Reflexes

Question 1

How do emergent properties arise?

Answer 1

1. tissuescells within multicellular organisms are specialised to perform specific functions. One cell by itself cannot usually carry out its function on a large enough scale to meet the demands of the organism. Instead, cells are part of tissues. A tissue is a group of cells that carry out a function together. There may be only one cell type, or several cell types that are specialised for different aspects of the tissues function
2. An organ is a group of tissues in an animal or plant that work together to carry out a specific function of life
3. Body systems are groups of organs working together

(interactions between all levels give rise to emergent properties)

Question 2

Hormonal signalling vs nervous signalling

Answer 2

Hormonal:
chemical
in the blood
widespread, but only certain target cells respond
target cells in any tissue
growth
development
reproduction
changes to metabolic rate
changes to solute concentration in blood
mood
slower
long duration- until hormone breaks down

Nervous:
electrical (nerve impulses)
in neurons
signal passes only to specific cells via synapses
muscles or glands
contraction of striated muscles used in locomotion
contraction of smooth muscles
change to the rate of cardiac muscle contraction
secretion by glands
very rapid
short duration- unless summation occurs

Question 3

What is the role of the brain?

Answer 3

The brain is the central integrating organ of our body. It receives information, processes it, stores some of it and sends instructions to all parts of the body to coordinate life processes. Information is received from sensory receptors, both in specialised sense organs such as the eye and also from receptor cells in other organs, such as pressure receptors in blood vessels. The brain can store information, for the short term or longer term. The capacity to store information is called memory and it is essential for learning. Processing of information leads to decision making by the brain. This may result in signals being sent to muscles or glands. Which cause these organs to carry out a response.

Question 4

How does the spinal cord coordinate unconscious processes?

Answer 4

The spinal cord is located inside the vertebral column. The area of tissue in the centre of the spinal cord is the grey matter. It acts as an integrating centre for unconscious processes.

Neurons bring information to the grey matter from the brain and sense organs. Motor neurons convey signals from the grey matter to muscles and glands. Interneurons pass impulses via synapses between neurons in the grey matter. The pattern of neurons and synapses determines how information is processed in the grey matter and what decisions are made.

The spinal chord only coordinates unconscious processes, especially reflexes. It can do this more quickly than if signals were conveyed to and from the brain.

Question 5

Conscious processes vs unconscious processes

Answer 5

Unconscious:
Can happen when asleep
Involuntary
Controlled by brain and spinal cord
glands and smooth muscle are controlled involuntarily

Conscious processes:
Only happen when awake
Voluntary
controlled by cerebral hemispheres of the brain
Striated muscle is controlled voluntarily

Question 6

How do we respond do stimuli?

Answer 6

Receptor cells, located in the skin and sense organs, detect changes in the external environment. For example, rod and cone cells in the retina of the eye detect light. Receptor cells then pass impulses to sensory neurons. Nerve endings of some sensory neurons. Nerve endings of some sensory neurons act as receptors for touch and heat, without the need for a separate receptor cell. There are also receptor cells inside the body that monitor internal conditions. Stretch receptors in striated muscle sense the state of contraction, allowing the brain to deduce the posture of the body. Stretch receptors in the walls of arteries give a measure of blood pressure. Chemoreceptors in the walls of blood vessels monitor concentrations of oxygen, carbon dioxide and glucose.

Question 7

What are nerves?

Answer 7

A nerve is a bundle of nerve fibres enclosed in a protective sheath. Nerves vary in size depending on the number of nerve fibres and how many of them are myelinated.

• the widest is the sciatic nerve (20mm across)
• the optic nerve contains up to 1.7 million nerve fibres
• small nerves may contain fewer than a hundred fibres

Most nerves contain nerve fibres of both sensory and motor neurons. However, some contain only sensory neurons and some contain only motor neurons

All organs of the body are served by one or more nerves.

Question 8

What is the reflex arc?

