3.6

Question 1

Survival and Response

Question 2

Why do organisms respond to changes in their environment

Answer 2

to increase their chances of survival
e.g. by preventing extinction through potential danger

Question 3

What are taxis and kinesis

Answer 3

simple responses that enable mobile organisms to stay in a favourable environment
taxis is a directional response to stimuli
kinesis is a non-directional response to stimuli

Question 4

What is kinesis affected by and give an example

Answer 4

The rate of movement of an organism is affected by the intensity of the stimulus
Flatworms called planarians possess a network of neurones and simple eye-like structures that have light-sensitive cells
Planarians display kinesis when removed from their usual dark environment
Planarians are found on the underside of stones, hidden from daylight
When a stone is removed or turned over the planarians begin to move in random directions
Once these random movements eventually bring them back into the darkness they stop moving
This type of responsive behaviour helps them to protect themselves from predators
In the scenario above, the light-sensitive cells are detecting light when the stone is overturned but the planarian has no way of detecting the nearest shaded space, therefore it moves randomly until the eye detects a low level or no light
The planarian uses kinesis to ensure it is in its favourable environment - darkness

Question 5

What is taxis affected by and give an example

Answer 5

The organism moves directly away from or towards the stimulus
A single-celled organism called Euglena which is commonly found in ponds exhibits taxis
It has chloroplasts for photosynthesis and a flagellum to help it swim
The flagellum has a receptor close to its base that is sensitive to light
Euglena swims directly towards the light, this is known as phototaxis
This behaviour is highly valuable as it brings the organism towards the light where it can photosynthesise

Question 6

investigating taixs and kinesis

Answer 6

Taxes and kineses behaviour in small animals can be studied using special apparatus
Choice chambers and mazes are common pieces of apparatus that are used
Woodlice and maggots are often the organisms studied
It can be difficult to distinguish taxis from kinesis in these experiments
The animals need to be observed during the experiment to see if turning frequency or movement rate changes in different environments
If movement is directional then the turning frequency would decrease when the organism detects the stimulus
Choice chambers
An experiment was conducted to investigate whether maggots exhibited negative phototaxis
This would mean that they moved away from bright light (not randomly)
One half of the transparent choice chamber was covered in an opaque material to prevent light from entering
30 maggots were placed into the chamber via the hole in the centre of the lid
10 minutes later the number of maggots found in each half of the chamber were counted
This was repeated several times
The results showed that there was always more maggots in the shaded half of the chamber at the end of the experiment
As the maggots were not observed during the experiment it can not be said whether kinesis or taxis has occurred
However, the results do conclude that maggots have the ability to detect bright light and respond by moving until they reach a more favourable environment

Question 7

6.1.1 sme

Answer 7

Organisms must respond to changes in their environment in order to survive, so that they can:
-find favourable conditions for living
-find food
-avoid being eaten (predattion)

-prvents extinction
For example, a red robin must find worms and insects to feed on and at the same time, they must also be watching out for predators such as crows
Detecting and responding to change
Responses to change can vary in complexity depending on the type of organism involved and the specific circumstances they are responding to
Responding to change requires detection
Detection involves a stimulus being detected by a receptor cell
There are different types of receptors
Some receptor cells produce electrical activity in nerve cells in response to stimuli
Other receptor cells secrete substances in response to stimuli
The nerve impulses sent by receptor cells travel to a coordinator
This is either the brain or the spinal cord
From the coordinators, the impulse is conducted to the specific effector that will produce the appropriate response
Using the earlier example of the red robin staying alert to predators:
A sudden movement by a crow (the stimulus) is detected by the receptors in the robin’s eye
The receptor cells send an impulse along the nerves and to the brain (coordinator)
The brain sends an impulse to the wing muscles (effectors) of the red robin so it can fly away (response)

Question 8

protective effect of a simple reflex

Question 9

effect of different concentrations of indoleacetic acid (IAA) on cell elongation in the roots and shoots of flowering plants as an explanation of gravitropism and phototropism in flowering plants.

Question 10

basic structure of a Pacinian corpuscle.

