Name: how catalysts lower reaction barriers
Uploaded: 2019-03-11T09:21:09.000Z
Duration: 1 min 26 s
Channel: JoVE

K12 Standards

Louisiana Student Standards for Science

High School

Chemistry

Matter And Its Interactions

HS-PS1-4

Activation Energy

- [Narrateur] L'énergie d'activation est l'énergienécessaire pour déclencher une réaction chimiqueentre les réactifs en présence. 
Elle provient souvent de l'énergie thermiquequi accélère le mouvement des moléculeset provoque des collisions entre celles-ci ;ce qui rompt les liaisons chimiques existantentre les réactifs. 
Ainsi, les réactifs originaux ont eu besoin d'énergielibre pour que la réaction puisse avoir lieu. 
 Représentéepar la partie ascendante de cette courbe,c'est la quantité d'énergie d'activation de la réaction. 
Au niveau du sommet de la courbe, appelé état de transition,les molécules non liées sont maintenantdans un état instable,car les atomes s'associent dans de nouvellesliaisons chimiques, ils libèrent del'énergie libre dans l'environnement,comme vous le voyez dans la portiondescendante de la courbe. 
Tandis que les êtres humains métabolisent du sucre etde la graisse pour avoir de l'énergie,si l'énergie thermique était utilisée pour briserces molécules, trop d'énergie libreserait produite sous forme de chaleur au point dedénaturer les protéines contenues dans les cellules. 
Au lieu de cela, des substances appelées catalyseurssont spécialement ajoutées pour réguler le taux demétabolisme, en l'accélérant, par exemple. 
Ainsi, un catalyseur biologique, une enzyme,réduit l'énergie d'activation nécessaire pour briserdes liaisons chimiques et fait en sorte que les réactionschimiques se produisent sans endommager la cellule.

Robinson, Peter K. « Enzymes : principes et applications biotechnologiques ». Essais en biochimie 59 (15 novembre 2015) : 1-41. [<a href="https://doi.org/10.1042/bse0590001 » target="__blank">Source</a>]

Énergie d'activation

L’énergie d’activation est la quantité minimale d’énergie nécessaire pour qu’une réaction chimique avance. Plus l’énergie d’activation est élevée, plus la vitesse de la réaction est lente. Cependant, l’ajout de chaleur à la réaction augmentera la vitesse car elle fait bouger les molécules plus rapidement et augmente la probabilité que des molécules entrent en collision. La collision et la rupture des liaisons représentent la phase ascendante d’une réaction et génèrent l’état de transition. L’état de transition est un état instable à haute énergie des réactifs. La formation de nouvelles liaisons chimiques et de nouveaux produits finaux ainsi que la libération d’énergie libre est la phase descendante de la réaction. Les catalyseurs augmentent la vitesse d’une réaction en abaissant l’énergie d’activation. Par exemple, en biologie, les enzymes qui métabolisent le sucre et les graisses augmentent la vitesse à laquelle leur dégradation se produit et en même temps, empêchent la surproduction d’énergie libre qui autrement dénaturerait les protéines dans la cellule.
 
 <h4>Les catalyseurs</h4>
 
 Un catalyseur est une substance qui augmente la vitesse d’une réaction en abaissant l’énergie d’activation et qui se régénère en cours de route. Un catalyseur fournit une voie ou un mécanisme alternatif pour que la réaction ait lieu et il accélère les réactions dans le sens direct et inverse. En biologie, les enzymes sont des exemples de catalyseurs parce qu’elles réduisent l’énergie d’activation nécessaire pour les réactions dans le métabolisme cellulaire.
 
 Par exemple, les humains métabolisent le sucre et la graisse pour l’énergie. Les enzymes sont indispensables pour que l'homme puisse décomposer ces molécules car si seule l'énergie thermique était utilisée, l'énergie libre libérée sous forme de chaleur provoquerait la dénaturation des protéines de la cellule. En outre, l’énergie thermique catalyserait de manière non spécifique toutes les réactions. Cependant, les enzymes ne se lient qu’à des réactifs chimiques spécifiques, appelés substrats, et elles réduisent leur énergie d’activation pour catalyser des réactions cellulaires sélectives.

L’énergie d’activation est la quantité minimale d’énergie nécessaire pour qu’une réaction chimique se poursuive. Plus l’énergie d’activation est élevée, plus le taux ...

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Biology

Biologie

Cellular Processes

Processus cellulaires

Metabolism

Métabolisme

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

how catalysts lower reaction barriers

Activation energy is the minimum energy a chemical reaction needs to keep going. When the activation energy is higher, the reaction usually happens more slowly. Adding heat can speed the reaction because molecules move faster and collide more often.

As reactants collide, bonds break and the reaction moves through an uphill phase. That step creates the transition state, which is an unstable, high-energy state of the reactants. After that, new bonds form, products appear, and free energy is released in the downhill phase.

Catalysts speed up reactions by lowering the activation energy. A catalyst is a substance that takes part in the reaction pathway and then regenerates itself. It offers an alternate mechanism and speeds up both the forward and reverse reactions.

In biology, enzymes are catalysts that help cells control metabolism. They help break down sugar and fat for energy, and they do it without releasing too much free energy as heat. That matters because excess heat could denature proteins in the cell.

