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Jonathan G.
Current Medical Student with 5 years of tutoring experience
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Chemistry
TutorMe
Question:

What is the difference between a covalent bond and an ionic bond? Give an example for each. What determines which bond occurs?

Jonathan G.
Answer:

A covalent bond is when two atoms bond together and share electrons between one another. A great example of this is diatomic elements such as Hydrogen(H2), Nitrogen(N2) or Oxygen(O2). An ionic bond is when an element donates or accepts a valence electron off another element. An example of this is when Sodium(Na+) reacts with Chloride(Cl-) to form SodiumChloride(NaCl), which is also known as table salt. The feature that determines the bond is the difference in electronegativity. Electronegativity of an element is essentially how strongly an element attracts electrons. If the difference in electronegativity in either element isn't large enough, then the bond is covalent.(Difference is usually 0 -1.7) If the difference in electronegativity is to great, that is >1.7 in electronegativity, then the bond is an ionic bond and one element will either accept or give up an electron. Note: Ionic bonds almost always occur between metals and non-metals, due to the massive differences in electronegativity.

Psychology
TutorMe
Question:

Please explain what needs to occur for an action potential to happen within a cell. Once the action potential occurs, please explain the phases of the action potential, and what ionic movement is occurring, why the ions are moving in that direction and how it affects the voltage inside the cell.

Jonathan G.
Answer:

The neuron is normally at a voltage of -70mV which is known as it's resting potential. However for an action potential to occur, the threshold must be hit which is around -55mV. Therefore graded potentials, known as EPSP's(Excitatory post synaptic potentials) must depolarize(more positive) the neuron in question which will drive the voltage of the cell towards -55mV. Once the -55mV is hit, then the action potential occurs which is all or none response. The first phase, voltage gated sodium channels open which allow sodium(Na+) to rush into the cell. The reason that sodium floods the neuron is because it follows it's concentration gradient(more sodium outside the cell then inside) and it's electrochemical gradient(inside of the neuron is more negative, recall -55mV), thus floods into the cell. Once the cell hit's around +30mV,the voltage gated sodium channels close and the voltage gated potassium channels open in the neuron. Potassium ions(K+) rush out of the cell because their concentration gradient (more K+ inside the cell) and electrochemical gradient (The neuron is now at +30). However the potassium channels close once the voltage hits below -70mV, the sodium/potassium pump along with the leak channels bring back the voltage to its original resting potential of -70mV.

Biology
TutorMe
Question:

1. What is the significance of the phospholipid bilayer that makes up the plasma membrane?

Jonathan G.
Answer:

1. The importance of phospholipid bilayer is that it controls the permeability of the cell. That is, the phospholipid bilayer which is composed of the hydrophilic head(exterior) and hydrophobic tails(interior) allows for the control of certain ions and molecules, such as sodium, potassium and H2O. A great example of this is water which does not have easy access past the phospholipid bilayer due to the fact that it can't diffuse pass the hydrophobic interior of the cell. Therefore certain channels or pores are created with proteins to control the movement of ions and molecules inside/outside of each cell.

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