Lecture 5 Flashcards
What are carbohydrates divided into?
Simple carbs and complex carbs
What are simple carbs divided into?
Monosaccharides and disaccharides
What are some characteristics of monosaccharides?
- most common is glucose
- naturally occurring
- cannot be hydrolyzed into a smaller unit
- considered a reducing sugar when the anomeric carbon is free
What are some characteristics of disaccharides?
- most common is sucrose
- two monosaccharides joined by an acetyl bond (glycosidic bond), formed between two hydroxyl groups, a glycosidic bond can also be alpha or beta
- configuration of the Oh group on the anomeric carbon determines whether the disaccharide is alpha or beta
What are complex carbs divided into?
Oligosaccharides and polysaccharides
What is a characteristic of oligosaccharides?
short chain monosaccharides
What are some characteristics of polysaccharides?
- long-chain monosaccharides
- homo and heteropolysaccharides
- starch (amylose and amylopectin) is rich in plants
- glycogen (animal)
- cellulose (plant)
What are monosaccharides divided into?
Triose
Pentose
Hexose
What are some characteristics of triose?
- metabolite of glucose
- 3 carbon monosaccharides
- not dietary
What are some characteristics of pentose?
- components of dna and rna
- 5 carbon monosaccharides
What are some characteristics of hexose?
- nutritionally the most important
- 6 carbon monosaccharides
What is stereoisomerism?
When two molecules have the same molecular formula and sequence but differ in 3D space due to chiral carbon atoms (L and D isoforms)
What are the two different types of stereoisomers?
Enantiomers (mirror image)
Diastereomers (not a mirror)
What is an aldose?
a monosaccharide with a carbon backbone with a carbonyl group on the endmost carbon
Where does counting for naming monosaccharides begin for aldose?
counting begins at the anomeric carbon for aldose
How do we determine a D vs an L stereoisomer?
Determined by the OH group on the highest chiral carbon
OH on the right = D
OH on the left = L
What is a chiral carbon?
A carbon that is bound to four other atoms
How to determine the number of stereoisomers for a molecule?
2n (where n = # of chiral carbons)
Why are D-monosaccharides important?
Nutritionally important because digestive enzymes are stereospecific for D sugars
What is a hemiacetal?
made from an aldose
What is a hemiketal?
made from a ketose
What are the rules for whats up and down in the haworth model?
- non-acetyl/non-ketal CH2OH always points up
- hydroxyl (-OH) groups
if its right in the fischer, its below in the haworth
if its left in the fischer, its above in the haworth - hemiacetal (alpha or beta)
alpha has the -OH group pointing down
beta has the -OH group pointing up
LAB vs RBA?
LeftAboveBeta
RightBelowAlpha
What is an aldose?
H-C=O
What is a ketose?
R-C-R
“
O
What are polysaccharides divided into?
Homopolysaccharides - more abundant in food
Heterpolysaccharides
What is the advantage of branching in polysaccharides?
allows structure to be compact, elasticity, and energy, more ends to cleave more glucose
What are the branching characteristics of amylose, amylopectin and glycogen
Amylose - no branching
Amylopectin - alpha 1-6 and alpha 1-4 bonds cause branching
Glycogen - lots of branching
What is dietary fibre?
- non-digestible complex CHO
- structural part of plants
- divided into insoluble and soluble fibres
What are three insoluble fibres?
cellulose
lignin
hemicellulose
What are three soluble fibres
pectins
gums
mucilages
What are some solubility characteristics of insoluble vs soluble fibre?
Insoluble:
- remains intact through intestinal tract (doesn’t dissolve in water)
- reduces transit time
- increases fecal bulk (more bowel movement)
Soluble:
- forms gel (does dissolve in water)
- delays gastric emptying, increases transit time
- slows down the rate of nutrient absorption and goes slower through the intestine
Why can’t digest dietary fibre?
we lack enzymes needed to break down dietary fibre (beta bonds can’t be broken by us)
What are some characteristics of cellulose?
- both a dietary fibre (naturally occurring in a food) and functional fibre (naturally occurring fibre that is added to a food that normally doesn’t have any cellulose)
- homopolysaccharide of beta 1-4 glucose units in a linear chain
- poorly fermented by human gut bacteria (because humans generally lack cellulose fermenting microbes in their gut microbiome (methanogens))
- rich in brain, legumes, nuts, peas, root vegetables and seeds
What are some characteristics of hemicellulose?
