According to the education in summer, the cell membrane can control substances entering and leaving the cell. Some substances in the cell can often penetrate the cell membrane, but some substances are not easy to penetrate the cell membrane, so the cell membrane can maintain the stability of substances in the cell. Boiled spinach leaves turn green in cold water because boiling water destroys the cell membrane of spinach mesophyll cells, making chloroplasts in spinach mesophyll cells release chlorophyll and turn green when exposed to water.