Answer 8

1. Receptor cells perceive the stimulus. Pain and heat are perceived directly by nerve endings of sensory neurons in the skin, so there is no need for a separate receptor cell. This is how the stimulus is perceived when the hand touches the hot object.
2. Sensory neurons receive signals, either from receptor cells or from their own sensory nerve endings and pass them as nerve impulses to the brain or spinal cord, via long axons. These axons end at synapses with interneurons in the grey matter area of the spinal cord or brain.
3. Interneurons are located in the grey matter of the brain and spinal cord. They have many branched fibres called dendrites, along which nerve impulses travel. Interneurons process signals brought by sensory neurons and make decisions about appropriate responses. They do this by combining impulses from multiple inputs and then passing impulses to specific other neurons. The decision-making process in a reflex action is very simple because there may be only one interneuron connecting a specific sensory neuron to the motor neuron that can cause an appropriate response.
4. Motor neurons receive signals via synapses with interneurons. If a threshold potential is achieved in a motor neuron, an impulse is passed along the axon which leads out of the CNS to an effector. The axon does not change its position or connections, so the impulse always travels to the same effector cells.
5. Effectors carry out the response to a stimulus when they receive the signal from a motor neuron. The two types of effector are muscle and glands. When the hand touched a hot object, the effectors are muscles which flex the arm at the elbow, pulling it away.

Question 9

What is the role of the cerebellum?

Answer 9

The cerebellum is a part of the brain. The cerebellum has important roles in the control of skeletal muscle contraction and balance. It does not make decisions about which muscles will contract, but it fine-tunes the timing of contractions. It allows very precise coordination of movements and helps us to maintain posture, for example when we are standing. It also helps us with activities requiring motor memory, such as riding a bike or typing on a keyboard.

Question 10

The role of melatonin in regulating sleep patterns

Answer 10

In a circadian rhythm, something happens once per 24 hours. There is a circadian rhythm in human behaviour with diurnal and nocturnal phases. The rhythm is set by groups of cells in the hypothalamus called the suprachiasmatic nuclei (SCN). They control the secretion of the hormone melatonin by the pineal gland. Melatonin secretion increases in the evening and drops to a low level at dawn. The hormone is rapidly removed from the blood by the liver, so blood concentrations rise and fall rapidly in response to rate of secretion.

The most obvious effect of melatonin is the sleep-wake cycle. High melatonin levels cause feelings of drowsiness and promote sleep throughout the night. Falling melatonin levels encourage waking at the end of the night.

A special type of ganglion cell in the retina of the eye detects light of wavelength 460-480nm and passes impulses to the cells in the SCN. The signals to the SCN the timing of dusk and dawn and allows it to adjust melatonin secretion so that it corresponds to the day-night cycle.

Question 11

Structure of a neuron

Answer 11

Neurons have a cell body with cytoplasm and a nucleus but they also have narrow outgrowths called nerve fibres along which nerve impulses travel. There are two types of nerve fibre:

• dendrites are short-branched nerve fibres, for example those used to transmit impulses between neurons in one part of the brain or spinal chord
• axons are very elongated nerve fibres, for example, those that transmit impulses from the tips of the toes or fingers to the spinal cord.

Question 12

What is the resting potential of a neuron?

Answer 12

The plasma membrane of a cell restricts ion movements, allowing concentration gradients to be maintained between inside and outside.

Differences in the concentrations of positively charged and negatively charged ions and an overall imbalance in charge cause a potential difference (voltage) across the membrane. Generally, the inside of cells is electrically negative compared with the outside, so the membrane potential, measured in millivolts, is a negative value.

Neurons typically have a membrane potential of -70mV while waiting to transmit an impulse. This is called the resting potential, though energy has to be expended to maintain it.

Sodium-potassium pumps transfer 3 Na+ ions across the membrane out of the neuron and at the same time transfer 2 K+ ions in. As this is active transport, it uses energy from ATP and reestablishes concentration gradients for both ions.

• There are negatively charged proteins inside the neuron which also adds to the charge imbalance

Question 13

What happens in an action potential?

