Stimuli And Response Flashcards
How do organisms increase their chance of survival?
By responding to the environment as they detect changes in external evironments.
What do animals respond to?
External and internal environment
Why do animals respond to their interal environment?
To make sure that the conditions are always optimal for their metabolism.
How do plants increase their chance of survival?
By responding to changes in their environment
What is a stimulus?
A change in the interal or external environment
What detect stimuli?
Receptors
What produces a response to a stimulus?
Effector
What can receptors be?
cells or proteins on cell surface membranes
What do receptors communicate with?
the nervous system
the hormonal system
What is the nervous system made up of?
A complex network of cells called neurones.
What are the three main neurones that make up a nervous system?
- Sensory neurones - transmit electrical/ nerve impulses from receptors to the central nervous system (CNS) - the brain and spinal cord.
- Motor neurones - transmits electrical/ nerve impulses from CNS to effectors.
- Relay neurones - transmits electrical/ nerve impulses between sensory neurones and motor neurones.
What is the central nervous system (CNS)?
the brain and spinal cord.
Explain the steps of the nervous system
- a stimulus detected by receptor cells.
- electrical impulse sent along a sensory neurone.
- when an electrical impulse reaches the end of the neurone, chemicals called neurotransmitters take the info across to the next neurone which sends an electrical impulse
- The CNS processes the information and sends impulses along motor neurones to an effector
Give an example of receptors
light receptors
What is the nervous system split into (2 different systems)
The central nervous system
The peripheral nervous system
Explain the central nervous system
Made up of the brain and spinal cord
explain the peripheral nervous system
made up of the neurones that connect the CNS to the rest of the body.
It has two different systems; the somatic nervous system (controls conscious activites) and the autonomic nervous system (controls unconscious activites).
Explain the somatic nervous system
controls conscious activites
Explain the autonomic nervous system
what is it split into
controls unconscious activites.
it has two divisons that have the opposite effects on the body.
- The sympathetic nervous system - gets the body ready for action. it’s the flight or fight system.
- The parasympathetic nervous system - calms the body down. it’s the rest and digest system.
What are reflexes?
Where the body responds to a stimulus without making a concious decision to respond.
Are reflexes rapid and automatic?
yes
Why are reflexes rapid and automatic?
they dont have to spend time deciding how to respond so the information travels fast from receptors to effectors.
How do simple reflexes help organisms protect the body?
Because they are rapid
What is a reflex arc?
The pathway of neurones linking receptors to effectors in a reflex
What receptors detect the heat stimulus
thermoreceptors
how would a hand respond to heat
- thermoreceptors in skin detects heat stimulus
- the sensory neurone carries impulses to the relay neurone
- the relay connects to the motor neurone
- the motor sends impulses to the effector
- your muscle contracts to withdraw your hand and stop it being damaged
if there’s a relay neurone involved in the simple reflex arc then it’s possible to _____ the reflex.
override
i.e. your brain could tell your hand to withstand the heat.
What is a nervous system communication
localised
short lived
rapid
how is a nervous system communication localised? //
what are secreted when an electrical impulse reaches the end of a neurone and where are they secreted onto?
neurotransmitters and they re secreted directly onto target cells (so nervous response is localised)
How is the response short lived in a nervous system
the neurotransmitters are quickly removed once the job is done.
How is the response rapid in a nervous system
the electrical impulse is really fast
How do plants respond to the environment to increase their chance of survival?
- They sense the direction of light and grow towards it to maximise light absorption for photosynthesis.
- They sense the gravity so their roots and shoots grow in the right direction.
- Climbing plants have a sense of touch so they can find things to climb up and reach the sunlight.
What is a tropism?
The response of a plant to a directional stimulus.
How do plants respond to a stimuli?
By regulating their growth
What is a positive tropism
The growth towards the stimulus
What is a negative tropism?
The growth away from stimulus
What is phototropism?
The growth of a plant in response to light.
Shoots are positively phototropic and grow towards the light. The IAA moves to more shaded parts of the shoots = increase in the conc. of IAA and causes te cells to elongate and the shoot bends towards light.