Answer 10

-not a separate cell, as they are found at the ends of sensory neurone axons

-made of many layers of membrane separated by a gel

-gel between the layers contains positively charged sodium ions (Na+)

-the section of axon surrounded by layers of membrane contains stretch-mediated sodium ion channels

• these open when sufficient pressure is applied

Question 11

how does deformation of stretch-mediated sodium ion channels in a Pacinian corpuscle leads to the establishment of a generator potential.

Answer 11

-an excess of positively charged sodium ions surrounds the axon
-pressure is exerted on the Pacinian corpuscle
-the layers of membrane become distorted and the stretch-mediated sodium channels in the axon membrane open
-sodium ions enter the axon via facilitated diffusion
-changes the electrical potential difference across the membrane
-leads to depolarisation
-establishes a generator potential
-the generator potential triggers impulses (action potentials) that travel along the sensory neurone to the central nervous system

Question 12

What are Pacinian corpuscles

Answer 12

-receptors that respond to changes in pressure
-are present in the skin of fingers, soles of the feet, joints, tendons and ligaments
-stimulating these receptors with excess pressure on skin leads to a generator potential

Question 13

Function of:
cornea
retina
iris
optic nerve
pupil
lens

Answer 13

cornea
transparent layer that retracts light as it enters eye

retina:
contains light receptors
-rods: detect light intensity
-cones: detect colour

iris:
controls how much light enters pupil

optic nerve
sensory neurone that carries impulses between the eye and brain

pupil
hole that allows light to enter the eye

lens
transparent disc that can change shape to focus light onto retina

Question 14

Sensitivity to light and colour

Question 15

Visual acuity

Question 16

Cone cells provide higher visual acuity

Answer 16

-one cone cell synapses with a single bipolar cell
-one bipolar cell synapses with a single ganglion cell

-if two cones are stimulated to send an impulse the brain is able to interpret these as two different spots of light

-cone cells detect only one of three colours (red, green or blue) , so the brain will receive information about the colour of light detected by the stimulated cone cell and where this light is

-this is because the brain knows which bipolar cell connects to which cone cell

Question 17

Rod cells provide lower visual acuity

Answer 17

-multiple rod cells synapse with a single bipolar cell

-multiple bipolar cells synapse with a single ganglion cell

-brain is not able to interpret which impulses are sent by specific rods

-if multiple rod cells connected to the same bipolar cell detect light, only one impulse from the bipolar cell is sent

-hence the brain receives a general, not specific, understanding of the fields of vision that are light or dark

Question 18

what is summation and the benefit of it

Answer 18

There is a benefit to how the rods are connected to the optical nerve
Each rod is very sensitive to light however a single stimulated rod is unlikely to produce a large enough generator potential to stimulate the bipolar cell for the conduction of nerve impulses
When a group of rods are stimulated at the same time the combined generator potentials are sufficient to reach the threshold and stimulate the bipolar cell for the conduction of nerve impulses onwards towards the optic nerve
This additive effect of rods is known as summation
Summation produces a less sharp image but enables organisms to see in much dimmer light than cones allow
Nocturnal animals tend to have mostly or solely rods present in their eyes

Question 19

Explain why the heart is considered myogenic

Answer 19

It contracts without any external stimulus

Question 20

Outline how heart rate is controlled and coordinated
the roles of the autonomic nervous system and effectors in controlling heart rate.

Answer 20

-sinoatrial node (SAN) is a group of cells in the wall of the right atrium
-SAN initiates a wave of depolarisation that causes the atria to contract
-the depolarisation is carried to the atrioventricular node (AVN - region of conducting tissue between atria and ventricles)
-after a slight delay, the AVN is stimulated and passes the stimulation along the bundle of His
-delay means that the ventricles contract after the atria
-the bundle of His is a collection of conducting tissue in the septum (middle) of the heart.
-the bundle of His divides into two conducting fibres, called Purkyne tissue, and carries the wave of excitation along them
-the Purkinje fibres spread around the ventricles and initiate the depolarization of the ventricles from the apex (bottom) of the heart
-makes the ventricles contract and blood is forced out of the pulmonary artery and aorta

Question 21

How does exercise affect heart rate??

