Multicellular organisms have different cells and tissues doing different things, but also the same signal molecules such as hormones travelling to each cell in the body. Equally, different cells have different receptor molecules, on their surface and inside the cell, and their DNA in the nucleus.

Communication therefore is the connection between these different receptors, whether they are present (if not, the signal simply won’t be read) or not, and the signal molecules. Moreso, signals themselves can cause a production or reduction of receptors in cells. The same signal molecules can also result in different outcomes in different tissues. Welcome to biology!

Since cells are separated by lipid membranes, the route taken for signal transmission can be categorised according to whether the signal molecules act at the cell surface (on the membrane), or straight into the cell. As such, the signals can be hydrophobic or hydrophilic.

Hydrophobic signals: thyroxine; sex hormones

Hydrophobic signals include the hormones thyroxine (secreted by the thyroid gland) and steroids such as testosterone and oestrogen. These chemicals are hydrophobic, like the plasma membrane inner core which contains the hydrophobic phospholipid tails.

They can make their way past cell membranes and act directly on receptors inside the cell. In the case of thyroxine, its receptor protein binds DNA directly, inhibiting the transcription of the gene for Na⁺/K⁺ATPase (the previously introduced membrane transport protein responsible for maintaining the ion gradient in nerve signal transduction, kidney function, etc.).