Name: glycolysis: energy investment steps
Uploaded: 2025-05-22T09:05:35.000Z
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K12 Standards

Florida Department of Education

High School

Life Science

SC.912.L.18

SC.912.L.18.8

Glycolysis: Preparatory Phase

Glucose is the primary source of energy in most cells of the human body. It is catabolized inside the cell cytosol by a series of enzymes to yield two molecules of pyruvate - a process called glycolysis.
Glycolysis is divided into two phases, based on whether ATP is catabolized or synthesized.
In the first phase of glycolysis, hexokinase phosphorylates glucose to produce glucose-6-phosphate, consuming one ATP.
Next, phosphoglucose isomerase converts glucose-6-phosphate into fructose-6-phosphate, which is further phosphorylated by phosphofructokinase into fructose-1,6-bisphosphate consuming another ATP.
Aldolase then cleaves fructose-1,6-bisphosphate to yield two 3-carbon molecules, glyceraldehyde-3-phosphate or G3P and dihydroxyacetone phosphate or DHAP.
Finally, the triosephosphate isomerase converts DHAP to G3P in a reversible reaction. So, in the first phase, glucose is converted into two G3P molecules by expending two ATP.

glycolysis: preparatory phase

In cellular metabolism (the complete breakdown of glucose to extract energy),&#160; glycolysis is the first step. Glycolysis takes place in the cytoplasm of both prokaryotic and eukaryotic cells. Glucose enters heterotrophic cells in two ways. One method is through secondary active transport, where the transport takes place against the glucose concentration gradient. The other mechanism uses a group of integral proteins called GLUT proteins, also known as glucose transporter proteins. These transporters facilitate the diffusion of glucose.
Glycolysis begins with the six-carbon ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvate. Glycolysis consists of two distinct phases. The first phase&#160; of the glycolysis pathway traps the glucose molecule in the cell and uses energy to modify it so that the six-carbon sugar molecule can be split evenly into the two three-carbon molecules. The second phase of glycolysis extracts energy from the molecules and stores it in the form of ATP and NADH&#8212;remember: this is the reduced form of NAD.
First phase of Glycolysis (Energy-Requiring Steps or Preparatory Phase)
Step 1. The first step in glycolysis is catalyzed by hexokinase, an enzyme with broad specificity, that catalyzes the phosphorylation of six-carbon sugars. Hexokinase phosphorylates glucose using ATP as the source of the phosphate, producing glucose-6-phosphate, a more reactive form of glucose. This reaction prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins. Also, because it is negatively charged, the phosphate&#160; can no longer leave the cell because charge will not allow it to cross the hydrophobic interior of the plasma membrane.
Step 2. In the second step of glycolysis, an isomerase converts glucose-6-phosphate into one of its isomers, fructose-6-phosphate (this isomer has a phosphate attached at the location of the sixth carbon of the ring). An isomerase is an enzyme that catalyzes the conversion of a molecule into one of its isomers. (This change from phosphoglucose to phosphofructose allows the eventual split of the sugar into two three-carbon molecules.)
Step 3. The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme phosphofructokinase. A second ATP molecule donates a high-energy phosphate to fructose-6-phosphate, producing fructose-1,6-bisphosphate. In this pathway, phosphofructokinase is a rate-limiting enzyme. It is active when the concentration of ADP is high; it is less active when ADP levels are low and the concentration of ATP is high. Thus, if there is &#34;sufficient&#34; ATP in the system, the pathway slows down. This is a type of end product inhibition, since ATP is the end product of glucose catabolism.
Step 4. The newly added high-energy phosphates further destabilize fructose-1,6-bisphosphate. The fourth step in glycolysis employs an enzyme, aldolase, to cleave fructose-1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.
Step 5. In the fifth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer, glyceraldehyde-3-phosphate. Thus, the pathway will continue with two molecules of a glyceraldehyde-3-phosphate. At this point in the pathway, there is a net investment of energy from two ATP molecules in the breakdown of one glucose molecule.
This text is adapted from <a href="https://openstax.org/books/biology-2e/pages/7-2-glycolysis">Openstax, Biology 2e, Section 7.2: Glycolysis</a>

In cellular metabolism (the complete breakdown of glucose to extract energy),&#160; glycolysis is the first step. Glycolysis takes place in the cytoplasm ...

Education

JoVE Core

Cell Biology

Unit 1

Introduction to Metabolism

KEY TERMS AND CONCEPTS

glycolysis: energy investment steps

Glycolysis begins with energy investment steps in the cytoplasm of both prokaryotic and eukaryotic cells. It is the first step in cellular metabolism, which is the complete breakdown of glucose to extract energy. In heterotrophic cells, glucose enters by secondary active transport or through GLUT proteins, which are glucose transporter proteins that help glucose diffuse into the cell.

The pathway starts with one six-carbon, ring-shaped glucose molecule and ends with two three-carbon molecules called pyruvate. Glycolysis has two phases. The first phase traps glucose in the cell and uses energy to prepare it for splitting. The second phase extracts energy and stores it as ATP and NADH, the reduced form of NAD.

In step 1, hexokinase catalyzes the phosphorylation of glucose. It uses ATP to add a phosphate and form glucose-6-phosphate. This makes glucose more reactive. It also keeps the molecule from continuing to interact with GLUT proteins, and the negative charge helps keep it from leaving the cell through the hydrophobic interior of the plasma membrane.

In step 2, an isomerase converts glucose-6-phosphate into its isomer, fructose-6-phosphate. An isomerase is an enzyme that changes a molecule into one of its isomers. This shift from phosphoglucose to phosphofructose helps the sugar split evenly into two three-carbon molecules later in the pathway.

Step 3 is the phosphorylation of fructose-6-phosphate by phosphofructokinase. A second ATP donates a high-energy phosphate and forms fructose-1,6-bisphosphate. Phosphofructokinase is a rate-limiting enzyme. It becomes more active when ADP levels are high and less active when ATP is high. This is end product inhibition, because ATP is the end product of glucose catabolism.

In step 4, aldolase cleaves fructose-1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. In step 5, another isomerase converts dihydroxyacetone phosphate into glyceraldehyde-3-phosphate. After these steps, the pathway continues with two molecules of glyceraldehyde-3-phosphate. At this point, glycolysis has used two ATP molecules, so there is a net investment of energy in breaking down one glucose molecule.

Glycolysis: Energy Investment Steps

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Glycolysis begins with energy investment steps in the cytoplasm of both prokaryotic and eukaryotic cells. It is the first step in cellular metabolism, which is the complete breakdown of glucose to extract energy. In heterotrophic cells, glucose enters by secondary active transport or through GLUT proteins, which are glucose transporter proteins that help glucose diffuse into the cell. The pathway starts with one six-carbon, ring-shaped glucose molecule and ends with two three-carbon molecules...

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key terms and concepts

Trace the energy investment steps of glycolysis, from glucose trapping to the split into two three-carbon molecules, with key enzymes and ATP use.

Glycolysis: Energy Investment Steps | JoVE K12

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Glycolysis: Energy Investment Steps

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