1. 2.4 Chemical Reactions and Enzymes
Everything that happens in an organism is based on chemical reactions

2. Chemical reactions
A chemical reaction is a process that changes, or transforms, one set of chemicals into another
The elements or compounds that enter into a chemical reaction are known as reactants
The elements or compounds produced by a chemical reaction are known as products

3. Baking soda + vinegar chemical reaction

4. Chemical reactions
Chemical reactions always involve changes in the chemical bonds that join atoms in compounds

5. Energy in reactions
Energy is released or absorbed whenever chemical bonds form or are broken. Because chemical reactions involve breaking and forming bonds, they involve changes in energy.

6. Energy changes
Chemical reactions that release energy often occur spontaneously. Chemical reactions that absorb energy will not occur without a source of energy.
An example of an energy releasing reaction is hydrogen gas burning, or reacting, with oxygen to produce water vapor.
2H₂ + O₂  2H₂O

7. Hindenberg

8. Energy changes

9. Energy changes

10. Energy changes

11. Activation energy
Even chemical reactions that release energy do not always occur spontaneously. If they did, the paper you are writing on might burst into flames.
Chemists call the energy that is needed to get a reaction started the activation energy.

12. Activation energy

13. Enzymes
A catalyst is a substance that speeds up the rate of a chemical reaction. Catalysts work by lowering a reaction’s activation energy.
Enzymes are proteins that act as biological catalysts. Enzymes speed up chemical reactions that take place in cells.
Enzymes work by lowering the activation energies.

14. Enzyme action

15. Enzyme action
For a chemical reaction to take place, the reactants must collide with enough energy so that existing bonds will be broken and new bonds will be formed.

16. The Enzyme-Substrate Complex
Enzymes provide a site where reactants can be brought together to react. Such a site reduces the energy needed for reaction
The reactants of enzyme-catalyzed reactions are known as substrates

17. The Enzyme-Substrate Complex

18. Regulation of Enzyme Activity
Many enzymes are affected by changes in temperature. Enzymes produced by human cells generally work best at temperatures around 37℃, the normal temperature of the human body.
Enzymes also work best at certain pH values and solute concentrations

19. Regulation of enzyme activity

20. Regulation of enzyme activity