[Y2] Organisms Respond To Changes In Their Internal and External Environment Flashcards
(206 cards)
1
Q
What is a stimulus?
A
A detectable change in the internal or external environment of an organism that leads to a response in the organism.
2
Q
What is the benefit of being able to respond to a stimulus?
A
- Increases the chance of survival for an organism.
- Organism have a greater chance if raising offspring.
- Thus passing on their alleles to the next generation.
3
Q
How are stimuli detected
A
By receptors specific to the type of stimuli.
4
Q
What is a coordinator?
A
Something that formulates a suitable response to a stimulus.
5
Q
How are responses produced to stimuli?
A
By an effector.
6
Q
What is the sequence of events in the nervous system?
A
stimulus → receptor → coordinator → effector → response
7
Q
What is taxes?
A
A simple response whose direction is determined by the direction of the stimulus
8
Q
How might a motile organism respond to a stimulus via taxis?
A
Moving its whole body:
- towards a favourable stimulus.
- away from an unfavourable one.
9
Q
Give an example of a positive taxis.
A
- Single-celled algae will move towards light.
- This increases their chances of survival since, being photosynthetic, they require light to manufacture their food.
10
Q
Give an example of a negative taxis.
A
- Earthworms will move away from light.
- This increases their chances of survival because it takes them into the soil, where they are better able to conserve water, find food, and avoid preditors.
11
Q
What are kineses?
A
A response where organisms change its speed at which it moves and the rate of change of direction.
12
Q
How might a kineses stimuli be different from a taxes one?
A
- Stimuli for kineses tends to be less directional.
- e.g. Humidity and temperature.
- does not always produce a clear gradient from one extreme to another.
13
Q
Give an example of kinesis.
A
- Occurs in woodlice.
- They lose water from the body in dry conditions.
- When they move into a dry environment from a damp one, they move rapidly in a straight line.
- This is so they go straight through the dry environment
- When they move into a damp environment from a dry one they move more slowly, and change direction more.
- This is so they travel in circles and spend more time in the dry environment.
14
Q
What are tropisms
A
A growth of part of a plant in response to a directional stimulus.
15
Q
What is:
- positive phototropism?
- negative phototropism?
- positive gravitropism?
- negative gravitropism?
A
- positive phototropism: Shoots grow towards the light
- negative phototropism: Roots grow away from light
- positive gravitropism: Shoots grow in the same direction as gravity acts.
- negative gravitropism: Roots grow in the opposing direction to which gravity acts.
16
Q
What do plant growth factors do?
A
- They exert their influence by affecting growth and, they may be made by cells located throughout the plant rather than in particular organs.
- Unlike animal hormones, some plant growth factors affect the tissue that releases them rather than acting ona distant target organ.
17
Q
Give an example of a plant growth factor.
A
Indoleacetin acid (IAA)
- belongs to the subgroup called auxins.
- among other things, IAA controls plant cell elongation.
18
Q
What is unilateral light?
A
A light that is detected from one side.
19
Q
What takes place in the response of shoots of flowering plants to unilateral light?
A
- Cells in the tip of the shoot produce IAA, which is then transported down the shoot.
- The IAA is initially transported evenly throughout all regions as it begins to move down the shoot.
- Light causes the movement of IAA from the light side to the shaded side of the shoot.
- A great concentration of IAA builds up on the shaded side of the shoot than the light side.
- As IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot, the cells on this side elongate more.
- The shaded side of the shoot elongates faster than the light side, causing the shoot tip to bend towards the light
20
Q
How does IAA differ in roots to its effect in shoots?
phototropism
A
- Promotes cell elongationín shoots
- Inhibits cell elongation in roots: therefore roots are negatively phototrophic.
21
Q
What takes place in the response of a horizontally-growing root to gravity?
A
- Cells in the tip of the shoot produce IAA, which is then transported along the root.
- The IAA is initially transported to all sides of the root
- Gravity influences the movement of IAA from the upper side to the lower side of the root.
- A greater concentration of IAA builds up on the lower side of the root than the upper side.
- As IAA inhibits the elongation of root cells and the is a greater concentration of IAA on the lower side, the cells on this side elongate less than those on the upper.
- The relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards in the direction of the force of gravity.
22
Q
How does IAA differ in shoots to its effect in roots?
gravitropism
A
- In shoots, the greater concentration of IAA on the lower side increases cell elongation and causes this side to elongate more than the upper side.
- As a result, the shoots grow upwards opposing the force of gravity.
23
Q
What effect does IAA have on plant cells?
A
Increases their plasticity (ability to streach).
24
Q
What type of cells/part of cell does IAA work in?
A
- Only on young cell walls.
- As they are able to elongate.
Cannot happen in older cell walls as…
- as the cell matures they develop greater rigidity, so older parts of the plant roots/shoots are unable to respond.
25
What is the proposed explanation of how IAA increases the plasticity of the cell?
The acid growth hypothesis.
- Involves active transport of hydrogen ions from the cytoplasm into spaces in the cell wall.
- Causes the cell wall to become more plastic allowing the cell to elongate by expansion.
26
What is a reflex arc?
The pathway of neurons involved in a simple nervous response to stimuli.
27
What are the two major divisions of the nervous system?
- Central nervous system (CNS): Brain and spinal cord.
| - Peripheral nervous system (PNS): pairs of nerves that originate from the brain or spinal cord...
28
How is the nervous system divided further (from the major divisions)
PNS:
- Sensory neurones: carry nerve impulses from receptors towards CNS
- Motor neurones: carry nerve impulses away from CNS to effectors.
29
How can the motor nervous system be further subdivided?
PNS => Motor:
- Voluntary nervous system: carries nerve impulses to body muscles, under conscious control.
- Autonomic nervous system: carries nerve impulses to glands, smooth muscle, cardiac muscle; controlled subconsciously.
