UNIT 2

Chapter 6: A Tour of the CellChapter 7: Membrane Structure &

FunctionChapter 8: An Introduction to Metabolism

The Chemistry of Life is Organized into The Chemistry of Life is Organized into Metabolic PathwaysMetabolic Pathways

The sum total of all an organism’s The sum total of all an organism’s chemical reactions is its chemical reactions is its metabolismmetabolism CatabolismCatabolism: breakdown of molecules, : breakdown of molecules,

releases energyreleases energy AnabolismAnabolism: construction of molecules, stores : construction of molecules, stores

energyenergy BioenergeticsBioenergetics is the study of how is the study of how

organisms manage their energy resourcesorganisms manage their energy resources

Metabolism is highly complex and Metabolism is highly complex and numerous metabolic pathways existnumerous metabolic pathways exist

DefinitionsDefinitions Kinetic energyKinetic energy Potential energyPotential energy

• Chemical energyChemical energy

Living Systems are Subject to Two Laws Living Systems are Subject to Two Laws of Thermodynamicsof Thermodynamics

ThermodynamicsThermodynamics is the study of energy is the study of energy transformationstransformations First Law of ThermodynamicsFirst Law of Thermodynamics: energy cannot : energy cannot

be created or destroyedbe created or destroyed Second Law of ThermodynamicsSecond Law of Thermodynamics: energy : energy

transformation must make the universe more transformation must make the universe more disordereddisordered• EntropyEntropy: a measure of disorder or randomness: a measure of disorder or randomness

Order can be increased locally, but there Order can be increased locally, but there is an unstoppable trend towards is an unstoppable trend towards randomization in the universerandomization in the universe Increased entropy usually in the form of heatIncreased entropy usually in the form of heat Heat is the most random state of energyHeat is the most random state of energy

Organisms do not violate the 2Organisms do not violate the 2ndnd law law Light energy or chemical energy goes in, and Light energy or chemical energy goes in, and

convert that energy into mostly heatconvert that energy into mostly heat Living organisms possess relatively low Living organisms possess relatively low

entropy compared to the universeentropy compared to the universe

Free EnergyFree Energy

Free energyFree energy is a system’s energy available is a system’s energy available to do workto do work

Reactions are considered Reactions are considered spontaneousspontaneous if if G is negativeG is negative

If If G = 0, the reaction is at equilibriumG = 0, the reaction is at equilibrium

G = GG = Gfinalfinal – G – Gstartstart

Metabolism and Metabolic Metabolism and Metabolic DisequilibriumDisequilibrium

Exergonic Exergonic reactions release energy and reactions release energy and occur spontaneouslyoccur spontaneously

G is negativeG is negative• CC66HH1212OO66 + 6 O + 6 O22 6 CO 6 CO22 + 6 H + 6 H22O + energyO + energy

EndergonicEndergonic reactions store energy and reactions store energy and are non-spontaneousare non-spontaneous

G is positiveG is positive

In natural (living) systems, equilibrium is In natural (living) systems, equilibrium is not reachednot reached

ATP (Adenosine TriPhosphate)ATP (Adenosine TriPhosphate)

In most cases, ATP is the immediate In most cases, ATP is the immediate source of energy for cellssource of energy for cells

The third phosphate group can be The third phosphate group can be hydrolyzed to produce ADP, a phosphate hydrolyzed to produce ADP, a phosphate group and energygroup and energy (7.3kcal/mole of ATP)(7.3kcal/mole of ATP)

Energy couplingEnergy coupling is a common “tactic” used is a common “tactic” used by cells to power endergonic reactions by cells to power endergonic reactions using exergonic onesusing exergonic ones Phosphate group hydrolyzed from ATP used Phosphate group hydrolyzed from ATP used

to to phosphorylatephosphorylate another molecule another molecule ATP can be regenerated by cells very ATP can be regenerated by cells very

rapidlyrapidly Working muscle cells use ~10million ATP Working muscle cells use ~10million ATP

molecules molecules per secondper second

Enzymes Work to Speed Reaction Enzymes Work to Speed Reaction RatesRates

EnzymesEnzymes are biological are biological catalystscatalysts that that lower the lower the energy of activationenergy of activation (E (EAA) for a ) for a

reactionreaction Enzymes are not Enzymes are not

altered by the altered by the reactionreaction They are free to They are free to

catalyze againcatalyze again

Enzymes are Designed to Work in Enzymes are Designed to Work in Specific ReactionsSpecific Reactions

A given enzyme will only work on one type A given enzyme will only work on one type of of substratesubstrate

Substrate will bind to Substrate will bind to active siteactive site of protein of protein

LactaseLactase

Lactose + HLactose + H22O O Glucose + Galactose Glucose + Galactose

Enzyme ActivityEnzyme Activity

Enzymes are proteins, and therefore are Enzymes are proteins, and therefore are subject to subject to denaturationdenaturation Enzymes possess Enzymes possess optimaoptima – conditions at – conditions at

which they function bestwhich they function best• Temperature and pHTemperature and pH• Some enzymes require Some enzymes require cofactorscofactors (inorganic (inorganic

substances) or substances) or coenzymescoenzymes (organic substances) to (organic substances) to promote catalytic activity promote catalytic activity

Some molecules prevent enzyme activity Some molecules prevent enzyme activity by binding to the enzymeby binding to the enzyme Competitive inhibitionCompetitive inhibition: inhibiting molecule : inhibiting molecule

binds to active site, preventing substrate from binds to active site, preventing substrate from bindingbinding

Non-competitive inhibitionNon-competitive inhibition: inhibiting : inhibiting molecules bind elsewhere on the enzyme, molecules bind elsewhere on the enzyme, which alters the enzymes conformation and which alters the enzymes conformation and the active sitethe active site

Metabolic ControlMetabolic Control

Allosteric enzymes Allosteric enzymes can be activated or can be activated or deactivated by an activator or inhibitordeactivated by an activator or inhibitor They bind to They bind to allosteric siteallosteric site on enzyme on enzyme

Most allosteric enzymes are comprised of Most allosteric enzymes are comprised of multiple polypeptidesmultiple polypeptides

Enzymes can be Enzymes can be inhibited by the inhibited by the products they createproducts they create Feedback inhibitionFeedback inhibition

In multiple subunit In multiple subunit enzymes, enzymes, cooperativitycooperativity can can amplify the enzyme’s amplify the enzyme’s response to response to substratessubstrates

END

Cell Membranes & Phospholipids

Phospholipids constitute cell membranes and their fatty acid tails determine membrane fluidity Unsaturated tails increase fluidity, saturated

tails decrease fluidityTemperature also plays a role:

Cholesterol also influences membrane movement Reduces membrane

fluidity

Warm: phospholipids move freely Cool: tight packing of phospholipids - solidify

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

Higher concentration of solutes = hypertonic Lower concentration of solutes = hypotonic

Hypotonic and hypertonic are relative terms 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