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Home Masters dr Krzysztof Michalak Wykłady z biochemii The second lecture Endorsers - Glycolysis, Cycle pentozowy, Los pyruvate, Krebs cycle

The second lecture Endorsers - Glycolysis, Cycle pentozowy, Los pyruvate, Krebs cycle


Lecture 2

In the second lecture of the series "Biochemistry in a nutshell," I would like today to introduce you the basic pathways of carbohydrate in the cells of our body. In other words, I tell you what happens after the entry of sugar into the cells? We start with carbohydrates, because their combustion is a kind of biochemical framework for the combustion of both proteins and fats. In this framework consists of three basic transformation pathways: glycolysis, the cycle and the Krebs cycle pentozowy. They will be presented in the most general, to minimize unnecessary at this stage of excessive detail. It seems important to first have a general view from the eyes of the whole, and then only to go into details.Handbook of Biochemistry, unfortunately, is inundated with details and we need a holistic view to a large extent to form themselves based on a thorough reading of multiple chapters. Combustion of proteins and fats, as well as the synthesis of cholesterol and fatty acids will be presented in a similar schematic for subsequent lectures. The starting point for the next consideration is the moment in which the various nutrients (B, T, W) has already got into the cell and must be incinerated or be used for intracellular synthesis. If you omit the entire stage of digestion, absorption into the blood and transport from the blood into cells.There will also be omitted for the time being, any chemical transformation leading to the formation of intracellular metabolism of starting points, namely, glucose, fatty acids and amino acids.


As for glucose, is, as I wrote the first lecture, it is a sugar with six carbon atoms, and thus belonging to a group of hexoses [i]. Of glucose converted into starch, which in practice means bread, cereals and potatoes, which are quantitatively the most important source of carbohydrates in traditional nutrition. Converted to glucose or galactose within the cell [ii], which is occurring sugar in milk.

Is somewhat different from fructose, the sugar present in fruit, sugar and in those with cukierniczki.There it turns to glucose, but she turns in her burning trail, bypassing an important step regulating the speed of this process. Due to the importance of this detail I remember about him today. Will be discussed in detail in a subsequent lecture.

Fats, although the blood are transported in various forms, inside the cell occur as a distributed, or as fatty acids and glycerol. As triglycerides [iii] only exist intracellularly in adipose tissue - as an energy reserve.

Protein while still in the digestive tract are distributed to individual amino acids, then through the blood reach the cells where they are further transformation.

Before we will discuss the fate of these three major nutrients, to bring you a little short construction of the cell. From the standpoint of combustion processes is necessary to award the intracellular two separate fluid spaces: the cytoplasm and mitochondria. The cytoplasm is the fluid that fills the cell.Immersed in the cytoplasm are other cellular organelles, which delimited pellicle space in which there are any special events or changes. Structure of this type of special importance in combustion processes are the mitochondria.

Mitochondria is one fairly unique intracellular structure. It is interesting, that has its own DNA, the genetic code to produce proteins from which it is built.

So it is as if this tiny micro-organism living in symbiosis with the cell.Reproduce inside our cells, providing instead of ATP, the energy for life processes of all cells. Since the mitochondria of sperm remains after its penetration into the egg destroyed as a foreign body, the entire mitochondrial DNA is always inherited only from the mother.

I write about mitochondria slightly wider, for it is in them is a lot of combustion of protein, fats and carbohydrates. Dual lipid membrane that separates the interior of the mitochondrion from the cytoplasm is an important barrier to the penetration of its different chemical compounds. There are specific transport mechanisms for some compounds, while some may pass freely through it. It is very important for the control of combustion processes and the processes of intracellular synthesis.Talking about each chemical reaction must always be clear whether it occurs in the cytoplasm or the mitochondrion. In "Harperze" this is not always adequately emphasized.

