Digestion and absorption
AQA
A-Level
Digestion is chemical hydrolysis: a water molecule is used to break a bond, the reverse of the condensation reaction that built the molecule. Starch, triglycerides and proteins are too large, and largely too insoluble, to cross the phospholipid bilayer of the epithelial cells lining the gut, so each is broken into smaller products first. Hydrolysis of glycosidic bonds in carbohydrates gives monosaccharides; hydrolysis of ester bonds in triglycerides gives a monoglyceride and fatty acids; hydrolysis of peptide bonds in proteins gives amino acids. Each bond type has its own class of enzyme, and each enzyme is specific because its active site is complementary to only one substrate. Digestion occurs in the lumen of the gut, and the small soluble products are then absorbed across the cell-surface membrane of the epithelial cells lining the ileum, passing into the blood or the lacteal.
9
Objectives
9
Exam Tips
13
Pitfalls
15
Key Terms
17
Mark Points
Subtopics in this area
During digestion, large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes.
Digestion in mammals of: carbohydrates by amylases and membrane-bound disaccharidases lipids by lipase, including the action of bile salts proteins by endopeptidases, exopeptidases and membrane- bound dipeptidases.
Mechanisms for the absorption of the products of digestion by cells lining the ileum of mammals, to include: co-transport mechanisms for the absorption of amino acids and of monosaccharides the role of micelles in the absorption of lipids.
Digestion and absorption Revision Guide
Learning Objectives
What you need to know and understand
- Name the bond hydrolysed and the products formed when starch, a triglyceride and a protein are each digested.
- Explain, using size and solubility, why a polymer must be hydrolysed before it can cross the cell-surface membrane of an ileum epithelial cell.
- Contrast hydrolysis with the condensation reaction that formed the bond, stating what happens to the water molecule in each case.
- Describe the complete digestion of starch, naming amylase, maltose and a membrane-bound disaccharidase.
- Explain how bile salts increase the rate of lipid digestion without hydrolysing any bonds themselves.
- Sequence endopeptidases, exopeptidases and dipeptidases, stating which bond each hydrolyses and why acting in that order speeds digestion up.
- Describe the sequence by which glucose is absorbed into an ileum epithelial cell, beginning with the sodium-potassium pump.
- Explain why micelles are needed for fatty acid absorption even though fatty acids can cross the bilayer unaided.
- Predict the effect of damaged microvilli on absorption, on the water potential of the lumen and so on the volume of water lost.
Marking Points
Key points examiners look for in your answers
- Hydrolysis breaks a bond by adding a molecule of water; credit a correct contrast with condensation, in which water is released as a bond forms.
- Name the bond hydrolysed in the named molecule: glycosidic in a carbohydrate, ester in a triglyceride, peptide in a protein.
- Name the small products: monosaccharides from starch, amino acids from protein, and a monoglyceride plus fatty acids from a triglyceride.
- Explain that only small, soluble molecules can be absorbed across the cell-surface membrane of the epithelial cells, so polymers must first be hydrolysed.
- Link enzyme specificity to an active site complementary in shape to the substrate, so each enzyme hydrolyses only its own bond type.
- Place digestion in the lumen of the gut and absorption across the epithelial cell-surface membrane, with products entering the blood or lacteal.
- Amylase hydrolyses glycosidic bonds in starch to produce maltose; a membrane-bound disaccharidase such as maltase then hydrolyses maltose to glucose.
- State that the disaccharidase is membrane-bound on the microvilli of the epithelial cells, not secreted into the lumen.
- Lipase hydrolyses the ester bonds of a triglyceride to produce a monoglyceride and two fatty acids.
- Bile salts emulsify lipids into smaller droplets, increasing the surface area on which lipase can act and so raising the rate of hydrolysis; they do not hydrolyse bonds themselves.
- Endopeptidases hydrolyse internal peptide bonds, creating shorter peptides with more ends; exopeptidases remove terminal amino acids; membrane-bound dipeptidases hydrolyse dipeptides to single amino acids.
