Plant And Animal Responses 5.1

Question 1

What are plant responses to herbivory?

Answer 1

-herbivory is a biotic stress factor

-some plants produce chemicals that provide a defence against herbivory:
->alkaloids- bitter tasting or toxic, either deterring or killing herbivores
->pheromones-a signal to nearby plants of the same species that they are under attack from herbivores, triggering other defences. A signal to attract a herbivorous insect’s natural predators

-some plants are sensitive to touch, leaves fold rapidly from touch, this can protect plants from herbivorous insects

Question 2

How do plants respond to abiotic stress?

Answer 2

Plants can respond to drought- by reducing water loss by transpiration e.g. shutting stomata, dropping leaves

Question 3

What are some examples of abiotic stress for plants?

Answer 3

-freezing
-drought
-increased soil water salinity
-presence of heavy metals

Question 4

Draw a table of the different tropisms in a plant include what they are in response to, type of stimulus and the advantage of tropism

Answer 4

Question 5

What are deciduous plants?

Answer 5

-deciduous plants lose their leaves in very hot and dry environmental conditions in order to reduce water loss
-deciduous plants in temperate climates also do this during winter when absorption of water is difficult due to frozen soils (As well as this photosynthesis is being limited by low temperatures and reduced light)

Question 6

What is the role of hormones in leaf loss in deciduous plants?

Answer 6

-hormones are responsible for this leaf loss:
->in temperate climates these hormones are produced in response to shortening day length in the autumn
->a layer of cells known as the ABSCISSION LAYER develops at the base of the leaf stalk- this is a layer of parenchyma cells with thin walls, making them easy to break
->the plant hormone ETHENE stimulates the breakdown of cell walls in this abscission layer, causing the leaf to drop off
->AUXIN also plays a role in leaf loss
-usually auxins inhibit leaf loss and are produced in young leaves, making the leaf insensitive to ethene
-the concentration of auxins in leaves decrease as they age until leaf loss can once again occur in response to ethene

Question 7

What is the role of hormones in stomatal closure?

Answer 7

-during times of water stress, the hormone abscisic acid (ABA) is produced by plants to stimulate the closing of their stomata
-guard cells have ABA receptors on their cell surface membranes
-ABA binds with these receptors inhibiting the proton pumps and therefore stopping the active transport of hydrogen ions out of the guard cells
-ABA also causes calcium ions to move into the cytoplasm of the guard cells through the cell surface membranes
-the calcium ions act as second messengers
->they cause channel proteins to open that allow negatively charged ions to leave the guard cells
->this stimulates the opening of further channel proteins that allow potassium ions to leave the guard cells
->the calcium ions also stimulate the closing of channel proteins that allow potassium ions to enter the guard cells
->the loss of ions increases the water potential of the guard cells
->water leaves the guard cells by osmosis
->the guard cells become FLACCID causing the stomata to close

Question 8

What is the role of hormones in seed germination?

Answer 8

-Abscisic acid has the opposite effect effect to gibberellins, they maintain dormancy by inhibiting amylase production
-the start of germination is therefore determined by the balance of abscisic acid and gibberellins present in the seed

Question 9

What is apical dominance?

Answer 9

-the auxins that are produced at the growing tip at the apex (top) of a plant stem cause the stem to grow upwards and also stop lateral ( side) buds from growing
->this is known as apical dominance

Question 10

Why is apical dominance beneficial for a plant?

Answer 10

-growing upwards towards the light is beneficial as it ensures the plant has access to more energy for photosynthesis
-most of the time, sideways growth is not useful to plants and so apical dominance caused by auxins ensures that growth is preferentially upwards

Question 11

What happens to apical dominance if the growing tip of a plant is removed?

Answer 11

-if the growing tip at the apex of a plant is removed, lateral buds grow from the top of the plant, as the source of auxins has been removed and there is no longer apical dominance
-however, with time the lateral shoots that grow from these lateral buds curl up towards the light, meaning the plant continues to grow in an upwards direction

Question 12

How is apical dominance shown experimentally?

Answer 12

• the apical bud of the first test plant is removed (decapitated)
  -this allows the lateral buds to grow
  -a second (genetically identical) plant is decapitated but this time the cut tip is immediately replaced with an agar block containing auxin
  -this restores the inhibition of lateral bud growth and no lateral buds grow

Question 13

What are Gibberellins?

