One particularly obvious category of response to the environment in organisms is movement. This is one of the most fundamental ways for a plant or an animal to enhance survival. It’s no wonder then, that much of this response is solidly built-in and automated, like a reflex.
There are 3 main examples of response: tropism, taxis and kinesis and reflex.
1. Tropism – think of a sunflower following the sun
The direct stimuli which contribute to plant survival (such as light, nutrients, gravity) are responded to by positive or negative tropisms. Tropisms get their names based on the stimulus they refer to. For example, a plant responding towards light would exhibit a positive phototropism.
Most plants also exhibit negative gravitropism which means they grow away from the strongest gravitational pull i.e. the ground.
This change in direction is enabled by different concentration of a plant hormone called indoleacetic acid (IAA) which stimulates cell elongation in the roots and shoots in flowering plants. IAA is an auxin.
All these ensure plants respond to their environment in a way that best maximises their survival prospects. More light enables better photosynthesis, while growing upwards exposes their leaves to more sunlight.
2. Taxis and Kinesis – think of a maggot jiggling aimlessly
What happens it you expose a maggot (not that you’d ever do such a thing) to an extreme temperature? Will it identify the nearest fire exit and calmly and orderly proceed to the assembly point? Will it pull out its emergency kit and offer first aid to its fellow maggots?
No I’m afraid. It will just jiggle about and move its squishy body utterly and completely aimlessly until it just so – hey presto – happens to get to a better place. It must have worked pretty well apparently! This maggoty aimlessness that characterises their attitude to life is called kinesis.
What if there’s a freshly decomposing, nicely rotten carcass lying around, emanating a stream of increasingly concentrated chemicals that give its characteristic smell? That seems like a good dinner for our maggots. But will they find it on time?
You bet they will. They may not have any sophisticated response to burning flames or death by ice, but they sure have a built-in putrid flesh detector (put-dar?). When they move specifically towards the ever-increasing chemical concentration from a carcass, it’s taxis they’re up to.
Kinesis – moving randomly hoping for the best (in response to a stressful stimulus)
Taxis – getting in a cab to your destination (ok!!! sorry. Taxis is moving specifically towards/away from a stimulus)
3. Reflex – think of sensory neuron –> interneuron –> motor neuron
The knee-jerk reflex is too famous so it shall be ignored on this occasion (also, what is the point of it anyway? I never recall it saving me from anything, do you?!). Instead we turn to the iris reflex responsible for controlling the amount of light entering your eyes.
(Fun experiment that’s also an opportunity for justified procrastination: go in the bathroom and keep covering an eye and uncovering quickly while staring in the mirror to see how quickly the pupil appears to change size!)
How does this reflex work? Most reflexes have multiple steps, but the outline goes something like this:
receptor –> sensory neuron –> integration centre –> motor neuron –> effector
In other words: the retina (receptor) at the back of the eyes senses the amount of light present. This information is relayed (sensory neuron) to a centre in the brain (integration centre) which then acts upon it by stimulating the motor neuron to carry out a response command to the effector – the iris muscles which contract.