L23

Question 1

What is the role of tight junctions

Answer 1

refer to L23

Question 2

Transcellular and paracellular transport

Answer 2

refer to L23

Question 3

Have an understanding of Epithelial structure and function .

Answer 3

refer to L23

Question 4

Understand how epithelial cells can mediate either absorption or secretion of a substance

Answer 4

refer to L23

Question 5

Understand Glucose absorption in the intestine and kidney

Answer 5

refer to L23

Question 6

Understand Glucose/galactose malabsorption syndrome

Answer 6

refer to L23

Question 7

Understand Glucosuria in the kidney

Answer 7

refer to L23

Question 8

Epithelia consists of cells arranged in….

Answer 8

continuous sheets in either single of multiple layers.

Question 9

There are two basic types of epithelial tissues:

Answer 9

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 10

Epithelia cells are classified by arrangement of cells into layers:

Answer 10

1. Simple = one cell layer thick
2. Stratified = many cell layers thick.
3. Pseudostratified = single layer of cells where all cells don’t reach apical surface;

Question 11

Epithelia cells are classified by shape of surface cells:

Answer 11

1. Squamous = flat;
2. Cuboidal = cube-shaped
3. Columnar = tall column
4. Transitional = shape varies with tissue stretching.

Question 12

Epithelia cells are classified by:

Answer 12

1. arrangement of cells into layers

2. shape of surface cells

Question 13

What are Tight junctions

composed of?

Answer 13

Tight junctions are composed of thin bands

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

Question 14

What do tight junctions do?

Answer 14

Tight junctions function both as a barrier to restrict the movement of substances through the
intercellular space between cells, and as a fence to prevent membrane proteins from diff using in
the plane of the lipid bilayer.

Question 15

Tight junctions separate the epithelial cells into two distinct membrane
domains:

Answer 15

1. Apical membrane (or luminal or mucosal) 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 16

Transport via epithelium can occur

Answer 16

paracellular pathway (via tight
junctions) 

or transcellular pathway (through the cell),

or via both pathways.

Question 17

What is Paracellular Transport?

Answer 17

Paracellular transport is governed by the laws of diff usion and the tightness of the tight junctions

Question 18

What is transcellular transport?

Answer 18

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

This transport can either be:
Absorption: transport from lumen to blood;
Secretion: transport from blood to lumen.

Question 19

What are the rules of Transcelluar Transport?

Answer 19

1. Entry and exit steps
2. Electrochemicial gradient
3. Electroneutrality
4. Osmosis

Question 20

Describe Glucose Absorption

Answer 20

1. Tight junctions divide cells into apical and basolateral membrane domains.
2. Na/K pump sets up ion gradients.
3. The sodium glucose symporter uses the energy of the Na+ gradient to actively accumulate
   glucose above its concentration gradient.
4. Facilitative glucose transporter mediates glucose exit across the basolateral membrane via
   passive diffusion down its gradient.
5. Na+ taken up via the sodium-glucose symporter and exits via the basolateral Na/K pump.
6. The transport of Na+ and glucose across the epithelium induces paracellular Cl-
   and water fluxes.

Question 21

Describe Oral Rehydration Therapy.

Answer 21

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, when given to dehydrated babies suffering from
diarrhoea, is responsible for saving millions of lives per year.

Question 22

Describe Glucose/Galactose Malabsorption Syndrome

Answer 22

Glucose/galactose malabsorption syndrome is caused by a mutation in the Sodium-dependent
Glucose (co-) Transporter (SGLT) protein. It results in accumulation of glucose and galactose in the
lumen of the small intestine. This produces an osmotic imbalance, which attracts water and results
in severe diarrhoea.

Question 23

What is the treatment for Glucose/Galactose Malabsorption Syndrome?

Answer 23

Treatment of glucose-galactose malabsorption involves the removal of glucose and galactose from
the diet. Fructose is used as a source of carbohydrate. This therapy uses a facilitative transporter
that is specific for fructose.

Question 24

Describe Glucose Reabsorption in the Kidney

HELLLLLLLLLLLLLPPPP

Answer 24

In the kidney glucose in the plasma is filtered and needs to be reabsorbed or it will appear in the urine

Question 25

What is Glucosuria? and how does it happen?

Answer 25

Glucosuria is an accumulation of glucose in the urine.

It occurs if the transport maximum of the
SGLT protein is exceeded. The commonest cause is diabetes mellitis because insulin activity is
defi cient and blood sugar is too high (over 200 mg/mL). In diabetes, the glucose symporter can
not absorb glucose fast enough and glucose appears in the urine.

Question 26

Epithelial tissues can be functional classified into:

Answer 26

1. Leaky epithelium – paracellular transport dominates

2. Tight epithelium – transcellular transport dominates

Question 27

Describe the purpose of changes in tight junction resistance?

Answer 27

Tight junction resistance changes in a proximal to distal direction in the GI tract and kidney

Question 28

Describe the features of proximal and distal in regards to changes in tight junction resistance

Answer 28

Proximal (para)
Leaky epithelium 
Low electrical resistance 
Low number of strands 
Bulk transport (paracellular) e.g. Duodenum, proximal tubule, stomach

Distal (trans)
Tight epithelium
High electrical resistance
High number of strands Hormonally controlled (transcellular) e.g. Colon, collect duc

Question 29

what can transcellualar transport be broken down into?

Answer 29

Absorption: transport from lumen to blood.

Secretion: transport from blood to lumen.

Question 30

is pd > pf or is pf > pd?

Answer 30

The permeability of water through the lipid bilayer is less than the permeability of water through a water channel

Question 31

What do the kinetics of Gluecose absorption in the small intestine look like?

Answer 31

lecture slides

Question 32

What does the kinetics of Gluecose in the urine look like?

Answer 32

lecture slides