Enzymes are also selective. They bind only to specific reactants called substrates and lower the activation energy for those reactions. Thermal energy alone would not be as selective and could drive many reactions at once, but enzymes support specific cellular reactions instead.

How Catalysts Lower Reaction Barriers

what is metabolism?

Metabolism, Energy, and Cell Reactions

Metabolism is the set of chemical reactions that happen in a cell. It includes anabolism, which builds molecules, and catabolism, which breaks them down. These reactions help cells manage energy and keep life processes running. Cells use metabolism to turn food into energy, make cell parts, and remove waste products. Food stores potential energy in the bonds between atoms. Catabolic pathways break large food molecules into smaller ones and release that energy. Anabolic pathways use that energy...

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second law of thermodynamics

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kinetic energy

Motion as a Source of Energy

Kinetic energy is the energy of motion. It appears when an object is moving and can do work or cause change. A waterfall is one clear example. Water behind a dam has potential energy, but when it rushes downward, that moving water has kinetic energy. When the water reaches the bottom and stops moving, its energy can become potential energy again. Kinetic energy also shows up in living systems. Plants absorb light particles, called photons, which move very fast and help create chemical energy...

potential energy

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Stored energy in motion and bonds is called potential energy. It depends on the structure of an object or the position of matter in space. Raising a kettlebell changes its location and increases its potential energy. A stretched rubber band also stores potential energy, which can later change into kinetic energy. Chemical bonds between atoms hold another form of stored energy called chemical energy. These bonds involve attractive forces, and that energy can change into other forms during a...

free energy

Gibbs Free Energy in Chemical Reactions

Gibbs free energy is a measure of useful energy that can be taken from a chemical reaction to do work. It is written as G and named for scientist Gibbs, who discovered it. This energy is what remains after entropy is considered. A reaction can either take in energy or release it. Reactions that absorb energy are called endergonic reactions. Reactions that release energy are called exergonic reactions. The direction of energy flow helps show which type of reaction is happening. Plants carry...

activation energy

Activation energy is the minimum energy a chemical reaction needs to keep going. When the activation energy is higher, the reaction usually happens more slowly. Adding heat can speed the reaction because molecules move faster and collide more often. As reactants collide, bonds break and the reaction moves through an uphill phase. That step creates the transition state, which is an unstable, high-energy state of the reactants. After that, new bonds form, products appear, and free energy is...

hydrolysis of atp

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phosphorylation

Phosphorylation and Protein Control

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induced-fit model

How Enzymes Adjust to Their Substrates

Enzymes speed up many chemical reactions in cells. They are biological catalysts, so they increase reaction speed without being consumed or permanently changed. They do this by lowering the activation energy needed to turn reactants into products. Enzymes are proteins that usually work by binding to a substrate, which is the reactant molecule they act on. They show substrate specificity, meaning they bind only certain substrates. This specificity depends mainly on the shape and chemical...

enzyme kinetics

Substrate Effects on Enzyme Rates

Enzyme kinetics examines how enzymes change the rate of a reaction. Enzymes speed reactions by lowering the activation energy of the reactants. The reaction rate is the speed at which an enzyme turns reactants into products, and it can be affected by several factors, including the number of available reactants. Scientists often study enzyme kinetics in a test tube with a fixed amount of enzyme. When more substrate, or reactant, is added, the reaction rate increases because the enzyme can make...

enzyme inhibition

Competitive vs Noncompetitive Inhibitors

Enzyme inhibitors are molecules that reduce enzyme activity by binding to enzymes. Cells use these inhibitors to help regulate normal enzyme function. Drugs and toxins can also block enzymes and change how they work. Some inhibitors bind at the active site, which is the part of the enzyme where the substrate normally fits. Competitive inhibitors do this by occupying the active site and preventing the substrate from binding. If the substrate concentration becomes high enough, it can outcompete...

feedback inhibition

How Cells Control Product Buildup

Cells use enzymes to run biochemical reactions and make the products they need. These reactions are happening constantly, and enzymes speed them up so they happen fast enough to matter. Enzyme activity also has to be regulated. Cells need the right amounts of starting materials, intermediate metabolites, and final products. If these substances build up too much or drop too low, the cell can be harmed. That is why cells use control systems to keep metabolic levels balanced. One important...

allosteric regulation

Allosteric Control of Enzyme Speed

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cofactors and coenzymes

Enzyme Helpers: Metals, Vitamins, and More

Enzyme helpers are needed for many enzymes to work properly. These helpers fall into two groups: cofactors and coenzymes. Cofactors are metallic ions, while coenzymes are non-protein organic molecules. Both can be tightly bound to an enzyme or attach only when the substrate binds. Cofactors are common in mature proteins, appearing in about 30% of them. They are often added as the enzyme folds and help with catalytic activity. Magnesium is one important cofactor, and it is needed by more than...

key terms and concepts

Learn how heat, transition states, catalysts, and enzymes affect reaction speed. See how lowering activation energy helps cells break down sugar and fat.

How Catalysts Lower Reaction Barriers | JoVE K12

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Tennessee Academic Standards

Energy

CHEM1.PS3.3

Minnesota Academic Standards in Science

9-12

3.1.1 9C.2.2.1.1

Alabama Course of Study Science

Physical Science

9-12.PS.1.1

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How Catalysts Lower Reaction Barriers

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