A2 Biology 4) CNS June 2024

Control & Coordination

Animals and plants are complex organisms, made up of trillions of cells, where different parts of the
organism perform different functions so it is essential that information can pass between these
different parts, so that their activities are coordinated.
In animals there are two types of information transfer
Ø Electrical impulses that are transmitted through nerves.
Ø Chemical messengers called hormones that travel in the blood.
Coordination in plants also involves
Ø Electrical impulses for fast responses.
Ø Hormones (also known as plant growth regulators) for coordinating slower responses.

Nervous System

The nervous system is made up of the brain & spinal cord, forming the central nervous system (CNS),
and the cranial and spinal nerves, which form the peripheral nervous system (PNS)

There are three types of neurons ( Nerve cell ).

1. Sensory neurons transmit impulses from receptors to the CNS.

2. Intermediate neurons transmit impulses from sensory to motor neurons inside the spinal cord.
3. Motor neurons transmit impulses from the CNS to effectors.

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Structure of a Motor Neuron

1. Cell body ( Soma ) that lies in the spinal cord or brain and contains
• Nucleus.
• Mitochondria.
• Rough endoplasmic reticulum & Ribosomes. ( Nissl’s bodies )
2. Dentrites which are thin short cytoplasmic processes extending from the cell body and they
conduct nerve impulses towards the cell body. ( ↑ SA for synapses )
3. Axon which is a long process extending from the cell body and it conducts nerve impulses away
from the cell body,
The cytoplasm contains mitochondria.
it ends with terminal branches ending with synaptic knobs containing mitochondria & vesicles
containing neurotransmitter.

± Myelin Sheath

is made when Schwann cells wrap themselves around the axon forming a sheath that is made largely
of lipids with some proteins.

The small, uncovered areas of axon between Schwann cells called Nodes of Ranvier. ( 2-3 𝜇m )
They occur about every 1–3 mm in human neurons.
Describe the Structure of a Motor Neuron

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The Reflex Arc

A reflex arc is the pathway along which impulses are transmitted from a receptor to an effector
without involving conscious regions of the brain.

The effector therefore responds to the stimulus before there is any voluntary response involving the
conscious regions of the brain. This type of reaction to a stimulus is called a reflex action.

It is a fast, automatic, innate response to a stimulus, the purpose of which is to avoid danger.

Nerve Impulses

Neurons transmit electrical impulses that travel very rapidly along the cell surface membrane from
one end of the cell to the other.
These impulses are not a flow of electrons like an electric current, rather, the signals are very brief
changes in the distribution of electrical charge across the cell membrane called action potentials,
caused by the very rapid movement of Na+ & K+ ions into and out of the axon.

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Resting Potential

Some axons in some organisms such as squids and earthworms are very wide, where it is possible to
insert tiny electrodes into their cytoplasm to measure the changes in electrical charge.

In a resting axon, it is found that the inside of the axon always has a slightly negative electrical
potential compared with the outside.
The difference between these potentials, “potential difference “is around −60 mV & −70 mV.
In other words, the electrical potential of the inside of the axon is between 60 and 70 mV lower than
the outside. This difference is the resting potential.

1. The resting potential is produced and maintained by the Na+ / K+ pumps in the cell membrane.
Pumping 3 Na+ ions out of the axon & pumping 2 K+ ions in the axon, against their concentration
gradients using ATP.
2. The membrane has Na+ & K+ leaky protein channels which are open all the time that will allow
the diffusion of K+ outward and Na+ inward, but there are more channels for K+ than Na+
therefore the membrane is more permeable for potassium.

resulting in an overall excess negative charge inside the neuron membrane than outside of -70 mv,
and the membrane is said to be polarized.
Describe How the Resting Potential is Produced

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Action Potential

It’s the change in electrical charge distribution across the neuronal membrane upon stimulation.

When the neuron is stimulated, some Na+ channels open, so that Na+ enters the axon & the
potential difference increases from -70 mv, depending on the stimulus, if enough sodium ions enters
so that the potential deference reaches - 50 mv ( threshold potential ), more voltage gated Na+
channels will open to allow more sodium influx into the axon down the electrochemical gradient,
further raising the potential difference up to +30 mv so the membrane is depolarized.

