Chapter 14: Response to Stimuli Flashcards
spec point 3.6.1
what is a stimulus?
- detectable change in the internal or external environment of an organism that produces a response
what is a taxis?
simple response that’s direction is determined by the direction of the
stimulus
how do organisms respond directly to stimulus?
moving its body either:
- ) Toward the stimulus (positive taxis)
- ) Away from the stimulus (negative taxis)
give an example of positive phototaxis and negative phototaxis
positive phototaxis: single-celled algae move towards light, this increases their chances of survival since they’re photosynthetic
negative phototaxis: earthworms move away from light, this increases chances of survival bc it takes them into soil, where they’re able to conserve water, find food and avoid some predators
what’s a kinesis?
- simple reponse where organism (doesn’t move towards/away from the stimulus, instead) changes speed at which it moves and rate at which it changes direction
• The more intense the stimulus the more rapid the movements
• Kinesis is important when the stimulus is less directional such as heat or
humidity
give an example of how kinesis occurs in an organism
- woodlice lose water from their bodies in dry conditions
- when they move from damp area to dry one, they move more rapidly and change direction more often
- increases chance of moving to favorable conditions i.e. damp area, prevents them drying out and increases chances of survival
what’s tropism?
- growth of part of a plant in response to a directional stimulus
explain positive and negative phototropism and gravitropism in relation to the shoots and roots of a plant
- shoots grow towards light (positive phototropism) and away from gravity (negative gravitropism) so that their leaves are in most favourable position to capture light for photosynthesis
- plant roots grow away from light (negative phototropism) and towards gravity (positive gravitropism). In both cases response increases probability that roots will grow into soil, where they’re better able to absorb water and mineral ions
give three examples of changes plants respond to
light (i.e. positively phototropic)
gravity (i.e. positively gravitropic)
water (i.e. positively hydrotropic)
what is IAA?
- Indoleacetic acid (IAA) is an auxin - a plant growth factor which is produced in small quantities
- controls plant cell elongation: causes elongation of shoot cells but inhibits elongation in root cells
Explain why shoots show positive phototropism
- cells in tip of shoots produce IAA, which is transported down shoot, IAA intially transported evenly throughout all regions as it starts to moves down shoot
- unilateral light causes movement of IAA from light side to shaded side of shoot, so greater conc builds up on shaded side than light side
- IAA causes elongation of shoot cells and bc of greater conc on shaded side of shoot, these cells elongate more
- so shaded side of shoot elongates faster than light side, causing shoot tip to bend towards the light
how does gravity affect IAA in plants?
- changes distribution of IAA carrier proteins that export IAA from cells
Explain why roots show positive gravitropism
Gravity causes IAA to accumulate on lower side of the root.
2. IAA inhibits elongation of root cells.
3. Cells on the upper side of the root
elongate faster, so the root tip bends
downwards
explain why shoots show negative gravitropism
- greater conc of IAA on lower side increases cell elongation
- causes this side to elongate more than upper side
- so shoot grows upwards from force of gravity
explain why roots show negative phototropism
- IAA controls bending of roots in response to light, it inhibits cell elongation in roots
- in roots elongation of cells greater on light side than on shaded side (greater conc of IAA on shaded side)
- so root bends away from light i.e. they’re negatively phototropic
explain why covering/removing tip of shoot prevents response (phototropic or gravitropic)
- tip must detect stimulus or produce messenger (IAA)
- its removal prevents any response
what are plant growth factors?
- hormone-like substances which control plant responses to external stimuli
- they exert their influence by affecting growth and they may be made by cells located throughout the plant rather than in particular organs
- they can affect the tissues that release them
eg indoleacetic acid (IAA)
Why do organisms need to Respond to Stimuli?
- for increasing chance of survival (predator/prey awareness, homeostasis, )
How do simple organisms Respond to Stimuli?
- Taxis
- Kinesis
Evidences for Tropism (positive phototropism in shoot)?
