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.

There are two types of synapses: electrical and chemical. Electrical synapses have gap junctions between adjacent cells and are usually found between cardiac muscle cells. Chemical synapses are more abundant and utilize neurotransmitters (such as acetylcholine) to transmit signals between adjacent cells. They are typically found in neuromuscular junctions of skeletal muscle cells.

Acetylcholine is a neurotransmitter found in neuromuscular junctions. Release of acetylcholine into the synaptic cleft allows acetylcholine to bind to its receptors on the muscle membrane. Once bound, acetylcholine activates a signaling cascade that eventually leads to muscle contraction. Circulating antibodies in Myasthenia gravis patients prevent this interaction between acetylcholine and its receptor, thus decreasing muscle contraction. One way to treat this disease is by administering drugs that increase the half-life of each acetylcholine molecule. The most common way to do this is by administering an acetylcholinesterase inhibitor. Acetylcholinesterase is an enzyme that breaks down acetylcholine molecules in the synaptic cleft; decreasing or inhibiting this enzyme will lead to increased acetylcholine concentration. This increased concentration will compete with the antibodies and facilitate muscle contraction. Decreasing calcium influx in the presynaptic neuron will decrease the release of acetylcholine into the synaptic cleft. This will make Myasthenia gravis symptoms worse.

Out of all the neurotransmitters listed, the only one that isn't a catecholamine is serotonin. This neurotransmitter is initially derived from the amino acid tryptophan, whereas the catecholamines are derived from the amino acid tyrosine.

Dopamine, norepinephrine, and epinephrine are all catecholamine neurotransmitters. In fact, in the metabolic pathway that produces these compounds, dopamine is an intermediate that can be converted into norepinephrine, which can subsequently be converted into epinephrine.

Transferases are not neurotransmitters and can be omitted from selection. Catecholamines and small-molecule transmitters are overlapping categories and are typically packed in small, clear core vesicles. Gasotransmitters are membrane permeable and do not require vesicles for release. Neuropeptides are unique in that they are large and are synthesized at the ER, and are packed in large, dense vesicles, and thus this is the correct answer.