Steroids do have a common cyclic skeleton, but they are not only present in animal tissue; they can be found in plants as well. This cyclic skeleton has a four-ring structure, but they are not aromatic rings. The progesterone receptor is found inside cells, although recent research has also confirmed its presence on plasma membranes. Aldosterone is a mineralocorticoid, not a glucocorticoid, which is indeed secreted by the adrenal gland.

If a person is deficient in 21-hydroxlyase, the steroid pathway converting progesterone to deoxycorticosterone will be unable to continue.  Therefore, the final product of that pathway, aldosterone, will not be made.  Aldosterone acts to retain salt, and so a lack of aldosterone causes significant salt loss in patients deficient in 21-hydroxlyase.  

Steroid hormones, not peptide hormones, travel through the cell membrane and into the nucleus to directly affect transcription of DNA. The only answer choice that is a steroid hormone is cortisol.

For this question, we're being asked to identify the rate-limiting step for the creation of steroid hormones. Remember, the rate-limiting step refers to the slowest step of the overall process.

First, it's important to remember that all of the steroid hormones are initially derived from cholesterol. The pathway leading to the biosynthesis of cholesterol is very long and complex, but does not represent the rate-limiting step for steroid hormone production.

Cholesterol is either obtained from the diet, or made in the cytosol of cells. Once here, the cholesterol needs to be translocated into the mitochondrial matrix for further processing. The enzyme StAR (steroid acute regulatory protein) is able to bind cholesterol and, through a poorly understood mechanism, it is able to carry it across the hydrophilic intermembrane space as well as both mitochondrial membranes. Once inside the matrix, an enzyme of the electron transport chain called cholesterol side chain cleavage enzyme () cleaves the side chain of cholesterol to produce the new compound called pregnenolone. Pregnenolone, in turn, is able to be converted into any of the other steroid hormones depending on the enzymes present.

It turns out that the rate-limiting step of this is the translocation of cholesterol into the matrix by StAR.

Steroid hormones and peptide hormones, due to their structural make up, have different methods of binding to their respective receptors. Steroid hormones are soluble in lipids and can therefore pass directly through the cell membrane to act upon an intracellular receptor. Peptide hormones, on the other hand, can not penetrate the membrane, and must bind to receptors on the surface of the phospholipid bilayer. The only hormone listed that is a steroid and would not bind to a receptor on the cell surface is aldosterone - the rest are peptide hormones.

Steroid hormones bind to intracellular and not transmembrane receptors.The steroid hormone-receptor complexes then bind to special DNA sequences in genes they regulate.These special DNA sequences are called hormone-responsive elements. Hormone-responsive elements usually reside in the promoter region of genes.

The steroid conversion pathway of vitamin D begins in the skin where cholecalciferol is synthesized by UV light. In the liver a hydroxyl group is added making 25-hydroxycholecalciferol. In the proximal tubule of the kidney, another hydroxyl group is added, forming the active form of vitamin D - 1,25-dihydroxycholecalciferol.

The correct answer is "spontaneously passing through the plasma membrane and forming a complex with a steroid receptor, which enters the nucleus and acts as a transcription factor." Glucocorticoids are steroid hormones, which are hydrophobic and thus can pass right through lipid membranes without having to interact with receptors in the plasma membrane. Transcortin and albumin are used to transport steroid hormones through the blood, but they do not enter the cell along with the hormone.

The release of calcium ions at the axon terminal is responsible for the exocytosis of vesicles carrying neurotransmitters. 

When an action potential reaches the synapse, choline enters the neuron. Once inside, the choline molecule binds to acetyl-CoA and forms acetylcholine, which is then packaged into vesicles. Upon calcium influx, the acetylcholine vesicles fuse with the synaptic membrane and release acetylcholine into the synaptic cleft. The acetylcholine molecules can now bind to receptors on the postsynaptic membrane and initiate an action potential in the postsynaptic neuron.