3.6 Organisms respond to changes in their internal and external environments

Question 1

action potential

Answer 1

An A.P. is a depolarisation of the cell membrane, where it reaches a potential difference of +40mV compared to the outside

Question 2

action potential- rest

Answer 2

the membrane is polarised at -60mV with some K+ channels open and all Na+ channels shut

Question 3

action potential- slight increase in potential difference

Answer 3

some Na+ channels open due to energy changes in the environemnt allowing some ions to diffuse down the conc gradient. membrane starts to depolarise

Question 4

action potential- great increase

Answer 4

once membrane reaches a threshold potential of around -50mV, voltage gated Na+ channels open to allow even more Na+ to flood in
causes membrane potential to rise sharply and become positive charged compared to outside
eventually reaches a peak of around +50mV

Question 5

action potential- decrease

Answer 5

all Na+ channels close and all K+ channels open

as K+ diffuse back into the cell, the membrane potential becomes more negative (repolarisation)

Question 6

action potential- lowest dip

Answer 6

as slightly more K+ channels are open than usual the potential difference overshoots slightly and becomes hyperpolarised

Question 7

action potential- refractory period

Answer 7

takes time for pumps to restore the ion concentrations for the next action potential so there is a period of time where the action potential cannot be stimulated
prevents depolarisation

Question 8

all or nothing principle

Answer 8

all action potentials are the same size all impulses are of the same amplitude
more intense stimulus=greater frequency of impulses

Question 9

speed of transmission

Answer 9

myelination and salutatory conduction
distance involved
axon diameter
temperature

Question 10

synapses

Answer 10

gaps between neurones

information is sent between neurones by chemical transmission

Question 11

synaptic transmission steps

Answer 11

step 1- calcium channels open 
step 2- neurotransmitter release 
step 3- sodium channels 
step 4- new action potential 
step 5- acetylcholinesterase 
step 6- remaking acetylcholine

Question 12

synaptic transmission- step 1

Answer 12

calcium channels open
incoming A.P causes depolarisation in synaptic knob
causes calcium channels to open and ions flood into the synaptic knob

Question 13

synaptic transmission- step 2

Answer 13

neurotransmitter release
influx of calcium ions causes synaptic vesicles to fuse with presynaptic membrane
releases neurotransmitter into a cleft

Question 14

synaptic transmission- step 3

Answer 14

neurotransmitter is released into the synaptic cleft
binds to the receptor sites on sodium ion channels
sodium ion channels open\

Question 15

synaptic transmission- step 4

Answer 15

new action potential
depolarisation inside postsynaptic neurone must be above a threshold value
if threshold is reached a new action potential is sent along the axon of the post synaptic neurone

Question 16

synaptic transmission- step 5

Answer 16

acetylcholinesterase
hydrolytic enzyme
breaks up acetylcholine into acetyl and choline

Question 17

synaptic transmission- step 6

Answer 17

ATP is released by mitochondria is used to recombine acetyl and choline
stored in synaptic vesicles
more acetylcholine can be made at the SER

Question 18

sensory receptors

Answer 18

These are specialised cells that detect a change in the surroundings to trigger a nerve impulse
They are energy transducers, as they convert a change in energy (a stimulus) into an electrical impulse

Question 19

pacinian corpuscles location

Answer 19

Abundant deep in skin – Fingers, soles of the feet, external genitalia. Occur in joints, ligaments and tendons to show a change of direction

Question 20

pacinian corpuscles and producing a generator potential

Answer 20

When pressure is applied the membrane becomes distorted and stretched
Sodium channels are then open and sodium ions diffuse into the neurone
The sodium ions change the potential of the membrane and it becomes depolarised producing a generator potential
The generator potential creates a nerve impulse (action potential) along the sensory nerve to the CNS

Question 21

transducers

Answer 21

Both rod and cone cells act as transducers by converting light energy into the electrical energy of a nerve impulse.

Question 22

rod cells

Answer 22

Cannot distinguish different wavelengths of light and therefore produce images only in black and white.
Rod cells are more numerous than cones.
share a single sensory neurone. Rod cells can therefore respond to light of very low intensity because a certain threshold value has to be exceeded before a generator potential is created in the bipolar cells to which they are attached.

Question 23

rod cells and bipolar cells

Answer 23

A number of rod cells are attached to a single bipolar cell (= retinal convergence), there is a much greater chance that the threshold value will be exceeded than if only a single rod cell were attached to each bipolar cell.
As a result, rod cells allow us to see in low light intensity (i.e. at night), although only in black and white.

Question 24

cone cells

Answer 24

Cone cells are of three different types, each responding to a different wavelength of light. Depending on the proportion of each type that is stimulated, we can perceive images in full colour.

Question 25

cone cells and bipolar cells

Answer 25

Each cone cell usually has its own bipolar cell connected to a sensory neurone. This means that often the generator potential is not exceeded. As a result, cone cells only respond to high light intensity and not to low light intensity.

Question 26

cone cells and fovea

Answer 26

Light is focussed by the lens on a point known as the fovea. The fovea therefore receives the highest intensity of light.

Therefore cone cells, but not rod cells, are found at the fovea. The concentration of cone cells diminishes further away from the fovea. At the peripheries of the retina, where light intensity is at its lowest, only the rod cells are found.

Question 27

cone cells and pigments

Answer 27

Cone cells contain a different pigment to rod cells (iodopsin). This requires a higher light intensity to be broken down and create a generator potential.

As cone cells are attached to their own bipolar cell, if 2 adjacent cells are stimulated, the brain receives 2 separate impulses.
Cone cells give very accurate vision, they have good visual acuity.