Roots are negatively phototropic and grow away from the light. IAA conc. increases on shaded side, so growth inhibited causing roots to bend away from the light.
What is gravitropism?
The growth of a plant in response to gravity.
Shoots are negatively gravitropic and grow upwards. IAA moves to the underside of shoots and causes the conc. on lower side to increase - cells elongate to the shoot and grows upwards.
Roots are positively gravitopic and grow downwards. The IAA conc. on lower side means growth inhibited so the roots grow downwards.
Where is IAA produced?
in tips of shoots and roots
what does IAA stand for?
Indoleacetic acid
What is IAA?
An auxin that is moved around the plant to control tropisms
How is IAA moved around?
it moves by diffusion and active transport over short distances, and via. phloem over long distance.
How is IAA moved around over short distances
diffusion and active transport
How is IAA moved around over long distances?
The phloem
What does the uneven distribution of IAA cause?
Uneven growth of the plant
How do plants respond to directional stimuli?
Using specific growth factors
What are growth factors?
Hormone-like chemicals that speed up or slow down plant growth.
They move to areas where neeeded in other parts of the plants
Where are growth factors produced?
In the growing regions of the plant (shoot tips, leaves)
What are auxins
growth factors
What do auxins do?
Stimulate the growth of shoots by cell elongation (makes the cell walls become loose and stretchy so cell gets longer).
What inhibits the growth in roots?
High concentrations of auxins.
What do simple mobile organisms (woodlice and earthworms) have?
simple responses to keep them in a favourable environment
What are examples of stimulus’
change in temperature
light intensity
pressure
What are the two simple responses of simple mobile organisms?
- Tactic
- Kinetic
What is a tactic response? (taxis)
The directional movement of organisms towards or away in response to a directional stimulus i.e. light
Give an example of tactic (taxis) responses
- Woodlice show a tactic response to light - move away from a light source so survives as concealed under stones during day safe from predators and in damp conditions.
What is a kinetic response (Kinesis)?
A non-directional movement in response to a stimulus (non-directional) i.e humidity
Give an example of a kinetic response (Kinesis)
- Woodlice show a kinetic response to humidity. In high humidity they move slowly and turn less often so they stay where they are.
- As air gets drier, they move faster and turn more often so move into a new area.
- That response helps them move from dry to more humid air and then stay put.
- That improves chance of survival - reducing water loss and helps keep them concealed.
What can you use to invesitgate animal responses?
Choice chambers
What is a choice chamber?
A container with different compartments in which you create different environmental conditions.
How can you use a choice chamber?
- Construct a choice chamber using the equipment; petridish, divider, base divided into two, fine mesh, choice chamber.
- Investigate effect of light intensity on woodlouse movement by covering one half of the lid with black paper. Put damp filter paper on both sides of the base so humidity constant.
- 10 woodlice on mesh in centre of the chamber and position lid on mesh so lined up with base.
- After 10 mins, take off lid and record number of woodlice in each side of chamber, Minimise amount of time the lid is off so environment conditions aren’t disturbed.
- Repeat the experiment after gently moving the woodlice back to the centre. Use a soft paint bursh to help. Most woodlice should end up back on the dark side (tactic response).
- To investigate humidity, place some damp filter paper in one side of the base and a desiccating agent in the other. Don’t cover the lid with paper. Put the lid back on and leave for 10 mins to stabilise.
Give one similarity and one difference between a taxis and a tropism. (2)
Similarity − directional response (to a stimulus) / movement towards / away from a stimulus;
Difference − taxis (whole) organism moves and tropism a growth (response).
What do these results show about the responses of the roots of tomato plants to gravity and salt? (3)
- Grow in direction of / towards (pull of) gravity;
- Grow away from salt;
- Salt has more effect (than gravity).
When measuring the mean percentage damage to maize plants, 60 plants from each test plot were selected at random and examined.
Describe how the maize plants could be selected at random.
- Set up tape measures on two sides of the plot / make grid of plot;
- Use random number table / calculator / generator;
- To generate coordinates;
The legume plants have nodules containing nitrogen-fixing bacteria on their roots. Explain how nitrogen-fixing bacteria could increase the growth of the maize.
