Carbohydrates Flashcards
How are carbohydrates classified and what are the three classifications?
Classified by degree of polymerisation.
- sugars (mono and disaccharides DP 1-2)
- oligosaccharides (DP 3-9)
- polysaccharides (DP > 9)
What is the general formula of a carbohydrate? And what is the formula for glucose?
General formula: (CH2O)x
Glucose: C6H12O6
Give 2 examples of monosaccharides and disaccharides.
Mono - glucose, galactose & fructose
Di - sucrose, lactose & maltose
Give an example of an oligosaccharide.
Maltodextrins
Give an example of a starch polysaccharide and a non-starch polysaccharide.
Starch - amylase, amylopectin & modified starches
Non-starch - cellulose, hemicellulose
What bond joins monosaccharides?
Glycosidic bond
In terms of monosaccharides, what do alpha or beta bonds determine?
Whether the bond faces up or down
Describe the two sub groups of oligosaccharides
Alpha Glucans: (alpha-1,4 or alpha 1,6 bonds) mostly derived from starch and digested and absorbed like polysaccharide alpha-Glucans.
Non-alpha Glucans: e.g. Peas, beans and lentils and not susceptible to pancreatic or brush border enzyme breakdown. They are non-digestible oligosaccharides.
What are the two subgroups of polysaccharides?
Alpha Glucans (starch) & non-alpha-Glucans (non-starch polysaccharides - NSP)
What determines the nutritional properties of carbohydrates?
The monosaccharide composition and type of linkage.
What do glyceamic carbohydrates do?
Provide glucose for metabolism as a result of digestion and absorption in the small intestine.
What do non-glyceamic carbohydrates do?
They are NSP (non-starch polysaccharides) and are not absorbed in the small intestine but are fermented to short chain fatty acids, CO2, H2 and methane in the large intestine.
What is the aim of carbohydrate digestion?
To liberate monosaccharides from disaccharides and polysaccharides.
Which carbohydrates are hydrolysed to monosaccharides and are absorbed in the small intestine?
- Most sugars
- Alpha-glucan oligosaccharides
- Starch (but some starch is resistant)
Which carbohydrates resist digestion and pass into the large intestine where they are fermented?
- Some sugars (e.g. Lactose)
- Non-alpha-glucan oligosaccharides
- Non-alpha-glucan (non-starch) polysaccharides
- Resistant starch
Summarise digestion of carbohydrates.
- Chewing breaks up food structure and mixes with saliva containing alpha-amylase)
- Alpha-amylase (which is produced by the salivary gland and pancreas and has an optimal pH range of 6.6-6.8) digests alpha-1,4 links in amylose and amylopectin.
- Amylase does not hydrolyse bonds at the ends of molecules or those next to alpha-1,6 branch points.
- Once swallowed, amylase is inhibited by low pH of stomach
- Once food is in small intestine (SI) alpha-amylase is secreted by acinar cells in the pancreas into the SI.
- Within the lumen, amylase hydrolyses alpha-1,4 glycosidic linkages.
- Starch is broken down into dextrins and maltose
- Branched starch is broken down into mixture of dextrin and glucose units.
- Remainder of digestion occurs close to intestinal surface, close to transporter proteins which is ideal for absorption.
- At the brush border of intestinal epithelium, enterocytes lining villi contain the enzymes: glucosidases, disaccharidases & oligosaccharidases which break down complimentary carbohydrates e.g. Sucrase breaks down sucrose.
- The resulting monosaccharides (e.g. Glucose, galactose and fructose are available for absorption)
- Carbohydrate which has not been digested in this way passes into the large intestine (e.g. NSP)
Name the sugar transporters on the apical membrane of the enterocytes.
Glucose and galactose (active transport): Sodium-glucose transport protein-1 (SGLT1)
Fructose (passive): GLUT5 protein
Name the sugar transporters on the basolateral membrane of the enterocytes.
Glucose, galactose and fructose: GLUT2 protein
Where are monosaccharides absorbed too?
Through the epithelial cells in the lumen of the small intestine into the portal vein which is linked to the liver.
Describe the active transport of glucose across epithelial cells.
- Active transport requires energy and a specific receptor or transporter.
- There is active transport of sodium across the apical membrane via the sodium/potassium pump.
- Decreased sodium inside the cell draws sodium across the apical membrane with glucose (SGLT1)
- Glucose then passes across basolateral membrane into blood vessels by facilitated transport (GLUT2)
Describe the facilitated diffusion of fructose.
- It passes across the apical membrane with GLUT5 protein
2. Passes across the basolateral membrane with GLUT2 protein.
What is the rate of uptake of glucose determined by?
Rate of uptake of glucose from the intestine is determined by the rate of hydrolysis or oligosaccharides and polysaccharides that are susceptible to brush border enzymes.
Summarise briefly the digestion and absorption of carbohydrates.
Salivary alpha amylase -> pancreatic alpha-amylase -> products of alpha-amylase digestion: maltose, maltotriose, alpha-dextrin and some glucose -> in intestinal cells brush border enzymes e.g. Sucrase, maltase, isomaltase & lactase digest resulting in monosaccharides -> these are absorbed into portal vein -> those that were resistant to pancreatic and brush border enzymes are sent to the colon for fermentation.
Why are non-glyceamic carbohydrates moved into colon?
- Enzymes needed to digest the carbohydrates:
- not present in small intestine
- cannot gain access to carbohydrate (e.g. Starch trapped in plant cell wall)
- do not digest carbohydrate rapidly enough - Monosaccharide transporters do not exist or function at a high enough rate.
