An Introduction to Metabolism
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Transcript An Introduction to Metabolism
UNIT 2
Chapter 6: A Tour of the Cell
Chapter 7: Membrane Structure & Function
Chapter 8: An Introduction to Metabolism
The Chemistry of Life is Organized into
Metabolic Pathways
The
sum total of all an organism’s
chemical reactions is its metabolism
Catabolism: breakdown of molecules,
releases energy
Anabolism: construction of molecules, stores
energy
Bioenergetics
is the study of how
organisms manage their energy resources
Metabolism
is highly complex and
numerous metabolic pathways exist
Definitions
Kinetic energy
Potential energy
• Chemical energy
Living Systems are Subject to Two Laws
of Thermodynamics
Thermodynamics
is the study of energy
transformations
First Law of Thermodynamics: energy cannot
be created or destroyed
Second Law of Thermodynamics: energy
transformation must make the universe more
disordered
• Entropy: a measure of disorder or randomness
Order
can be increased locally, but there
is an unstoppable trend towards
randomization in the universe
Increased entropy usually in the form of heat
Heat is the most random state of energy
Organisms
do not violate the 2nd law
Light energy or chemical energy goes in, and
convert that energy into mostly heat
Living organisms possess relatively low
entropy compared to the universe
Free Energy
energy is a system’s energy available
to do work
Free
G = Gfinal – Gstart
Reactions
are considered spontaneous if
G is negative
If G = 0, the reaction is at equilibrium
Metabolism and Metabolic
Disequilibrium
Exergonic reactions release energy and
occur spontaneously
G is negative
•
C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy
Endergonic reactions store energy and
are non-spontaneous
G is positive
In
natural (living) systems, equilibrium is
not reached
ATP (Adenosine TriPhosphate)
In
most cases, ATP is the immediate
source of energy for cells
The
third phosphate group can be
hydrolyzed to produce ADP, a phosphate
group and energy
(7.3kcal/mole of ATP)
coupling is a common “tactic” used
by cells to power endergonic reactions
using exergonic ones
Energy
Phosphate group hydrolyzed from ATP used
to phosphorylate another molecule
ATP
can be regenerated by cells very
rapidly
Working muscle cells use ~10million ATP
molecules per second
Enzymes Work to Speed Reaction
Rates
Enzymes
are biological catalysts that
lower the energy of activation (EA) for a
reaction
Enzymes are not
altered by the
reaction
They are free to
catalyze again
Enzymes are Designed to Work in
Specific Reactions
A
given enzyme will only work on one type
of substrate
Lactase
Lactose + H2O Glucose + Galactose
Substrate will bind to active site of protein
Enzyme Activity
Enzymes
are proteins, and therefore are
subject to denaturation
Enzymes possess optima – conditions at
which they function best
• Temperature and pH
• Some enzymes require cofactors (inorganic
substances) or coenzymes (organic substances) to
promote catalytic activity
Some
molecules prevent enzyme activity
by binding to the enzyme
Competitive inhibition: inhibiting molecule
binds to active site, preventing substrate from
binding
Non-competitive inhibition: inhibiting
molecules bind elsewhere on the enzyme,
which alters the enzymes conformation and
the active site
Metabolic Control
Allosteric
enzymes can be activated or
deactivated by an activator or inhibitor
They bind to allosteric site on enzyme
Most
allosteric enzymes are comprised of
multiple polypeptides
Enzymes
can be
inhibited by the
products they create
Feedback inhibition
In
multiple subunit
enzymes,
cooperativity can
amplify the enzyme’s
response to
substrates
END
Cell Membranes & Phospholipids
Phospholipids constitute cell membranes and
their fatty acid tails determine membrane
fluidity
Unsaturated tails increase fluidity, saturated
tails decrease fluidity
Temperature
plays a role:
also
Warm: phospholipids move freely
Cool: tight packing of phospholipids - solidify
Cholesterol also
influences membrane
movement
Reduces membrane
fluidity
Cells can alter the lipid
composition of their
membranes to suit
environmental needs
Fluid Mosaic Model
Membranes possess a variety of different
proteins embedded in the phospholipid
bilayer
There are two main types of membrane
proteins: peripheral and integral
(transmembrane)
Peripheral proteins are not embedded in the
membrane itself, they are bound to proteins
found in the membrane
The Role of Proteins in Membranes
Proteins help provide structure and support
for cells
They also perform numerous other functions
Cell-to-cell recognition is achieved by integral
proteins and the carbohydrates bound to
them
Membrane carbohydrates usually branched
oligosaccharides
Cells can be distinguished from one another
Membrane’s Molecular Organization
Allows for Selective Permeability
Molecules and ions are constantly moving
across cell membranes
Oxygen, carbon dioxide, sugars, amino acids,
ions (K+, Na+, Ca2+, Cl-)
Passage is not indiscriminate, membranes
are selectively permeable
Dependent upon interaction with hydrophobic
core of membrane
Transport proteins may assist molecules
across membrane
Some Transport Across a Membrane
Does Not Require Energy
Transport across a membrane may occur
without energy (passive) or energy may be
required (active)
Diffusion is the simplest form of passive
transport
Requires a concentration gradient to occur
Even though a concentration gradient may
exist, some molecules may not be able to
pass through the membrane
Facilitated diffusion involves the use of a
transport protein
Some simply provide channels for molecules
Others change
conformation to move
molecules
Active Transport Requires Energy
Cells sometimes need to move molecules
against their concentration gradients
Active transport requires the cell to “spend”
some of its energy, usually in the form of ATP
Sodium/Potassium pump (Na+/K+ pump)
3 Na+ move out, 2 K+ move in
Sodium/Potassium Pump
Osmosis
Osmosis is the passive diffusion of water
across a selectively permeable membrane
Concentration differences in solutions required
Hypotonic and hypertonic are relative terms
Higher concentration of solutes = hypertonic
Lower concentration of solutes = hypotonic
Ex. Human cells are hypertonic to distilled water,
but they are hypotonic to sea water
If no concentration differences exist, solutions
are isotonic
In the case of osmosis, the type of solutes
present does not matter – only the total
amount of solutes
Osmosis will
continue until
both solutions
are isotonic
END