S1: Alimentary Transport 2

Question 1

Functions of the alimentary tract

Answer 1

• Absorption of nutrients, salts and water

- Colon absorbs 90% of water from the contents that arrive at it, to produce a semi-solid material

Question 2

How is water absorbed and how can malfunction lead to problems?

Answer 2

• Fluids secreted into small intestine are absorbed back into blood
• Absorption is achieved by transport of ions e.g. Na+ and water follows by osmosis
• Malfunction can lead to nutritional, salt and water imbalance.
• Diarrhoea is caused by abnormal water absorption in the intestine

Question 3

What are the two internal neuronal plexuses in the gut that form the enteric nervous system?

Answer 3

1. Submucosal Meissner plexus which regulates the digestive glands
2. Myenteric Auerbach plexus which is connected with gut motility

Question 4

Compare the intrinsic and extrinsic control of the gut

Answer 4

INTRINSIC CONTROLS

• Nerve plexuses near the GI tract initiate short reflexes
• Short reflexes are mediated by local enteric plexuses (gut brain)

EXTRINSIC CONTROLS

• Long reflexes arising within or outside the GI tract
• Involve CNS centres and extrinsic autonomic nerves
• Parasympathetic reflexes

Question 5

What is the outer layer of the gut called?

Answer 5

The outer layer of the gut is covered by the serosa which is continuous with the mesentery containing blood vessels,lymph vessels and nerve fibres.

Question 6

What detects different chemical substances in the gut lumen?

Answer 6

Sensory neurones connected to mucosal chemoreceptors detect different chemical substances in the gut lumen

Question 7

What do stretch receptors do?

Answer 7

They respond to tension in the gut wall caused by the food and chyme.

Question 8

What are paravertebral ganglia?

Answer 8

Symptathetic trunk/ganglia

Question 9

What is absorbed in the upper small intestine?

Answer 9

All dietary nutrients, water and electrolytes

Question 10

What gives the small intestine a large surface area?

Answer 10

Epithelial folds
Villi
Microvilli

Question 11

What are villus cells and crypt cells?

Answer 11

• Villus cells are involved with absorption

- Crypt cells are involved with secretion and sit at the base between fingers

Question 12

What is crypt of lieberkuhn?

Answer 12

The epithelia of the villi extend down into the lamina propria where they form crypts. Many important cells reside in the crypts, including those involved in host defence and signalling.

The crypt cells also contain stem cells that replenish the epithelial cells further up the villi

Question 13

What form are carbohydrates digested in?

Answer 13

Carbohydrates can only absorbed in the form of monosaccharides

Question 14

How is complex CHO reduced to disaccharides?

Answer 14

It starts in the mouth by amylase

If it enters the duodenum then pancreatic amylase will break it down

Question 15

What do specific brush border enzymes do in carbohydrate digestion?

Answer 15

It converts disaccharides to monosaccharides (galactose and glucose)

Question 16

Name some carbohydrase

Answer 16

Sucrase acts on sucrose –> Glucose and fructose

Lactase acts on lactose –> Galactose and glucose

Glucoamylase acts on glucose oligomers –> Glucose

Maltase acts on maltose –> Glucose and glucose

Question 17

What does insufficient lactase?

Answer 17

Lactose intolerance, bacteria ferment the unbroken lactose leading to excess gas generation and intestinal discomfort

Question 18

Describe protein digestion

Answer 18

Proteins arrive in the small intestine as polypeptides and these are denatured by gastric acid and broken down by pepsin in the stomach.

Proteins are broken down into oligopeptides and amino acids by pepsins (pancreatic peptidases)

Oligopeptides (consist 2-20 amino acids) are broken down into amino acids and di/tripeptides (done by Di-peptidases is brush border peptidases)

Single amino acids and tri/bipeptides can be absorbed by the enterocyte and be broken down by cytoplasmic peptidases into AA.

AA, di/tripeptides can pass into the blood.

