Transport across membranes

• what are membranes around cells and organelles

  plasma membranes (acts as a partially permeable barrier)
• What are the 4 components that make surfaces membranes


  1. phospholipid bilayer
  2. Proteins
  3. Glycoproteins & Glycolipids
  4. Cholesterol
• What is an intrinsic protein (channel protein)

  Protein molecule spanning the whole phospholipid bilayer.

  
  

  they are fluid/ water filled space which allows water soluble ions
• Is a Na+/K+ pump active or passive transport?

  active transport and is a protein
• What is an extrinsic protein (carrier protein)

  Protein molecule spanning 1/2 of the structure, it provides mechanical support

  
  

  transports ions, glucose and amino acids by changing shape
• What is the cell membrane made of?

  phospholipid bilayer is the whole structure which includes:

  • intergral protein (adds more rigidity to stop the cell membrane from moving too much)
  • Fatty Acid hydrophobic tail & Phospholipid Hydrophilic Head
  • Cholesterol (offers rigidity)
  • Protein Channel / Carrier Protein
  • Glycogen protein (acts as a receptor)
  • Glycolipid
• What is the fluid mosaic model?

  states that a membrane is a fluid structure with a "mosaic" of various proteins embedded in it

  
  

  • the phospholipid bilayer moves to give the membrane flexibility (e.g - phagocytes)
  • mosaic is how the different shaped proteins, glycoproteins & cholesterol are embedded in the phospholipid bilayer
• What is the phospholipid bilayer made up of and whats its function?

  Feature:

  • Hydrophilic heads facing out and hydrophobic tails facing in which the majority of most membranes consists of

  
  

  Functions:

  • Allow small uncharged (non polar) lipid soluble molecules through it
  • provides flexibility as constantly moving
• what is cholesterol made up of... and its function?

  Feature:

  • a lipid that sits with phospholipids in the core of the membrane

  
  

  Functions:

  • Increase strength & stability of the membrane
  • Prevents leakage of water & ions due to their hydrophobic nature
• What is glycoprotein made up of... and whats is its function?

  Feature:

  • Glycogen attached to a membrane protein

  
  

  Function:

  • Acts as receptors for hormones
  • Antigens for cell recognition
  • Points for cell adhesion
• What is glycolipid made up of... and whats its function?

  Feature:

  • Glycogen attached to a membrane phospholipid

  
  

  Function:

  • Acts as receptors for hormones
  • Antigens for cell recognition
  • Points for cell adhesion
• What are the 3 factors affecting membrane permeability?

  1. Temperature
  2. Solvents (alkali / alcohol solutions)
  3. pH
• At a low temperature, on a graph which shows the effect of temperature on membrane permeability, the membrane permeability is very high, why?

  • The phospholipids do not have much energy,
  • they are packed together closely
  • channel & carrier proteins denature
  • this increases permeability

  
  

  this is because the ice crystals may form and pierce the membrane which makes it high permeable when it thaws
• As the temperature increases, on a graph which shows the effect of temperature on membrane permeability, the membrane permeability increases, why?

  • The phospholipids can move
  • they aren't packed together closely
  • membrane is partially permeable
  • as temperature increases, the phospholipids move more
  • they have more energy which increase permeability
• As it reaches high temperature , on a graph which shows the effect of temperature on membrane permeability, the membrane permeability is very high, why?

  • phospholipid bilayer begins to break down
  • the membrane becomes more permeable
  • water puts pressure on the cell membrane
  • channel proteins & carrier proteins denature in the membrane
  • it cannot control what enters and leaves the cell
• Required Practical: investigating how temperature affects membrane permeability

  1. Use a scalpel to carefully cut five equal sized pieces of beetroot (Make sure you do your cutting on a cutting board.) Rinse the pieces to remove any pigment released during cutting.
  2. Add the five pieces to five different test tubes, each containing 5 cm3 of distilled water. Use a measuring cylinder or pipette to measure the water.
  3. Place each test tube in a water bath at a different temperature, e.g. 10 °C, 20 °C, 30 °C, 40 °C, 50 °C, for the same length of time (measured using a stopwatch).
  4. Remove the pieces of beetroot from the tubes, leaving just the coloured liquid using tweezers
  5. Now you need to use a colorimeter — a machine that passes light of a specific wavelength through a liquid and measures how much of that light is absorbed. Many colorimeters use filters to make sure the light passing through the liquid is at the desired wavelength.
  6. Firstly, switch the colorimeter on and allow five minutes for it to stabilise.

  Then set up the colorimeter so you're using a blue filter (or a wavelength of about 470 m).
• Required practical continued

  1. Add distilled water to a cuvette so it is three quarters full (a cuvette is a small container that fits inside a colorimeter - see Figure 7). Put the cuvette into the colorimeter. Two of the cuvette's sides may be ridged or frosted — you need to make sure you put the cuvette into the colorimeter the correct way, so that the light will be passing through the clear sides. Calibrate the machine to zero.
  2. Next, use a pipette to transfer a sample of the liquid from the first test tube to a clean cuvette — again it should be about three quarters full.
  3. Put the cuvette in the colorimeter and read and record the absorbance of the solution.

  10. Repeat steps 8-9 for the liquids in the remaining four test tubes (using a clean pipette and cuvette each time).

  11. You're now ready to analyse your results — bear in mind, the higher the absorbance reading, the more pigment released, so the higher the permeability of the membrane.
• How does solvents affect membrane permeability?

