Glycolysis

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Model and understand aerobic respiration demonstrating the flow of matter and energy out of a cell and explain energy transfer systems. Also, compare aerobic respiration to alternative processes of glucose metabolism.

Article includes ideas, images, and content from Troy Smigielski (2021-10)

Glycolysis

(Someone should come up with a spicer name for this!)

Glycolysis is the first step in cellular respiration. Of the three steps it is different in a couple of ways:

• It does not require oxygen, which makes it anaerobic.
• It occurs in the cytoplasm (the cytosol) of the cell; not the mitochondrion.

In cellular respiration, you’ll notice that if the process is anaerobic, then it will happen outside of the mitochondrion.

Conversely, if it is aerobic, it will happen in the mitochondria.

Before we get too far, let's recall a few things. Keep in mind that…

… glucose is one of the reactants of cellular respiration. Where does the glucose come from? You will recall that plants make glucose as a product of photosynthesis. 

Don't forget the main purpose of cellular respiration (of which glycolysis is a step) is to convert the energy stored in glucose into energy stored as ATP. In other words, cellular respiration breaks down glucose and turns it into ATP.

If this process breaks down glucose, what does our starting material have to be?

Glycolysis starts with one glucose molecule.

Okay… fancy word… Lysis means to break.

So, glycolysis means “glucose broken”.

Using 2 ATP, the glucose molecule is broken into two 3-C molecules. (Wait! I thought we were trying to MAKE ATP. Are we doing this backwards?)

Next, 4 ATP and electrons are extracted from the two 3-C molecules. The electrons are placed onto NAD+, and this creates NADH, which is one of the electron carriers in cellular respiration.

(So… NADH… is that like the NADPH in photosynthesis?)

Just as the NADPH in photosynthesis carried electrons, the NADH carries electrons throughout cells as needed. NADH, is an electron carrier.

Electron carriers function to carry electrons around the cell. It is these NADH molecules that are used to transport electrons to the ETC (Step 3).

This extraction of ATP and electrons turns the 3-C molecules into pyruvate (pyruvic acid).

In basic terms, glycolysis turns glucose into 2 pyruvate.

But, it also makes some ATP and and NADH.

Staring with one glucose molecule at the beginning, by the end of glycolysis, the following have been produced:

• 2 pyruvate (2 pyruvic acid)
• 2 NADH
• 2 ATP (2 where used up and 4 were created, so you end up with a net gain of 2, because math. )

During glycolysis, 2 ATP molecules are used. However, by the end of the process, 4 ATP molecules are created. So, if you lose 2 but gain 4, there is a net gain of +2 ATP.

What happens after glycolysis?

Normally, your body uses oxygen after glycolysis to go on to the Krebs Cycle. However, when oxygen is not available, your body resorts to fermentation.

Fermentation

Since fermentation happens in the absence of oxygen, is it aerobic or anaerobic?

Since fermentation happens when no oxygen is available, that makes this process anaerobic. Considering it does not require oxygen, this also happens in the cytoplasm of the cell. If a process is anaerobic, it will happen in the cytoplasm.

But… why? Why is this a thing?

In order for an organism to survive, it needs energy. The goal of fermentation is to keep glycolysis going so your body can still get at least 2 ATP because you have no other source of ATP without oxygen.

To do this, fermentation converts NADH back into NAD+ so that glycolysis can start again in order to produce those 2 ATP. 

If the NADH molecules cannot go to the ETC, then they are essentially useless. So, your body finds a use for them.

There are 2 main types of fermentation in nature:

• Alcoholic - occurs in bacteria and yeast
• Lactic acid - occurs in humans (animals)

In humans, the lactic acid pathway provides your body with small amounts of ATP during intense exercise. Remember, this is anaerobic, so it happens when your muscles’ oxygen demand is greater than the oxygen supply.

So, if skeletal muscles are used (such as during vigorous exercise) when there is not enough oxygen, they produce lactic acid instead of carbon dioxide.

Because this process only produces 2 ATP, this cannot keep you going for a long time.

As this pathway continues, lactic acid builds up in the body. This gives you a burning sensation and can cause nausea and vomiting.

Um… I suppose there must be some way to get rid of lactic acid build up?

To get rid of this lactic acid buildup, you simply need to rest and breathe.