There are four levels of protein structure. these figures show primary, secondary, tertiary, and quaternary protein structure. which level(s) of protein structure may be stabilized by covalent bonds?

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

read more on -

allowed for the prediction of the properties of missing elements.

the electromagnetic energy from the sun arrives in the form of visible and invisible light.