3.6 Flashcards
Survival and Response
Why do organisms respond to changes in their environment
to increase their chances of survival
e.g. by preventing extinction through potential danger
What are taxis and kinesis
simple responses that enable mobile organisms to stay in a favourable environment
taxis is a directional response to stimuli
kinesis is a non-directional response to stimuli
What is kinesis affected by and give an example
The rate of movement of an organism is affected by the intensity of the stimulus
Flatworms called planarians possess a network of neurones and simple eye-like structures that have light-sensitive cells
Planarians display kinesis when removed from their usual dark environment
Planarians are found on the underside of stones, hidden from daylight
When a stone is removed or turned over the planarians begin to move in random directions
Once these random movements eventually bring them back into the darkness they stop moving
This type of responsive behaviour helps them to protect themselves from predators
In the scenario above, the light-sensitive cells are detecting light when the stone is overturned but the planarian has no way of detecting the nearest shaded space, therefore it moves randomly until the eye detects a low level or no light
The planarian uses kinesis to ensure it is in its favourable environment - darkness
What is taxis affected by and give an example
The organism moves directly away from or towards the stimulus
A single-celled organism called Euglena which is commonly found in ponds exhibits taxis
It has chloroplasts for photosynthesis and a flagellum to help it swim
The flagellum has a receptor close to its base that is sensitive to light
Euglena swims directly towards the light, this is known as phototaxis
This behaviour is highly valuable as it brings the organism towards the light where it can photosynthesise
investigating taixs and kinesis
Taxes and kineses behaviour in small animals can be studied using special apparatus
Choice chambers and mazes are common pieces of apparatus that are used
Woodlice and maggots are often the organisms studied
It can be difficult to distinguish taxis from kinesis in these experiments
The animals need to be observed during the experiment to see if turning frequency or movement rate changes in different environments
If movement is directional then the turning frequency would decrease when the organism detects the stimulus
Choice chambers
An experiment was conducted to investigate whether maggots exhibited negative phototaxis
This would mean that they moved away from bright light (not randomly)
One half of the transparent choice chamber was covered in an opaque material to prevent light from entering
30 maggots were placed into the chamber via the hole in the centre of the lid
10 minutes later the number of maggots found in each half of the chamber were counted
This was repeated several times
The results showed that there was always more maggots in the shaded half of the chamber at the end of the experiment
As the maggots were not observed during the experiment it can not be said whether kinesis or taxis has occurred
However, the results do conclude that maggots have the ability to detect bright light and respond by moving until they reach a more favourable environment
6.1.1 sme
Organisms must respond to changes in their environment in order to survive
They can only survive if they are successful at:
Finding favourable conditions for living
Finding food
Avoiding being eaten
If these vital requirements are not met then a species will die out or go extinct
For example, a red robin must find worms and insects to feed on and at the same time, they must also be watching out for predators such as crows
Detecting and responding to change
Responses to change can vary in complexity depending on the type of organism involved and the specific circumstances they are responding to
Responding to change requires detection
Detection involves a stimulus being detected by a receptor cell
There are different types of receptors
Some receptor cells produce electrical activity in nerve cells in response to stimuli
Other receptor cells secrete substances in response to stimuli
The nerve impulses sent by receptor cells travel to a coordinator
This is either the brain or the spinal cord
From the coordinators, the impulse is conducted to the specific effector that will produce the appropriate response
Using the earlier example of the red robin staying alert to predators:
A sudden movement by a crow (the stimulus) is detected by the receptors in the robin’s eye
The receptor cells send an impulse along the nerves and to the brain (coordinator)
The brain sends an impulse to the wing muscles (effectors) of the red robin so it can fly away (response)
protective effect of a simple reflex
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.
basic structure of a Pacinian corpuscle.
-not a separate cell, as they are found at the ends of sensory neurone axons
-made of many layers of membrane separated by a gel
-gel between the layers contains positively charged sodium ions (Na+)
-the section of axon surrounded by layers of membrane contains stretch-mediated sodium ion channels
- these open when sufficient pressure is applied
how does deformation of stretch-mediated sodium ion channels in a Pacinian corpuscle leads to the establishment of a generator potential.
-Pacinian corpuscles are stimulated by pressure on the skin, which establishes a generator potential
-this happens due to the movement of charged ions across the membrane
-usually an excess of positively charged sodium ions (Na+) surrounds the axon
-pressure is exerted on the Pacinian corpuscle
-the layers of membrane become distorted and the stretch-mediated sodium channels in the axon membrane open
-sodium ions enter the axon via facilitated diffusion
-changes the electrical potential difference across the membrane
-leads to depolarisation
-establishes a generator potential
-the generator potential triggers impulses (action potentials) that travel along the sensory neurone to the central nervous system
What are Pacinian corpuscles
-receptors that respond to changes in pressure
-are present in the skin of fingers, soles of the feet, joints, tendons and ligaments
-stimulating these receptors with excess pressure on skin leads to a generator potential
Function of:
cornea
retina
iris
optic nerve
pupil
lens
cornea
transparent layer that retracts light as it enters eye
retina:
contains light receptors
-rods: detect light intensity
-cones: detect colour
iris:
controls how much light enters pupil
optic nerve
sensory neurone that carries impulses between the eye and brain
pupil
hole that allows light to enter the eye
lens
transparent disc that can change shape to focus light onto retina
Sensitivity to light and colour
Visual acuity
