If the egg cell membrane did not change after fertilisation, a second sperm cell could enter the egg.
Why would this be a problem?
the fertilised egg would have too many chromosomes
the fertilised egg would lose its store of nutrients
the fertilised egg would have too few mitochondria
the fertilised egg would keep a haploid nucleus
41 exam-style questions on Edexcel GCSE Biology 1.1 Cells and cell structure, covering 1.1.1 Sub-cellular structures and functions, 1.1.2 Specialised cells and their adaptations, and 1.1.3 Microscope technology and cell structures. Each one has a worked solution and a mark scheme showing where the marks go.