Ions Vitamins and Minerals

Question 1

Types of Transport

Name the two types of transport and what it encompasses

Answer 1

Types of Transport

§ The two broad types of transport are para- and trans-cellular.

Question 2

Define paracellular

Answer 2

through tight junctions and lateral intercellular spaces

Question 3

Define transcellular

Answer 3

through the epithelial cells

Question 4

Membrane transport

Answer 4

Types of Transport - Summary
§ Active Transport – Requires energy.
o Primary – Linked directly to cellular metabolism (ATP powered).
§ E.G. Sodium Potassium ATPase.
o Secondary – Derives energy from concentration gradient of another substance actively transported.
§ E.G. Bicarbonate/Chloride counter-transport, SGLT-1.
§ Facilitated transport – Enhances rate a substance can flow down a concentration gradient à equilibrium.
§ E.G. GLUT-5, GLUT-2. Remember, enterocytes absorb GLUCOSE & GALACTOSE via 2nd active transport (SGLT-1 and sodium) and FRUCTOSE by facilitated diffusion (GLUT-5) - APICAL. Exit of glucose then via facilitated diffusion (GLUT-2) - BASAL.

Question 5

Types of carrier mediated transport

Answer 5

uniport
symport
antiport

Question 6

ion channels

- the different types + what can they be open and closed by?

Answer 6

voltage gated 
ligand gated (extracellular ligand) 
ligand gated (intracellular ligand) 
mechanically gated

Question 7

greatest amount of water is absorbed in the?

Answer 7

small intestine, jejunum
8L by small intestine
and 1.4L by large intestine

Question 8

What is standing gradient osmosis driven by?

Answer 8

Na+

Standing gradient osmosis contributes to water reabsorption

Question 9

where does the 8L of water coming from?

Answer 9

This water comes from:
o Ingestion (2L) and saliva (1.2L).
o Gastric secretions (2L) and bile (0.7L).
o The pancreas (1.2L) and the intestine (2.4L).

Question 10

Water Absorption – Standing Gradient Osmosis
Sodium gets INTO the cells via different methods depending on location of the enterocyte:
4 ways

Answer 10

Water Absorption – Standing Gradient Osmosis
§ Sodium gets INTO the cells via different methods depending on location of the enterocyte:
o Proximal bowel – Counter-transport for H+.
o Jejunum – Co-transport with amino-acid, monosaccharides.
o Ileum – Co-transport with Cl-.
o Colon – Restricted movement through ion channels.

Question 11

Cl- is co-transported with? in the?

K+ diffuses in via?

Answer 11

Cl- is co-transported with Na+ in the ILEUM and exchanged with HCO3- in the COLON.
§ K+ diffuses IN paracellularly in the small intestine and OUT in the colon

Question 12

Water absorption

Answer 12

Water Absorption – Standing Gradient Osmosis
§ High intracellular sodium is controlled by:
o Active transport of Na+ into lateral intracellular spaces by Na+/K+ATPase.
§ Cl- and HCO3- transports into the intracellular space by electrochemical gradient of Na+.
§ High blood concentration of ions pulls water.

Question 13

Most absorption of calcium occurs in
A calcium deficiency INCREASES
What two things stimulate absorption of calcium.
How much calcium do we absorb?
Compare the intracellular and extracellular conc of calcium

Answer 13

§ Most absorption of calcium occurs in the duodenum and ileum.
§ A calcium deficiency INCREASES the gut’s ability to absorb calcium.
§ Vitamin D and parathormone stimulate absorption of calcium.
§ Even though we ingest 1-6g of calcium a day, we only absorb about 0.7g of it (it’s regulated).
§ Resting, there is a LOW (nm) intracellular concentration and a HIGH (micromole) extracellular concentration.

Question 14

Absorption of calcium via 2 types of transporter:

Answer 14

Absorption via 2 types of transporter:
o Facilitated diffusion – IMcal.
o Ion channel.
§ Binds to calbindin to make it inactive.
§ Calcium pumped out of basolateral surface by:
o Ca2+ATPase (PMCA).
§ HIGH affinity but LOW capacity.
§ Maintains the low intracellular concentration.
o Na+/Ca2+ exchanger.
§ LOW affinity but HIGH capacity.

Question 15

Calcium Absorption –

• deficiency called?
• what vitamin is key?
• Function of 1,25 dihydroxy Vitamin D3?

