Carbohydrate metabolism – part one

Carbohydrate metabolism

Part one

 In carbohydrate metabolism we will study, the types of carbohydrate, how they are digested and absorbed, then we will study the cycles of carbohydrate oxidation which generate the energy required for life…. these cycles are:

  1. Glycolysis  = Glucose oxidation (i.e. break down of glucose to get energy).

  2. Frucose metabolism.

  3. Galactose metabolism.

  4. Gluconeogensis = formation of Glucose form non-carbohydrate sources.

  5. Kerbs cycle.

  6. Glycogen Metabolism including breaking and formation of glycogen.

  7. Pentose phosphate pathway = which generate NADH

A) Types of carbohydrates:

1. Monosaccharides

==> They are those carbohydrates that cannot be hydrolyzed into simpler carbohydrates.

==> They may be classified as trioses (3-carbon sugar), tetroses (4-carbon sugar), pentoses (5-carbon sugar), hexoses (6-carbon sugar), or heptoses (7-carbon sugar).

2. Disaccharides:are condensation products of two monosaccharide units. Examples are maltose and sucrose.

3. Oligosaccharidesare condensation products of 2 to 10 monosaccharides; Example: maltotriose

4. Polysaccharidesare condensation products of more than ten monosaccharide units; examples are the starches and dextrins, which may be linear or branched polymers.

B)   Carbohydrate digestion:

After eating, digestion begins as follow:

1) In the mouth, salivary amylase, hydrolyze starch partially into a mixture of dextrins and maltose.

2) In the stomach, salivary amylase continues hydrolysis of starch only for few minutes then stop because the pH becomes acidic due to the presence of the HCl in stomach and this is unfavourable condition for the amylase to work.

3) In the intestine, pancreatic amylase, completes the digestion of starch into maltose with little isomaltose and maltotriose which are then hydrolyzed in the intestine into glucose. Fructose and Galactose.

Starch + H2O   ===== Amylase ====>  Dixterns + Maltose

Dixterns + H2O  ===== Amylase ====> Maltose + Isomaltose + Maltotriose

Maltotriose + H2O ===== Maltase====>Maltose + Glucose

 Maltose + H2O  ======== Maltase====> Glucose + Glucose

Isomaltose + H2O===== Isomaltase====> Glucose + Glucose

Sucrose + H2O========== Sucrase=====>Glucose + Fructose 

Lactose + H2O ========Lactose=======>Glucose + Galactose

4. Sucrose and the enzymes that complete hydrolysis into Monosaccharides presents in the mucus layer of the intestine

5. The net result of carbohydrate hydrolysis is glucose, fructose and Galactose



C) Absorption:

– The polysaccharides and oligosaccharides are not absorbable so, they must be converted to Monosaccharides.

– Monosaccharides are principally absorbed from the duodenum then pass into the blood through the hepatic portal vein to the liver where:

  • Part of these monosaccharides is stored as glycogen and part is oxidised through glycolysis to produce energy

  • Part is oxidised through the pentose phosphate pathway to regenerate NADPH which, together with glucose itself, is used in synthesis of such molecules as amino acids, nucleotides, fats and cholesterol

  • Part is oxidised to produce energy (ATP) which is used in the anabolism processes.

Before we know the mechanism by which the glucose is absorbed into the cells we must know the composition of the cell membrane through which the glucose passes.

this part will help any one who don’t study biology or cell before to be able to understand biochemistry

Composition of the cell membrane:

==>  As from the previous picture, the cell membrane is composed of 2 layers each of them consists of a layer of phospholipids.

==>  Each phospholipid molecule consists of a polar hydrophilic head and a non-polar hydrophobic tail.

==> How the bilayer membrane is formed: And as we know, the extracellular and intercellular fluid are polar, therefore the polar heads are arranged towards the polar fluids and the non-polar tails arranged toward themselves  inward the membrane where there are a hydrophobic interaction between them and thus forming a phospholipids bilayer membrane

==> And across the membrane there are transport proteins and receptor proteins

==> Function of the cell membrane:-

  • Protection of cells.

  • On the other hand, it represents a barrier which prevents entry of some molecules into cells such as polar molecules but it uses other mechanism by which the molecules enter the cells such as Na-K pump through which ions are transported into the cell.

In case of glucose:

Any polar molecules can’t pass through the inner non-polar layer of cell membrane so, how glucose enter the cell through the cell membrane while it is polar?

===> The answer is that glucose has 2 mechanism to enter the cells:

1) The passive diffusion or transport:

===> Definition: it transport of biochemical and other atomic or molecular substance across the cell membrane into the cell from higher concentration region to a lower concentration region without any need for energy. (i.e. the substance enter the cell only if its concentration outside the cell is more than its concentration inside the cell.)

===> It depends on the concentration gradient.

===> It doesn’t need energy.

===> Depends on the permeability of the cell membrane.

===> Types:

  • Simple diffusion: in which the substance are transported from higher concentration to lower concentration region through the phospholipids bilayer without any need for energy and without using the transmembrane proteins (carriers/transporters/channels/pores).

  • Facilitated diffusion or passive-mediated-transport: in which the substance are transported from higher concentration to lower concentration region through the phospholipids bilayer using the transport proteins  and without any need for energy.

–    Fructose and pentose use this mechanism.

–    Glucose is transported by this mechanism into brain, kidney and liver.

2) Active diffusion or transport

===> Definition: it means transport of a substance from region of lower concentration to a region of higher concentration (i.e. against the concentration gradient) utilizing energy from ATP molecules and carrier proteins.

===> It doesn’t depend on the concentration gradient.

===> It needs energy.

===> This mechanism applied by the cell to accumulate high concentration of molecules that the cell needs such as ions, glucose and amino acids.

===>Doesn’t depend on the permeability of the cell membrane because this mechanism transports the substance through the transport proteins and carrier proteins.

===> It has 2 types:

  • Primary active transport: in which the cell uses chemical energy such as ATP.

  • Secondary active transport: in which the cell uses electrochemical gradient such as sodium and potassium dependent ATP bump

===> It is the mechanism by which glucose is transported from intestinal tract

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