| Stimulus meaning | - A detectable change on the internal or external environment of an organism that leads to a response in the organism |
| Advantage to organisms of being able to respond to stimuli | - Increases chance of survival
- Greater chance of reproducing having offspring, passing alleles to next gen
- So always a selection pressure favoring organism with more appropriate responses |
| Flow diagram of events | Stimulus-receptor-coordinator-effector-response |
| How stimuli are detected | - By receptors
- They are specific to one type of stimulus |
| Coordinator | - A coordinator formulates a suitable response to a stimulus
- Coordination may be at the molecular level or involve a large organ such as a brain |
| Effectors | - A response is produced by an effector
- This response may be at the molecular level or involve the behavior of a whole organism
- One means of communication in large, multicellular organisms occurs via chemicals called hormones, which is a relatively slow process in animals and plants
- Or the nervous system |
| The nervous system | - Usually have many different receptors and control effectors
- Each receptor and effector is linked to a central coordinator
- The coordinator acts like a switchboard, connecting info. from each receptor with the appropriate effector |
| Taxes meaning | - Directional response
- Organism moes its whole body either towards a favorable stimulus or away from the unfavorable one
- Towards the stimulus (positive taxes) or away from the stimulus (negative taxes) and also by the nature of the stimulus |
| How taxes increases chance of survival | - Positive phototaxis - faster rate of photosynthesis - produce more glucose
- Positive chemotaxis - respiration
- Negative phototaxis - reduce water loss, avoid predators |
| Kineses | - Random movement due to an adverse stimulus
- Frequency of turning is proportional to the strength of the stimulus
e.g. woodlice in light |
| How kineses increases chance of survival | - Woodlice lose water from their bodies in dry conditions
- When they move from damp area into dry one , they move more rapidly and change direction more often
- This increase chance of moving back into damp area |
| Tropisms | - Diectional response to stimuli involving growth
- Plant part grows towards (positive response) or away (negative response) to the stimulus |
| How tropisms increases chance of survival | - Plant shoots grow towards light (positive phototropism) and away from gravity (negative phototropism) to maximise light absob. ]
- Plant roots grow away from light (negative phototropism) and towards gravity (positive phototropism) ab. water + mineral ions |
