysis: the breaking of a chemical bond between two molecules involving the use of a water molecule.
• Amylase: an enzyme that catalyses the hydrolysis of starch into maltose.
• Membrane-bound disaccharidase: an enzyme (e.g. maltase) attached to the cell-surface membrane of the epithelial cells lining the ileum that hydrolyses a disaccharide into monosaccharides.
• Endopeptidase: an enzyme that hydrolyses peptide bonds in the interior (middle) of a polypeptide chain.
• Exopeptidase: an enzyme that hydrolyses peptide bonds at the ends of a polypeptide chain, removing single amino acids or dipeptides.
• Dipeptidase: a membrane-bound enzyme that hydrolyses dipeptides into single amino acids.
• Bile salts: molecules made by the liver that emulsify lipids, increasing the surface area for lipase action.
• Emulsification: the breaking up of large lipid globules into smaller droplets, increasing the surface area for lipase.
• Lipase: an enzyme that catalyses the hydrolysis of triglycerides into monoglycerides and fatty acids.
• Micelle: a tiny structure of bile salts, monoglycerides and fatty acids that carries the products of lipid digestion to the epithelial cell membrane.
• Co-transport: a mechanism in which a molecule (glucose or an amino acid) is carried across a membrane through a carrier protein together with sodium ions moving down their concentration gradient.

Specification

• I can state that during digestion large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes.
• I can describe the digestion of carbohydrates by amylases and membrane-bound disaccharidases.
• I can describe the digestion of lipids by lipase, including the action of bile salts.
• I can describe the digestion of proteins by endopeptidases, exopeptidases and membrane-bound dipeptidases.
• I can explain the co-transport mechanism for the absorption of amino acids and of monosaccharides in the ileum.
• I can explain the role of micelles in the absorption of lipids in the ileum.

Related notes

• Surface area to volume ratio
• Mass transport in animals and plants
• Transport across cell membranes
• Carbohydrates
• Lipids
• Proteins