CP - Traffic across cells: Epithelial transport of glucose

Question 1

What do epithelial tissues consist of?

Answer 1

Cells arranged in continuous sheets in either single or multiple layers

Question 2

What do epithelial tissues cells sit on?

Answer 2

Basement membrane

Question 3

What do epithelial tissues form?

Answer 3

The boundary between the body’s organs or between the body and external environment

Question 4

What are epithelial tissues subject to?

Answer 4

Physical breakdown and injury, therefore, they undergo constant and rapid renewal

Question 5

What are the 2 basic types of epithelial tissues?

Answer 5

1. Covering and lining epithelium, e.g. epidermis of skin, lining of blood vessels and ducts
2. Glandular epithelium, e.g. thyroid, adrenal and sweat glands

Question 6

What are epithelial cells separated from their neighbours by?

Answer 6

Lateral intercellular space

Question 7

What are epithelial cells held together at their luminal edges by?

Answer 7

Tight junctions

Question 8

What are tight junctions composed of?

Answer 8

Thin bands that encircle the cell and make contact with thin bands from adjacent cells

Question 9

In electron microscopy, how are tight junctions appeared?

Answer 9

It appears that the membranes are fused together

Question 10

In freeze fracture, how are tight junctions appeared?

Answer 10

It appears as an interlocking network of ridges in the plasma membrane

Question 11

What do tight junctions act as?

Answer 11

1. A barrier to restrict movement of substances through the intercellular space between the cells
2. A fence to prevent membrane proteins from diffusing in the plane of the lipid bilayer

Question 12

What 2 distinct membrane domains are epithelial cells separated into?

Answer 12

1. Apical (or luminal or mucosal) membrane that faces the lumen of the organ or body cavity
2. Basolateral membrane that adheres to the adjacent basement membrane and interfaces with the blood

Question 13

The distinct membrane domians means that different ____ ____ can be inserted into either the ____ or ____ membrane

Answer 13

transport proteins, apical, basolateral

Question 14

What pathways can transport occur in epithelial cells? (and what are these via)

Answer 14

Paracellular pathway (via tight junctions) or transcelullar pathway (through the cell) or via bothways

Question 15

What is paracellular transport?

Answer 15

Governed by the laws of diffusion and the tightness of tight junctions (doesn’t involve transport proteins)

Question 16

How can the electrical resistance to ion flow through tight junctions be measured?

Answer 16

Ohms law, the higher the electrical resistance to ion flow, the greater the no. of tight junction strands holding the cell together (the tighter the tight junctions, the lower the current flow)

Question 17

How does tight junctions resistance changes?

Answer 17

Tight junctions resistance changes in a proximal to distal direction in the gastrointestinal tract and kidney

Question 18

Proximal (close to the stomach) consists of:

Answer 18

• Leaky epithelium
• Low electrical resistance
• Low no. of strands
• Bulk transport (paracellular)
• e.g. Duodenum, proximal tubule

Question 19

Distal (away from the stomach) consists of:

Answer 19

• Tight epithelium
• High electrical resistance
• High no. of strands
• Hormonally controlled (transcellular)
• e.g. Colon, collect duct

Question 20

What can epithelial tissues be functional classified into?

Answer 20

• Leaky epithelium (paracellular transport dominates)

- Tight epithelium (transcellular transport dominates)

Question 21

What is transcellular transport?

Answer 21

Epithelial cells use primary and econdary active transport often in combination with passive diffusion through ion channels to produce transport across the epithelial tissues

Question 22

Transcellular transport can either be:

Answer 22

• Absorption: transport from lumen to blood

- Secretion: transport from blood to lumen

Question 23

Transepithelial transport can be broken down into the following areas that need to be considered:

Answer 23

1. Entry and exit steps: the entry step for absorption is the apical membrane but for secretion, it’s the basolateal membrane
2. Electrochemical gradient: is the entry step for passive or active
3. Electroneutrality: movement of +ve or -ve ion will attract a counter ion
4. Osmosis: net movement of ions will establish a difference in osmolarity that will cause water to flow by osmosis

Question 24

Epithelial cells use different collections of ____ and ____ to mediate either _____ or ______

Answer 24

transporters, channels

absorption, secretion

Question 25

What do primary active transporters set up?

Answer 25

Ion gradients

Question 26

The entry step is often? (what type of transport)

Answer 26

Secondary active transport

Question 27

The exit step is often?

Answer 27

Passive diffusion

Question 28

In glucose absorption, glucose and Na are cotransported across the apical membrane by what transport protein?

Answer 28

SGLT (Sodium Glucose Symporter) transport protein

Question 29

In glucose absorption, how does glucose exit the basolateral membrane?