30
What is the spinal cord?
A column of nervous tissue that runs along the back and lies inside the vertebral column fro protection.
31
What are the characteristics of a reflex?
- Rapid.
- Short-lived.
- Localised.
- Involuntary.
32
Why is a reflex sometimes a spinal reflex?
One of the neurones involved is in the spinal cord.
33
What happens during a spinal reflex? (use hand on hot surface example)
- Stimulus: heat from the hot object.
- Receptor: temperature receptors in the skin generates a nerve impulse in the sensory neurone.
- Sensory neurone: passes nerve impulses to the spinal cord.
- Coordinator (intermediate neurone): links the sensory neurone to the motor neurone in spinal cord.
- Motor neurone: carries nerve impulses from the spinal cord to muscles in the upper arm.
- Effector: muscles in the upper arm, which is stimulated to contract.
- Response: pulling hand away from the hot object.
34
Why is reflex action important?
- Involuntary ∴ don't require a decision from the brian:
- This allows it to continue to carry out more complexes responses.
- Brain not overloaded with situations that require the same response.
- Some impulses are sent to brain, so it is informed and can override if necessary.
- Protect the body from harm, do not need to be learned.
- Fast, as neuron pathway is short with very few (1 or 2) synapses. (synapses are the slowest link).
- Absence of any decision-making process means that action is rapid.
35
What is the difference between sensory reception and sensory perception?
- Sensory reception is the function of receptors to a spesific type of stimuli.
- Sensory perception involves making sense of information from the receptors.
36
What are the features of sensory reception (as illustrated by the Pacinian corpuscle?
- Is specific to a single type of stimulus: responds only to mechanical pressure.
- Produces a generator potential by acting as a transducer: transduces mechanical energy of the stimulus into a generator potential.
37
What is a generator potential?
When receptors in the nervous system convert the energy of the stimulus into a nervous impules.
38
What does the Pacinian corpuscle respond to?
Mechanical stimuli, such as pressure.
39
Where are Pacinian corpuscles found?
Deep in skin and more abundant:
- on the fingers
- on the sole of feet
- on external genitalia.
- in joints, ligament and tendons (allow organisms to know which joints are changing direction.)
40
Describe the structure of the Pacinian corpuscle.
- A single sensory neuron at the centre of layers of tissue.
| - Each layer of tissue is separated by a gel.
41
How does the Pacinian corpuscle's structure transduce mechanical energy from the stimulus into a generator potential?
- In the central neurons plasma membrane stretch-mediated sodium channels are found.
- Their permeability to sodium changes when they are deformed.
- At its resting state the channels are too narrow to allow sodium ions to pass along them, so the neuron has a resting potential.
- When pressure is applied, the deformation causes the sodium channels to stretch, widening the channel so sodium ions can diffuse into the neuron.
- The influx of sodium ions depolarises the membrane, producing a generator potential.
- This creates an action potential that passes along the neuron to the CNS (via other neurons).
42
(In mammals) where are light receptors located?
The retina.
43
How many light receptors are there?
Millions.
44
How many types of light receptor are there? Name them.
Two.
Rods and cones.
45
How are rods and cones transducers?
They convert energy carried by light into electrical energy.
46
What are the properties of rod cells?
- One type.
- Cannot distinguish between different wavelengths (black and white image).
- More numerous than cones.
- Many connected to a single sensory neuron in the optic nerve.
- Rod-shaped
- More are located at the periphery of the retina and absent at the fovea
- Give poor visual acuity
- Sensitive to low light intensity.
47
What are the properties of cone cells?
- Three types: each responding to different wave lengths.
- Can respond to different wavelengths (coloured image).
- less numerous than rods.
- One connected to a single sensory neuron in the optic nerve.
- Cone-shaped
- Fewer are located at the periphery of the retina and concentrated at the fovea.
- Give good visual acuity
- No sensitive to low light intensity
48
How do rod cells allow us to see at low light intensities (e.g. at night)?
- Rods respond to low light and many connect to the same bipolar neuron.
- Due to retinal convergence there is a much higher chance that the threshold value is exceeded than with one.
- Once the threshold is met there is enough energy for the pigment rhodopsin to be broken down.
- This summation allows us to see low light intensities (e.g. at night).
49
Why do rods cells have a poor visual acuity?
- Many rod cells link to the same bipolar neuron.
- So light revied by cells sharing the same neurone will only generate a single impulse regardless of how many neurones are stimulated.
- Therefore the brain cannot distinguish between the separate sources of light that stimulated the rods.
- So two dots close together cannot be resolved and will appear as a single blob.
50
How do cone cells allow us to see different colours?
- There are three different types of cones (RGB) that respond to different wavelengths of light (and have there own type of iodopsin).
- depending on the proportions of each cone stimulated, we can perceive images in full colour (from red to violet).
51
Why do cones only respond to high-intensity light?
- Cones are connected to there own bipolar neuron, so cannot combine to exceed the threshold to create a generator potential.
- As well as this, high energy is needed to break down its pigment, iodopsin to stimulate a generator potential.
- Therefore only high-intensity light will be enough to overcome the threshold needed to break down iodopsin and create a generator potential.
52
Why do cone cells give good visual acuity?
- Each cone cell has its own connection to a single bipolar neuron.
- If two adjacent cones are stimulated the brain will receive two separate impulses.
- This means it is able to distinguish between two sources of light.
- So two dots close together can be resolved and will appear clearly.
53
Describe the distribution of rods and cones on the retina and explain why this is the case?
- There distribution on the retina is uneven.
- Light is focused opposite to the pupil on the retina (known as the fovea).
- Thus the fovea receives the highest intensity light.
- So cones are found at the fovea.
- Their concentration diminishes the further away, to the point where only rods are found at the peripheries of the retina where light intensity is at its lowest.
54
What is the autonomic nervous system responsible for?
