Cell Membranes - The basics

• What do cell surface membranes surround?


  Cells
• What are cell surface membranes?


  Barrier between the cell and its environment
• What is it called when you can let some molecules through but not others?

  Partially permeable
• How can substances move across the cell-surface membrane?

  Diffusion

  Osmosis

  Active transport
• What can the cell surface membrane be sometimes called?


  Plasma membrane
• What is the basic structure of all cell membranes?

  Composed of lipids (mainly phospholipids) proteins and carbohydrates
• What was the fluid mosaic model suggested to describe?


  Arrangement of molecules in the membrane
• What does the term "fluid mosaic model" mean?


  Fluid = Phospholipid molecule can move around within each layer, means layer is flexible and can change shape

  Mosaic = Membrane is studded with protein molecules, arrangement of protein varies like the tiles of a mosaic
• What are glycoproteins?


  Proteins that have carbohydrates attached to them.
• What are glycolipids?


  Lipids with a carbohydrate attached.
• What do phospholipids form?


  A barrier to dissolved substances
• What are two features of phospholipids?


  Hydrophilic head and hydrophobic tail
• What is hydrophilic?


  Attracted to water.
• What is hydrophobic?


  Repelling water.
• What do the molecules automatically do?


  Arrange themselves into a bilayer - head face out towards water on either side of the membrane
• Why can't water soluble substances like ions and polar molecules diffuse through the centre of the bilayer?


  Centre of bilayer is hyrdrophobic
• What can diffuse through centre of the bilayer?


  Small, non-polar substances like carbon dioxide and water
• What does cholesterol give to the membrane?


  Stability
• Where is cholesterol present?


  A lipid present in all cell membranes (except bacterial cell membranes)
• Where does cholesterol fit between?


  Between the phospholipids
• Where does cholesterol bind to?


  Hydrophobic tails of the phospholipids causing them to pack more closely
• What happens when cholesterol causes phospholipids to pack more closely together?


  Restricts the movement of the phospholipid making the membranes less fluid and more rigid
• What does cholesterol help maintain the shape of?


  Animal cells which don't have cell walls
• What else does cholesterol have?


  Hydrophobic regions
• Why is it good that cholesterol also has hydrophobic regions?


  Able to create a further barrier to polar substances moving through the membrane
• What are cell surface membranes affected by?


  Temperature