B cells migrate to the lymphoid follicles. They are directed there by chemokine interactions between CXCL13, which is secreted by follicular dendritic cells (FDCs).

TLS Online TPP Program

#Id: 9265

#Section 2: General Biology

TLS Online TPP Program

#Id: 9266

#Section 2: General Biology

N2 constitutes about 80% of the earth’s atmosphere, but the stability of its triple bond renders it inert and metabolically inaccessible to most organisms.

TLS Online TPP Program

#Id: 9267

#Section 2: General Biology

Diazotrophs are able to catalyze the enzymatic reduction of this compound to ammonia (NH3) using the enzyme nitrogenase

TLS Online TPP Program

#Id: 9268

#Section 2: General Biology

Nitrogenase is a complex consisting of the MoFe protein (dinitrogenase or component I) a heterotetramer, and the Fe protein (dinitrogenase reductase or component II) a homodimer.

TLS Online TPP Program

#Id: 9269

#Section 2: General Biology

Fe protein 

contains 2 identical subunits (dimer) – nif H

Each subunit has MW of 30 ~ 72 kD.

Each subunit contains an Fe-S cluster (4Fe and 4S2-).

TLS Online TPP Program

#Id: 9270

#Section 2: General Biology

MoFe protein 

2 pairs of identical subunits (tetramer, nif D & nif K)

A total of MW 180 ~ 235kD

Each subunit has 2 Mo-Fe-S clusters.

Both Fe proteins and MoFe proteins are sensitive to O2.