How Cells Work

Enzymes at Work

There are all sorts of enzymes at work inside of bacteria and human cells, and many of them are incredibly interesting! Cells use enzymes internally to grow, reproduce and create energy, and they often excrete enzymes outside their cell walls as well. For example, E. coli bacteria excrete enzymes to help break down food molecules so they can pass through the cell wall into the cell. Some of the enzymes you may have heard of include:

  • Proteases and peptidases - A protease is any enzyme that can break down a long protein into smaller chains called peptides (a peptide is simply a short amino acid chain). Peptidases break peptides down into individual amino acids. Proteases and peptidases are often found in laundry detergents -- they help remove things like blood stains from cloth by breaking down the proteins. Some proteases are extremely specialized, while others break down just about any chain of amino acids. (You may have heard of protease inhibitors used in drugs that fight the AIDS virus. The AIDS virus uses very specialized proteases during part of its reproductive cycle, and protease inhibitors try to block them to shut down the reproduction of the virus.)
  • Amylases - Amylases break down starch chains into smaller sugar molecules. Your saliva contains amylase and so does your small intestine. Maltase, lactase, sucrase (described in the previous section) finish breaking the simple sugars down into individual glucose molecules.
  • Lipases - Lipases break down fats.
  • Cellulases - Cellulases break cellulose molecules down into simpler sugars. Bacteria in the guts of cows and termites excrete cellulases, and this is how cows and termites are able to eat things like grass and wood.

Bacteria excrete these enzymes outside their cell walls. Molecules in the environment are broken down into pieces (proteins into amino acids, starches into simple sugars, etc.) so they are small enough to pass through the cell's wall into the cytoplasm. This is how an E. coli eats!


Inside a cell, hundreds of highly specialized enzymes carry out extremely specific tasks that the cell needs to live its life. Some of the more amazing enzymes found inside cells include:

  • Energy enzymes - A set of 10 enzymes allows a cell to perform glycolysis. Another eight enzymes control the citric-acid cycle (also known as the Krebs cycle). These two processes together allow a cell to turn glucose and oxygen into adenosine triphosphate, or ATP. In an oxygen-consuming cell like E. coli or a human cell, one glucose molecule forms 36 ATP molecules (in something like a yeast cell, which lives its life without oxygen, only glycosis occurs and it produces only two ATP molecules per glucose molecule). ATP is a fuel molecule that is able to power enzymes by performing "uphill" chemical reactions.
  • Restriction enzymes - Many bacteria are able to produce restriction enzymes, which recognize very specific patterns in DNA chains and break the DNA at those patterns. When a virus injects its DNA into a bacterium, the restriction enzyme recognizes the viral DNA and cuts it, effectively destroying the virus before it can reproduce.
  • DNA-manipulation enzymes - There are specialized enzymes that move along DNA strands and repair them. There are other enzymes that can untwist DNA strands to reproduce them (DNA polymerase). Still others can find small patterns on DNA and attach to them, blocking access to that section of DNA (DNA-binding proteins).
  • Enzyme-production enzymes - All of these enzymes have to come from somewhere, so there are enzymes that produce the cell's enzymes! Ribonucleic acid (RNA), in three different forms (messenger RNA, transfer RNA and ribosomal RNA), is a big part of the process.

A cell really is nothing but a set of chemical reactions, and enzymes make those reactions happen properly.