Sections 2.4 & 2.5

CHEMICAL REACTIONS &

ENZYMES

2.4 Chemical Reactions•Key Concept:•Life depends on chemical reactions.

Bonds break and form during chemical reactions.

•Chemical reactions change substances into different ones by breaking and forming chemical bonds.• Reactants are changed during a chemical reaction.• Products are made by a chemical reaction.

• Endothermic and Exothermic Reactions

Chemical reactions release or absorb energy.

•Activation energy is the amount of energy that needs to be absorbed to start a chemical reaction

Exothermic reactions release more energy than they absorb.

• Reactants have higher bond energies than products.• Excess energy is released by the reaction.• Energy “exits” the reaction. (Exo = exit) Ex: Cellular

Respiration, making ice cubes

Endothermic reactions absorb more energy than they release.•Reactants have lower bond energies than products.•Energy is absorbed by the reaction to make up the difference.•Energy goes into the reaction. (Endo = “into”) Ex: Photosynthesis, melting ice cubes

STOP & REVIEW• _________ __________change substances into different ones by breaking and forming chemical bonds.• Chemical Reactions• _____________ are changed during a chemical reaction.• Reactants• __________ are made by a chemical reaction.• Products• ___________ ____________ is the amount of energy that needs to be absorbed to start a chemical reaction.• Activation Energy• ______________ reactions release more energy than they absorb.• Exothermic• ___________ reactions absorb more energy than they release.• Endothermic

2.5 Enzymes•Key Concept:•Enzymes are catalysts for chemical reactions in living things.

A catalyst lowers activation energy.•Catalysts are substances that speed up chemical reactions•Decrease activation energy• Increase reaction rate

Enzymes allow chemical reactions to occur under tightly controlled

conditions.•Enzymes are catalysts in living things.•Enzymes are needed for almost all processes.•Most enzymes are proteins.

Disruptions in homeostasis can prevent enzymes from

functioning.•Enzymes function best in a small range of conditions.•Changes in temperature or pH can break hydrogen bonds.•An enzyme’s function depends on its structure.

An enzyme’s structure allows only certain reactants to bind to the

enzyme.•Substrates: reactants that bind to an enzyme•Active site: area on the enzyme where substrates bind

Lock and key model shows how enzymes function

•Substrates are brought together at the active site of the enzyme•Enzyme weakens the bonds between the substrates.•Reaction forms a product that is released from the enzyme.

STOP & REVIEW• _________ are catalysts for chemical reactions in living things.• Enzymes• ________ are substances that speed up chemical reactions by• Catalysts• Decreasing __________ ______• Activation energy• Increasing ___________ _______• Reaction rate• Most enzymes are _______________• proteins• Changes in __________ or _____ can break hydrogen bonds.• Temperature or pH• An enzyme’s function depends on its _________• Structure

• __________: reactants that bind to an enzyme• Substrates• ________ _____: area on the enzyme where substrates bind• Active Site