Answer 13

-a group of hormones that help plants grow by stimulating cell division and elongation in the stem

Question 14

What is some experimental evidence for the role of Gibberellins in stem elongation?

Answer 14

-dwarf plant varieties have been shown to have very low levels of Gibberellins- due to a mutation involved in the synthesis of Gibberellins
-under experimental conditions treating these dwarf varieties with Gibberellins results in them growing to the same height as normal varieties

Question 15

What is the experimental evidence for the role of gibberellins in seed germination?

Answer 15

-gibberellins stimulate germination and the breaking of dormancy
->seeds of mutant varieties of plants that do not produce gibberellins can be induced to germinate if gibberellins are applied
->seeds of certain lettuce varieties that require light in order to germinate can be made to germinate in the dark if gibberellins are applied

Question 16

What is geotropism?

Answer 16

-a directional growth response to gravity

Question 17

What is phototropism?

Answer 17

-a directional growth response to light

Question 18

What is a coleoptile?

Answer 18

-a sheath that surrounds the young growing shoot of grass plants

Question 19

Describe Darwins experiment about plants phototrophic response

Answer 19

• Darwin discovered that removing the tip of a coleoptile stopped the phototrophic response to a unidirectional light source from occurring
  -To ensure this was not simply due to the wounding caused to the plant, he covered the tip of a coleoptile with an opaque cover or ‘cap’ instead, to block out the light. This also stopped the phototropic response from occurring, showing that the tip of the coleoptile was responsible for detecting light

Question 20

Describe Boysen- Jensen’s experiment on the phototrophic response

Answer 20

Boysen-Jensen found that if he replaced the cut tip back on top of the coleoptile and inserted a gelatin block as a barrier in between, the phototropic response was restored
This showed that the stimulus for growth was a chemical (hormone), which was able to travel through the gelatin block
Bosen-Jensen then inserted a mica barrier (mica is impermeable to chemicals) halfway through the coleoptile just below the tip, first on the lit side and then on the shaded side
When the mica barrier was inserted into the lit side, the phototropic response occurred
When the mica barrier was inserted into the shaded side, the phototropic response did not occur
This confirmed that the stimulus for growth was a chemical (hormone) and showed that it was produced at the tip, before travelling down the coleoptile on the side opposite to the stimulus (i.e. the shaded side)
It also showed that the stimulus acted by causing growth on the shaded side (rather than inhibiting growth on the lit side)

Question 21

Describe Paáls experiment

Answer 21

-Paál cut off the tip of a coleoptile and then replaced it off-centre in the dark
The side of the coleoptile that the tip was placed on grew more than the other side, causing the coleoptile to curve (similar to a phototropic response)
This showed that, in the light, the phototropic response was caused by a hormone diffusing through the plant tissue and stimulating the growth of the tissue

Question 22

Describe Went’s experiment

Answer 22

Went placed the cut tip of a coleoptile on a gelatin block, allowing the hormones from the tip to diffuse into the block
The block was then placed on the coleoptile, off-centre and in the dark
As in Paál’s experiment, the side of the coleoptile that the block was placed on grew more than the other side, causing the coleoptile to curve
The greater the concentration of hormone present in the block, the more the coleoptile curved

Question 23

If the shoot is only illuminated from one side the auxins move towards the ________ ________ of the shoot thus causing elongation of the ___________ _________ only which results in the bending of the shoot towards the light

Answer 23

Shaded part
Shaded side

Question 24

When the shoot is illuminated from all sides the auxins are distributed ________ and move ________ the shoot tip thus causing elongation of cells across the zone of elongation

Answer 24

Evenly
Down

Question 25

What are some commercial uses of plant hormones?

Answer 25

-to control ripening ->ethene can be used to speed up fruit ripening in apples tomatoes and citrus fruit ->Gibberellins delay senescence in citrus fruits, extending the time fruits can be left unpicked and making them available for longer in shops -auxins are used in rooting powder to encourage plant growth -auxins are used as hormonal weed killers -they promote shoot growth so much that the stem cannot support itself, buckles and dies

Question 26

What is the central nervous system?