Question 19

What are the 3 sites of protein digestion?

Answer 19

1. Gut lumen e.g. pancreatic proteases, Pepsin in gastric juice
2. Brush border peptidases e.g. Di-pepitdases
3. In enterocytes e.g. cytoplasmic peptidases

Question 20

What membrane of the enterocyte is more permeable to amino acids?

Answer 20

The basolateral membrane is more permeable to amino acids than is the brush border membrane. Therefore diffusion is more important for the basolateral transport, especially for amino acids with hydrophobic side chains

Question 21

Name some dietary lipids

Answer 21

• Tryglycerides are the major dietary lipids
• Phospholipids
• Cholesterol
• Fat soluble vitamins (A, D, E, K)

Question 22

What is steatorrhoea?

Answer 22

Excess fat in faeces

Gallstones, pancreatitis, Crohn’s disease and liver disease can lead to fat malabsorption leading to steatorrhoea.

Question 23

Explain fat digestion

Answer 23

• Salivary lipase digests a small fraction of the TGs
• Gastric lipase breaks down approx 10-30% of fats
• The remainder is digested by pancreatic lipase which binds to the surface of small emulsion particles

Bile salts from the gall bladder and liver emulsify the fat into droplets allowing them to enter suspensions. As well as micelle formation (allow it to be absorbed)

Question 24

How is fat emulsified?

Answer 24

The large fat droplet is broken down by muscular actions

Bile salts prevent small droplets from combining

The surface area where lipase can act is increased

Question 25

Explain absorption of nutrients

Answer 25

Almost all nutrients enter the blood stream by crossing enterocytes (absorptive cells of small intestine)

There are 2 routes by which molecules enter the blood stream from the small intestine:

• Transcellular route: Across plasma membrane of enterocyte (through cell)
• Paracellular route: Across tight junctions between the enterocytes (between cells)

Large organic molecules eg. AA, glucose cannot enter the blood stream via tight junctions of the paracellular route so they take the transcellular route

Nutrients e.g. CL- enter via paracellular transport

Question 26

Explain carbohydrate absorption

Answer 26

• Glucose and galactose are rapidly absorbed into enterocyte by a secondary active transport process
• Fructose and mannose are slowly absorbed into enterocyte with passive transport on a GLUT 5 carrier

Monosaccharides such as glucose can be actively transported with sodium across the epithelium. Sodium binds to the transport protein and this allows glucose or galactose to bind - this is a co-transporter called SGLT-1. The glucose/galactose and Na+ enter the enterocyte together. SGLT-1 only transports a combination of sodium and either glucose or galactose, it won’t transport one alone.

Inside the enteroyte, GLUT-2 transports glucose, fructose and galactose into the capillaries within the villus.

Fructose and mannose are slowly absorbed by passive transport on a facilative GLUT-5 carrier into enterocyte. GLUT5 is specific for fructose with no ability to transport glucose or galactose.
So GLUT-5 and GLUT-2 work in series to promote the absorption of fructose.

Question 27

Explain protein absorption

Answer 27

There are 2 mechanisms:

1. Amino acids are transported on a sodium coupled carrier system (co-transport). There are seperate carriers for different types of amino acids
2. Some di/tri peptides are transported on a carrier system using inwardly directed H+ gradient

The amino acids are then actively transported into the capillary

Question 28

Explain fat absorption

Answer 28

Fats are diffuse into lacteals which are lymphatic vessels present in each villus (rather than capillaries). They drain from the intestines and flow into the blood stream via the lymphatic system

• Fats have to broken down into fatty acids and glycerol (monoglycerides) before being absorbed in entercytes.

In the enterocyte, triglycerides are formed from fatty acids and monoglycerides. The triglycerides are then packaged with cholesterol, phospholipids and proteins into chylomicrons which are transported into the lacteals.

Short chain free fatty acids will enter the portal vein and combine with bile in the liver to form micelle, through action of lipases.