topic 6 - organisms respond to changes in their internal and external environments

Question 1

what is a stimulus

Answer 1

any change in the internal or external environment

Question 2

describe the simple reflex arc

Answer 2

-receptor detects stimulus
- the sensory neuron carries the impulse to a relay neuron which connects to a motor neuron which then sends impulses to an effector
-it only goes to the spinal chord so no thinking of how to respond
-it is fast and involuntary so it prevents injury and allows escape from predators

Question 3

what is a tropism?

Answer 3

growth of part of a plant in response to a directional stimulus
they can be a positive tropism which is growth towards the stimulus or a negative tropism which is growth away from the stimulus

Question 4

how does IAA cause phototropism in the shoots

Answer 4

-cells in shoot tip produce IAA
-it is transported down the shoot by diffusion and is initially transported evenly throughout all regions
-light causes movement of the IAA from the light side to the shaded side of the shoot
-the IAA concentration increases in the shaded side of the shoot which causes cell elongation on the shaded side
-the shaded side elongates faster so the tip bends towards the light

Question 5

how does IAA result in gravitropism in roots

Answer 5

-cells in root tip produce IAA
-it is transported along the root by diffusion and is initially transported to all sides
-gravity causes the IAA to move from the upper side to the lower side
-the IAA concentration increases on the lower side of the roots which inhibits cell elongation of root cells on the lower side
-the lower side elongates less so roots bend downwards to the force of gravity

Question 6

define taxis

Answer 6

a directional movement response where the animal moves towards or away from the stimulus

Question 7

define kinesis

Answer 7

-non-directional random response to non-directional stimulus
-the more unpleasant the stimulus the faster the rate of turning seen in the animal movement
-this means the animal is more likely to find favourable conditions

Question 8

how does the pacinian corpuscle lead to an action potential

Answer 8

-the pacinian corpuscle contains a sensory nerve ending which is wrapped in layers of connective tissue called the lamellae
-applying pressure cause the lamellae to become distorted
-this causes the sensory neuron’s cell membrane to stretch which causes the stretch mediated sodium ion channels to deform so they are open
-sodium ions diffuse into the neuron depolarising the membrane and creating a generator potential
-if the generator potential reaches the threshold it triggers and action potential

Question 9

describe the differences in sensitivity to light between rods and cones

Answer 9

-rods are more sensitive to light
-many rods are connected to one bipolar neuron so many weak generator potentials combine to reach the threshold and trigger and action potential (spatial summation)

-cones are less sensitive to light
-one cone joins to one neuron so it takes more light to reach the threshold and trigger an action potential

Question 10

describe the difference in visual acuity between rod and cone cells

Answer 10

-rods give lower visual acuity
-many rods are connected to one bipolar neuron so many neurons generate one action potential regardless of the number of neurons stimulated
-therefore they cannot distinguish between seperate sources of light

cones give higher visual acuity
-one cone joins to one neuron
-if two adjacent cones are stimulated the brain receives two seperate impulses
-therefore it distinguishes between 2 separate sources of light

Question 11

how does the heart control regular beating of the heart

Answer 11

-the sinoatrial node/SAN is in the wall of the right atrium
-the SAN initiates the heartbeat
-the SAN sends waves of electrical activity across the atria causing the right and left atria to contract at the same time
-non conducting collagen tissue prevents the waves of electrical activity from being passed directly to the ventricles
-the waves of electrical activity as passed on to atrioventricular node/AVN
-the AVN delays the impulses to allow the atria to empty before the ventricles contract
-the AVN sends a wave of electrical activity down the bundle of HIs made of purykne tissue
-the purkyne tissue causes ventricles to contract from base up

Question 12

explain how baroreceptors detect and respond to a rise in blood pressure

Answer 12

-baroreceptors detect high blood pressure
-increases the frequency of impulses to the medulla
-this increases the frequency of impulses along the parasympathetic neurons to the SAN
-acetylcholine is released at the synapse and binds to receptors on the SAN
-this decreases the rate of production of electrical waves by the SAN so decreases the heart rate

