BIO2202 Plant Physiology 1 Nutrition Lecture 3

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BIO2202 Plant Physiology 1Nutrition - Lecture 3

• Passive and Active Transport of Solutes

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Passive Transport

• Occurs when a solute molecule diffuses across a
  membrane, down a chemical potential gradient
• tends towards equilibrium i.e.
• internal concentration external concentration

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Passive Transport

• Equilibrium rarely obtained - why??
• Due to dynamics of cell metabolism
• solutes constantly being broken down, removed for
  storage, incorporated into large biomolecules,
  compartmentalised

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Active Transport

• Occurs when a solute moleculecrosses a membrane
  against the chemical potential gradient
• Requires an energy input from the cell
• Achieved by coupling ion uptake with a second
  process that releases energy e.g. ATP hydrolysis

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µj µj RT ln Cj zjFE VjP µj the
electrochemical potential (J.mol-1) µj
electrochemical pot. under standard conditions R
universal gas constant T absolute
temperature Cj the concentration of the ion
j zj charge on j F Faradays constant E
overall electrical potential of solution The
last term can generally be ignored Vj partial
molar volume of j P pressure potential
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• For sucrose
• Potential inside cell is µsi µs RT ln
  Csi
• Potential outside cell is µso µs RT ln
  Cso
• The chemical potential difference is therefore
• ?µs µsi - µso
• ?µs RT(lnCsi - lnCso)
• ?µs RT.ln Csi
• Cso

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• For potassium
• ?µK (RT.lnC Ki zFEi) - (RT.lnCKo
  zFEo)
• ?µK RT.ln KI zF(Ei - Eo)
• Ko

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Electrical Neutrality

• Living cells obey the principal of electrical
  neutrality in which bulk solutions always contain
  equal numbers of positive and negative ions.
• While an electrical diffusion potential implies
  that the distribution of charges across the
  membrane is uneven, the actual number of ions is
  insignificant in chemical terms.
• A membrane potential of about 100mV results
  from the presence of only a single extra anion
  out of every 100,000. These extra ions are found
  adjacent to the membrane surface, not in the bulk
  solution.

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• At Equilibrium
• µjo µji
• µj RT ln Cjo zjFEo µj RT ln Cji
  zjFEi
• zjFEi - zjFEo RT ln Cjo - RT
  ln Cji
• Ei - Eo RT . ln Cjo
• zjF Cji
• The Nernst Equation
• Ei - Eo 2.3RT .log Cjo
• zjF Cji