- removing or covering shoot tip prevents tropism [tip causes tropism]
- placing micin (prevents movement of chemicals e.g. IAA) across shoot inhibits tropism [tropism caused by movement of chemicals]
- placing gelatine (prevents movement of electrical signals) across shoot does not affect tropism [tropism not caused by movement of electrical signals]
- if shoot tip is moved to one side, that side grows faster and the shoot bends the other way [IAA promotes growth in shoot]
- when in light or darkness the overall levels of IAA remain the same [light does not inhibit or breakdown IAA but rather redistributes it]
define the following:
a. Phototropism
b. Geotropism
c. Hydrotropism
d. Chemotropism
Phototropism: directional growth in response to a light source.
Geotropism: directional growth in response to gravitational pull.
Hydrotropism: directional growth in response to a water source.
Chemotropism: directional growth as a result of chemicals in the environment.
how is IAA transported?
- in one direction
- away from tip of shoots and roots (where it’s produced)
explain the affect of IAA on plant cells walls
- increases plasticity of young cell walls (ones able to elonagte)
- response only in young cell walls, bc cells mature and develop greater rigidity (therefore older parts of shoot/root won’t be able to respond)
explain the acid growth hyopthesis
- light causes IAA to be transported towards shaded side
- IAA causes active transport of H+ ions into cells wall, decreases pH of cell wall
- decreased pH is optimum pH for expansins to loosen walls
- wall less rigid, allows walls to elongate as they take up water
IAA moves towards shaded side of shoots when the light is what?
UNIDIRECTIONAL
light coming from single direction
what is the acid growth hypothesis?
NOT explain
proposed explanation of how IAA increases plasticity of cells
contrast mammalian hormones and plant growth factors
CASTS
Concentration:
mammalian hormone response not always dependent on conc, PGF response proportional to conc
Action:
mammalian hormone binds to complementary proteins in/on target cell, PGF can affect all cells
Synthesis:
mammalian hormone synthesised by specialised glands, PGF synthesised by various tissues in growing regions
Transport:
mammalian hormone transported through circulatory system, PGF transported by diffusion or phloem translocation
Speed:
mammalian hormone is faster-acting (homeostasis), PGF is slower acting (plant growth)
advantage of taxis and kinesis
maintain mobile organism in optimum environment
many organisms respond to temperature and,humidity via kinesis rather than taxis. Why?
Less directional stimuli; often no clear
gradient from one extreme to the other.
how could a student recognise kinesis in an organism’s movement?
- Organism crosses sharp division between favourable & unfavourable environment: turning increases
(return to the original favourable environment). - If organism moves considerable distance into unfavourable environment: turning slowly decreases; begins to move in long, straight lines; sharper turns (lead organism to new environment).
Outline what happens in a simple reflex
arc
STIMULUS → RECEPTOR → SENSORY NEURON → RELAY NEURON/CO-ORDINATOR (intermediate neuron) → MOTOR NEURON → EFFECTOR → RESPONSE
Suggest two advantages of simple reflexes
- Rapid;
- Protect against damage to body tissues;
- Do not have to be learnt;
- Help escape from predators;
- Enable homeostatic control;
what is a reflex
involuntary response to a stimulus
A response is controlled by a…
coordinator e.g. brain
A response (e.g. hormone secretion) is produced by an…
effector
The two modes of communication between cells in large multicellular organisms are…
- Hormonal communication
2. Nervous communication
The two major divisions of the nervous system are…
central nervous system (CNS)
peripheral nervous system
the central nervous system consists of…
the brain
the spinal cord
the peripheral nervous system consists of…
sensory neurons
motor neurons
role of sensory and motor neurons
sensory = carry nerve impulses (electrical signals) from receptor towards CNS motor = carry nerve impulses away from CNS to effectors
the motor nervous system is further subdivided into…
- voluntary nervous system
- autonomic nervous system
autonomic nervous system (ANS) further subdivided into…
sympathetic and parasympathetic nervous system
what is the role of the voluntary and autonomic nervous system ?
voluntary = carries nerve impulses to body muscles and under voluntary (conscious) control autonomic = carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control (involuntary/subconscious)
what is the spinal cord?