Question 13

explain how chemoreceptors respond to low pH or high carbon dioxide concentration

Answer 13

-chemoreceptors detect rise in carbon dioxide or fall in pH
-increases the frequency of impulses to the medulla
-this increase frequency of impulse along the sympathetic neurons to the SAN
-Noradrenaline is secreted at the synapse and binds to receptors on the SAN
-this increases the rate of production of electrical waves by the SAN so increases the heart rate

Question 14

explain how a resting potential is established

Answer 14

-the sodium-potassium pump moves 3 sodium ions out of the axon and 2 potassium ions into the axon
-this creates a sodium ion electrochemical gradient because there are more positive sodium ions outside the cell than inside
-the neuron membrane is more permeable to potassium than sodium so potassium moves out the axon by facilitated diffusion
-the inside of the axon is more negatively charged compared to the outside of the axon

Question 15

how is an action potential generated

Answer 15

stimulus:
- a stimulus excites the neuron cell membrane causing sodium channels to open so sodium ions diffuse into the neuron
-this increases the potential difference from -70mv to a less negative value inside of the neuron
depolarisation:
-if the potential difference reaches the threshold of -55mv more sodium ion channels open and diffuse rapidly into the neuron
repolarization:
-at around 40 mv the voltage gated sodium channels close and the voltage gated potassium ions open
-the potassium ions diffuse down their concentration gradient out of the axon so the inside of the membrane becomes more negative again
hyperpolarization:
-potassium ion channels are slow to close so there is an overshoot of potassium ions
-too many potassium ions diffuse out of the neuron so the potential difference becomes more negative than the resting potential

Question 16

why does a myelinated axon conducts impulses faster than a non-myelinated axon

Answer 16

-in myelinated axon the depolarisation only occurs at the nodes of ranvier
-the neurons cytoplasm conducts enough electrical charge to depolarise the next node so saltatory conduction occurs with the impulse jumping from node to node
-this means the impulse does not travel along the whole length of the axon membrane

Question 17

describe the structure of a myelinated motor neurone

Answer 17

-long fibre known as an axon
-the axon is insulated by a fatty sheath called myelin sheath
-the myelin is made of specialised cells called schwann cells
-small uninsulated sections along its length called nodes of ranvier
-the cell bodies contain a nucleus and have many extensions called dendrites

Question 18

describe the refractory period

Answer 18

-the ion channels reset
-the sodium potassium pump return the membrane to resting potential
-another action potential cannot be generate in this time

Question 19

describe the processes that occur at a cholinergic synapse

Answer 19

-an action potential arrives at the synaptic knob of the presynaptic membrane
-this stimulates voltage gated calcium channels to open so calcium ions diffuse into the synaptic knob
-the influx of calcium ions causes the synaptic vesicles to fuse with the presynaptic membrane
-the vesicles release acetylcholine into the synaptic cleft
-the acetylcholine diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane
-sodium ion channels open and sodium ions diffuse into the post synaptic neuron causing depolarisation of the membrane
-if the threshold is reached and action potential is generated

Question 20

explain why synapses result in unidirectional nerve impulses

Answer 20

-neurotransmitters only made in pre synaptic neuron
-neuron receptors are only on post synaptic membrane

Question 21

explain temporal summation of synapse

Answer 21

-one presynaptic neuron releases neurotransmitters many times over a short period
-this results in a build up of sufficient neurotransmitters to reach the threshold to trigger an action potential

Question 22

compare transmission across cholinergic synapses compared to neuromuscular junctions

Answer 22

-cholinergic synapse is a junction between a neuron and another neuron whereas a neuromuscular junction is between a neuron and a muscle
-there is an action potential in the neuron but no action potential in the muscle
-no summation occurs in the muscle
-muscle response is always excitatory
-some neuromuscular junctions have different neurotransmitters