Involuntary (subconscious) activities of the internal muscles and glands.
55
State and explain the two divisions of the autonomic nervous system.
Sympathetic:
- Stimulates effectors and speeds up activity.
- to help cope with stressful situations by heightening awareness and preparing for activity.
Parasympathetic:
- Inhibits effectors and slows down any activity.
- to control activities under normal resting conditions, and conserving energy and replenishing the body's reserves.
56
What word describes the relation between the sympathetic and parasympathetic nervous systems?
They are antagonistic to each other.
- This fact means they are able to work in tandem to regulate internal glands and muscles.
57
What does myogenic mean and what is it used to describe?
- When a muscles contractions are regulated from within the muscle itself.
- Used to describe the cardiac muscle.
58
What is the opposite of myogenic?
Neurogenic.
59
What about the heart makes it myogenic?
It has a sinoatrial node (SAN) where the initial stimulus for contraction originates.
60
What are the sequences of events that control the basic heart rate?
- A wave of electrical excitation spreads out from the sinoatrial node (SAN) across both atria.
- This causes both atria to contract.
- The atrioventricular septum (a layer of non-conductive tissue) prevents the wave crossing to the ventricles.
- Between the atria, the wave of excitation enters the atrioventricular node (AVN).
- After a short delay the AVN conveys a wave of electrical excitation between the ventricles along the bundle of His (made up of Purkyne tissue fibres).
- At the base of the ventricles the bundle of His branches off into smaller fibres of Purkyne tissue.
- This releases the wave of excitation, causing contractions of both ventricles quickly from the apex upwards.
61
What is the resting heart rate of a typical adult human?
70 bpm
62
How are changes to heart rate controlled?
By the region of the brain known as the medulla oblongata.
- Increasing heart rate is done via the sympathetic nervous system linked to the sinoatrial node.
- decreasing heart rate is done via the parasympathetic nervous system linked to the sinoatrial node.
63
What type of stimuli may cause a change in heart rate?
- Chemical (e.g. O₂/CO₂ concentration)
| - Pressure (e.g. blood pressure)
64
How do chemoreceptors affect the heart rate, and then how does this return to normal?
(use an increase in CO₂ conc in examples)
- When blood CO₂ conc is higher than normal its pH is lower (as CO₂ forms an acid).
- Chemoreceptors in the wall of the carotid arteries and aorta detect this and increase the frequency of nervous impulses to the medulla oblongata.
- The centre of the medulla oblongata that increases heart rate then increases the frequency of impulses via the sympathetic nervous system to the SA node.
- This increase the rate of production of electrical waves by the sinoatrial node, increasing the heart rate.
- This increases blood flow causing more CO₂ to be removed by the lungs, returning blood CO₂ to normal.
- As a result blood pH returns to normal and chemoreceptors reduce the frequency of nerve impulses to the medulla oblongata.
- The medulla oblongata reduces the frequency of impulse to the SA node, leading to the heart rate returning to normal.
65
What happens when blood pressure is higher than normal?
- Baroreceptors in the wall of the carotid arteries and aorta detect a change in blood pressure
- They then transmit more nerve impulses to the centre in the medulla oblongata that decreases heart rate.
- This centre sends impulses via the parasympathetic nervous system to the SA node.
- This decreases the rate of production of electrical waves by the SA node, decresing the heart rate.
66
What are the features of the hormonal system?
- Communication by chemicals called hormones.
- Transmission by via blood.
- Slow transmission.
- Hormones travel to all parts of the body, but only target cells respond.
- Response is widespread.
- Slow response
- Long-lasting response
- Effect may be permanent and irreversible.
67
What are the features of the nervous system?
- Communication by nerve impulses.
- Transmission by neurones.
- Rapid transmission.
- Impulses travel to specific parts of the body.
- Response is localised.
- Rapid response
- Short-lived response
- Effect usually be temporary and reversible.
68
What are neurotransmitters?
- Chemicals secreted in target cells for the nervous system.
69
What are neurones specialised for?
- Neurones (nerve cells) are specialised cells adapted to rapidly carry electrochemical changes.
- These are called nerve impulses and act from one part of the body to another.
70
What are mammalian motor neurones made up of?
A cell body:
- contains usual cell organelle, including a nucleus and many RER (for the production of proteins and neurotransmitters).
Dendrons:
- extensions of the cell body that subdivide into smaller branched fibres (dendrites) that carry impulses to the cell body.
An axon:
- a single long fibre that carries nerve impulses away from the cell body.
Schwann cells:
- surround the axon, protecting it and providing electrical insulation.
- carry out phagocytosis and play a part in nerve regulation.
A myelin sheath:
- forms a covering to the axon and is made up of membrane-bound Schwann cells
- its membrane is rich in lipids known as myelin.
- neurones with myelin sheaths is called a myelinated neurone.
Nodes of Ranvier:
- between two adjacent Schwann cells where there is no myelin sheath.
- 2-3μm long and occur every 1-3mm (in humans).
71
What are the three types of neuron (and their function/features)?
Sensory neurones:
- transmit impulses from a receptor to an intermediate or motor neuron.
- have one dendron that is very long that carries impulses towards the cell body
- have one axon that carries it away from the cell body.
Motor neurones:
- transmit nervous impulses from an intermediate or relay neuron to an effector.
- have one long axon
- have many short dendrites.
Intermediate or relay neurones:
- Transmits impulses between neurones.
- have numerous short processes.
72
When the electric potential difference across an axon membrane is reversed, what has happened?
It has gone from its resting potential to an action potential.
73
How is the movement of ions across the axon membrane controlled at its resting potential?
- The phospholipid bilayer prevents ions from diffusing across it.
- Channel proteins span the bilayer, these have ion channels that pass through them, and some have gates which can be opened or closed to regulate the facilitated diffusion of ions.
- Pumps (sodium-potassium pump) that actively transport ions into and out of the axon.
