Carbohydrates Flashcards
What is a carbohydrate?
Organic molecule with the general formula (CH2O)X
What are carbohydrates classified by, give the 3 categories
Classified by the degree of polymerisation
Sugars (mono and disaccharides) (DP 1-2)
Oligosaccharides (DP 3-9)
Polysaccharide (DP >9)
Explain what degree of polymerisation (DP) is
The degree of polymerisation or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule
Explain alpha and beta linkage and where they are found, explain the strength
The linkage found in disaccharides, oligosaccharides and polysaccharides are classified as alpha or beta. These are the bonds between two monosaccharides
The alpha-linkages are easily digested by the human body
Beta-linkages are stronger because they are more stable and are not easily digested by the human body
Explain the structure of lactose and why type of carbohydrate is it
Lactose is a disaccharide found in milk and it consists of a beta D-galactose that is attached to an alpha D-glucose. The linkage between these two sugars is a beta- 1-4, glycosidic bond
Name the sub-groups of Oligosaccharides (3-9) and give examples
Malto-oligosaccharides (alpha-glucans): Maltodextrins
Non-alpha-glucan oligosaccharides: Raffinose, Stachyose, Polydextrose, Inulin
Name the sub-groups of Polysaccharides (>9) and give examples
Starch (alpha-glucans): Amylose, Amylopectin, Modified starch
Non-starch polysaccharides: Cellulose, Hemicellulose, Pectin, beta-glucan, Plant gums, Mucilage
Name the sub-groups of Sugars (1-2) and give examples
Monosaccharides: Glucose, Fructose, Galactose
Disaccharides: Sucrose, Lactose, Maltose, Trehalose
Polyols (sugar alcohols): Sorbitol, Mannitol, Lactitol, Xylitol, Isomalt
Explain what polypols are and where they are found
A specific group if sugar alcohols that are formed via the catalytic hydrogenation of CHOs. They are found naturally in certain fruits, vegetables, and mushrooms; however, they are also used as sugar-free sweeteners in products such as chewing gum, candies, and beverages
Name the 2 classification of carbohydrates based on physiological behaviour and explain both
Glycaemic carbohydrates - provides glucose for metabolism as a result of digestion and absorption in the small intestine
Non-glycaemic carbohydrates - fermented to short-chain fatty acids, CO2, H2 and methane in the large intestine
Explain absorption of glycaemic and non-glycaemic carbohydrates
Glycaemic - absorbed from the small intestine and provide CHO for metabolism
Non-glycaemic - not ingested in the small intestine but pass into the large intestine. They do not increase the blood concentration of glucose
Explain the function of Glycaemic Carbohydrates
Provide 3.8 kcal/g energy
Explain the function of Non-Glycaemic Carbohydrates
Prebiotic
Heart function
Helps to prevent constipation and stomach pain
Explain what a prebiotic and probiotic is
Probiotics are foods or supplements that contain live microorganisms intended to maintain or improve the “good” bacteria (normal microflora) in the body. Prebiotics are foods (typically high-fiber foods) that act as food for human microflora.