Answer 26

-the central nervous system (CNS) is the central part of the nervous system -it is composed of the brain and spinal cord

Question 27

The spinal cord has many ___________ neurones making up the central grey matter The spinal cord also contains large numbers of ___________ neurones making up an outer region of white matter. These _____________ neurones carry ______ _________ up and down the spinal cord for rapid communication over long distances The spinal cord is protected by the ___________ ________. Between each of the vertebrae, peripheral nerves enter and leave the spinal cord carrying action potentials to and from the rest of the body

Answer 27

Non-myelinated Myelinated Myelinated Action potentials Vertebral column

Question 28

Much of the brain is composed of _________ neurones,which have multiple connections enabling complex neural pathways. Most of these cells are ______________ cells and the tissue looks grey in colour. This is known as __________ ________

Answer 28

Relay Non-myelinated Grey matter The

Question 29

What is the role of the Peripheral nervous system?

Answer 29

-the role of the peripheral nervous system (PNS) is to ensure rapid communication between sensory receptors, the CNS and effectors

Question 30

What is the Peripheral nervous system composed of?

Answer 30

-the sensory and motor neurones -these are normally bundled together in a connective tissue sheath to form nerves

Question 31

What is somatic nervous system?

Answer 31

-the motor neurones under conscious control -these neurones are mostly myelinated so that responses can be rapid -there is always one single motor neurones under conscious connecting the CNS to the effector

Question 32

What is the autonomic nervous system?

Answer 32

-part of the nervous system responsible for controlling the involuntary motor activities of the body -it consists of motor neurones that conduct action potentials from the CNS to effectors that are not under voluntary control e.g. glands, cardiac muscle and smooth muscle in the walls of the blood vessels and the airways -the control of many of these effectors does not require rapid responses and the neurones are mostly non-myelinated. There are at least 2 neurones involved in the connection between the CNS and the effector. These neurones are connected at small swellings called ganglia

Question 33

What can the autonomic nervous be further divided into?

Answer 33

The autonomic nervous system can be further divided into the SYMPATHETIC SYSTEM- which prepares the body for activity, controls the ‘flight-or-fight’ responses PARASYMPATHETIC SYSTEM- which conserves energy, controls the ‘rest and digest’ system ->they are antagonistic systems, as the action of one opposes the action of the other -changes to the internal conditions or stress leads to changes in the balance of stimulation between the two systems leading to an appropriate response

Question 34

What are the differences between the sympathetic and parasympathetic system?

Answer 34

Question 35

Label this diagram

Answer 35

Question 36

What is the cerebrum?

Answer 36

-the region of the brain dealing with higher functions such as conscious thought, conscious actions, emotional responses, intelligence, reasoning, judgement, decision making and factual memory -it is divided into 2 cerebral hemispheres, which are connected via major tracks of neurones called corpus callosum. The outermost layer of the cerebrum consists of a thin layer of nerve cell bodies called the cerebral cortex

Question 37

What is the cerebral cortex divided into?

Answer 37

The cerebral cortex is subdivided into areas responsible for specific activities and body regions: ->sensory areas receive action potentials indirectly from the sensory receptors. The sizes of the regions allocated to receive input from different receptors are related to the sensitivity of the area that the inputs are received from ->association areas compare sensory inputs with previous experience, interpret what the input means and judge an appropriate response ->motor areas send action potentials to various effectors (muscles and glands) the sizes of the regions allocated to deal with different effectors are related to the complexity of the movements needed in the parts of the body Motor areas on the left side of the brain control effectors on the right side of the body and vice versa

Question 38

What is the cerebellum?

Answer 38

-region of the brain coordinating balance and fine control of muscle movement -to do this it must receive information from many sensory neurones (e.g. retina, balance organs in the inner ear) and process the information accurately

Question 39

What is the cerebrum and cerebellum connected by?

Answer 39

They are connected by the pons

Question 40

What is the medulla oblongata?

Answer 40

-region of the brain that controls physiological processes -the medulla oblongata controls the non-skeletal muscles (the cardiac muscle and involuntary smooth muscles) by sending action potentials out through the autonomic nervous system. -the medulla oblongata contains centres for regulating several vital processes, including: ->the cardiac centre, which regulates heart rate ->the vasmotor centre, which regulates circulation and blood pressure ->the respiratory centre, which controls rate and depth of breathing These centres receive sensory information and coordinate vital functions by negative feedback

Question 41

What is the hypothalamus?

