Which substance is a waste that would normally diffuse across the placenta from the embryo to the mother? a) urea, b) glucose, c) oxygen, d) amino acid

The correct answer is option (A) urea.

Placenta is the organ connecting the fetus with the wall of the uterus for its development. It grows into the wall of the uterus and joins the fetus by the umblical cord. It helps in the exchange of gases, transport of nutrients and the elimination of wastes.

Nutrients like glucose and amino acids reach the developing fetus from the mother through the placenta via active and passive transport. Exchange of gases involving the uptake of oxygen from the maternal blood and the elimination of carbon dioxide into the mother's blood occurs through the placenta. Waste products like urea are excreted and transferred into the maternal blood by diffusion across the placenta.

There is a basic observation that similar sequences from the same evolutionary family often adopt similar protein structures, which forms the foundation of homology modeling. So far it is the most accurate way to predict protein structure by taking its homologous structure in PDB as template.