Bioenergetics and Membrane Transport

1
Bioenergetics and Membrane Transport

• Reading Becker, ch. 5, pp. 112-120
• ch. 8, pp. 218-220

2
Energy

• The ability to do work
• The ability to cause change
• Types of change
• Change in solute across a membrane
• Change in ion and charge across a membrane
• Changes in chemical structure (making and
  breaking bonds)
• Changes in location of cells or subcellular
  structures (movement)
• Changes in temperature (production of heat)
• Production of light (bioluminescence)

3
Energy

• Measured in units of calories (cal)
• 1 cal amount of energy required to raise temp
  of 1 g of H2O 1ºC at a pressure of 1 atmosphere
• Also expressed as joules
• 1 cal 4.184 joules

4
The Cell is an Open System

• Can exchange energy with its surroundings

5
First Law of Thermodynamics(Law of Conservation
of Energy)

• Energy can be neither created nor destroyed, but
  can be converted from one form to another
• For a closed system (such as the universe as a
  whole)
• Total energy in all forms is the same before
  and after any reaction.
• For an open system (such as the cell)
• Energy out energy in energy stored
• OR
• Energy stored energy in energy out

6
Change in Internal Energy (DE)

• Energy stored energy in energy out
• Energy stored Internal energy E
• DE change in stored energy after a process
  has occurred
• DE Eproducts Ereactants

7
Change in Heat (DH)

• Difference in stored energy (DE) before and after
  a biological reaction is almost always due to a
  change in heat
• During a biological reaction, heat is either lost
  or consumed
• Change in stored energy can be approximated as
  the change in heat
• DE DH

8
Change in Heat (DH)

• Heat content is known as enthalpy
• DH the change in enthalpy accompanying a
  biological reaction
• DH Hproducts Hreactants

9
Change in Heat (DH)

• DH Hproducts Hreactants
• If Hproducts lt Hreactants, energy has been lost
  in the form of heat
• negative DH (decrease in enthalpy)
• exothermic reaction
• If Hproducts gt Hreactants, energy has been taken
  up by products in the form of heat
• positive DH (increase in enthalpy)
• endothermic reaction

10
Change in Heat (DH)
11
The Second Law of Thermodynamics

• In every physical or chemical change, the
  universe tends toward greater disorder or
  randomness
• The system may tend toward more order, but the
  surroundings become more disordered because of
  the input of energy required from outside

12
Entropy

• Entropy a measure of randomness or disorder
• S
• Change in entropy DS

13
Entropy

• For every biological reaction, entropy in the
  universe increases
• DSuniverse is positive
• Within a system, entropy can either increase
  (DS), decrease (-DS), or stay the same as the
  result of a reaction

14
Free Energy

• Free Energy a measure of whether or not a
  biological reaction will occur spontaneously
• i.e., requires energy (non-spontaneous) or does
  not require energy (spontaneous)
• G
• Change in Free Energy DG
• A measure of energy released or utilized in a
  biological reaction

15
Change in Free Energy

• DG DH TDS,
• where T is temp in degrees Kelvin (K ºC 273)
• Spontaneous reactions
• DGsystem lt 0
• Exergonic or energy-yielding
• Non-spontaneous reactions
• DGsystem gt 0
• Endergonic or energy-requiring

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DG and Transport Across Membranes

• Uncharged solutes, S
• Uncharged solutes will cross membranes down their
  concentration gradients by either simple or
  facilitated diffusion, with rate of diffusion
  dependent upon the solutes size and polarity and
  the presence of specific transport proteins
• Uncharged solutes can be pumped across membranes
  up their concentration gradients by active
  transport, dependent on the presence of specific
  pumps

17
DG and Transport Across Membranes

• For uncharged solutes, S
• DGinward RTlnSinside
• Soutside
• DGoutward RTlnSoutside
• Sinside
• Where R 1.987 cal/mol-K
• T absolute temp in K
• S is expressed in mol/L

18
DG and Transport Across Membranes

• Charged solutes (ions) are affected by both the
  concentration gradient and the electrochemical
  gradient
• DGinward RTlnSinside zFVm
• Soutside
• Where R 1.987 cal/mol-K
• T absolute temp in K
• S is expressed in mol/L
• z the charge on the ion
• F 23,062 cal/mol-K
• Vm membrane potential in volts
• Membrane potential the voltage across a
  membrane that is caused by the unequal
  distribution of charge across the membrane
• For most animal cells, membrane potential is
  between -60 and -90 mV