Answer 41

-the part of the brain that coordinates homeostatic responses -it contains its own sensory receptors and acts by negative feedback to maintain a constant internal environment E.g. temperature regulation- the hypothalamus detects changes in core body temperature and receives sensory input from temperature receptors in the skin. It will initiate a response to temperature change that regulate body temperature within a narrow range. These responses may be mediated by the nervous system or by the hormonal system (via the pituitary gland) Osmoregulation- the hypothalamus contains osmoreceptors that monitor water potential in the blood. When the water potential changes the osmoregulatory centre initiates responses that bring about a reversal of this change. The responses are mediated by the hormonal system via the pituitary gland

Question 42

What is the pituitary gland?

Answer 42

-endocrine gland at the base of the brain, below but attached to the hypothalamus ->the anterior lobe produces and secretes many hormones. They are released into response to releasing factors (which are hormones) produced by the hypothalamus ->the posterior lobe stores and releases hormones made in the hypothalamus. The posterior lobe is linked to the hypothalamus by specialised neurosecretory cells. Hormones such as ADH which are manufactured in the hypothalamus, pass down the neurosecretory cells and are released into the blood from the pituitary gland

Question 43

What is a reflex action?

Answer 43

A response to a change in the environment that does not involve any processing by the brain to coordinate the movement

Question 44

What three neurones do most reflex pathways consist of ?

Answer 44

Sensory neurone -> relay neurone -> motor neurone

Question 45

What is the survival value to reflex actions?

Answer 45

A reflex may be used to get out of danger, to avoid damage to part of the body or it may be used to maintain balance

Question 46

What do doctors use the knee jerk reflex to assess?

Answer 46

to assess whether the nervous system of a patient is working properly or not Doctors use a specialised hammer to hit a ligament between the knee cap and the tibia, and the leg of the patient will involuntarily straighten in a small kicking motion

Question 47

The knee jerk reflex consists of the following components …

Answer 47

Stimulus - stretching of the quadriceps muscle caused by pressure on the ligament; this pressure is created by the hammer Receptor - stretch receptors in the quadriceps muscle Coordinator - the spinal cord Effector - the quadriceps muscle Response - contraction of the quadriceps muscle, causing the leg to straighten

Question 48

What is the nervous pathway of the knee jerk reflex?

Answer 48

In the nervous pathway of the knee-jerk reflex: The stretch receptors send impulses down a sensory neurone which connects directly, via a synapse, with a motor neurone in the spinal cord Note that although relay neurones are normally present in the nerve pathways of reflex actions, there is no relay neurone in the knee-jerk reflex The motor neurone then carries the impulses to the effector (the quadriceps muscle), which contracts

Question 49

How does the knee jerk pathway show us why reflex actions are fast and automatic?

Answer 49

Nerve impulses are delayed by synapses If these impulses are transmitted via the brain, as occurs in voluntary actions, they have to travel across many synapses In this reflex action the signal only has to cross a single synapse, allowing for a very rapid response Connections from the spinal cord to the brain will still allow information about the stimulus (the pressure caused by the hammer) to be sent to the brain but by the time it receives and processes this information, the response will have already occurred This is why the response happens automatically - the brain has no chance to make a decision

Question 50

What is the mechanism of the ‘fight or flight’ response?

Answer 50

Sensory neurones detect environmental stimuli associated with danger and send impulses to the brain The amygdala (a small region of the brain located in the cerebrum) sends impulses to various other parts of the brain, including the hypothalamus The hypothalamus is stimulated to send impulses via the sympathetic nerves to the adrenal glands This causes the adrenal medulla to secrete the hormone adrenaline Adrenaline stimulates target organs and tissues to increase sensory awareness, making the organism more alert and so improving its ability to respond to danger At the same time, the hypothalamus also releases a peptide hormone that stimulates the anterior pituitary gland to release ACTH (adrenocorticotropic hormone) ATCH is transported to the adrenal glands via the bloodstream This causes the adrenal cortex to secrete the hormone cortisol Cortisol stimulates target organs and tissues to increase blood pressure, blood glucose ensuring the tissues have sufficient glucose and oxygen needed for rapid response Cortisol also suppresses the immune system

Question 51

What is adrenaline?

Answer 51

The hormone adrenaline is secreted from the adrenal glands (and sometimes the medulla oblongata) It is often during times of stress or aggression that this hormone is secreted, which is why it is sometimes referred to as the 'fight-or-flight' hormone Adrenaline is transported via the bloodstream and it has a rapid effect on cells

Question 52

What are the effects of adrenaline?

