3.6 Organisms respond to changes in the environment Flashcards Preview

AQA A level Biology > 3.6 Organisms respond to changes in the environment > Flashcards

Flashcards in 3.6 Organisms respond to changes in the environment Deck (158)
Loading flashcards...

3.6.1 Stimuli, both internal and external, are detected and lead to a response

What is a stimulus?

A detectable change in the internal or external environment


What are receptors?

Any structure able to respond to change


What is a co-ordinator?

The “switchboard” connecting information from the receptor to the appropriate effector


What is an effector?

Causes a response (muscle or gland)


What is a response?

The output/change in behaviour


What are the stages from a stimulus to a response?

1. Stimuli e.g. bright light
2. Receptor e.g. eye
3. Central Nervous System (Brain and spinal cord)
4. Effector (muscle for nervous response or gland for hormonal response) e.g. eye muscles
5. Response e.g. pupil constricts


How is selection an example of stimulus and response?

Organisms that survive have a greater chance of raising offspring
Organisms alleles are passed to next generation
Selection pressure favours organisms with appropriate responses


What are the three types of response an organism could have to a stimulus?

1. Taxis (tactic response)
2. Kinesis (kinetic response)
3. Tropism (Tropic response)


What is a kinetic response?

When a whole organism moves
Change in rate of movement
In response to a change in intensity of a stimulus

e.g. woodlouse moving from dry environment to moist area


What is a tactic response?

Movement of entire organism/cell
In response to and directed by stimulus
Positive taxis (towards +) or negative taxis (away from -)

e.g. phototaxis = movement towards/away from light
Chemotaxis = movement towards/away from chemicals


What is a tropic response?

Movement of part of a plant
Directed by a stimulus
Growth response

e.g. Hydrotropism = movement due to water
Geotropism = movement due to gravity
Phototropism = movement due to light


What are the types of plant responses?

Phototropism - response to LIGHT
Hydrotropism - response to WATER
Geotropism - response to GRAVITY


How do plants respond?



What is the main growth factor causing cell elongation?

IAA - The hormone Indoleactic Acid


What was Darwin's experiment in 1880?

Had 5 shoots
One as a control, one had tip removed, one had tip covered by opaque cap, one had tip covered by transparent cap and one had base covered by opaque shield


The shoot is positively phototropic
No response when tip is covered so light must be detected by tip
Transparent cap has no effect on phototropism as see through
Opaque shield still allow light to reach tip so has no effect on phototropism


Tip must be responsible for detection of light or production of messenger
Any response is prevented by removal


What was the Boysen-Jensen experiment in 1913?

Wanted to prove that response was due to chemicals produced in tip not an electrical signal initiated to tip

Mica = electrical conductor that does not allow chemicals to diffuse through it
Gelatin = conducts chemicals but not electricity


One tip had thin impermeable barrier of mica on lighted side
Another tip had mica inserted on shaded side
Third tip had tip removed, gelatin block inserted and tip replaced


First tip movement of chemical down shaded side and bends towards light
Second tip movement of chemicals down shaded side is prevented by mica. No response to light
Third tip had movement of chemical down shaded side. Bends towards light


Mica allows IAA to pass down shaded side only as increased growth on shaded side
Mica also allows electrical impulse to pass down the shoot but not chemicals.
There was no response so message is chemical
Gelatin allows chemicals but not electricity
Therefore bending is due to chemicals


What was the Paal experiment in 1919?


Shoots in darkness and tips removed then replaced but displaced to one side


Shoots bend towards side where no tip is present


Bending of shoot tips is a chemical factor (IAA) not an electrical impulse
This chemical is produced in the shoot tip and causes the elongation of plant cells in the shaded side


What was the Briggs experiment?


In experiment one one tip is in light, the other in darkness and IAA is collected from both shoots and amounts compared
In experiment two, a thin glass plate is placed to separate the two sides of the shoot. IAA is collected either side of glass plate and measured
In experiment three, the glass plate is placed so that the lateral transfer of IAA is possible at the tip. IAA is collected either side of the glass plate and measured


Experiment one, shoot with light bends towards light. Shoot in darkness has no bending.
IAA amounts in each shoot is approximately the same
In experiment two, the shoot does not bend and amount of IAA collected is approximately the same either side of the glass plate
In experiment three, the shoot bends towards light with most of IAA collected from the shaded side


Experiment one = shoot still bends so IAA is not destroyed by light
Experiment two = Glass prevents lateral movement so equal growth
Experiment three = IAA is transferred from light to dark side so IAA is produced in the tip


Why was a glass plate used in the Briggs experiment?



What are the two main divisions of the nervous system?

1. Central Nervous system
- Brain
- Spinal cord

2. Peripheral Nervous system
- pairs of nerves from the CNS travelling to limbs and organs
- sensory and motor neurone
- relays messages from CNS to effector
- 2 main divisions = Somatic (conscious involving the brain) and Autonomic (subconscious reflex actions)


What is a reflex action?

