Carbohydrates Flashcards

1
Q

simple carbohydrates

A

monosaccharides
- most common is glucose
- naturally occurring
- cannot be hydrolyzed into a smaller unit
- “reducing sugar” when the anomeric carbon is free
disaccharides
- most common is sucrose
- 2 monosaccharides joined by an acetyl bond (glycosidic)

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2
Q

complex carbohydrates

A

oligosaccharideds
- short polymers
polysaccharides
- homo and hetero exist
- glycogen (animal), starch and cellulose (plant)

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3
Q

ratio of organic molecules in carbohydrates

A

CHO = 1:2:1

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4
Q

most common monosaccharides

A

Triose (3C)
- metabolites of glucose
- broken down into trioses in glycolysis
Pentose (5C)
- components of DNA and RNA
- sugars in nucleic acids
Hexose (6C)
- nutritionally the most important
- give our body energy

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5
Q

what is an anomeric carbon

A

the carbon that is apart of the carbonyl (C=O) group

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6
Q

what is an aldose

A

a sugar that contains an aldehyde (e.g. D-glucose)

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7
Q

what is a ketose

A

a sugar that contains a ketone (e.g D-fructose)

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8
Q

what is sterioisomerism?

A

CHO with the same molecular formula and sequence but differ in 3D space due to chiral carbonds
- come in D and L forms
- enzymes used for digestion can recognize stereoisomers (steriospecific)

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9
Q

what is a chiral carbon?

A

a carbon attached to 4 DIFFERENT atoms or groups

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10
Q

different types of isomers

A

enantiomers: mirror image
diasteromers: not mirror images

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11
Q

Fischer projection

A
  • linear form of a sugar
  • counting of carbons begins at the anomeric carbon for an aldose
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12
Q

Stereoisomers and Fischer projections

A
  • D or L for is determined by the -OH group on the highest chiral carbon (furthest from C=O)
  • OH on the right = D
  • OH on the left = L
  • the number of stereoisomers is 2^n (n = # of chiral carbons)
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13
Q

why are D monosaccharides nutritionally important?

A

digestive enzymes are stereospecific for D sugars

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14
Q

converting a linear sugar to a cyclical sugar

A
  • the anomeric carbon allows the linear structure to turn cyclical
  • carbon 1 becomes a new chiral center
  • hemiacetal: made from an aldose
  • hemiketal: made from a ketose
  • reaction occurs spontaneously (no enzymes involved and reversible)
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15
Q

alpha and beta configurations of cyclical sugars

A
  • alpha = OH on anomeric carbon is down
  • beta = OH on anomeric carbon is up
  • beta is more common
  • alpha is more nutritionally important for us
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16
Q

Haworth model of sugars

A
  • the non-acetyl/ketal Ch2OH always points up (C#6)
  • OH group right in fischer = down in haworth, left in fischer = up in haworth
  • alpha = OH on C1 is down, beta = OH on C1 is up
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17
Q

disaccharides

A
  • the most common oligosaccharide
  • 2 monosaccharides attached by a glycosidic bond
  • glycosidic bond can be alpha or beta
  • configuration of -OH group on the anomeric carbon determines whether the disaccharide is alpha or beta
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18
Q

common disaccharides

A

sucrose (alpha(1-2))
- found in sugar cane and fruits
- glucose + fructose
- non-reducing (both anomeric carbons are used)
lactose (beta(1-4))
- found in milk
- galactose + glucose
- reducing - free anomeric carbon
maltose (alpha(1-4))
- found in beer and liquor
- glucose + glucose
- reducing - free anomeric carbon

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19
Q

polysaccharides

A

long strings or branches of monosaccharides (min 6) attached by glycosidic bonds
homo: all the same monosaccharides
hetero: different monosacharides
- both found in nature but homo ar more common in food

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20
Q

advantage of branching in polysaccharides

A
  • provides a larger number of ends from which to cleave glucose when energy is needed
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21
Q

what is dietary fibre

A
  • non-digestible CHO
  • structureal part of plants
    2 types: soluble and insoluble
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22
Q

insoluble fibre

A
  • includes cellulose, lignin and hemicellulose
  • remains intact through the intestinal tract
  • doesn’t dissolve in water
  • reduced transit time (moves quick through gut)
  • increased fecal bulk
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23
Q

soluble fibre

A
  • includes pectins, gums and mucilages
  • forms a gel in intestinal tract
  • dissolves in water
  • delays gastric emptying (increased transit time)
  • slows the rate of nutrient absorption
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24
Q

dietary fibres and solubility

A

characteristics of solubility are…
water holding ability (ability of a fibre to hold water, becoming a viscous solution)
absorptive ability (ability of a fibre to bind enzymes and nutrients)

