Carbohydrates 1-2 Flashcards

1
Q

Define the major carbohydrates in the diet.

A

Monosaccharides, Disaccharides, Maltose, Lactose, Sucrose, Polysaccharides, Starch, Glycogen

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2
Q

Describe, in outline, the digestion and absorption of the main carbohydrates…

A
  • Monosaccharides are the simplest form of carbohydrates that are easily absorbed in intestine
  • A disaccharide (also called a double sugar or biose) is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose, lactose, and maltose.
  • Maltose (C12H22O11), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α-bond between the glucose units and is a two-unit member of the amylose homologous series which is the major structural component of starch.
  • Lactose is a sugar found in milk and milk products. Lactose intolerance happens when your small intestine does not make enough of a digestive enzyme called lactase. Lactase breaks down the lactose in food so your body can absorb it.
  • Polysaccharides, such as starch, chitin, glycogen, and cellulose, can be broken down into monosaccharides. This occurs through the process of hydrolysis, which uses water to break the bonds between monosaccharides.
  • Starch digestion involves the breakdown by α-amylase to small linear and branched malto-oligosaccharides, which are in turn hydrolyzed to glucose by the mucosal α-glucosidases, maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI).
  • Glycogen: The process of glycogen breakdown can occur either in the cytosol or in the lysosomes. In the cytosol, the glycogen phosphorylase catalyzes the nonreducing ends of glycogen branches, releasing glucose-1-phosphate. Its action stops 4 glucose residues before an α1→6 junction.
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2
Q

Describe and contrast the actions and functions of the enzymes hexokinase and glucokinase.

A

Hexokinase can phosphorylate a broad range of hexose sugars, has a higher affinity for glucose, and is inhibited by glucose-6-phosphate. Glucokinase is specific for glucose, has a lower affinity, and is regulated by glucokinase regulatory protein (GKRP).

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3
Q

Describe the synthesis and degradation of glycogen.

A

Insulin activates glycogen synthase and therefore increases glycogen synthesis in the liver and muscle. When blood glucose levels decrease, glucagon inhibits glycogen synthase and activates glycogen phosphorylase in order to break down glycogen in the liver.

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4
Q

Compare and contrast the functions of glycogen in skeletal muscle and liver.

A

Muscle it is converted into ATP and lactate after glycolysis and phosphorylation.

In the liver it is to regulate blood sugar levels.

ergo….
The liver breaks down glycogen to maintain adequate blood glucose levels, whereas muscles break down glycogen to maintain energy for contraction.

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5
Q

Describe the function and process of glycolysis.

A

Glycolysis is a series of 10 reactions that extract energy from glucose by splitting it into two three-carbon molecules called pyruvates.

  1. Phosporylation of glucose using hexokinase as the catalyst and 1 ATP
  2. Conversion of G-6-P to F-6-P
    using phosphohexose isomerase as a catalyst. The reaction can go either way due to low free energy
  3. Phosphorylation of F-6-P to F-1,6-bisP. Uses 1 ATP and a kinase catatlyst. After this point the glucose molecule is soley destined for glycolysis. Catalyst – phosphofructokinase-1 (PFK-1).
  4. Cleavage of F-1,6-bisP this has a small free energy change and so is reversible it uses a catalyst called aldolase. Catalyst – fructose 1,6-bisphosphate aldolase (or aldolase for short).
  5. Interconversion of triose sugars. Catalyst – triose phosphate isomerase.
    6.Oxidation of G-3-P to 1,3-bisPG
    7.P transfer from 1,3-bisPG to ADP
    8.Conversion of 3-PG to 2-PG, Catalyst – phosphoglycerate mutase
    9.Dehydration of 2-PG to PEP, catalyst enolase.
    10.Transfer of P from PEP to ADP, catalyst pyruvate kinase
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6
Q

Describe the functions of the lactate dehydrogenase- catalysed reactions.

A

Lactate dehydrogenase (LDH or LD) is an enzyme found in nearly all living cells. LDH catalyzes the conversion of pyruvate to lactate and back, as it converts NAD+ to NADH and back. A dehydrogenase is an enzyme that transfers a hydride from one molecule to another. Ergo it helps your cells make energy

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7
Q

Describe the fate of blood lactate.

A

In mammals, lactate is intramuscularly oxidized or converted to glycogen in the muscle and/or liver.

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8
Q

Describe the precursors for, functions of and process of gluconeogenesis.

A

Gluconeogenesis refers to the synthesis of glycogen or glucose from non-sugar precursors such as glycerin, lactic acid, and amino acids. Hepatic glycogen formation is the main product of gluconeogenesis.

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9
Q

Describe, in brief, the fates of absorbed galactose and fructose.

A

The liver takes them up and converts galactose to glucose, breaks fructose into even smaller carbon-containing units, and either stores glucose as glycogen or exports it back to the blood

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10
Q

Describe the functions of the pyruvate dehydrogenase-
catalysed reactions.

A

The pyruvate dehydrogenase complex (PDC)3 catalyzes the oxidative decarboxylation of pyruvate with the formation of acetyl-CoA, CO2 and NADH (H+) (1,–3). The PDC occupies a key position in the oxidation of glucose by linking the glycolytic pathway to the oxidative pathway of the tricarboxylic acid cycle.

Shortened version:
Pyruvate dehydrogenase (PDH) is a multienzyme complex that catalyzes the oxidative transformation of pyruvate to acetyl coenzyme A (CoA), CO2, and reducing equivalents in the form of NADH.

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