74
What is the charge of an axon relative to its outside at resting potential?
Negative.
75
In humans what is the resting potential?
-65mV
76
How is the axon made polarised at its resting potential?
- Sodium-potassium pumps actively transport sodium ions out.
- Sodium-potassium pumps actively transport potassium ions in.
- Active transport of Na⁺ out > the active transport of K⁺ in. (3:2)
- Higher conc of Na⁺ in tissue fluid around axon than in the cytoplasm.
- Higher conc of K⁺ in the cytoplasm than tissue fluid.
- So there is an electrochemical gradient.
- Thus K⁺ move back out via facilitated diffusion through open K⁺ channel proteins.
- But most Na⁺ channel proteins are closed, so less diffuse back in.
77
If a stimulus is great enough and provides enough energy what happens to the potential difference across the axon membrane?
It becomes depolarised.
From -65mV to +40mV.
78
Why does depolarisation have an effect?
It changes the shape of channel proteins in the axon membrane, opening or closing them.
This is why they are called voltage-gated channels.
79
Can depolarisation occur across the whole axon membrane?
No, only a particular point on the axon membrane.
80
Describe what happens during an action potential.
- At resting potential, some K⁺ voltage-gated channels are open (the ones that are permanently open), but the Na⁺ voltage-gated channels are closed.
- A stimulus provides energy that causes some Na⁺ voltage0gated channels to change shape and open.
- Na⁺ can now diffuse into the axon along its electrochemical gradient.
- As Na⁺ are positive this causes the p.d across the membrane to reverse.
- As Na⁺ ions diffuse into the axon, more Na⁺ channels open, causing an even greater influx.
- Once at +40mV the voltage-gated Na⁺ channels close and the remaining closed K⁺ voltage-gated channels begin to open.
- The electrical gradient that previously prevented the outflow of K⁺ is reversed.
- Thus more K⁺ diffuse out.
- This starts repolarisation.
- This causes a temporary overshoot of the electrical gradient, with the axon's inside being more negative (hyperpolarisation).
- This causes the closable K⁺ gates to close and the activities of the Na⁺-K⁺ pumps once again cause Na⁺ to be pumped out and K⁺ in.
- This re-establishes the resting potential of -65mV and the axon is now repolarised.
81
What is a travelling wave of depolarisation?
An action potential.
82
How does an action potential a travelling wave of depolarisation?
As one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for the depolarisation of the next region of the axon.
83
Why do action potentials pass along myelinated neurones faster than along unmyelinated ones?
- Myelinated axons have a fatty sheath of myelin around them.
- This acts as an electrical insulator as it prevents action potentials from forming along it's 1-3mm interval.
- The localised circuit can only arise between adjacent nodes of Ranvier, so the action potentials jump from node to node.
- This is known as saltatory conduction.
('saltare' means to jump in Latin)
84
What factors affect the speed of an action potential?
- The myelon sheath.
- The diameter of the axon.
- Temperature.
85
How does the diameter of an axon affect the speed at which it transmits an action potential?
- Greater diameter = faster speed of conductance.
- As less leakage of ions from large axons.
(the leaks would make the membrane potential harder to maintain)
86
How does the temperature of an axon affect the speed at which it transmits an action potential?
- Higher temp = faster speed of conductance.
- As higher temp = increased rate of diffusion.
- But the energy needed for active transport comes from respiration. This is controlled by enzymes.
- Enzymes function more rapidly at higher temps but only to a certain point.
- Above this temp and enzymes and the plasma membrane proteins will denature and impulses will fail to conduct.
- Enzymes also control the sodium-potassium pump.
87
What is the all-or-nothing principle?
- There is a certain threshold value which triggers an action potential.
- Below this, no action potential (and thus no impulse) is generated (Nothing).
- At/Above this and the action potential generated is more or less the same size (All).
88
If the all-or-nothing principle is true, how can organisms perceive the size of a stimulus?
- by the number of impulses passing in a given time.
(larger stimulus, more impulses per unit time)
- by having different neurones with different threshold values.
(brain interprets the number and type of neurones that pass impulses as a result of a given stimulus)
89
What happens after an action potential?
The refractory period.
90
What is the refractory period?
- When the inward movement of sodium ions are prevented because the Na⁺ voltage-gated channels are closed.
- During this time it is impossible for a further action potential to be generated.
91
What are the advantage (purposes) of the refractory period?
- It ensures action potentials are propagated in one direction only.
- It produces discrete impulses.
- It limits the number of action potentials
92
What is a synapse?
Where one neurone communicates with another or with an effectors
93
How is information transmitted across a synapse?
Via chemicals known as neurotransmitters.
94
What gaps separate neurones?
The synaptic cleft.
95
What is the presynaptic neurone?
The neuron that releases the neurotransmitter.
96
What is the synaptic knob?
The axon of the presynaptic neurone
97
What cell structure is found in numbers at the synaptic knob and why?
Mitochondria and endoplasmic reticulum.
- Are used in the manufacturing of neurotransmitters that takes place in the axon.
98
What is the role of the synaptic vesicle?
Stores neurotransmitters.
99
What are features of synapses?
- Unidireactionality.
- Summation.
- Inhibition.
100
What is spatial summation?
- When a number of different presynaptic neurones release enough neurotransmitter to exceed the threshold value of the postsynaptic neurones to trigger an action potential.
101
What is temporal summation?
- When a single presynaptic neurone releases neurotransmitter many times over a very short period.
- If its concentration exceeds the threshold value of the postsynaptic neurone, an action potential is triggered.
102
How do inhibitory synapses work?
- A neurotransmitter is released that binds to chloride ion protein channels on the postsynaptic neurone.
- This causes the channel to open.
- Cl⁻ moves into the postsynaptic neurone by facilitated diffusion.
- Nearby potassium protein channels are also opened.
