Chemical Digestion

Question 1

Describe chemical digestion in the stomach

Answer 1

Chief cells secrete pepsinogen. Pepsinogen is activated when it comes into contact with hydrochloric acid forming pepsin.

Question 2

What is an endopeptidase?

Answer 2

E.g. pepsin, an endopeptidase enzyme which hydrolyses peptide Bonds in the centre of polypeptide Chains, forming shorter polypeptide chains. This forms more free ends for exopeptidase enzymes to act on.

Question 3

What enzymes are attached to the epithelial cells of villi?

Answer 3

Dipeptidase, maltase, sucrase and lactase. This is part of chemical digestion in ileum

Question 4

Dipeptidase

Answer 4

Hydrolyses dipeptides to amino acids

Question 5

Maltase

Answer 5

Hydrolyses maltose into alpha glucose

Question 6

Sucrase

Answer 6

Hydrolyses sucrose into glucose and fructose

Question 7

Lactase

Answer 7

Hydrolyses lactose into beta glucose

Question 8

Chemical digestion in the buccal cavity (mouth)

Answer 8

Salivary amylase from the salivary glands hydrolyse amylose in starch to maltose and glycogen to smaller polysaccharides

Question 9

Chemical digestion in the pancreas

Answer 9

Exocrine cells in the pancreas secrete trypsinogen. Secretes exopeptidase and endopeptidase enzymes. Also secretes pancreatic amylase and lipase

Question 10

Trypsin

Answer 10

An endopeptidase

Question 11

What are exopeptidase enzymes?

Answer 11

Enzymes that hydrolyze peptide bonds at the ends of polypeptide Chains, producing even shorter polypeptide chains , dipeptides and amino acids.

Question 12

The livers role within chemical digestion

Answer 12

The Liver secretes bile,

Question 13

What does lipase do?

Answer 13

Hydrolyses triglycerides to fatty acids and glycerol

Question 14

pancreatic amylase

Answer 14

hydrolyses amylose in starch to maltose

Question 15

How is trypsinogen activated?

Answer 15

It is activated when it comes into contact with enterokinase in the duodenum forming trypsin.

Question 16

Why is pepsin secreted in an inactive form?

Answer 16

So that its active form does not start digest the cells that produce it

Question 17

Pancreatic juice

Answer 17

Contains bicarbonate ions and sodium hydrogen carbonate ions which aid in neutralisation of the stomach acid

Question 18

Chemical digestion in the duodenum

Answer 18

carbohydrate and protein digestion is completed by enzymes fixed in the membrane of the epithelial cells of the mucosa. These enzymes include maltase, lactase, galactase and dipeptidase.

Question 19

What happens to excess amino acids?

Answer 19

They are taken to the Liver where deamination occurs, producing a respiratory substrate and urea which is then excreted

Question 20

What happens to lipids once they are digested?

Answer 20

They are used for membranes and hormones and the excess are stored as fat.

Question 21

Enzymes that break down carbohydrates, proteins and lipids

Answer 21

carbohydrases, proteases, lipases,

Question 22

Chemical digestion of starch pathway

Answer 22

-> maltose -> glucose

Question 23

Chemical digestion of protein pathway

Answer 23

-> polypeptides -> dipeptides -> amino acids

Question 24

Where does protein digestion take place?

Answer 24

lumen of the stomach, small intestine, mucosa of the small intestine

Question 25

enzyme protein -> polypeptide

Answer 25

endopeptidase

Question 26

enzyme polypeptide -> dipeptide

Answer 26

exopeptidase

Question 27

enzyme dipeptide -> amino acid

Answer 27

dipeptidase - usually embedded in membrane

Question 28

where does lipid digestion occur?

Answer 28

Lumen of the small intestine. It is only here that lipase and bile occur

Question 29

How does the stomach prepare lipids for digestion?

Answer 29

It churns the solid lipids into a fatty liquid made of fat droplets . No chemical digestion takes place because the pH is too acidic for lipase activity.

