Signals transmitted in the body can be either chemical or electrical.

Nerve cells

Nerve cells use electrical impulses. Receptor cells pick up stimuli from the outside world and pass this information on in the form of electrical impulses to neurones. Neurones then pass the information onto between each other in the form of chemical neurotransmitters. This causes a rapid and localised response.

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Hormones

Hormones are chemical messengers composed of proteins. They’re produced in the glands of mammals and released into the blood stream where they travel to their target cells. The result is a long-lasting and widespread response. They control a wide variety of bodily functions including water and glucose levels and gamete production.

Plant hormones

Plants use chemical substances to coordinate growth. They’re called growth factors or growth hormones and they diffuse from areas which are growing to other tissues in the plant. This is important for regulating growth in response to directional stimuli or tropisms and they’re able to promote, inhibit or modify the growth of the plant.

Auxins are one of the most important types of plant hormone. Flowering plants use the auxin indoleacetic acid (IAA). In the shoot tip IAA promotes growth. Shoots are phototrophic so grow towards the sun. The shoot in direct sunlight contains little IAA. However, the shaded part of the shoot contains more IAA so cell growth is promoted in this area. This causes the shoot to bend towards the light.

Roots exhibit geotropic movement so they grow in the direction of gravity. In the roots IAA has the opposite affect: a high concentration inhibits growth.

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Control in cells and in organisms

By detecting stimuli and using the appropriate effectors multicellular organisms can control their various tissues and organs. Organisms are able to respond to both external and internal stimuli. For animals, these can help them to avoid particular environments and keep their internal conditions at an optimum for metabolism.

Cells, for example, control their genome transcription and translation. All cells within an organism contain the same genetic code but only a part of it is translated. This means that different cells can carry out different functions which allow tissues and organs to be formed. Being able to manipulate DNA has led to medicinal and technological advances.

You should now read through the rest of our section on Chemical and Electrical coordination, as well as test yourself when you are finished.