Here is a beginning.....Made bold an important aspect comparing ALA for instance to marine sources of n3's. To get the beneficial aspects of DHA/EPA optimally, this is an important thing to know.
The omega-3 polyunsaturates are a family of polyunsaturated fatty acids. Fatty acids are the building blocks from which edible oils and fats are made up. The polyunsaturates are characterised by the presence of certain structural features which mark them out as being different from other fatty acids. The polyunsaturates are essential nutrients for humans and indeed most if not all animals. There are two important families of polyunsaturates, the omega-3 family, and the omega-6 family. Both are needed in the human diet for optimal health. The omega-6 family is found mainly in vegetable oils such as sunflowerseed, sesame, corn, soy etc. The omega-3 family is found mainly in fish, and especially in the oil rich fish such as herring, mackerel, sardines etc. Small amounts of the omega-3's are found in eggs. Certain vegetable oils such as linseed (flaxseed), rapeseed (canola) and walnut oil contain a type of omega-3, known as the short chain type (alpha-linolenic acid). To be useful to the human body, these short chain omega-3s must first be converted to their long chain form. While the human body can in theory do this conversion, in practice, modern diets and lifestyles reduce the effectiveness of the conversion systems. Roughly 10 times more of the short chain omega-3s must be consumed to have the same effect as the long chain form from fish oil.