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25
Q

why is dietary fibre beneficial for our gut health

A

dietary fibre feeds our gut microbiota, which reduced inflammation; less stress on host

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26
Q

cellulose

A
  • both a dietary fibre (naturally occuring) and functional fibre (natural but added
  • homopolysaccharide of B-1,4 glucose units in a linear chain
  • poorly fermented by human gut bacteria ( because humans lack cellulose-fermenting microbes in their gut microbiome
  • rich in bran, legumes, nuts, peas, etc.
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27
Q

hemicellulose

A
  • heteropolysaccharide that varies between plants
  • a mixture of alpha and beta glycosidic linkages
  • can contain both pentoses and hexoses (xylose is most common)
  • 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 and fruits
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28
Q

pectin

A
  • both a dietary and functional fibre
  • part of the primary cell wall of plants
  • backbone of unbranched alpha-1,4-linked D galacturonic acid
  • stable at low pH
  • highly fermented by gut bacteria (good bulking agent in animal feed)
  • rich in fruits
29
Q

resistant starch

A
  • four main types: 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
30
Q

what are the health benefits of fibre

A
  • maintains function and health of the gut
  • insoluble fibre decreases constipation: stimulates muscle contraction to break down waste, decreases the risk of bacterial infection
  • soluble fibre increases satiety: delays gastric emptying, slows down nutrient uptake
31
Q

soluble fibre and disease risk

A
  • decreases cardiovascular disease risk by lowering blood cholesterol (gel binds cholestrol in the small intestine and caries it out of the body)
  • lowers the risk of type II diabetes by binding some glucose in the digestive tract
32
Q

carbohydrate digestion: mouth

A
  • a-amylase gets the ball rolling by breaking a-1,4-glycosidic bonds
  • produces only a few monosaccharides
  • cellulose and lactose are resistant since they have a-1,6-bonds (branch in structure)
33
Q

carbohydrate digestion: stomach

A
  • a-amylase digestion continues until pH drops, and then is inactivated by stomach acid
  • at this point the pool of dietary CHO consists of small polysaccharides and maltose
34
Q

carbohydrate digestion: small intestine

A
  • a-amylase digestion continues in the pancreas
  • active at neutral pH
  • a-1,6-bonds are resistant and eventually produce isomaltose
  • bulk of digestion happens here
35
Q

brush border enzyme activity

A
  • by the time sugars reach the BBM there are disaccharides
    specific enzymes break sugars down further…
    isomaltase (alpha-dextrinase): isomaltose - 2 glucose
    maltase: maltose - 2 glucose
    invertase (sucrase): sucrose - glucose + fructose
    lactase: lactose - glucose + galactose
36
Q

lactose intolerance

A
  • no lactase present in the intestine
  • bacteria ferments the lactose and makes byproducts that cause discomfort for the individual
  • age, ethnicity and genetics influence lactase enzyme activity
37
Q

what are entrocytes?

A
  • ## polarized cells that take up monosaccharides into the intestinal lumen for absorption
38
Q

what happens to glucose after it is taken up by entrocytes?

A
  • small amounts leak back out into the lumen from the entrocyte
  • small amounts diffuse into the blood through the basolateral membrane
  • majority gets transported into the blood through glut2
39
Q

transport of monosaccharides from the lumen into the blood

A
  • glucose and galactose depent on basolateral Na-K ATPase activity
  • ATP targets the ion pumps, not the glucose transporters
  • ion pumps prevent Na+ from building up in the cell which maintains gradient
  • if you don’t have sodium you cant take up glucose
  • fructose is taken up by facilitated transport (GLUT5 on apical surface)
  • all enter the blood through GLUT2
40
Q

functions of carbohydrates in the body

A
  • glucose is the primary source of energy for cells (proper function of CNS and red blood cells)
  • carbohydrates “spare” proteins: prevents breakdown of protein for energy (allows it to concentrate on building, repairing and maintaining body tissue)
  • carbohydrates prevent ketosis: when limited, fats are broken down for energy which leads to production of ketone bones causing the bodies pH to become slightly acidic
41
Q

