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Part 1: What is going on inside a cell?

I. Life depends on chemical reactions.

II. Chemical reactions occur when molecules bump into each other and react.

III. Synthesis is when two or more atoms combine to form a molecule (bigger group of atoms)

III. It’s not so easy.  Molecules are moving in the cell’s cytoplasm. That makes it tougher for them to get together.

Cytoplasm is the fluid that fills a cell, that all the cell’s organelles are floating in.

Dancing Queen molecules in cytoplasm

IV. Secondly, they are also spinning.  That won’t make it easy, will it?

Rotating molecule

V. They have to get together in exactly the right way, like a key fits into a lock

Lock-and-Key Model of Enzyme Action


VI. We can’t just throw a key at a lock – it has to go in the right place, at the right angle.

Lock and Key enzyme analogy

VII. Molecules really do act like a lock-and-key, but they don’t look quite like this.
Below is a more realistic pic.

Substrate Binding to the Active Site of an Enzyme


VIII. It gets more challenging, because there are many different lock and key molecules, inside every cell, all at the same time.

This is not what cells look like simplification

IX. This is how densely proteins are packed together in your cell’s cytoplasm.
Most molecules are either a lock or a key.

Densely packed proteins in cytoplasm

Here’s a great video Inner Life of a Cell | Protein Packing. XVIVO Scientific Animation

X. What if we wait for the right molecules to randomly bump into each other, at just the right angle? That would take a long time.

That’s where enzymes come in: They can get molecules to get together, at the right angle, much faster.


Part 2: What are proteins made of?

Amino acids are the building blocks of protein.

Think of them as like Legos.

A single block of something is a “monomer”

Proteins are a bunch of amino acids bonded together.

Many monomers bonded together are a “polymer.”

Monomer Polymer Lego analogy

And then this chain of amino acids folds up into a shape.
Every protein has its own shape.

Related labs

Hands-on Biochemistry: Beads, Pipe Cleaners, and the clearest way to teach monomers and polymers!


Part 3: How do molecules bind with enzymes?

There are active sites on the enzyme; these are locations which bind the compounds.

Once these compounds are bound they will react with each other

Once reacted, the enzyme releases the compounds.

The enzyme itself is unchanged, and can be used over and over again.

This lets reactions occur thousands of times faster than if we didn’t have enzymes.

Example: Induced fit and conformational change

Part IV. How do we change the speed of enzyme activity?

Idea: Enzymes are folded into a precise shape – so anything that alters the shape will reduce its ability to work.

What can change the shape of an enzyme?


Enzymes have an optimum temperature at which they function most efficiently.

For humans this temp. is 37 C, 98.6 F

If the temp decreases, then the enzyme shrinks and its shape changes.

If the temp increases, then the enzyme expands and its shape changes.

pH (The level of acidity or base of the solution)

Enzymes have an optimum pH

A high pH causes free H ions to attach to the enzyme, changing its shape in one way.

pH action on enzyme

A low pH causes some H atoms to be pulled off of the enzyme, changing its shape in another way.



Part V: How are enzymes regulated?

(Section to be written)

Examples: When are enzymes used

(to be written)


Learning Standards

National Science Education Standards

Most cell functions involve chemical reactions. Food molecules taken into cells react to provide the chemical constituents needed to synthesize other molecules. Both breakdown and synthesis are made possible by a large set of protein catalysts, called enzymes. The breakdown of some of the food molecules enables the cell to store energy in specific chemicals that are used to carry out the many functions of the cell.

National Research Council. 1996. National Science Education Standards. Washington, DC: The National Academies Press. https://doi.org/10.17226/4962.

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)

1.3 Explain the role of enzymes as catalysts that lower the activation energy of biochemical reactions. Identify factors, such as pH and temperature, that have an effect on enzymes. (Biology)

New York The Living Environment Core Curriculum

2.2c Different enzymes can be used to cut, copy, and move segments of DNA. Characteristics produced by the segments of DNA may be expressed when these segments are inserted into new organisms, such as bacteria.

5.1f Biochemical processes, both breakdown and synthesis, are made possible by a large set of biological catalysts called enzymes. Enzymes can affect the rates of chemical change. The rate at which enzymes work can be influenced by internal environmental factors such as pH and temperature.
5.1g Enzymes and other molecules, such as hormones, receptor molecules, and antibodies, have specific shapes that influence both how they function and how they interact with other molecules.



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