Answer 13

1. Depolarisation

Sodium channels in the membrane open, allowing Na+ ions to diffuse into the neuron down the concentration gradient via facilitated diffusion. Entry of Na+ ions reverses the charge imbalance of the membrane, so the inside becomes positive relative to the outside. The rise is from roughly -70mV to +40mV

1. Repolarisation

This happens immediately after depolarisation and is due to the closing of the sodium channels and opening of potassium channels. Potassium ions diffuse out of the neuron down their concentration gradient, so the inside of the neuron becomes negative again relative to the outside. The potassium channels remain open until the membrane potential has fallen to -80mV

1. Hyperpolarisation

Due to a lag in closing the potassium ion channels, the membrane potential overshoots resting potential and becomes more negative. The sodium-potassium pumps then reestablish the membrane potential and their respective concentration gradients.

N.b. This can only happen if the resting potential has been met/ exceeded (all or nothing principle)

Question 14

What is a synapse?

Answer 14

A synapse is a junction between two cells in the nervous system. The area between two synapses is called the synaptic cleft

Question 15

What happens when an action potential reaches the presynaptic membrane?

Answer 15

Synaptic transmission is the sequence of events between the arrival of an impulse at the presynaptic membrane and the initiation of an impulse in the postsynaptic membrane. The first stages of synaptic transmission result in release of neurotransmitter. Arrival of a nerve impulse in the transmitting neuron causes depolarisation of the presynaptic membrane. This causes depolarisation of the presynaptic membrane. This causes calcium ion channels to open and as the concentration of Ca2+ is higher outside the presynaptic membrane, they diffuse in. Influx of Ca2+ causes vesicles containing neurotransmitter to move to the presynaptic membrane and fuse with it, releasing neurotransmitter into the synaptic cleft by exocytosis.

Question 16

What can change velocity of an action potential?

Answer 16

Axons are circular in transverse cross section with a plasma membrane enclosing cytoplasm. In humans the diameter in most cases is about 1 micrometer. Nerve impulses travel along axons with this basic structure at a speed of about 1m/s. Two features can increase speed of nerve impulses significantly:

1. larger axon diameter- less resistance, therefore faster conduction of nerve impulses
2. myelination- allows saltatory conduction (jump from one node of ranvier to another)

Question 17

what occurs in excitatory synapses?

Answer 17

Release of neurotransmitter from the presynaptic membrane triggers the stages of synaptic transmission that culminate in an action potential in the postsynaptic membrane

• molecules of neurotransmitter diffuse across the synaptic cleft. this happens extremely rapidly because the distance is so short
• the neurotransmitter binds to receptors in the postsynaptic membrane. the binding causes Na+ channels to open. In many cases the receptor itself acts as the ion channel
• Na+ ions diffuse down their concentration gradient across the postsynaptic membrane into the receiving neuron, causing the membrane potential to become less negative.
• if the potential rises from -70mV to -50mV, it triggers an action potential.
• a change in potential that is large enough to stimulate an action potential is an excitatory postsynaptic potential.

Acetylcholine is a widely used neurotransmitter at synapses between neurons. It is also used at synapses between neurons and muscle fibres. Acetylcholine is synthesised from choline and acetyl groups in the transmitting neuron. It binds to a receptor in the postsynaptic membrane which also acts as the channel for Na+ ions. Acetylcholine is broken down in the synaptic gap by enzyme acetylcholinesterase.

Question 18

all living organisms are adapted for movement

Answer 18

Movement is one of the functions of life and all organisms have adaptations for it.

• in every organism there are internal movements such as peristalsis in the gut, or ventilation of the lungs. there are movements in the cytoplasm of unicellular organisms
• motile organisms move their entire body from one place to another. this is locomotion, with each motile organism adapted to their own method of locomotion
• sessile organsisms remain in a fixed position. most plants are sessile, with roots fixed in the soil

Question 19

How are action potentials propagated?

Answer 19

A nerve impulse is an action potential that travels from one end of an axon to the other. This movement happens because an action potential in one part of an axon triggers an action potential in the next part, which is called propogation of the nerve impulse. It is due to local currents.

