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Summary Topic 6: Organisms respond to changes in their internal and external environments £4.16
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Summary Topic 6: Organisms respond to changes in their internal and external environments

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3.6.1.1 Survival and response Organisms increase their chance of survival by responding to changes in their environment. In flowering plants, specific growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli.

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  • February 1, 2025
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3.6.1.1 Survival and response



Organisms increase their chance of survival by responding to changes
in their environment.



In flowering plants, specific growth factors move from growing regions
to other tissues, where they regulate growth in response to directional
stimuli.

● Plants respond to external stimulus to increase their chance of survival
○ ie exhibit tropisms
■ growth response controlled by direction stimulus
■ phototropisms
■ direction of growth determined by direction of light
■ shoots of plants are positively phototrophic
■ roots are negatively phototrophic and grow away
from the light



The effect of different concentrations of indoleacetic acid (IAA) on cell
elongation in the roots and shoots of flowering plants as an explanation
of gravitropism and phototropism in flowering plants.

● Plant growth is controlled by indoleacetic acid
○ auxin produced in tips and shoots of flowering plants
○ distribution of IAA around the plant controls tropisms
■ unevenly distributed IAA
■ uneven growth
○ When shoot illuminated from all sides
■ auxins distributed evenly and move down shoot tip
■ causing elongation of cells across zone of elongation
○ When shoot is illuminated from one side
■ auxins distributed evenly and move down shoot tip
■ causing elongation of cells across zone of elongation

, ○ Whereas if shoot only illuminated from one side
■ auxins move towards shaded part of shoot
■ causing elongation of shaded side only
■ bending of shoot towards light
○ Gravitropism
■ roots is the opposite
■ IAA will build up on lower side of root
■ inhibiting growth
■ causing cells on upper cells to grow faster
■ causing root to bend downwards



Taxes and kineses as simple responses that can maintain a mobile
organism in a favourable environment.

● Taxes
○ response that involves movement in specific direction
○ positive taxis towards stimulus
■ mobile bacteria moving to area where there is higher concentration of
glucose
○ negative taxis away from stimulus
● Kinesis
○ response that involves movement
■ random directions
○ Both speed and frequency of direction change increase
○ response carried out in order to increase chance organism enter different
conditions more rapidly
■ place woodlouse in dry area it speeds up and change direction more
frequently to enter damp conditions



3.6.1.2 Receptors

● Detect changes in internal and external environment
● many types of receptors
○ each specific to a particular kind of stimuli
■ photoreceptors
■ detect changes in light
■ mechanoreceptors

, The Pacinian corpuscle should be used as an example of a receptor to
illustrate that:

• respond only to specific stimuli




The basic structure of a Pacinian corpuscle.

● single sensory neurone located in centre of connective tissue called lamellae
○ forms layers separated by a gel
● located deep in the skin
○ found mostly on
■ fingers
■ soles of feet
■ external genitalia



Deformation of stretch-mediated sodium ion channels in a Pacinian
corpuscle leads to the establishment of a generator potential.

● contains stretch-mediated sodium channels in cell surface membrane
○ when not under pressure
■ channels closed
○ under pressure
■ become deformed
■ open and allow rapid influx of sodium ions to occur
■ positive charge of sodium ions changes membrane
potential
■ causing membrane to become depolarised
■ results in generator potential being
created which goes on to create an action
potential in the axon



The human retina in sufficient detail to show how differences in
sensitivity to light, sensitivity to colour and visual acuity are explained
by differences in the optical pigments of rods and cones and the
connections rods and cones make in the optic nerve.

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