Carbohydrates Flashcards

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1
Q

Monosaccharides

A

Simple sugar units (6C-hexoses), e.g. glucose and fructose found in fruit and honey.

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2
Q

Disaccharides

A

Formed by 2 monomers linked by glycosidic bonds between the OH group and the anomeric C. E.g. maltose and lactose which are reducing sugars found in beer and milk. E.g. sucrose a table sugar making up 25% of our intake. There is no free anomeric C so is non-reducing.

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3
Q

Polysaccharides

A

Polymers of medium to high molecular weight. They are distinguished by the identity of repeating units, length, type of bonding amd amount of branching exhibited.
Homopolysacchrarides; single monomeroc species.
e.g. starch which contains 2 types of glucose (amylose and amylopectin) with many non-reducing ends branching every 24-30 residues, found in cereals, potatos, rice. E.g glycogen formed by many glucose sub-units with branching every 8-1w residues found in meat. 90% in liver and skeletal muscle.
Heteropolysaccharides; 2+ monomer species

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4
Q

Cellulose and hemicellulose

A

Found in plant cell walls and aren’t digested.

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5
Q

Oligosaccharides

A

Alpha1-6 galactose found in peas, beans and lentils which are not digested.

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6
Q

Digestion of carbohydrates

A
  1. Mouth; amylose hydrolyses a1-4 bonds of starch.
  2. Duodenum; pancreatic amylase hydrolyses a1-4 bonds of starch.
  3. Jejunum; final digestion by mucosal cell-surface enzymes. Isomaltase hydrolyses a1-6 bonds, glucoamylase removed glucose sequentially from non-reducing ends, sucrase hydrolyses sucrose and lactase hydrolyses lactose. The main products are glucose, galactose and fructose.
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7
Q

Absorption of carbohydrates

A

Monosaccharides;

  1. Glucose is absorbed through an indirect ATP-powered process. ATP driven Na+ pump maintains low cellular [Na+] so glucose can continually be moved into epithelial cells even against a concentration gradient.
  2. Galactose is similarly absorbed to glucose utilising gradients to facilitate transport.
  3. Fructose binds to channel protein GLUT5 and moves down it’s concentration gradient.

Cellulose and hemicellulose; cannot be digested by the gut but it increases faecal bulk and decreases transit time for food to pass through the gut .

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8
Q

Hexokinase

A

Found in other tissues. It has a low Km and a low Vmax.
Hexokinaseis the initial enzyme ofglycolysis, catalyzing the phosphorylation of glucose by ATP to glucose-6-P. It is one of the rate-limiting enzymes ofglycolysis.
Due to it’s high affinity even at low levels of [glc] it can grab it effectively and due it’s low Vmax tissues are easily satisfied.

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9
Q

Glucokinase

A

Found in the liver, it has a high Km and a low Vmax. Glucokinase(EC 2.7.1.2) is an enzyme that facilitates phosphorylation of glucose to glucose-6-phosphate. Due to its low affinity to glucose when blood [Glc] is normal then the liver won’t grab it all leaving it for other cells. If blood [Glc] is high the liver will phosphorylate it quickly and traps it in the liver.

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10
Q

The synthesis of glycogen

A

Step 1.
UDP (carrier molecule) glucose&raquo_space;glycogenin» Glucose&raquo_space;glycogen synthase» extended glucose chains

Step 2
Glc chains&raquo_space;glycogen branching enzyme» glycogen. This enzyme breaks the chains and re-attaches them via (a1-6) bonds to give branch points.

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11
Q

Degradation of glycogen

A

Glycose monomers are removed one at a time from the non-reducing ends as G-1-P. As glycogen is heavily branched this happens rapidly.
Glycogen&raquo_space;glycogen phosporylase» G-1-P»de-branching transferase» removed 3Glc residues and non-reducing end (a1-4)
G-1-P»glucosidase» glc by breaking (a1-6). Leaving an unbranched chain which can be further degraded or built upon. We very rarely fully degrade glycogen as it is so big and we are constantly adding to it as well as degrading it.

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12
Q

Function of glycogen in the liver

A

A decrease in [Glc] causes glycogen to be broken down to form glucose which is release into the blood.

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13
Q

Function of glycogen in skeletal muscle

A

A decrease in [blood Glc] causes glycogen to be broken down via glycolysis to form lactate produces ATP. This is substrate-level phosphorylation.

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14
Q

Function of glycolysis

A

The cellular degredation of glucose to yield ATP as an energy source through substrate level phosphorylation. Anaerobically is creates lactic acid to produce ATP. Aerobically it forms pyruvate.

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15
Q

Lactate dehydrogenase and pyruvate dehydrogenase

A

Lactate degydrogenase catalyses pyruvate to lactate. Pyruvate dehydrogenase catalyses pyruvate to acety CoA. These enzymes are required to re-generate NAD+ maintaining redox balance.

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16
Q

Descrive the fate of blood lactate

A

Lactate is produced in the absence of O2 in working skeletal muscle or in RBCs that don’t contain mitochodria. Lactate is converted back to glc in the liver by gluconeogenesis whoch repays the O2 debt run up by muscles.

17
Q

Precursors for gluconeogenesis

A

Lactate, pyruvate, glucerol (TAG), alanine, glutamine, CAC intermediates.

18
Q

Function of gluconeogenesis

A

The process of synthesising glucose from non-carbohydrate sources. It produces glucose after fasting or lots of exercise.

19
Q

Location of gluconeogenesis

A

Liver

20
Q

Gluconeogeneis steps

A
  1. Conversion of pyruvate or lactate to phosphoenolpyruvic acid (PEP) via oxaloacetate.
  2. Hydrolysis to cleave F-1,6-bisP into F-6-P. (Cytoplasm)
  3. Dephosphorylation/hydrolysis of G-6-P to Glc (lumer on ER)
21
Q

Fate of absorbed galactose and fructose

A

Both are common dietary carbohydrates however the body does not have catabolic pathways for them. Most fructose is metabolised by the liver and they can both enter glycolysis at various points.

22
Q

Function of the pentose-phosphate pathway

A
  • It produces NADPH (captures electrons) for all organisms.
  • In the liver it is used for FA synthesis, steroid synthesis and drug metabolism.
  • mammary gland: FA synthesis
  • Adrenal cortex: steroid synthesis
  • RBC: antioxidant -It produces pentose (5C sugar)
  • is a precursor of ATP, RNA and DNA. It metabolises small amount of dietary pentoses.
  • digestion of nucleotides.

The NADPH links the pentose-phosphate pathway (catabolic) with other anabolic processes.