Principles of coordination
- The nervous system uses nerve cells to pass electrical impulses along their length. They stimulate their target cells by secreting chemicals, known as neurotransmitters, directly onto them. This results in rapid communication in the body between specific paesof an organism. The responses produced are often short lived and restricted to a localised region of the body.
- The hormonal system produces chemicals that are transported in the blood plasma to their target cells, which they then stimulate. This results in a slower, less specific form of communication between parts of an organism. The responses are often long lasting and wide spread but produces a slower response.
At the cellular level, however, they are complemented by a further form of coordination. This involves substances known aschemical mediators. These are chemicals that are released from certain mammalian cells and have an effect on cells in their immediate vicinity. They are typically released by infected released by infected or injured cells and cause small arteries and arterioles to dilate. This leads to a rise in temperature and swelling of the affected area - the so-called 'imflammatory response'.
Two expamples of chemical mediators are:
- histamine, whichis stored in certain white bloodcells and released following injury or in response to an allergen, such as pollen. It causes dilation of small arteries and arterioles and increased permeability of capillaries, leading to localised swelling, redness and itching.
- prostglandins, which arefound in cell membranes and also cause dilation of small arteriesand arterioles. Their release following injury increase the permeability of capillaries. They also affect bloodpressure and neurotransmitters(substances involved in the transmission of nerve impulses). In so doing, they affect pain sensation.
Plant Growth factors
In order to survive, plants must respond to changes in both their external and internal environments. For example plants must respond to: Light – stems grow towards the light
- gravity – plants need to be firmly anchored in the soil, roots are sensitive to gravity and grow in the direction of its pull.
- - water – almost all plant roots grow towards water, in order to absorb it for use in photosynthesis and other metabolic processes, as well as for support.
Plants respond to external stimuli by means of plant hormones or,more correctly, plant growth factors. The latter term is more descriptive because:
- - They exert their influence by affecting growth
- - Unlike animal hormones, they are made by cells located throughout the plant rather than in particular organs.
- - Unlike real animal hormones, some plant growth factors affect the tissuesthat release them rather than acting on a distant target organ.
Control of tropisms
Tropism is a growth movement of a plant in response to a directional stimulus. In the case of light, we can observe that a young shoot will bend towards the light that is directed at it from one side. This response is due to the following sequence of events.
- 1) Cells in the tip of the shoot produce IAA, which is then transported down the shoot
- 2) The IAA is initially transported to all sides as it begins to move down the shoot.
- 3) Light causes the movement of IAA from the light side to the shaded side of the shoot.
- 4) A greater concentration of IAA builds up on the shaded side of the shoot than on the light side
- 5) As IAA causes elongation of cells and there is a greater concentration of IAA on the shaded side of the shoot, the cells on this side elongate more.
- 6) The shaded side of the shoot grows faster, causing the shoot to bend towards the light.