Question 30

Describe lipid digestion, including the definition of emulsification

Answer 30

In the duodenum, bile salts from the liver bind onto the fat droplets and this splits them into smaller droplets - emulsification. It is a physical process which increases the SA for lipase. Lipase from the pancreas catalyses the breakdown of lipids into fatty acids and glycerol.

Question 31

Example of extracellular digestion in humans

Answer 31

salivary glands, gastric glands and pancreas all secreting enzymes that pass into the lumen of the gut for digestion to take place OUTSIDE OF THE CELLS that make up the gut wall.

Question 32

Example of intracellular digestion in humans

Answer 32

Maltase and dipeptidases are found on the outer membranes and the cytoplasm (for dipeptidase) of the epithelial cells lining the small intestine. Digestion takes place INSIDE OF THE CELLS.

Question 33

How are maltose and dipeptides digested extracellularly?

Answer 33

Maltose/dipeptides molecules diffuse towards the outer membrane of the microvilli, where they bind to the enzyme maltase/dipeptidase. Once it is digested into 2 glucose molecules/amino acids, the molecules pass through the membrane and into the cytoplasm.

Question 34

Where do absorbed lipids go?

Answer 34

In the villi, they go into lacteal

Question 35

Where do most absorbed products of digestion go? (excl. lipids)

Answer 35

Transported into the blood vessels, by the epithelial cells of the villi. They are then transported directly to the liver where they are processed. (hepatic portal vein…?

Question 36

By what process are glucose and amino acids absorbed?

Answer 36

Partly facilitated diffusion (moved across membrane through channel protein ) and partly active transport (uses carrier protein and ATP).

Question 37

By what process are the products of lipid digestion absorbed?

Answer 37

Simple diffusion

Question 38

Describe how glucose (and amino acids) enter the villi cells and capillaries

Answer 38

Na+/K+ pump transports Na+ ions out of the cell by active transport. Glucose and sodium ions bind to the carrier protein and the Na+ ions diffuse into the cells (due to low Na+ conc.), carrying the glucose with it in facilitated diffusion. Inside the cell, the glucose and sodium ions leave the carrier protein. The glucose concentration rises and glucose diffuses to the other end of the cell, where it moves into the capillaries by facilitated diffusion to go to the liver.

Question 39

To what are lipids broken down into

Answer 39

They are broken down to fatty acids, glycerol and monoglycerides (partially digested lipids). fatty acids will vary in the length of their chain.

Question 40

Describe how lipids enter the villi cells and capillaries

Answer 40

Short fatty acids diffuse directly into the blood from the lumen if the ileum VIA THE EPITHELIUM. Longer chain fatty acids, monoglycerides and glycerol diffuse into the epithelial cells and enter the SER, where they recombine to form triglycerides. These are packaged into vesicles with cholesterol + phospholipid to form ‘water-soluble fat droplets, called chylomicrons’. These are transferred to the lacteal by exocytosis. The chylomicrons then travel in the lymph system where they are transferred to the sub clavian vein from the throracic duct.

Question 41

How do dimers (maltose, sucrose, lactose, dipeptides) get broken down in the ileum?

Answer 41

They are hydrolysed by cell membrane proteins.

Question 42

How do monomers enter the epithelium?

Answer 42

Enter by co-transport (active transport as well as facilitated diffusion…?) and then pass into the blood by facilitated diffusion.

Question 43

Fate of glucose

Answer 43

respiration & excess is converted to glycogen or fat for storage.

Question 44

Fate of lipids

Answer 44

used to produce phospholipids in the cell membranes, hormones or excess stored as fat

Question 45

Fate of amino acids

Answer 45

absorbed by cells for protein synthesis, excess can’t be stored so is ‘deanimated’ - NH2 group is removed - and converted to urea for excretion. The remaining C, H and O is converted to carbohydrate and is respired or stored.