Local currents are movements of Na+ ions by diffusion between one part of an axon that has depolarized and the adjacent part that is still polarized. Depolarization is due to an influx of Na+ ions, which increases the Na+ concentration inside the axon and reduces it outside.
This causes sodium ions to diffuse along the axon, both inside and outside the membrane. Diffusion inside the axon is from the depolarized to polarized parts and outside the axon it is in the opposite direction.
These local currents make the potential in the region that is still polarized less negative. If the potential across the membrane rises from -70 mV to the threshold potential of -50mV, voltage gated sodium ion channels start to open, triggering an action potential.

Question 20

Depolarisation and repolarisation

Answer 20

Depolarization (opening of voltage-gated Na+ channels)
Sodium channels start to open if membrane potential rises from the resting potential of -70 mV to the threshold potential of -50 mV. Sodium ions diffuse into the axon through pores that have opened, raising the membrane potential and causing yet more Na+ channels to open. This is an example of positive feedback and results in the very rapid rise in membrane potential. Sodium channels only remain open for one to two milliseconds before closing again, but in this time enough Na+ ions diffuse inwards for the membrane to become depolarized, with the potential typically rising from a negative value of -50 mV to a positive potential of +30 to +40 mV.

Repolarization (opening of voltage-gated K+ channels)
The positive membrane potential that develops during depolarization causes voltage-gated potassium channels to open.
As with sodium channels, the opening only persists for one to two milliseconds, before the channels close. Even in this short time, enough K+ ions diffuse out of the axon to repolarize the axon. The membrane potential returns to -70 mV and may briefly overshoe by becoming more negative than this, before the sodium-potassium pump re-establishes concentration gradients.

Question 21

What is saltatory conduction?

Answer 21

Action potentials only occur at nodes of ranvier (gaps between schwann cells). Instead of being propogated continuously along the axon, as in unmyelinated axons, the nerve impulse jumps from one node of ranvier to the next in myelinated nerve fibres.

This is saltatory conduction. It speeds up propogation of the nerve impulse greatly.

Question 22

What are exogenous chemicals?

Answer 22

Exogenous chemicals come from outside the body. They can enter through the skin, the lungs or the gut. They can also be injected. Pesticides and drugs are two groups of exogenous chemicals. In both groups there are chemicals that affect synaptic transmission, either by blocking or promoting it, such as neonicotinoids and cocaine.

Question 23

What are the actions of neonicotinoid pesticide?

Answer 23

Neonicotinoid pesticides are synthetic compounds similar to nicotine. They bind to acetylcholine receptors in cholinergic synapses in the central nervous system of insects, causing the Na+ channel in the receptor to open. Acetylcholinesterase does not break down neonicotine, so the binding is irreversible and the Na+ ion channel remains open. An excess of Na+ enters the receiving neuron, overstimulating it and blocking normal synaptic transmission. The consequence in insects is paralysis and death. Neonicotinoids are therefore very effective insecticides, but they can cause harm to bees and other insects with important roles in ecosystems or in agriculture

Question 24

What is the action of cocaine?

Answer 24

Cocaine acts at synapses that use dopamine as a neurotransmitter. It binds to dopamine reuptake transporters, which are the membrane proteins that pump dopamine back into the presynaptic neuron Because cocaine blocks these transporters, dopamine builds up in the synaptic cleft and the postsynaptic neuron is continuously excited. Cocaine is therefore an excitatory or stimulant psychoactive drug that gives feelings of euphoria that are unrelated to a reward activity such as eating.

Question 25

What is the perception of pain?

Answer 25

Pain receptors in the skin and other parts of the body detect stimuli such as the chemical substances in a bee’s sting, excessive heat or the puncturing of skin by a hypodermic needle. These receptors are the endings of sensory neurons that convey impulses to the central nervous system.
The nerve endings associated with pain receptors have channels for positively charged ions, which open in response to a stimulus such as high temperature, acid, or certain chemicals such as capsaicin in chili peppers.
Entry of positively charged ions causes the threshold potential to be reached and nerve impulses then pass through the sensory neuron to the spinal cord. Interneurons in the spinal cord relay the impulse to the cerebral cortex.

