module 6; organisms respond to changes in their internal + external environments Flashcards
what is a stimulus?
it’s a detectable change in the environment
what are receptors?
cells that detect a stimulus (changes in environment)
how do organisms increase their chance of survival?
organisms increase their chance of survival by responding to stimuli via different response mechanisms
what is tropism?
tropism is the term given to when plants respond, via growth, to stimuli
explain tropism in plants
tropism can be positive or negative, growing towards or away from a stimulus - plants respond to light & gravity
what factors control tropism
controlled by specific growth factors
e.g. indoleacetic acid
what is IAA?
its a type of auxin & can control cell elongation in shoots & inhibits growth of cells in the roots
it’s made in the tip or the roots & shoots but can diffuse to other cells
what is phototropism?
it’s the term given to the tropisms where the plant is responding to light
how does phototropism improve the shoot of a plant’s survival?
as light is needed for the LDR in photosynthesis, the plants grow & bend towards the light - this is positive phototropism
describe phototropism in the shoots of plants
- shoot tip cells produce 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 conc of IAA there
- the cells on the shaded side to elongate more & results in the plant bending towards the light source
describe phototropism in the roots of a plant
roots do not photosynthesise so they do not require light, they must anchor the plant deep in the soil
in roots a high conc of IAA inhibits cell elongation, causing root cells to elongate more on the lighter side and so the root bends away from the light - this is negative tropism
describe gravitropism in the shoots of a plant
IAA will diffuse from the upper side to the lower side of the shoot
if a plant is vertical, this causes the plant cells to elongate & the plant grows upwards
if the plant is on its side, it will cause the shoot to bend upwards - this is negative tropism
describe gravitropism in the roots of a plant
IAA moves to the lower side of roots so that the upper side elongates & the roots bend down towards gravity & anchor the plant in - this is positive gravitropism
what is a reflex?
it’s a rapid, automatic response to protect you from danger
what is a reflex arc made up of?
made of 3 neurons:
sensory neuron
relay neuron
motor neuron
why are reflexes rapid responses?
as there are only 2 synapses
state examples and explain simple responses
taxes & kinesis are simple responses which keep organisms within the favourable conditions of their environment (light, moisture & chemicals)
what is taxes?
it’s where an organism will move its entire body towards a favourable stimulus or away from an unfavourable stimulus
what is kinesis?
an organism changes the speed of movement and the rate it changes direction
explain the types of taxes
positive taxes - when organisms move towards a stimulus
negative taxes - when an organism moves away from a stimulus
explain the types of kinesis
if an organism moves from an area where there are beneficial stimuli to an area with harmful stimuli, its kinesis response will be to increase the rate at which it changes direction to return to the favourable conditions
if negative stimuli surround an organism, the rate of turning decreases to keep it moving in a relatively straight line to increase the chances of it finding a new location with favourable conditions
what causes a response?
each receptor responds only to specific stimuli & this stimulation of a receptor leads to the establishment of a generator potential which can cause a response
state the 3 receptors
- pacinian corpuscle
- rods
- cones
where is the pacinian corpuscle receptor found?
pressure receptors located deep in the skin, mainly in the fingers & feet
describe the structure of the pacinian corpuscle receptor
the sensory neurone in the pacinian corpuscle has special channel proteins in its plasma membrane
explain the function & importance of the pacinian corpuscle’s membrane
the membranes have a stretch-mediated sodium channels
these open & allow Na⁺ ions to enter the sensory neurone only when they are stretched & deformed
how does pressure effect the plasma membrane of the pacinian corpuscle
when pressure is applied it deforms the neurone plasma membrane, stretches & widens the Na⁺ channels so Na⁺ diffuses which leads to the establishment of a generator potential
state the types of photoreceptors the retina contains
rods & cones
how do rod cells process images?
rods process images in black & white
how is the generator potential in rod cells created?
to create the generator potential, the pigment of rod cells (rhodopsin) must be broken down by light energy
what sensory neurone are rod cells connected to & why?
connected to one sensory neurone - retinal convergence, as they can detect light of very low intensity
what is a property of rod cells?
low visual acuity
why do rod cells have low visual acuity?
as the brain cannot distinguish between the separate sources of light that stimulated it
how do cone cells process images?
cone cells process images in colour
what is the difference between the types of cone cells?
there are 3 types of cone cells - these are different as they contain different types of iodopsin pigment (red, green & blue) which all absorb different wavelengths of light
how is iodopsin pigment broken down?
