Why is fluorine considered an electron-withdrawing group despite being a center of large electron density?
This seemingly complicated question can be answered by boiling it down to periodic table trends. As we know, as move left right, and from bottom to top of the period table, electronegativity increases. As such, we can understand why fluorine is the most electronegative atom. Being so electronegative means that fluorine consistently dominates covalent bonds it takes part in, so is always found pulling electrons away from atoms it's bonded to (carbon, oxygen, sulfur etc.). Thus, despite having 7 valence electrons and pulling electrons away from atoms it is bonded to, the electronegative tendencies of the atom prevent it from sharing its electrons. In fact, you could say that the logic of the question is reversed. Fluorine's electron's don't make it electron-donating. It's electronegativity causes it to have a large center of electron density and thus, make it electron-withdrawing.
If the Citric Acid Cycle takes place in the mitochondrial matrix, why do some sources show that one particular step takes place very close to the inner mitochondrial membrane? What step is this and why does it take place at this unique location, when compared to the other steps of the cycle?
This step is the conversion of succinate to fumarate with the help of the enzyme succinate dehydrogenase. In addition to this conversion, this reaction also produces FADH2 from FAD. The reason that this step occurs so close to the inner mitochondrial membrane is because FADH2 feeds electrons into Complex II of the electron transport chain. FADH2 is an electron carrier, like NADH, but is different in that it feeds into Complex II and reduces quinone. Ultimately, the electrons donated by FADH2 help generate ATP for the cell, and regenerates more FAD for the Citric Acid Cycle to continue.
Find the slope of the line perpendicular to the line y = (1/3)x - 7
To begin, we can first dissect what the question is actually asking for us by looking at some of the key words. The first key word is "slope", which we know is the [(change in y)/(change in x)] of a particular line. The second is "perpendicular", which is particularly important because it is a characteristic of the line that we want to find. Two lines are perpendicular if their slopes multiply to -1. So, we know that we are looking for a line with a slope that when multiplied by the slope of y = (1/3)x - 7, gives us -1. Now, we can look to y = (1/3)x - 7 to find the slope of this line. We see that this line is written in y = mx+b form, where m is the slope. As such, we find that the slope of this line is (1/3). Now, we need to find a number that when multiplied by (1/3), gives -1. We can set up an equation: (1/3)s = -1. We can divide both sides by (1/3) and we see that s = -3. Thus, the slope of the line perpendicular to the line y = (1/3)x - 7 is -3.