Unit 6: organisms respond to changes in their internal and external environments Flashcards
stimulus
a change in the internal or external environment
receptor
detects a stimulus
-specific to one type of stimulus
coordinator
formulates a suitable response to a stimulus
effector
produces a response towards the stimulus
taxis
a simple directional response determined by the direction of the stimulus
positive taxis
motile organism moves towards the favorable stimulus
negative taxis
motile organism moves away from the unfavorable stimulus
positive phototaxis
organism moving towards light
e.g. single celled algae
kinesis
form of response in which the organism does not move towards or away from the stimulus. instead it changes at the speed it moves and the rate at which it changes direction
kinesis in favourable conditions
movement will be slower and direct, so the organism spends longer here
kinesis in unfavourable conditions
movement is faster and involve more turns, to find the favourable condition
tropism
the growth of a plant in response to a directional stimulus
positive phototropism and negative gravitropism
plant shoots grow towards light and away from gravity
negative phototropism and positive gravitropism
plant shoots grow away from light and towards gravity
what do plants respond do
-light, shoots grow towards light because it is needed for photosynthesis
-gravity, plants need to be firmly anchored into the soil. roots are sensitive to gravity and grow in the direction of its pull
-water, almost all plants grow towards water (e.g. positively hydroptropic) in order to absorb it for use in photosynthesis and other metabolic processes
what is plant growth controlled by
indoleacetic acid (IAA) which is an important auxin produced in the tips and shoots of flowering plants. the distribution of IAA around the plant controls tropisms.
IAA on the effect of phototropism
when the shoot is illuminated from all side, the auxins are evenly distributed and move down the shoot tip thus causing elongation of cells across the cells of elongation.
whereas, if the shoot is only illuminated from one side, the auxins move towards the shaded part of the shoot thus causing elongation of the shaded side only which results in the bending of the shoot towards the light
IAA on the effect of gravitropism
gravitropism in the roots is the opposite. IAA will build up on the lower side of the root. In roots IAA inhibits growth, therefore causing the cells on the upper side to grow faster, causing the root to bend downwards
explain how the movement of IAA in shoots helps plants to survive
more IAA moves towards the shaded side of shoots than the light side when light is unidirectional. in repose to this uneven distraction of IAA, the cells on the long acted side elongate faster than those on the light side and the shoot bends towards the light
-this ensures that the shoot and the leaves attached to it have a greater chance of being illuminated
-as light is essential for photosynthesis, the process by which organic material for respiration is manufactured, the plant has a greater chance of survival
what are the nervous systems two major division
central nervous system- made up of brain and spinal cord
peripheral nervous system- made up of pairs of nerves that originate from either the brain or the spinal cord
what is the peripheral nervous system divided into
sensory nervous system- carry nerve impulses from the receptors the cns
motor nervous system- carry nerve impulses from the cns to the effectors
what is the motor nervous system divided into
voluntary nervous system- which carries nerve impulses to body muscles and is under voluntary control
autnomic nervous system- carries nerve impulses to glands, smooth muscle and cardiac muscle. under involuntary control
what is a reflex
type of involuntary response to a sensory stimulus
what is a reflex arc
the pathway of neurons involved in a reflex
order of the reflex arc
stimulus, receptor, sensory neuron, coordinator, motor neuron, effector, response
importance of reflex arcs
-they are involuntary and therefore do not require the decision making powers of the brain. allows the brain to be free to carry out more complex responses
-they protect the body from harm, present from birth and do not have to be learnt
-they are fast, because the neuron pathway is short with very few synapses where neurons communicate with each other
features of a Pacinian corpuscle
-is specific to a single type of stimulus, it only responds to mechanical pressure
-produces a generator potential by acting as a transducer. Pacinian corpuscle transduces the mechanical energy of the stimulus into a generator potential
basic structure of a Pacinian corpuscle
-single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule
-stretch mediated sodium channels on plasma membrane
-capillary runs along base layer of tissue
what stimulus does a Pacinian corpuscle respond to and how
-pressure deforms membrane, causing stretch mediated sodium ion channels to open
-if influx of sodium raises membrane to threshold potential (and the membrane becomes depolarised), a generator potential is produced
-action potential moves along the sensory neuron
2 types of photoreceptor cells located in the retina
-cone cells
-rod cells
location of rod cells
distribution more at the periphery of the retina
-rod cells are in abundance because light intensity is lowest
explain the low intensity response of rod cells
rhodopsin can break down the pigment in rod cells to create a generator potential. there is enough energy from low intensity light to cause this breakdown.
-the stimulation of multiple rod cells can be combined to exceed the threshold value and therefore create a generator potential
explain the visual acuity of rod cells
in perception, the brain cannot distinguish between the separate sources of light due to multiple rod cells only linking to one bipolar cell therefore it is only able to send a single nerve impulse
location of cone cells
concentrated at the fovea as this is what receives the highest intensity of light
explain the high intensity response of cone cells
each cone cell has their own sperate bipolar cell, so the stimulation of multiple cone cells cannot be combined to exceed the threshold value which produces a generator potential.
