AN INTRODUCTION TO METABOLISM

Chapter 3

Metabolism and Energy

The sum of all the reactions that take place in our cells.

Reactions require ENERGY Energy is the ability to do work (cellular work) Different types of energy

Chemical, Electrical, Mechanical, Light, Thermal Each form of energy can be converted to other forms

Energy can exist in two states Kinetic energy

Occurs as a result of motion (molecules, ions moving in solution)

Potential energy Stored within an object (chemical bonds, glucose)

Energy Obeys Laws

The First Law of Thermodynamics The total amount of energy in any closed system is

constant. Energy cannot by created or destroyed; it can only be

converted from one form to another. If a physical system gains an amount of energy, another

physical system must experience a loss of energy of the same amount

Photosynthesis Sunlight converts to chemical energy

Cellular Respiration Convert chemical energy into mechanical energy (muscle

contraction)

Conversion of Energy

Depends on Breaking and formation of chemical bonds in a

chemical reaction Energy is absorbed when breaking bonds Energy is released when forming bonds

Combustion of methane (CH₄) Mole •6.022 x 10²³ of atoms of a certain element

•Gives each element on the periodic table its certain atomic mass

1 mol of methane

2 mol of oxygen

1 mol of carbon dioxide

2 mol of water

Bond Energy

Measure of the strength or stability of a covalent bond.

Measured in (kJ/mol)Different amounts of energy are

required to break different types of bonds

Activation Energy The minimum amount of energy

needed to break bonds and start a chemical reaction (match needed to start combustion reaction)

Chemical Reaction

Activation energy requiredReaches transition state

Temporary condition in which bonds in the reactants have reached breaking point and new bonds are ready to form in products.

Two types Exothermic reaction

Net release of energy Endothermic reaction

Net absorption of energy

BEabsorbed – BEreleased = Net energy released/absorbed

Energy Obeys Laws

The Second Law of Thermodynamics In every energy transfer or conversion, some

of the useful energy in the system becomes unusable and increases the entropy of the universe.

Entropy A measurement of disorder in a system Always increases whenever there is a

chemical reactionIncreases when

Solids react to form liquids or gases Liquids react to form gaseous products Total number of product molecules is greater

than the total number of reactant molecules

Spontaneous vs Non-Spontaneous Change

Predict whether a chemical reaction will occur without the continuous input of additional energy

Spontaneous Will continue to occur on its own (lit match) No additional energy required Three Factors to determine whether a reaction will happen

spontaneously (favourable) Energy changes - exothermic changes – release of energy Entropy – increase High temperatures

Non-spontaneous Will not continue to occur on its own (boiling pot of water) Additional energy is required Factors determining whether a reaction will not happen (not

favoured) Energy changes – endothermic – absorption of energy Entropy decreases – increasing order Low temperatures

Favoured vs Not Favoured

Gibbs Free Energy

Energy that is not lost during a reaction Represented by the symbol GApplies to both chemical and physical processesAlways a reduction in the amount of free energy after

completion of process

Responsible for (living organisms) Synthesis of molecules Reproduction Movement

Represented by ∆G = Gfinal state – Gintitial state

∆G = Gfinal state – Gintitial state

Difference in the free energy of the final state of molecules to the free energy of the initial state

Negative values (-∆G) (spontaneous) Free energy of products is less than free energy of reactants Gives off free energy C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O ∆G = -2870 kJ/mol glucose oxidized Exergonic reaction – releases free energy

Positive values (+∆G) (non-spontaneous) Free energy of products is more than free energy of reactants Must gain free energy to occur This reaction cannot happen on its own 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ ∆G = +2870 kJ/mol glucose formed Endergonic reaction – requires energy

Exergonic vs Endergonic

Every type of cell in every organism continuously carries out thousands of these reactions

Coupled Reactions

Transfer of energy from an exergonic reaction to an endergonic reaction

Free energy from exergonic reaction drives endergonic reaction

When combined free energy is released and both reactions occur spontaneously

A → B + C ∆G = - 5 kJ/mol D + E → F ∆G = +4 kJ/mol

Catabolic vs Anabolic

Two types of pathways Catabolic

Complex molecules are broken down into simpler compounds

Release free energy -∆G

Anabolic Build complicated

molecules from simpler molecules

Consumes free energy + ∆G

ATP

Adenosine triphosphate Supplies energy that powers nearly every

cellular functionEnergy “currency”

Where Does Energy From ATP Come From?

Consists of Nitrogenous base

(adenine) 5 carbon sugar

(ribose) 3 phosphate

groupsFree energy

comes from the three negatively charges phosphate groups

Hydrolysis of ATP

Catabolic reaction Phosphate group is

broken off using water Two products formed

Adenosine diphosphate (ADP)

Inorganic phosphate (Pi)

H⁺ released into solution

ATP + H₂O → ADP + Pi

∆G = -30.5 kJ/mol

ATP and Energy Coupling

Release of a phosphate group (ATP) attaches itself to a reactant molecule - phosphorylation

Molecule gains free energy becoming more reactive

Enzyme brings ATP molecule and reactant molecule together – allow for transfer of phosphate group

Regeneration of ATP

ATP molecules must be generated for cells to keep functioning

Cells generate ATP by combining ADP with Pi

Reaction called ATP synthesis Requires free energy

Where do we obtain energy to create ATP? Food

Breakdown of carbohydrates, fats proteins All are sources of free energy

ATP Cycle

ATP is hydrolyzed and resynthesized at least 10 million times per second in our cell

Enzymes and Activation Energy

Metabolism in organisms would be slow if enzymes were not present

Just because a reaction can proceed on its own does not mean that it will proceed.

Speed at which a reaction occurs is increased by enzymes

Almost all enzymes are proteins RNA molecules can also function as enzymes

Only function of enzyme Lower the potential energy level of the transition state.

Catalyzed reactions written with enzyme name above reaction arrow Name of enzyme ends in “-ase” and is usually based on substrate

What Provides Activation Energy for A Chemical Reaction?

Thermal Energy – combustion reactions Combustion of propane

A small spark is enough energy for some reactants to overcome the activation barrier

Increase in temperature is bad in biology Destroy structural components of

some proteins and DNA Speed up all of the reactions in a

cell

Enzymes Lower Activation Energy

How do Enzymes Reduce Activation Energy?

Important – Substrate molecules need to be in transition state for a reaction to proceed.

Enzymes can achieve this in 3 different ways Bring molecules together (a) Expose reactant molecules to

altered charge environments (b) Active site contains ionic groups

with (+) or (-) charges Change the shape of the substrate

(c) Weakens its chemical bonds

reducing energy required to break them (induced fit model)

Food as Fuel

C –H bonds Position of electrons to atomic nuclei of carbon and

hydrogen Electrons are approx equidistant from two small nuclei

(high energy)Farther away an electron is from the nucleus the

more potential energy it has Size of nucleus affects potential energy

Electrons are more strongly attracted to a larger nucleus than

a smaller one

Energy Changes During Oxidation

Oxidation of glucose (exergonic) Transfer of electrons to O₂ Controlled oxidation

Cells are able to capture more free energy and produce less waste thermal energy

Occurs through a series of enzyme-catalyzed reactions Energy released transfers to energy-carrying molecules

Energy Carriers

Dehydrogenases (Class) Facilitate transfer of high-energy

electrons from food molecules NAD⁺

Nicotinamide adenine dinucleotide Important in metabolic processes Remove two hydrogen atoms from

a substrate molecule NAD⁺ becomes reduced to NADH Other H⁺ is released into cytosol

Facilitate the synthesis of ATP