Small Intestine Physiology

Question 1

Define ‘facilitated diffusion’. How is it different from active transport?

Answer 1

Absorption takes place down a [] gradient, but a membrane carrier protein passively allows a certain molecule to pass.
Active transport requires an energy input and will often transport against a [] gradient.

Question 2

Describe the digestion of starch.

Answer 2

As starch cannot be absorbed, it is broken down by amylase, some salivary but mainly pancreatic in the small intestine. Breakdown products are:
Maltose
Maltotriose
Sucrose
Lactose
These are further hydrolysed on the microvillous membrane to form glucose galactose fructose, which can be absorbed.

Question 3

Describe digestion of protein.

Answer 3

Pancreatic enzymes breakdown to amino acids. The enzymes secreted as pro-enzymes, and converted in presence of protein. 
Pancreatic pro-enzymes:
-Trypsinogen
-Chymotrypsin
-Elastase

[Fun fact: some di- and tripeptides absorbed intact by active transport]

Question 4

Main constituents of dietary fat?

Answer 4

Triglycerides
Cholesterol
Fat-soluble vitamins

Question 5

What is a ‘micelle’?

Answer 5

An aggregation of the products of fat digestion, monoglycerides, fatty acids and cholesterol, which bile-derived substances solubilise.
Hydrophilic ends of molecules face distally from micelle.
Bile salt micelles already exist, become mixed micelles when combined with monoglycerides etc.

Question 6

Where are bile salts reabsorbed?

Answer 6

At the distal end of the ileum.

Question 7

Name the main causes of steatorrhoea.

Answer 7

Malabsorption of fats, caused by:

• Liver disease (gallbladder dysfunction, bile salts lacking)
• Pancreatic disease
• Drugs like Orlistat (lipase inhibitor)
• Small intestine disease (mucosal lesion, coeliac, tropical sprue, giardiasis)
• Ileal disease: Crohn’s

Question 8

Describe the water and electrolyte absorption of the following:

1. Jejunum
2. Ileum
3. Colon

Answer 8

1. Large amounts of water/electrolytes absorbed, coupled with absorption of sugars, amino acids, bicarbonate in UPPER jejunum.
2. Further absorption in ileum/colon, more active sodium transport, not coupled to solute absorption.

Question 9

What are the 3 routes that proteins provide to move molecules across membranes?

Answer 9

1. Large pores (made of protein subunits, allow bulk flow of water, ions, some larger molecules)
2. Transporter molecules (use direct metabolic energy to move molecules against chemical/osmotic gradients)
3. Ion channels (allow passage of particular ions)

Question 10

Transport proteins can derive energy from which two sources?

Answer 10

1. Metabolic (active transport)

2. Conc. gradient (facilitated diffusion)

Question 11

Define the following types of transport protein:

1. Uniporter
2. Symporter
3. Antiporter

Answer 11

1. Moves a particular molecule in one direction
2. Moves several different molecules in one direction
3. Moves different molecules in different directions