Topic 14 - Response to Stimuli

• A stimulus is a detectable change in the internal or external environment of an organism that leads to a response in the organism.
• The ability to respond to stimuli is a characteristic of life and increases the chances of survival for an organism
• Organisms that survive have a greater chance of raising offspring and passing their alleles onto the next generation.
• There is always a selection pressure that favours organisms with the most appropriate responses to stimuli.
• Stimuli are detected by receptors, and each receptor is tailored to a specific stimulus.
• A coordinator formulates a suitable response to a stimulus, and can be on a molecular level or a large organ (such as the brain).
• A response is produced by an effector, and again, this response can be on a molecular level or involve the behaviour of the entire organism.
• Large, multicellular organisms can communicate via hormones, although hormonal communication is fairly slow.
• In addition to hormonal communication, animals possess a rapid nervous system, containing many different receptors and control effectors.
• Each receptor and effector in the nervous system is linked to a central coordinator.
• The sequence of events in the nervous system occurs as follows: stimulus -> receptor -> coordinator -> effector -> response.
• A taxis is a simple response whose direction is determined by the direction of the stimulus. As a result, an organism will respond directly to an environmental stimulus by moving its whole body towards a favourable stimulus, or away from an unfavourable stimulus.
• Taxis are classified according to whether the movement is towards the stimulus (positive taxis) or away from the stimulus (negative taxis).
• An example of a positive phototaxis is single-celled algae. They will move towards light so they can absorb this for photosynthesis to manufacture their food.
• An example of a negative phototaxis is earthworms. They will move away from light to increase their chances of survival because it takes them into the soil. In the soil, they are better able to conserve water, find food and avoid some predators.
• An example of a positive chemotaxis is some species of bacteria. They will move towards a region where glucose is more highly concentrated to increase their chances of survival, since glucose is their food source.
• A kinesis is a form of response where an organism doesn't move towards or away from a stimulus. Instead, it changes it's speed and the rate at which it changes direction.
• In a kinesis, the rate of turning for the organism will increase if it crosses a distinct line between favourable and unfavourable conditions. This raises its chances of a rapid return to the favourable environment.
• In a kinesis, if an organism is a considerable distance into an unfavourable environment, the rate of turning will decrease so that the organism moves in a straight line before it turns sharply into favourable conditions.
• An example of a kinesis is woodlice. They lose water from their bodies in dry conditions, so move more quickly and change direction more frequently when in favourable damp conditions. This way, the woodlouse is more likely to stay within these conditions.
• If after some time a woodlouse is still in dry, unfavourable conditions, it will move rapidly in straight lines, to increase its chances of moving into a new damp area. This prevents them from drying out, increasing their chances of survival.
• A tropism is the growth of part of a plant in response to a directional stimulus. The plant part will grow towards (positive tropism) or away from (negative tropism) the stimulus.
• Plant shoots grow towards light (positive phototropism) and away from gravity (negative gravitropism) so their leaves are in the most favourable position to absorb light for photosynthesis.
• Plant roots grow away from the light (negative phototropism) and towards gravity (positive gravitropism). This response increases the probability that the roots will grow into the soil, where they are better able to absorb water and mineral ions.
• Plants respond to several changes in their external and internal environments. These include light, gravity and water.
• Plant roots grow in the direction of gravity to anchor themselves in the soil (positive gravitropism).
• Plant roots grow towards water (positive hydrotropism) to absorb it for photosynthesis, support and other metabolic processes.
• Plant responses to external stimuli involve hormone-like substances known as plant growth factors.
• Plant growth factors exert their influence by affecting growth in cells throughout the entire plant, rather than in one organ. Unlike animal hormones, plant growth factors can also affect the tissues that release them, rather than just distant organs.
• Plant growth factors are produced in small quantities. For example, indoleacetic acid (IAA) is an auxin, and has a role in controlling cell elongation.
• Positive phototropism firstly involves cells in the shoot tip producing IAA, which is transported evenly down the shoot. Next, light causes IAA to move from the light side to the shaded side of the shoot, creating a higher concentration on the shaded side. IAA promotes elongation in shoot cells, causing the cells on the shaded side to elongate faster than the light side. The shoot tip will bend towards the light.
• IAA also controls the bending of roots in response to light. However, IAA inhibits elongation in the root cells, causing them to bend away from the light (negative phototropism).
• Positive gravitropism firstly involves cells in the root tip producing IAA, which is transported evenly down the root. Next, gravity causes IAA to move from the upper side to the lower side of the root (falls), creating a higher concentration on the lower side. IAA inhibits elongation in root cells, causing the cells on the lower side to elongate less than the upper side. The less-inhibited elongation on the upper side causes the root to bend downwards towards the force of gravity.
• Negative gravitropism involves a greater concentration of IAA collecting on the lower side of the shoot, meaning that there is more cell elongation on the lower side than the upper side. As a result, the shoot bends upwards away from gravity.
• The transport of IAA is in one direction, away from the roots and shoots where it is produced. It has multiple effects in plant cells, such as increasing the plasticity of young cell walls. As these cells mature, they become more rigid, and therefore the roots/shoots become less able to bend.
• The acid growth hypothesis explains how IAA increases the plasticity of cell walls. It involves the active transport of hydrogen ions from the cytoplasm into spaces in the cell wall, causing the cell wall to become more plastic. Consequently, the cell can elongate by expansion.
• The nervous system has two main divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The central nervous system (CNS) is made up of the brain and spinal cord.
• The peripheral nervous system is made up of the pairs of nerves that originate from either the brain or the spinal cord.
• The peripheral nervous system is divided into sensory neurones and motor neurones.