Involuntary response to a stimuli


Why are reflexes so important?

They are immediate (fast)
Protective response
Do not involve conscious part of brain
Innate (not learnt)
Automatic (unconscious)
Only has one course of action

- Therefore, the brain can focus on complex behaviours
- Escape predators, gain food or mates
Organism can survive and reproduce
Advantageous allele can be passed on


What is a reflex arc?

The pathway of neurones involved in a reflex action


Describe the process of a reflex arc

1. Stimuli - detectable change in environment
2. Receptor detects the stimuli
3. Sensory neurone carries electrical message from receptor to CNS
2. Intermediate neurone links sensory and motor neurone in CNS
3. Motor neurone passes electrical message to effector
6. Effector is stimulated to respond
7. Response e.g. move hand away from heat


How do rod cells generate a potential (retinal conversions)?

To create a generator potential the pigment (rhodopsin) inside rod cells must be broken BUT

- Threshold value must be exceeded
- Shared neurons ensures an additive effect of each lower light intensities
- Ensures a generator potential is achieved


What are the disadvantages of retinal convergence?

Only generates one impulse regardless of number of rod cells stimulated
Cannot distinguish between different sources of light/stimuli
Low visual acuity


What controls our heart beat?

Two nodes:

Sinoatrial node (SAN) - natural pacemaker
Atrioventricular node (AVN)

Together they create the cardiac cycle


Explain the cardiac cycle

SAN sends an electrical impulse across the atria
Atria contracts
Electrical activity travels to AV node
After pause to allow ventricles to fully fill with blood, AV sends impulse down Bundle of His
B of H conducts impulse through AV septum to bottom of ventricle
Small fibres called purkinje fibres branch out throughout ventricle walls
Ventricles contract from base up (apex) as more muscle in ventricle walls to increase pressure and push blood further


What is the importance of purkinje fibres?

To ensure every muscle contracts
Smaller branching network which sends nerve impulses to cells in ventricles of heart


Explain why heart rate changes

Varing oxygen demands e.g. during exercise
Heart must speed up flow of blood
Provide more glucose and oxygen
For respiration - provide the energy


What is the medulla oblongata

A cone shaped neuronal mass
Responsible for involuntary functions
Contains the cardiac, respiratory and vomiting centres
Controls breathing, heart rate and blood pressure


What are the two centres of the medulla oblongata?

Increases heart rate
- linked to SAN by the sympathetic nervous system

Decreases heart rate
- linked to SAN by parasympathetic nervous system


What are the two types of receptors in medulla oblongata?

Chemoreceptors in carotid arteries
- chemical changes in blood

Baroreceptors/pressure receptors in carotid arteries and aorta
- pressure changes in blood


What happens when blood pressure is high?

Nervous impulse is sent to centre in medulla
The centre sends an impulse via parasympathetic nervous system to the SA node
This decreases the rate at which the heart beats


What happens when blood pressure is low?

Nervous impulse is sent to centre in medulla
The centre sends an impulse via the sympathetic nervous system to the SA node
This increases the rate at which the heart beats


Explain the role of the medulla oblongata in controlling blood pH (chemoreceptors)

Increase in CO2 concentration causes a decrease in blood pH
This is detected by chemoreceptors in carotid arteries
Increases frequency of nerve impulses to centre in medulla
Impulse sent via sympathetic nervous system to SA node
This increases heart beat


3.6.2 Nervous Coordination

What are nerve cells?

Highly specialised cells adapted to rapidly carry electrochemical charges (nerve impulses)
Also called neurons


What is the structure of a nerve cell?

Cell Body
Myelin Sheath
Schwann Cells
Node of Ranvier
Axon Terminals


What is the function of a schwann cell?

Individual cells which protect the neurone
Provide electrical insulation
Aid in the regeneration of damaged axons
Carry out phagocytosis to remove cell debris


What is the function of dendrites?

Extensions of cell body
Carry impulses towards the cell body
Increase surface area


What is the function of the nodes of ranvier?

Gaps where there is no myelination
Increase speed


What is the function of the cell body?

Contains the nucleus (which provides mRNA for protein synthesis)
Contains a large amount of rough endoplasmic reticulum (RER makes protein to repair neurone if damaged and makes neurotransmitters)


What is the function of the axon?

Collect and carry the nerve impulse away from the cell body (spreads nerve impulse and is a single long fibre)


What is the function of the myelin sheath?

Multiple cells wrapped around the axon
Increase speed of impulse
Cell membrane = myelin
Multiple layers = myelin sheath


What is the structure of a sensory neuron?

Sensory neuron transmits nerve impulse from receptor to CNS/intermediate neuron

- One dendron carries impulse towards cell body
- One axon carries it away as only going to one destination
- Cell body usually appears as an extension of the axon
- One nerve impulse is transmitted to the CNS for coordination


What is the structure of a motor neuron?

