GI enzyme, UC + Crohns Flashcards
What do the intestines absorb?
Nutrients
Water
Electrolytes
What conditions are required for absorption?
Very large surface area
Slow movement
Mostly active so ATP required
Describe the structure of the small intestine?
Mucosa folded into villi, separated by crypts
Cells multiply in the crypts and migrate towards tips of villi, maturing as they go - acquire capacity to absorb, micro-villi on luminal surface increases surface area enormously (brush border)
They are then shed from the villus tips
Mucosa therefore constantly renewed ‘unstirred layer’
What do the villi do?
Increase surface area
Cells on them are very active
Secrete enzymes into brush border:
- Forms unstirred layer
- Almost digested nutrients diffuse into unstirred layer where trapped enzymes complete digestion
- Steadily releasing small molecules for absorption
What are the 2 forms of starch?
Amylose - straight chains with alpha 1,4 bonds
Amylopectin - branched with alpha 1,6 bonds at branches
What does alpha amylase do?
Acts at alpha 1,4 bonds
Yield maltose from amyloses
Yields alpha limit dextrins from amylopectins
Where are alpha amylases secreted?
In saliva and by pancreas
How is glucose created?
Isomatase breaks down branched molecules at a-1,6 bonds
Maltase breaks down maltose to glucose
Sucrase
Lactase
How is glucose absorbed in the intestine?
Absorbed actively using energy from sodium gradient
Glucose enters mucosal cells via Na/glucose transporter - SGLT1 (also transports galactose)
Leaves cell to ECF y facilitated diffusion - GLUT2 transporter
How are fructose and lactose absorbed?
Facilitated diffusion not linked to Na+
What stimulates maximum water uptake?
Na+ generates osmotic gradient and drive uptake of glucose which also generates a gradient of its own - Oral rehydration fluid
What enzymes digest proteins to amino acids?
Protein -> oligopeptides in stomach by pepsin from chief cells (likes bonds near aromatic side chains
Trypsin in the duodenum breaks bonds near basic side chains
Chromotrypsin breaks bonds near aromatic side chains
Carboxypeptidase breaks C terminal amino acids with basic side chains
How are proteins absorbed into the body?
Both amino acids and small peptides absorbed but not proteins in adults. Brush border enzymes break down oligopeptides further. Amino acids taken up by active and passive processes
What types of Na+/amino acid co-transporters are there?
Small neutral amino acids Neutral amino acids, basic amino acids and cystine Acidic amino acids Imino-aminio acids Beta amino acids
Describe the process of absorption of amino acids.
Some uptake of amino acids by facilitated passive diffusion
Dipeptides and tripeptides taken up by active mechanism associated with active pumping of H+ into lumen
Return H+ by co-transport with peptide
How are electrolytes and water absorbed in the intestines?
Sodium taken up via diffusion into cell and active transport across basolateral membrane.
Chloride follows
Osmotic gradient from all absorption leads to uptake of water
Describe the uptake of calcium.
About 700mg absorbed of 6g consumed
Enters cell by facilitated diffusion (low intracellular concentration)
Pumped out of basolateral membrane by Ca2+ ATPase
Both process require vitamin D and are stimulated by parathyroid hormone
Describe the uptake of iron.
About 20mg/day absorbed mostly in haem or related pigments
Gastric acid important for Fe absorption - solubise iron complexes
Stomach also secretes gastroferrin - solubises iron
Mucosal cells secrete transferrin which binds Fe2+ in the lumen.
Complex taken into cells by endocytosis
Fe2+ liberated and exported to blood where it binds again to transferrin
How are vitamins absorbed in the intestines?
Water soluble vitamins absorbed largely by passive diffusion (C and Bs)
Vit B12 absorbed with co-factor in terminal ileum only (intrinsic factor secreted by stomach mucosa
What happens when a person is B12 deficient?
Pernicious anaemia. Stomach damage, terminal ileum removed