-iodopsin requires a higher intensity for its breakdown and creation of a generator potential
explain the visual acuity of cone cells
high visual acuity because each cone cell is connected to its own bipolar cell, which means that if two adjacent cone cells are stimulated, the brain receives two separate impulses. the brain cannot distinguish between the two separate sources of light that stimulated the cone cells
what is visual acuity
the clarity of vision, determined by the individuals ability to recognise small details with precision
what are the colours seen by rod + cone cells
rod- cannot distinguish different wavelengths of light so black and white only
cone- each cone cell is sensitive to a specific range of wavelengths so can see in full colour (three types of cone cells)
define myogenic
the hearts ability to initiate its own contraction
pacemaker of the heart
sinoatrial node located in the wall of the right atrium
-this initiates a wave of electrical stimulation which causes the atria to contract roughly at the same time
describe the control of heart rate
-the SAN initiates a heart beat and sends a wave of electrical impulses across the atria, causing them to contract
-after a short delay to allow the atria to empty before the ventricles contract, the AVN sends a wave of electrical impulses down the bundle of His
-bundle of His receives electrical activity from the AVN and conducts a wave of excitation to the apex of the heart
-purkyne tissues branch off the bundle of His, the wave of excitation is released from the purkyne tissues causing the ventricles to contract quickly at the same time from the bottom upwards
what region of the brain controls heart rate
medulla oblangata
2 divisions of the autonomic NS that control heart rate
sympathetic- increases heart rate
parasympathetic- decreases heart rate
control by chemoreceptors
they are sensory receptors found in the wall of the carotid arteries that detect changes in the concentration of chemicals in the blood
-when the blood has increased levels of co2, the concentration of H+ ions also increases which causes the pH levels to fall
-chemoreceptors detect this and increase the frequency of nervous impulses to the centre in the medulla oblongata
-this centre increases the frequency of impulses via the sympathetic nervous system to the sinoatrial node, this increases the rate of production of electrical waves by the SAN, increasing heart rate
-the increased blood flow, leads to more co2 being removed by the lungs and so the co2 concentration in the blood returns to normal
-pH levels rise back to normal, chemoreceptors reduce the frequency of nerve impulses to the medulla oblongata
-which reduces the frequency of impulses to the SAN therefore a reduction in heart rate
control by baroreceptors
pressure receptors found within the walls of the carotid arteries
-when blood pressure is higher than normal, pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that decreases heart rate. this centre sends impulses down the parasympathetic NS to the SAN which decreases heart rate
-when blood pressure is lower than normal, pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that increases heart rate. centre sends signals down the sympathetic NS to the SAN which increases heart rate
two main forms of coordination
nervous system- uses nerve cells to pass electrical impulses along their length. they stimulate their target cells by secreting neurotransmitters directly onto them.
hormonal system- produces hormones that are transported in the blood plasma to their target cells, which have specific receptors on their cell surface membranes and the change in hormone concentration stimulates them
comparisons of hormonal and nervous systems
hormonal:
-communication is by chemicals (hormones)
-transmission is by the blood stream
-transmission is usually relatively slow
-hormones travel to all parts of the body, but only target cells respond
-response is slow and often long lasting
-effect may be permanent and irreversible
nervous:
-communication is by nerve impulse
-transmission is by neurons
-transmission is very rapid
-nerve impulses travel to specific parts of the body
-response is rapid and short lived
-effect is usually temporary and reversible
neurons
specialised cells adapted to rapidly carrying electrochemical changes called nerve impulses from one part of the body to another
cell body
contains all the usual cell organelles, including a nucleus and large amounts of rough ER. this is associated with the production of proteins and neurotransmitters
dendrons
extensions of the cell body which subdivide into smaller branched fibers, called dendrites, that carry nerve impulses towards the cell body
axon
single long fibre that carries nerve impulses away from the cell body
Schwann cells
surround the axon, protecting it and providing electrical insulation. they also carry out phagocytosis
myelin sheath
forms a covering to the axon and is made up of the membranes of the Schwann cells. neurons with a myelin sheath are called myelinated neurons
nodes of Ranvier
constrictions between adjacent Schwann cells where there is no myelin sheath
sensory neurons
nerve impulses from a receptor to an intermediate/motor neuron
-one long dendron and one axon
-cell body in the middle
motor neurons
nerve impulses from an intermediate/relay neuron to an effector
-long axon and many short dendrites
-cell body at one end
intermediate/relay neurons
impulses between neurons with numerous short processes
define resting potential
when the inside of the axon is negatively charged relative to the outside of the axon, when this occurs the axon is polarised
establishing a resting potential
-sodium ions ae actively transported out of the axon by the sodium potassium pumps
-potassium ions are actively transported into the axon by the sodium potassium pumps
-active transport of sodium is greater than that of potassium because 3 sodium ions move out for every 2 potassium ions in
-as a result, there are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm, more potassium ions are in the cytoplasm than in the tissue fluid. this creates an electrochemical gradient
-sodium ions begin to diffuse back naturally into the axon while the potassium ions diffuse back out of the axon
-most of the gated channels for diffusion of sodium ions are closed but for potassium are open, maintaining the electrochemical gradient causing resting potential
define action potential
when the negative charge inside the membrane becomes the positive charge, depolarising the axon, occurs when transmitting a nerve impulse
-simply means that the axon membrane is transmitting a nerve impulse
establishing an action potential
-at resting potential, some potassium voltage gated channels are open but the sodium voltage gated channels are closed
-the energy of the stimulus causes some of the sodium voltage gated channels in the axon membrane to open so sodium ions diffuse into the axon along their electrochemical gradient
-sodium ions are +ively charged, so this diffusion causes a reversal in the potential difference across the membrane
-as more ions diffuse, more sodium ion channels open, causing a greater influx of the ions
-once the action potential of around +40mV exists, voltage gates close on the sodium ion channels and those on potassium ion channels open
-the electrical gradient which was preventing further further outward movement of the potassium ion is now reversed so even more channels open, starting repolarisation of the axon due to movement of potassium ions
-this causes hyperpolarisation of the axon due to the inside being more negative than usual
-potassium ion channel gates close and the sodium potassium pump moves sodium out and potassium’s in
-this re-establishes the resting potential so the axon is repolarised