Transmits the nerve impulse from the intermediate neuron/CNS to effector

- Long axon to reach out to effectors far away
- Many short dendrites to transmit nerve impulse to multiple cells/effectors


What is the structure of an intermediate neuron?

Transmits the nerve impulse between neurons

- Lots of short dendrites
- Dendrites on both sides to spread nerve impulse between two neurons
- Short axon as not transmitting nerve impulse very far


What is a nerve impulse?

A self-propagating wave of electrical disturbance that travels along the surface of the axon membrane

Specifically it is the temporary reversal of electrical potential difference across the axon membrane


What is an ion?

Chemical that can carry an electric charge


What are the important ions in the nervous system?

Sodium (Na+)
Potassium (K+)


What controls the movement of ions?

Phospholipid bilayer
- Non-polar fatty acid tails repel charged molecules, water or too large molecules
Intrinsic Proteins
- Ion channels allow specific ions to pass
- Have specific tertiary structure which only allows certain ions to pass
Gated Channels
- Na+ and K+ gated channels control amount of movement (at certain times depending on voltage)


What is a resting potential?

When the outside of the axon membrane has a positive charge and the inside has a negative charge


How is a resting potential established?

Na+ are actively pumped out of the axon by sodium-potassium pumps
K+ are actively pumped into the axon by sodium potassium pumps
For every 3 Na+ pumped out, 2 K+ move in
There are more Na+ outside (tissue fluid) than K+ inside axon cytoplasm. A chemical gradient is formed
Due to the gradient, Na+ try to move back in and K+ try to move out, down their concentration gradient
However, Na+ gates are shut, but K+ gates are open
So only K+ can move and they leave the axon
At this point the membrane is 100x more permeable to K+
This causes K+ to diffuse in faster than Na+ can diffuse in, causing outside of axon to become positively polarised, and inside of axon to become negatively polarised
But now, due to massive positive charge outside axon (electrical gradient), some K+ are compelled to move back inside. They are attracted to negative charge and repelled by positive outside
Some of K+ do move back in, but an equilibrium is formed, where there is no more net movement of ions
The electrical and chemical gradient becomes balanced, and resting potential is established


What is an action potential?

When the energy of a stimuli causes a temporary reversal of charge on the axon membrane
Outside of membrane becomes negative and inside positive


How does an action potential occur along a non-myelinated neurone?

1. Stimuli causes sodium voltage gated channels to open
2. Sodium diffuse down their electrochemical gradient which stimulates more sodium channels to open until +40mV is reached
3. At +40mV sodium channels close and voltage gated potassium channels open
4. This causes an electrical gradient = more potassium gates open. Axon is repolarised -65mV
5. Movement of potassium out causes overshoot of electrical gradient -70mV
6. Sodium Potassium pump (3 Na+ out, 2 K+ in) restores resting potential (-65mV). Axon is repolarised


How is an action potential passed along the axon?

1. The Resting Potential
- Sodium outside, potassium inside (polarised -65mV)
2. Initiation of the FIRST action potential
- Influx of sodium ions (reversal of charge +40mV)
- Action potential is initiated
3. Stimulation of the NEXT action potential
- Acts as a stimuli causing sodium channels to open further along the axon
- Depolarisation occurs
- New action potential is initiated
4. Stimulation of the NEXT action potential
- Behind sodium gated channels close, potassium open
- Potassium leave the axon along their electrochemical gradient
5. Repolarisation of the axon
- The axon is propagated for the 3rd time
- The continued outward movement of K+ causes the charge behind the action potential to return to its resting potential (-65mV)
- The area of the 1st action potential had been repolarised
- Now the area of the second action potential, is removing potassium ions in the same way
6. Getting back to normal
- Everything has shifted to the right
- The area of the 1st action potential is pumping OUT sodium (-65mV)
- That part of the axon is now ready to receive a new stimulus and strat the whole process off again


What factors affect the speed of an impulse?

1. Myelination (saltatory conduction)
2. Temperature
3. Diameter of the axon


How does myelination affect the speed of an impulse?

Myelin sheath is an electrical insulator
This prevents an action potential forming
Action potentials can only occur at Nodes of Ranvier
This results in node hopping (saltatory conduction)


What is saltatory conduction?

Action potential propagation along myelinated axons from one node of ranvier to next node


Describe the process of saltatory conduction

1. Sodium channels open and sodium diffuses in
2. Reversal of charges, action potential occurs
3. Sodium ions diffuse along concentration gradient
4. Voltage gated sodium channels opens further along axon
5. Na+ diffuse in
6. New AP occurs
7. AP moves along the axon


How does temperature affect the speed of an impulse?

Higher temperatures increase the speed of a nerve impulse
Particles have more kinetic energy so diffuse quicker
Increased enzyme actions
More energy available for active transport (Na/K pump)
Action potential is established quicker
Temperature too high = changes tertiary shape of protein channels


How does diameter of axon affect speed of an impulse?

