Energy and Metabolism

Chapter 6

2

Flow of Energy

Energy: the capacity to do work

-kinetic energy: the energy of motion

-potential energy: stored energy

Energy can take many forms:

mechanical electric current

heat light

3

Flow of Energy

Most forms of energy can be converted to heat energy.

Heat energy is measured in kilocalories.

One calorie = the amount of heat required to raise the temp of water by 1oC

1 kilocalorie (kcal) = 1000 calories

4

Flow of Energy

Potential energy stored in chemical bonds can be transferred from one molecule to another by way of electrons.

oxidation: loss of electrons

reduction: gain of electrons

redox reactions are coupled to each other.

5

6

Laws of Thermodynamics

First Law of Thermodynamics – energy cannot be created or destroyed

-energy can only be converted from one form to another

For example:

sunlight energy chemical energy

photosynthesis

7

Laws of Thermodynamics

Second Law of Thermodynamics: disorder is more likely than order

entropy: disorder in the universe

The 2nd Law of Thermodynamics states that entropy is always increasing.

8

9

Laws of Thermodynamics

Free energy: the energy available to do work

-denoted by the symbol G (Gibb’s free energy)

enthalpy: energy contained in a molecule’s chemical bonds

free energy = enthalpy – (entropy x temp.)

G = H - TS

10

Laws of Thermodynamics

• Chemical reactions can create changes in free energy:

G = H - T S

• When products contain more free energy than reactants – G is positive.

• When reactants contain more free energy than products – G is negative.

Delta G

• Gibbs Free Energy Equation

• ΔG = ΔH - TΔS

• where

• ΔG = Gibbs Free Energy, in kJΔH = enthalpy changeT = temperature, in KelvinΔS = entropy change (in kJ · K-1)

11

DELTA G

• IF ΔG < 0   The reaction is spontaneous ΔG > 0  The reaction is nonspontaneous ΔG = 0   The reaction is at equilibrium.Free energy changeis the net driving forceof a chemical reaction—whether the reactionwill be spontaneousor not 12

DELTA G PROBLEM• Notice the case where ΔG = 0. Equilibrium is a concept we will be discussing in depth in another

module.

• Generally you will find that most exothermic reactions are spontaneous, even if entropy decreases (becomes more ordered), because enthalpy contributes more to ΔG than does entropy. The exceptions are reactions occurring at high temperatures.

• Example.

• Calculate ΔG for the following reaction at 25°C. Will the reaction occur (be spontaneous)? How do you know?

• NH3(g) + HCl(g) → NH4Cl(s)

• Also given for this reaction:

• ΔH = -176.0 kJΔS = -284.8 J·K-1

• Solution

• We will calculate ΔG using the formula

• ΔG = ΔH - TΔS

• but first we need to convert units for ΔS and temperature to Kelvin:

• ΔS = -284.8 J·K-1 = -0.2848 kJ·K-1K = 273 + °C = 273 + 25 = 298 K Now we can solve our equation:

• ΔG = -176.0 - (298)(-0.2848)ΔG = -176.0 - (-84.9)ΔG = -91.1 kJ

• Since ΔG < 0 the reaction will be spontaneous.13

14

Laws of Thermodynamics

Chemical reactions can be described by the transfer of energy that occurs:

endergonic reaction: a reaction requiring an input of energy

- G is positive

exergonic reaction: a reaction that releases free energy

- G is negative

15

16

Laws of Thermodynamics

Most reactions require some energy to get started.

activation energy: extra energy needed to get a reaction started

-destabilizes existing chemical bonds

-required even for exergonic reactions

catalysts: substances that lower the activation energy of a reaction

17

18

Energy Currency of Cells

ATP = adenosine triphosphate

-the energy “currency” of cells

ATP structure:

-ribose, a 5-carbon sugar

-adenine

-three phosphates

19

20

Energy Currency of Cells

ATP stores energy in the bonds between phosphates.

Phosphates are highly negative, therefore:

-the phosphates repel each other

-much energy is required to keep the phosphates bound to each other

-much energy is released when the bond between two phosphates is broken

21

Energy Currency of Cells

When the bond between phosphates is broken:

ATP ADP + Pi

energy is released

ADP = adenosine diphosphate

Pi = inorganic phosphateThis reaction is reversible.

22

23

Energy Currency of Cells

The energy released when ATP is broken down to ADP can be used to fuel endergonic reactions.

The energy released from an exergonic reaction can be used to fuel the production of ATP from ADP + Pi.

24

Enzymes

Enzymes: molecules that catalyze reactions in living cells

-most are proteins

-lower the activation energy required for a reaction

-are not changed or consumed by the reaction

25

Enzymes

Enzymes interact with substrates.

substrate: molecule that will undergo a reaction

active site: region of the enzyme that binds to the substrate

Binding of an enzyme to a substrate causes the enzyme to change shape, producing a better induced fit between the molecules.

26

27

28

Enzymes

Multienzyme complexes offer certain advantages:

1. The product of one reaction can be directly delivered to the next enzyme.

2. The possibility of unwanted side reactions is eliminated.

3. All of the reactions can be controlled as a unit.

29

Enzymes

Not all enzymes are proteins.

Certain reactions involving RNA molecules are catalyzed by the RNA itself.

ribozymes: RNA with enzymatic abilities

For example, the ribosome is a ribozyme.

30

Enzymes

Enzyme function is affected by its environment.

Factors that can change an enzyme’s 3-dimensional shape can change its function.

-for example, pH, temperature, regulatory molecules

31

Enzymes

Temperature -enzyme activity may be increased with

increasing temp, up to the temp optimum-temperatures too far above the temp

optimum can denature the enzyme, destroying its function

pH – most enzymes prefer pH values from 6 to 8.

32

33

Enzymes

Inhibitors are molecules that bind to an enzyme to decrease enzyme activity.

-competitive inhibitors compete with the substrate for binding to the same active site

-noncompetitive inhibitors bind to sites other than the enzyme’s active site

34

Enzymes

Allosteric enzymes exist in either an active or inactive state.

-possess an allosteric site where molecules other than the substrate bind

-allosteric inhibitors bind to the allosteric site to inactivate the enzyme

allosteric activators bind to the allosteric site to activate the enzyme

35

36

Metabolism

Metabolism: all chemical reactions occurring in an organism

Anabolism: chemical reactions that expend energy to make new chemical bonds

Catabolism: chemical reactions that harvest energy when bonds are broken

37

Metabolism

Some enzymes require additional molecules for proper enzymatic activity.

These molecules could be:

-cofactors: usually metal ions, found in the active site participating in catalysis

-coenzymes: nonprotein organic molecules, often used as an electron donor or acceptor in a redox reaction

38

Metabolism

Biochemical pathways are a series of reactions in which the product of one reaction becomes the substrate for the next reaction.

Biochemical pathways are often regulated by feedback inhibition in which the end product of the pathway is an allosteric inhibitor of an earlier enzyme in the pathway.

39

40