activation energy extra energy required to destabilize existing bonds and initiate a chemical...

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

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

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ΔG

Ener

gy R

elea

sed

Ener

gy S

uppl

ied

Free

Ene

rgy

(G) Activation

energy

Activationenergy0

uncatalyzedcatalyzed

Course of Reaction

Product

Reactant

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

3

4


ΔG

Ener

gy R

elea

sed

Ener

gy S

uppl

ied

Free

Ene

rgy

(G) Activation

energy

Activationenergy0

uncatalyzedcatalyzed

Course of Reaction

Product

Reactant

5

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

6


AMP

CORE

O

O–

O

O

O

HH H

H

O

CC

NN

N

C

N

C

CHH

P O–

O P

O P O

ADP

ATP

Triphosphategroup

O–

CH2

High-energybonds

a.

Adenine NH2

Ribose

OHOH

b.

7

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

8


+

+

Pi

Energy fromexergoniccellularreactions

ATP H2O

ADP

Energy forendergoniccellularprocesses

9

Enzymes: Biological Catalysts• 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

10


Active site

a. b.Enzyme Enzyme–substrate complex

Substrate

Active site

• Pockets or clefts for substrate binding• Forms enzyme–substrate complex• Precise fit of substrate into active site• Applies stress to distort particular bond to

lower activation energy– Induced fit

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12


1. The substrate, sucrose, consists of glucose and fructose bonded together.

2. The substrate binds to the active site of the enzyme, forming an enzyme– substrate complex.

3. The binding of the substrate and enzyme places stress on the glucose– fructose bond, and the bond breaks.

4. Products arereleased, andthe enzyme isfree to bind othersubstrates.

BondGlucose

Fructose

Active site

Enzymesucrase

H2O

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ATP• Cells use ATP to drive endergonic reactions– ΔG (free energy) = –7.3 kcal/mol

• 2 mechanisms for synthesis1. Substrate-level phosphorylation• Transfer phosphate group directly to ADP• During glycolysis

2. Oxidative phosphorylation• ATP synthase uses energy from a proton gradient

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PEP

– ADP– ADP

Enzyme Enzyme

– ATP– ATP

Adenosine

Pyruvate

PP

PPPP

PP

PP

Adenosine

PP


15

Oxidation of Glucose

The complete oxidation of glucose proceeds in stages:

1. Glycolysis2. Pyruvate oxidation3. Krebs cycle4. Electron transport chain & chemiosmosis

16

Outermitochondrial

membrane

Intermembranespace

Mitochondrialmatrix

FAD O2

Innermitochondrial

membrane

ElectronTransport Chain

ChemiosmosisATP Synthase

NAD+

Glycolysis

Pyruvate

Glucose

PyruvateOxidation

Acetyl-CoA

KrebsCycle

CO2

ATPH2O

ATP

e–

e–

NADH

NADH

CO2

ATP

NADH

FADH2


H+

e–

17

Glycolysis

• Converts 1 glucose (6 carbons) to 2 pyruvate (3 carbons)

• 10-step biochemical pathway• Occurs in the cytoplasm• Net production of 2 ATP molecules by

substrate-level phosphorylation• 2 NADH produced by the reduction of NAD+

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NADH must be recycled

• For glycolysis to continue, NADH must be recycled to NAD+ by either:

1. Aerobic respiration– Oxygen is available as the final electron acceptor– Produces significant amount of ATP

2. Fermentation– Occurs when oxygen is not available– Organic molecule is the final electron acceptor

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Krebs Cycle

• For each Acetyl-CoA entering:– Release 2 molecules of CO2 – Reduce 3 NAD+ to 3 NADH– Reduce 1 FAD (electron carrier) to FADH2 – Produce 1 ATP– Regenerate oxaloacetate

20

Electron Transport Chain (ETC)

• ETC is a series of membrane-bound electron carriers

• Embedded in the inner mitochondrial membrane

• Electrons from NADH and FADH2 are transferred to complexes of the ETC

• Each complex– A proton pump creating proton gradient– Transfers electrons to next carrier

21


Mitochondrial matrix

NADH + H+

ADP + PiH2O

H+ H+

2H+ + 1/2 O2

Glycolysi s

Pyruvate Oxidatio n

2

KrebsCycle ATP

Electron Transport ChainChemiosmosis

NADH dehydrogenase bc1 complexCytochrome

oxidase complex

Innermitochondrial membrane

Intermembrane space

a. The electron transport chain

ATPsynthase

b. Chemiosmosis

NAD+

Q

C

e–

FADH2

H+H+

H+H+

e–22 e–22

ATP

FAD

22

Chemiosmosis

• Accumulation of protons in the intermembrane space drives protons into the matrix via diffusion

• Membrane relatively impermeable to ions• Most protons can only reenter matrix through