Small axon causes ions to leave easily
Harder to build up ions in axon
Harder to establish electrical and chemical gradients
Membrane potentials are difficult to maintain


What is the refractory period?

The time taken for Na+ influx to be possible again

1. Na+ channels open and action potentials start
2. Na+ close when maximum voltage is reached during depolarisation (+40mV)
3. Potassium gated channels open
4. Na+ channels won’t open until resting potential is reached (REFRACTORY PERIOD)


Why is the refractory period important?

1. Ensures action potentials are in one direction
- Area before action potential will be in refractory period so axon is hyperpolarised so influx of sodium will not reach threshold and therefore a new action potential cannot occur because the axon must be at rest (-65mV)
- This prevents a message backwards
- It is going in one direction to send stimulus to coodinator, effector etc

2. Ensures action potentials are discrete (separate)
- Area behind in refractory period = AP cannot occur
- Refractory period takes time which separates messages as rest has to occur
- Messages sent to brain are discrete and not muddled
- This ensures correct message is received/delivered and a correct response is coordinated
- Intensities can change

3. Ensures action potentials are limited in number at one time
- AP = fixed distances apart due to refractory period separating them
- Cannot occur ‘behind’ one another
- Axon is a fixed length therefore only a certain number of action potentials will ‘fit’
- This means different intensities of stimuli can be responded to and different responses can be initiated


What is the structure of a synapse

Myelin sheath
Axon of presynaptic neurone
Membrane of presynaptic neurone
Smooth endoplasmic reticulum
Calcium ion channels
Calcium (Ca2-) ions
Synaptic vesicles containing acetylcholine *neurotransmitters)
Membrane of post-synaptic neurone
Sodium ion channels
Synaptic cleft


What are neurotransmitters?

Chemical messages that are specific to receptors on post synaptic neurone that enable synaptic transmission


What does the response to the arrival of a neurotransmitter depend on?

1. The cell
2. The cells location
3. The neurotransmitter involved


What is a cholinergic synapse?

A synapse that relies on the neurotransmitter acetylcholine
Causes polarisation and generation of an action potential


What are the steps involved in synaptic transmission?

1. Incoming action potential causes depolarisation in pre-synaptic neuron
2. This causes calcium gated channels to open
3. Causes an influx of calcium ions into presynaptic knob
4. Calcium ions cause vesicles containing neurotransmitters (acetylcholine) to move
5. Vesicles fuse with membrane of presynaptic knob
6. Neurotransmitters are released into synaptic cleft and diffuse across synapse
7. Acetylcholine binds to sodium channels
8. Sodium channels open
9. Influx of sodium ions
10. New action potential generated if threshold is met


What are neuroreceptors?

Chemical gated ion channels in post synaptic neuron membrane

- Specific (binding site for neurotransmitter involved)
- Usually closed
- When neurotransmitter bind undergo a conformational change
- Causes an influx of sodium ions


What is acetylcholine esterase?

A hydrolic enzyme (hydrolysis)
Located on membrane
Breaks up acetylcholine into acetyl (ethanoic acid) and choline

After hydrolysis acetyl and choline diffuse back across cleft into presynaptic neurone
Neurotransmitters are recycled and repackaged


What are the features of chemical synapse?

1. Transmit impulses in one direction to a precise location
- Towards the effector, coordination centre, next neuron

2. Protect the system against overstimulation
- Limited neurotransmitters released = prevents fatigue

3. Act as junctions between one neurone and another allowing:
- A single impulse to be transmitted to multiple neurones
(one stimuli = multiple simultaneous responses e.g. pain = verbal and muscular response)
- A number of impulses to be combined at a synapse
(additive effect = summation to reach threshold)


What is summation?

The additive effect of low frequency action potentials to produce sufficient neurotransmitters to trigger an AP across the synapse


What are the two forms of summation?

1. Spatial
2. Temporal


What is spatial summation?

Definition: Different presynaptic neuronscome together to trigger ONE action potential

- Multiple presynaptic neurons at one synapse
- A single neuron does not release enough neurotransmitters so multiple neurones release neurotransmitters to ensure threshold is exceeded
- Action potential is initiated


What is temporal summation?

Definition: Single presynaptic neuron releases neurotransmitters many times over a short period to exceed the threshold

- One neuron per presynaptic neuron
- Low frequency APs = not release enough neurotransmitters = no threshold
- High frequency Aps = neurotransmitters are released multiple times over a short period = threshold met
- The higher the frequency of AP = quicker the threshold is exceeded


What is an inhibitory synapse?

A synapse in which the nerve impulse in a presynaptic cell results in a reduced likelihood of an action potential initiated in post synaptic cell


What happens at an inhibitory synapse?

1. Neurotransmitters diffuse across
2. Causes chloride ion (Cl⁻) channels in post synaptic membrane to open
3. Cl⁻ ions diffuse across postsynaptic membrane
4. Membrane becomes more negative than at resting (hyperpolarisation)


How do drugs affect synaptic transmission?

