Chemistry

Home > Chemistry > Options > The Biochemistry of Movement > The Biochemistry of Movement: 7. Glycolysis, the first stage of respiration

Syllabus reference (October 2002 version)
7. Glycolysis is the first stage of the decomposition of glucose to release energy	Students learn to: identify that the enzymes of glycolysis are found in cell cytoplasm and that glucose is the raw material for glycolysis summarise the energy release in glycolysis and identify the form in which this energy is captured identify the end product of glycolysis as 2-oxopropanoate (pyruvate) discuss the role of the oxidation of fatty acids in the inhibition of the pyruvate conversion to acetyl CoA	Students: process information from a simplified flow chart of biochemical pathways to analyse the total energy output from glycolysis

identify that the enzymes of glycolysis are found in cell cytoplasm and that glucose is the raw material for glycolysis

The breakdown of glucose in respiration begins in the cytoplasm of cells. Respiration is a multistep process and each step is controlled by its own specific enzyme. The first series of reactions from glucose to pyruvic acid is called glycolysis.

process information from a simplified flow chart of biochemical pathways to analyse the total energy output from glycolysis

Process information in the Biochemical Pathways Flowchart in small chunks by identifying the relevant parts and constructing your own chart with supplementary notes.
Examine the Biochemical Pathways Chart to identify the series of glycolysis reactions and the ATP produced in those reactions. Analyse information provided on the whole Flowchart to include ATP released from the oxidation of the reduced compounds NADH and FADH₂.

summarise the energy release in glycolysis and identify the form in which this energy is captured

In the conversion of glucose to pyruvic acid, there is some energy released and trapped as ATP. There is a net conversion of two ADP molecules to two ATP molecules for each glucose molecule broken down. Each glucose molecule C₆H₁₂O₆ must lose 4H to form two pyruvic acid C₃H₄O₃.
Two hydrogen atoms must be removed in forming each pyruvic acid. These atoms attach to a hydrogen carrier called NAD⁺. This cation is the oxidized form and is easily reduced by two H atoms. The reduced form of the carrier is symbolised as NADH and a free H⁺ is formed.
NAD⁺ + 2H (from an organic molecule) → NADH + H⁺
Since the amount of the NAD⁺ carrier available in a cell is limited, the NADH molecule must be oxidised again and recycled or else glycolysis will stop.
Three ATP are formed for each NADH oxidised back to NAD⁺ in the mitochondria. This process in the mitochondria is called oxidative phosphorylation because oxygen is required and phosphorylation is the addition of a phosphate group to change ADP to ATP.
Energy is captured in the form of ATP.
The total energy release in glycolysis of one glucose molecule to two pyruvic acid molecules by including ATP produced by oxidative phosphorylation of NADH + H⁺ is ____ATP Answer

identify the end product of glycolysis as 2-oxopropanoate (pyruvate)

Many trivial names persist in Biochemistry for historical reasons. The systematic name for pyruvic acid is 2-oxopropanoic acid. Similarly, lactic acid is 2-hydroxypropanoic acid.
At the pH of the cytoplasm, pyruvic acid actually exists as the anion CH₃COCOO^- called pyruvate (2-oxopropanoate). This is also the case for other weak organic acids in the cell such as lactic acid.
The –COOH group is a weak acid and is in the anion form of -COO^- at pH 7.4. However, for the sake of consistency, these compounds will be given their acid name and formula here.
Discuss situations where the human body would produce acetyl CoA from fatty acids rather than glucose. An athlete or PDHPE student could be a good person to have the discussion with.

discuss the role of the oxidation of fatty acids in the inhibition of the pyruvate conversion to acetyl CoA

The 3-carbon pyruvic acid changes to CO₂ and a 2-carbon acetyl group CH₃CO which attaches to Coenzyme A forming acetyl CoA, CH₃COCoA. ( Note that Co in CoA stands for the coenzyme, a vitamin A derived molecule, and not for the element cobalt.) The conversion of pyruvic acid to acetyl CoA reduces a NAD⁺ which leads to the release of three ATP in a mitochondrion.
Depending on the availability of fuels to the muscle cell, fatty acids can be used for energy production especially by type 1 skeletal muscle cells. The breakdown of fatty acids to produce energy results in an increase in the compound acetyl CoA. With the buildup of acetyl CoA, pyruvic acid is no longer converted to acetyl CoA in glycolysis. Conversion of pyruvic acid to acetyl CoA stops until the acetyl CoA is used.
By analysing the chart, students can see that pyruvic acid is a pivotal point in the respiratory pathway. It can be directed anaerobically to lactic acid during rapid contraction, thus regenerating NAD⁺ and enabling glycolysis to continue. Alternatively, it can be converted to acetyl CoA and on to complete breakdown to CO₂ and H₂O. In this case, the NADH is further metabolised in the mitochondria, recycling NAD⁺ and producing many more ATP molecules.
Discuss situations where the human body would produce acetyl CoA from fatty acids rather than from glucose. An athlete or PDHPE student could be a person to have a discussion with.