MetabolismLecture 5, part 1

Fall 2008

Metabolism

Metabolism• All the biochemical process within an organism

that maintain life and contribute to growth• Emergent properties

– The whole is greater than the sum of its parts– New properties emerge with each step

upward in the hierarchy of life• Cellular metabolism arises from interactions

between molecules within the orderly environment of the cell

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Metabolism

• Metabolic pathway– Series of chemical reactions

• Catabolic pathway– Breaks down a complex molecule into simpler

compounds– Creates energy

• Anabolic pathway– Builds a complex molecule from simpler compounds– Consumes energy

• Bioenergetics– Study of how energy flows through living organisms

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Energy

Energy• The capacity to cause change• The ability to rearrange a collection of matter

Two main types• Kinetic energy• Potential energy

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EnergyKinetic energy• Energy of motion

– Does work by imparting motion to other objects

• Thermal energy (heat)– Type of kinetic energy– Amount of energy associated with the random

movement of atoms and molecules– Temperature

• Measure of how much thermal energy a molecule possesses

• The faster the molecule, the more collisions, the higher the temperature

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Energy

Potential energy• Stored energy• Based on location or structure• Chemical energy

– Form of potential energy available for release in a chemical reaction

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Chemical Energy

Chemical energy• Energy stored in chemical bonds

– Released when bonds between molecules broken

• Produces heat, kinetic energy and waste products• Amount of kinetic energy produced is how efficient

the process is– Car: 25% kinetic energy – rest is lost as heat– Cellular respiration: 40% cellular work, rest is used for

body heat

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Review Chemical Bonds, pg. 38-41

• Covalent

• Ionic

• Hydrogen

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Energy Transformation

• Energy can change from potential to kinetic and back

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Energy Transformation

• Energy can change from potential to kinetic and back

Fig. 8.2

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Energy & ThermodynamicsThermodynamics• Study of energy transformations that occur in a

collection of matterFirst Law of Thermodynamics (Principle of Conservation

of Energy)– Energy is neither created nor destroyed, only

converted from one form to another– Amount of matter & energy in the universe remains

the same– Energy is always conserved

• Can be converted from one form to another.

e.g. photosynthesis converts energy from the sun into plant biomass.

• Energy quantity stays the same

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Fig.8.3

Energy & ThermodynamicsSecond Law of Thermodynamics

– When energy is changed from one form to another, some of the useful energy is always degraded to lower-quality, more dispersed, less useful energy.

• Usually heat– e.g. cellular respiration

glucose + oxygen = carbon dioxide + water + energy (+ heat)

• Energy quality is changed

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Fig.8.3

Energy & Entropy• Entropy

– amount of disorder in a group of molecules– Heat - high disorder, high entropy, less useful form of

energy

Second Law of Thermodynamics (revisited)• Every energy transformation increases the

entropy of the universe

• Cells are not disordered – use energy to fight entropy

• Organisms are “islands of low entropy in an increasingly random universe”

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How do chemical reactions happen?

• Reactants– starting materials

• Products– resulting materials

• Balanced equations• Matter is neither created nor destroyed, only rearranged

– Breaking and forming of chemical bonds

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How do chemical reactions happen?

3H2 + N2 → 2NH3 ←

• Reaction is reversible• Chemical equilibrium

– A dynamic but stable state of a reversible chemical reaction in which the forward reaction and reverse reaction proceed at the same rate, so that the concentrations of reactants and products remain constant

– Changing chemical equilibrium• Changing concentration of reactants or products• Changes in temperature (e.g., gas to liquid)

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How do chemical reactions happen?

What makes a chemical reaction spontaneous?– Proceed on their own without any

continuous external influences (energy)

Reactions tend to be spontaneous if:1. the products have lower potential

energy than the reactants2. when the product molecules are less

ordered than the reactant molecules

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What makes a chemical reaction spontaneous?

1. Reactions tend to be spontaneous if the products have lower potential energy than the reactants

• Products have lower potential energy if their electrons are held more tightly than electrons of reactants

• More electronegative– Electronegativity– The tendency of an atom to attract electrons towards

itself

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What makes a chemical reaction spontaneous?

• Enthalpy (ΔH)– Measure of difference in energy between

reactants and products– When reaction is exothermic ΔH is negative

• Exothermic– Chemical reaction that releases heat

• Endothermic– Chemical reaction that absorbs heat

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What makes a chemical reaction spontaneous?

2. Reactions tend to be spontaneous when the product molecules are less ordered than the reactant molecules

• Entropy (S)– amount of disorder in a group of molecules– Δ S is positive when products are less

ordered than reactants– Spontaneous reactions increase entropy

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Free Energy• Physical and chemical process proceed in direction that

results in lower potential energy (negative ΔH) and increased disorder (positive ΔS)

Gibbs free-energy change (ΔG)• Free energy

– The portion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system (e.g., living cell)

ΔG = ΔH - T ΔS• T= temperature in Kelvin

– Temperature becomes more important in determining free-energy change as thetemp of molecules increases

Fig. 8.5

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Free Energy

ΔG = ΔH - T ΔS• If ΔG is less than 0, reaction is

spontaneous = exergonic– Net release of free energy

• If ΔG is greater than 0, reaction is not spontaneous = endergonic– Absorbs free energy from its

surroundings– Stores free energy in molecules

Fig. 8.6

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Free Energy & Equilibrium

• When ΔG is zero, reactions are at equilibrium– Free energy decreased– Systems cannot spontaneously move away

from equilibrium

• Living cells not at equilibrium– Products become reactants in other metabolic

pathways

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ATP & Cellular Work

Three main types of work• Mechanical

– E.g. moving cilia, contracting muscles• Transport

– Transport of molecules across cell membrane• Chemical

– Promoting chemical reactions that do not happen spontaneously (endergonic)

Most cellular work done by ATP

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ATP & Cellular Work

ATP (adenosine triphosphate)• Adenine (nitrogenous base)• Ribose (sugar)• 3 phosphate groups

– Phosphate groups negatively charged

– Repelling of charges = high potential energy

• Unstable molecule– Hydrolysis breaks bond of

terminal phosphate group– Products: Adenosine

diphosphate (ADP) and inorganic phosphate

– Exergonic: releases energy• 7.3 kcal per mole ATP

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Fig. 5.5

ATP & Cellular Work

Phosphate Transfer• Phosphorylation

– Transfer of the phosphate group from ATP to some other molecule

• This phosphorylated molecule undergoes a change that performs work– More reactive/less stable– Conformation change– Phosphorylated = molecule

that receives the phosphate group

Fig. 8.11

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ATP & Cellular Work

Energy coupling• Transfer of energy

from processes that yield energy (exergonic) to processes that consume energy (endergonic)

Fig. 8.10

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ATP & Cellular Work

ATP recycling• ATP used continuously by organism• ADP+ inorganic P brought together again via

cellular respiration– Very rapid - 10 million ATP molecules spent

& regenerated per second per active muscle cell

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Fig. 8.12