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lactic acid during exercise??

I do CCEA AS biology
I'm struggling to answer why the level of lactic acid during exercise period increases and why it continues to rise in the blood even after the exercise has stopped??

Can anyone help me??

Reply 1

saraht97

During exercise we use up oxygen supplies, so have to respire anaerobicaly in the cell cytoplasm which produces lactic acid.
Once we stop exercising we still have to do anaerobic respiration as there is still not enough oxygen - this continues until oxygen debt filled and lactic acid oxidised to carbon dioxide and water.

When the cells are respiring, due to high levels of excersize, there will come a point that the body is not able to pump enough 02 and Glucose around to respiring cells by the circulatory system fast enough.
What the body does to increase the speed these reach cells is increase heart rate, which increases blood pressure. However, once 02 availability is low, the cells must respire without o2.

This is by Anaerobic respiration, Lactate fermentation.
Normally In glycoloysis (which occurs in cells) Glucose is broken to to two molecules of pyruvate and this is then transported to the Link Reaction. However, the Link Reaction, along with the Krebs Cycle and Oxidative Phosphorylation cannot occur without the presence of O2 (As it acts as the final electron acceptor in the e- transport chain, forming H20)
So if the e- transport chain stops, so does the Link reaction and the Krebs cycle (despite the fact they don't need O2) It's like a production line. Once one stage is halted, so is everything else.

So Reduced NAD which is reduced in glycolysis (from the breakdown of Glucose to Pyruvate, remember) has to release it Hydrogen somewhere. And since it obviously can't donate its e- to the electron transport chain (which already has its own crap going on, what with being temporarily down), and it's Hydrogen ions therefore cant be used to make ATP by chemisomosis (as electron transport chain uses energy from electrons Reduced NAD donated to PUMP Hydrogen ions to the other side of the Inner Mitochondrial membrane (yeah, we are here now), and thus a proton/PH/Electrochemical gradient is CREATED! Those H+ are desperate to get out of the intermembrane space they are in, and trouble is..they can't diffuse through the inner mitochondrial membrane - cos its impermeable to them (Drats!) but not to worry, ATP Synthase, ever the knight in shining armour is on hand. It promises to allow hydrogen ions safe passage way through the intermembrane space, back to the beloved mitrochondrial matrix (where they started out) in exchange for the energy of this mass movement to Create ATP! HURRAH!

So.. back to Anaerobic respiration.. none of that can happen, (with me so far) cos, no O2, no e- transport chain e-, no O2 as final electron acceptor..yada yada yada.
So Reduced NAD (yes, it has a very important role yet) decides, hey!.. I need to get regenerated init, so I can go and help glycolysis continue (cos it like produced a net gain of 2 ATP molecules! - which are desperately needed by the body, so you know, muscle contraction during excersize can continue!)
So it decided to like dump its hydrogen atoms to Pyruvate which is like 'yeah sure, I'l accept'
and once it does.. totally cool.... it CONVERT into LACTATE!

But..alas. All is not so Good. Because lactate is quite bad. It forms Lactic acid, which reduced the PH of the enviroment of the cells, (Buffers try and resist this, but if the person has excersized a hell of a lot, and thus produced a larrge amount of lactate, - PH decreases inside the cell) This is bad news for those enzymes, such as NAD, which become denatured, cos of the 3D bonds disrupted, and active site getting changed.

Lactate is transported to the liver right, and stored there until excersize is over, and then converted back to Pyruvate, so that AEROBIC respiration can occur when O2 is present.