- K⁺ move out of the postsynaptic neurone into the synapse.
- Due to the movement of these ions the inside of the postsynaptic neurone becomes more negative, and the outside more positive.
- This decreases the membranes potential to -80mV.
- The hyperpolarisation makes it less likely that a new action potential will be created because a larger influx of sodium ions are needed to produce one.
103
What are the advantages of the synapse when transmitting information from one neurone to another?
- It allows a single stimulus to create a number of responses.
- It allows nerve impulses from receptors reacting to different stimuli to contribute to a singe response.
104
How are neurotransmitters released?
When an action potential reaches the synaptic knob the membrane of the vesicle that stores neurotransmitter fuses with the presynaptic membrane to releases the neurotransmitter.
105
Why do synapses only work in one direction?
- Neurotransmitters are only made in the presynaptic neurone.
- The specific receptor proteins for neurotransmitters are only on the postsynaptic neurone.
106
What is a cholinergic synapse?
A synapse in which the neurotransmitter is acetylcholine.
107
Where are cholinergic synapses more commonly found?
Inside vertebrates between neuromuscular junctions.
108
Describe the process of transmission across a cholinergic synapse.
- A new action potential at the presynaptic neurone causes calcium ion protein channels to open.
- Ca⁺ enters the synaptic knob via facilitated diffusion.
- The influx of Ca⁺ causes synaptic vesicles to fuse with the presynaptic membrane.
- This releases acetylcholine into the synaptic cleft.
- Due to the short diffusion pathway, acetylcholine moves across the clef very quick.
- It then binds to receptors sites on sodium-ion protein channels on the postsynaptic neurone.
- This causes them to open, allowing Na⁺ to diffuse into the postsynaptic neurone.
- This generates a new action potential.
- Acetycholineesterases then hydrolyse the acetylcholine into choline and ethanoic acid, which then diffuses back across the cleft into the presynaptic neurone.
- Sodium-ion protein channels close in the absence of acetylcholine in the receptor site.
- ATP from mitochondria then recombine acetylcholine and stores it in the synaptic vesicles for further use.
109
What are the advantages of acetylcholine being hydrolysed after use?
- Prevents the continuous generation of a new action potential in the postsynaptic neurone (so leads to discrete transfer of info across a synapse).
- Allows for more acetylcholine to be made in the presynaptic neurone (it recycles it).
110
What are the three types of muscles in the body?
- Cardiac muscle.
- Smooth muscle (found in walls of blood vessels and gut).
- Skeletal muscles.
111
How is skeletal muscle different from other muscle?
- Makes up the bulk of body muscle (invertebrates).
| - Is attached to bones and acts under voluntary, conscious control.
112
List the structures of muscle?
- Whole muscle
- Bundle of muscle fibres (made up of nerves, blood capillary, and...)
- Single muscle fibres (fused cells that share a nucleus and cytoplasm, called sarcoplasm)
113
What makes up muscle fibers?
Myofibrils: made up from actin and myosin.
114
What is a sarcomere?
The distance between two adjacent Z-lines.
115
Whys do myofibrils appear striped?
Due to their alternating I bands (light) and A-bands (dark).
116
What is the light coloured region at the centre of the A band called?
H-zone.
117
Other than myofibrils what other proteins in found in muscles?
Tropomyosin.
118
What are the two types of muscle fibre?
- Slow-twitch.
| - Fast-twitch.
119
What are features of slow-twitch muscle fibres?
- Contract slower but over a longer period.
- Adapted for endurance.
- Adapted for aerobic respiration.
- Large store of myoglobin
- Rich supply to blood vessels.
- Numerous mitochondria
120
What are features of fast-twitch muscle fibres?
- Contract fast but over a short period.
- Adapted for explosiveness/intense exercise.
- Thicker and more numerous myosin filaments
- High concentration of glycogen
- High concentration of enzymes involved in anaerobic respiration to provide ATP rapidly.
- Stores phosphocreatine, that can rapidly generate ATP from ADP in anaerobic conditions.
121
What is a neuromuscular junction?
The point where a motor neuron meets a skeletal muscle fibre.
122
What would happen if there was only one neuromuscular junction on a muscle
- it would take time for a wave of contraction to travel across.
- not all fibres would contract simultaneously and the movement would be slow.
123
What is a motor unit?
- when all muscle fibres supplied by a singe motor neuron act together as a single functional unit.
- more force needed = more motor unit are stimulated.
124
What happens when a nerve impulse is received?
- The synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine.
- Acetylcholine diffuses to the postsynaptic membrane of the muscle fibre.
- It alters its permeability to Na⁺, which enters rapidly, depolarising the membrane.
125
What happens to acetylcholine after use?
- It is broken down by acetylcholinesterase to ensure the muscles aer not over-stimulated.
- The cholinen and ethanoic acid diffuses back into the neuron.
- Here they recombine using energy provided by the mitochondria.
126
What are the similarities between the neuromuscular junction and a synapse?
- both have neurotransmitters that are transported by diffusion.
- both have receptors, that on binding with the neurotransmitter, cause an influx of sodium ions.
- both use a sodium-potassium pump to depolarize the axon.
- Use enzymes to breakdown the neurotransmitter.
127
What are the differences between the neuromuscular junction and a synapse?
- Neuromuscular junction (NJ) can only be excitatory, whilst a synapse (S) can also be inhibitory.
- NJ only links neurons to muscles, whilst a S links neurons to neurons, or neurons to other effector organs.
- Only motor neurons are involved in NJ, whist motor, sensory and intermediate can be involved in S.
- The action potential ends at a NJ, whilst a new action potential may be produced along another neuron (postsynaptic).
- At NJ acetylcholine binds to receptors on membrane of muscle fibres, whilst acetylcholine binds to receptors on membrane of postsynaptic neuron in S.
128
What does the contraction of a skeletal muscle do?
