Transport across membranes

Diffusion

= net movement of molecules/ ions from region of higher to lower conc until evenly distributed

• no net movement

• equilibrim

• passive: energy comes from natural, inbuilt motion of particles

  • not external sources e.g. ATP

• particles constantly in motion due to possessing KE

• random motion of particles: constantly bouncing off one another and other objects e.g. container vessel

• most substances don’t easily pass across membrane

  • e.g. charged ions and polar molecules don’t diffuse easily due to hydrophobic nature of fatty acid phospholipid tails

  • can pass: small, non-polar molecules e.g. O2 and CO2

Factors affecting rate

concentration gradient

• steeper = increased diffusion

• = difference in concentration

• e.g. blood capillary/ vessels: O2/CO2 conc

Temperature

• >KE = > movement = faster diffusion

SA

• increased through folding e.g. microvilli in intestine/ cristae in mitochondria

• increased size = decreased SA:Vol = decreased rate

diffusion distance

• > distance = < diffusion

• e.g. blood capillaries 1 cell thick, alveoli walls very thin

properties of molecules/ ions

• large = slower, > energy required

• non-polar = quicker, as soluble in non-polar phospholipid bilayer

facilitated diffusion

= with aid of proteins

• for charged ions e.g. Na+ and Cl- and large polar molecules e.g. glucose and amino acids

• through transmembrane channels and carrier proteins

• passive, down conc grad

Channel proteins

• integral proteins forming water-filled hydrophobic channels, allowing water-soluble ions to pass

• selective dur to diameter and charged groups

• some always open, some gated and open only in presence of specific ion

  • or specific voltage

Carrier proteins

• span membrane

• when molecule e.g. glucose present, binds to protein

  • causing it to change shape so molecule released to other side of membrane

• no external energy needed

• direction of movement of molecules diffusing across membrane depends on relative conc on each side of membrane

  • but net movement always down conc grad

osmosis

= net movement of water from region of high water potential (dilute_ to region of lower water potential (concentrated) through a selectively permeable membrane

• involved solutions: solution dissolved in solvent

• water moves down water potential gradient until dynamic equilibrium established and no more net movement of water

• solute and water molecules in random motion due to KE

• selectively permeable plasma membrane only allows water molecules across it, not solute

• water pot of pure water

  • at standard temp and P = 0kPa

    • 0 = highest possible WP

  • > solute = > -ve WP (i.e. <0kPa)

Osmosis: animal cells

water in = lysis

• hypotonic solution i.e. higher WP than RBC

• dilute/ lower solute conc

• because has no cell wall

Isotonic: dynamic equilibrium

• equal WP

water out = crenation

• hypertonic solution

• > concentrated than blood cell

• lower WP in solution than RBC

Osmosis: plant cells

water in = turgid + large vacuole

• has limited expansion, so pressure builds up resisting further entry of water

• protoplast (all but central vacuole adn cell wall) kept pushed against cell wall

• allows plant to stand up = support + strength

water out = flaccid + plasmolysis + small vacuole

• protoplast pulled away from cell wall

• vol of cell decreases

• plant wilts

RP3: water potential of plant tissue

Determining water potential of potato tuber cells

1. label 6 boiling tubes with conc of sucrose

2. use 1.0 moldm-3 sucrose solution to make up 20cm3 of sucrose solution of each of following concs: 0.0, 0.2, 0.4, 0.6, 0.8, 1.0

3. Put boiling tubes with sucrose solution in water bath at 30C

   • use thermometer to check temps in all tubes reach 30C

4. using potato chip cutter, cut 6 chips from potato tuber

   • remove peel and use ruler, scalpel and tile to cut all chips to same length

   • blot potato chips dry (don’t squeeze)

5. weight initial masses of each chip

6. transfer potato chips to boiling tubes in water bath

7. remove chips after 20 mins

   • blot chips dry and reweigh for final masses

8. calc change in mass and calc % change in mass

9. plot graph to determine conc of sucrose which has same water potential as potato tuber cells

Active transport

= movement of molecules and ions through cell membrane from region of lower conc to higher conc using ATP (from respiration) and carrier proteins

How energy released from ATP

• majority of energy stored in ATP between 2nd and 3rd phosphate bond

  • high energy bonds = unstable with low activation energy, so are easily broken

• phosphorylation = adding phosphate group to ADP to form ATP

  • condensation reaction

  • catalysed by ATP synthase

  • (energy added)

• removal of phosphate group to form ADP and release energy

  • hydrolysis reaction

  • catalysed by ATP hydrolase

  • (energy released)

ATP+H2O → ADP +Pi + E

Pi = inorganic phosphate

E = energy

reversible reaction

Process of active transport

1. molecule/ ion binds to receptor sites on carrier protein

2. ATP binds to protein causing it to split into ADP and Pi

   • energy released used to change shape of protein and open to opposite side

3. molecule/ ion released to other side of membrane

4. phosphate molecule released, causes protein to revert to original shape

   • allowing process to

   • ADP and Pi will recombine during respiration to form ATP

Active transport vs facilitated diffusion

• both use carrier proteins

• f.d. down conc grad

• a.t. against conc grad

• carrier proteins have very specific tertiary structures with specific binding sites so will only transport specific substances to membrane

co-transport

= coupled movement of substances across cell membrane via carrier protein

• 2 types of molecules moved across membrane at same time (movement of one dependent on other)

• involves combination of facilitated diffusion and active transport