1. Some drugs work by creating fewer action potentials (inhibitory) by:
- Inhibiting the release of neurotransmitters
- Blocking the sodium/potassium channels
- Inhibiting acetylcholine esterase which blocks channels
- E.g. GABA inhibits or slows the brains function and promotes sleepiness and reduces stress

2. Some drugs work by stimulating the nervous system and create more action potentials by:
- Mimicking the neurotransmitter
- Stimulating the release of more neurotransmitter
- Inhibiting the breakdown of neurotransmitter (block the enzyme)
- E.g. Serotonin believed to help regulate mood and social behaviour, appetite and digestion, sleep, memory, sexual desire and function


3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors

What are the three main connective tissues?

1. Tendon
- Connect bone to muscle
2. Ligament
- Connective tissue that connects bone to bone
3. Cartilage
- Connective tissue found between bones (shock absorber)


What are the three types of muscles?

1. Cardiac Muscle
- Present in the heart
- Acts involuntarily (myogenic)

2. Skeletal Muscle
- Attached to bones
- Vast majority of muscle
- Discontinues contraction
- Controlled voluntarily

3. Smooth Muscle
- Involuntary muscle
- Found in gut, blood vessel walls and iris of eye
- Slow and weak contraction


How do muscles work?

Muscles act by:
- Receiving a nerve impulse
- Working in antagonistic pairs (one muscle (prime mover) contracts and other (antagonistic) relaxes). You cannot stimulate contraction of two antagonistic muscles at same time
- Pulling bones


Describe the structure of a skeletal muscle

Myofibrils (made from thick and thin filaments) bunch together to form individual muscle fibres
These bunch together to form bundle of muscle fibres which group to form a whole muscle


What are the two types of muscle fibres?

Slow twitch fibres
Fast twitch fibres


What are the features of slow twitch fibres?

Slower speed of contraction
Low power of contraction but can contract for a prolonged period of time
Used for endurance activities
Carries out aerobic respiration
E.g. calf muscles


What are the features of fast twitch fibres?

Fast soeed of contraction
Powerful contraction as contains the most myofibrils
Used for intense activity over short periods
Carries out anaerobic respiration
E.g. Biceps


What are adaptations of slow twitch fibres?

Large amount of myoglobin (red molecule that stores oxygen)
- Supplies oxygen for aerobic respiration
Good supply of glycogen
- Storage of glucose for aerobic respiration
Good blood vessel network
- Good oxygen and glucose supply and removes CO2 to prevent build up of lactic acid
Many mitochondria
- Site of respiration to release ATP
Small diameter
- Faster diffusion of oxygen and glucose and short diffusion pathway to remove CO2
Tend to be darker in colour
- Better blood supply for oxygen


What are the adaptations of fast twitch fibres?

Thicker and more myosin filaments
High concentration of enzymes involved in anaerobic respiration
A store of phosphocreatine
- Provides energy (energy buffer)

BUT they tire quickly because:
- Fewer blood vessels
- Run out of oxygen more quickly
- Thicker so larger diffusion distance


What are muscle fibres made up of?

Individual cells fused together to make one long cell

Nuclei = multinucleated
Cytoplasm = sarcoplasm (surround myofibrils)
Cell membrane = sarcolemma


Why do nuclei need to be dotted all over the muscle fibres?

To allow transcription to occur for mRNA
All muscle cells controlled


What organelle will be in large number in the sarcoplasm?

- Release energy through respiration for contraction of muscles and protein synthesis
- Protein synthesis
- Storage of calcium ions for contraction


What are the two types of protein filaments in myofibrils?

1. Actin
- Thinner
- Made up of strands coiled around each other

2. Myosin
- Thicker
- Consists of rod shaped filaments with bulbed heads
- Bulbed heads project outwards


What do myofibrils consist of?

Multiple sarcomeres
Actin and myosin filaments
Banding pattern
Causes myofibrils to appear striped


What are the two main components in sarcomere?

1. Dark bands
- Actin and myosin overlap
- Anisotropic bands (A bands)
- Depends on myosin length

2. Light bands
- No overlap
- Isotropic bands (I bands)


What is the ultrastructure of a muscle?

One sarcomere = Z line to Z line
A band = Overlapping of both myosin and actin filaments
I band = Only actin filaments
H zone = Only myosin filaments


What happens to the ultrastructure during contraction?

I band narrows (as only actin)
Z lines move closer together
H zone narrows (overlaps of actin and myosin)
A band remains the same (A band is determined by length of myosin filaments that do not change length)


What is a neuromuscular junction?

Where a motor neurone meets skeletal muscle fibre
ALWAYS a cholinergic synapse
- Neurotransmitter = acetyl choline
- Enzyme hydrolysing neurotransmitter = acetylcholine esterase


What is the all or nothing principle?