Moves the skeleton.
129
Why do muscles act antagonistically to each other?
- Muscles can only pull, so having two acting antagonistically allows motion in both directions.
130
When muscles contract, what happens to the sarcomere?
- I-band becomes narrower.
- Z-line moves closer together (the sarcomere shortens).
- H-zone becomes narrower.
- A-band remains the same width.
131
What proof discounts the claim that muscle contractions are due to the filaments themselves shortening?
- A-band remains the same width (which is determined by the myosin filaments)
- Thus myosin filaments do not become shorter.
132
What is proof for the sliding filament mechanism?
- Myofibrils appear darker in colour when the actin and myosin filaments overlap.
- Therefore we should see more overlap of actin and myosin when a muscle is contracted.
133
Describe the three main proteins involved in the sliding filament mechanism?
- Myosin, is made up of two types of protein:
- fibrous protein arranged into a filament made up of several hundred molecules (the tail).
- globular protein formed into two bulbous structures at one end (the head).
- Actin is a globular protein that is arranged into long chains that are twisted around one another to form a helical strand.
- Tropomyosin forms long thin threads that are wound around actin filaments.
134
What are the three stages of the sliding filament mechanism?
- Muscle stimulation.
- Muscle contraction.
- Muscle relaxation.
135
What happens during the muscle stimulation stage of the sliding filament mechanism?
- An action potential reaches many neuromuscular junctions simultaneously,
- causing calcium ion protein channels to open and calcium ions to diffuse into the synaptic knob.
- The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane.
- This releases their acetylcholine into the synaptic cleft.
- Acetylcholine diffuses across the synaptic cleft and binds with receptors on the muscle cell-surface membrane.
- This causes depolarisation.
136
What happens during the muscle contraction stage of the sliding filament mechanism?
- An action potential travels through T-tubules, and branch throughout the sarcoplasm.
- The action potential reaches the sarcoplasmic reticulum, opening up Ca⁺ protein channels.
- Ca⁺ diffuse into the sarcoplasm causing tropomyosin to pull away from the actin filament binding site.
- ADP attaches to myosin heads, changing their angle, pulling the actin filament along with.
- It releases ADP.
- ATP attaches to myosin head, detaching it from the actin filament.
- Ca⁺ activates ATPase to hydrolyse ATP to ADP.
- The energy from this returns the myosin head to its original position.
- Myosin attaches to ADP and then reattaches further along the actin filament.
- As long as the conc of Ca⁺ in the myofibrils is high this will continue.
- more on page 375
137
What happens during the muscle relaxation stage of the sliding filament mechanism?
- After nervous nervous stimulation, Ca⁺ are actively transported back into the sarcoplasmic reticulum using the energy from the hydrolysis of ATP.
- This allows tropomyosin to block actin filaments again.
- Myosin heads are unable to bind to actin filaments and contraction ceases.
- The muscle relaxes
- At this point the force from an antagonistic muscle can pull actin filaments out from between myosin to a point.
138
What is energy from ATP needed from during muscle contraction?
- to move the myosin head.
| - to reabsorb calcium ions into the endoplasmic reticulum via active transport.
139
Why is there a need to anaerobically generate ATP in contracting muscles?
- Energy is often needed in large quantities.
- (e.g. escaping a predator)
- The body cannot inhale enough oxygen to meet this demand sometimes.
- Therefore ATP must be made anaerobically.
140
How is the anaerobic ATP demand helped to be met?
- By phosphocreatine.
| - By more glycolysis.
141
What is the function of phosphocreatine?
- Stored in the muscles, it acts reserve supply of phosphate.
- It is immediately available to combine with ADP to reform ATP.
142
How is phosphocreatine replenished?
- Using phosphate from ATP when the muscle is relaxed.
143
Define homeostastis?
The maintenance of a constant internal environment.
144
Whys is homeostasis essential for the proper functioning of organisms?
- Enzymes that control biochemical reactions are sensitive to change.
- Changes to the water potential of blood and tissue fluids may cause cells to shrink or burst.
- A constant blood glucose conc ensures a reliable source of glucose for respiration by cells.
- Organism that are more able to maintain a constant internal environment are more independent to changes in the external environment.
145
What are the stages involved in any sefl-regulating system?
- Optimum points.
- Receptors.
- Coordinator.
- Effector.
- Feedback mechanisms.
146
What is meant by positive feedback?
- When a deviation from an optimum causes changes that result in an even greater deviation from the normal.
147
What is meant by positive feedback?
When a stimulus returns the system to its original (optimum) level, and prevent any overshoots.
148
Why are there separate negative feedback mechanisms that control departures from the normal in either direction?
To give a greater degree of homeostatic control.
149
What characteristics do hormones have in common?
- Produced in glands, which secrete them into the blood (endocrine glands).
- Carried in blood plasma to the target cell.
- Have specific receptors on their cell-surface membrane that are complimentary to the cell in which they act.
- Are effective in very low concentrations but often widespread and long-lasting.
150
Describe the second messenger model (with blood glucose conc as an example).
- Adrenaline binds to transmembrane protein receptors within cell-surface membrane of liver cells.
- Causing the protein to change shape on the inside membrane.
- Leading to the activation of adenyl cyclase that converts ATP to cAMP.
- cAMP acts as a second messenger that binds to protein kinase enzyme, changing its shape and activating it.
- This catalyses the conversion of glycogen to glucose that moves out of the liver cell by facilitated diffusion, into the blood via channel proteins.
151
What cells make up the panceurs?
- Mostly cells that produce digestive enzymes.
| - hormone producing, islets of Langerhans.
152
Describe the cells of the islets of Langerhans?
- α cells: Larger and produce glucagon.
| - β cells: Smaller and produce insulin.
153
What are hepatocytes?
Cells that make up the liver.
154
What processes associated with regulating blood sugar take place in the liver?
