3.6 organisms respond to changes in their environment

define stimulus

define stimulus

a detectable change in the external or internal environment of an organism that leads to a response

advantages of reflex arc

• greater chance of survival, raising offspring, and passing on alleles to the next generation

• it doesn’t overload the brain as it’s a subconscious response

• protects the body from harm

• quick due to the short neuron pathway and absence of the decision-making process

reflex arc steps:

stimulus > receptor > coordinator > effector > response

taxes (taxi definition)

a simple response whose direction is determined by the direction of the stimulus

positive and negative taxis

positive taxis: movement towards the stimulus
negative taxis: movement away from the stimulus

kinesis definition

a form of response where the organism doesn’t move towards or away from the stimulus - instead it changes the speed of its movement and the rate at which it changes direction

tropisms definition

the growth of the part of the plant in response to a directional stimulus

shoots: positive phototropism, negative gravitropism
roots: negative phototropism, positive gravitropism

what controls tropisms (plant cell growth)

IAA (indole-3-acetic acid)

positive phototropism in flowering plants:

• cells in the tip produce IAA, which move down and evenly distribute through the plant

• light causes movement of IAA from light to shaded side

• a greater conc of IAA builds up on the shaded side of the shoot

• the cells on the shaded side elongate faster, causing the shoot tip to bend towards the light

gravitropism in flowering plants

• cells in the tip of the root produce IAA which moves along the root

• gravity causes movement of IAA to the lower side of the root

• IAA stops elongation of root cells and as there is greater concentration of IAA on the lower side the upper side elongates quicker causing the root to bend downwards by the force of gravity

the spinal chord

consists of a column of nervous tissue along the back, inside the column for protection

the three neurons

sensory, relay, motor neurons.

the 7 steps of the reflex arc:

1. stimulus

2. receptor

3. sensory neuron

4. coordinator (relay neuron)

5. motor neuron

6. effector

7. response

what is the pacinian capsule

a sensory receptor that responds to changes in mechanical pressure

features of a pacinian capsule

• specific to a single type of stimulus

• produces a generator potential, that converts energy from the stimulus into nerve impulses

• receptors convert one form of energy into another (producing generator potential)

where are pacinian capsules located

fingers, soles of feet, external genetalia, ligaments, joints, tendons

structure of pacinian capsule

• plasma membranes have channel proteins

• the sensory neurone ending has stretched-mediated sodium channels in the membrane

• when these channels are deformed (by pressure) they become permeable to sodium ions

function of the pacinian capsule

• resting state: the stretching mediated sodium channels are too narrow

• when pressure is applied the channels deform causing the membrane to stretch which widens the sodium channels allowing sodium ions to diffuse into the neutron

• the influx of sodium ions depolarises the neuron

• this creates an action potential

structure of the eye

features of ROD CELLS (6)

1. rod-shaped

2. greater number compared to cone cells

3. more at the periphery of the retina, absent at the fovea

4. poor visual acuity (clarity)

5. sensitivity to low-intensity light

   1. one type only

features of CONE CELLS (6)

1. cone-shaped

2. fewer number compared to rod cells

3. fewer at periphery of the retina, concentrated at the fovea

structure of receptors in the eye

where are light receptors found

on the retina

how do rods and cones act as transducers

by converting light energy into an impulse

functions of ROD cells

• breaks down rhodopsin to generate an action potential

• many rods connect to a single neuron (only sending one impulse) = brain can’t distinguish between separate sources of light that stimulated the impulse = poor visual acuity

functions of CONE cells

• breaks down iodopsin to generate an actions potential

• each type of cone cells contains an specific type of iodopsin that’s sensitive to different light wavelengths

  • each cone cell connected to its own neuron = high visual acuity

where are the most cone cells found and why

the fovea

• lens focuses the light onto the fovea, which receives the most amount of light, so this is where the most amount of cone cells are found

what is the cardiac muscle called tissue of the heart called

myogenic tissue

what two systems control the organ systems

the nervous and hormonal system

hormonal system

• communication by hormone chemicals

• transmission by the blood system

• transmission is relatively slow

• hormones travel to all parts of the body, but only to the target cells response

• response is widespread

• response is slow

• response is long-lasting

• effects may be permanent and irreversible

nervous system

• communication is by nerve impulses

• tranmission is by neurones

• transmission is very rapid

• nerve impulses travel to specific parts of the body

• response is localised

• repsonse is rapid

• respsone is short-lived

• effect is usually temporary and reversible elc

cell body

nucleus and large amount of RER associated with production of proteins and neutron

where is the pacemaker of the heart

in the wall of the right atrium there is a region of specialised fibres called the sinoatrial node

what does the sinoatrial node do

initiates a wave of electrical stimulation which causes the atria to contract at roughly the same time

why does the ventricle only contract when the atria have finished

due to the presence of tissue at the base of the atria which is unable to conduct the wave of excitation

path of the electrical wave

the electrical wave entually reaches the atrioventricular node between the two atria

this passes the wave of excitation to the ventricles, down the bundle of His to the apex of the heart

the bundle of His branches into Purkyne fibres which carry the wave upwards - this causes the ventricles to contract therefore emptying them

resting potential

difference in charge across the membrane when a neurone is not firing - (-70mv)

how is resting potential maintained

• by keeping more pos ions outside the cell than inside

  1) done by using a Na+/K+ pump, ATP pumps 3Na and 2K in - a neurone with a more pos charge outside than inside creating a negative resting potential

  2) membrane also has more protein channels for K+ than Na+ and due to the concentration of K+ being higher inside, it diffuses out of the neurone, making the resting potential even lower

  3) many neg charge molecules also present inside the cell, and the membrane is impermeable to them, so neurone is more negative inside

action potential

rapid change in potential difference across the membrane caused by changes in permeability of cell surface membrane to Na+ and K+

a synapse is a junction between

two neurones, a neurones and an effector