Answer 15

Calcium Absorption – Vitamin D Deficiency = rickets, osteoporosis.
§ Essential for normal Ca2+ absorption.
§ 1, 25-dihydroxy Vitamin D3 is taken up by enterocytes and functions to:
o Enhance transport of Ca2+ through cytosol.
o Increase level of calbindin.
o Increase number of Ca2+ATPase in membrane for faster extrusion from cell.

Question 16

Iron is vital for?

Iron in excess is?

Answer 16

Iron is vital for oxygen transport (in haem) and oxidative phosphorylation (ETC).
§ Iron in excess however is TOXIC so it must be regulated (no mechanism for excreting iron).
o We ingest ~15-20mgday-1 but only absorb 0.5-1.5mgday-1.

Question 17

Iron is present in the diet as?

Answer 17

*inorganic iron (Fe3+ ferric, Fe2+ ferrous)
§ We CANNOT absorb Fe3+ directly (like we do Fe2+) so we absorb it via:
· Fe3+ + insoluble salt binding (e.g. hydroxide, phosphate, bicarbonate).
· Vitamin C reduction from Fe3+ to Fe2+.

*as part of heme group (haem group)- haemoglobin, myoglobin and cytochrome

Question 18

Fe3+ insoluble salts with?

Answer 18

hydroxide
phosphate
HCO3-

Question 19

Heme

-How do we absorb this?

Answer 19

We absorb this intact into the enterocyte via haem carrier protein 1 (HCP-1) which endocytoses via receptor mediated endocytosis. Fe2+ is then liberated by haem oxygenase

Question 20

Iron uptake

Answer 20

Haem can bind to Haem Carrier Protein 1 (HCP-1) and is then absorbed by receptor mediated endocytosis, Haem Oxygenase (HO) then liberates Fe2+.

Duodenal Cytochrome B (Dcytb – membrane enzyme) can reduce Fe3+ à Fe2+ which then passes into cytosol via Divalent Metal Transporter 1 (DMT-1) with H+ ions.

Question 21

Iron – Release
Into Blood:

Excretion:

Answer 21

Iron – Release
Into Blood:
o Ferric enters the blood via ferroportin (FP).
o Hephaestin (HP) is a copper dependant ferroxidase that converts Fe2+ à Fe3+.
o Fe3+ binds to apotransferrin and travels in blood as transferrin (TF).
o Hepcidin supresses FP.

Excretion:
o Ferric binds to apoferratin to form ferritin micelle. Fe2+ then oxidises to Fe3+ which creates a protein shell.
o The micelle is lost when the enterocytes are shunted off the tip of the villi.
o Increase of iron conc. in cytosol increases ferritin synthesis.

Question 22

Vitamins – define
Vitamins are predominantly absorbed by?
What are fat soluble vitamins? and how are they transported to the brush border?
Specific transport mechanisms exist for vitamin?

Answer 22

Vitamins – Organic compounds that cannot be manufactured by the body but are vital to metabolism.
§ Vitamins are predominantly absorbed by passive diffusion.
§ Fat soluble vitamins (e.g. A, D, E, K) are transported to the brush border by micelles while K is actively taken up.
§ Specific transport mechanisms exist for vitamin C, B1, B12 and folic acid.

Question 23

Vitamin B12

• liver contains what store?
• impaired absorption of Vit B12 leads to?

Answer 23

Vitamin B12:
§ Liver contains a large store (2-5mg).
§ Impaired absorption of Vitamin B12 retards the maturation of RBCs = pernicious anaemia.

Question 24

Vitamin B12

• most are bound to what in food?
• what does the B12 bind to in the stomach and why?
• what happens in the duodenum?

Answer 24

Vitamins – Vitamin B12
§ Most Vitamin B12 is bound to proteins in food.
§ Stomach low pH and pepsin releases B12 from food but the B12 is easily denatured by HCl.
§ B12 therefore binds to R protein (haptocorrin) which is released by parietal cells and in saliva.
o R proteins are then digested in the duodenum.

Question 25

In the duodenum what happens after R proteins are broken down? and why?

Answer 25

Vitamins – Vitamin B12
§ B12 binds to Intrinsic Factor (IF) secreted by parietal cells.
o IF is resistant to digestion.
§ B12/IF complex binds to cubulin (cub) receptor in distal ileum and is endocytosed (possibly by receptor mediated methods).
§ B12/IF complex broken down in mitochondria, B12 binds to transcobalamin II (TCII) and travels to liver in the blood.
TCII receptors on the Liver allow uptake of complex.
§ Proteolysis then breaks down the TCII inside the cell