Nah definitely no need to memorise the definitions. It's unlikely that you'll actually be asked to define terms except for either in the multiple choice or you'll be given information and then you'll get a question, for example you'll have a diagram of a cell with arrows pointing to different structures within the cell and the question might be 'what is the role of structure X' or something like that. It won't be like 'Define these terms; mitohondria, chloroplast, endocytosis, immunity, etc.'
They don't need to be worded exactly correctly - but they do need to refer to key terms. For mitochondria, you'd need to say something about it being the site of anaerobic respiration, but it doesn't really matter how you word it.
Edit: Sine beat me to it, but extra answers can't hurt
