Transport Across Membranes 3.2.3

• How can movement across membranes occur by?

  -simple diffusion-facilitated diffusion (carrier and channel proteins)-osmosis-active transport -co-transport
• Two types of transport across membranes?

  Passive transport= does not require any external energyActive transport= requires energy in the form of ATP
• Diffusion


  The net movement of particles from a region of high to low concentration until evenly distributed (down a concentration gradient)When concentration reaches equilibrium, diffusion continues however net movement is zero.
• Simple diffusion


  Small, lipid- soluble, uncharged particles can diffuse (eg. Oxygen and carbon dioxide)

  
• Facilitated diffusion

  Channel proteins= small, water-soluble molecules can diffuse (mainly ions or polar molecules)Carrier proteins= larger, water-soluble (eg. Glucose, amino acids)

  
• Rate of simple and facilitated diffusion

  
• Factors which affect rate of diffusion

  -temperature -concentration gradient-surface area-diffusion distance-size of molecule-movement of molecules
• Active transport


  The movement of particles into or out of a cell from a region of lower to higher concentration (against its concentration gradient) using ATP and carrier proteins

  
• Process of active transport

  1)the molecule or ion to be transported across the membrane binds to specific carrier proteins at the binding site2)ATP binds to the other side of carrier protein at the ATP binding site 3)this causes it to hydrolyse into ADP and Pi (releases the ADP)4)this causes the carrier protein to change shape and open to the other side of the membrane, releasing the molecule or ion5)the inorganic phosphate (Pi) is released from the carrier protein, causing it to return to its original shape 6)process can then be repeated
• The sodium- potassium pump (Na+/K+ pump)

  
• Process of the Na+/K+ pump

  1) 3 sodium ions bind to specific binding sites on the carrier protein , and ATP binds to the other side 2) ATP is hydrolysed into ADP and inorganic phosphate 3) ADP is released which causes carrier protein to change shape and open up on the other side (outside) of cell to release the 3 sodium ions 4) whilst carrier protein is open , 2 potassium ions bind to other specific binding sites 5) The inorganic phosphate is released which causes the protein to return to it's original shape 6) this releases the 2 potassium ions . 7) Process can be repeated
• Co-transport of sodium with glucose/ amino acids

  
• Process of co-transport of sodium and amino acids

  1)The Na+/K+ pump transports sodium ions into the blood stream to create a concentration gradient (low concentration in the epithelial cell, high concentration in the ileum lumen)2)Due to the concentration gradient, sodium ions transport down the concentration gradient via facilitated diffusion4)Due to co-transport, amino acids can transport against their concentration gradient, into the epithelial cell using energy from the diffusion of sodium ions5)Amino acids can be transported into the blood stream.
• What is osmosis?

  The passive movement (diffusion) of water from a region of higher water potential to a region of lower water potential, through a selectively permeable membrane
• Explain water potential

  ➔ It is basically a measure of how free water molecules are to 'do work' ie. to move (their potential to do osmosis)!➔ Water always moves from a high water potential to low water potential (down a water potential gradient)➔ It is measured in units of pressure (e.g. kPa, KiloPascals)100% pure water has Ψ = 0
• Effect of solute on water potential

  100% pure water has Ψ = 0

  
• Define hypotonic, isotonic, hypertonic

  
• Osmosis in plant cells

  Hypertonic: turgidIsotonic: incipient plasmolysis Hypotonic: plasmolyses

  
• Osmosis in animal cells

  Hypotonic: lyses Isotonic: normalHypertonic: shrivels

  
• Suggest how a student could produce a desired concentration of solution from stock solution

  ● volume of stock solution = required concentration x final volume needed / concentration of stock solution● volume of distilled water = final volume needed - volume of stock solution
• turgid vs. plasmolysis


  Plasmolysis= refers to the process in which plant cells lose water in a hypertonic solution, whileTurgidity= refers to the state of plant cells being swollen due to high fluid content

  
• The water potential equation

  
• What is bulk transport?

  The movement of large molecules using transport vesicles.
• Exocytosis


  Allows substances to exit a cell1)vesicles fuse with plasma membrane2)releases contents out of cell

  
• Endocytosis


  Allows substances to enter a cell1)captures a substance from outside of the cell2)membrane folds over the substance to engulf it3)vesicles buds off into the cell

  
• What is phagocytosis and pinocytosis?

  Phagocytosis (cell eating) describes the ingestion of large particles such as cell debris and whole microorganisms by means of large vesicles.Pinocytosis (cell drinking) describes the internalization of extracellular fluid and small macromolecules by means of small vesicles.
• What does bulk transport require?

  -vesicles-metabolic energy (ATP) - active method of transport