2 Energy  Why cells need energy – driving energetically unfavourable reactions – the need for electron and energy transfer  The laws of thermodynamics (Freeman Chp2, 32 – 37)  Endothermic and exothermic reactions (Freeman Chp 2, 32 – 37)  Photosynthesis and respiration as endothermic and exothermic reactions

2 Why do cells need energy? ReactantsProducts Energy Electrons These reaction are governed by the laws of thermodynamics Other reactions produce energy ProductsReactants Energy Electrons 1. to drive energetically unfavourable reactions 2. to balance the need for electron and energy transfer

2 defining energy: The first law of thermodynamics states that energy can never be destroyed but can be transformed from one type to another – as in the ATP example above. 1. Energy is the capacity to do work and exists as potential energy or kinetic energy. 3. For example, the molecule ATP (adenosine triphosphate) has potential energy stored in its molecular structure. 2. When it is actively involved in reactions this potential energy is converted into kinetic energy altering chemical bonding of other molecules

2 EpEp EkEk E p (lower)

What determines if a reaction will require or produce energy? Reaction will be spontaneous (exothermic) if the potential energy of the products is lower than that of the reactants. Reaction will be spontaneous (exothermic) if the resultant products are less ordered that the reactants Second law of thermodynamics: reactions proceed to maximise entropy and minimise potential energy, order leads to disorder….. 2

order --  disorder 2 nd Law of Thermodynamics disorder ----  order gives off energy requires energy

2

Gibbs free energy change = change in potential energy + change in entropy  G =  H + T  S T is temperature in Kelvin as entropy directly related to temperature Decrease in potential energy & increase in entropy = exothermic, spontaneous 2

Increase in potential energy & decrease in entropy = endothermic, not spontaneous 2

Some reactions also involve the transfer of electrons These are referred to as redox reactions Atom that loses electron(s) is oxidised. Atom that gains electron(s) is reduced. 1 st and 2 nd laws of thermodynamics apply to redox reactions 2e’H+H+ reductant 2

2 Carbohydrates (e.g. glucose C 6 H 12 O 6 ) CO 2 Complex Simple Reduced Oxidised Respiration Exothermic – releases energy and/or electrons Energy Electrons

2 CO 2 + H 2 O Carbohydrates (e.g. glucose C 6 H 12 O 6 ) SimpleComplex Oxidised Reduced Photosynthesis Endothermic – requires energy and/or electrons Energy Electrons

2 It’s more complex than this! Respiration overall is exothermic, but some of the reactions are endothermic (require energy). Photosynthesis overall is endothermic, but some of the reactions are exothermic (produce energy).

 Energy and Electron carriers in biological systems  Enzymes (Freeman, Chpt 3, p 63-71)  Phosphorylation (Freeman, Chpt 9, p )  Overview of respiration (Freeman, Chpt 9)

3 Energy and Electron carriers in biological systems: Metabolism requires the exchange of: Energy Electrons Simple carbon units glucose + phosphate  glucose-6-phosphate Requires 14 kJ mol -1 Decrease in entropy & increase in potential energy Energy O HO— P — O - OH

supplier  energy supplier Glucose + phosphate + energy  glucose-6-phosphate + spent Re-energized Acid anhydride bond 3 Adenosine triphosphate

31 kJ mol -1 3

Glucose + phosphate + ATP  glucose-6-phosphate + ADP 14 – 32 = -17 kJ mol-1 Heat 3

Related compounds power some biosynthetic pathways: GTP – Guanosine triphosphate UTP – Uridine triphosphate CTP – Cytidine triphosphate 3 GTP ATP

ATP ADP + Pi Energy -31 kJ mol -1 Adenosine triphosphate Adenosine diphosphate Acid anhydride bonds of ATP are made: by the light reactions of photosynthesis respiration Phosphorylation De-phosphorylation Energy 31 kJ mol -1 3

ADP + Pi ATP Adenosine triphosphate Adenosine diphosphate Phosphorylation accomplished in two ways: substrate phophorylation via electron transport and ATPsynthase (enzyme) Phosphorylation Inorganic phosphate Acid anhydride bond 3

vocabulary Resp 2 entropy order disorder thermodynamics energy (heat) energy (biolocical) endothermic exothermic phosphorylation dephosphorylation oxidation reduction redox