- column of nervous tissue that runs along back and lies inside vertebral column for protection
name and discuss features of a reflex response
- rapid (there’s no decision making process)
- innate (protect body from harm, don’t have to be learnt)
- fast (bc neuron pathway is short w/ few synapses)
- involuntary (therefore brain not overloaded and free to carry out more complex responses)
Suggest a suitable statistical test to determine whether a factor has a significant effect on the movement of an animal in a choice chamber
Chi squared
What features are common to all sensory
receptors?
● Act as energy transducers which
establish a generator potential.
● Respond to specific stimuli.
Describe the 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 Na+ channels on plasma membrane.
● Capillary runs along base layer of tissue.
what is a pacinian corpuscle?
- sensory receptor in skin that detect nd responds toCHANGES in pressure
how does a pacinian corpuscle produce a generator potential?
- by acting as a transducer (converts to nerve impulse)
how does a pacinian corpuscle work?
- when mechanical pressure is applied (pressure deforms membrane) stretch mediated sodium channels widen
- this allows sodium ions to diffuse into the neurone
- if influx of Na+ raises membrane to threshold potential, a generator potential is produced (pacinian corpuscle transduces mechanical energy of stimulus into generator potential)
- an action potential (nerve impulse) is created
- action potential passes along sensory neurone and other neurones until the CNS and a response is coordinated
where do pacinian corpuscles occur?
- occur deep in skin, most abundant in fingers, soles of feet and external genitalia
- also occur in joints, ligaments and tendons: where they enable organisms to know which joints are changing direction
why is there no influx of Na+ ions at the stretch-mediated sodium channels of the pacinian corpuscle at resting potential?
- at normal (resting) state, stretch-mediated Na+ channels of membrane around pacinian corpuscle too narrow to let Na+ ions pass along them
- when they’re deformed (pressure applied), membrane around neurone becomes stretched which widens sodium channels and Na+ ions diffuse into neurone
name the two photorecpetor cell located in the retina
cone cells
rod cells
Where are rod and cone cells located in the retina?
Rod: evenly distributed around periphery
but NOT in central fovea
Cone: mainly central fovea no
photoreceptors at blind spot
rod and cone cells act as….
….transducers by conserving light energy into electrical energy of a nerve impulse
How does the Retina of the Eye work?
- photoreceptors (detects light) so the brain can generate an image
- detected by retina (located at back of eye)
- made of Cone and Rod cells
- Cone Cells detect high light intensity only, produces colour image, with high visual acuity
- Rod Cells can detect low light intensity, produces black and white image, with low visual acuity
- Cone Cells located in centre of retina (fovea) - site of high light intensity
- Rod Cells located in periphery of retina
why do cone cells have high visual acuity?
- 1 cone cell connects to 1 bipolar neurone which connects to one sensory neurone (therefore no summation of light can take place so only detects bright light)
- because 1 cone cell connects to 1 bipolar neurone which connects to one sensory neurone, each stimuli can be distinguished = high visual acuity
why do rod cells have low visual acuity?
- a few rod cells connect to 1 bipolar neurone which connects to 1 sensory neurone (therefore summation of light can take place so can detect low light intensity)
- because a few rod cells connect to 1 bipolar neurone which connects to one sensory neurone, the stimuli will be merged together = low visual acuity
what is meant by visual acuity?
Resolving power/ability to distinguish small objects/close objects/tell close 1
objects apart
properties of rod cells
- detect light across middle of visible light spectrum
- more sensitive to low light intensities (detect light in v low intensity)
- use pigment rhodopsin to detect light
- more abundant than cone cells
- located more towards periphery of retina (not present at fovea)
- multiple rod cells connect to a single bipolar cell
- provide poor visual acuity
properties of cone cells
- three types of cone cells, which respond to red, green and blue light
- comparing responses from each type pf cone receptor allows for colour vision
- use pigment iodopsin to detect light
- fewer numbers than rod cells
- conc at fovea, fewer at periphery at the retina
- cone cells connect to their own bipolar cell
- provide good visual acuity
why do rod cells only allow images to be black and white?
