Digestion and absorption

Question 1

Process of amino acids being absorbed into the blood stream from the ileum

Answer 1

• Sodium ions actively pump out of the epithelial cell by the sodium-potassium pump using ATP
• This lowers the concentration of sodium ions in the epithelial cell
• This causes sodium ions to diffuse down the concentration gradient from the ileum into the epithelial cell via a co-transporter protein (facilitated diffusion)
• Amino acids are co-transported into the epithelial cell with the sodium ions
• This causes the concentration of amino acids inside the epithelial cell to increase, so they diffuse down their concentration gradient into the blood via a co-transporter protein

Question 2

Why does the absorption of amino acids into the blood from the ileum require oxygen?

Answer 2

• Absence of oxygen means that ATP isn’t available
• This means that a concentration gradient isn’t established
• Oxygen is needed for active transport for the sodium-potassium pump

Question 3

What occurs in the digestive tract?

Answer 3

Triglycerides are hydrolysed by pancreatic lipase, releasing free fatty acids and monoglycerides

Question 4

Key issue in the absorption of fats

Answer 4

Lipids are poorly soluble in the aqueous environment of the digestive tract

Question 5

Role of emulsification

Answer 5

The breaking up of fat globules into much smaller emulsion droplets

Question 6

What are the small emulsion droplets coated with?

Answer 6

Bile salts and phospholipids

Question 7

Role of bile salts

Answer 7

Prevent emulsion droplets from re-associating

Question 8

Advantage of smaller emulsion droplets

Answer 8

Increases surface area so increases rate of digestion

Question 9

Process of emulsification and digestion

Answer 9

• Digestion is greatly aided by emulsification; the break down of fat globules into much smaller emulsion droplets
• Motility (movement) in the small intestine breaks fat globules apart into smaller emulsion droplets which are coated with bile salts and phospholipids (prevent emulsion droplets from re-associating)
• Emulsion droplets are where digestion occurs

Question 10

Advantage of emulsification

Answer 10

Greatly increases surface area where water soluble pancreatic lipase can work to digest triglycerides

Question 11

What makes up micelles?

Answer 11

Monoglycerides and fatty acids + bile salts and phospholipids

Question 12

Role of micelles

Answer 12

Transport poorly soluble monoglycerides and fatty acids to the surface of the epithelial cell where they can be absorbed

Question 13

Advantage of micelles

Answer 13

Reduce diffusion distance so increase rate of absorption

Question 14

What can be absorbed?

Answer 14

Only freely dissolved monoglycerides and fatty acids

Question 15

Advantage of fatty acids and monoglycerides being non-polar

Answer 15

Able to just diffuse across the plasma membrane of the epithelial cell

Question 16

What are chylomicrons made of?

Answer 16

Fatty acids + monoglycerides + proteins

Question 17

What takes place in the Golgi apparatus?

Answer 17

Monoglycerides and fatty acids are combined with proteins and packaged into chylomicrons

Question 18

Role of chylomicrons

Answer 18

Lipoproteins which are designed for the transport of lipids in circulation

Question 19

Where are chylomicrons released?

Answer 19

At the lower surface of the epithelial cell via exocytosis

Question 20

Why can’t chylomicrons enter typical capillaries?

Answer 20

They’re too large

Question 21

What do chylomicrons enter?

Answer 21

Lacteals (lymphatic capillaries that poke up into the centre of each villus)

Question 22

How do chylomicrons flow?

Answer 22

Via lymphatic vessels which drain into the general circulation at the large veins in the chest

Question 23

Which enzyme hydrolyses triglycerides in chylomicrons?

Answer 23

Lipoprotein lipase

Question 24

Process of fat absorption

Answer 24

• Micelles break down and feed pool of dissolved monoglycerides and fatty acids
• Dissolved monoglycerides and fatty acids enter epithelial cell via diffusion
• Monoglycerides and fatty acids are combined with proteins and packaged into chylomicrons
• Chylomicrons enter circulation via the lacteals where they deliver absorbed triglycerides to the body’s cells
• Monoglycerides and fatty acids released from digestion of triglycerides then diffuse into cells