its only broken down of there is a high light intensity - so action potentials can only be generated with enough light
explain why can’t we see colour when its dark
one cone cell connects to one (bipolar cell)sensory cell and therefore there has to be a high enough light intensity to break down enough of the iodopsin pigment to be able to trigger an action potential
what is a property of cone cells
they have high visual acuity
why do cone cells have high visual acuity?
as each cone is connected to one bipolar cell, the brain can distinguish between separate sources of light detected
explain the distribution of rods & cones across the retina
it’s uneven as light is focused by the lens on the fovea - which will receive the highest intensity of light
where are most of the cone cells located in the retina & why?
near the fovea, as it has the highest light intensity reaching the retina because that is where the light is focused by the lens
why is it an advantage that most cone cells are located in the fovea?
as the cone cells will only be able to detect the colour lights in higher light intensities
where are most rod cells located in the retina & why?
found further away from the fovea, as they can detect light even at low light intensities
explain why the cardiac muscle is described as myogenic
the cardiac muscle is myogenic; it contracts on its own accord without any nervous system input (however, the speed at which it contracts is controlled by the nervous system) but the rate of contraction is controlled by a wave of electrical activity
state the parts of the heart involved in controlling the heart rate & where they are located
- sinoatrial node (SAN) - located in the right atrium (known as pacemaker)
- atrioventricular node (AVN) - located near the border of the right & left ventricle within the atria
- the bundle of His - runs through the septum
- the purkyne fibres - in the walls of the ventricles
how do the different parts of the heart control heart rate?
SAN releases a wave of depolarisation across the atria, causing it to contract.
AVN releases another wave of depolarisation when the 1st reaches it. A non-conductive layer between the atria & ventricles prevents the wave of depolarisation from travelling down to the ventricles.
Instead, the bundle of His conducts the wave of depolarisation down the septum & the purkyne fibres.
As a result, the apex & the walls of the ventricles contract. There is a short delay before this happens, whilst the AVN transmits the 2nd wave of depolarisation.
This allows enough time for the atria to pump all the blood into the ventricles. Finally, the cells repolarise & the cardiac muscle relaxes.
what part of the brain controls heart rate & briefly state how
the medulla oblongata - via the autonomic nervous system
what nervous systems, connected to the medulla oblongata, control heart rate
there are 2:
1. a centre linked to the sinoatrial node to increase heart rate via the sympathetic nervous system
2. another that decreases heart rate via the parasympathetic nervous system
how is heart rate increased using the sympathetic nervous system?
If impulses travel via the sympathetic nervous system that will cause the SAN to release the waves of depolarisation more frequently & therefore increase heart rate
how is heart rate decreased using the autonomic nervous system?
If more impulses are sent down the parasympathetic nervous system that will cause the SAN to release the waves of depolarisation less frequently & therefore it decreases the heart rate.
what does heart rate change in response to?
pH & blood pressure
how are the stimuli that change heart rate detected & where are they found?
detected by chemoreceptors & pressure receptors which are found in the aorta & carotid artery
state what occurs to the pH of the blood during times of high respiratory rate & why
the pH of the blood decreases due to the production of CO₂ or lactic acid
why must excess acid be removed from the blood?
excess acid must be removed from the blood rapidly to prevent enzymes from denaturing
how is excess acid removed from the blood?
by increasing the heart rate (more impulses vis the sympathetic nervous system to SAN), so CO₂ can diffuse out into the alveoli more rapidly
explain the effect of high blood pressure on the arteries
if the blood pressure is too high this can cause damage to the walls of the arteries
what are the mechanisms that reduce blood pressure?
more impulses via the parasympathetic nervous system to decrease the heart rate
what is the effect of blood pressure on respiring cells?
low blood pressure causes an insufficient supply of oxygenated blood to respiring cells & removal of waste
how does low blood pressure effect heart rate?
low blood pressure results in more impulses via the sympathetic nervous system to increase the heart rate
describe the structure of a myelinated motor neurone
the cell body - contains the organelles found in a typical animal cell, proteins & neurotransmitters chemicals are made here
dendrites - carry action potentials to surrounding cells
axon - is the conductive, long fibre that carries the nervous impulse along the motor neurone
schwann cells - wrap around the axon to form the myelin sheath, which is a lipid & so it does not allow charged ions to pass through it. there are gaps between the sheath called nodes of ranvier.
what is resting potential?
when a neurone is not conducting an impulse, there is a difference between the electrical charge inside & outside the neurone
explain why the voltage of resting potential -70mV?