When impulses reach sensory areas of the cerebral cortex, the sensation of pain is perceived. Signals are transmitted to the prefrontal cortex allowing our perception of pain and the environment around you.

Question 26

What is inhibitory neurotransmission?

Answer 26

Many neurotransmitters stimulate action potentials in the postsynaptic membrane, but some have the opposite effect. They make the membrane potential more negative (lower than - 70 mV) when they bind to receptors in the postsynaptic membrane. This hyperpolarization makes it more difficult for the postsynaptic neuron to reach the threshold potential, so nerve impulses are inhibited, rather than stimulated. GABA (gamma-amino butyric acid) is an example of an inhibitory neurotransmitter. When it binds to its receptor, a chloride channel opens, causing hyperpolarization of the postsynaptic neuron by an influx of Cl- ions to the receiving neuron. A transient lowering of membrane potential caused by neurotransmitters such as GABA and acetylcholine is known as an inhibitory postsynaptic potential.

Question 27

Excitatory vs inhibitory neurotransmitters

Answer 27

excitatory:
channel for positively charged ions (usually Na+)
rises from -70mV, so becomes less negative
action potential

inhibitory
for negatively charged ions
falls from -70 (hyperpolarisation)
inhibits action potential

Question 28

What is summation?

Answer 28

A single neuron can form synapses with many other neurons. A receiving (postsynaptic) neuron may receive neurotransmitter from many transmitting (presynaptic) neurons. Some neurons in the brain have synapses with hundreds or even thousands of transmitting neurons.
The neurotransmitter at each specific synapse is always the same and may be excitatory or inhibitory.

Question 29

What is spatial summation?

Answer 29

This occurs when there are multiple presynaptic neurons.
When neuron A and neuron B release a large amount of neurotransmitter. This leads to a large enough influx of Na+ ions to meet and exceed the threshold potential. Inducing an action potential

Question 30

What is temporal summation?

Answer 30

High frequency of action potentials from neuron A leads to a summative effect, so there is a large enough amount of neurotransmitter in the synaptic cleft to open ligand gated ion channels, so large influx of Na+ by facilitated diffusion, so the membrane potential meets and exceeds the threshold potential, stimulating an action potential in the postsynaptic neuron

Question 31

What is consciousness?

Answer 31

If we are conscious of something, we are aware of it. We do not have to be actively thinking about something to be aware of it, so we can be simultaneously aware of many things. This state of complex awareness is known as consciousness. There is agreement that it exists, but philosophers and scientists have not reached agreement on how to define it.

Sleep is a state of reduced or partial consciousness. General anaesthetics, used during surgery, make us unconscious but the mechanisms of action of these drugs are not well understood, so they do not reveal much about the physiological basis of consciousness. Perhaps the most that we can say with certainty is that consciousness is a property that emerges from the interaction of individual neurons in the brain. Consciousness is thus an example of an emergent property.
Emergent properties are the result of interactions between the elements of a system. When we recognize that a system is more than the sum of its parts, we are acknowledging the existence of emergent properties.
Two biological examples are the catalytic activity of enzymes and flight in birds.

Question 32

What is the structure of a sarcomere? What does it look like under contraction?

Answer 32

Two types of protein filament are arranged in a regular pattern with sarcomeres- thin actin filaments and thick myosin filaments. The dark band in the centre of a sarcomere contains many parallel myosin filaments. The ends of these myosin filaments overlap with six equidistant actin filaments. The light bands at the ends of sarcomeres contain actin filaments but not myosin. Each actin filament is attached to a Z-disc at one end and overlaps with myosin filaments at the other end.
(Contraction is due to actin being pulled towards centre of sarcomere by myosin)

(Dark band remains the same length during contraction, but light bands shorten)

The contraction of sarcomeres is due to the sliding of actin and myosin filaments. Myosin has heads that can attach to the binding sites on actin.