- For each enzyme, link bond hydrolysed to product formed, e.g. peptide bond to amino acids, ester bond to monoglyceride and fatty acids.
- Describe the sodium-potassium pump actively removing sodium ions from the epithelial cell into the blood, using ATP to create a concentration gradient.
- Explain that sodium ions re-enter the cell down their concentration gradient through a co-transporter protein, carrying glucose or an amino acid with them.
- State that glucose is moved against its own concentration gradient into the cell and then leaves the cell into the blood by facilitated diffusion.
- Explain that micelles carry monoglycerides and fatty acids to the epithelial cell membrane, where they break down and release them to diffuse directly through the phospholipid bilayer.
- Link the loss of microvilli, or fewer co-transport, carrier or channel proteins, to a reduced rate of absorption.
Examiner Tips
Expert advice for maximising your marks
- 💡Use the verb hydrolysed and name the bond; a vague 'digested into smaller bits' does not show the mechanism.
- 💡Follow one named molecule through, such as starch to maltose to glucose, rather than writing generally about food.
- 💡If asked why digestion is necessary, answer in terms of size and solubility for absorption, not in terms of releasing energy.
- 💡Build each digestion point from bond plus enzyme plus product, so write all three each time.
- 💡Place membrane-bound enzymes (disaccharidases, dipeptidases) in the cell-surface membrane of the epithelial cells, not in the lumen.
- 💡For proteins, sequence endopeptidases, exopeptidases and dipeptidases, stating the bond each hydrolyses and why the order speeds digestion.
- 💡Start every co-transport answer at the sodium-potassium pump; the sequence only makes sense in that order.
- 💡Damage to the ileum lining requires two ideas: fewer co-transport proteins and reduced surface area, followed by the water potential consequence.
- 💡Clearly distinguish between the emulsification of lipid droplets by bile salts and the subsequent formation of micelles from digested monoglycerides and fatty acids.
Common Mistakes
Pitfalls to avoid in your exam answers
- Writing that molecules are 'broken down' when the mark requires hydrolysed, or using hydrolysis without mentioning water. Correction: state that a water molecule is added to break the bond.
- Saying large molecules are too big to cross the membrane without naming the small products. Correction: name the products, e.g. starch to maltose to glucose.
- Describing water being released during digestion. Correction: water is released in condensation, not hydrolysis; hydrolysis uses water.
- Naming the wrong bond, such as peptide bonds in starch or glycosidic bonds in a triglyceride. Correction: match bond to molecule: glycosidic in carbohydrate, ester in triglyceride, peptide in protein.
- Claiming digestion happens inside the epithelial cells. Correction: digestion occurs in the gut lumen; membrane-bound enzymes act at the epithelial surface.
- Saying bile salts digest or hydrolyse lipids. Correction: bile salts emulsify lipids, increasing surface area for lipase; they hydrolyse nothing.
- Writing that amylase breaks starch straight into glucose. Correction: amylase gives maltose, then a membrane-bound disaccharidase gives glucose.
- Saying disaccharidases are secreted into the lumen. Correction: they are membrane-bound on the microvilli of epithelial cells.
- Swapping endopeptidase and exopeptidase. Correction: endopeptidases cut internal peptide bonds; exopeptidases remove terminal amino acids.
- Writing 'protease' throughout when the question names specific enzyme groups. Correction: name endopeptidases, exopeptidases and dipeptidases as appropriate.
- Saying the micelle itself is absorbed or enters the epithelial cell. Correction: Micelles break down outside the cell; only the monoglycerides and fatty acids diffuse across the membrane.
- Calling co-transport active transport of glucose, with ATP said to act directly on the glucose carrier. Correction: It is indirect active transport; ATP is used by the sodium-potassium pump, not the co-transporter.
- Explaining diarrhoea as water loss without a water potential argument. Correction: State that the water potential of the lumen falls, or that of the cells rises, so water leaves the cells by osmosis.