After 1 millisecond these Na+ channels close and K+ voltage gated channels open to allow K+ efflux
down its electrochemical gradient, thus returning the potential difference to the resting potential of
-70 mv, and the membrane is repolarized.

In fact, the potential difference across the membrane briefly becomes even more negative than the
normal resting potential where the membrane is said to be hyperpolarized.
Describe How the Action Potential is Generated
The sodium–potassium pump continues to pump Na+ out and K+ in, and this helps to maintain the
distribution of Na+ & K+ across the membrane so that many more action potentials can occur.

Note 1 if the stimulus is weak thus didn’t open many Na+ channels, the increase in potential
difference will not reach the threshold potential so no action potential will be generated
this is called the All or Nothing Law

Note 2 Na+ influx after the threshold potential is reached is an example of positive feedback.

Note 3 during the hyperpolarized period, the neuron cannot respond to any new stimulus, this
period is called the refractory period.

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Transmission of Action Potential

The temporary depolarization of the membrane where the action potential is, causes a local circuit
to be set up between the depolarized region and the resting regions on either side of it.
Where Na+ moves sideways inside the axon towards the negatively charged region, thus opening the
voltage gated Na+ channels generating action potentials in them.

In myelinated neurons, the action potential travels 50 times faster ( 100 m/s ) than in unmyelinated
neurons, as the myelin sheath speeds up the rate of impulse conduction by insulating the axon.
Where action potentials do not occur in parts of the axon surrounded by myelin sheath, thus can
only occur at the nodes of Ranvier & the local circuits are set up between the nodes so that the
action potentials jump from one node to the next, this is called saltatory conduction.

Describe How the Action Potential is Transmitted

Note 1 In the lab, when an electrical current is applied at a point along the axon, it will create an
action potential at that point in an axon’s cell surface membrane.
That action potential will trigger action potentials in the membrane on both sides of it.

Note 2 In the body, action potentials have a one way transmission that begin at one end and ‘new’
action potentials are generated ahead and not behind, since the regions behind will still be
hyperpolarized so the Na+ voltage-gated channels are unresponsive. ( Refractory period ).

Note 3 Axon’s diameter also affects the speed of conduction where its directly proportional.

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Synapses
Where two neurons meet, they do not quite touch, there is a very small gap, about 20 nm wide,
between them called the synaptic cleft.
The parts of the two neurons near to the cleft, plus the cleft itself, make up a synapse.
Impulses cannot jump across the synapse, instead a neurotransmitter is released from the
presynaptic neuron to stimulate the postsynaptic neuron.

Neurons can be classified according to the type of neurotransmitter they release into mainly
Ø Cholinergic neurons that release acetylcholine ( ACh )
Ø Adrenergic neurons that release noradrenaline.

When action potential arrives at the terminal branches of the presynaptic neuron, it causes the
opening of voltage gated Ca2+ channels leading to Ca2+ influx into the neuron down the
electrochemical gradient, which will trigger the movement of vesicles containing ACh inside the
neuron to move towards the presynaptic membrane & fuse with it, releasing Ach by exocytosis into
the synaptic cleft, where Ach will diffuse towards the postsynaptic membrane. ( 0.5 ms )

When the Ach binds with it’s complementary receptor on the postsynaptic membrane, it opens a
Na+ channel ( Ligand Gated Channel ) allowing the influx of Na+ into the postsynaptic neuron leading
to its depolarization.

ACh is then hydrolyzed by acetylcholinesterase enzyme into acetate & choline.

The choline is taken back into the presynaptic neuron, where it is combined with acetyl coenzyme A
to form ACh once more, which will be then transported into the presynaptic vesicles.

Describe How a Nerve Impulse crosses a Cholinergic Synapse

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Note if ACh remained bound to the postsynaptic membrane, the Na+ channels would remain open
leading to the continuous depolarization of the postsynaptic membrane, thus it must be hydrolyzed
by acetylcholinesterase.
Although synapses slow down the rate of transmission of a nerve impulse since it has to travel along
two or more neurons, but it has some major roles.