6 processes in carbohydrate metabolism

A
  • glycogenesis
  • glycolysis
  • hexose monophosphate shunt
  • gluconeogenesis
  • glycolysis
  • Krebs cycle
42
Q

what happens when glucose enters a cell

A
  • glucose becomes glucose-6-phosphate (G6P)
  • G6P undergoes either glucogenesis, hexose monophosphate shunt, or glycolysis based on needs
  • enters glycogenesis for energy storage
  • enters glycolysis for energy production
  • enters hexose monophosphate shunt to generate precursors for biogenesis
43
Q

what is glycogenesis?

A
  • the formation of glycogen from glucose
  • G6P becomes G1P and then glycogen synthase turns it to glycogen by removing a phosphate
44
Q

what does the liver do to glycogen

A

breaks down glycogen to release glucose into the blood when needed

45
Q

what role does glycogen play with insulin

A
  • insulin activates glucokinase, hexokinase and glycogen synthase
  • insulin favours the uptake and clearance of blood sugar to form glycogen
46
Q

what is glycogenin

A
  • an enzyme that serves as a scaffold on which to attach glucose molecules to build glycogen
  • 30 000+ glucose molecules can be contained in a single glycogen structure
  • process requires energy
47
Q

what is glycogenolysis?

A
  • the breakdown of glycogen to make glucose
  • glycogen becomes glucose-1-phosphate by glycogen phosphorylase (breaks a-1,4-bond to release monomers)
  • G1P becomes G6P
  • G6P is converted to glucose by glucose-6-phosphotase and has 2 fates…
    1. in the liver (ONLY) G6P is dephosphorylated to glucose and is released into the blood
    2. G6P can go onto glycolysis
48
Q

what is the role of glucagon?

A
  • present when blood sugar is low
  • activates glycogen phosphorylase to make glucose from glycogen
49
Q

state of flux between glycogenesis and glycogenolysis

A

low blood sugar
- promotes glucagon release by pancreas
- glucagon release stimulates the breakdown of glycogen in the liver
- glycogen is broken down to glucose and released from the liver to increase blood sugar
high blood sugar
- promotes insulin release from the pancreas
- insulin either travels to tissue cells OR stimulates glycogen formation in the liver
- glucose is taken up from the blood and converted to glycogen to decrease blood sugar

50
Q

what are the main ways to produce energy in the cell?

A
  1. substrate level phosphorylation
    - transfer of a high-energy phosphate group from one molecule to another
    - usually attached a phosphate to ADP to make ATP
    - the only way red blood cells can get energy
  2. oxidative phosphorylation
    - happens in the ETC of the mitochondria
    - O2 is required and increased the proton motive force
51
Q

what is glycolysis?

A
  • breaking glucose to generate energy (ATP)
  • happens in the cytoplasm by enzymes
  • endpoint depends on anaerobic vs aerobic cell
  • one glucose generates 2 pyruvates
52
Q

important molecules and enzymes in glycolysis

A

glucokinase/hexokinase: converts glucose to G6P
phosphofructokinase: produces fructose-1,6-bisphosphate
- phosphofructokinase is split into 2 molecules of glyceraldehyde-3-phosphate
- end product is 2 pyruvate molecules

53
Q

what is the importance of phosphofructokinase in glycolysis?

A
  • the first committed (irreversible step) in glycolysis
  • ATP and glucagon in the liver inhibit this enzyme (no need to breakdown glucose) and therefore the committed step
54
Q

net energy yield from 1 glucose in glycolysis

A

2NADH and 2ATP ~ 8 ATP
- NADH will go on to the ETC
- 2 ATP are put in and 4 are generated, therefore net gain of 2

55
Q

fate of pyruvate after glycolysis

A

anaerobic = kreb’s cycle
aerobic = lactate

56
Q

Anaerobic metabolism of glucose: lactate production

A
  • occurs in muscles during prolonged exercise and in red blood cells
  • pyruvate is converted into lactate in the cell’s cytosol
  • regenerates NAD+ to continue glycolysis
  • net of 2 ATP is produced when glucose is converted to lactate
57
Q

