glycolysis

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Biochemistry notes…. After studying biochemistry for two weeks I must say I have developed a love-hate relationship with this subject kind of like with the anatomy.I love learning about the metabolism of carbohydrates, everything makes sense, every puzzle belongs on the right place. But I cannot say the same for enzymes. I just hate haye that part of biochemistry. Learning about enzyme kinetics, alosteric enzymes…. it’s just not fun at all for me. Maybe I haven’t found the method of studyng enzymes. Any suggestions? My biochem exam is in three weeks . So for the next two weeks I will have to learn everything about vitamins, cellular respiration(which hopefully I will finish today), gluconeogenesis, pentose phosphate pathway and everything about the metabolism of lipids. So fantastic?! Am I right? the perks of being a med student

Originally posted by datgifarchive

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Hey guys! For my first studyblr original post, I wanted to explain glycolysis. I found it way easier to learn it in terms of “energy-requiring” and “energy-releasing” phase, so here it is :) Hope you like it

Cell Respiration: Glycolysis, Pyruvate Processing, and The Citric Acid Cycle

glycolysis is the first of a set of series of reactions that oxidizes glucose in cellular respiration. it occurs in the cytoplasm of the cell.

start with glucose.

1. phase 1 of glycolysis begins. one molecule ATP is used, dephosphorylated to ADP and Pi. glucose changes into glucose-6-phosphate.

2. glucose-6-phosphate becomes fructose-6-phosphate (5-carbon sugar instead of 6-carbon sugar)

3. one molecule ATP is used, dephosphorylated to ADP and Pi. allosteric inhibition via ATP binding to enzyme phosphofructokinase when ATP levels are too high (feedback inhibition). phosphofructokinase catalyzes change of fructose-6-phosphate to fructose-1,6-biphoshphate.

4 and 5. fructose-1,6-biphosphate splits into two molecules: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate

6. phase 2 of glycolysis begins. two molecules NAD+ are reduced to become NADH. two glyceraldehyde-3-phosphates become two 1,3-biphosphoglycerates. 

7. two molecules ADP are phosphorylated to two molecules ATP. this is energy payoff of two ATP molecules used in phase 1. the two 1,3-biphosphoglycerates become two 3-phosphoglycerates.

8. two 3-phosphoglycerates become two 2-phosphoglycerates.

9. two 2-phosphoglycerates become two molecules phosphoenolpyruvate.

10. two molecules ADP are phosphorylated to two molecules ADP. two molecules phosphoenolpyruvate are dephosphorylated to two molecules pyruvate.

end product of glycolysis: 2 NADH, 2 ATP, 2 pyruvate.

pyruvate moves from cytoplasm into mitochondrial matrix.

it loses a carbon via carbon dioxide, is oxidized when an NAD+ is reduced to NADH, and reacts with coenzyme A to form acetyl CoA.

citric acid cycle takes place in mitochondrial matrix and is the final set of reactions of glucose oxidation. cycle revolves twice for each molecule of glucose.

start with acetyl CoA.

1. input of water. allosteric regulation via ATP which inhibits acetyl CoA becoming citrate when ATP levels are too high. 

2. citrate becomes isocitrate.

3. NAD+ reduced to NADH. loss of a carbon via carbon dioxide. isocitrate becomes alpha-ketoglutarate. allosteric inhibition via ATP when ATP levels are too high; competitive inhibition via NADH when NADH levels are too high.

4. NAD+ reduced to NADH. loss of a carbon via carbon dioxide. alpha-ketoglutarate becomes succinyl-CoA. allosteric inhibition via ATP when ATP levels are too high; competitive inhibition via NADH when NADH levels are too high.

5. ADP is phosphorylated to become ATP (or GTP to GDP). succinyl-CoA becomes succinate.

6. FAD+ is reduced to FADH2. succinate becomes fumarate.

7. input of water. fumarate becomes malate.

8. NAD+ is reduced to NADH. malate becomes oxaloacetate and one rotation of the citric acid cycle is finished.

Glycolysis

The name “glycolysis” is delightfully fitting: glyco means “carbohydrate”, and lysis means “splitting”. That’s exactly what this first step in cellular respiration does: split a carbohydrate. Glycolysis occurs in the cytoplasm so it doesn’t need any special organelle, which means that every living organism can do it. It also doesn’t require oxygen—remember that, because it’ll be important later on.

