Sensory Reception

Control in Cells and Organisms Multicellular organisms are able to respond to stimuli that originate both from outside and inside their bodies. This helps them avoid harmful environments whilst also maintaining an internal environment that provides optimum conditions for their metabolism. Organisms control their activities through a combination of growth factors, hormones and nervous impulses. At a cellular level, control is achieved by regulating which genes are expressed and when this takes place.

STIMULUS RECEPTOR A detectable change in the internal or external environment of an organism Acts like a switchboard. It receives information from areas of the body and then activates certain cells/tissue or organs of the body to produce a response COORDINATOR Cells or organs that are able to detect a change in the environment (internal or external) EFFECTOR Cells, tissue or even organs that carry out a specific response designed to increase chances of survival.

Answers Stimulus – A detectable change in the internal or external environment of an organism. Receptor – Cells or organs that are able to detect stimuli e.g. rod/cone cells in eye. Receptors transform the energy of a stimulus into a form of energy that can be processed by the organism and leads to a response.

Answers Coordinator – Acts like a switchboard. It receives information from receptors then activates certain areas of the body to produce a response. Differs from organism to organism but in most complex organisms – hypothalamus region of the brain. Effectors – Cells, tissue or even organs that carry out a specific response designed to increase chances of survival. Effectors in higher order organisms usually take form of muscles or glands.

Stimulus and Response Organisms are better adapted for survival if they are able to respond to changes internally or externally. The ability to respond to changes in the environment increase the chances of survival of an organism. For example, they may help to detect and move away from harm such as predators and extremes of temperature. They may help an organism move towards a food source Having a better chance of survival, increases the chances of reproducing successfully and passing ones alleles to the next generation. There is selection pressure favouring organisms that are more able to produce appropriate responses.

Respones in Simple Organisms Taxis – simple response determined by the DIRECTION of the stimulus. If a motile organism experiences the stimulus it will either move towards or away from the direction of the stimulus. Positive taxis – moving towards (stimulus must be favourable) Negative taxis – moving away (unfavourable stimulus)

Examples of Taxes Single-celled algae will move towards light (POSITIVE PHOTOTAXIS). Earthworms will move away from light (NEGATIVE PHOTOTAXIS) because it will take them into soil that is moist (won’t dry out), less conspicuous to predators (dark) and more likely to find food. Some species of bacteria will move towards a region that has a high concentration of glucose (POSITIVE CHEMOTAXIS). Glucose is their food source.

KINESES A form of a response in which an organism does not move towards or away from a stimulus. Woodlice show Instead, the more unpleasant the stimulus, the more rapidly it moves and more frequently it changes direction. (Changing direction more often will not increase it’s chances of moving in a circle because woodlice have the ability to make a 2 nd turn in the opposite direction to their previous turn). This type of response increases the chances of moving into a more favourable environment.

Kineses Kineses are important when the stimulus is non-directional e.g. temperature or humidity. Woodlice are prone to desiccation (drying out) because they lose water from their bodies quite easily in dry conditions. When in a dry area they will move more rapidly and change direction more often. When back in a moist area, they will slow down and change direction less often.

Tropisms A growth movement of a plant in response to a directional stimulus. In almost all cases the plant part (stem, root etc.) grows towards (positive tropism) or away from (negative tropism) the stimulus.

Examples of Tropisms Plant shoots grow towards light (positive phototropism) and so that their leaves are in most favourable conditions for photosynthesis. Plant roots grow away from light (negative phototropism) and towards gravity (positive geotropism). These increase chances of roots growing into soil where they are better able to absorb water and minerals. Plant roots also grow towards water (positive hydrotropism) so that, within the soil, root systems will develop where there is most water.

Questions For each of the following statements, name the type of response described (taxis, kinesis, tropism) and the survival value of the response. 1. Some species of bacteria move away from the waste products that they produce. 2. The sperm cells of a moss plant are attracted towards a chemical produced by the female organ of another moss plant. 3.The young stems of seedlings grow away from gravity.