Glycolysis (Cellular Respiration) — Summary & Steps

What Is Glycolysis?

Glycolysis is a set of anaerobic reactions, ones that does not need oxygen to be performed. It takes glucose and breaks it down into pyruvate. This has the result of creating two ATP, which can release energy.

Overall, this process creates four ATP, but it uses two ATP early in the reaction. This means that there is a net gain of two ATP.

The overall equation for this pathway is:

1 glucose + 2 ATP ⟶ 2 pyruvate + 2 NADH + 4 ATP + 2 H2O

What Are the Steps? What Enzymes Are Used?

Step 1: The process starts with phosphorylation of glucose, a 6-carbon sugar, into glucose-6-phosphate. The enzyme hexokinase removes a high energy phosphate group from an ATP and places it on a glucose, which results in an ADP and the glucose-6-phosphate.

Step 2: The molecule is made into an isomer by phosphoglucose isomerase, turning G6P into fructose-6-phosphate.

Step 3: F6P is phosphorylated. The enzyme phosphoglucose isomerase removes a phosphorus from an ATP and places it on F6P, which results in an ADP and fructose-1,6-biphosphate.

Step 4: The enzyme aldolase breaks F6P into G3P and DHAP (dihydroxyacetone phosphate).

Step 5: DHAP is converted into G3P by triose phosphate isomerase.

Step 6: The enzyme G3P-dehydrogenase removes hydrogens from each G3P and phosphorylates the molecules. In doing this, it converts two NAD+ into two molecules of NADH. This step creates two molecules of 1,3-biphosphoglycerate.

Step 7: The enzyme phosphoglycerate kinase removes a phosphorus from the 1,3-biphosphoglycerates. This converts two ADP into two molecules of ATP. This step creates two molecules of phosphoglycerate.

Step 8: The enzyme phosphoglycerate mutase moves the phosphorus into the center of the molecule.

Step 9: The enzyme enolase changes the orientation of the molecule. This step creates two molecules of phosphoenolpyruvate.

Step 10: Pyruvate kinase removes the final molecules of phosphorus, forming ATP molecules. This step creates two molecules of pyruvate, which can then be used in other reactions such as the Krebs cycle.

The glycolytic pathway can occur with or without oxygen, but it is slightly different in anaerobic conditions.

To begin, a molecule of glucose and two molecules of ATP are used to make two molecules of pyruvic acid (or pyruvate), four NADH, and four ATP. Now I'm gonna skip right to the part that's different when oxygen isn't around. If you'd like to see a detailed outline of the reactions that take place during glycolysis, check out one of the other explanations in this section.

Without Oxygen:

NADH is normally an electron donor molecule, meaning it transports and donates electrons from glycolysis to the electron transport chain. This process helps the cell make large amounts of ATP.

The problem is that the electron transport chain only works when oxygen is around. When oxygen isn't present, NADH returns electrons to the pyruvic acid which turns it into NAD$^+$. The NAD$^+$ is now an electron acceptor. This means that it can be used to carry out more glycolysis. It won't make as much ATP as the electron transport chain, but it will help keep the cell going. The pyruvic acid is then converted to other products, like alcohol or lactate, in a process called fermentation.

What Does Glycolysis Produce?

Glycolysis is the first step in cellular respiration and it's the only one that doesn't require oxygen. It is anaerobic (it happens in the absence of oxygen).

In this pathway:

1 molecule of $\color{#E1528D}{\text{glucose}}$ is broken down and a net of 2 adenosine triphosphate ($\color{#FFA400}{\text{ATP}}$) are produced.

• 4 $\color{#FFA400}{\text{ATP}}$ are actually produced, but it costs the cell 2 $\color{#FFA400}{\text{ATP}}$ to carry out the reactions. So, the leftover gain of $\color{#FFA400}{\text{ATP}}$ is 2. That's why we say "net total (or yield) of 2 $\color{#FFA400}{\text{ATP}}$".

It's important to keep track of the number of carbons involved in these reactions. Note that $\color{#E1528D}{\text{glucose}}$ has $\color{#E1528D}{\text{6 carbons}}$ (6C). When it is split into 2 $\color{#13C2C2}{\text{pyruvate}}$ molecules, each of those molecules has $\color{#13C2C2}{\text{3 carbons}}$ (so there's still a total of six).

The glycolytic pathway occurs in the cytoplasm, which means it doesn't require an organelle like the other two steps of cellular respiration do. That's why, this process also occurs in prokaryotes, like bacterial cells.

Here's a short summary of this process:

Key Vocabulary

1. Cellular respiration
2. Glycolysis
3. ATP
4. Anaerobic

What Is Glycolysis?

Glycolysis is the first step in cellular respiration. One molecule of glucose is broken down into two pyruvic acid molecules, releasing ATP and NADH. Remember that ATP and NADH are sources of energy for the cell.

This process takes place in the cytoplasm of a cell. The rearrangement of the bonds within ATP, NADH, and the original glucose molecule releases energy that the cell can harvest.

Glycolysis literally means "splitting sugars". The prefix glyco- means sugar, while the suffix -lysis means to dissolve, split, or decompose. (Remember lysosomes?) The breaking down of glucose to harvest chemical energy (ATP) is a process known as catabolism. These steps do not require any oxygen, so it is an anaerobic process. Anaerobic means "without air", while aerobic means "with air".

Step 1:

The first step in glycolysis requires two molecules of ATP to kick off. These two ATP molecules each lose a phosphate group to break the glucose into two pyruvic acid molecules, becoming adenosine diphosphate (ADP).

Step 2:

In the next step, high-energy electrons are removed from the pyruvic acid molecules and combined with the electron carrier NAD$^+$ (nicotinamide adenine dinucleotide) to form NADH. NADH holds onto the electrons and carries them to the electron transport chain.

Step 3:

At the same time, four ADP molecules gain phosphate groups and are transformed back into ATP.

So, at the end of glycolysis, we have gained two ATP molecules (two in, four out), two NAPH molecules, and two pyruvic acid molecules.

$$\color{#8925ae}{\text{Vocabulary Review:}}$$

$\color{#8925ae}{\text{Glycolysis}}$ : A process that converts glucose molecules into usable energy

$\color{#8925ae}{\text{Cellular Respiration}}$ : A set of complex chemical reactions that provides a cell with energy

$\color{#8925ae}{\text{ATP}}$ : A molecule that stores energy in its bonds. The cell is able to break these bonds to release energy

$\color{#8925ae}{\text{Phosphorylation}}$ : Enzymes are able to break a bond between phosphate groups on ATP, turning it into ADP. This releases energy that the cell can use

$\color{#8925ae}{\text{Anaerobic}}$ : A reaction that does not need oxygen to occur

(Video) Glycolysis Rap

by sciencemusicvideos

Glycolysis is an anaerobic process that takes place in the cytoplasm.

Overall, it takes a sugar molecule, called glucose and breaks it down into 2 pyruvate molecules, NADH, and 4 ATP (a net yield of 2 ATP).

Remember the steps with Mr. W's rap music video!

There are three main parts:

Investment:

In this part, 2 ATP molecules are needed to get the reactions of the pathway going.

Cleavage:

During this part, glucose (a 6-carbon sugar) is split into two 3-carbon molecules called pyruvate.

Harvest:

This is where the reactions produce 4 ATP. Since we originally needed an investment of 2 ATP, the total yield (or net) value is actually only 2 ATP.