as there are more +ve ions (Na⁺ & K⁺) outside compared to inside the axon and therefore inside the neurone is more -ve
how is a resting potential maintained?
by a sodium-potassium pump, involving active transport & ATP
describe the movement of ions & which type of transport helps with their movement
at the pump there 2 K⁺ ions are being pumped into the axon and 3 Na⁺ ions are being actively transported out of the axon
this creates an electrochemical gradient gradient causing K⁺ ions to diffuse out & Na⁺ ions to diffuse in
the membrane is more permeable to K⁺ so more are moved out resulting in the -70 mV
what is an action potential?
it’s when the neurone’s voltage increases beyond a set point from the resting potential - this generates a nervous impulse
explain what causes an increase in voltage (depolarisation) & the movement of action potential
an increase in voltage is due to the neurone membrane becoming more permeable to Na⁺
once an action potential is generated it moves along the axon like a mexican wave
explain how action potential is generated
- Resting potential: A stimulus provides the energy that can cause the Na⁺ ion voltage-gated channels in the axon membrane to open.
- Start of depolarisation: This causes the Na⁺ ions to start diffusing in. The K⁺ ion channel is permanently open. As some Na⁺ ions are moving there is an increase in voltage.
- Depolarisation: If the stimulus is large enough to cause enough Na⁺ ion channels to open then there will be enough Na⁺ ions to cause an increase beyond the threshold potential. If there is an increase beyond -55 mV then the action potential will always be generated. There are even more voltage-gated Na⁺ ion channels so the voltage will start to increase causing more Na⁺ channels to open (+35 to 40 mV). There is then a decrease when it reaches this voltage the voltage-gated Na⁺ ions close (no more +ve ions enter but K⁺ ions move out)
- Repolarisation: Causes more K⁺ ion channels to open so more K is diffusing out. Eventually, due to many +ve ions diffusing out, we get to the resting state but it overshoots (-80 mV)
- Hyperpolarisation: K⁺ ions are now able to move out but all the other gates are closed.
where does action potential occur?
along the axon (graph of action potential occurs multiple times along the axon at every node of ranvier)
how does the process of action potential differ between a myelinated neurone & an unmyelinated neurone
along the axon of a myelinated neurone, an action potential occurs at every node of the ranvier until the end of the neurone and then a synapse
along the axon of an unmyelinated neurone, an action potential occurs at every position on the axon as there aren’t any myelin sheaths, so it would take longer for action potentials to reach the end of the neurone
what is the all-or-nothing principle?
If the depolarisation does not exceed -55 mV, an action potential & an impulse are not produced (nothing).
Any stimulus that does trigger depolarisation to -55 mV will peak at the same max voltage (all) - bigger stimuli increase the frequency of action potentials.
why is the all-or-nothing principle important?
as it makes sure that animals only respond to large enough stimuli, rather than responding to every slight change in the environment
what is the refractory period?
after an action potential has been generated, the membrane enters a refractory period when it can’t be stimulated, as the Na channels are recovering & can’t be opened
explain the importance of the refractory period
- It ensures that discrete impulses are produced. An action potential cannot be generated immediately after another & this makes sure that each is separate.
- It ensures that action potentials travel in one direction. This stops the action potential from spreading out in 2 directions which would prevent a response.
- It limits the number of impulse transmissions. This is important to prevent overreaction to a stimulus.
state the factors affecting the speed of conductance
- myelination & saltatory conduction
- axon diameter
- temperature
how does myelination & saltatory conduction affect the speed of conductance?
the action potential jumps from node to node (saltatory conduction), which means the action potential travels along the axon faster
how does the axon diameter affect the speed of conductance?
a wider diameter increases the speed of conductance
a wider diameter means that there is less leakage of ions & therefore action potentials travel faster
how does temperature affect the speed of conductance?
a higher temp increases the speed of conductance because:
1. the ions diffuse faster
2. the enzymes involved in respiration work faster - therefore there is more ATP for active transport in the Na⁺/K⁺ pump
what is a synapse?
it’s the gap between the end of an axon of one neurone and the dendrite of another neurone
what occurs across the synapse?
the action potential is transmitted as neurotransmitters that diffuse across the synapse
explain what occurs at the synapse
- An action potential arrives at the synaptic knob. Depolarisation of the synaptic knob leads to the opening of Ca2+ channels & Ca2+ diffuses into the synaptic knob.