1. Synapses ensure one-way transmission.

Ø This is because neurotransmitter is released on one side and its receptors are on the other.
2. Prevent overloading the brain with sensory information.
Ø If the depolarization of the postsynaptic membrane does not reach the threshold, no impulse
is sent in that neuron so that impulses with low frequencies do not travel from sensory
neurons to reach the brain.
3. Synapses allow the interconnection of nerve pathways.
Ø Allowing a wider range of behaviours than could be generated in a nervous system, since
each individual neuron can pass impulses to many neurons.
4. Synapses are involved in memory and learning.
Ø Where new synapses form between the neurons passing the new information.

State the Roles of Synapses

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Receptors
In a lab we can stimulate an action potential in a neuron by applying a small electric current.
In normal life, however, action potentials are generated by a wide variety of stimuli, such as light,
pressure (touch), sound, temperature or chemicals.

A cell that responds to stimuli by initiating an action potential is called a receptor cell.
Receptor cells are transducers where they convert energy in one form such as light, heat or sound
into electrical energy in a neuron.

Receptor Sense Form in which energy is received

rod or cone cells in retina sight light
taste buds on tongue taste chemical potential
olfactory cells in nose smell chemical potential

Some receptors, such as chemoreceptors in the taste buds, are specialized cells which detect a
specific type of stimulus and influence the electrical activity of a sensory neuron.
Other receptors as touch receptors in the skin are simply the ends of the sensory neurons
themselves.

Chemoreceptors

The tongue is covered in many small bumps or papillae, each having many taste buds.
Within each taste bud are between 50 and 100 chemoreceptor cells.
There are different types of chemoreceptors, each type can detect a certain type of chemicals giving
us different taste sensations.
There are five tastes: sweet, sour, salt, bitter and umami (savoury).

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Chemoreceptors in the taste buds that detect salt are directly influenced by sodium ions.

1. Na+ diffuse through selective protein channels in the microvilli.

2. Increasing the + ve charge inside the cell creating a receptor potential.
3. When the receptor potential exceeds the receptor threshold potential, it will stimulate the
opening of voltage gated Ca2+ channels.
4. Causing Ca2+ influx that will stimulate exocytosis of vesicles containing neurotransmitter from the
basal membrane.
5. The neurotransmitter stimulates an action potential in the sensory neuron that transmits
impulses to the taste center in the cerebral cortex of the brain.
Describe How Salt Chemoreceptors generate Action Potentials

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When a stimulus is applied to a receptor, it must have a minimum intensity for the receptor
potential to become large enough to trigger an action potential in the sensory neuron.

The smallest receptor potential required for an action potential to be generated is called the
receptor threshold potential.

Weaker stimulus ( Subthreshold ) can cause only local depolarization in the receptor cell without
evoking an action potential in the sensory neuron.

Further increase in stimulus intensity above threshold, doesn’t give a larger action potential with
higher amplitude, instead stronger stimuli will produce
• More frequent action potentials.
• Stimulate more sensory neurons.

Note 1 The brain interpret the frequency of action potentials arriving along the axon of a sensory
neuron, and the number of neurons carrying action potentials, to get information about the strength
of the stimulus being detected.

Note 2 The nature of the stimulus, whether it is light, heat, touch or so on, is deduced from the
position of the sensory neuron bringing the information.

Note 3Threshold levels in receptors rarely stay constant all the time. With continued stimulation,
they often increase so that it requires a greater stimulus before receptors send impulses along
sensory neurons.

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Skeletal Muscles

A muscle such as a biceps is made up of thousands of muscle fibers.

The cytoplasm of the muscle fiber is known as Sarcoplasm which contains

1. Many nuclei ( Multinucleated ) thus called a syncytium.
2. Many cylindrical structures called Myofibrils that extend along the complete length of the muscle
fiber.
3. Many mitochondria packed between the myofibrils.
4. Many endoplasmic reticulum known as Sarcoplasmic reticulum ( SR )
which form a network around myofibrils, the membranes of which have a large number of
Ca2+ pumps & channels.

