Digestion and absorption ✅

explain what happens in digestion

• Large (insoluble) biological molecules hydrolysed to smaller (soluble) molecules

  ● That are small enough be absorbed across cell membranes into blood

describe the digestion of starch in mammals

1. amylase (produced by salivary glands) hydrolysis starch to maltose

2. membrane bound maltase (attached to cells lining ileum) hydrolyses maltose to glucose

3. hydrolysis of glycosidic bond

describe the digestion of disaccharides in mammals

● Membrane-bound disaccharidases hydrolyse disaccharides to 2 monosaccharides:

○ Maltase - maltose → glucose + glucose

○ Sucrase - sucrose → fructose + glucose

○ Lactase - lactose → galactose + glucose

● Hydrolysis of glycosidic bond

describe the digestion of lipids in mammals, including action of bile salts

1. bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets

2. this increases surface area of lipids for increase lipase activity

3. lipase (made in pancreas) hydrolyses lipids into monoglycerides + fatty acids

4. hydrolysis of ester bond

define hydrolysis

• splitting up of a large molecule into smaller molecules by the addition of water

define absorption

process by which small, soluble molecules produce by digestion are taken up from the small intestine and into the bloodstream

define assimilation

• process by which substances absorbed from the small intestine after digestion are taken into the cells of the body and built up into useful substances

describe the digestion of proteins by a mammal

Endopeptidases - hydrolyse internal (peptide) bonds

within a polypeptide → smaller peptides

○ So more ends / surface area for exopeptidases

● Exopeptidases - hydrolyse terminal (peptide) bonds at

ends of polypeptide → single amino acids

● Membrane-bound dipeptidases - hydrolyse (peptide)

bond between amino acids in dipeptides → 2 amino acids

● Hydrolysis of peptide bond

name 2 places where starch is produced and as a result 2 places it is released

1. produced in salivary glands, released in mouth

2. produced in pancreas, released into duodenum

what are disaccharides hydrolysed by

membrane bound disaccharidases

Suggest why membrane-bound enzymes are important in digestion

• membrane bound enzymes are located on cell membranes of epithelial cells lining the ileum

• (by hydrolysing molecules at the site of absorption they) maintain concentration gradients for absorption

describe the pathway for absorption of products of digestion in mammals

lumen of ileum → cells lining ileum → blood

describe the absorption of amino acids and monosaccharides in mammals

1. Na⁺ actively transported from epithelial cells lining ileum to blood (by Na⁺/K⁺ pump)

2. establishing a concentration gradient of Na⁺ (higher in lumen than epithelial cell)

3. Na⁺ enters epithelial cell down its conc grad with a monosaccharide or amino acid against its concentration gradient

4. via a co-transporter

5. monosaccharide or amino acid moves down a conc grad into blood via facilitated diffusion

describe the absorption of lipids by mammals, including the role of micelles

● Bile salts combine with monoglycerides and fatty acids to form micelles

○ Micelles make monoglycerides and fatty acids (more) soluble in water

○ Micelles carry fatty acids and monoglycerides to cells lining the ileum, where they break down to release them

○ This maintains a high concentration of fatty acids and monoglycerides near cells lining the ileum

● Monoglycerides / fatty acids are absorbed (into epithelial cell) by diffusion (as they’re lipid soluble)

● Triglycerides reformed in (epithelial) cells and aggregate into globules

● Globules coated with proteins forming chylomicrons which are then packaged into vesicles

● Vesicles move to cell membrane and fuse with it, releasing chylomicrons via exocytosis

○ Chylomicrons enter lymphatic vessels and eventually return to blood circulation

describe the absorption of glucose in mammals

1. Na⁺ actively transported from epithelial cells lining ileum to blood (by Na⁺/K⁺ pump)

2. establishing a concentration gradient of Na⁺ (higher in lumen than epithelial cell)

3. Na⁺ enters epithelial cell down its conc grad with glucose against its concentration gradient

4. via a co-transporter

5. glucose moves down a conc grad into blood via facilitated diffusion

what is a micelle

aggregate of monoglycerides, fatty acids and bile salts

explain how the active transport of amino acids out of epithelial cell allows for amino acids to diffuse into epithelial cell (2)

1. creates a low concentration of amino acids

2. between lumen and cell

explain why sodium ions and amino acids are not absorbed from the lumen of the small intestine when active transport can’t occur (2)

• no more pump

• no longer a concentration gradient established

• no facilitated diffusion of Na⁺ ions into cell, amino acids absorption requires diffusion of Na⁺ ions into cell

explain the importance of microvilli

increase surface area for absorption

describe the role of rough endoplasmic reticulum in the formation of chylomicrons (2)

1. proteins are synthesised

2. involvement of ribosomes

3. proteins transported

4. in vesicles to golgi

suggest how chylomicrons leave the epithelial cell (2)

1. via exocytosis

2. because too large to leave by other methods

what makes up a chylomicron

made of triglycerides and protein

which vessel do the chlyomicrons enter after leaving the cell

lymphatic vessel