- rod cells can’t distinguish different wavelengths and therefore lead to images being seen only in black and white
why do rod cells allow us to see in low light intensity?
- certain threshold value must be exceeded before generator potential is created in bipolar cells that they’re connected to
- number of rod cells connected to single bipolar cell (retinal convergence)
- so much greater chance that threshold value exceeded (than if only 1 rod cell connected to each bipolar cell)
- due to summation, so rod cells let us see in low light intensity (although B and W)
role of rhodopsin
- found in rod cells
- to create generator potential, rhodopsin must be broken down
(enough energy from low intensity light to cause breakdown)
why do cone cells respond to high intensity light and not low light intensity?
- certain threshold value needs to be exceeded so generator potential generated
- each cone cell connected to 1 bipolar cell, so stimulation of no. of cone cells can’t be combined to exceed threshold
- less chance of exceeding threshold and generating generator potential
- ## so cone cells only respond to high intensity light, not low intensity
role of iodopsin
- found in cone cells
- requires high intensity light for its breakdown
- so only high intensity light will provide enough energy to break it down and create a generator potential
why is each cone cell sensitive to different specific range of wavelengths?
- there are three different types of cone cells
- all containing specific type of iodopsin
why are most cone cells found in fovea?
- bc light is focused on fovea, so it receives the highest intensity of light
- so cone cells found there
why are most rod cells found at the peripheries of retina?
bc light intensity is lowest here
compare and contrast the differences between rod cells and cone cells of the eye
- rod cells are rod shaped, cone cells are cone-shaped
- rod cells are in greater number than cone cells
- rod cells more distributed at periphery of retina, absent at fovea; cone cells fewer at periphery and conc at fovea
- rod cells give poor visual acuity, cone cells give good visual acuity
- rod cells sensitive to low-intensity light, cone cells not sensitive to low-intensity light
- rod cells contain rhodopsin, cone cells contain iodopsin
- one type of rod cell, three types of cone cells all responding to different wavelengths of light
name and describe the two divisions of the autonomic nervous system (ANS)
sympathetic nervous system:
stimulates effectors and speeds up activity, helps cope with stressful situations by heightening awareness and preparing body for activity (fight or flight)
parasympathetic nervous system:
inhibits effectors, so slows down activity, controls activity under resting conditions; it’s concerned with conserving energy and replenishing body’s reserves
describe structural differences between motor neurone and sensory neurone
motor: 1. long axon 2. no receptor 3. cell body termninal and has dendrites 4 many short dendrons
sensory:
- short axon
- receptor (attached to)
- cell body at the side; no dendrites
- one long dendron
what is a nerve impulse?
- movement of an action potential along a neurone
- action potential = change in membrane potential
- charge in one section of the neurone changes from negative (polarised) -> positive (depolarised) -> back to negative (hyperpolarised)
how can the actions of the sympathetic and parasympathetic nervous system be described?
antagonistic
- if one system contracts a muscle, then the other relaxes it
the heart is myogenic, what does this mean?
it’s heart beat is intiated by the SAN (doesn’t require signal from nerves)
i.e. contractions intiated within the muscle itself (heart can contract and relax on its own)
muscle of the heart known as….
cardiac muscle
what is neurogenic?
- opposite of myogenic
- nerve impulses from outside initiate contraction of muscles
what is the SAN?
- sinoatrial node
- distinct group of cells in wall of right atrium
- initial stimulation that determines beat of heart originates here
why is the sinoatrial node referred to as the pacemaker of the heart?
- it generates electrical impulses that set the normal rhythm and rate in a heart
- so SAN is the ‘natural pacemaker of the heart’
The process of contraction in the heart begins in the sino-atrial node. Discuss how the SAN sets the rhythm of the heartbeat
[6 MARKS}
- SAN transmits regular waves of electrical activity
- This causes the atria to
contract - Collagen/ non-conductive tissue (atrioventricular septum), prevents the contraction (electrical wave) from passing to the ventricles
- waves of electrical activity are transferred to the AVN
- after a short delay, AVN passes waves of electrical activity to the bundle of His
- which then conducts the electrical activity to the Purkyne fibres/purkinje fibres
- wave of excitation released from purkyne tissue, this causes the ventricles to contract from the bottom up.
state the roles of acetycholine and noradrenaline (their affects)
– acetylcholine – acts on the parasympathetic nervous system and slows the heart rate
- noradrenaline – act on the sympathetic nervous system to increase the heart rate
Which part of the brain unconsciously controls the SAN?