The cell surface membrane of muscle fiber is known as Sarcolemma which has
1. Protein receptors for ACh & Na+ channels.
2. Membrane infoldings which run close to the SR called T – tubules.

Describe the Structure of Skeletal Muscle Fiber

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Structure of Myofibrils

The myofibril is made of parallel groups of thick protein filaments lying between thin filaments.
The thick filaments are made of myosin protein, while the thin filaments are made of actin protein.
The arrangement of these filaments gives the myofibril the appearance of striations, consisting of

“I” band → the lighter bands made of thin actin filaments only.
“A” band → the darker bands made of thick myosin filaments with some overlap with actin.
“H” band → the part of the “A” band that is made of thick myosin filaments only.
“Z” line → central protein that provides attachment for actin filaments.
“M” line → central protein that provides attachment for myosin filaments.

The part between two “Z” lines is called a Sarcomere.

Myosin is a fibrous protein with globular heads pointing away from the “M” line, each head has an
ATPase enzyme that hydrolyzes ATP to release energy.

Actin is globular protein made of two chains that are twisted together to form the thin filament.
Also twisted around the actin chains is a fibrous protein called tropomyosin that blocks the myosin
binding sites.
Another protein, troponin, is attached to the actin chain at regular intervals

Describe the Structure of Myofibrils

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Sliding Filament Theory

In presence of Ca2+ & ATP the muscle will contract, where the sarcomeres get shorter.

Ø Calcium ions
bind to troponin of the thin filament, inducing a change in its shape which causes the tropomyosin to
move, exposing the myosin binding sites on the actin filaments.

Ø ATP
1. Hydrolyzed by the myosin heads by the ATPase enzyme so that myosin heads bind to actin
forming cross bridges.
2. Then the myosin heads tilt, pulling the actin filaments towards the “M” line, ADP & Pi detach
during the power stroke.
3. A new ATP molecule binds to the myosin head leading to the detachment from the actin, and the
myosin head returns to the starting position.

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Muscle Contraction

Skeletal muscles contract when they receive an impulse from a motor neuron since its neurogenic.

Describe the Mechanism of Muscle Contraction

When there is no longer any stimulation from the motor neuron, there are no impulses conducted
along the T-tubules.
Calcium pumps move Ca2+ back into stores in the sarcoplasmic reticulum.
As Ca2+ leave their binding sites on troponin, tropomyosin will cover the myosin binding sites on the
thin filaments.

When there are no cross-bridges between thick and thin filaments, the muscle is in a relaxed state.
There is nothing to hold the filaments together so any pulling force applied to the muscle will
lengthen the sarcomeres so that they are ready to contract (and shorten) again.

Note Each skeletal muscle in the body has an antagonist – a muscle that restores sarcomeres to their
original lengths when it contracts. For example, the triceps is the antagonist of the biceps.

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A contracting muscle uses a lot of ATP. The very small quantity of ATP in the muscle fibers in a
resting muscle is used up rapidly once the muscle starts to contract.

More ATP is produced by respiration – both aerobic respiration inside the mitochondria and, when
that cannot supply ATP fast enough, also by lactic fermentation in the sarcoplasm.

Muscles also have another source of ATP, produced from a substance called creatine phosphate.
They keep stores of this substance in their sarcoplasm.
A phosphate group can quickly and easily be removed from each creatine phosphate molecule and
combined with ADP to produce more ATP, when they are used up during contraction:

creatine phosphate + ADP → creatine + ATP

Later, when the demand for energy has slowed down or stopped, ATP molecules produced by
respiration can be used to ‘recharge’ the creatine:

creatine + ATP → creatine phosphate + ADP

However, if energy is still being demanded by the muscles and there is no ATP spare to regenerate
the creatine phosphate, the creatine is converted to creatinine and excreted in urine.

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Glands
A gland is a group of cells which synthesizes & secretes 1 or more substances.
• Exocrine gland
They secrete their substances ( not hormones ) into a duct.
Like salivary glands that secrete saliva into the mouth through salivary ducts.

• Endocrine gland
Ductless glands that secrete hormones directly into blood capillaries inside the gland.
Like adrenal glands that secrete adrenaline directly into blood.

