Metabolism - Lecture Twenty-Four Flashcards
Absorption of carbohydrates and proteins
What are the specialised structures that create a vast surface area for absorption?
Villi and microvilli (brush border)
Sugar transport
Sugars are highly water soluble and cannot simply diffuse across cell membranes so they require specific transporter proteins that are anchored in the membrane to form ‘pores’
What are the two types of sugar transport?
Active transport - against the concentration gradient (requires ATP energy)
Facilitative transport - down the concentration gradient
What sites does the GLUT 1 transporter distribute?
Everywhere
What sites does the GLUT 2 transporter distribute?
Liver, pancreas, kidney and intestinal epithelia
What sites does the GLUT 3 transporter distribute?
Brain
What sites does the GLUT 4 transporter distribute?
Muscle and adipose tissue
What sites does the SGLT 1 transporter distribute?
Intestinal epithelia
What does glucose transport by SGLT 1 involve?
Simultaneous transport of sodium ions
Absorption of peptides
There is very little absorption of peptides longer than four amino acids
How does the absorption of di- and tri- peptides in the small intestine occur?
By co-transport with H+ ions via membrane transporter PepT1
What happens to di- and tri- peptides once they are absorbed into the small intestine?
They are further digested into the individual amino acids by cytoplasmic peptidases and exported from the epithelial cells into the blood circulation
Absorption of amino acids from the gastrointestinal tract
From the lumen of the small intestine by transepithelial transport.
Transepithelial transport
Transepithelial transport is a semi-specific Na+ dependent transport system
Na+ dependent carries
Transport both Na+ and an amino acid
Types of Na+ dependent carriers
Neutral AA, proline, hydroxyproline, acidic AA, basic AA (Lys, Arg) and cistine
Uptake of intact proteins from gastrointestinal tract
Occurs only in a few circumstances, e.g. new born animals, such as uptake of immunoglobulins in colostral milk- acquisition of passive immunity
Lactose intolerant
Lactase enzyme deficiency (genetic basis), causes bloating, flatulence and diarrhoea due to fermentation of lactose by intestinal bacteria so they need to avoid lactose in diet
Examples of disease affecting digestive organs
Pancreatitis, stomach (or peptic) ulcers, cystic fibrosis and coeliac disease
Pancreatitis
Leads to inappropriate activation of zymogens (proenzymes), resulting in “self-digestion”
Stomach (or peptic) ulcers
Due to the breakdown of the mucosa which normally protects against protease action
Cystic fibrosis
Causes malabsorption and thick mucous secretions which block the pancreatic duct and secretion of pancreatic enzymes and can be aided by taking supplements containing pancreatic enzymes i.e. ‘pancreatin’, a pancreas extract
Coeliac disease
Causes malabsorption and is a disease of the small intestine where the body reacts against gluten protein that is present in wheat. Antibodies react with transglutaminase, villi are flattened and nutrients are not absorbed which leads to gastrointestinal symptoms.
Digestion of dietary nucleic acid polymers
DNA and RNA are subject to partial acid hydrolysis in the stomach
Intestinal endonuclease enzymes
Hydrolyse the phosphodiester bonds linking individual nucleotides
Exonuclease enzyme
Release individual nucleotides (nucleoside monophosphate
How are individual nucleotides absorbed?
Via nucleotide transporters
