So building on question 2), what kind of forces hold lipids together?
For question 3), I meant what I said. I had encountered a question earlier asking if membrane proteins were only of a quaternary structure I think I've just presumed that quaternary structured proteins are more common than their tertiary counterparts.
With question 4), I'm still confused as to why all RNA looks the same. I know that RNA has four different nitrogenous bases, but isn't the RNA sequence in each tRNA just 3 bases? Concerning the picture, why is it impossible to differentiate between RNA types when tRNA has a base sequence of 3 nitrogenous bases? I'm missing something about the structure of tRNA!
As a general rule: All proteins have primary, secondary and tertiary structures; but not all proteins have quartenary structures. The primary structure refers to the sequence of amino acids in the polypeptide chain. The secondary structure refers to the characteristic 3D shape of a protein, given rise to by different R groups and their arrangement on either side of the peptide bonds. The tertiary structure of a protein is it's overall geometric shape, a modified secondary structure really, and is what allows a protein to carry out its function. A quarternery structure refers to proteins made of more than one polypeptide chain. For example, hemoglobin is made up of 4 polypeptide chains, and is globular (one of the two main classes of quarternery structured proteins). However, realise that not all proteins can have more than one polypeptide chain. Therefore, not all proteins can have a quarternery structure.
Regarding your question on RNA, I think you are referring to
codons. A codon is the initial nucleotide from which translation starts. Codons are sets of 3 adjacent nucleotides that t-RNA transfers from mRNA in the process of synthesising amino acids. All RNA still have the 4 bases (AUGC), but they read only in groups of 3 when being translated into an amino acid. RNA have the same structure but different function? I'll let someone else answer that.