Phosphorus is an essential nutrient for plants and animals. phosphorus normally occurs in nature as part of a phosphate ion (po4)3. as you can see from the diagram of the phosphorus cycle, phosphorus is a component of plant fertilizers. two of the critical roles phosphorus plays in life on earth is that it is a component of atp as well as all
a) amino acids.
b) carbohydrates.
c) cell membranes.
d) cellular cytoplasm.

The correct answer is (c) cell membrane.

Phosphorous is the second most important component in the body after calcium. The purpose of cell membrane is to hold the different components of the cell together and protect it from the outer environment. There are two important parts of phospholipids. The head region and tail region. The head region is a phosphate molecule which is hydrophilic in nature and the two tails are made up of fatty acids which are hydrophobic in nature. When the cell membrane is exposed to watery environment mixed with electrolyte, the phosphate group faces the watery environment and the tails hideaway being hydrophobic in nature. This helps in transportation of polar and non-polar molecules across membrane. Besides this function phosphorous is also a structural component of bones and teeth (calcium phosphate).

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.  

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answer; both surfaces of the membrane are nonpermeable