6. Organisms Respond To Changes In Their Internal And External Environment Flashcards
Define stimulus
A detectable change in the environment
Define receptors
Detect changes in internal and external environment of an organism
Define effectors
Muscles or glands that bring about change in response to a stimulus
Why is it important that organisms can respond to a stimulus?
It increases their chances of survival
Define tropism
A plants directional response, via growth, to a stimulus
Gravitropism is?
Plants response to gravity
Phototropism is?
Plants response to light
What is IAA
- plant growth factor
- type of auxin
Describe phototropism in shoots
Shoots are positively phototropic
- shoot tip cells produced IAA, causing cell elongation
- the IAA diffuses to other cells
- if there is unilateral light, the IAA will diffuse towards the shaded side of the shoot resulting in a higher concentration of IAA there
- the cells on the shaded side elongate more and results in plant bending towards the light source
Describe phototropism in roots
- Roots are negatively phototropic
- a high concentration of IAA inhibits cell elongation causing root cells to elongate more on the lighter side and so the roots bend away from light
Gravitropism in shoots
- shoots are negatively gravitropic
- IAA will diffuse from the upper side to the lower side of the shoot
- if a plant is vertical, this will cause the plant cells to elongate and the plant grows upwards
- if a plant is on its side it will cause the shoot to bend upwards
Gravitropism in roots
- roots are positively gravitropic
- IAA moves to the lower side of roots so that the upper side elongates and the root bends down towards gravity and anchors the plant in
What is a reflex?
A rapid automatic response to protect you from danger
What 3 neurones make up the reflex arc
- Sensory neurone
- Relay neurone
- Motor neurone
(Only 2 synapses and that’s why its such a rapid response)
Define taxis
An organism will move its entire body towards a favourable stimulus or away from an unfavourable stimulus
Define kinesis
An organism changes the speed of movement and the rate it changes direction
What is positive and negative taxis
Positive taxis is when and organism move towards a stimulus
Negative taxis is when an organism moves away from
What is a receptor?
Detects stimuli
Each receptor responds only to a specific stimuli and this stimulation of a receptor leads to the establishment of a generator potential which causes a response
What is the pacinian corpuscle
- pressure receptor located deep in skin, found in fingers and feet
- the sensory neurone in the pacinian corpuscle has a special channel protein in its plasma membrane
Describe how a generator potential is established
- membranes of the pacinian corpuscle have stretch mediated sodium channels
- these open and allow Na+ to enter the sensory neurone only when they are stretched and deformed
- when pressure is applied it deforms the neurone plasma membrane, stretches and widens the Na+ channels
- so Na+ diffuses in which leads to a generator potential
Describe rod cells
Process images in black and white
How to create a generator potential in rod cells?
- the pigment of rod cells (rhodopsin) has to be broken down by light energy
- detects light of low intensity because rod cells connect to one sensory neurone (retinal convergence
- so this creates a low visual acuity
Describe cone cells
- process images in colour
- 3 types that contain different types of iodopsin pigment (red, green and blue, that absorb different wavelengths of light)
How to create a generator potential in cone cells
- iodopsin breaks down only if there is high light intensity
- because only one cone cells connects to a bipolar cell so has to be high intensity
- so it creates a high visual acuity
What is the distribution of rod and cones like
- uneven
- light is focused by the lens on the fovea (highest intensity of light)
- most cone cells located near fovea
- rod cells further away
Differences between rod and cone cells
ROD CELLS
- rod shaped
- pot visual acuity (B&W only)
- high visual sensitivity
- multiple rod cells connected to one bipolar neurone
CONE CELLS
- cone shaped
- good visual acuity (colour vision)
- low visual sensitivity
- one cone cell connected to one bipolar neurone
What does myogenic mean
The cardiac muscle contracts by itself but the rate of contraction is controlled by wave of electrical activity
Describe the sinoatrial node (SAN)
Located in the right atrium and called the pacemaker
Describe the atrioventricular node (AVN)
Located near the border of the right and left ventricular within the atria still
Describe the bundle of His
Runs through the septum
Describe the purkyne fibres
In the walls of the ventricles
Describe how the SAN and AVN control myogenic stimulation of the heart
- SAN releases a wave of depolarisation causing it to contract
- AVN releases another wave of depolarisation
- bundle of His conducts the wave of depolarisation down the septum and the purkyne fibres
- walls of ventricles contract
What controls the heart rate?