Hormones

They are cell signaling molecules that are secreted by endocrine glands into the blood, where they
travel to reach their target cells with specific complementary receptors.

Type of Hormone Solubility Site of Receptor Examples

Insulin
Protein Water Soluble On cell surface membrane Glucagon
ADH
Estrogen
Steroid Lipid Soluble Inside the cytoplasm or nucleus Progesterone
Testosterone

Note Hormones have short life in the body as they are broken down by enzymes in the blood or
cells, or lost in the urine.

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Electrical Communication in Plants

Plant cells have electrochemical gradients across their cell surface membranes as animal cells.
They have resting potentials, which can get depolarized after a certain stimulus has been applied.
Plant action potentials spread from cell to cell along the plasma membrane through plasmodesmata.

The Venus Fly Trap

It’s a carnivorous plant that obtains nitrogenous compounds by trapping & digesting insects.

The insect trap is a specialized leaf which is divided into two convex lobes, with a midrib in between.
The outer edges of the lobes have stiff hairs that interlock to trap the insect inside.
The inside of each lobe is red and has nectar-secreting glands to attract insects.
The surface of the lobes has enzyme secreting glands for the digestion of trapped insects.
Each lobe has 3 stiff sensory hairs that when deflected, the leaf responds by closing the trap.

Describe the Structural Adaptations of a Venus Fly Trap

The deflection of sensory hairs (trigger hairs) opens Ca2+ channels in cells at the base of the hair
leading to calcium influx generating a receptor potential.
- If two hairs are stimulated within 20 to 35 seconds.
- or one hair is touched twice within 20 to 35 seconds.
→ The action potential travels across cells shutting close the trap within 0.5 seconds & changing the
shape of the lobes into concave, trapping the insect.

The continuous stimulation of the sensory hair by the trapped insect results in further calcium influx
into gland cells, stimulating the exocytosis of enzyme containing vesicles to digest the insect.

Describe the Action of a Venus Fly Trap

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Venus fly traps have two adaptations to avoid closing unnecessarily and wasting energy.
1. The stimulation of a single hair does not trigger closure, this prevents the traps closing when it
rains or when a piece of debris falls into the trap.
2. The large gaps between the stiff hairs that form the ‘bars’ of the trap allow very small insects to
crawl out, the plant would waste energy digesting a very small ‘meal’.

Chemical Communication in Plants

Plant hormones or plant growth regulators like Abscisic acid, Auxins & Gibberellins are chemicals
produced in plants that affect their growth & responses to environmental conditions.
They are produced in small quantities, and can move through the plant from site of synthesis
1. From cell to cell directly by diffusion or active transport.
2. Transported in the phloem or xylem.
Some hormones have different effects
• in different tissues.
• in different species.
• at different stages of plant development.
• when they are present in different concentrations.
Some plant hormones may interact together either synergistically or antagonistically.

State the General Features of Plant Hormones

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Auxins

Are a group of several chemicals made by the plant, of which the principal one is indole 3-acetic acid
which we refer to as ‘auxin’.
Auxin is synthesized in the growing tips (meristems) of shoots and roots, where the cells are dividing.
It is transported back down the shoot, or up the root, by
1. active transport from cell to cell.
2. in phloem sap to a lesser extent.

Note At meristems, such as those at shoot tips and root tips, Growth occurs in three stages:
1) Cell division by mitosis. 2) Cell elongation by absorption of water. 3) Cell differentiation.

Auxin controls growth by stimulating cell elongation.

Note Auxin stimulates the growth of the tips & inhibits the growth of the side stems.

Auxin binds to a receptor protein on the cell surface membrane, which stimulates
1) Opening of K+ channels, that allow K+ influx, decreasing the water potential inside the cell so
water moves in by osmosis down water potential gradient, increasing the turgor pressure on the
cell wall in order to stretch it.
2) ATPase proton pumps to pump H+ from the cytoplasm into the cell wall, lowering its PH
→ Acidic PH activates Expansins proteins that loosen the non covalent interactions between
cellulose microfibrils & cell wall matrix, allowing microfibrils to slide over each other.