- Glycogenesis.
- Glycogenolysis.
- Gluconeogenesis.
155
What is glycogenesis?
- The conversion of glucose into glycogen.
| - When blood glucose is high the liver can store glycogen.
156
What is glycogenolysis
- The breakdown of glycogen to glucose.
| - When blood glucose is low the liver can restore it back to normal.
157
Gluconeogenesis?
- The production of glucose from sources other than carbohydrates.
- When glycogen stores are exhausted, the liver can produce glucose from molecules such as glycerol and amino-acids.
158
What is the main consequence of low blood glucose?
- Cells deprived of energy, leading to death.
159
What is the main consequence of high blood glucose??
- Lowers water potential of the blood, causing dehydration.
160
What factors affect blood glucose?
- Diet: as glucose hydrolysed from carbohydrates (like starch, maltose, lactose and sucrose.)
- Glycogenolysis
- Gluconeogenesis.
161
What hormones help maintain blood glucose?
- Insulin.
- Glucagon.
- Adrenaline.
162
What type of protein is insulin?
Globular
| made from 51 amino acids.
163
What do almost all body cells have (with insulin in mind)?
Glycoprotein receptors on their cell-surface membrane that is complimentary to insulin.
164
What happens once insulin binds to glycoprotein receptors?
- Changes the tertiary structure of glucose transport carrier proteins, causing them to change shape and open. So, more glucose makes into the cell by facilitated diffusion.
- Vesicles with the carrier proteins fuse with the cell-surface membrane, so increases the number of glucose transport channels.
- Enzymes are activated that convert glucose to glycogen and fat.
165
What are the ways in which blood glucose concentrations are lowered?
- Increasing the rate of absorption of glucose into the cell (especially in muscles).
- Increasing the respiratory rate of cells, using up more glucose, increasing the uptake of glucose from the blood.
- Increasing the rate of conversion of glycogenesis in the cells of the liver and muscle.
- Increasing the rate of conversion of glucose to fat.
166
What does the lowering of blood glucose concentration lead to?
(still above optimum)
β-cells to reduce their secretion of insulin.
| negative feedback
167
Other than glucagon, what hormone helps increase blood glucose concentrations?
Adrenalin.
168
How does adrenalin increase blood glucose concentration?
- by attaching to protein receptors on the cell-surface membrane of targeted cells.
- by activating enzymes that cause the breakdown of glycogen to glucose in the liver
169
As the concentration of insulin in the blood increases, the secretion of insulin...
reduces.
170
What is the relation between insulin and glucagon?
They act antagonistically.
171
Summarise what happens to return the blood glucose to optimal levels?
- Rise in BGconc detected by β-cells of the pancreas.
- They produce/secrete insulin into blood.
- This increases cellular respiration, converts glucose to glycogen, converts glucose to fat, and absorbs glucose into cells.
- BGconc falls.
- Fall in BGconc detected by α-cells of the pancreas.
- They produce/secrete glucagon into blood.
- This coverts glycogen to glucose and amino acids to glucose.
- An uncontrolled quantity of glucose may also enter from the intestines (after digestion).
- BGconc rises
(cycle repeats to maintain optimal conc)
172
What are the alternative names for diabetes types I and II?
Type I: insulin-dependent
Type II: insulin-independent
173
Describe features of Type I diabetes?
- Body is unable to produce insulin.
- Normally begins in childhood.
- May be caused by the body's immune system attacking β-cells of the islets of Langerhans.
- develops quickly over a few weeks.
174
What the signs and symptoms of Type I diabetes?
Signs:
- High BGconc.
- Glucose in urine.
- Need to urinate excessively.
- Genital itching/ regular episodes of thrush.
- Weight loss.
- Blurred vision.
Symptoms:
- Tiredness
- Increased thirst and hunger
175
Describe features of Type II diabetes?
- Due to glycoprotein receptors on body cells being lost or losing their responsiveness to insulin.
- May also be due to an inadequate supply of insulin from the pancreas.
- Usually develops in over 40s, however, poor diet and obesity may lead to it.
- develops slowly with less severe/unnoticed symptoms.
176
How is type I diabetes treated?
- Injecting insulin, two or four times a day.
| - With the does matched to the glucose intake.
177
How are insulin injections matched to glucose intake?
Using biosensors.
178
Why is insulin not taken by mouth?
It would be digested in the alimentary canal (digestive system).
179
How is type II diabetes treated?
- by regulating the intake of carbohydrate in the diet, and matching this to the amount of exercise done.
- In some cases supplemented by injections or drugs that stimulate insulin production or slows down the rate of glucose absorption.
180
Whta is osmoregulation?
The homeostatic control of water potential of the blood.
181
what organ carries out osmoregulation?
Kidney.
182
What is a nephron?
The functional unit of the kidney.
183
What structures make up the mammalian kidney?
- Fibrous capsule: Outer membrane.
- Cortex: lighter coloured outer region made up of renal (Bowman's) capsules.
- Medulla: darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels.
- Renal pelvis: funnel-shaped cavity that collects urine into ureter.
- Ureter: tube that carries urine to bladder.
- Renal artery: supplies kidney with blood from heart via aorta.
- Renal vein: returns blood to heart via vena cava.
184
List the structures of the nephron.
- Renal (Bowman's capsule.
- Proximal convoluted tubule.
- Loop of Henle.
- Distal convoluted tubule.
- Collecting duct.
185
Describe the renal (Bowman's) capsule.
- closed end at start of nephron.
- Cup-shaped.
- surrounded by blood capillaries (glomerulus).
- Inner layer made up of podocytes.
186
Describe the proximal convoluted tubule.
A series of loops surrounded by blood capillaries.
- Walls are made of epithelial cells that have microvilli.
187
Describe the loop of Henle.
- Long, hairpin loop that extend from cortex into medulla and back again.
- Surrounded by capillaries.
