OCR Biology Help:

ik theses sound like a silly question but I'm struggling to wrap my head around these:
what's a subunit? is it the same as the tertiary structure in a protein?

And for the oCR spec, do we need to know LDL's and HDL's fats?

Dont need to know about low density and high density lipids - it did come in a past paper, but all of the information you need to know was given, it was more of an application of your knowledge to a unknown topic.
Primary structure - sequence of amino acids joined by peptide bonds (amide bonds, you will learn about amides in year 13 chemistry if you do it)
Secondary structure - alpha helix, or beta plated sheet. Formed by hydrogen bonds between the non-R group parts of the amino acid
Tertiary structure - the polypeptide chain forms a 3D structure, with ionic bonds, hydrogen bonds, hydrophobic and hydrophilic interactions, sulphide bonds
Quaternary structure - made from 2 or more polypeptide chains. Each polypeptide chain that makes up a quaternary structure is called a subunit. A polypeptide chain has a 3D structure.

Haemoglobin is a quaternary protein - made from 4 subunits. Each polypeptide chain has a haem group bonded to it.
A protein can have a tertiary structure (one polypeptide chain) or a quaternary structure (multiple polypeptide chain), depending whether the polypeptide chain will interact with other polypeptide chains