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

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Hypotonic and hypertonic are relative terms

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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