Cellular Respiration Part II: Glycolysis

Curriculum Framework f. Cellular respiration in eukaryotes involves a series of coordinated enzyme-catalyzed reactions that harvest free energy from simple carbohydrates.

Big Energy Events of Respiration Glycolysis (breaks down glucose into two molecules of pyruvate) The citric acid cycle (completes the breakdown of glucose) Oxidative phosphorylation (accounts for most of the ATP synthesis) Glycolysis, Citric Acid cycle and oxidative phosphorylation are the three big energy events of respiration. In this session, we will look more closely at the events of glycolysis.

Big Energy Events of Respiration Electrons carried via NADH Electrons carried via NADH and FADH2 Glycolysis Pyruvate oxidation Citric acid cycle Glucose Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION Figure 9.6 An overview of cellular respiration. ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation

Electrons carried via NADH Electrons carried via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Pyruvate oxidation Glucose Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION Figure 9.6 An overview of cellular respiration. ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

Mitochondrion Oxygen Water Carbon dioxide ATP Fuel (glucose) Next we will watch a video clip that describes cellular respiration and more specifically glycolysis. In the video, oxygen is shown as red, water is red and white, carbon dioxide red and blue, glucose is represented by the ring shape and ATP by the yellow star burst. (Teachers Only show from time 0 to 1:35 of the video) ATP Fuel (glucose)

GLYCOLYSIS Curriculum Framework 2.F.1 Glycolysis rearranges the bonds in glucose molecules, releasing free energy to form ATP from ADP and inorganic phosphate, and resulting in the production of pyruvate.

Glycolysis Glycolysis is thought to be one of the oldest metabolic pathways. This energy harvesting process occurs in the cytoplasm of both prokaryotic and eukaryotic cells.

Mitochondrion ATP Glucose GLYCOLYSIS Let’s take a closer look at how ATP is produced from a molecule of glucose—our fuel. Only the carbon skeleton is shown to keep things simple. The first step is called glycolysis, and it takes place outside the mitochondria. To begin the process, some energy has to be invested.

Next, the molecule is split in half, forming pyruvate (pyruvic acid)

Glycolysis Represented schematically

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases Energy investment phase Energy payoff phase Glycolysis occurs whether or not O2 is present Emphasize WHERE each portion of the process takes place. Help student connect the fact that since glycolysis occurs in the cytoplasm, ALL cells carry out glycolysis. No special “equipment” required!

Electron carrier (NADH) Now, the molecule NAD+, an electron carrier, picks up electrons and hydrogen atoms from the carbon molecule, becoming NADH. Keep track of the electron carriers—they play an important role by transporting electrons to reactions in the mitochondria. Electron carrier (NADH)

ATP Pyruvic acid In the final steps of glycolysis, some ATP is produced, but not much—for every glucose molecule, only two net ATPs are produced outside the mitochondrion. However, glycolysis has produced pyruvic acid, which still has a lot of energy available.

Glycolysis Activation energy input Splitting the fructose Harvesting the energy 2 NADH + H+, 2 ATP Two pyruvates (pyruvic acid)

Emphasize the energy “count” with regard to ATP. How many invested Emphasize the energy “count” with regard to ATP. How many invested? (2) How many harvested? (4) The point is there is a net gain of 2 ATP and the first step is endergonic (not spontaneous, thus requires energy)

Substrate-Level Phosphorylation Enzyme Enzyme ADP P Substrate ATP ATP produced during glycolysis is produced through substrate level phosphorylation which is one of the two common pathways of energy transfer in cells. The second method, oxidative phosphorylation will be addressed as we move further into the process of cellular respiration. Product

Inputs Outputs Glycolysis Glucose 2 Pyruvate  2 ATP  2 NADH This image summarizes the overall energy transfer results of glycolysis.

Curriculum Framework 2.F.2. Pyruvate is transported from the cytoplasm to the mitochondrion, where further oxidation occurs.

What happens to the pyruvate?

Oxidation of Pyruvate to Acetyl CoA Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle This step is carried out by a multienzyme complex that catalyzes three reactions

Outer mitochondrial membrane Pyruvic acid ACETYL COA FORMATION Let’s follow this pyruvic acid molecule into a mitochondrion to see where most of the energy is extracted. Inner mitochondrial membrane

Carbon dioxide As the molecule enters the mitochondrion, one carbon is removed, forming carbon dioxide as a by-product.

Electron carrier (NADH) Electrons are stripped, forming NADH.

Coenzyme A attaches to the 2-carbon fragment, forming acetyl CoA.

MITOCHONDRION CYTOSOL CO2 Coenzyme A NAD NADH + H Acetyl CoA 1 3 2 Glycolysis produces pyruvic acid in the cytosol. The conversion to Acetyl CoA is finished inside the mitochondrion of the cell. NAD NADH + H Acetyl CoA Pyruvate Transport protein

The fate of pyruvate is dependent upon the type of cell involved as well as the availability of oxygen. Today we have looked at the pathway of glucose in the presence of oxygen. We will look at alternate routes later in the unit.

Electrons carried via NADH Electrons carried via NADH and FADH2 Figure 9.6-3 Electrons carried via NADH Electrons carried via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Pyruvate oxidation Glucose Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION Summarize your learning. ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

Closing thoughts… What process is represented by A? What process is represented by B? C and D are products of A. What could they represent? What process is occurring at B? What does E represent? What is the fate of E? What process is represented by A? Glycolysis What process is represented by B? Pyruvate oxidation C and D are products of A. What could they represent? ATP, NADH What does E represent? carbon dioxide What is the fate of E? diffuses out of cell, picked up by bloodstream, delivered to lungs and breathed out… Next time we will trace the fate of the remaining carbons…..

Created by: Debra Richards Coordinator of Secondary Science Programs Bryan ISD Bryan, TX