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a model of an enzyme: on a black background, blue and red and gray and yellow balls crowded together

A model of an enzyme reacting with another molecule and breaking it into two pieces. Each little ball represents an atom.This enzyme, lysozyme, breaks up bacteria in your blood to keep you from getting sick.

What are enzymes made of?

An enzyme is a kind of big, complicated protein molecule, made mainly of hydrogen and carbon atoms, but with some other atoms as well.

What are proteins?
What are hydrocarbons?
More about amino acids
Evolution of cells
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When did enzymes evolve?

Both prokaryote and eukaryote cells use enzymes (ENN-zimes), so the first enzymes probably evolved around four billion years ago, together with the first living cells.

The Hadean Eon
Prokaryote cells

How do cells make enzymes?

Prokaryote cells assemble enzymes in their cytoplasm. The cell’s DNA molecule uses enzymes to assemble RNA molecules, and these RNA molecules then in turn assemble more kinds of enzymes.

What is DNA?
Where did RNA come from?

What do enzymes do?

The cell pushes some of these enzymes outside the cell to digest food by breaking it apart into smaller pieces that can get through the cell membrane. Other enzymes can digest smaller molecules floating in the cytoplasm inside the cell. Some enzymes grab two small molecules and attach them together to make a larger molecule, like a lipid to fix the cell membrane.

How do cell membranes form?
Cell digestion
What is cytoplasm?

When the enzyme is done, it lets the new molecule go, and it’s ready to grab two more molecules and do it again. Each kind of enzyme has its own shape, and it will only work with molecules that fit into it exactly, like having the right puzzle piece in a puzzle, or the right key for a lock. That way each kind of enzyme can do its job and not interfere with anything else in the cell.

Eukaryote cells and enzymes

Eukaryote cells make enzymes in the endoplasmic reticulum, following instructions from the cell’s RNA. Then those enzymes, in turn, build more enzymes. The enzymes float out of the endoplasmic reticulum into the Golgi bodies and then into the lysosomes, where they break down large molecules of food or garbage.

Eukaryote cells
Endoplasmic reticulum
Golgi bodies

Or the enzymes float into vacuoles, or into the cell’s nucleus where the enzymes help repair the cell’s DNA or build more RNA molecules. Some enzymes float around in the cytoplasm where they can break up viruses or germs that attack the cell.

How do new enzymes evolve?

New enzymes appear accidentally, when a old kind of enzyme accidentally gets broken or forms wrong. Usually these broken enzymes are no good to the cell, and sooner or later the enzymes in a lysosome break them down and recycle their parts.

What is a virus?
Natural selection
History of disease

But sometimes it turns out that the broken enzyme happens to be the right shape to match up with, for instance, a new virus that is trying to invade the cell and kill it. Maybe the new enzyme can break up that virus and kill it instead, saving the cell.

What is measles?

If the new enzyme turns out to be useful, then cells that make that enzyme will survive better, and the cells that don’t make that enzyme will die of the disease that virus causes. Measles is an example of a disease that we’re now better at surviving that we used to be. Your own cells each make more than a thousand different kinds of enzymes.

Enzymes that work outside of cells

Even in eukaryotes, some enzymes still leave the cell to digest food outside it. In your own body, you’ll find some of those enzymes in your stomach or in your saliva (spit).

What does a stomach do?

Those enzymes break down your food so it can get into your cells through the cell membrane.

Learn by doing – enzymes, bread and spit

Bibliography and further reading about the parts of a cell:

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