The strength by which a nerve or muscle fibre responds to a stimulus is independent of the strength of the stimulus


Explain the process of synaptic transmission at neuromuscular junction

Action potential causes depolarisation of presynaptic neuron
This causes calcium gated channels to open and influx of calcium ions into presynaptic knob
This causes vesicles containing acetyl choline (neurotransmitters) to move
Vesicles fuse with membrane of muscle fibre and neurotransmitters diffuse across synapse
Acetylcholine binds to complementary sites on sodium channels
Sodium gated channels on membrane of muscle fibre open causing influx of sodium ions into muscle fibre which depolarises membrane
Action potential generated if threshold is met


How do muscles contract?

Action potentials travel deep into muscle fibre through system of tubules called T Tubules that branch through sarcoplasm and cause calcium ions to be released


Compare and contrast transmission across a cholinergic synapse and transmission across a neuromuscular junction

Both cholinergic and neuromuscular junction have neurotransmitter acetyl choline
In cholinergic the neurotransmitter binds to sodium gated channels on membrane of post synaptic knob whereas on neuromuscular junction it binds to sodium channels of muscle fibre (sarcolemma)
Cholinergic can be excitatory or inhibitory whereas neuromuscular is only excitatory
Cholinergic connect neurons to neurons or effectors whereas neuromuscular is neurons to muscles
Cholinergic is all three neurons may be involved whereas neuromuscular only involves motor neurons
In cholinergic an action potential is propagated whereas neuromuscular an action potential ends


What are the two types of protein filament?

1. Tropomyosin
- Long and thin fibrous strands

2. Troponin
- Globular protein involved in muscle contraction


What are the steps of muscle contraction (sliding filament model)?

Calcium channels open in plasma membrane and the sarcoplasmic reticulum
Calcium ions are released from SR and bind to troponin
This causes tropomyosin to change shape and free myosin binding sites
Myosin with attached ADP binds to actin binding site and flexes
Actin-myosin cross bridges form, pulling the actin (sliding mechanism) = power stroke
ATP binds to bulbous head causing it to detach
Hydrolysis of ATP (ATPase) gives the energy for myosin to cock its head and return to normal position
Myosin with ADP attached reattaches further along actin


What are the steps of muscle relaxation?

Calcium ions are actively transported back into the sarcoplasmic reticulum
Troponin reverts to original shape
Tropomyosin blocks binding sites
Myosin heads cannot bind
Muscle relaxes


Why is energy important in muscle contraction?

In order to move myosin heads
To reabsorb calcium ions into the ER


Where does the energy come from?

Oxidative phosphorylation in mitochondria
Anaerobically – phosphorylation using phosphocreatine (PCr)


What is phosphocreatine?

A buffer supply of energy stored in muscle used to restore ATP

- A reserve supply of phosphate
- Broken down when energy is needed
- Combines with ADP to reform ATP
- Is immediately available
- Replenished using phosphate from ATP when muscle is relaxed


3.6.4 Homeostasis is the maintenance of a stable internal environment.

What is homeostasis?

The maintenance of a constant internal environment


Explain why the control and maintenance of internal conditions is important for an organism

Allows organisms to live in and adapt to changeable environments without affecting the internal environment

If too high, enzymes denature
Effects rate of diffusion/movement of molecules
Effects proteins
Has effect on transpiration

Sugar Levels
Affects water potentail
Respiration (lowers ATP = no active transport)

Oxygen levels
Affects ability to meet respiratory needs

Water levels
Cells may shrivel (plasmolysided) or burst (lysis)

Composition of blood
CO2 and O2 concentrations

Tissue Fluid
Must supply cells with nutrients and remove waste


What are the stages of homeostatic control systems?

1. Stimuli (change to the system)
2. Receptor (detects variation)
3. Control Unit (co-ordinates response)
4. Effector (bring about change)
5. Output (returns body back to set point)
6. Feedback loop (tells receptor about change)


What is thermoregulation?

The control of internal body temperature


What is an ectotherm?

Organism which maintains a proportion of their heat from sources outside of their body
e.g. lizards and snakes


What is an endotherm?

Organism which derives heat from sources (metabolic activities) inside of the body
e.g. mammals and birds


What is the hypothalamus?

The control unit for most responses
Links nervous system and endocrine system via the pituitary gland
Production of most hormones


What is the feedback loop for thermoregulation to increase body temperature?

1. Stimuli = change in blood temperature
2. Cold receptors in skin
3. Control unit (hypothalamus) so heat gain centre
4. Response is
- Vasoconstriction of arterioles
- Shivering
- Piloerection (hairs stand up)
- Increased metabolism


What is the feedback loop for thermoregulation to decrease body temperature?

1. Stimuli (change in body temperature)
2. Warm receptors in skin
3. Control unit (hypothalamus) heat loss centre
4. Response is
- Vasodilation of arterioles
- Sweating
- Pilorelaxation (hairs lie flat)
- Decreased metabolism


What is negative feedback?

