Why are these sentences involving 一部, 一部分, and 部分 ungrammatical?

Question

I've been trying to figure out the differences between 一部, 一部分, and 部分 using GOO's thesaurus. It indicates that 部分 is used for one of several parts, while 一部 and 一部分 are for parts of a whole. And then it seems to indicate that while 一部 and 一部分 are the same, that 一部分 is just a bit more formal.

This makes sense to me, but then the examples with which are valid confuse me. The 4 examples are:

1. 葉先の…が白い (all valid)
2. 積み荷の…がくずれる (一部 and 一部分 are valid, but 部分 is invalid)
3. …の人にしか伝わらない (一部 and 一部分 are valid, but 部分 is invalid)
4. この文章で大切な…はどこですか (部分 is valid, 一部分 is questionable, and 一部 is invalid)

This definitely confuses me. The first feels like 部分 should be invalid, since the tip of a leaf can't be broken into multiple parts. The second, on the other hand, feels like it can be broken up into parts (a box/container of cargo), so it doesn't make sense why 部分 is invalid (it feels backwards). 3 explicitly implies the person is one of many others, so it again feels backwards. And the last is probably the most confusing of all of them. Sure, 部分 should work, since there are several parts to a sentence, but why is 一部 invalid, and more importantly, why is 一部分 semivalid? Shouldn't it always be as valid as 一部, apart from formality?

Answer 1

A J-J dictionary obviously needs to explain every word in Japanese, but this matter could be made much clearer if they did in English.

積み荷の一部 a part of cargo
積み荷の部分 (the) cargo part

Xの一部 means that X is the whole to the part, while in Xの部分 the noun X is an extra description of 部分 that forms apposition. This is because what comes immediately before 部分 is interpreted as the qualifier, and 一部 = 一つの部分 "a part; one part".

When you want to mention the qualifier to the part and the whole thing at once, e.g. "many parts of the government", you'd say:

政府の多くの部分
[the whole] の [qualifier] (の) 部分

Usually, in a situation where you have to use the word "part (of the whole)", there should be always some kind of qualifier gets in, but a rare exception is "a part of —", for which Japanese has a special word.

As a side note, while 一部分 is a complete synonym of 一部 when it means "a part of", 部 has other meanings besides "part" which cannot be replaced by 部分, such as 一部リーグ "(League) Division 1".

Edit:

この文章で大切な一部分はどこですか

As the dictionary says, 一部 is not acceptable here (perhaps comparable to saying "*important a part" in English). The reason why 一部分 is sometimes valid is that 一 can work as an emphasis marker added to 部分 (i.e. it is not a "single word" any more). So it means "important one part", or practically "the most important part", instead of "an important part".