Almost all the reactions that occur in the mitochondria, are focused on the synthesis of ATP [iv] (adenozynotrójfosforowy acid). This compound is the most important in our body energy carrier. He has joined up to 3 high-energy phosphate groups. After removing a phosphate group is formed ADP (adenozynodwufosforowy acid). After disconnecting the second phosphate group is formed AMP (adenozynomonofosforowy acid). If any reaction or process in the body requires energy supply, in parallel with it there is a breakdown of the rule of ATP to ADP and phosphate free. If the decay is supplied too little energy to the appropriate incident response, ATP may disintegrate into AMP and pyrophosphate [v] providing a larger, double portions of energy. Briefly, phosphate-H2PO4 group will be determined in the reactions by P.

After this little introduction przydługawym finally move on to the proper topic of this lecture, the burning of carbohydrates.


The main trail is called the combustion of carbohydrates. pathway of glycolysis, which occurs in the cytoplasm of the cell. In this pathway a molecule C6H12O6 glucose is converted into two molecules of pyruvic acid, CH3-CO-COOH. Pyruvic acid (also called pyruvate in short) is the most important nodal point in intracellular metabolism. It is this molecule that easily penetrates into the mitochondria would there be further changes.

Further intermediates on the path from glucose to pyruvate in the glycolysis pathway are:

1st glucose-6-phosphate. As I said at the first lecture, the combination of the carbon chain of the phosphate group forms a high-energy bond. Joining a phosphate group to the 6-carbon glucose it requires consumption of 1 molecule of ATP. The presence of a phosphate group makes the molecule, having had more energy, easier to enter into further reactions.

2nd fructose-6-phosphate. Followed by rearrangement of atoms within the molecule. At this point, after phosphorylation, the trail enters fructose.

3rd Fructose-1 ,6-diphosphate. Followed by subsequent attachment of a phosphate group at the expense of the next molecule of ATP.

4th gliceraldehydo-3-phosphate (2x). Followed by disintegration of 6-carbon chain to two 3-carbon chains. All other changes are double compared to the initial molecule of glucose.

5th 1,3-dwufosfoglicerynian (2x). Followed by separation of two hydrogen atoms connected with the connection of another phosphate group. The hydrogen atoms are transferred to a complex molecule, whose main role is to transport hydrogen. In short, we call it NAD [vi]. (Transport of hydrogen is an important element of change and will be discussed separately). Joining a phosphate group, this time does not require ATP.

6th 3-fosfoglicerynian (2x). Disconnection of a phosphate group is coupled with recovery of ATP.

7th 2-fosfoglicerynian (2x). Transfers a phosphate group from the third carbon to the other.

8th fosfoenolopirogronian (2x). Followed by separation of water molecules.

9th pyruvate (2x). There is a disconnection of a phosphate group linked with the synthesis of ATP molecules.

To not go into excessive detail, this trail can be globally represented by the following equation:

C6H12O6 (glucose) + 2 NAD + 2 ADP + 2 P ® 2C3H4O3 (pyruvate) + 2 + 2 ATP NADH2

In the glycolysis pathway are consumed at the beginning of two ATP molecules to attach phosphate groups, but then recovered are 4 molecules.Total produced so two molecules of ATP and 2 molecules NADH2.Hydrogen from NADH2 be transported to the mitochondria, and there be burned with oxygen. The emergence was 6 with 2 molecules of ATP molecules NADH2.

2 NADH2 + O2 + 6 ADP + 6 P ® 2 NAD + 2 H2O + 6 ATP

In total, so at this stage of combustion of glucose from a glucose molecule produced eight molecules of ATP.

Pentose phosphate pathway

This is an alternative route bypassing the combustion of glucose allows certain stages of glycolysis.Like glycolysis, it occurs entirely in the cytoplasm of the cell. For a better understanding of this somewhat complex route, consider the fate of three molecules of glucose:

1st glucose-6-phosphate (3x). (In brief: glucose-6P). As in the glycolysis reaction.

2nd 6-fosfoglukonian (3x). There is a disconnect two hydrogen atoms from each molecule and transfer them to NADP [vii]. (It is very important to distinguish whether the hydrogen is transferred to the NAD or NADP on, but more on that later).

3rd ribulose-5P (3x). After another two hydrogens are transferred to NADP, is also disconnected CO2 molecule. Rybuloza is a 5-carbon sugar, and therefore belongs to pentoses. (Because of the phosphate cycle is always connected to the last carbon cyferka sugars in the names of the next intermediate is also a quantity of carbon atoms in the molecule).

