Digestion

Carbohydrate Digestion

• Amylase hydrolyses starch into the disaccharide maltose; it is produced in the salivary glands and the pancreas.
• Membrane-bound disaccharidases, such as maltase, are located on the cell-surface membranes of the epithelial cells in the ileum to break disaccharides into monosaccharides like glucose.

Protein Digestion

• Endopeptidases hydrolyse peptide bonds within the central region of a polypeptide chain, creating more 'ends' for other enzymes to act upon.
• Exopeptidases remove terminal amino acids from the ends of polypeptide chains, progressively shortening the chain into dipeptides or single amino acids.
• Dipeptidases are membrane-bound enzymes that specifically hydrolyse dipeptides into individual amino acids for immediate absorption.

Lipid Digestion

• Lipases hydrolyse lipids into monoglycerides and fatty acids by breaking ester bonds.
• This process is significantly enhanced by bile salts, which perform emulsification rather than chemical digestion.

• Bile salts, produced in the liver and stored in the gall bladder, are essential for efficient lipid digestion through a process called emulsification.

• Emulsification involves breaking down large lipid globules into many tiny lipid droplets, which increases the total surface area available for lipase enzymes to act upon.

• Once hydrolysed, the fatty acids and monoglycerides associate with bile salts to form micelles, which are tiny spherical structures that transport the insoluble lipids to the epithelial cell membrane.

• Micelles are dynamic; they constantly break up and reform, allowing their non-polar contents to diffuse directly through the phospholipid bilayer of the intestinal cells.

• The absorption of glucose and amino acids relies on a co-transport mechanism driven by a sodium ion ($Na^+$) gradient.

• Active transport of sodium ions out of the epithelial cell into the blood via the sodium-potassium pump creates a low concentration of $Na^+$ inside the cell.

• Sodium ions then move down their concentration gradient from the intestinal lumen into the cell via a co-transporter protein, carrying a glucose molecule or amino acid with them.

• This is considered indirect active transport because, while the co-transporter itself is passive, the entire process requires ATP to maintain the initial sodium gradient.

• Once inside the cell, the high concentration of nutrients allows them to move into the blood via facilitated diffusion through specific channel proteins.

• It is critical to distinguish between physical breakdown (emulsification) and chemical breakdown (hydrolysis).

• Membrane-bound enzymes (like maltase and dipeptidases) are distinct from lumen enzymes (like amylase and pepsin) because they ensure the final products of digestion are produced exactly where they can be immediately absorbed.

• Terminology Precision: Never use the word 'amount' when describing experimental results; always specify 'volume' or 'concentration' of the solution.

• Bile Misconception: Always state that bile salts emulsify lipids to increase surface area; never refer to bile as an enzyme or say it 'digests' lipids chemically.

• pH Sensitivity: When discussing enzyme activity (like amylase), remember that pH affects the tertiary structure and the shape of the active site, which is why buffers are used in experiments.

• Absorption Logic: If asked why absorption is 'active', focus on the sodium-potassium pump requiring ATP to maintain the gradient, even if the co-transporter itself is passive.