Module 5 Section 1: Communication and Homeostasis Flashcards
1
Q
What is the nervous system made up of
A
2
Q
What is the CNS composed of and what neurons does it contain
A
Brain and spinal cord
Contains relay neurons
3
Q
What is the PNS composed of and what neurons does it contain
A
Composed of cranial nerves, spinal nerves, peripheral nerves
Contains sensory neurons and motor neurons
Acts as interface between CNS and environment and transmits electrical impulses to and from the CNS
4
Q
What is the peripheral nervous system
A
Made up of the neurons that connect the CNS to the rest of the body
Has two different functional systems: somatic nervous system and autonomic nervous system
5
Q
What is the somatic nervous system
A
Controls conscious conscious activities
E.g. running and playing videos games
6
Q
What is the autonomic nervous system
A
Controls unconscious activities
E.g. digestion
Got two subdivisions that have opposite effects on the body
7
Q
What is the sympathetic nervous system
A
Gets the body ready for fight or flight
Sympathetic neurons release the neurotransmitter noradrenaline from the adrenal glands
Causes heart rate to increase and allows rapid blood and glucose supply to respiring muscles
Allows high intensity activities like running from a predator to be an immediate response
8
Q
What is the parasympathetic nervous system
A
Calms the body down in rest and digest system
Parasympathetic neurons release the neurotransmitter acetylcholine
9
Q
What are neurons
A
Neurons are specialised cells that conduct electrical impulses within the nervous system
10
Q
What is a nerve
A
A nerve is a bundle of many neuron fibres enclosed within a protective sheath
11
Q
What are nerve fibres
A
These are the long axons of neurons together with any associated tissues
12
Q
Different components of a nervous system reaction
A
Stimulus: change in internal or external environment (e.g. touching hot dish)
Receptor: detect stimulus (e.g. sense organ)
Coordinator: formulates a suitable response to a stimulus
Effector: produces a response
13
Q
What do sensory receptors act as
A
Sensory receptors act as transducers
Convert energy of a stimulus into electrical energy
14
Q
Pathway of a nerve impulse (conscious action)
A
15
Q
All the receptors and what they are sensitive to
A
Receptors in eyes are sensitive to light
Receptors in ear are sensitive to sound
Receptors in tongue are sensitive to chemicals
Receptors in nose are sensitive to chemicals
Receptors in skin are sensitive to touch, pressure, pain and temperature
16
Q
What features are found in all neurons
A
Neurons have a long fibre know as an axon
They have a cell body that contains the nucleus and other cellular structures
End of the axon (axon terminal) contains nerve endings
The axon terminal allows neurons to connect to many other neurons and receive impulses from them, forming a network for easy communication
17
Q
3 types of neurons
A
Sensory neurons
Relay neurons
Motor neurons
18
Q
What are sensory neurons
A
Transmits the electrical impulses from receptors to CNS
Have one dendron which carries impulses to the cell body and one axon which carries the impulse away from the cell body
19
Q
What are motor neurons
A
Transmits electrical impulses from the CNS to the effector (muscle/gland)
20
Q
What relay neurons
A
(Intermediate) transmits electrical impulses within CNS, connect sensory and motor neurons
21
Q
Adaptations of neurons
A
High branched: thin dendrites extend from the cell body and communicate with other neurons to allow electrical impulses to pass from one to the other
Myelin sheath: insulates the axons to ensure the impulses travel rapidly along the axon
22
Q
Myelin sheath
A
Axons can be myelinated and electrically insulated by a myelin sheath (fatty substance)
Has small uninsulated sections along the length (nodes of ranvier)
Myelin sheath made up of specialised cells called Schwann cells
Means that electrical impulses can jump from one node to the other so impulses can travel much faster
Each time they grow around the axon, a double layer of phospholipid bilayer is laid down
23
Q
How does action potential travel faster down myelinated sheath
A
Between Schwann cell are patches of bare membrane called the nodes of ranvier
Sodium ion channels are concentrated at the nodes
In a myelinated neuron, depolarisation only happens at the nodes of ranvier (where Na+ can get through the membrane)
Neurons cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse ‘jumps’ from node to node
Called saltatory conduction (100x faster than non-myelinated)
24
Q
A
25
Where are many unmyelinated neurons
Many nerves in the CNS are myelinated
They make up the grey matter in the brain and spinal cord
26
How does action potential travel along unmyelinated neurons
Impulses travel as a wave along the whole length of the axon membrane
Slower than saltatory conduction
27
How many stimuli are receptors adapted to
Receptors are specific to only one type of stimulus
E.g. light, pressure, glucose concentration
Receptors can be cells or proteins
28
What is an example of receptors
Pacinian Corpuscles - pressure receptors in the skin
29
How do receptors work