4th ksylulozo-5P (2x), ribose-5P (1x). Rearrangement of atoms within molecules.

5th ksylulozo-5P (1x), sedoheptulozo-7P (1x), gliceraldehydo-3P (1x).Rearrangement of atoms between molecules.

6th ksylulozo-5P (1x), fructose-6P (1x), erytrozo-4P (1x). Further rearrangement of atoms between molecules.

7th fructose-6P (2x), gliceraldehydo-3P (1x). Further rearrangement of atoms between molecules.Both the resulting compounds are intermediate products of glycolysis. So they can be burned, including for this cycle. You can also get the opposite reaction to glycolysis, which may be rebuilding glucose-6P, which closes the pentose cycle. glucose-6P can then re-enter the reactions of this cycle.

Picture this cycle appears to be probably as far as not too bright. For our needs, so just remember the reaction of combustion of a sum of glucose molecules after passing the appropriate number of times this cycle, which is much simpler in the receipt:

C6H12O6 + 12 NADP + 6 H 2 O ® 12 NADPH2 + 6 CO2

Here we see that 1 molecule of glucose without oxygen, using extra oxygen and hydrogen contained in water, breaks down into carbon dioxide and hydrogen. A large amount of hydrogen formed in this cycle (transmitted by NADP, but not NAD) is then used for the synthesis of fatty acids and cholesterol. Notice also interesting that the resulting hydrogen in this series comes in the middle of sugar and half water. It will not be so factually wrong, if somewhat ironically and playfully write that adipose tissue, the synthesis of which is consumed, arises from the water. Of course participation is required an excess of carbohydrates ☺. (But let no one case did not think that as a drinking water limit, you can eat sugar with impunity! ☺)

Pyruvate tract

But back to the tone a bit more serious. As I said, pyruvate compound is crucial in intracellular metabolism. It is the final stage of pre-combustion of glucose, and (as we shall see in a subsequent lecture), most amino acids. It is this compound that freely permeates the cytoplasm to mitichondrium, would there be further changes. The mitochondrion has two possibilities of change.The first one, leading to burning, the conversion of acetyl-coenzyme A [viii]. In chemical reactions, this compound is written in short as CH3-CO-CoA or simply acetyl-CoA. During this reaction, CO2 is disconnected, and the two hydrogens are transferred to NAD to later in the course of combustion with oxygen to produce three molecules of ATP:


Acetyl-CoA is the second very important, crucial link in the intracellular metabolism. It is a molecule that can be either burned in the mitochondria, or get out of the mitochondrion and to be consumed for the synthesis of fatty acids or cholesterol in the cytoplasm of the cell.

But returning to the pyruvate: second possibility that can happen to him outside the reversal of acetyl-CoA is conversion to oxaloacetate. The primary goal of this transformation is to provide intermediates for the Krebs cycle, which in a moment. But what are the more distant consequences, we will find out on one of the next lecture, because apparently a very complex matter, in a more detailed analysis will be very nice to simplify. However, this will be explained when discussing the burning of amino acids.

Krebs Cycle

Krebs cycle is a sequence of reactions in the mitochondria, allowing the burning of a molecule of acetyl-CoA. Krebs cycle is the first part of the combustion of acetyl-CoA, which occurs during the distribution of hydrogen H2 and carbon dioxide CO2. Hydrogen is then used to synthesize ATP, which occurs during its combustion with oxygen. CO2 is removed.

