Lecture 7 Cell Respiration: Glycolysis and Anaerobic Respiration

cell respiration

The series of chemical reactions, some of which require oxygen and some of which do not, that produce the bulk of cellular energy

why do cells do cellular respiration

All cells, and thus, all living things must undergo some form (or forms) of cellular respiration in order to produce the necessary energy to promote normal cellular processes

electron carriers

“carry” electrons in the form of Hydrogen (H) atoms to the electron transport chain, in order to provide the electromotive force (emf) to promote oxidative phosphorylation

NADH

nicotinamide adenine dinucleotide; an electron carrier derived from niacin (Vitamin B₃) and oxidized to become NAD⁺

FADH₂

flavin adenine dinucleotide; an electron carrier that is derived from riboflavin (Vitamin B₂) and oxidized to become FADH and FAD

oxidation

An atom/ molecule is oxidized when it loses an electron

reduction

An atom/ molecule is reduced when it gains an electron

exchange of Hydrogen atoms

indicates which molecules are oxidized or reduced

Glycolysis

A form of anaerobic respiration that takes place in all cells (specifically in the cytosol)

A step of 10 constitutive reactions that take place in the cytosol of a cell and catabolize a single molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon sugar). ATP and the electron carrier NADH is produced during this phase of (anaerobic) respiration

irreversible reactions of glycolysis

• Reaction 1, catalyzed by the enzyme hexokinase. 

glucose + ATP → glucose-6-phosphate + ADP 

• Reaction 3, catalyzed by the enzyme phosphofructokinase-1.

fructose-6-phosphate + ATP → fructose-1,6,-bisphosphate + ADP 

• Reaction 10, catalyzed by the enzyme pyruvate kinase. 

phosphoenolpyruvate + ADP → pyruvate + ATP 

Investment Phase of glycolysis

Reactions 1 - 5

2 ATP molecules are used and no ATP molecules are produced

2 glyceraldehyde-3-phosphate (G3P) molecules are produced, effectively doubling the reactions that take place in the subsequent phase

Playoff phase of glycolysis

Reactions 6 - 10

4 ATP molecules are produced (2 for each G3P molecule), leading to the net “gain” of ATP for the cell of 2 ATP per original glucose molecule metabolized. 

2 pyruvate (pyruvic acid) molecules are produced. The fate of these pyruvate molecules depends upon the presence or absence of oxygen in the cell: 

• If oxygen is present, pyruvate will be converted to Acetyl-CoA and shuttled into the mitochondria for aerobic respiration. 

• If oxygen is absent, pyruvate will be used as the substrate to produce lactate (lactic acid) or ethanol (alcohol), depending upon the type of cell and enzymes present. 

shunting in glycolysis

cells can bypass several steps of the glycolysis pathway, and in addition, the reactants and products from various steps can be utilized for other pathways, effectively shunting the glycolysis pathway. 

• The product from reaction 1 of glycolysis, glucose-6-phosphate, can be used as the substrate for glycogen synthesis or gluconeogenesis. It can also be used to generate 5-carbon sugars via the pentose phosphate pathway. 

• The products from reaction 5 of glycolysis, glyceraldehyde-3-phosphate, can be used to synthesize glycolipids (specifically triglycerides). 

• The final products from glycolysis, pyruvate, can be used to synthesize amino acids like Alanine, as well as fatty acids. 

pyruvate oxidation

if O₂ is present, pyruvate molecules produced at the end of glycolysis are likely to be transported into the innermost compartment of the mitochondria, known as the matrix, to be converted into Acetyl-CoA, the necessary substrate for the Krebs Cycle

NADH and CO₂ molecule is also produced during this reaction

Acetyl-CoA

a two-carbon molecule with the large coenzyme A attached, crucial for the Krebs Cycle

CO₂ production

In the process of removing a carbon from pyruvate, Carbon Dioxide (CO₂) is produced and eventually removed from the cell (and the body via exhalation)

lactic acid fermentation

In situations where oxygen is not present (and/ or in the case of many prokaryotic cells that cannot undergo aerobic respiration), pyruvate is converted to lactate (lactic acid)

Highly active muscle tissue that cannot obtain enough oxygen at a fast enough rate to allow for aerobic respiration 

In this process, NADH is recycled to NAD⁺, allowing the cell to continue to undergo glycolysis to produce ATP

Alcohol Fermentation

process by which (in the absence of oxygen) pyruvate is converted first to an intermediate acetaldehyde, and then eventually to alcohol (ethanol)

Carbon Dioxide (CO₂) is produced during these two reactions (this is what makes bread rise and bubbles form during fermentation), and NADH is recycled to NAD⁺