Question 22

Outline the role and location of chemoreceptors and baroreceptors

Answer 22

Chemoreceptors detect the concentration of oxygen in the blood. They are also sensitive to changes in pH resulting from the carbon dioxide dissolved in the blood (its reacts with the water to form carbonic acid), which is an indication of oxygen availability

Baroreceptors detect changes in blood pressure

Both types of receptors are found in the aortic and carotid bodies.

Question 23

Nervous coordination

Question 24

Outline the structures of the three types of neurones

Answer 24

Schwann cells wrap around axon to form myelin sheath (lipid)
Charged ions can’t pass through
Gaps between = nodes of Ranvier

sensory neurone has its cell body in the middle and has a dendron and axon

motor neurone has its cell body at the start and only has a long axon

Question 25

Describe the two types of motor neurones

Answer 25

Somatic supplies skeletal muscle = under conscious control

Autonomic supplies cardiac muscle, smooth muscle, glands = under subconscious control

Question 26

Define action potential

Define nerve impulse

Question 27

Outline what happens during an action potential

Answer 27

stimuli causes Na+ ions to enter the start of the neurone

makes membrane potential less negative

if it reaches threshold (-50mV), Na+ channels open

therefore more Na+ ions diffuse into the neurone, therefore membrane potential becomes positive (depolarised)

the membrane potential reaches +40mV

then the Na+ channels close, the K+ channels open

therefore K+ ions diffuse out, therefore membrane potential becomes negative (repolarised)

too many K+ ions move out, so the membrane potential becomes more negative than normal (hyperpolarised, known as the refractory period)

-resting potential is achieved when it slowly begins to increase again

Question 28

What affects action potentials

Answer 28

frequency of impulses
high frequency = larger stimuli
Low frequency = smaller stimuli

Question 29

Factors affecting speed of nervous impulses

Answer 29

Temperature
-higher temp = higher kinetic energy
-faster rate of diffusion of ions
-enzymes involved in respiration also work faster, so more ATP produce
-more active transport in the Na+/K+ pump
-faster nerve impulse

Axon diameter
-wider diameter
-neurone less leakage of ions
-APs travel faster
-faster nerve impulse

Myelination:
-Schwann cells wrap around axon
-insulates axon preventing AP
-therefore AP only occurs in gaps – called node of Ranvier
-so AP jumps from node to node = saltatory conduction
-faster nerve impulse

Question 30

What is a synapse
outline the stages of synaptic transmission

Answer 30

-connection between 2 different neurones
-sends nerve impulse across the synaptic cleft using neurotransmitters (e.g. acetylcholine)

Process:
-AP arrives in end of presynaptic neurone
-voltage-gated Ca2+ channels open
-Ca2+ ions enter presynaptic neurone
-causes vesicles containing neurotransmitter to move to presynaptic membrane
-vesicle binds to membrane releasing neurotransmitter into cleft
-neurotransmitter diffuses across cleft
-binds to complementary receptors on postsynaptic membrane
-Na+ channels open, Na+ ions enter
-if threshold is reached, AP occurs

To return to rest
-enzyme used to break down neurotransmitter
-e.g. acetylcholinesterase breaks down acetylcholine into ethanoic acid and choline
-diffuses back into presynaptic neurone
-ATP used to reform neurotransmitter into vesicle and actively transport Ca2+ ions out

Question 31

Properties of synapses

Answer 31

unidirectionality = AP/nerve impulse travels in one direction, from pre to post, pre has the neurotransmitter, post has the receptors

filters out low level stimuli = low level stimuli do not release enough neurotransmitter, therefore not enough Na+ ion channels open, therefore not enough Na+ ions enter postsynaptic neurone for threshold to be reached, therefore no AP produced

inhibitory = normal synapses are excitatory (cause AP), some can be inhibitory – prevent action potential from occurring by making postsynaptic neurone hyperpolarised