Subsequent reactions of this cycle are as follows:

1st oxaloacetate + acetyl-CoA + H2O ® citrate + CoA

(Transfer of acetyl residues from CoA to oxaloacetate)

2nd citrate ® izocytrynian

(Transformations inside the molecule)

3rd izocytrynian + NAD + ® szczawiobursztynian NADH2

(Transfer of the first two carbons of NAD)

4th szczawiobursztynian ® a-ketoglutarate + CO2

(Disconnection of a molecule of CO2)

5th a-ketoglutarate + CoA + NAD ® succinyl-CoA + CO2 + NADH2

(Disconnect the CO2 molecule and two subsequent transfer of hydrocarbons on the NAD, temporarily taking part in this molecule CoA)

6th succinyl-CoA + H2O + ADP + P ® succinate + CoA + ATP

(Disconnection CoA is coupled with one molecule of ATP synthesis)

7th succinate + FAD + FADH2 ® fumarate

(Separation of two consecutive carbons, this time to participate in this other conveyor: FAD [ix])

8th fumarate + H2O ® malate

(Connection of the water molecules)

9th malate + NAD + oxaloacetate NADH2 ®

(Subsequent transfer to NAD bons, return to the starting oxaloacetate)

These reactions can be summarized as follows:

CH3CO-CoA + 3 H20 + 3 NAD + FAD + ADP + P

® CoA + 2CO2 + 3 + FADH2 + NADH2 ATP.

Since one molecule can be obtained NADH2 three ATP molecules, while the FADH2 can be obtained only 2 ATP molecules, including a complete combustion cycle acetyl-CoA gives a total of 12 molecules of ATP (3 × 3 with NADH2 + 1 × 2 FADH2 + 1 directly Krebs cycle).

If you are adding up all of the ATP molecule that may arise when burning a sugar molecule, then we get: 8 ATP (glycolysis) + 2 × 3 ATP (pyruvate ® acetyl-CoA) + 2 × 12 ATP (Krebs cycle).Together this gives 38 ATP molecules.

To compare the energy value of different compounds will, however, makes more sense to calculate the amount of ATP formed from the complete combustion of 100 grams of the substance. In the case of glucose obtained from 100g of 21.1 mol [x] ATP.

That would be enough when it comes to today's lecture. I invite you to the next, which is a continuation of today, which I will discuss the changes of intracellular pathways for synthesis and fat burning. Talk to you soon.

[And] hexose - a group of simple sugars, which is composed of six carbon atoms. The second important group are pentose sugars, the sugars made up of five carbon atoms. All hexoses are chemical formula C6H12O6. They differ among themselves while the spatial distribution of OH groups and the group = O.

[Ii] Galactose is a simple sugar like glucose. Its main source is milk. The milk is disaccharide - lactose, which is broken down into glucose and galactose.Similarly, sucrose decomposes into glucose and fructose.

[Iii] triglyceride molecule is composed of one molecule of glycerol and three fatty acid molecules (see Lecture 1)

[Iv] ATP - adenozynotrójfosforowy acid. Is composed of adenine, ribose and three phosphate molecules are connected in series ..

[V] pyrophosphate is a combination of two phosphate groups. In short, is designated as the PP.

[Vi] NAD - nikotynamidoadeninowy dinucleotide. Her character is hydrogenated NADH2. It is one of the two main conveyors of hydrogen in chemical reactions.

[Vii] NADP (nicotinamide adenine dinucleotide phosphate) is the second (next to NAD), an important conveyor of hydrogen in the cell. Occurs only in the cytoplasm. The figure is hydrogenated NADPH2.

[Viii] coenzyme A is a complex molecule, whose formula does not make sense to present. At its end is the group-SH, which reacts with-OH group of acetic acid and formed CH3-CO ~ S-CoA (acetyl-coenzyme A). During this reaction, followed by disconnection of water molecules. The central role of CoA molecules are moving acetic acid (acetyl residues). It plays an important role in the synthesis of fatty acids. To not have any problems with the counting of hydrocarbons in chemical reactions it is desirable to distinguish terminal-SH group of this complex molecule and save it as: CoA × SH.

[Ix] FAD - adenine dinucleotide flavonoids. One of conveyers of hydrogen inside the mitochondrion.Among others mediates the transfer of hydrocarbons from NAD to oxygen.

[X] 1 mol, as I said, this 6023 × 1023 pieces (like a dozen is 12 pieces.)

Author: Krzysztof Piotr Michalak. All rights reserved.

The second lecture Endorsers - Glycolysis, Cycle pentozowy, Los pyruvate, Krebs cycle

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