Karina Alvarez Week 2

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This week's assignment is to evaluate this primary research article.

Biological Terms

In order to fully understand the paper, I researched the terms that were not very familiar to me. These terms are defined here:

  • glutamate: major fast excitatory neurotransmitter in the mammalian central nervous system (Source)
  • glutamine: a crystalline amino acid occurring in proteins; important in protein metabolism (Source)
  • dehydrogenase: enzyme that oxidizes a substrate by transferring hydrogen to an acceptor that is either NAD/NADP or a flavin enzyme; removes hydrogen from its substrate (Source)
  • proline: amino acid abundant in collagens more than other proteins (Source)
  • Northern analysis: a procedure similar to the southern blot analysis, used mostly to separate and identify rNA fragments (Source)
  • permease: general term for a membrane protein that increases the permeability of the plasma membrane to a particular molecule by a process not requiring metabolic energy (Source)
  • oligonucleotides: polymers made up of a few nucleotides; in genetics, a short sequence synthesized to match a region where a mutation is known to occur, and then used as a probe (Source)
  • transferase: an enzyme that transfers a specific grouping from one molecule to another (Source)
  • flux: the total amount of a quantity passing through a given surface per unit time (Source)
  • metabolites: any substance produced by metabolism or by a metabolic process (Source)


Main Result

The main result of this study is that the concentration of ammonia, rather than the rate of input, is the limiting factor in the growth of Saccharomyces cerevisiae. This was based off much previous research indicating that the concentration of ammonia, more than any other nitrogen sources (including proline and urea, leads to more rapid growth in Saccharomyces cerevisiae.


To specify the source of increased growth, flux and concentrations were differentiated to determine which played a bigger role, if any, in the growth of S. cerevisiae. S.cerevisiae, SU32 strand, was grown in continuous cultures with feeds containing differing levels of ammonia concentration but fixed glucose concentrations at a fixed rate.


One of the key pieces of data was that the rate of ammonia assimilation in the bacteria was approximately constant. This constant greatly reduced the number of variables and allowed the rates of assimilation to be compared only to those with differing concentrations of ammonia and glucose.


The figures relay the data collected in the study. Below are the descriptions of what the figures represent.

Figure 1

This figure exhibits the physiological parameters of S. cerevisiae SU32 at differing concentrations of ammonia in the feed (but at constant flux for all of them).

  • X axis: NH4+ concentration (mM)
  • Y axis - left: residual NH4+ concentration (mM)
  • Y axis - right: biomass (gl^-1)

This graph represents that increases concentrations of ammonia correlate with increases in biomass (in grams liter^-1).

  • X axis: NH4+ concentration (mM)
  • Y axis - left: O2 consumption, CO2 production (mmol(g^-1)(h^-1))
  • Y axis - right: respiratory quotient

This graph shows that as ammonia concentration increases, O2 consumption decreases while respiratory quotient decreases as well.

  • X axis: NH4+ concentration (mM)
  • Y axis: levels of a-ketoglutarate, glutamate, glutamine in (micromolg^-1)

These graphs show that as ammonia concentration decreases, levels of a-ketoglutarate decrease, levels of glutamate increase, and levels of glutamine increase.

Figure 2
  • X axis: NH4+ concentration (mM)
  • Y axis: % expression

This graph shows that as ammonia concentrations increase, the percentage of expression of nitrogen related genes generally decreases.

Figure 3
  • X axis: NH4+ concentration (mM)
  • Y axis: levels of NADPH-GDH, levels of NAD-GDH, levels of GS transferase (micromol(min^-1)(mg^-1))

This graph reveals that as ammonia levels increase, levels of NADPH-GDH decrease, levels of NAD-GDH increase, and levels of GS transferase decrease.


This paper does not explore whether or not other limiting nutrients would interfere and play a bigger role than the source of ammonia. In future studies, one could research how these limiting nutrients play a role in differing levels various other nutrients, including ammonia.


This study implies that ammonia concentration is the regulator, rather than the flux. This may lead to further studies on ammonia sensors in other organisms, like nitrogen-fixing bacteria involved in legumes grown for human consumption, that could lead to increased productivity and more efficient growing of food, especially if the ammonia levels play a bigger role than levels of glucose.

Helpful Links

BIOL398-04/S15:Week 2
BIOL398-04/S15:Class Journal Week 2

Karina Alvarez Main Page: Karina Alvarez
Math 388-01 Course Page: BIOL398-04/S15