3.2.3 Transport across cell membranes

• Give two reasons why a cell membrane is required:1)For shape or protection - to keep cell intact.

  2) It is semi-permeable/permeable.
• Explain what it means when something is permeable:It allows substances to pass through it
• Why do we need a cell membrane?

  • compartmentalization
  • regulates transport of material in & out (endocytosis & exocytosis)
  • has surface antigens
  • contains receptors for chemical signals
  • may be site of chemical reactions
• What is the phospholipid made of?

  Formed from a glycerol molecule, a phosphate group and two fatty acid chains.
• What does amphipathic mean?

  They have one end that is hydrophilic (attracted to water) and one that is hydrophobic (repels water).
• What does selectively permeable mean?

  They will allow certain substances through, but not others.
• Active Transport

  Movement of a substance from a region where it is in a low concentration to a region where it is in a high concentration. The process requires the expenditure of metabolic energy form of ATP.
• Bilayer

  A membrane consisting of two layers of phospholipids.
• Carrier protein

  A protein completely spanning the phospholipid bilayer which bind to ions or molecules then change shape in order to move these molecules across the membrane.
• Cell Membrane

  A selectively permeable membrane surrounding the cell and controlling the entry and exit of materials.
• Cholesterol

  Lipid that is an important component of cell-surface membranes because it adds strength. Excess in the blood can lead to atheroma.
• Fluid mosaic model


  The arrangement of the various molecules of the cell-surface membrane. Fluid because the individual phospholipid molecules can move relative to one another and mosaic because the proteins vary in shape, size and pattern.
• Glycolipid 


  A carbohydrate covalently bonded with a lipid. They act as recognition sites, help maintain stability of the membrane and help cells attach to one another.
• Glycoprotein


  Carbohydrate chains attached to a protein (often extrinsic) which are part of the cell surface membrane. They act as recognition sites, help cells to attach to one another and allows cells to recognize one another.
• Partially permeable 


  Also called semi-permeable. A partially permeable membrane allows water and other small molecules to pass through, but not larger molecules such as starch.
• Permeability 


  How permeable a substance is depends on the size, polarity and charge of the molecule.If it is small, non-polar and fat soluble it is very permeable and can pass through the cell membrane.
• Phospholipid 


  Triglyceride in which one of the three fatty acid molecules is replaced by a phosphate molecule. Phospholipids are important in the structure and functioning of plasma membrane.
• Plasma membrane 


  Membranes consisting of a phospholipid bilayer found around and within all cells. The cell-surface membrane is the plasma membrane that surrounds cells.
• Protein channels


  A protein completely spanning the phospholipid bilayer which form water-filled tubes to allow water-soluble ions to diffuse across the membrane.