Define total carbohydrate, dietary fiber, and total sugar
Total Carbohydrate = All types of CHO found in the food or beverage
Dietary fiber = Dietary fiber is the part of plants that you eat but don’t get digested in your small intestine
Total Sugar = Sugars that occur naturally and are added artificially
Explain the process of digestion and absorption of carbohydrates
Entering the mouth:
A mixture of mono, di, oligo, and polysaccharides
Different susceptibility to digestion in the small intestine
Digestion and Absorption
Entering the bloodstream:
Mainly glucose
Small traces of fructose and galactose
Traces of other monosaccharides
Explain the action of alpha-amylase in digestion
The action of alpha-amylase:
Produced by salivary glands and pancreas
Activity inhibited by a low pH in the stomach
Digested alpha-1,4 links in amylose and amylopectin
Does not hydrolyze bonds at the end of molecules or those next to alpha-1,6 branch point
Amylose into maltose
Amylopectn into Oligosaccharides
Explain the different methods of absorption in the small intestine
Sugar transporters on enterocytes:
Apical membrane
Glucose and galactose - Sodium-glucose transporter protein-1 (SGLT1)
Fructose:
GLUT5 protein (passive transport)
Basolateral membrane
Glucose, Galactose, and Fructose
GLUT2 protein
Monosaccharides are thus transported to blood vessels linking to the portal vein to liver
Explain how the different transporter protein works in the small intestine during absorption
SGLT1:
Glucose and Na+
Glucose is transported from the intestinal lumen to epithelial cells to a GLUT2 transporter protein
Sodium is transported from the intestinal lumen to epithelial cell to a Na+/K+ Pump
GLUT2:
Glucose is transported from epithelial cell to portal blood
GLUT5:
Fructose is transported from the intestinal lumen to epithelial cells and then into another GLUT5 transporter at the epithelial cell to transport the fructose to the portal blood
Explain how glucose is actively transported in the small intestine
Active transport of sodium across basolateral membrane (Na+/K+-ATPase)
Decreased sodium inside the cell draws sodium across the apical membrane with glucose (SGLT1)
glucose passes across the basolateral membrane into blood vessels by facilitated transport (GLUT2)
Explain the facilitated diffusion of fructose in the small intestine
Across the apical membrane (GLUT5)
Across the basolateral membrane (GLUT2)
Explain why some carbohydrates undergo the process of fermentation
Enzymes are needed to digest the CHO that are not present in the small intestine
Enzymes can not gain access to the CHO trapped inside plant cell walls
Enzymes do not digest the CHO rapidly enough (affected by particle size and transit time)
Or monosaccharide transporters do not exist or are functional at a high enough rate
Explain the process of fermentation of carbohydrates
Virtually all CHO that enter the large intestine will be fermented by bacteria
Bacteria produce hydrolytic enzymes that break disaccharides, oligosaccharides, and polysaccharides into monosaccharides
Bacteria internalize monosaccharides
CHO is metabolized to pyruvate by the glycolysis pathway
Pyruvate fermented to short-chain fatty acids (SCFA)
Anaerobic process
Butyrate = energy source for colonic epithelial cell
Propionate = absorbed and transported to the liver
Acetate = is absorbed and passes to the liver and then the bloodstream - metabolized by skeletal and cardiac muscle and the brain
The total energy provided by fermentation in humans is about 5%
Explain the bacteria in the colon and explain what a symbiotic relationship is
Colon contains a complex ecosystem of over 400 known species of bacteria
Symbiotic relationship – they obtain substrates for growth from the host and return bi-products of their metabolism to the host
Name the digestive enzymes
Lactose
Sucrase
Maltase
How are glucose, galactose, fructose, and sugar alcohols absorbed?
Glucose and Galactose - active transport
Fructose-facilitated diffusion
Sugar alcohols - simple diffusion
Guve the dietary requirement for total carbohydrates, free sugars, intrinsic, milk sugars, starch, and dietary fiber according to SACN
Total Carbohydrates = 50% of dietary energy
Free sugars = < 5% of total energy
Intrinsic, milk sugars and starch = 45% of total energy
Dietary fibre = 30g/day
Name some of the carbohydrates in order of increasing cariogenicity
Sucrose (highest)
Fructose, Glucose, Maltose
Lactose, Galactose
Maltodextrins, Polysaccharides
Sorbitol, Xylitol
Name the conditions low dietary fiber is associated with
Constipation
Diverticular disease
Hiatus hernia
Bowel cancer
Name the characteristics of dietary CHO and functional GI tract (FGI) carbohydrates and give examples of the carbohydrates that have these characteristics and describe 1 problem of these CHO
Fermentable
Oligosaccharides
Disaccharides
Monosaccharides
Polyols
Include: Fructose, Lactose, Fructans, Galactans, Sorbitol, Mannitol, Xylitol, Malitol
Problem: Poorly absorbed in the small intestine, small and cosmetically active molecules, rapidly fermented by GI bacteria
Explain the symptoms of consuming too many FGI CHOs
Osmotic load
Increased water delivery
Increased gas production
Motility change
Bloting
Pain and discomfort
Wind
