Nerve cells send chemical signals to other cells.

The individual cells in a multicellular organism must be able to communicate with each other. Structures in the human ear convert sound waves into a nerve impulse; nerve cells then transmit those impulses from one cell to the next. When it reaches the brain, the nerve impulse sends a message to the adrenal gland to release the hormone adrenalin. This hormone travels through the blood and causes a person's heart to beat faster. Nerve impulses and hormones are different kinds of intercellular communication, each of which depends on the receptor molecules found on the membranes of cells.

Cells have receptor proteins on their membranes. These proteins extend across the lipid bilayer so that part of the molecule lies both inside and outside the cell. The portion of the molecule on the exterior of the cell has a unique shape that allows it to bind only to certain special molecules, in the same way a key fits a lock. In a nerve cell, the "key" is called a NEUROTRANSMITTER. In other kinds of cells, the "key" is called a HORMONE. When the correct signal protein links to the lock, it causes the part of the protein inside the cell to change shape. This change activates a signal inside the cell and generates a response from it.

This ability to communicate between cells is crucial to large multicellular organisms because it allows organisms to coordinate the activity of all their cells.

Lock and key model of cell communication

Here concludes this tour of a cell. Stops along the way have included a large number of the structures and functions of the cell. Remember that the cell is an incredibly complex unit of life. Why?

It needs to carry out all the functions of life. It must have many levels of organization, utilize energy, maintain stable internal conditions, grow, reproduce, and respond to its environment. This is achieved as many types of molecules (mostly proteins) enter the cell, communicate with it, leave the cell, and are processed by the cell. There's no doubt about it: cells are actually far more complicated than factories.

Soon, we'll take a closer look at some of the functions of a cell and investigate how plant cells transform energy, how all cells utilize cellular energy, and how cells grow and divide. Then, we'll examine the contents of the nucleus of the cell in great detail and see why DNA is such a powerful molecule.

Today, breakthroughs in genetic research occur every week. Some are a source of great controversy; others are a source of great hope. None of them would be possible without an understanding of cell theory.

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