- Nitrogen-fixing bacteria convert nitrogen (in the air) into ammonium compounds (in the soil) which are converted into nitrates / nitrification occurs;
- Maize uses nitrates (in soil) for amino acid / protein / ATP / nucleotide production;
Explain how this distribution of IAA causes the root to bend.

- (IAA) at bottom of root / where IAA concentration high inhibits expansion / elongation (of cells);
- (IAA) at top of root / where IAA concentration low leads to expansion / elongation (of cells);
What is the push-pull stimulus?
A push stimulus drives the pest away from the crop plant.
A pull stimulus attracts the pest towards a different species of plant or to a pesticide.
What are receptors specific to?
One particular stimulus
What do receptors in the nervous system convert the energy of the stimulus into?
Electricial energy used by neurones
- What happens at the receptors resting state?
- What is the potential difference called when a cell is at rest?
- When a nervous system receptor is in its resting state, there’s a difference in charge between the inside and outside of the cell - this is generated by ion pumps and ion channels. This means that there’s a voltage across the membrane. Voltage is also known as potential difference.
- The potential difference when a cell is at rest is called its resting potential.
What happens when a stimulus is detected?
What is the change in potential difference called?
When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell - altering the potential difference. The change in potential difference due to a stimulus is called the generator potential.
A bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger change in potential difference - so a bigger generator potential is produced.
If the generator potential is big enough, what does it trigger?
If the generator potential is big enough it’ll trigger an action potential - an electrical impulse along a neurone.
When is an action potential triggered?
An action potential is only triggered if generator potential reaches a certain level called the threshold level.
Action potentials are all one size so strength of stimulus is measured by frequence of action potentials.
If the stimulus is too weak the generator potential won’t reach the threshold, so there’s no action action potential.
The definition of resting potential
Potential difference when cells at rest
Definition of the generator potential
The change in potential difference due to a stimulus
What are pacinian corpuscles?
Pressure receptors in your skin.
They are mechanoreceptors that detect mechanical stimuli.
What does a bigger stimulus cause
the membrane to be excited so bigger movement of ions and bigger change in potential difference = bigger generator potential.
Where is the end of a sensory neurone found?
In the pacinian corpuscles
What is in the pacinian corpuscles?
Sensory nerve ending.
What is the sensory nerve ending wrapped in?
layers of connective tissues called lamellae.
Describe how a Pacinian corpuscles produces a generator potential when stimulated
- When it is stimulated, the lamellaes deformed and press on the sensory nerve ending.
- That causes the sensory neurone’s cell membrane to stretch, deforming the stretch-mediated sodium ion channels.
- Channels open and sodium ions diffuse into cells/ membrane creating a generator potential.
- The Na+ have a positive charge so causes the membrane to become depolarised,
- If the GP reaches threshold, AP triggered.
When observing an organism’s behaviour in a choice chamber (sections with different conditions, and a way for organisms to move between different sections), explain how a taxis would differ from a kinesis.
In a taxis, the organism would go straight to favourable conditions and away from unfavourable conditions. In a kinesis, the organism would set off randomly in any direction but it would speed up or turn less if in unfavourable conditions, and it would slow down or turn more in favourable conditions.
What are photoreceptors?
They are light receptors in your eye. The retina contains the photoreceptor cells which detect light.
Explain how light enters the eye and is controlled
Light enters through the pupil.
The amount of light that enters is controlled by the muscles of the iris.
Light rays are focused by the lens onto the retina, which lines the inside of the eye. The retina contains the photoreceptor cells.
The fovea is an area of the retina where lots of photoreceptors.
Nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve, which is a bundle of neurones.
Where the optic nerve leaves the eye = blind spot so no photoreceptor cells so not sensitive to light.
What is the blind spot?
Where the optic nerve leaves the eye
How do photoreceptors work?
- Light enters the eye, hits the photoreceptor and is absorbed by light sensitive optical pigments.
- Light bleaches the pigments causing a chemical change and altering the membrane permeability to soidum ions.
- A generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.
- Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.
What is the order of nervous communication in the eye?
