What do I write about my passion for my subject?

Your story about how you chose a subject isn't important. Why you are interested in it now is. However, if you want to do biology, there is nothing stopping you from applying for a generic grad scheme - many don't require a specific degree subject, just a grade. Are you sure you want to study economics for 3 years?

Well the reason why I love both biology and business studies equally is because it teaches me how things work. Biology teaches me about how life itself came to existence and works, whereas business studies teaches me about the business side of it. But both my biology teacher and business teacher are telling me to go for economics because they think I will do really good in it and it would be more beneficial for me to go to economics rather then just doing some normal business degree. :/

Your teachers are not the ones who have to study the subject for 3 years. It's not a good idea to apply for a subject just because your teachers have suggested it to you, particularly if you have other subjects in mind that you want to do.

Do you understand what studying economics would involve, compared to studying business? Would you miss studying biology?

Science degrees are generally well respected, and don't have to lead you down specialist career paths.

However, if you are sure you want to go for economics, you just have to explain why the subject interests you now. Admissions tutors don't need to know why you first decided to study the subject, or how that happened, but you do need to be able to demonstrate and interest in the subject and an understanding of it.