Biochemistry I - Introduction to Biological Energy Flow - Quick Guides | Biochemistry

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Study GuideBiochemistry I–Introduction to Biological Energy Flow1. Enzyme CatalystsAlmost all biochemical reactions in living cells need help to occur efficiently. This help comes fromenzymes, which are specializedbiochemical catalysts.For example, the biological synthesis of ammonia does not happen on its own at a useful rate. Itrequires aspecific enzymeto make the reaction proceed properly.Most enzymes areproteins, and they act astrue catalysts. This means they:•Speed up chemical reactions•Arenot used upduring the reaction•Can carry out the same reactionover and over againBecause enzymes are so efficient and reusable, they make it possible for thousands of chemicalreactions to occur quickly and in a controlled way inside living cells.Key TakeawayEnzymes are protein catalysts that make life’s chemistry possible by speeding up reactions withoutbeing consumed in the process.2.Space and Time Links in the CellAt any given moment,thousands of chemical reactionsare happening inside a single cell. Even asimple bacterium must perform many tasks at the same time. It has to:•Copy its DNA•Make new enzymes•Break down carbohydrates to release energy•Produce building blocks for proteins and nucleic acids•Bring nutrients into the cell and remove waste productsAll of this happenssimultaneously and efficiently.

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Study Guide2.1Metabolic PathwaysEach major task in the cell is carried out through a series of enzyme-driven steps called ametabolicpathway.•A pathway is a sequence of reactions•Each step is catalyzed by aspecific enzyme•The product of one step becomes the substrate for the nextThese reactions occur in a precise order, ensuring that the final product is made correctly.2.2Channeling Reactions for EfficiencyCells also controlwhere and whenreactions occur. In many cases, the molecules moving through apathway arechanneled directly from one enzyme to the next, instead of freely mixing in the cell.This organization has important advantages:•Increases efficiency•Prevents interference between pathways•Allows tight regulation of metabolismExample: Muscle CellsIn muscle cells, glucose is used for different purposes:•Some glucose provides energy formuscle contraction•Other glucose is used topower ion transport across the cell membraneThese two uses are kept separate. The glucose molecules do not mix randomly because they aredirected throughdifferent enzyme pathwaysin specific locations.Key TakeawayCells manage enormous chemical complexity by organizing reactions in bothspace and time. Byusing enzyme pathways and directing substrates through specific routes, cells can carry out manyessential processes at once without confusion or waste.

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Study Guide3.Energy Flow and Thermodynamics3.1What Is Thermodynamics?Thermodynamics is the branch of chemistry and physics that studiesenergy flowin physical andchemical systems. In simple terms, it helps us understand how energy moves, changes, anddetermines whether reactions can happen.3.2The Laws of ThermodynamicsFirst Law of Thermodynamics: Energy Is ConservedTheFirst Law of Thermodynamicsstates thatenergy cannot be created or destroyed. It can onlychange from one form to another.For example, chemical energy in food can be converted into mechanical energy when musclesmove—but the total amount of energy stays the same.3.3Second Law of Thermodynamics: Direction of ReactionsTheSecond Law of Thermodynamicsexplainswhy some reactions happen and others do not.•Spontaneous reactionsoccur naturally without extra energy input.•These reactions are associated with anincrease in entropy, which means an increase indisorder.What Is Entropy?Entropy can be thought of asenergy that is spread out and not useful for doing work.For example:•The oceans contain huge amounts of thermal energy from moving water molecules.•However, this energy is too dispersed to power a boat.•To move the boat, we need usable energy like fuel or wind.

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Study Guide3.4Free Energy: Usable Energy in a SystemThe energy thatcanbe used to do work is calledfree energy.The change in free energy for a reaction is given by:ΔG =ΔH − TΔSWhere:•ΔG= change in free energy•ΔH= change in enthalpy (heat content)•T= temperature in Kelvin•ΔS= change in entropyWhat Does This Equation Mean?•A reaction releases usable energy whenΔG is negative.•Another way to state the Second Law is:Reactions proceed in the direction that lowers free energy.3.5Free Energy and Chemical EquilibriumThe free energy change of a reaction understandard conditions(all reactants and products at 1 Mconcentration) is related to the equilibrium constant:ΔG° = −RT ln(Kₑq)Where:•R= gas constant•T= temperature in Kelvin•Kₑq= equilibrium constantWhat This Tells Us:•IfKₑq > 1, the reaction is favored.oln(Kₑq) is positiveoΔG° is negativeoThe reaction isexergonic(releases energy)

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Study Guide•IfKₑq < 1, the reaction is unfavored.oln(Kₑq) is negativeoΔG° is positiveoThe reaction isendergonic(requires energy)3.6Standard States in BiochemistryIn chemistry, standard free energy changes are measured with all substances at1 M concentration.However, this does not work well for biological systems.Why Biochemistry Uses a Different Standard State•Many biomolecules are unstable in strong acid.•Living cells operate aroundpH 7.0, not pH 0.Because of this, biochemistry uses abiochemical standard state:•pH = 7.0•Standard free energy is written asΔG°′•The prime (′) indicates this special biological condition3.7Free Energy Under Real Cellular ConditionsInside cells, reactant and product concentrations aremuch lower than 1 M. To calculate free energyunder these real conditions, we use:ΔG =ΔG°′ + RT ln(Π[Products] /Π[Reactants])Where:•Π[Products]= concentrations of all products multiplied together•Π[Reactants]= concentrations of all reactants multiplied togetherIf a substance appears more than once in the reaction, its concentration is raised to the appropriatepower—just like in equilibrium expressions.

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Study Guide3.8Special Cases•Standard conditions (1 M):ΔG =ΔG°′•At equilibrium:ΔG = 0At equilibrium, the equation becomes:ΔG°′ = −RT ln(Kₑq)Here,Kₑqis the equilibrium constant for the reaction atpH 7.0.Key Takeaway•Energy is conserved but becomes less useful as entropy increases.•Free energy tells us whether a reaction can occur.•NegativeΔG means a reaction is spontaneous.•Equilibrium and free energy are closely connected.•Biochemistry uses a special standard state to match real biological conditions.4.Free-Energy Calculations in Biochemical Reactions4.1Free Energy in BiochemistryIn biochemistry, free-energy changes are usually reported asstandard free energies of hydrolysis.This is because many important cellular reactions involve breaking bonds using water.Hydrolysis of Glucose-6-PhosphateConsider the hydrolysis of glucose-6-phosphate:Glucose-6-phosphate + H₂O → Glucose + PᵢUnder standard biochemical conditions, this reaction has:•ΔG°′ = −4.0 kcal/mol (−16.5 kJ/mol)

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