Question 33

What is muscle contraction (the sliding-filament model)?

Answer 33

The contraction of sarcomeres is due to the sliding of actin and myosin filaments. Myosin has heads that can attach to the binding sites on actin.

Without electrical stimulation, the myosin binding sites on actin filaments are blocked by a thin and fibrous protein subunit called tropomyosin. Tropomyosin runs alongside the actin filament, preventing the binding of myosin to actin, and hence preventing contraction of the muscle.

Tropomyosin is associated with another protein subunit called troponin. When a muscle is stimulated by a motor neuron, calcium ions are released from the sarcoplasmic reticulum (a specialised endoplasmic reticulum that is found in the sarcoplasm of muscle) of muscle cells. These calcium ions bind to troponin, causing it to undergo a conformational change. As a result of this conformational change, troponin moves tropomyosin away from the myosin binding sites, allowing myosin to bind to actin.

ATP is required in muscle contraction. It binds to the myosin head, causing it to detach from its binding site on an actin filament. ATP is hydrolysed into ADP and Pi and the myosin head changes position, moving towards the next myosin binding site on the actin filament. Once in this position, the myosin head then attaches to the new binding site, forming a cross bridge. The myosin head then returns to its original position, pulling the actin filament towards the centre of the sarcomere in a process called the power stroke. This cycle is repeated when another ATP molecule binds to the myosin head, allowing it to detach from the actin filament.

Question 34

How do muscles relax?

Answer 34

When muscles relax, potential energy is stored by titin, an elastic protein that has the largest polypeptides so far discovered (over 34,000 amino acids long in humans). Titin releases potential energy when it recoils during muscle contractions. This increases the amount of force that muscles can exert. Titin has two other roles. It connects the end of myosin filaments in sarcomeres to the Z disc and holds each myosin filament in the correct position in the centre of six parallel actin filaments. It also prevents overstretching of the sarcomere.

Energy is needed to stretch titin and therefore to lengthen a muscle. Lengthening of muscles happen when they relax. Muscles can only exert force when they contract, so a muscle cannot supply the energy it needs to lengthen. The energy has to be provided by another muscle that is known as the antagonist. Despite the name, an antagonistic pair of muscles work together, with the contraction of each member of the pair providing the energy needed to lengthen the titin molecules in the other as it relaxes

Question 35

What is quorum sensing?

Answer 35

Unicellular organisms can send chemical signals to each other. This can help to adjust activity to population density. A chemical signal is secreted at a low rate by all cells in the population and diffuses freely between cells. Each cell has receptors to which the signalling molecules bind. If the proportion of receptors in a cell with signalling molecules bound to them rises above a threshold, indicating high population density, changes in the activity of the cell are triggered. This type of control is quorum sensing.

Question 36

Example of quorum sensing: vibrio fischeri

Answer 36

Vibrio fisheri cells secrete a chemical signal, which acts as an autoinducer. The autoinducer is free to diffuse between cells. It binds to a receptor protein (LuxR) in the cytoplasm. LuxR-autoinducer complexes bind to a specific position in the cell’s DNA that induces the transcription of genes coding for the production of luciferase. This enzyme catalyses an oxidation reaction that releases energy in the form of bioluminescence. Light is not emitted in a low population density of free living Vibrio fidcheri because the concentration of autoinducer is low. The amount of light emitted would be insignificant and energy would be wasted.