Answer 52

Adrenaline is transported via the bloodstream and it has a rapid effect on cells It can have a range of effects on a number of different cell types: In the eyes it stimulates the muscles in the irises to contract, causing the pupils to dilate It increases the diameter of the bronchioles by relaxing smooth muscle. This helps to increase the airflow to the alveoli Adrenaline decreases the amount of blood flowing to the gut and skin via vasoconstriction A higher blood pressure occurs due to increased resistance from vasoconstriction It increases the amount of blood flowing to the brain and muscles via vasodilation Heart rate and stroke volume (volume of blood pumped per beat) increase as a result of adrenaline Adrenaline stimulates the breakdown of glycogen into glucose in the liver cells via enzymes, causing the blood glucose concentration to increase

Question 53

How does adrenaline increase the concentration of blood glucose?

Answer 53

Adrenaline binds to specific receptors on the membrane of liver cells This causes the enzyme adenylyl cyclase to change shape and become activated Active adenylyl cyclase catalyses the conversion of ATP to the second messenger, cyclic AMP (cAMP) cAMP binds to protein kinase A enzymes, activating them Active protein kinase A enzymes activate phosphorylase kinase enzymes by adding phosphate groups to them Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes Active glycogen phosphorylase enzymes catalyse the breakdown of glycogen to glucose This process is known as glycogenolysis The enzyme cascade described above amplifies the original signal from adrenaline and results in the releasing of extra glucose by the liver to increase the blood glucose concentration back to a normal leve

Question 55

Where is the adrenal gland?

Answer 55

At the top of the kidney

Question 56

What is involved in controlling the heart rate?

Answer 56

There is a specific region of the brain that plays a vital role in controlling the heart rate This cardioregulatory centre in the brain is called the medulla The medulla is found at the base of the brain near the top of the spinal cord The medulla is made up of two distinct parts: The acceleratory centre, which causes the heart to speed up The inhibitory centre, which causes the heart to slow down Both centres are connected to the sinoatrial node (SAN) by nerves These specific nerves are different from the nerves that control conscious activities. They make up the autonomic nervous system

Question 57

How does the acceleratory centre control the heart rate?

Answer 57

Aceleratory centre- increased heart rate Once the acceleratory centre has been activated impulses are sent along the sympathetic neurones to the SAN Noradrenaline is secreted at the synapse with the SAN Noradrenaline causes the SAN to increase the frequency of the electrical waves that it produces This results in an increased heart rate

Question 58

How does the inhibitory centre control the heart rate?

Answer 58

Inhibitory centre- heart slows down Once the inhibitory centre has been activated impulses are sent along the parasympathetic neurones to the SAN Acetylcholine is secreted at the synapse with the SAN This neurotransmitter causes the SAN to reduce the frequency of the electrical waves that it produces This reduces the elevated heart rate towards the resting rate

Question 59

How are the acceleratory and inhibitory centres activated?

Answer 59

Question 60

Where are the hormones noradrenaline and adrenaline secreted from and. What do they cause?

Answer 60

-both secreted by the adrenal glands and they both cause an increase in heart rate

Question 61

Where is the hormones thyroxine secreted from and what does it cause?

Answer 61

-thyroxine is produced by the thyroid gland -it causes an increase in heart rate

Question 62

Make a table showing the effect of the sympathetic nerves, parasympathetic nerves, noradrenaline, adrenaline and thyroxine on the heart rate

Answer 62

Question 63

What is a method to investigate the effect of caffeine on the heart rate of humans? What are the limitations

Answer 63

Use the heart rate monitor to record the heart rates of each individual while at rest These results can be used to calculate the mean heart rate of the group as well as the range of resting heart rates for the group prior to caffeine consumption Use the measuring cylinder to pour 200ml of the caffeine solution into a cup (one for each individual) Ask each individual to consume the drink Wait 15 minutes Use the heart rate monitor to record the heart rates of each individual while at rest These results can be used to calculate the mean heart rate of the group as well as the range of resting heart rates for the group after caffeine consumption It is important that the individuals taking part are not moving and all sat down so that these variables are controlled Repeat the heart rate measurements every 15 minutes for 2 hours This will show the duration for which caffeine has an effect on the heart rate Present the results in a graph or table form Limitations Care needs to be taken when selecting the caffeine concentration Too much caffeine can be dangerous, while too little may show no effect It is very difficult to ensure that individuals within a sample group are sufficiently similar By using the same group for the before and after measurements this is not as much of an issue If multiple groups were used, with some consuming caffeine and some not, then this would be a major issue and care would need to be taken to find individuals with a similar life history (same sex, age, weight and height etc.)

