During digestion, large biological molecules are hydrolysed to smaller…

The idea

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.

What earns the marks

• 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.

Common mistakes and how to put them right

• Writing that molecules are 'broken down' when the mark requires hydrolysed, or using hydrolysis without mentioning water.

  Put it right: 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.

  Put it right: Name the products, e.g. starch to maltose to glucose.

• Describing water being released during digestion.

  Put it right: 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.

  Put it right: Match bond to molecule: glycosidic in carbohydrate, ester in triglyceride, peptide in protein.

• Claiming digestion happens inside the epithelial cells.

  Put it right: Digestion occurs in the gut lumen; membrane-bound enzymes act at the epithelial surface.

Exam tips

• 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.