Initiating corrective mechanisms whenever the internal environment deviates from its normal or acceptable level
Returns conditions back to the norm
Reverses any changes
Opposes the stimuli
Self regulating

e.g. temperature control, control of blood sugar levels, changes in heart rate


What is positive feedback?

A deviation from normal conditions is amplified, leading to a further deviation

e.g. blood clotting, oxytocin causing more contractions, adrenaline (fight or flight)


How does glucose enter our body?

1. Diet (carbohydrate break down)
2. Glycogenolysis
3. Gluconeogenesis


What is glycogenolysis?

Breakdown of glycogen into glucose


What is glycogenesis?

Formation of glycogen by converting excess glucose


What is gluconeogenesis?

Production of glucose from glycerol and amino acids (liver)


Why do blood glucose concentrations vary?

Diet, not eating constantly, rate of use (exercise, stress, metabolic rate, muscle:fat)


What organs are responsible for controlling glucose levels?

- Digestie enzymes
- Glycogenolysis, glycogenesis, gluconeogenesis


Why is it important to monitor blood glucose concentrations?

Hyperglycemic (glucose levels too high)
- Lowers w.p. of blood = dehydration
- Causes muscle to break down, weight loss, tiredness
Hypoglycaemic (glucose levels too low)
- Cells deprived of energy and die
- Causes sweating, hunger, irritability, double vision


Why is the pancreas important?

1. Digestive enzymes
- Protease, amylase, lipase

2. Production of hormones
- Contains the islets of langerhans
- Insulin (decrease blood sugar levels)
- Glucagon (increase blood sugar levels)


What are the islets of Langerhans?

Hormone producing cells
Alpha cells – glucagon
Beta cells – Insulin
Work antagonistically (negative feedback)


Why is the second messenger model used?

The hormones are too large, not lipid soluble and polar so cannot pass through the phospholipid bilayer
Instead second messenger model is used


What is the second messenger model?

Adenylate cyclase is activated
This converts ATP to cyclic AMP (second messenger)
cAMP activates kinase enzyme
Glucogenolysis occurs (glycogen to glucose)


Describe how insulin reduces the concentration of glucose in the blood

Beta cells detect an increase in blood glucose concentration
Insulin is secreted into the blood
Insulin binds to glycoprotein receptors on cell surface membrane
Activates carrier proteins to open glucose channels
Increased permeability of the muscle/cell/liver to glucose (facilitated diffusion)
Glucose enters the cell and is converted to glycogen (glycogenesis) and some to fats plus some used in respiration


Describe how glucagon increases the concentration of glucose in the blood

Alpha cells detect a drop in blood glucose
Secrete glucagon into blood plasma
Only liver cells have glucagon receptors so only they respond
Glucagon binds to target cells
Activates kinase enzymes in the cells which convert glycogen to glucose (glycogenolysis)
AND gluconeogenesis synthesises glucose from glycerol and amino acids
Glucose is released into the blood plasma
Blood sugar levels rise


What is type 1 diabetes?

Early onset (usually childhood)
Do not produce enough insulin
Immune system attacks beta cells
Treated with insulin injections
Regulated/monitored using a biosensor


What is type 2 diabetes?

Produce insulin but cells do not respond
Glycoprotein receptors lose their responsiveness to insulin or there is a reduced supply of insulin from pancreas
Often correlated with obesity
Treated by monitoring carbohydrate intake and exercise


What are the symptoms of diabetes?

High blood glucose levels
Glucose in urine
- Glucose cannot be reabsorbed due to concentration gradient
Thirst anf hunger
- Decreased water potential of blood
- Mor ewater lost
Urinate frequently
- Lack of glucose absorbed/water potentail
Very tired
- Lower respiratory rate as cells cannot absorb glucose for respiration
Weight loss
- Less glucose = less energy for growth and repair so cells start to break down
Blurred vision caused by lens of eye changing shape
- Over time affect nerves and lead to poor blood circulation


What is osmoregulation?

The homeostatic control of water potential in the blood

The balance of water and mineral ions/salts
Controlled by the kidneys


What is the role of the kidney?

Control the amount of water in our body
Clean the blood
Remove any unwanted water, waste or toxins


What is the structure of the kidney?

Renal artery for dirty blood
Renal vein for clean blood
Medulla (middle)
Cortex (outside)


What is the function of each part of the kidney?

- Inner region made up of loops of Henle, colleting ducts and blood vessels
Convoluted tubule
- A series of loops surrounded by blood capillaries. Walls are made up of epithelial cells with microvilli
- A tube that carries urine to the bladder
- A many branched knot of capillaries from which fluid is forced out of the blood
Renal Vein
- Returns blood to the heart via the vena cava
- Outer region made up of renal capsules (Bowman’s capsule), convoluted tubules and blood vessels
Loop of Henle
- Long hairpin loop extending from the cortex into the medulla
Bowman’s capsule
- A cup shaped structure at the start of the nephron, surrounding into a mass of blood capillaries (glomerulus)
Renal Artery
- Supplies the kidney with blood from the heart via the aorta
Collecting duct
- A tube with several distal convoluted tubules from several nephrons empty. Increases in width as it empties into the pelvis of the kidney


What is the nephron?