Question 64

Describe the meaning of the term tropism (1 mark)

Answer 64

-directional growth in response to a stimulus

Question 65

Give one example of an abiotic stress and the corresponding plant response

Answer 65

Question 66

What are the names of the cell surface membrane, cytoplasm and endoplasmic reticulum in a muscle fibre?

Answer 66

Cell surface membrane = sarcolemma Cytoplasm = sarcoplasm Endoplasmic reticulum = sarcoplasmic reticulum (SR)

Question 67

What is the structure and function of involuntary (smooth) muscle?

Answer 67

-involuntary muscle is smooth muscle that contracts without conscious control -it consists of small elongated cells/ spindle shaped fibres that contain one nucleus -Similar to skeletal muscle it contains both actin and myosin filaments however it does not have any banding or striation -it is controlled by the autonomic nervous system -involuntary muscle is found in the walls of tubular structures such as the digestive system and blood vessels -it contracts slowly and regular;y and does not tire quickly -the muscles is usually arranged in longitudinal and circular layers that oppose each other

Question 68

What is the structure and function of cardiac muscle?

Answer 68

-muscle found in the heart walls -It is myogenic, meaning that it can contract without external stimulation via nerves or hormones. This allows the heart to beat at its own regular intervals (the length of the intervals can be regulated by the nervous system and endocrine system) -It does not tire or fatigue so it can contract (beat) continuously throughout an individuals life -unlike skeletal muscle it has cross-bridges which enable it to contract in a squeezing direction (skeletal muscle can only contract in one direction as in skeletal muscle Myofibrils and filaments all lie longitudinally in the muscle) -The cardiac muscle fibres form a network that spreads through the walls of the atria and ventricles -Cardiac muscle fibres are connected to each other via specialised connections called intercalated discs There is a large number of mitochondria present in the muscle fibres. These are needed to provide the large quantity of ATP needed for continual contractin

Question 69

What is the structure and function of skeletal muscle?

Answer 69

-skeletal (Striated muscle) is muscle under voluntary control -the muscle is arranged in antagonistic pairs -muscle cells form fibres, each fibre is mutinucleate (contains many nuclei) and is surrounded by a sarcolemma -The sarcolemma has many deep tube-like projections that fold in from its outer surface: -These are known as transverse system tubules or T-tubules -These run close to the SR -The sarcoplasm contains mitochondria and myofibrils ->The mitochondria carry out aerobic respiration to generate the ATP required for muscle contraction -Myofibrils are bundles of actin and myosin filaments, which slide past each other during muscle contraction -The membranes of the SR contain protein pumps that transport calcium ions into the lumen of the SR -voluntary muscle contracts quickly and powerfully it also fatigues quickly

Question 70

What are myofibrils?

Answer 70

Myofibrils are located in the sarcoplasm Each myofibril is made up of two types of protein filament: Thick filaments made of myosin (mysosin has more letters so is thicker) Thin filaments made of actin These two types of filament are arranged in a particular order, creating different types of bands and lines

Question 71

Make a table listing the part of the myofibril and its description

Answer 71

Question 72

Answer 72

Question 73

Describe the action of neuromuscular junctions

Answer 73

When an impulse travelling along the axon of a motor neurone arrives at the presynaptic membrane, the action potential causes calcium ions to diffuse into the neurone This stimulates vesicles containing the neurotransmitter acetylcholine (ACh) to fuse with the presynaptic membrane The ACh that is released diffuses across the neuromuscular junction and binds to receptor proteins on the sarcolemma (surface membrane of the muscle fibre cell) This stimulates ion channels in the sarcolemma to open, allowing sodium ions to diffuse in This depolarises the sarcolemma, generating an action potential that passes down the T-tubules towards the centre of the muscle fibre These action potentials cause voltage-gated calcium ion channel proteins in the membranes of the sarcoplasmic reticulum (which lie very close to the T-tubules) to open Calcium ions diffuse out of the sarcoplasmic reticulum (SR) and into the sarcoplasm surrounding the myofibrils Calcium ions bind to troponin molecules, stimulating them to change shape This causes the troponin and tropomyosin proteins to change position on the thin (actin) filaments The myosin-binding sites are exposed to the actin molecules The process of muscle contraction (known as the sliding filament model) can now begin

Question 74

What are the thin filaments within a myofibril made up of?

