Metabolism
Lecture 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
1
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
2
Energy
Energy
• The capacity to cause change
• The ability to rearrange a collection of matter
Two main types
• Kinetic energy
• Potential energy
3
Energy
Kinetic 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
4
Energy
Potential energy
• Stored energy
• Based on location or structure
• Chemical energy
– Form of potential energy available for
release in a chemical reaction
5
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
6
7
Review Chemical Bonds, pg. 38-41
• Covalent
• Ionic
• Hydrogen
8
Energy Transformation
• Energy can
change from
potential to
kinetic and
back
9
Energy Transformation
• Energy can change
from potential to
kinetic and back
Fig. 8.2
Energy & Thermodynamics
Thermodynamics
• Study of energy transformations that occur in a
collection of matter
First 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
Fig.8.3
10
Energy & Thermodynamics
Second 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
Fig.8.3
11
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”
12
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
13
14
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)
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 reactants
2. when the product molecules are less
ordered than the reactant molecules
15
16
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
17
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
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
18
19
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 the
temp of molecules increases
Fig. 8.5
20
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
21
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
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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23
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
Fig. 5.5
24
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
25
ATP & Cellular Work
Energy coupling
• Transfer of energy
from processes that
yield energy
(exergonic) to
processes that
consume energy
(endergonic)
Fig. 8.10
26
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
Fig. 8.12