the cell membrane

• Cell membrane


  Separates the internal and external environments of the cell
• Cell membrane


  • Made of two general types of molecules: lipids (fats, particularly phospholipids) and proteins (help with transport of materials, cell-to-cell communication and structure)
  • Provides a selectively permeable layer that surrounds and defines the cell
• Cell membrane controls the exchange of materials

  1. Diffusion

  2. Osmosis

  3. Active transport

  4. Endocytosis

  5. Exocytosis
• Osmosis


  Movement of water from a low solute concentration to a high solute concentration

  
• Passive transport


  Molecules move from high concentration to low concentration without input of energy
• Active transport


  Molecules move from low concentration to high concentration against the gradient, requires energy input
• Active transport


  Utilises specialised membrane transport proteins called protein pumps and an energy source known as ATP
• Endocytosis


  Substances moving into the cell, membrane engulfs substance and forms a membrane-bound vesicle

  
• Exocytosis


  Substances moving out of the cell, substances packaged into membrane-bound vesicles and fuse with cell membrane to release contents
• Surface area to volume (SA:V) ratio

  Larger surface area with smaller volume maximises exchange of substances

  
• Cells are usually very small and this maximises their surface area to volume (SA:V) ratio and optimizes rates of diffusion and exchange of materials
• The structure of the membrane means that some substances cannot cross the membrane easily
• Factors that influence substance movements

  • Size
  • Charge
  • Solubility in water
• Molecules are constantly moving (particle theory)
• In liquids and gases, molecules will space themselves so there's an even concentration
• Molecules move from areas of high concentration to low concentration


  This happens passively, without the need for energy
• Diffusion


  Passive movement of molecules from a region of high concentration to low concentration across the cell membrane
• Facilitated diffusion


  Passive movement of substances through a membrane transport protein
• Aquaporins


  Specialised membrane transport proteins that allow water to move across the cell membrane
• Water moves from a region of low solute concentration to high solute concentration
• Water tries to make the internal cell environment or outside environment less concentrated
• Charged molecules and larger molecules need to move from low concentration to high concentration
• This requires energy input and is termed active transport