Explain the Acid Growth Hypothesis

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Gibberellins

They are plant growth regulators that are synthesized in most parts of plants.
They are present in high concentrations in young leaves, seeds & stems.

Role of Gibberellins in seed germination

When the seed is shed from the parent plant, it is in a state of dormancy, as it contains very little
water and is metabolically inactive.

Note This is useful because it allows the seed to survive in adverse conditions, such as through a cold
winter, only germinating when the temperature rises in spring.

The seed contains

1. Embryo, which will grow to form the new plant.
2. Endosperm, which is an energy store containing starch.
3. Aleurone layer, which is a protein rich layer.
4. Testa, which is a tough, waterproof, protective layer surrounding the seed.

During germination, the seed absorbs water which stimulates the production of gibberellins by the
embryo.
Gibberellins diffuse to the aleurone layer where it stimulates transcription of the genes coding for
amylase. (by destroying the DELLA proteins)
Amylase hydrolyzes starch in the endosperm into maltose, which is converted into glucose.
Glucose diffuses to the embryo, where it is respired to provide energy needed for embryo growth.

Explain the Role of Gibberellins in Germination of Wheat & Barely Seeds

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Role of Gibberellins in stem elongation

The height of plants is controlled by their genes.

For example, tallness in pea plants is affected by a gene with two alleles
Ø A dominant allele, Le, when present, the plants can grow tall.
Ø A recessive allele, le, where homozygous plants (lele) always remain short. ”Dwarf Plants“

The dominant allele of this gene regulates the synthesis of the last enzyme in a pathway that
produces an active form of gibberellin ( GA1 ).
Active gibberellin stimulates cell division and cell elongation in the stem.

A substitution mutation in this gene gives rise to a change from alanine to threonine in the primary
structure of the enzyme near its active site, producing a non-functional enzyme.
This mutation has given rise to the recessive allele ( le ).

Note Applying active gibberellin to plants which would normally remain short, such as cabbages, can
stimulate them to grow tall.

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 Cambridge International AS & A Level Biology 9700 syllabus for 2022, 2023 and 2024. Subject content
A2 Biology 4) CNS June 2024

15 Control and coordination

All the activities of multicellular organisms require coordinating, some very rapidly and some more slowly. The
nervous system and the endocrine system provide coordination in mammals. Coordination systems also exist in
plants.
15.1 Control and coordination in Learning outcomes
mammals Candidates should be able to:
1 describe the features of the endocrine system with reference to
the hormones ADH, glucagon and insulin (see 14.1.8, 14.1.9 and
14.1.10)
2 compare the features of the nervous system and the endocrine
system
3 describe the structure and function of a sensory neurone and a
motor neurone and state that intermediate neurones connect
sensory neurones and motor neurones
4 outline the role of sensory receptor cells in detecting stimuli and
stimulating the transmission of impulses in sensory neurones
5 describe the sequence of events that results in an action
potential in a sensory neurone, using a chemoreceptor cell in a
human taste bud as an example
6 describe and explain changes to the membrane potential of
neurones, including:
• how the resting potential is maintained
• the events that occur during an action potential
• how the resting potential is restored during the refractory
period
7 describe and explain the rapid transmission of an impulse in a
myelinated neurone with reference to saltatory conduction
8 explain the importance of the refractory period in determining
the frequency of impulses
9 describe the structure of a cholinergic synapse and explain how
it functions, including the role of calcium ions
10 describe the roles of neuromuscular junctions, the T-tubule
system and sarcoplasmic reticulum in stimulating contraction in
striated muscle
11 describe the ultrastructure of striated muscle with reference to
sarcomere structure using electron micrographs and diagrams
12 explain the sliding filament model of muscular contraction
including the roles of troponin, tropomyosin, calcium ions and
ATP
15.2 Control and coordination in Learning outcomes
plants Candidates should be able to:
1 describe the rapid response of the Venus fly trap to stimulation
of hairs on the lobes of modified leaves and explain how the
closure of the trap is achieved
2 explain the role of auxin in elongation growth by stimulating
proton pumping to acidify cell walls
3 describe the role of gibberellin in the germination of barley (see
16.3.4)

Back to contents page www.cambridgeinternational.org/alevel 35

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