来自《耶鲁大学开放课程：生物医学工程探索 第15集 第15集》

• 
  
  even though they're small,
  尽管很小
• 
  
  charged ions like sodium
  还有带电的离子 比方说钠离子
• 
  
  cannot penetrate through a lipid bilayer,
  都是不能穿透脂质双分子层的
• 
  
  so there's no way for sodium to get through this lipid bilayer.
  因此 钠离子无法穿透脂质双分子层
• 
  
  Even though there's a very large difference in concentration,
  尽管胞内外钠离子浓度差异巨大
• 
  
  these molecules of sodium on the outside can't diffuse inside
  胞外钠离子却无法扩散入胞内
• 
  
  because they can't permeate through the lipid bilayer.
  因为钠离子无法穿越脂质双分子层
• 
  
  Now,potassium,notice,
  再来看钾离子 注意
• 
  
  has a very high concentration inside the cell,120 mmol,
  胞内钾离子浓度很高 120mmol
• 
  
  and outside the cell the concentration is low,
  胞外则很低
• 
  
  4.5 mmol,so again a big difference in potassium concentration
  仅4.5mmol 同样也是差异巨大
• 
  
  but going in the opposite direction.
  只不过内外相反了