[1 mark]
medulla oblongata
Where is the atrioventricular node found?
Between the walls of the two ventricles next to the semi lunar valves
what does the SAN do? what does the AVN do?
SAN:
Causes the heart to speed up or slow down
Causes the atria to contract
AVN:
Responsible for passing electrical impulses from the atria to the ventricles
what is the ‘Bundle of His’ ?
What is Purkyne tissue?
Bundle of ‘His’:
group of muscle fibres responsible for conducting the waves of electrical activity between the ventricles to the apex
Purkyne Tissue:
Fine muscle fibres in the right and left ventricles which conduct electricity from the Bundle of His
what are baroreceptors?
- receptors
- in the aorta and carotid arteries
- stimulated by high and low blood pressure
Describes the heart’s response to high blood pressure
1) Baroreceptors in the aorta and carotid arteries detect high blood pressure
2) Impulses sent to the medulla which sends impulses to SAN via parasympathetic nervous system
3) Acetylcholine secreted which binds to receptors on SAN
4) Heart rate slows down
Describe the heart’s response to low blood pressure
- detected by baroreceptors in the carotid arteries and aorta
- sends impulses to medulla oblongata
- medulla oblongata sends impulses to SAN via the sympathetic nerves
- stimulates release of noradrenaline, causing the heart rate to increase
benefit = increasing heart rate leads to an increase in blood pressure (so enough blood can reach the brain)
how does CO2 affect blood pH
- as CO2 combines with water, it forms carbonic acid, (which dissociates and) makes the blood acidic. So CO2 in the bloodstream lowers the blood pH.
why does exercise (increases heart rate) decrease pH of blood?
more respiration, more CO2 produced as waste product, blood more acidic
Describe the heart’s response to low blood O2, pH or high CO2 (e.g. from exercising)
- exercise = muscle contraction, which requires respiration so waste product CO2 is released into blood
- this lower pH of blood (more acidic)
- detected by chemoreceptors in carotid arteries
- sends impulses to medulla oblongata
- medulla oblongata sends impulses to SAN via the sympathetic nerves (medulla increases frequency of impulses to SAN which leads to increase in HR)
- releases noradrenaline, causing the heart rate to increase
benefit = increase blood flow to lungs to remove CO2 and take in O2 (rate of gas exchnage and ventilation increased)
Describe the heart’s response to high blood O2/pH (or low CO2)
- chemoreceptors in aorta and carotid arteries/ medulla oblongata detect chemical change
- impulses sent to the medulla oblongata and sends them to SAN via the parasympathetic neurones (medulla reduces frequency of impulses to SAN which leads to reduction in HR)
- acetylcholine released
- heart rate decreases and O2/ pH return to normal
what two centres does the medulla oblongata have?
- one centre that increases heart rate (linked to SAN via sympathetic nervous system)
- another centre that decreases heart rate (linked to SAN via pasrasympathetic nervous system)
name the process by which IAA moves from the growing regions of a plant shoot to other tissues
diffusion
name the process by which IAA moves from the growing regions of a plant shoot to other tissues [1 MARK]
diffusion
when a young shoot is illuminated from one side, IAA stimulates growth on the shaded side. Explain why growth on the shaded side helps to maintain the leaves in a favorable environment. [2 MARKS]
- causes plant to grow/bend towards light/positive phototropism
- (light) required for photosynthesis
how does an increase in temperature explain the increased uptake of NAA (a substance similar to IAA) by leaf of plants?
- More kinetic energy;
- Faster movement of molecules;
- More diffusion;
explain how the resting potential of -70mV is maintained in the sensory neurone when no pressure is applied
- Na+ ions actively transported out while K+ ions pumped in