The filtering unit of kidney which performs the job of filtering and fluid balance


What is the structure of the nephron?

See diagram


What are the four stages of osmoregulation in the nephron?

1. Ultrafiltration
2. Reabsorption
3. Maintenance of a gradient of sodium ions
4. Reabsorption


Where does ultrafiltration occur?

At the glomerulus
It forms the glomerulus filtrate


Describe the process of ultrafiltration

Afferent arteriole is wider than the efferent arteriole
This difference in diameter of blood vessels causes hydrostatic pressure to build up in the glomerulus
This causes ultrafiltration at the Bowman’s capsule through the pores in the membrane
- Water
- Glucose
- Mineral Ions
- Urea (enabled by small size)
Remain in:
- Red Blood Cells
- Proteins


How is the movement of the filtrate out of the glomerulus restricted?

Capillary epithelial cells
Connective tissue (lining capillaries)
Epithelial cells of the renal capsule
Low hydrostatic pressure
Lower water potential of the blood


How is the Bowman’s capsule adapted to prevent the resistance?

1. Podocytes
- Specialised epithelial cells
- Form gaps
- Shorter diffusion pathway

2. Gaps in epithelial cells
- Shorter diffusion pathway
- Easy passage of molecules


What is selective reabsorption?

The reabsorption of certain molecules back into the blood. This includes glucose, some ions and water via co-transport.


What is co-transport?

The movement of one molecule (sodium) coupled with another (i.e. glucose) in the same direction, through the same carrier protein


Explain how materials are reabsorbed into the blood via a co-transport mechanism

Sodium ions are actively transported out of the cell
This lowers the Na+ concentration inside the cell
Na+ diffuse from lumen into the cell via carrier proteins (facilitated diffusion)
Pull with it another molecule such as glucose (co-transport)
Molecule concnetration increases
Molecues diffuse into the blood alongside some water


Where does selective reabsorption take place?

Proximal Convoluted Tubule


Where does the counter-current multiplier take place?

Loop of Henle in ascending and descending limbs


What are the features of the descending limb?

Travels into the medulla
Thin walls
Highly permeable to water


What are the features of the ascending limb?

Travels back to the cortex
Thicker walls
Impermeable to water


Describe the process of maintaining a sodium ion gradient in the loop of Henle

• Near the top of the ascending limb, Na+ ions are actively transported out into the medulla
• The ascending limb is impermeable to water so water stays inside the tubule
• This creates a low water potential in the medulla, because there is a high concentration of ions
• Due to lower water potential in medull than descending limb, water moves out of descending limb into the medulla by osmosis
• This makes the filtrate more concentrated as the ions cannot diffuse out since the descending limb is not permeable to them
• The water in the medulla is reabsorbed into the blood through the capillary network
• Near the bottom of the ascending limb Na+ ions diffuse out into the medulla, further lowering the water potential in the medulla
• The ascending limb is impermeable to water so it stays in tubule
• Water moves out of distal convoluted tubule by osmosis and is reabsorbed into blood
• There is a high ion concentration in the medulla which lowers the water potential
• This causes water to move out of the collecting duct by osmosis
• The water in the medulla is reabsorbed into the blood by capillaries


Why do water levels vary?

External temperature
Fluid intake
Salt intake
Drug intake (ecstasy and alcohol)


How do we control water potential?

A hormone ADH
Secreted from the posterior pituitary gland
Acts of the DCT and CD
Concentrates urine


Decscibe the process of increasing the water potential of the blood

Decreased water potential in the blood
Osmoreceptor cells in hypothalamus detect change (cells lose water and shrink)
Stimulates neurosecretory cells in the hypothalamus
Increased action potentials to the posterior pituitary gland
Increased ADH production
ADH sectreted into blood
Travels in the blood to the DCT and CD
Walls of CD become more permeable to water
More water is reabsorbed into the blood


Describe the process of decreasing the water potential in the blood

Too much water
Increased water potential of the blood
Osmoreceptor cells in hypothalamus detect change (cells take in water so expand/swell)
Does not stimulate neurosecretory cells
Decreased action potentials passing to the posterior pituitary gland
ADH is not secreted into the blood
No/less ADH to the DCT and CD
Permeability of CD decreases so less water is reabsorbed into the blood


How does ADH work?

ADH binds to complementary receptors on DCT and CD (as ADH cannot pass through membrane)
Activates enzyme phosphorylase on inside
Aquaporin (vesicle containing water channels) fuse with membrane
Increased permeability to water
Water leaves CD
Increases water potential gradient
More concentrated urine produced