Question 37

Types of signalling groups

Answer 37

1. Hormones
   - produced by groups of specialised cells in glands
   - secreted directly into blood capillaries, so the secreting glands are endocrine and ductless
   - transported by blood to all parts of the body, which may take up to a minute
   - bind to receptors either inside or on the surface of target cells, triggering changes to the cells activities, either by promoting or inhibiting specific processes
   - persist for minutes or even hours in the body after being secreted, so can have long lasting effects
   - target one or multiple types of target cell, which can be in one or multiple parts of the body, so a single hormone can have complex and widespread effects on the body
2. Neurotransmitters
   - transmit signals across synapses
   - secreted when a nerve impulse reaches the end of a presynaptic neuron
   - diffuse across the synaptic cleft
   - bind to receptors in the plasma membrane of postsynaptic neurons, which either excites or inhibits transmission of an impulse
   - persist for only a fraction of a second before being removed from the synaptic cleft, so the effects are short lived
   - do not generally diffuse out of the synapse so convey the signal to one specific postsynaptic neuron only
3. Cytokines
   - a group of small proteins that act as signalling chemicals
   - secreted by a wide range of cells
   - one cytokine may be secreted by different cell types
   - one cell type may secrete different types of cytokine
   - some can be secreted by almost any cell in the body
   - do not normally travel as far as hormones and instead bind to receptors in the plasma membrane of a target cell
   - binding causes cascades of signalling inside the target cell, leading to changes in gene expression and thus cell activity
   - one cytokine can bind to several types of receptor and so have multiple effects
   - have cell signalling roles in inflammation and other responses of the immune system, in control of cell growth and proliferation and in the development of embryos
4. Calcium ions
   - pumped out of cells by calcium pumps in the plasma membrane, so intracellular concentrations are low
   - diffuse into cells through voltage gated or ligand gated ion channels in the plasma membrane
   - cause muscle fibres to contract
   - cause presynaptic neurons to release neurotransmitter into the synapse

Question 38

What are the requirements of signalling chemicals

Answer 38

Signalling systems using hormones and neurotransmitters have evolved repeatedly and a wide range of chemical substances have become the signalling chemicals. The requirements for a signalling chemical are:

• distinctive in shape and chemical properties so the receptor can distinguish between other chemicals
• small and soluble enough to be transported

Question 39

What are the effects of signalling molecules?

Answer 39

Signalling molecules vary greatly in how far they are transported and thus how widespread their effects are. Neurotransmitters released by presynaptic neurons may only diffuse 20 to 40 nanometres to reach the postsynaptic neuron and that is the only cell they signal to, so a neurotransmitters effects are very localised.

In contrast, hormones are transported long distances in the blood, from the gland that secretes them to target cells in any part of the body.

Question 40

What are the signal transduction pathways?

Answer 40

Binding of a signalling chemical to a receptor causes a sequence of interactions in the cell, called a signal transduction pathway. These pathways are very varied as they have evolved repeatedly, rather than having a common origin. The main differences are between transduction pathways for transmembrane and intracellular receptors:

1. Transmembrane receptors

Binding of a signalling chemical to the outer side of a transmembrane receptor causes reversible changes to its structure. In particular the inner side that is in contact with the cytoplasm becomes catalytically active and causes production of a secondary messenger within the cell. This conveys the signal to effectors within the cell that carry out responses

1. Intracellular receptors

Binding of signalling chemicals to intracellular receptors results in the formation of an active ligand-receptor complex. In most cases this then acts as a regulator of gene expression by binding to DNA specific sites resulting in the transcription of particular genes being either promoted or inhibited

Question 41

What are G-protein coupled receptors?

Answer 41

G protein-coupled receptors (GPCRs) are a large and diverse group of transmembrane receptors. They convey signals into cells using a second protein located in the plasma membrane called a G protein. The three subunits of G protein (alpha, beta and gamma) assemble on the surface of the receptor that faces the cytoplasm. A molecule of GDP, bound to the alpha subunit, maintains the G protein in an inactive state.

The receptor has a binding site for a ligand on the side facing the extracellular environment. When the ligand binds, conformational changes are triggered in the receptor, which cause changes in the coupled G protein. This results in dissociation of GDP from the alpha subunit, allowing GTP to bind in its place. Binding of GTP activates the G protein, which separates into its subunits and dissociates from the receptor. The activated G protein subunits cause further interactions within the cell leading to actions that are the ceils response to the signal brought by the ligand

Question 42

What are epinepherine receptors?