Question 32

Skeletal Muscle

Question 33

types

Answer 33

Skeletal

Smooth

Cardiac

Question 34

structure

Answer 34

basic structure = sarcomeres

made up of actin and myosin, actin is thin and has tropomysosin wrapped around it, myosin is thick and has heads, when the sarcomere contracts the whole muscle contracts, contracts/shortens by the sliding filament mechanism

many sarcomeres = myofibril

many myofibrils = muscle fibre

surrounded by a membrane called sarcolemma

contains myofibrils, fluid called sarcoplasm and tubes called sarcoplasmic reticulum

many muscle fibres = bundle

many bundles = whole muscle

Locations in a Sarcomere?
A band = location of myosin [no change in contraction]
I band = location between the myosin [shortens in contraction]
H zone = location between the actin [shortens in contraction]
Z line = end line of sarcomere [moves closer together in contraction]

Question 35

Overall function of skeletal muscles?

Answer 35

-moves the body skeleton
-when the muscle contracts (shortens) the tendon pulls on joints causing movement

Question 36

what occurs in Sliding Filament Mechanism?

Answer 36

how the sarcomere shortens

the myosin heads pull the actin inwards

the somatic motor neurone connects to the skeletal muscle via a neuro-muscular junction

one motor neurone connects to a few muscle fibres = motor unit

  (benefit = simultaneous muscle contraction and can control strength of contraction) 

releases acetylcholine that binds to complementary receptors on the muscle fibre membrane (sarcomere)

Na+ channels open, Na+ ions enter the muscle fibre causing depolarisation

wave of depolarisation travels through sarcoplasmic reticulum

causes release of Ca2+ ions into the sarcoplasm (fluid surrounding sarcomeres/myofibril)

this moves the tropomyosin on the actin

exposes binding sites on the actin

myosin heads now bind to the actin (form actin-myosin cross bridge)

a power stroke occurs, the myosin pulling the actin inwards

ATP attaches to myosin head so it detaches

ATP brokendown by ATPase to release energy

causes myosin head to go back to its original position

so it reattaches, pulling the actin further inwards

Question 37

Establishing resting potential

Answer 37

-sodium potassium pump pumps 3Na+ out and 2K+ in
-creates electrochemical gradient
-causes K+ ions to diffuse out (from high to low concentration)
-Na+ ions diffuse in (higher outside than in)

-cell membrane is more permeable to K+ ions than Na+, as there are more K+ ion channels,so more facilitated diffusions
-also some Na+ ion channels only open when the voltage is high enough, but K+ ion channels are always open

-70mV inside the neurone

Question 38

What is the all or nothing principle

Answer 38

If the depolarisation doesn’t exceed -55mV, then an action potential and impulse are not produced

Any stimulus that triggers depolarisation to-55mV will always peak at the same maximum voltage

Bigger stimuli = higher frequency of AP

Important as animals only respond to large stimuli

Question 39

Importance of refractory period

Answer 39

1. Discrete APs that don’t overlap so you can process the info in more detail
2. Ensures that it only travels in one direction
3. Limits the number of impulses transmitted, so not as overwhelming to senses

Question 40

What is summation?

Answer 40

-rapid build-up of neurotransmitters in synapse
-helps generate AP
Spatial:
-many different neurones collectively trigger new AP by combining the neurotransmitter they release to exceed the threshold value

Temporal:
-one neurone repeatedly releases neurotransmitter over short period of time to add up to enough to exceed the threshold value

Question 41

Inhibitory synapses

Answer 41

-cause chloride ions to move into the postsynaptic neurone and potassium ions to move out
-hyperpolarisation occurs as membrane potential decreases
-hence AP highly unlikely

-potsynaptic neurone becomes more negative/hyperpolarisation

-More sodium ions required (to reach threshold)
OR
Not enough sodium ions enter (to reach threshold);
- depolarisation/action potential; highly unlikely

Question 42

Neuromusclar junction vs cholinergic synapse

Answer 42

Neuromusclar junction:
-unidirectional as receptors for neurotransmitters are only on post-synaptic membrane
-only excitatory
-connects motor neurone to muscles
-end point for AP
-acetylcholine binds on receptors on muscle fibre membranes

