Positively charged molecules are often more easily endocytosed by cells than negatively charged molecules. which of these best explains this observation? a. the hydrocarbon tails of membrane lipids are hydrophilic. b. the hydrocarbon tails of membrane lipids are hydrophobic. c. the phosphate group of membrane lipids are positively charged. d. the phosphate group of membrane lipids are negatively charged.

Answer is D. The phosphate group of membrane lipids are negatively charged.

The cell or plasma membrane is a bilayer of phospholipids. The two phospholipids layers are opposite to each other. Each phospholipid molecule consists of a phosphate group and a lipid. The phosphate form inner and outer layers of hydorphilic and are negatively charged 'heads' and lipid forms the tail, hydrophobic and non-polar. Since phosphate is negatively charged it repels negatively charged molecules and do allow them to pass through it.

showing a photograph would make this a hell of alot easier

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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