How is a fetus able to extract O2 from its mother's blood?
A. Fetal blood has a higher concentration of hemoglobin than maternal blood.
B. Fetal hemoglobin has a lower affinity for O2 than maternal hemoglobin.
C. The maternal blood mixes with the fetal blood in the placenta to provide O2.
D. The pO2 of fetal blood is higher than the pO2 of maternal blood.
E. Fetal hemoglobin has a higher affinity for O2 than maternal hemoglobin.

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Proteins are the workhorses of the cell, controlling virtually every reaction within as well as providing structure and serving as signals to other cells. proteins are long chains of  amino acids  , and the exact sequence of the amino acids determines the final structure and function of the protein. instructions for that sequence are encoded in  genes  . to make a particular protein, a messenger ribonucleic acid (mrna) copy is made from the gene (in the process called  transcription  ), and the mrna is transported to the  ribosome  . protein synthesis, also called  translation  , begins when the two ribosomal subunits link onto the mrna. this step, called initiation, is followed by elongation, in which successive amino acids are added to the growing chain, brought in by transfer rnas (trnas). in this step, the ribosome reads the  nucleotides  of mrna three by three, in units called  codons  , and matches each to three nucleotides on the trna, called the anticodon. finally, during termination, the ribosome unbinds from the mrna, and the  amino acid  chain goes on to be processed and folded to make the final, functional protein