Cholinergic synapse:
-unidirectional as receptors for neurotransmitters are only on post-synaptic membrane
-could be excitatory/inhibitory
-connects 2 neurones
-new AP is generated in the next neurone
-acetylcholine binds to receptors on post-synaptic membrane of a neurone

Question 43

Sliding Filament theory sme

Answer 43

-during muscle contraction, sarcomeres within myofibrils shorten as the Z discs are pulled closer together

Process:
-action potential arrives at the neuromuscular junction
-calcium ions are released from the sarcoplasmic reticulum
-calcium ions bind to troponin molecules, stimulating them to change shape
-causes troponin and tropomyosin proteins to change position on the actin (thin) filaments
-myosin binding sites are exposed on the actin molecules

-globular myosin heads bind with these sites, forming cross-bridges between the two types of filament
-formation of the cross-bridges causes the myosin heads to spontaneously bend (releasing ADP and inorganic phosphate), pulling the actin filaments towards the centre of the sarcomere
-causes the muscle to contract a very small distance
-ATP binds to the myosin heads producing a change in shape that causes the myosin heads to release from the actin filaments

-ATP hydrolase hydrolyses ATP into ADP and inorganic phosphate
-causes the myosin heads to move back to their original positions (known as the recovery stroke)
-myosin heads are then able to bind to new binding sites on the actin filaments, closer to the Z disc
-myosin heads move again, pulling the actin filaments even closer the centre of the sarcomere,
-causes the sarcomere to shorten once more and pulling the Z discs closer together
-ATP binds to the myosin heads once more in order for them to detach again
-as long as troponin and tropomyosin are not blocking the myosin-binding sites and the muscle has a supply of ATP
-process repeats until the muscle is fully contracted

Question 44

Outline the structure of the two types of myofibrils (sme)

Answer 44

-thick filaments within a myofibril contain myosin molecules
myosin are ffibrous protein molecules with a globular head
-fibrous part anchors the molecule into the thick filament
-in the thick filament, many myosin molecules lie next to each other with their globular heads all pointing away from the M line

-thin filaments within a myofibril contain actin molecules
-globular protein molecules
-many actin molecules link together to form a chain
-two actin chains twist together to form one thin filament
-a fibrous protein known as tropomyosin is twisted around the two actin chains
-another protein known as troponin is attached to the actin chains at regular intervals

Question 45

Outline the role of ATP and phosphocreatine in muscle contraction (sme)

Answer 45

-ATP supply required for:
-the initial movement of myosin heads (powerstroke0
-the return movement of myosin heads that causes the actin filaments to slide
-the return of calcium ions back into the sarcoplasmic reticulum occurs via active transport

-resting muscles have a small amount of ATP stored that will only last for 3/4 seconds of intense exercise
- mitochondria present in the muscles fibres are able to aerobically respire and produce ATP but this is slow and can take a considerable amount of time
Anaerobic respiration, which is faster than aerobic, still takes 10 seconds before it even begins to produce any ATP
Phosphocreatine is a molecule stored by muscles that can be used for the rapid production of ATP
A phosphate ion from phosphocreatine is transferred to ADP
ADP + phosphocreatine → ATP + creatine
Different muscle fibre types contain different limited amounts of phosphocreatine
It allows for muscles to continue contracting for a short period of time until the mitochondria are able to supply ATP
For example, it would be utilised by the muscles of a 100m sprinter as sprinting involves an intense level of muscle contraction
For prolonged activity, once the supply of phosphocreatine has been used up then the rate of muscle contraction must equal the rate of ATP production from both aerobic and anaerobic respiration

Question 46

Describe the roles of calcium ions and ATP in the contraction of a
myofibril.

Answer 46

Calcium ions diffuse into myofibrils from (sarcoplasmic) reticulum;
2. (Calcium ions) cause movement of tropomyosin (on actin);
3. (This movement causes) exposure of the binding sites on the
actin;
4. Myosin heads attach to binding sites on actin;
5. Hydrolysis of ATP (on myosin heads) causes myosin heads to
bend;
6. (Bending) pulling actin molecules;
7. Attachment of a new ATP molecule to each myosin head
causes myosin heads to detach (from actin sites).