EFB325 Cell Physiology

Glycolysis and fermentation

The flow of energy through an ecosystem proceeds from the phototrophs to the chemotrophs

• capture of light energy and conversion to chemical energy is by phototrophs
• cells in phototrophs and chemotrophs then use chemical energy to do cellular work

Energy is used to do cellular work

A fundamental concept we want to learn about Cell Physiology is how cells accomplish work

Cellular work can take different forms:

• synthesis
• mechanical
• concentration of molecules
• electrical
• heat generation
• bioluminescence (light emission)

A living cell is continuously utilizing energy to do some form of work

• that energy ultimately comes from the sun and is captured as chemical energy
• all subsequent work by cells occurs through the release of that chemical energy during reactions

Metabolic pathways are a series of enzyme-catalyzed reactions linked together by the chemical compounds that are the substrates and products of those reactions

• those reactions are regulated so that they proceed toward the proper direction and at the proper rate (can be by regulating enzyme activity)
• we call the chemical compounds involved in these reactions metabolic intermediates (also called metabolites)
• many metabolic intermediates will be used in more than one metabolic pathway or process
• we can describe some pathways as linear and others as cycles: a cycle is a pathway in which one of the original primary substrates is regenerated at the end of the pathway

Metabolic pathways can function either . . .

• in the synthesis of cellular components (more order, less entropy)=anabolic reactions
• or in the breakdown of compounds to provide energy for anabolic reactions=catabolic

There are many different catabolic pathways, but the most important is the breakdown of carbohydrate (glucose)

• cellular breakdown occurs in many small steps, each capturing a small portion of the energy released, rather than burning which would release a large amount of heat all at once
• common pathway in nearly all organisms

The links between catabolic and anabolic pathways are ATP/ADP and NAD⁺/NADH

• the free energy released by catabolic reactions is captured when ADP accepts Pi to form ATP
• or when NAD⁺ is reduced by addition of two high-energy electrons (and a proton, remember reduction can also be described as hydrogenation) to form NADH

(NAD=nicotinamide adenine dinucleotide)

In overview, the breakdown of carbohydrates can be divided into four main processes/pathways (under aerobic conditions):

• breakdown of polymers (glycogen, starch), disaccharides (sucrose, lactose) to glucose
• glycolysis (in the cytosol)
• citric acid cycle (in the matrix of the mitochondria)
• oxidative phosphorylation (electron transport and ATP synthesis, occurs in the inner membrane of the mitochondria)

Glycolysis

• Pathway of breakdown of glucose (6-carbon sugar) to 2 molecules of pyruvate (3-carbon)

Key concepts:

• glycolysis occurs in the cytosol
• the initial addition of energy (using 2 ATP per glucose) drives the reaction forward-the reactions are exergonic (negative DG)
• subsequent addition of Pi sets up the ability to produce 2 ATPs per 3-carbon compound (4 ATPs per glucose=2 net ATPs produced per glucose->pyruvate)
• this transfer of phosphates directly from the triose phosphates, which are high-energy molecules, to ADP (to form ATP) is called substrate level phosphorylation
• G-3-P is oxidized in a step where NAD⁺ is reduced to NADH, therefore to keep performing glycolysis, the cell needs to regenerate NAD⁺ by oxidizing NADH

Key steps:

• Glucose is phosphorylated by ATP-this adds a "packet" of energy to the sugar molecule (G-6-P)
• Isomerization to fructose-6-P =>fructose can also enter the pathway at this point (like when sucrose is broken down to glucose and fructose)
• Fru-6-P is phosphorylated a second time by ATP to form Fru-1,6-bisP (a second packet of energy is added)
• The 6-carbon sugar-phosphate compound (F-1,6-bP) is then split into 2 3-carbon compounds (Glyceraldehyde-3-P=a triose phosphate)
• G-3-P is oxidized in a reaction that adds Pi and reduces NAD⁺ to NADH + H⁺ to produce 1,3-bisphosphoglycerate
• 1,3-bisPGA is a high energy molecule-it releases a P to ADP to produce ATP and 3-PGA
• 3-PGA is changed to phosphoenolpyruvate, a very high energy molecule
• PEP releases a P to ADP to form ATP and pyruvate

The path of oxidation of NADH (and regeneration of NAD⁺) depends on whether oxygen is present or not

• in the presence of oxygen: NADH is oxidized in the mitochondria by aerobic respiration (this generates ATP by oxidative phosphorylation)
• in the absence of oxygen (anaerobic): NADH is oxidized through fermentation

Pyruvate is a "branch point"-either to go to the mitochondria for aerobic respiration or to be used in fermentation

• different organisms use different pathways for fermentation
• pyruvate can be reduced to either: lactic acid or to ethanol and CO₂
• in both cases, NADH is used to reduce pyruvate and is oxidized to NAD⁺
• there is no NET oxidation through fermentation, but 2 ATPs are produced per glucose

Some organisms are obligate anaerobes or obligate aerobes

• some bacteria and fungi are facultative=can switch between respiration & fermentation
• in vertebrates, when skeletal muscle cannot be supplied with enough O₂ to do aerobic respiration, the cells use lactic acid fermentation to produce ATP; the lactic acid travels to the liver, where it is regenerated into glucose
• plant roots, when submerged (anaerobic), perform ethanol fermentation to make ATP
• fermentation of beer and wine is accomplished by yeast under anaerobic conditions to produce ethanol and CO₂ (also by bread yeast)

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