The medulla oblongata controls heart rate via the autonomic nervous system
What part of the nervous system increases and decrease heart rate?
A centre linked to the sinoatrial node increases heart rate via the sympathetic nervous system. The sinoatrial also decreases heart rate via the parasympathetic nervous system
Changes in blood pressure is detected by what?
Baroreceptors
Changes in blood pH are detected by?
Chemoreceptors
- detect changes in the conc. of CO2
Describe the resting potential
- when a neurone is not conducting an impulse there is a difference between the electrical charge inside and outside the neurone (resting potential). There are more + ions Na+ and K+, outside compared to inside, so the neurone is more negative. Resting potential = -70mv
How is a resting potential maintained?
- maintained by a sodium-potassium pump involving active transport and ATP
- the pump moves 2K+ ions in and 3Na+ ions out
- creates an electrochemical gradients causing K+ to diffuse out and Na+ to diffuse in
- the membrane is more permeable to K+ so more are moved out = -70mV
How is an action potential generated
- A STIMULUS EXCITES THE NEURONE
- voltage gated sodium ions channels on the axon open
- Na+ moves into the axon by facilitated diffusion
- inside of neurone less negatively charged - THE NEURONE IS DEPOLARISED
- if the influx of Na+ exceeds the threshold potential (-55mV) an action potential is triggered
- more Na+ ion channels open and more Na+ moves into neurone
- neurone becomes positively charged (40mV) - NEURONE IS REPOLARISED
- Na+ channels close and voltage gated potassium ion channels open
- K+ moves out neurone by facilitated diffusion
- neurone becomes negatively charged - NEURONE UNDERGOES HYPERPOLARISATION
- K+ ion channels close slowly
- = and overshoot where to many K+ ions leave neurone
- axon becomes more negative - NEURONE IS RETURNED TO ITS RESTING POTENTIAL
- sodium potassium pump corrects ion imbalance
- Na+ is pumped out of and K+ pumped into axon
- resting potential is maintained until neurone is triggered by another stimulus
Why can action potentials not be triggered during the refractory period?
- the axon is incapable of repeating the action potential
- both Na+ and K+ ion channels take time to recover from the action potential and the ion channels are closed
- the refractory period limits number of times a neurone can be stimulated (this ensures impulses are unidirectional)
Describe the all or nothing principle
- if depolarisation does not exceed -55mV, an action potential and impulses are not produced (nothing)
- any stimulus that does not trigger depolarisation to -55mV will always peak at the same max voltage (all)
- bigger stimuli increases the frequency of action potentials
Factors affecting the speed of conduction
Myelination and saltatory conduction
- action potential jumps from node to node (saltatory conduction) meaning the action potential travels along the axon faster
Axon diameter
- wider diameter increases speed of conduction
- so less leakage of ions so action potential can move faster
Temperature
- higher temp increases speed
- ions diffuse faster
- enzymes involved in respiration work faster so there is more ATP for active transport in the Na+/K+ pump
Describe the cell body in the myelinated motor neurone
Contains the organelles found in a typical animal cells.
Proteins and neurotransmitters chemicals are made here
Describe dendrites in the myelinated motor neurone
Carry action potentials to surrounding cells
Describe the axon in the myelinated motor neurone
Conductive, long fibre that carries the nervous impulse along the motor neurone
Describe Schwann cells in the myelinated motor neurone
Wrapped around the axon to form the myelinated sheath (lipid so does not allow charged ions to pass through)
What is the resting potential
When a neurone is not conduction an impulse, there is a difference between the electrical charge inside and outside of the neurone.
There are more + ions , Na+ and K+ outside compared to inside so therefore the inside of the neurone is more negative (-70mV)
How to establish a resting potential after a response to a stimulus and how it is maintained after no stimulus a resting potential ?