- heteropolysaccharide that varies between plants
- a mixture of alpha and beta glycosidic linkages (partly broken down)
- can contain both pentoses and hexoses (xylose is the most common monosaccharide in hemicellulose)
- exists as both branched and linear structures
- the solubility and fermentability of hemicellulose depends on the sugar composition
- found in bran, whole grains, nuts and some vegetables/fruits
What are some characteristics of pectin?
- both a dietary and functional fibre
- heterogeneous polysaccharide
- part of the primary cell wall of plants
- backbone of unbranched alpha 1-4 linked-D galacturonic acid (able to be broken down)
- stable of low pH
- highly fermented by gut bacteria (considered to be a good bulking agent in animal feeds)
- rich in fruits, such as apples, oranges, lemons and grapefruit
What are some characteristics of resistant starch?
- four main types, termed RS1 - 4 (found in different foods)
- typically found in plant cell walls
- resistant to amylase activity
- conveys some advantages of both soluble and insoluble fibres (green banana has resistant starch)
What are the health benefits of fibre?
- maintains function and health of the gut
- lower constipation (insoluble fibre)
- stimulates muscle contraction to break down (push fecal contest through GIT)
- decreases risk of bacterial infections
- higher satiety (soluble fibre)
- delays gastric emptying
- slows down nutrient uptake
How can soluble fibre decrease cardiovascular disease risk?
- by lowering blood cholesterol
removes cholesterol from stomach and into intestine (absorbs it), only about 10% of cholesterol, mostly reduces LDL cholesterol
Where are the three places that carbohydrate digestion takes place in the body?
Mouth
Stomach
Small Intestine
What occurs in the mouth for carbohydrate digestion?
- alpha amylase (salivary) breaks down an alpha 1,4-glycosidic bonds - very specific
- produces only a few monosaccharides
- cellulose and lactose are resistant, as are alpha 1,6 bonds
What occurs in the stomach for carbohydrate digestion?
- alpha amylase digestion continues until pH drops, then enzyme is inactivated by HCl stomach acid
- at this point, the pool of dietary CHO consists if small polysaccharides and maltose
What occurs in the small intestine for carbohydrate digestion?
- alpha amylase (secreted from the pancreas)
- active at a neutral pH
- alpha 1,6 bonds are resistant and eventually produce isomaltose
What are the four enzymes that function at the brush border?
Maltase - breaks maltose into two glucose
Alpha dextrinase (breaks down alpha 1,6 glycosidic bonds) - (also known as isomaltase) breaks isomaltose into two glucose
Invertase (also known as sucrase) - breaks sucrose into glucose and fructose
Lactase - breaks lactose into glucose and galactose
What is the function of the enzymes at the brush border?
enzymes break disaccharides at brush border to be absorbed by epithelial cells
What happens to the lactose in the large intestine when a person is lactose intolerant?
Normal - lactose is broken down into glucose and galactose by lactase
Intolerance - Lacks lactase, therefore lactose is not broken down and becomes bacteria which is fermented, this causes gases and irritation
What is the process of monosaccharides absorption in the intestine?
Glucose and Na+, Cl- and H2O can enter the blood from the intestinal lumen in multiple ways. Firstly, they can be transported through enterocytes (intestinal cells) through SGLT1 (sodium glucose transport 1), which will pass them down to glucose shuttle or NaK atpase shuttle, this process doesn’t require ATP. Secondly some molecules can go through the sides of the epithelial cells. This process works on a concentration gradient, meaning it doesn’t require ATP
What are some characteristics of monosaccharide absorption through the intestine into the blood?
- very efficient
- nearly all monosaccharides are taken up by enterocytes
- what happens to the glucose them: small amounts leak back into the intestinal lumen, small amount diffuse into blood through the basolateral membrane, majority is transported into blood through GLUT2
- transport of glucose and galactose from lumen into blood is dependent on basolateral Na-K ATPase activity
- in contrast, fructose taken up by facilitated transport
How do glucose, galactose and fructose enter the blood?
basolateral GLUT2
What are the three factors of a nutrient that determine the transport mechanism that is used?
- solubility
- concentration gradient
- molecular size
What are the three functions of carbohydrates in the body?
- glucose is the primary source of energy for cells (essential for proper functioning of cells in the CNS and RBC)
- carbohydrates spare protein
- prevents breakdown of protein for energy
- allows protein to concentrate on building, repairing and maintaining body tissue
- carbohydrates prevent ketosis
- when carbs are limited, fats are broken down for energy, this leads to ketone body prod., causing high acidity
