3.6.1.1 Survival & Response

Organisms increase their chance of survival by responding to changes in their environment. In flowering plants, specific growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli. The effect of different concentrations of indoleacetic acid (IAA) on cell elongation in the roots and shoots of flowering plants as an explanation of gravitropism and phototropism in flowering plants. Taxes and kineses as simple responses that can maintain a mobile organism in a favourable environment. The protective effect of a simple reflex, exemplified by a three-neurone simple reflex. Details of spinal cord and dorsal and ventral roots are not required.

what is a stimulus?

a detectable change in the internal or external environment of an organism that leads to a response

what are receptors?

specialised structure that detects a specific type of stimulus

what are coordinators?

coordinates information from the receptors and sends instructions to the effectors to form a suitable response

what is an effector?

an organ, tissue, or cell that produces a response to a stimulus

what are the two types of coordination in animals?

hormonal or nervous

what is a taxis?

a response that involves movement in a specific direction

what is a positive taxis?

towards the stimulus - eg: bacteria moving to an area where there is a higher concentration of glucose

what is a negative taxis?

away from the stimulus

what a kinesis?

• a response that involved movement, but in random directions

• speed and frequency of direction change increase

• carried out in order to increase chance that organism with enter different conditions more rapidly

why do plants respond to external stimuli?

• to increases chances of survival

what are tropisms?

• growth of part of a plant in response to a directional stimulus

what is IAA?

• indoleacetic acid

• belongs to a group of substances called auxins

• controls plant cell elongation

how are shoots positively phototrophic?

1. cells in the tip of shoot produces IAA, which is transported down the root by diffusion

2. IAA initially transported evenly throughout all regions

3. light causes 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 shoot cells, cells on the shaded side elongate more

6. shaded side elongated faster than light side causing root tip to bend towards the light

how are roots positively gravitropic?

1. cells in the tip of the root produce IAA → transported along the root

2. IAA initially transported to all sides of the root

3. gravity influences movement of IAA from upper to lower side

4. greater concentration of IAA accumulates on the lower side

5. as IAA inhibits the elongation of root cells so cells on the lower side elongate less than ones on upper side

6. this relatively greater elongation of cells on upper side causes root to bend downwards towards the force of gravity

outline the process of a reflex arc

stimulus → receptor → sensory neurone → relay neurone in CNS → motor neurone → effector → response

what are the advantages of a simple reflex?

• rapid response to potentially dangerous stimuli

• instinctive