188
Describe the distal convoluted tubule.
- Series of loops surrounded by blood capillaries.
- Walls made of epithelial cells.
- Fewer capillaries than the PCT.
189
Describe the collecting duct.
- Tube into which a number of DCTs from a number of nephrons empty.
- Lined with epithelial cells and become increasingly wide as it empties into the pelvis of the kidney.
190
List the blood vessels that are associated with the nephron
- Afferent arteriole.
- Glomerulus.
- Efferent arteriole.
- Blood capillaries.
191
Describe the afferent arterioles?
- Tiny vessels that arise from the renal artery.
| - Supplies nephron with blood.
192
Describe the glemerulus?
- Many branched knot of capillaries that fluid is forced out of the blood.
- capillaries recombine to form efferent arteriole.
193
Describe the efferent arterioles?
- Tiny vessels that leave the renal capsule.
- Smaller diameter than afferent arteriole.
- Carries blood away from renal capsule and branches to form blood capillaries.
194
Describe the blood capillaries?
- Concentrated network of capillaries that surround PCT, loop of Henle and DCT.
- reabsorb mineral salts, glucose, and water.
- Merge together into venules that merge to form renal veins
195
What are the stages that the nephron carries out during osmoregulation?
- the formation of glomerular filtrate by ultrafiltration.
- reabsorption of glucose and water by the proximal convoluted tubule.
- Maintenance of a gradient of sodium ions in the medulla by the loop of Henle.
- Reabsorption of water by the distal convoluted tubule and collecting ducts.
196
Describe the process of the formation of glomerular filtrate by ultrafiltration.
- A build-up of hydrostatic pressure takes place in the glomerulus,
- as a result of the diameter of the afferent arteriole being greater than that of the efferent arteriole.
- As a result, water glucose and mineral ions are squeezed out of the capillary.
- This forms the glomerular filtrate.
- Blood cells and large proteins are unable to pass across the renal capsule as they are too large.
197
What resists the movement of the glomerular filtration out of the glomerulus?
- Capillary endothelial cells.
- Connective tissue and endothelial cells of the blood capillary.
- Epithelial cells of the renal capsule.
- The hydrostatic pressure of the fluid in the renal capsule space.
- The low water potential of the blood in the glomerulus.
198
What reduces the resistance to the movement of the glomerular filtration out of the glomerulus?
- Podocytes allow filtrate to pass beneath them and through gaps between their branches. So filtrate passes between them rather than through them.
- The endothelium of the glomerular capillaries has spaces between cells, so fluid can pass between rather than through.
199
How is the proximal convoluted tubule adapted to reabsorb substances into the blood?
By having epithelial cells that...
- have microvilli to provide a large SA:V to reabsorb substances from the filtrate.
- have infolds at their base to give a large SA:V to transfer reabsorbed substances into blood capillaries.
- has a high density of mitochondria to provide ATP for active transport.
200
How does proximal convoluted tubule reabsorb substances into the blood?
- Na ions are actively transported out of the cell lining the PCT.
- Na conc in the cells lowered.
- Na ions diffuse from the lumen of PCT into epithelial clinging cells through specific carrier proteins by facilitated diffusion.
- As this happens another molecule (glucose, amino acids, chloride ions etc), are co-transported along with it.
- These molecules can then diffuse into the blood.
201
Describe the two regions of the loop of Henle?
- Descending limb: narrow, with thin walls that are highly permeable to water.
- Ascending limb: wider, with thick walls that are impermeable to water.
202
How does the loop of Henle work?
- Na ions are actively transported out of the ascending limb using ATP provided by the many mitochondria in the cell of its wall.
- This lowers the water potential in the region of the medulla between the two limbs.
- Water passes out of the descending limb into the interstitial space, by osmosis.
- This decreases its water potential, reaching a minimum at the tip o the loop.
- Na ions diffuse out of the filtrate as it moves up the ascending limb, and are actively transported higher up.
- As a result, the water potential gets progressively higher.
- This means in the interstitial space between the ascending limb and the collection duct, there is a gradient of increasing ion concentration from the cortex to the medulla.
- As a result, water in the filtrate moving down the collection duct passes out by osmosis through aquaporins, and this osmotic gradient is maintained.
- The gradient is also maintained as water moves into blood capillaries which transported it away.
(Counter-current multiplier)
203
What is the role of the distal convoluted tubule?
- To make final adjustments to the water and salts that are reabsorbed.
- To control the pH of the blood by selectively reabsorbing ions.
204
What may cause a lowered water potential?
- Too little water being consumed.
- Sweating occurring.
- Large amounts of ions, e.g sodium ions, being taken in.
205
Describe how the body regulates water potential in response to a fall in the water potential.
- Osmoreceptors in the hypothalamus detect the fall in water potential.
(as they shrink when the water potential of blood is too low)
- This causes them to produce ADH and secrete them into capillaries at the posterior pituitary gland.
- ADH binds to specific protein receptors on the cell-surface membrane of the DCT and collecting duct.
- This activates phosphorylase within the cell, causing vesicles to fuse with the cell-surface membrane.
- These vesicles carry plasma membrane that has numerous aquaporins, as a result the permeability to water of the cell-surface membrane increases.
- The permeability to urea also increases, which moves into the interstitial space further lowering the water potential of fluid around the duct.
- Both combined lead to more water leaving by osmosis and re-entering the blood.
- This prevents the water potential of the blood from decreasing any further (will not increase it though)
- Osmoreseptors also send ner impulse to thrust sensor in the brain in order to seek out more water and increase the water potential.
- Osmoreseptors detect a rise and send fewer impulse to the pituitary gland.
- Thus less ADH is released; the permeability of the collecting duct decreases and urea reverts to its former state.
206
What may cause an increased water potential?
- Large volume water being consumed.
| - Salts used in metabolism or excreted not being replaced in the diet.
