Lecture 27- Absorption

Question 1

What is absorption?

Answer 1

Absorption is the (Net) passage of substances from the GI lumen across the lining of the intestine (epithelial cells) into the interstitial fluid and then into the blood or lymph

Question 2

Where does absorption occur? and what is absorbed in each location?

Answer 2

-Mouth, esophagus, stomach (minimal: lipid soluble substances)
-Small intestine (primary site: 90% of water and sodium
and all nutrients)
-Large intestine (9% of water and sodium)

Question 3

What is the affect of motility on absorption?

Answer 3

-Correct rate of propulsion via mechanisms such as storage and peristalsis allows efficient digestion
and subsequent absorption
-Segmentation exposes products of digestion
to absorptive surfaces (epithelium)

Question 4

What is the affect of surface area on absorption and how is this modified in the small intestine?

Answer 4

-Greater the surface area the faster the rate of absorption
-Anatomical adaptations maximizes the surface area
(Length of intestine (6m), Circular folds, Villi, Microvilli)

Question 5

What is the problem presented with absorption in the GI tract?

Answer 5

• As it is continuous with the outside world a barrier to the internal body is required via the epithelium to prevent harmful microbiota/ bacteria getting in
• However, we also need to get nutrients across so a balance needs to be found

Question 6

What are the two pathways for absorption?

Answer 6

• Paracellular via the “Gap” between cells

- Transcellular across the cell membranes through the cytoplasm

Question 7

Describe the paracellular pathway for absorption…

Answer 7

• Solutes do not cross cell membranes instead pass between the cells so is regulated by tight junctions which bind the cells together
• Relatively non-selective in that all a solute requires is to be small to get through
• Passive and therefore requires a gradient (from high in lumen to low in interstition/ blood)

Question 8

Describe the transcellular pathway for absorption…

Answer 8

• Solutes must cross two cell membranes
• Cell membranes are lipid bilayers and so if solute is not lipid soluble it will require a transport protein embedded in the membrane

Question 9

What are two ways to maximize absorption across the surface area available?

Answer 9

• Reduces nutrients into smallest possible unit (chemical digestion)
• Specific transport proteins. These absorb what is required efficiently as uses active transport to work against a concentration gradient.

Question 10

How is it ensured that substances are removed from the interstitial fluid? Why is this important?

Answer 10

-Large blood flow to intestine as well as the arrangement in villi of blood vessels and lacteal
prevents build up in interstitial fluid (fast diffusion into blood stream)
-This is important as otherwise the gradient of high in the lumen and low in the interstition would not be maintained.

Question 11

Why is it important that be reabsorb water?

Answer 11

• What we drink only replaces what we lose from sweat, breathing, urine and faeces
• We secret a lot more in to the GI tract
• We therefore have to reabsorb into to negate loses

Question 12

How is water absorbed?

Answer 12

• Osmosis: The Passive movement of water (high to low concentration gradient) from lumen into blood
• This Osmotic gradient set up by absorption of salts and nutrients

Question 13

What are the two ways in which sodium can be absorbed? Why is the second pathway needed?

Answer 13

• Passive movement via paracellular pathway
• Active transport via the cells (transcellular pathway requiring transporters embedded in the cell membrane)

-Relying on passive transport alone is not efficient as it requires a specific concentration gradient to exist. Therefore, most often use the cellular pathway for absorption of protein.

Question 14

What are the three ways to actively transport sodium?

Answer 14

• Na+ transport alone
• Na+ transport coupled to monosaccharides (e.g. glucose, galactose)
• Na+ transport coupled to amino acids

Question 15

How is sodium transported alone?

Answer 15

• Na+ moves into cell from the lumen down its gradient (high to low). This requires transport proteins either in the form of a Na+ channel or Na+/H+ exchanger. (passive)
• To get out of cell into the interstition needs to go from low to high concentration. This is against the concentration gradient so active transport is used requiring ATP to power Na+,K+-ATPase.

Question 16

How does carbohydrate absorption occur?

Answer 16

• Monosaccharides (glucose, galactose)
• At apical membrane (from lumen into cell) process is active as requires going against the concentration gradient. Cotransport with Na which has favorable concentration gradient occurs using a sodium-glucose co-transporter.
• To then transport across the basal membrane into the interstition is passive and just requires a glucose carrier

Question 17

How many mechanisms are there for the absorption of amino acids?

Answer 17

• 2 Mechanisms

- Passive or active absorption

Question 18

Describe passive absorption of amino acids and explain why it is inefficient?

Answer 18

-Amino acids diffuse down concentration gradient via
paracellular pathway
-Is inefficient as relies on the desirable concentration gradient being present. If relied on this method. We would not obtain enough nutrients.

Question 19

Describe the active absorption of proteins?

Answer 19

• Occurs via the transcellular pathway
• For di and tri peptides movement across the apical membrane (lumen into the epithelial cells) is done via an hydrogen dependent cotransporter. These peptides are then converted to individual amino acids by Cytoplasmic peptidases before reaching the basal membrane.
• For amino acids (already simplest unit) movement across the apical membrane is done via Na+ - dependent cotransport
• To move amino acids across the basolateral membrane into the interstition a amino acid carrier is required conc gradient is favorable

Question 20

Why are lipids special in their absorption?

Answer 20

• They are lipid soluble so can freely pass across membranes (do not need a transporter)
• Instead they are delivered to the brush border by micelles and the Fatty acids and monoglycerides diffuse into the cell

Question 21

In lipid absorption is the whole micelle absorbed?

Answer 21

No, just the Fatty acids and monoglycerides contained in it

Question 22

What happens once the fatty acids and monoglycerides are inside the cell?

Answer 22

• Resynthesized into triglycerides
• Packaged into chylomicrons
• Chylomicrons exit cell by exocytosis (too large to do it any other way)
• Chylomicrons enter the lacteals

Question 23

Where does the absorption of lipids occur?

Answer 23

The jejunum of the small intestine

Question 24

What occurs in bile salt absorption?

Answer 24

• The bile salts in the micelles are eventually absorbed
• This occurs after fat absorption is complete in the ileum
• The active transport process involves an Na+ dependent bile acid cotransporter. The salts then travel through hepatic portal vein and are reabsorbed into the liver
• In the colon passive absorption occurs and this is where 95% of bile salts are absorbed

Question 25

How are fat soluble vitamins absorbed? What are some examples?

Answer 25

• Vitamins A, D, E, K

- Absorbed with fats

Question 26

How are water soluble vitamins absorbed? What are some examples?

Answer 26

• Na+ dependent absorption

- Vitamin C

Question 27

What is a special case in vitamin absorption?

Answer 27

• Vitamin B12
• Binds to intrinsic factor which is produced in stomach by parietal cells
• Specific transporter for Intrinsic factor vitamin B12 in the ileum

Question 28

What may result if not enough intrinsic factor is produced?

Answer 28

• Loss of intrinsic factor means Vitamin B12 can’t be absorbed/ not enough of it
• Vitamin B 12 is important for cell replication specifically red blood cells
• Therefore, what will result is Pernicious anemia where a person does not enough red blood cells to carry oxygen around their body