Energetics

1

Adapted by G. Cornwall, Ph.D. From Raven’s Biology, McGraw Hill Publishing

Chapter 6

2

Flow of Energy

• Thermodynamics– Branch of chemistry concerned with energy

changes

• Cells are governed by the laws of physics and chemistry

• Energy – capacity to do work– 2 states

1. Kinetic – energy of motion

2. Potential – stored energy

– Many forms – mechanical, heat, sound, electric current, light, or radioactivity

3

• Heat the most convenient way of measuring energy.– 1 calorie = heat required to raise 1 gram of water 1ºC

– calorie or Calorie?

• Energy flows into the biological world from the sun

• Photosynthetic organisms capture this energy

• Stored as potential energy in chemical bonds

4

5

Redox reactions• Oxidation

– Atom or molecule loses an electron

• Reduction– Atom or molecule gains an electron– Higher level of energy than oxidized form

• Oxidation-reduction reactions (redox)– Reactions always paired

Laws of thermodynamics

• First law of thermodynamics– Energy cannot be created or

destroyed– Can only change from one

form to another– Total amount of energy in the

universe remains constant– During each conversion,

some energy is lost as heat

6

7

• Second law of thermodynamics– Entropy (disorder) is continuously increasing– Energy transformations proceed

spontaneously to convert matter from a more ordered/ less stable form to a less ordered/ more stable form

8

Free energy

• G = Energy available to do work

• G = H – TS H = enthalpy, energy in a molecule’s chemical

bonds T = absolute temperature S = entropy, unavailable energy

ΔG = ΔH - TS

• ΔG = change in free energy• Positive ΔG

– Products have more free energy than reactants

– H is higher or S is lower– Not spontaneous, Requires input of

energy– Endergonic

• Neagtive ΔG – Products have less free energy than

reactants– H is lower or S is higher or both– Spontaneous (may not be

instantaneous)– Exergonic

9

Activation energy• Extra energy required to destabilize existing

bonds and initiate a chemical reaction• Exergonic reaction’s rate depends on the

activation energy required– Larger activation energy proceeds more slowly

10

• Rate can be increased 2 ways1. Increasing energy of reacting molecules (heating)

2.Lowering activation energy

11

Catalysts

• Substances that influence chemical bonds in a way that lowers activation energy

• Cannot violate laws of thermodynamics– Cannot make an endergonic reaction

spontaneous

• Do not alter the proportion of reactant turned into product

12

13

ATP• Adenosine triphosphate• Chief “currency” all cells

use• Composed of

– Ribose – 5 carbon sugar– Adenine– Chain of 3 phosphates

• Key to energy storage• Bonds are unstable• ADP – 2 phosphates• AMP – 1 phosphate – lowest

energy form

14

ATP cycle• ATP hydrolysis drives endergonic reactions

– Coupled reaction results in net –ΔG (exergonic and spontaneous)

• ATP not suitable for long term energy storage– Fats and carbohydrates better– Cells store only a few seconds worth of ATP

Card Quiz A

Oxidation is the ____________ and reduction is the ________.

loss of electrons, gain of electrons gain of protons, loss of protons loss of protons, gain of protons loss of electrons, gain of protons

Card Quiz A

The first law of thermodynamics states that energy can be –

Created Destroyed Converted Lost

Card Quiz A

The energy in a system that is able to do work. Enthalpy Entropy Free energy Kinetic energy

Card Quiz A

A reaction with a positive ∆G is – Exergonic Endergonic Enthalpic Energertic

Card Quiz A

Spending ATP involves hydrolyzing it into ADP and inorganic P. The energy released can drive other chemical reactions.

This is true This is false

Card Quiz A

A catalyst speeds up chemical reactions. How do catalysts do this?

Decreasing entropy Altering ∆G Consuming reactants Lowering activation energy

Card Quiz Answers

Green Red Red Red

Green Red

22

Enzymes• Most enzymes are protein

– Some are RNA

• Shape of enzyme stabilizes a temporary association between substrates

• Enzyme not changed or consumed in reaction• Carbonic anhydrase

– 200 molecules of carbonic acid per hour made without enzyme– 600,000 molecules formed per second with enzyme

Active site• Pockets or clefts for substrate binding• Forms enzyme-substare complex• Precise fit of substrate into active site• Applies stress to distort particular bond to lower activation

energy– Induced fit

23

24

• Enzymes may be suspended in the cytoplasm or attached to cell membranes and organelles

• Multienzyme complexes – subunits work together to form molecular machine– Product can be delivered easily to next

enzyme– Unwanted side reactions prevented– All reactions an be controlled as a unit

25

Nonprotein enzymes

• Ribozymes• 1981 discovery that certain reactions

catalyzed in cells by RNA molecule itself2 kinds

1. Intramolecular catalysis – catalyze reaction on RNA molecule itself

2. Intermolecular catalysis – RNA acts on another molecule

Enzyme function• Rate of enzyme-catlyzed reaction depends on

concentrations of substrate and enzyme• Any chemical or physical condition that affects

the enzyme’s 3 dimensional shape can change rate– Optimum temperature– Optimum pH

26

27

Inhibitors

• Inhibitor – substance that binds to enzyme and decreases its activity

• Competitive inhibitor– Competes with substrate

for active site

28

• Noncompetitive inhibitor– Binds to enzyme at a site

other than active site– Causes shape change

that makes enzyme unable to bind substrate

Cofactors & Coenzymes

• Cofactors: assist in enzyme function– Often metal ions found in active site– Zinc in corboxypeptidase– Required in the diet in small amounts

• Coenzymes: a cofactor that is an organic molecule (non-protein)– Many are vitamins essential in our diets

• Often coenzymes and cofactors are involved in capturing and transferring electrons

29

30

Allostery• Allosteric enzymes – enzymes exist in active

and inactive forms• Most noncompetitive inhibitors bind to allosteric

site – chemical on/off switch• Allosteric inhibitor – binds to allosteric site and

reduces enzyme activity• Allosteric activator – binds to allosteric site and

increases enzyme activity

31

Metabolism

• Total of all chemical reactions carried out by an organism

• Anabolic reactions / anabolism– Expend energy to build up molecules

• Catabolic reactions/ catabolism– Harvest energy by breaking down molecules

32

Biochemical pathways

• Reactions occur in a sequence

• Product of one reaction is the substrate for the next

• Many steps take place in organelles

Feedback inhibition• End-product of pathway binds to an

allosteric site on enzyme that catalyses first reaction in pathway

• Shuts down pathway so raw materials and energy are not wasted

33

Card Quiz B

Small organic molecules that assist in enzymatic functions are –

Coprolites Cofactors Activators Coenzymes

Card Quiz B

Feedback inhibition occurs when the final product acts as an inhibitor to the first enzyme in the pathway.

This is true This is false

Card Quiz B

Carbon monoxide (CO) binds to the hemoglobin protein at the oxygen binding site. Once the CO binds, the oxygen can no longer be transported. What does this describe?

Non-competitive inhibition Feedback inhibition Allosteric inhibition Competitive inhibition

Card Quiz B

Increasing the temperature increases the rate of an enzyme-catalyzed reaction. Once a critical temperature is reached, the reaction stops. Why does this happen?

The concentration of reactants drop The enzymes have all been consumed in the

reaction The increase in temperature alters the pH The polypeptide chains in the enzyme denature

Card Quiz Answers

Yellow Blue Red Red