Photoreceptors -> bipolar neurone -> optical neurone
What are the two types of photoreceptors?
Rods
Cones
Where are rods found?
In peripheral parts of retina
Where are cones found?
Packed together in fovea.
How are rods and cones sensitive to different wavelengths of light?
Different optical pigments
Which photoreceptor gives information in colour
cones
Which photoreceptor gives information in black and white?
rods
What are the different optical pigments in three types of cones?
- Red sensitive
- Green sensitive
- Blue sensitive
When do you see different colours?
When they’re stimulated in different proportions
Why would you be able to see yellow on cones?
Because the frequency of light falls between red and green and stimulates both red and green sensitive cones . The brain converts it into yellow.
Explain sensitivity in rods.
Very sensitive to light. They fire action potentials in dim light. This is because many rods join one bipolar neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential.
Action potential created
Explain sensitivity in cones
Less sensitive than rods. They only fire action potentials in bright light. This is because one cone joins one neurone so takes more light to reach the threshold and trigger an action potential.
No action potenital
Explain visual acuity in rods
Gives low visual acuity because many rods join the same neurone, which means light from two points close together can’t be told apart.
Action potential
Explain visual acuity in cones
High visual acuity because cones are close together and one cone joins one bipolar neurone. When light from two points hits two cones, two action potentials (one from each cone) go to the brain - so you can distinguish two points that are close together as two separate points.
Action potential
Compare rods and cones
Rods
- Mainly located in the peripheral parts of the retina.
- Gives info. in black and white.
- Many rods join one bipolar neurone.
- High sensitivity to light.
- Give low visual acuity.
Cones
- Mainly located in fovea.
- Give info. in colour.
- One cone joins one bipolar neurone.
- Low sensitivity to light.
- Give high visual acuity.
What do cardiac muscles control?
The regular beating of the heart
What is the cardiac muscle known as?
Myogenic
What does myogenic mean?
It can contract and relax without receiving signals from nerves.
What is the structure of the nerve system split into?
Central nervous system
Peripheral nervous sytem
What is sinoatrial mode (SAN)?
Small mass of tissue in the wall of the right atrium. It generates electrical impulses that cause the cardiac muscles to contract.
Explain the control of the heartbeat
- The process starts in the sinoatrial node (SAN) which is the wall in the right atrium.
- The SAN is like a pacemaker, it sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls.
- That cause the right and left atria to contract at the same time.
- A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles.
- Instead, these waves of electrical activity are transferred from the SAN to the atrioventricular node (AVN).
- The AVN is responsible for passing the waves of electical activity on to the bundle of His. But, there’s a slight delay before the AVN reacts, to make sure the atria have emptied before the ventricles contract.
- Bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity between the ventricles to the aprex (bottom) of the heart. The bundle splits into finer muscle fibres in the right and left ventricle walls, called Purkyne tissue.
- The Purkyne tissue carries the waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract stimultaneously from the bottom up.
What does the control of heart rate involve?
The brain and autonomic nervous system.
The rate at which the SAN fires is unconsciously controlled by what part of the brain?
Medulla oblongata
What do animals need to do to respond to internal stimuli?
Alter their heart rate
What are stimulis detected by?
pressure receptors and chemical receptors
What are pressure receptors called?
Baroreceptors
Where are baroreceptors found and what are they stimulated by?
The aorta and the carotid arteries
They are stimulated by high or low blood pressure
What are chemical receptors called?
Chemoreceptors
Where are chemical receptors found?
What do they monitor?
In the aorta, carotid arteries and medulla.
They monitor the oxygen level in the blood and carbon dioxide and pH (which are indicators of O2 level).
Where are electrical impulses sent from receptors?
To the medulla along sensory neurones.
What does the medulla do?
Processes the information and sends impulses to the SAN along parasympathetic or sympathetic neurones (which are part of the autonomic nervous system).
What causes the heart rate to speed up or slow down?
Stimulis detected by receptors
What detects high blood pressure and what process occurs to return the blood pressure to normal?
- Baroreceptors detect high blood pressure.
- Impulses are sent to the medulla which send impulses along parasympathetic neurones. They secrete acetylcholine, which binds to receptors on the SAN.