Here’s what happens: 1 glucose molecule (a six-carbon sugar) is split into two three-carbon sugars, which are oxidised and arranged to form 2 molecules of pyruvate, 2 NADH, and 4 ATP.

Really, there are six steps in glycolysis, but they involve a whole bunch of enzymes that we don’t need to worry about (if you study further biochemistry, you’ll have to worry about it, so good luck with that). What we need to know is that the whole process can be split into two short phases: energy investment and energy payoff. Glycolysis can’t just create ATP from nothing—it actually invests two ATP molecules in order to run the processes to get more back.

In the energy investment phase, a phosphate group is taken from each ATP molecule and attached to the 6-carbon glucose molecule. This process is called phosphorylation, and causes the ATP molecules to become ADP. The glucose molecule is then split in half, forming two 3-carbon sugars with a phosphate attached to each. These are called Glyceraldehyde-3-Phosphate (G3P).

In the energy pay-off stage, the G3P molecules are given an inorganic phosphate group each, and simultaneously transfer one hydrogen atom each to two molecules of NAD+, creating two molecules of NADH (a coenzyme that carries electrons). The G3P molecules are therefore oxidised (because they lose electrons) and the NAD+ is reduced (because they gain electrons).

Four ATP molecules are then produced by substrate-level phosphorylation, which is a process where phosphate groups are given directly to ATP. (Note: Be aware that there’s a difference between substrate-level phosphorylation and oxidative phosphorylation; we’ll talk about it soon).

So, the debt of the investment phase is paid off—glycolysis used up two molecules of ATP and got four back, giving us a net profit of 2 ATP.

These phosphate groups were taken from our G3P molecules, and once they’re gone, our 3-carbon sugars rearrange to become two 3-carbon molecules of pyruvate. The carbon bonds of pyruvate have a lot of chemical energy stored in them, and in the next few stages of cellular respiration, I’ll show you how this energy is extracted.

Here’s a breakdown of what we’ve done:

  • 6-carbon glucose is broken down into two 3-carbon pyruvate molecules.
  • 2 NAD+ have been reduced to 2 NADH.
  • 2 ATP have been invested, yielding 4 ATP—with a net gain of 2 ATP.

At this stage, we come to a crossroads. Up until this point, we haven’t needed oxygen to do anything, but now there are two options: if oxygen is present, we can go onto the citric acid cycle and complete aerobic respiration. If oxygen isn’t present, we can go onto fermentation.

Further resources: Khan Academy: Glycolysis (Khan Academy literally got me through my bio class so excuse me if I link it a lot)

MEMORIZE URGHGGHHH

glucose ——hexokinase—–> glucose-6-phosphate

glucose 6 phosphate——-glucose-6-phosphate isomerase—-> fructose-6-phosphate

fructose-6-phosphate—-phosphofructokinase—–> fructose-1,6-bisphosphate

fructose-1,6-bisphosphate—–aldolase—> dehydroxyacetone phosphate + glyceraldehyde-3 phosphate

dehydroxyacetone phosphate ——triosephosphate isomerase —> glyceraldehyde 3- phosphate

2 glyceraldehyde 3- phosphate —–glyceraldehyde 3 phosphate dehydrogenase—–> 1,3- bisphosphoglycerate

1,3 bisphosphoglycerate —–phosphoglycerate kinase —-> 3 phosphoglycerate

3 phosphoglycerate—-phosphoglycerate mutase—-> 2 phosphoglycerate

2-phosphoglycerate—-enolase—> phosphoenolpyruvate

phosphophenolpyruvate—-pyruvate kinase—-> pyruvate

Taking Bio in 9th Grade: The mitochondria is the powerhouse of the cell.

Taking AP Bio in 11th Grade: The mitochondria is the powerhouse of the cell because it takes glucose and oxygen and breaks them down through the process of glycolysis and produces a net of 2 ATP and 2 pyruvate which in turn goes through a transition reaction and the Kreb’s Cylce which in turn produces another 2 ATP and 10 NADH which then goes through the Electron Transport Chain and produces approximately 34 ATP and that is why it is considered the powerhouse of the cell