  Solvents like ethanol/methanol can dissolve lipids in the phospholipids bilayer increasing the membranes permeability
• What is diffusion?


  The net & passive movement of particles from an area of high concentration to an area of low concentration across a partially permeable membrane until they are net distributed
• What factors that affect the rate of diffusion, and how?

  1. The concentration gradient:

  • steeper concentration gradient = faster rate of diffusion

  (this can occur in ventilation & blood circulation- they maintain a steep O2 concentration gradient between alveoli)

  
  

  1. The thickness of exchange surface:

  • The less cells that make up the exchange surface = Th shorter the distance the molecules have to travel so the faster to rate of diffusion.

  (this occurs in the alveoli of the lungs & villi of the small intestines = both lined with a single layer of flattened cells so shorter diffusion pathways)

  
  

  1. The Surface Area of the exchange surface:

  • the larger the surface area = the faster the rate of diffusion

  (thats why there is lots of alveoli in the lungs and lots of microvilli on villi in the small intestines)
• What factors affect Facilitated Diffusion/Simple Diffusion?

  1. Number of channel/carrier proteins in membranes (FD ONLY)
  2. Size & charge of diffusing particles (FD ONLY)
  3. Temperature- the kinetic energy of particles diffusing membrane protein structure and fluidity of the two phospholipids (FD & SD)
• What is faciltated diffusion?

  The net & passive movement of particles from an area of high concentration to an area of low concentration across a carrier/channel proteins.

  
  

  they allow polar molecules and water (aquaporins)
• What is a carrier protein?

  a transport molecule, change shape to allow the molecule to pass across the membrane

  
  

  they pass through large molecules and polar molecules
• What is a channel protein?

  A protein which creates a fluid filled pore in the cell membrane through which (polar molecules) ions and water can pass.
• Why does facilitated diffusion have a steeper curved line on a graph compared to a straight line

  The rate of facilitated diffusion increases steeply at the start - as the the concentration gradient becomes steeper as the extracellular glucose concentration increases.

  
  

  the rate of uptake eventually levels off (plateau) because equilibrium has reached - steeper concentration gradient but all of the glucose carrier proteins are occupied so carrier proteins becomes the limiting factor
• Simple diffusion and facilitated diffusion both move particles down their concentration gradient across a cell membrane. Suggest how you could determine whether a particular particle is being transported by simple or facilitated diffusion in an experimental setting. Explain your answer

  1. Set up a range of increasing extracellular particle concentrations.
  2. Measure the rate of uptake in each beaker
  3. if the rate of uptake plateaus the concentration increases
  4. Facilitated diffusion is occurring as all carrier proteins are occupied
• What is osmosis?


  is the net passive movement of water or other solvent through a selectively permeable plasma membrane from a region of HIGH water potential to a region of LOW water potential
• What is an isotonic solution?

  Contains equal potentials of water on both sides.
• Describe the water potential of each solution (lower) in relation to inside the red blood cells

  The water potential is lower inside the RBC than outside the RBC as it is crenated.

  There is a steeper concentration gradient

  Water has moved out of the RBCs by osmosis
• Describe the water potential of each solution (the same) in relation to inside the red blood cells


  The solution has the same water potential so water diffuses in and out at equal rates by osmosis which is called isotonic
• Describe the water potential of each solution (higher) in relation to inside the red blood cells

  The solution is a higher water potential so water has diffused into the RBC by osmosis causing the cell to swell up and burst
• What are the 3 factors affecting the rate of osmosis and how does it affect it?

  1. Surface area (folded membranes) - larger surface area leads to a faster rate
  2. Water potential gradient - steeper gradient leads to a faster rate
  3. Temperature - higher Kinetic energy of water molecules leads to a faster rate
• If the water potential is higher outside the plant cell, what state does the plant cell become and why?

  turgid because the protoplast (outer cell surface membrane) swells as the water potential outside enters the cell
• if the water potential is equal outside & inside the plant cell, what state does the plant cell become and why?

  there is no change because the water neither enters or leaves this causing the cell to be incipient plasmolysis (isotonic)
• if the water potential is lower outside the plant cell, what state does the plant cell become and why?

  The cell shrinks because water leaves the cell, therefore the plant cell becomes plasmolysed (shrunken)
• What is active transport?

  the movement of ions or molecules across a cell membrane against the concentration gradient using CARRIER PROTEINS, and requiring energy in the form of ATP.
• How does active transport work in the body?

  1. Any incoming molecule will bind complementary to the binding site of the carrier protein
  2. ATP attaches to it
  3. The carrier protein changes shape and hydrolysis of ATP into ADP + P, it releases energy to move molecules against the concentration gradient
• What 3 factors affect the rate of active transport?

  1. Number of carrier proteins
  2. Number of mitochondria present
  3. Presence of respiration inhibitors
• In the production of urine, glucose is initially lost from the blood but is then reabsorbed into the blood by cells in the kidneys. Explain why it is important that this reabsorption occurs by active transport rather than by diffusion?

  After most glucose is absorbed by facilitated diffusion there is no longer a concentration gradient so active transport help to absorb the remaining molecules against the concentration gradient: to ensure the body can use it for aerobic respiration to release energy
• where in the small intestine is large food molecules digested in?

  duodenum by enzymes released from the pancreas