- That causes the heart rate to slow down to reduce blood pressure back to normal.
What detects low blood pressure and what process occurs to return the blood pressure to normal?
- Baroreceptors detect low blood pressure.
- Impulses are sent to the medulla, which sends impulses along smypathetic neurones. These secrete noradrenaline (a neurotransmitter), which binds to the receptors on the SAN.
- Speeds up heart rate to increase blood pressure back to normal.
What detects high blood oxygen levels, low carbon dioxide levels and high pH levels and what process occurs to return the blood pressure to normal?
- Chemoreceptors detect chemical changes in the blood.
- Impulses sent to medulla, which sends impulses along parasympathetic neurones. These secrete acetylcholine, which binds to receptors on the SAN.
- Heart rate decreases to return oxygen, carbon dioxide and pH levels back to normal.
What detects low blood oxygen levels, high carbon dioxide levels and low pH levels and what process occurs to return the blood pressure to normal?
- Chemoreceptors detect chemical changes in the blood.
- Impulses sent to medulla, which sends impulses along sympathetic neurones. These secrete noradrenaline, which binds to receptors on the SAN.
- Heart rate increases to return oxygen, carbon dioxide and pH levels back to normal.
What is the effector that controls heart rate?
Cardiac muscles
Role of the Purkyne tissue
the Purkinje fibres carry the contraction impulse from both the left and right bundle branch to the ventricles.
Damage to the myelin sheaths of neurones can lead to problems controlling the contraction of muscles.
Suggest one reason why.
- Action potentials travel more slowly / don’t travel
- So delay in muscle contraction / muscles don’t contract / muscles
- contract slow(er);
Explain how the resting potential of –70 mV is maintained in the sensory neurone when no pressure is applied.
- Membrane more permeable to potassium ions and less permeable to sodium ions;
- Sodium ions actively transported / pumped out and potassium ions
Explain how applying pressure to the Pacinian corpuscle produces the changes in membrane potential recorded by microelectrode P.
- (Pressure causes) membrane / lamellae to become deformed / stretched;
- Sodium ion channels in membrane open and sodium ions move in;
- Greater pressure more channels open / sodium ions enter.
The membrane potential at Q was the same whether medium or heavy pressure was applied to the finger tip. Explain why.
- Threshold reached
- Causes maximal response / all or nothing principle.
Multiple sclerosis is a disease in which parts of the myelin sheaths surrounding neurones are destroyed.
Explain how this results in slower responses to stimuli.
- Less / no saltatory conduction / action potential / impulse unable to ‘jump’ from node to node;
- More depolarisation over length / area of membranes.
Explain how a rise in blood pressure results in a decrease in the rate of heartbeat
- Baroreceptors
- Electrical impulse sent from medulla
- Across the parasympathetic nerve
- to SAN;
- release of ACh / inhibits SAN / decreases impulses from SAN;
- decreases impulses to AVN / decreased stimulation of AVN / decreases impulses from AVN;
Some drugs inhibit the transmission of nerve impulses to the heart. Explain how these drugs reduce high blood pressure. 2 marks
- inhibit impulses in sympathetic nerves / from cardio-acceleratory centre;
- SAN not stimulated / noradrenaline not released so heart rate lowers / does not increase;
rod cells enable us to see in conditions of low light intentsity. explain how
Many rods join one bipolar neurone
Light strikes several rod cells
explain how cone cells enable us to distinguish between objects close together
One cone cells join one bipolar neurone. Light from two points hits two cones so two action potential go to the brain to distinguish two points.
Explain how one axon of a neurone can be stimulatory while another axon of the same neurone can be inhibitory
Diff. neurotransmitters produced
Describe how rod cells act as transducers of energy
- transducers converts one form of energy into another.
- light energy to chemical to electrical.
- causing AP in neurones.
What is meant by visual acuity?
Ability to distinguish two close points/ objects apart
Explain how pressure on the pacinian corpuscles produces the change in membrane potential recorded by microelectrode A
- Pressure deforms sensory nerve cell and causes sodium channels to open.
- Increase in pressure opens more channels
- causing depolarisation
