Transport across cell membranes

• Protein carriers


  - Bind with a molecule, e.g. glucose, which causes a change in the shape of the protein - this change in shape enables the molecule to be released to the other side of the membrane

  
• Protein channels


  - Tubes filled with water enabling water-soluble ions to pass through the membrane - selective

  - channel proteins only open in the presence of certain ions when they bind to the protein
• Fluid mosaic model


  - Describes the lateral movement of membranes

  - with scattered embedded intrinsic and extrinsic proteins

  - membrane contains glycoproteins, glycolipids, phospholipids and cholesterol
• Phospholipids in membranes


  - Phospholipids align as a bilayer

  - hydrophilic heads are attracted to water - hydrophobic tails repelled by water

  
• Cholesterol


  - Present in eukaryotic organisms to restrict lateral movement of the membranes

  - adds rigidity to membrane - resistant to high temperatures & prevents water + dissolved ions leaking out
• Selectively permeable membrane


  - Molecules must have specific properties to pass through plasma membrane

  - lipid soluble (hormones e.g. oestrogen)

  - very small molecules

  - non-polar molecules (oxygen)
• Simple diffusion


  -Net movement of molecules from an area of higher concentration to an area of lower concentration

  - until equilibrium is reached

  - passive DOWN concentration gradient
• Facilitated diffusion


  - Passive process using protein channels/carriers

  - down the concentration gradient

  - used for ions and polar molecules e.g sodium ions - and large molecules e.g. glucose
• Osmosis


  - Net movement of water

  - from an area of higher water potential to an area of lower (more negative) water potential

  - across a partially permeable membrane

  -Down water potential gradient
• Water potential


  - The pressure created by water molecules

  - measured in kPa

  - represented by symbol ψ

  - pure water has a water potential of 0kPa

  - the more negative the water potential, the more solute must be dissolved
• Hypertonic solution


  - When the water potential of a solution is more negative than the cell

  - water moves out of the cell by osmosis

  - both animal and plant cells will shrink and shrivel
• Hypotonic solution


  - When the water potential of a solution is more positive (closer to zero) than the cell

  - water moves into the cell by osmosis

  - animal cells will lyse (burst)

  - plant cells will become turgid
• Isotonic


  - When the water potential of the surrounding solution is the same as the water potential inside the cell

  - no net movement in water

  - cells would remain the same mass
• Active transport


  - The movement of ions and molecules from an area of lower concentration to an area of higher concentration using ATP and carrier proteins

  - carrier proteins act as selective pumps to move substances

  -Against concertation grdient
• Role of carrier protein in active transport


  - When molecules bind to the receptor - ATP will bind to protein on inside of membrane and is hydrolysed to ATP / Pi

  - protein changes shape and opens inside membrane
• Co-transport

  -The movement of two substances across a membrane together, when one is unable to cross the membrane itself

  - involves a co-transport protein (type of carrier protein) involves active transport

  - e.g. absorption of glucose/amino acids from lumen of intestines