- Sodium-potassium pump, involving active transport and ATP
- The pump moves 2K+ ions in and 3Na+ ions out
- creating an electrochemical gradient causing K+ to diffuse out and Na+ to diffuse in
- the membrane is more permeable to K+ so more are moved out = -70mV
How is an action potential generated if there is a stimulus
- when the neurones voltage increases beyond a set point from the resting potential
- increase of voltage (depolarisation), due to to a neurone membrane becoming more permeable to Na+
- once generated it moves along the axon
How is an action potential generated
- a stimulus provides energy that causes the Na+ voltage gated ion channels in the axon membrane to open = sodium ions diffuse in
- K+ ion channel is open but the voltage increases because of sodium ions
- if the stimulus is large enough to cause lots of sodium ion channels to open there will be enough sodium ions diffusing in to cause an increase beyond threshold (-55mV)
- as the voltage starts to increase = more sodium ions channels to open = 35-40mV
- repolarisation = more sodium channels open so potassium diffuses out = resting potential
Describe synapses
Gaps between end of the axon of one neurone and the dendrite of another
Action potential is transmitted as neurotransmitters diffuse across synapse
Describe the function of a synapse
- Action potential arrives at synaptic knob —> depolarises membrane
- Depolarisation opens V-gated Ca2+ channels = Ca2+ influx
- Ca2+ causes exocytosis of ACh
- ACh released into synaptic cleft
- ACh binds to receptor which is a ligand-gated Na+ channel on the post synaptic membrane. When ACh binds, it opens the channel and Na+ ions diffuse into post synaptic neurone
- Influx of Na+ depolarises membrane of post synaptic neurone = action potential
- AChE enzyme breaks down ACh into acetate and choline
What is summation?
Rapid build up of neurotransmitters in the synapse to generate an action potential
What is spatial summation?
Multiple pre-synaptic neurones release neurotransmitters = enough to exceed threshold to generate action potential in post synaptic neurone
What is temporal summation?
One pre-synaptic neurone releases neurotransmitter at a higher frequency = enough to exceed threshold to generate action potential in post synaptic neurone
What is an inhibitory synapse?
Cause chloride ions to move into the post synaptic neurone and potassium ions to move out. This makes the membrane potential to decrease to -80mV (hyperpolariasation) so action potential unlikely
Neurone-neurone synapse (cholinergic synapse) vs neuromuscular junction
Neurone-neurone
- excitatory or inhibitory
- links neurones to neurones or neurones to other effectors
- can occur between sensory intermediate and motor neurones
- if post-synapse is another neurone a new action potential generated
- many different neurotransmitters available
Neuromuscular junction
- only excitatory
- only links neurones to muscles (effector)
- only occurs between motor neurones and muscle
- end of neural pathway so action potential ends here
- ACh bind to receptors on membrane of muscle fibre
Describe the structure of skeletal muscles
- made up of many muscle fibres (sarcoplasm)
- sarcoplasm contains lots of mitochondria and endoplasmic reticulum
- fibres made up of hundreds of myofibrils
- myofibrils made of 2 myofilaments, actin and myosin
What is actin?
Thin filament made up of 2 twisted strands and a protein called tropomyosin is wound around it
What is myosin?
Thick filament that has 2 protruding head which stick out at the end of each molecule
What leads to muscle contraction
Interaction between actin and myosin
Describe the sliding filament theory
- Action potential from neuromuscular junction travels deep into muscle fibre through T-tubule
- T-tubules are in contact with sarcoplasmic reticulum, which actively transports Ca2+ ions into it from the sarcoplasm
- Action potential opens Ca2+ channels in the sarcoplasmic reticulum —> Ca2+ ions diffuse into sarcoplasm
- Ca2+ ions bind to tropomyosin and cause it to move —> exposes actin binding sites
- ADP molecules attach to the myosin heads —> allows the heads to bind to the actin filament and form cross-bridges
- Myosin heads change angle —> pulls actin filament along the myosin filament and ADP is released
- ATP attaches to each myosin head —> detaches from actin filament
- ATPase hydrolyses ATP-ADP providing energy to return the myosin head to its original position
- Myosin heads again have an ADP molecule attached so it can reattach itself further along the actin filament
Describe fast twitch muscle fibres
- contract rapidly and powerfully for a short period
- many thick myosin filaments
- lots of glycogen
- enzymes involved in glycolysis and phosphocreatine
Describe slow twitch muscle fibres
- contract slowly and less powerfully but for a longer period
- lots of myoglobin —> stores O2 for aerobic respiration
- lots of vasculature —> deliver glucose + O2
- lots of mitochondria
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