3.6.1.1 Survival and response

A stimulus is a detectable change in the internal/external environment of an organism that leads to a response in the organism. This increases the organisms chance of survival (e.g the ability to detect and move away from prey, or detect and move towards a food source).

So we need to know quite a bit about plants (in particular, flowering plants). Plants have no nervous system so their responses to external stimuli involve hormone-like substances known as plant growth factors. They are produced in small quantities and an example is IAA which is an auxin. IAA controls plant elongation. Growth factors can move from growing regions to other tissues (meaning that they are not confined to the tissue they were produced in) and bring about a change in growth in the following way:

1. Cells in the shoot tip produce IAA
2. the IAA is transported down the shoot evenly throughout all regions
3. Light causes the movement of IAA from the light side to the shaded side of the shoot so a greater concentration of IAA builds up on the shaded side
4. The cells on the shaded side elongate more as there is a greater concentration of IAA here
5. The shaded side elongates faster than the light side which causes the shoot tip to bend towards the light

IAA inhibits root growth:

1. Cells in the root tip produce IAA
2. the IAA is transported initially to all sides of the root
3. Gravity influences the movement of IAA from the upper side to the lower side
4. A greater concentration builds up on the lower side meaning the cells here elongate less than those on the upper side
5. This causes the root to bend down towards gravity.

IAA has a number of effects on plant cells including increasing the plasticity of their cell walls. This response only occurs in young cell walls where cells are able to elongate. As cells mature they develop greater rigidity meaning that older parts of the shoot/root will not respond. The acid growth hypothesis is the current way in which we think IAA increases the plasticity of cells. It involves the active transport of hydrogen ions from the cytoplasm into the cell walls causing the cell wall to become more plastic allowing the cell to elongate.

A taxis is a simple response whose direction is determined by the direction of the stimulus. As a result a motile organism responds directly to the environmental changes by moving away from an unfavourable one. Taxis are classified according to whether the movement is towards (positive) or away from (negative taxis) the stimulus and by the nature of the stimulus. Some examples are as follows:

• Positive phototaxis - single celled algae move towards light to increase their chances of survival as they are photosynthetic
• Positive chemotaxis - some bacteria move towards a region of high glucose concentration. This increases their chance of survival because they use glucose as a food source.

A kinesis is a form of response in which the organism does not move towards or away from a stimulus. Instead it changes its speed and also the rate it changes direction. If it crosses a line between favourable and unfavourable conditions its rate of turning increases which raises its chances of quickly returning to a favourable environment. If it has moved a considerable distance in an unfavourable environment its rate of turning decreases so it moved in a long straight line before sharply turning as this is likely to bring the organism into a new region with favourable conditions.

A tropism is the growth of part of a plant in response to a directional stimulus. The type of response is named after the stimulus and the direction the organism moves relative to it. Examples include:

• positive phototropism - plant shoots grow towards the light so their leaves are in a more favourable position for photsynthesis
• Plant roots grow away towards gravity (positive gravitropism). This increases their probability to find water and mineral ions.

Stimuli are detected by receptors which are specific to one type of stimulus. A coordinator formulates a suitable response, which is produced by an effector. Animals also have a nervous system in addition to hormonal communication. This has many different receptors and effectors which are linked to a central coordinator.

The spinal cord is a column of nervous tissue running along the back which lies inside the vertebral column for protection. Pairs of nerves emerge at intervals along the spinal cord. A reflex is rapid, short-lived, and localised. The pathway of neurones involved is known as a reflex arc:

1. stimulus 
2. receptor
3. sensory neurone
4. relay neurone/coordinator
5. motor neurone
6. effector
7. response

Reflex actions are important as they make survival more likely. They are important for the following reasons:

• involuntary so do not require the decision powers of the brain so the brain is free to carry out more complex responses
• they protect the body from harm and are effective from birth so do not have to be learnt
• they are fast as the neurone pathway is short with very few synapses
• the absence of the decision making process also makes it fast