Answer 42

Epinephrine receptors are located in the plasma membranes of target cells. They are an example of G protein-coupling. Binding of epinephrine to the receptor activates G protein, which activates the enzyme adenylyl cyclase in the membrane. this enzyme converts ATP In the cytoplasm into cyclic AMP (a secondary messenger), Cyclic AMP initiates a sequence of reactions that amplify the signal, so responses to the binding of epinephrine happen very rapidly inside the cell. For example, liver cells break down glycogen arid release glutose into the blood within seconds of recelving an epinephrine signal.

Question 43

What is tyrosine kinase activity?

Answer 43

Protein kinases are enzymes that transfer phosphate groups from ATP to proteins (phosphorylation). For example, tyrosine kinase transfers phosphate to tyrosine (an amino acid) in specific proteins. Phosphorylation of proteins in many cases activates them and removal of the phosphate makes them inactive again.
Example: insulin receptors
The insulin receptor is a large transmembrane protein with two tails extending into the cytoplasm that are tyrosine kinases, Binding of insulin a site on the extracellular portion of the protein causes conformational changes that connect the two intracellular tails to form a dimer. Each tall then phosphorylates the other tail. This activates the insulin receptor, which trigger a chain of events inside the cell (signal transduction), that eventually results in vesicles containing glucose transporters to move to the plasma membrane and fuse with it. The transporters thus inserted into the plasma membrane are channel proteins that allow uptake of glucose into the cell by facilitated diffusion. The glucose can then be used as a substrate in cell respiration.

Question 44

What are the effects of oestradiol and progesterone?

Answer 44

Oestradiol and progesterone are female sex hormones that are secreted by cells in the ovary and by the corpus luteum and placenta during pregnancy. They are both steroid hormones. so diffuse into target cells through the plasma membrane and bind to specific receptors in the
cytoplasm.
Oestradiol has a broad range of effects both in the ovary and the uterus. It also affects production and release of gonadotropin-releasing hormone (GnRH) by the hypothalamus (a part of the brain). Just before and during ovulation, oestradiol binds to a receptor in the cytoplasm of cells in the hypothalamus. The resulting oestradiol-receptor complex enters the nucleus where it enhances the transcription of GRH mRNA. GRH triggers the secretion of the sex hormones LH and FSH by the anterior pituitary gland adjacent to the hypothalamus.

Progesterone promotes the development and maintenance of the endometrium (uterus lining) so that it can support a developing foetus. Progesterone binds to a receptor in the cytoplasm of uterus cells. The resulting progesterone-receptor complex enters the nucleus and causes the transcription of specific genes. Through changes to gene expression, progesterone causes cells in the uterus wall to divide repeatedly, leading to thickening of the endometrium.

Question 45

What are positive feedback and negative feedback

Answer 45

In both positive and negative feedback systems, amounts of the product of a pathway affect how much more is produced by the pathway. The product usually interacts with an early stage in the pathway.

Question 46

Negative feedback pathways

Answer 46

In negative feedback, the product inhibits more of its own production, so more product leads to less production, and less to more.
An example is regulation of thyroxin secretion by the thyroid gland in the neck. Cells in the hypothalamus secrete thyrotropin releasing hormone (TRH) which stimulates cells in the anterior pituitary gland to secret thyroid stimulating hormone (TSH) which stimulates thyroid gland cells to secrete thyroxin. High levels of thyroxin in the blood inhibit TRH secretion by the hypothalamus and low levels of thyroxin stimulate secretion of TRH.

Question 47

Positive feedback pathways

Answer 47

In positive feedback, the product stimulates more of its own production, so more product leads to more production, and less to less.
An example is the LH surge during the ovulatory phase of the menstrual cycle. Oestradiol, secreted by developing follicles in the ovary, stimulates GnRH secretion by the hypothalamus, which stimulates LH secretion by the anterior pituitary gland, which in turn stimulates more secretion of oestradiol by the developing follicles. This feedback loop results in a rapid rise in the concentration of the hormone LH, which causes follicles to become mature and release their eggs on about day 14 of the cycle.