Answer 29

By facilitative diffusion mediated by the GLUT (Facilitative Glucose Transporter) protein

Question 30

In glucose absorption, how is the energy for glucose entry provided and maintained?

Answer 30

Provided by the Na gradient, which is maintained by the Na pump

Question 31

In glucose absorption, what does the movement of glucose across the epithelium create?

Answer 31

An osmotic imbalance, which drives the absorption of water

Question 32

In step 1 of glucose absorption in the small intestine, how do tight junctions divide the cells?

Answer 32

Into apical and basolateral membrane domains and forms a barrier to the movement of some molecules via paracellular pathway

Question 33

In step 2 of glucose absorption in the small intestine, what sets up the ion gradients?

Answer 33

Na/K pump, which is set up at the basolateral membrane

Question 34

In step 3 of glucose absorption in the small intestine, SGLT uses the energy of Na+ gradient to do what?

Answer 34

To actively accumulate glucose above its concentration, where the entry step is in apical membrane (example of carry mediated transport and secondary transport)

Question 35

In step 4 of glucose absorption in the small intestine, what mediates glucose exit across the basolateral membrane and how does glucose exit?

Answer 35

GLUT and via passive diffusion down its gradient (as there is more glucose inside the cell than outside in the blood)

Question 36

In step 5 of glucose absorption in the small intestine, what happens to Na+?

Answer 36

Na+ is taken up via the SGLT and exits via the basolateral Na/K pump

Question 37

In step 5 of glucose absorption in the small intestine, what happens if Na+ is not removed?

Answer 37

The Na gradient dissipates (due to pump leak hypothesis). As soon as the Na is inside the cell, energy is used to remove it, maintaining electrochemical gradient for Na movement and maintaining ability to keep accumulating glucose above its conc. gradient. Thus, it can passively diffuse across basolateral membrane and Na is recycled to maintain electrochemical gradient

Question 38

In step 6 of glucose absorption in the small intestine, the transport of Na+ and glucose across the epithelium causes what to happen?

Answer 38

It induces paracellular Cl- and water fluxes (as Na+ and glucose attract Cl- and water, by the paracellular pathway to preserve electroneutrality and cancel out the osmotic difference so, net result is accumulation of glucose, Na, Cl and water)

Question 39

What is oral rehydration therapy?

Answer 39

The ability of glucose to enhance the absorption of Na+ and hence Cl- and water is exploited in oral rehydration therapy. A simple sugar solution is given to dehydrated babies suffering from diarrhoea

Question 40

What is the Glucose/Galactose Malabsorption Syndrome caused by?

Answer 40

A mutation in the SGLT protein (at the apical membrane)

Question 41

What does the Glucose/Galactose Malabsorption Syndrome result in?

Answer 41

The accumulation of glucose and galactose in the lumen of the small intestine. This produces an osmotic imbalance, which attracts water (the associated increase in lumen osmolarity induces a water efflux) and increased water flow results in severe diarrhoea

Question 42

How is Glucose/Galactose Malabsorption Syndrome treated?

Answer 42

Involves the removal of glucose and galactose from the diet and fructose is used as a source of carbohydrate. This therapy uses a facilitative transporter (GLUT5) that is specific for fructose

Question 43

In the small intestine during fasting, how does glucose enter and exit the epithelial cells?

Answer 43

Glucose enters from the blood via facilitated diffusion and exit via passive diffusion

Question 44

What is stimulated by oral rehydration therapy?

Answer 44

Isotonic fluid absorption

Question 45

What happens to glucose in the kidney?

Answer 45

Glucose in the plasma is filtered and needs to be reabsorbed or it will appear in the urine

Question 46

What is glucosuria?

Answer 46

The accumulation of glucose in the urine

Question 47

How does glucosuria occur?

Answer 47

The glucose conc. in the plasma exceeds the transport max. for glucose uptake in the proximal tubule of the kidney (glucose absorption is either impaired or transport is saturated)

Question 48

What is the most common cause of glucosuria? and why?

Answer 48

Diabetes mellities as insulin activity is deficient and blood sugar is too high. In diabetes, the glucose symporter can’t absorb glucose fast enough and glucose appears in the urine

Question 49

What happens to all filtered glucose until the renal threshold is reached?

Answer 49

All filtered glucose is reabosrbed until the rental threshold is reached

Question 50

What happens once the renal threshold is reached?

Answer 50

Glucose appears in the urine

Question 51

What does the renal threshold reflect?

Answer 51

The transport max. of SGLT (the glucose conc. in the plasma when glucose uptake becomes saturated)