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Nervous system

Pass electrical impulses along nerves - stimulate target cells by releasing neurotransmitter

Rapid - short lived -restricted to one region of the body - response is localised / rapid / temporary / reversible

Hormonal system

Hormones transported in the blood plasma - Travel to all parts of the body but only taget organs respond

Slower - less specific - long-lasting - widespread - effect may be permanent / irreversible

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Chemical mediators

Chemical mediators - chemicals released from certain mammalian cells that have an effect in their immediate vicinity. Typically released by infected/injured cells > arteries/arterioles dilate > imflammatory response (raised temp / swelling)

Histamine - stored in certain white blood cells - released in response to injury / allergen. Dilation of small arteries and arterioles > increased permeability of capillaries > swelling / redness / itching.

Prostaglandins - found in cell membranes > small arteries/arterioles dilate > increased permeability of capillaries > affects blood pressure and neurotransmitters > affect pain sensation.

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Plant growth factors

Light - stems = phototropic

Gravity - roots = positively geotropic / shoots = negatively geotropic

Water - roots = positively hydrotropic

Plant growth factors

> Exert influence by affecting growth

> Made by cells located throughout the plant (not organs)

> Can affect the tissues that release them (not just target tissues)

> Released in small quantities

> Effective close to the tissue that releases them

> Eg: IAA (Indoleacetic acid) causes plant cells to elongate

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A young shoot will bend towards the light which is directed at it from one side

1. Cells in tip of shoot produce IAA > transported down the shoot

2. IAA initially transported to all sides as it moves down shoot

3. Light causes IAA to move from light side to shaded side of shoot

4. Greater concentration on shaded side than light side

5. The cells on shaded side elongate more

6. Shaded side of shoot grows faster > bends towards the light

Also causes bending of roots in the direction of gravity > decreases growth in root cells

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