ATP synthase– Uses energy of gradient to make ATP from ADP + Pi

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ADP + Pi

Catalytic head

Stalk

Rotor

H+

H+

Mitochondrialmatrix

Intermembranespace

H+ H+

H+

H+H+

ATP


24


H2O

CO2

CO2

H+

H+

2H+

+1/2 O2

H+

e–

H+

32 ATPKrebsCycle

2 ATP

NADH

NADH

FADH2

NADH

PyruvateOxidation

Acetyl-CoA

e–

QC

e–

Glycolysis

Glucose

Pyruvate

Photosynthesis

Chapter 8

25

26

Photosynthesis Overview

• Energy for all life on Earth ultimately comes from photosynthesis

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

• Oxygenic photosynthesis is carried out by– Cyanobacteria– 7 groups of algae– All land plants – chloroplasts

Chloroplast

• Thylakoid membrane – internal membrane– Contains chlorophyll and other photosynthetic

pigments– Pigments clustered into photosystems

• Grana – stacks of flattened sacs of thylakoid membrane

• Stroma lamella – connect grana• Stroma – semiliquid surrounding thylakoid

membranes27

28


Vascular bundle Stoma

Cuticle

Epidermis

Mesophyll

Chloroplast

Inner membraneOuter membrane

Cell wall

1.58 mm

Vacuole

Courtesy Dr. Kenneth Miller, Brown University

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Stages

• Light-dependent reactions– Require light1.Capture energy from sunlight2.Make ATP and reduce NADP+ to NADPH

• Carbon fixation reactions or light-independent reactions– Does not require light3.Use ATP and NADPH to synthesize organic

molecules from CO2

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O2

Stroma

Photosystem

Thylakoid

NADP+ADP + Pi

CO2

Sunlight

PhotosystemPhotosystem

Light-DependentReactions

CalvinCycle

Organicmolecules

O2

ATP NADPH

H2O


31

Pigments

• Molecules that absorb light energy in the visible range

• Light is a form of energy• Photon – particle of light– Acts as a discrete bundle of energy– Energy content of a photon is inversely

proportional to the wavelength of the light• Photoelectric effect – removal of an electron

from a molecule by light

32

Light-Dependent Reactions

1. Primary photoevent– Photon of light is captured by a pigment molecule

2. Charge separation – Energy is transferred to the reaction center; an excited

electron is transferred to an acceptor molecule3. Electron transport– Electrons move through carriers to reduce NADP+

4. Chemiosmosis– Produces ATP

Capt

ure

of li

ght e

nerg

y

33

Chloroplasts have two connected photosystems

• Oxygenic photosynthesis• Photosystem I (P700)– Functions like sulfur bacteria

• Photosystem II (P680)– Can generate an oxidation potential high enough to oxidize

water

• Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

Chemiosmosis

• Electrochemical gradient can be used to synthesize ATP

• Chloroplast has ATP synthase enzymes in the thylakoid membrane– Allows protons back into stroma

• Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

34

Production of additional ATP

• Noncyclic photophosphorylation generates– NADPH– ATP

• Building organic molecules takes more energy than that alone

• Cyclic photophosphorylation used to produce additional ATP– Short-circuit photosystem I to make a larger

proton gradient to make more ATP35

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Carbon Fixation – Calvin Cycle

• To build carbohydrates cells use• Energy– ATP from light-dependent reactions– Cyclic and noncyclic photophosphorylation– Drives endergonic reaction

• Reduction potential– NADPH from photosystem I– Source of protons and energetic electrons

37

Calvin cycle

• Named after Melvin Calvin (1911–1997)• Also called C3 photosynthesis

• Key step is attachment of CO2 to RuBP to form PGA

• Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco

38

• There are four basic mechanisms for cellular communication1. Direct contact2. Paracrine signaling3. Endocrine signaling4. Synaptic signaling

• Some cells send signals to themselves (autocrine signaling)

39

Signal transduction• Events within the cell that occur in response to a

signal• When a ligand binds to a receptor protein, the cell

has a response• Different cell types can have similar response to the

same signal– Glucagon example

• Different cell types can respond differently to the same signal– Epinephrine example

40

Kinase cascade

• Mitogen-activated protein (MAP) kinases– Important class of cytoplasmic kinases– Mitogens stimulate cell division– Activated by a signaling module called a

phosphorylation cascade or kinase cascade– Series of protein kinases that phosphorylate each

other in succession– Amplifies the signal because a few signal

molecules can elicit a large cell response

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Response

Signal

Receptor

Activator

ActiveInactive

Inactive

Inactive

Active

Active

MKKMKK

MKMK

MKKKMKKK

Ras

PP

P

PP

P

MAP kinase cascade

Firstkinase

Secondkinase

MAPkinase

Responseproteins

Cellularresponse

ResponseproteinsResponseproteins

MKKK

MKK

MK

MKKK

MKK

MK

a.

42

Signal amplification

Signal

Receptor

Activator

Cellular responses

MKK MKK MKK

MK MK MK MK

MKKK MKKK

Response proteins

b.

MKKK

MKK

MK

MKKK

MKK MKK

MK MK MK

Response proteins


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