Answer 74

-the thin filaments within a myofibril are made up of actin molecules ->these are 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 called tropomyosin is twisted around the two actin chain ->another protein called troponin is attached to the actin chains at regular intervals

Question 75

What are the thick filaments within a myofibril made up of?

Answer 75

-they are made up of myosin molecules ->these are fibrous protein molecules with a globular head ->the fibrous part of the myosin molecule 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

Question 76

During muscle contraction sarcomeres within myofibrils __________ as the actin and myosin filaments move past each other The ___ zone get shorter. Therefore the _ lines move closer and the sarcomere gets _________

Answer 76

- shorten -H -Z -shorter

Question 77

Describe the sliding filament of muscular contraction

Answer 77

An action potential arrives at the neuromuscular junction Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm by diffusion Calcium ions bind to troponin molecules, stimulating them to change shape This causes troponin and tropomyosin proteins to change position on the actin filaments Myosin binding sites are exposed on the actin molecules The globular heads of the myosin molecules bind with these sites, forming cross-bridges between the two types of filament The myosin heads bend and pull the actin filaments towards the centre of the sarcomere, causing the muscle to contract a very small distance The movement of the myosin heads is known as the power-stroke When the myosin heads bend, it releases a molecule of ADP ATP binds to the myosin head, allowing it to detach from actin The myosin head acts as an ATPase enzme, hydrolysing ATP into ADP and Pi; the energy released during this reaction allows the myosin head to return to its original position The myosin head can now bind to a new binding site on the actin filaments The myosin heads move again, pulling the actin filaments even closer to the centre of the sarcomere and causing the sarcomere to shorten further As long as troponin and tropomyosin are not blocking the myosin-binding sites and the muscle has a supply of ATP, this process repeats until the muscle is fully contracted

Question 78

What is the role of ATP in muscle contraction?

Answer 78

ATP binding allows myosin to detach from actin and ATP hydrolysis allows the myosin heads to return to their original shape; both of these processes are essential to allow muscle contraction to repeat The return of calcium ions to the sarcoplasmic reticulum occurs via active transport

Question 79

How is the supply of ATP maintained in muscles by creatine phosphate?

Answer 79

-the mitochondria in the muscle fibres are able to respire aerobically and produce ATP but this is slow -creatine phosphate is a molecule stored by muscles that can be used for the rapid production of ATP ->a phosphate ion from creatine phosphate is transferred to ADP - to make ATP and creatine -creatine phosphate allows for muscles to continue contracting for a short period of time until the mitochondria are able to supply ATP -for prolonged activity once the supply of creatine phosphate has been used up then the rate of muscle contraction must equal the rate of ATP production from both aerobic and anaerobic respiration

Question 80

What are some factors which increase the heart rate?

Answer 80

-low pH caused by high CO2 concentration- detected by chemoreceptors located in the carotid arteries, aorta and the brain. The receptors send impulses to the medulla oblongata where the cardiovascular centre is located -stretch receptors respond to muscle movement e.g. during Excercise -decrease in blood pressure -adrenaline

Question 81

What prevents muscle contraction from continuously occurring?

Answer 81

-in order to prevent muscle contraction from being continually stimulated by a single impulse -the acetylcholinesterase enzyme present in the synaptic cleft breaks down the acetylcholine molecules -calcium ions are also pumped back into the SR once the sarcolemma, T tubules and SR are no longer polarised -the movement of calcium ions terminates muscle contraction

Question 82

Compare the cholinergic synapses and neuromuscular junctions

Answer 82

Question 83

Which is skeletal muscle, involuntary muscle and cardiac muscle?

Answer 83

Question 84

In humans, when core body temperature falls below 35degrees Celsius, positive feedback causes this decrease in core body temperature to continue. This process is known as HYPOthermia. Explain how positive feedback could accelerate the process of hypothermia (4marks)

Answer 84

Question 85

Answer 85