- Vesicles containing neurotransmitters move towards & fuse with the presynaptic membrane. Neurotransmitters are released into the synaptic cleft.
- Neurotransmitter diffuses down a conc gradient, across the synaptic cleft, to the postsynaptic membrane; neurotransmitter binds by complementary (of shape) to receptors on the surface of the postsynaptic membrane.
- Na⁺ ion channels on the postsynaptic membrane open & Na⁺ diffuses in. If enough Na⁺ diffuses, above the threshold, the postsynaptic membrane becomes polarised.
- Neurotransmitters are released from the receptor; the Na⁺ channel closes & the postsynaptic neurone can re-establish resting potential; the neurotransmitter is transported back into the presynaptic neurone where it’s recycled.
give an example of a synapse & its neurotransmitter
cholinergic synapse
acetylcholine
explain why acetylcholine needs to be broken down by an enzyme
Acetylcholine will bind to its receptor at the synapse (process of synaptic transmission occurs) but the neurotransmitters don’t remain permanently bound as if they did the Na⁺ channels would be continually open which would trigger an action potential & therefore a response even though the stimulus wasn’t present anymore
what enzyme breaks down acetylcholine & how
acetylcholine esterase breaks acetylcholine into choline & acetate which can then be reabsorbed into the presynaptic neurone & be reused
what is summation?
it’s the rapid build-up of neurotransmitters in the synapse to help generate an action potential
state the methods of summation that help generate action potential
- spatial summation
- temporal summation
what is spatial summation?
many different neurones collectively trigger a new action potential by combining the neurotransmitter they release to exceed the threshold value
what is temporal summation?
one neurone releases neurotransmitter repeatedly over a short time to add up to enough to exceed the threshold value
what do inhibitory synapses cause?
they cause Cl ions to move into the postsynaptic neurone & K ions to move out
explain the impact that inhibitory synapses have on action potential
inhibitory synapse makes the membrane potential decrease to -80 mV (hyperpolarisation) and so an action potential is highly unlikely
what is the neuromuscular jucntion?
it’s a synapse that occurs between a motor neurone & a muscle - it’s very similar to a synaptic junction
state a similarity between the neuromuscular junction & the cholinergic synapse
they are both unidirectional due to the neurotransmitter receptors only being on the postsynaptic membrane
state the differences between the neuromuscular junction & the cholinergic synapse
neuromuscular junction:
only excitatory
connects a motor neurone to muscles
this is the endpoint for the action potential
acetylcholine binds to receptors on muscle fibre membranes
cholinergic synapse:
could be excitatory/inhibitory
connects 2 neurones, which could be sensory, relay or motor
a new action potential is generated in the next neurone
acetylcholine binds to receptors on postsynaptic membrane of a neurone
how do muscles create movement?
muscles act in antagonistic pairs against an incompressible skeleton to create movement - can be automatic or controlled by conscious thoughts
what are myofibrils made up of?
it is made up of fused cells that share nuclei & cytoplasm, known as sarcoplasm - high number of mitochondria
what is a sarcomere made up of?
made up of 2 proteins; myosin & actin which form a sarcomere (which forms myofibrils)
describe what happens to the length of the different bands when a muscle contracts
The A band shows myosin length, which never changes (it’s constant) as the myosin itself (apart from the heads) never moves.
The H zone is where myosin doesn’t have any actin overlapping - which will change when the muscle contracts as the actin slides in so the H zone decreases. The I band also changes in size as in the I band there is only actin, so when the muscle is relaxed there is a bigger I band but when it contracts the I band decreases as the actin slides closer together (there is more of an overlap with myosin)
Z lines indicate the start & end of one sarcomere - as the actin slides in, the Z lines become closer
explain the sliding filament theory
- When an action potential reaches a muscle, it stimulates a response
- Ca ions enter & cause the protein tropomyosin to move & uncover the binding sites on actin
- Whilst ADP is attached to the myosin head, the myosin heads to the actin to form a cross-bridge
- The angle created in this cross-bridge creates tension - resulting in the actin filament being pulled & sliding along the myosin. The ADP molecules are released.
- An ATP molecule then binds to the myosin head &. causes it to change shape slightly. It detaches from the actin.
- Within the sarcoplasm, an enzyme ATPase is activated by Ca ions, to hydrolyse the ATP on the myosin head into ADP & release enough energy for the myosin head to return to its original position.
- This process repeats continually whilst Ca ions remain high & so the muscle remains stimulated by the nervous system.