Anaerobic metabolism of glucose: ethanol production

A
  • doesn’t happen in the body
  • basis of fermentation when you make wine and beer
  • yeast breaks down pyruvate into CO2 and ethanol
  • regenerates NAD+ which allows glycolysis to continue
58
Q

The cori cycle

A
  • occurs when oxygen isn’t available in the muscle leading to the production of lactate
  • how lactate is transported back to the liver where gluconeogenesis allows for the conversion of pyruvate back to glucose
  • for 2 molecules of lactate to form 1 glucose the cell consumes 6 ATP
  • not sustainable because more energy is being consumed than produced
59
Q

what is the hexose monophosphate shunt?

A
  • important for NADPH production and ribose synthesis from glucose (G6P)
  • occurs in the cytoplasm of a cell
  • molecules produced will go on to perform other functions in the cell such as biosynthesis
60
Q

Hexose monophosphate shunt: oxidative phase

A
  • G6P releases NADPH and becomes 6-PG
  • 6-PG releases NADPH and CO2 to give ribulose-5-phosphate which will go onto the nonoxidative phase
  • NADPH produced can be used for fatty acid biosynthesis in the liver, oxidative phosphorylation and oxidative defence system in red blood cells
  • G6P dehydrogenase is the rate limiting step which converts the first intermediate - can be inhibited by NADPH
61
Q

hexose monophosphate shunt: nonoxidative phase

A
  • ribulose-5-phosphate is converted to ribose-5-phosphate which can do 1 of 2 things
    1. go onto nucleotide synthesis
    1. create C3-C7 intermediates which become F6P
  • all cells use this phase for nucleotide synthesis to continue
62
Q

Pyruvate dehydrogenase

A
  • the “gatekeeper” to Kreb’s which converts pyruvate to acetyl CoA
  • happens in the mitochondria (pyruvate is committed to staying there)
  • CoA-SH is added to pyruvate (3C) and NAD+ comes in and NADH + CO2 comes out to give acetyl CoA (2C)
  • happens twice per 1 glucose
  • net energy yield: 2NADH ~ 6ATP
63
Q

pyruvate dehydrogenase complex

A
  • several enzymes and cofactors are required to convert pyruvate to acetyl CoA
  • 4 vitamins (micronutrients): thiamine, Niacin, Riboflavin, pantothenic acid
64
Q

the Kreb’s cycle

A
  • over 90% of the energy in food is released in this biochemical process
  • final catabolic pathway for products of protein, lipid and carbs
  • takes place in the mitochondrial matrix
  • some AAs can be converted to acetyl CoA or other compounds in the kreb’s cycle in a fasted state if your body needs to break down protein for energy
  • energy yield of 1 cycle: 3NADH, 1FADH2, 1GTP ~ 12ATP
65
Q

how can pyruvate get converted to Oxaloacetate (instead of acetyl CoA)?

A
  • pyruvate carboxylase converts pyruvate (3C) to oxaloacetate (4C)
  • required the input of 1 ATP
  • this step is inhibited if enough Acetyl CoA (2C) is present
66
Q

How much energy can you get from one glucose molecule?

A

1 NADH ~ 3ATP, 1 FADH2 ~ 2ATP, 1 GTP ~ 1ATP
- glycolysis: 8 ATP
- pyruvate dehydrogenase: 6 ATP (since x2 pyruvate)
- Kreb’s cycle 24 ATP (since x2 acetyl CoA)
TOTAL ENERGY: 38 ATP
- theoretical maximum possible since some NADH and FADH2 go onto contribute to other processes

67
Q

What is gluconeogenesis?

A
  • making glucose from non-CHO sources
  • active when glucose is needed - such as a fasting state
  • very active in the liver, can also happen in the kidney during starvation
  • doesn’t happen in muscle or adipose tissue since they lack the enzymes
  • when lactate travels from muscle back to the liver (cori cycle) gluconeogenesis can take place
68
Q

gluconeogenesis: mitochondria

A
  • lactate gets converted back to pyruvate which enters the mitochondrial via pyruvate translocase
  • Pyruvate carboxylase converts pyruvate to oxaloacetate
  • oxaloacetate cannot leave the mitochondria so it is converted to malate via malate dehydrogenase, the crosses the membrane and is converted back to oxaloacetate in the cytosol