BSC 314 Lecture Notes - Lecture 25: Oxidative Phosphorylation, Glycolysis, Acetaldehyde
27 Jun 2018
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Anaerobic Respiration: Fermentation
A supply and demand problem arises among cells when glycolysis produces more
NADH than can be utilized or when NAD + supplies are diminished or oxygen is
unavailable. NADH production in glycolysis is a way to dispose of electrons and
hydrogen; the NADH needs the electron transport chain with its terminal oxygen
acceptor and NAD +is needed to complete the conversion of PGAL to pyruvate. If the
pathway is disturbed, organisms remedy the problem generally in one of two ways.
Lactate fermentation
Animals, protists, and many bacteria and fungi make lactate and release two molecules
of ATP, enough to regenerate some NAD + and keep glycolysis running (but utilizing
only a small portion of the energy of the glucose). Yogurt and cheese makers employ
bacteria that respire this way and harvest the tasty byproducts of the reactions.
Alcohol fermentation
Most plant cells and yeasts (fungi) breakdown pyruvate to acetaldehyde, releasing
CO 2. The acetaldehyde is then reduced by NADH to ethanol (ethyl alcohol). The
CO 2 makes bread rise, and ethanol is used by brewers and distillers to make alcoholic
beverages of all kinds.
Thermodynamically, this is a poor use of glucose. Over 90 percent of the energy of
glucose remains in the two alcohol molecules; fermentation has removed only about 7
percent. The ATP captures about one quarter of that, with the rest released as heat.
Respiration: Energy for Plant Metabolism
Respiration is the process through which energy stored in organic molecules is released
to do metabolic work. A stepwise process conducted in all living cells, it is controlled by
enzymes, and releases carbon dioxide and water.
Breathing, the inspiration and expiration of air by animals, is not the same as
respiration. Both animals and plants respire, but plants neither breathe nor have
specialized respiratory systems as do animals. In plants, gases diffuse passively into
the plant (through the stomata or directly into the epidermal cells) where they come into
contact with the moist cellular membranes and then move in water along diffusion
gradients between and within cells. No special carriers (such as the hemoglobin of
human blood) or organs (such as lungs or gills) aid in the diffusion.
Glucose is the originating molecule for respiration; other reserve foods either follow
different utilization pathways or, in the case of complex carbohydrates, are broken down
to glucose before undergoing respiratory oxidation.
Respiration can be divided into the following stages (see Figure ):
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Document Summary
A supply and demand problem arises among cells when glycolysis produces more. Nadh than can be utilized or when nad + supplies are diminished or oxygen is unavailable. Nadh production in glycolysis is a way to dispose of electrons and hydrogen; the nadh needs the electron transport chain with its terminal oxygen acceptor and nad +is needed to complete the conversion of pgal to pyruvate. If the pathway is disturbed, organisms remedy the problem generally in one of two ways. Animals, protists, and many bacteria and fungi make lactate and release two molecules of atp, enough to regenerate some nad + and keep glycolysis running (but utilizing only a small portion of the energy of the glucose). Yogurt and cheese makers employ bacteria that respire this way and harvest the tasty byproducts of the reactions. Most plant cells and yeasts (fungi) breakdown pyruvate to acetaldehyde, releasing. The acetaldehyde is then reduced by nadh to ethanol (ethyl alcohol).