When a receptor is in a resting state (not being stimulated), there’s a difference in charge between the inside and outside of the cell - generated by ion pumps and ion channels
Means that there’s a voltage across the membrane (potential difference)
Potentiation difference when a cell is at rest is called it’s resting potential
When a stimulus is detected, the cell membrane is excited and become more permeable, allowing more ions to move in and out of the cell which alters the potential difference
The change in potential different due to a stimulus is called the generator potential
Bigger stimuli excites membrane more, causing a bigger movement of ions and a bigger change in potential difference - so a bigger generator potential is produced
If a generator potential is big enough it will trigger an action potential (nerve impulse) along a neuron
An action potential is only triggered if the generator potential reaches a certain level called the threshold level
If stimulus is too weak the generator potential won’t reach the threshold, so there’s no action potential
30
What are pacinian corpuscles
These are mechanoreceptors that detect mechanical stimuli e.g. pressure and vibrations
Found in skin
Composed of a sensory neuron which is wrapped in layers of lamellae
When the pacinian corpuscle is stimulated the lamellae deform and press on the sensory nerve ending
This causes sodium ion channels to open and sodium ions diffuse into the cell, creating a generator potential
If the generator potential reaches the threshold - it triggers an action potential along the sensory neuron
31
What are dendrons
Short extensions which come from the cell body
These extensions divide into smaller and smaller branches known as dendrites
They are responsible for transmitting electrical impulses towards the cell body
32
Features of sensory receptors
Specific to a single type of stimulus
Act as transducers to convert a stimulus into a nerve impulse
33
How do sensory receptors act as transducers
Detect a range of different stimuli including light, heat, sound or pressure
The receptor converts the stimulus into a nervous impulse
This is a generator potential
34
What electrical potential is the inside of a resting axon
Resting axons (one not transmitting impulses) always have a negative electrical potential compared to the outside of the axon
This is called the resting potential
Around -70mV
35
What is the potential difference of an axon at resting potential
This potential difference is usually about -70mV
The inside of the axon has an electrical potential about 70mV lower than the outside
36
What factors contribute to maintaining the resting potential
Sodium-potassium pump active transports sodium ions and potassium ions (requires ATP)
Differential membrane permeability
Maintained by keeping more positive ions outside the cell than inside
This means that the inside is more negatively charged, it is polarised
37
How do the cell surface membrane of neurons allow ions to come in and out
Cell surface membrane has selective protein channels that allow sodium and potassium ions to move across the membrane by facilitated diffusion
Channels are less permeable to sodium ions than potassium ions
Means that potassium ions can diffuse back down their concentration gradient, out of the axon at a faster rate than sodium ions
38
What does the sodium-potassium pump do
Uses ATP to pump 3 sodium ions out of the cell and 2 potassium ions into the cell
SOPI (Sodium Out Potassium In)
39
What is the generator potential
When a stimulus is detected, the cell membrane is excited and becomes more permeable
This allows more ions to move in and out of the cell
This alters potential difference
Change in potential difference is due to a stimulus is called the generator potential.
The bigger the stimulus the bigger the generator potential that is produced
40
What is the all or nothing nature of action potentials
Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is
If the threshold isn’t reached, an action potential will not fire
A bigger stimulus won’t cause a bigger action potential, but it will causes them to fire more frequently
So if the brain receives a high frequency of action potentials, it interprets this as a big stimulus and responds accordingly
41
What is an action potential
An action potential occurs via a brief change in the distribution of electrical charge across the cell surface membrane
42
How are action potentials caused and how does this work
Action potentials are caused by the rapid movement of sodium ions and potassium ions across the membrane of the axon
There are voltage-gated channel proteins which open and close depending on the electrical potential (or voltage) across the axon membrane and allow Na+ and K+ ions to pass through
They are closed when the axon membrane is at its resting potential
43
Steps of how action potential occurs
Stimulus
Depolarisation
Repolarisation
Hyperpolarisation
Resting potential
44
What happens when the stimulus is presented in the action potential process
Stimulus excites the neuron cell membrane
Causes sodium ion channels in the membrane to open
Membrane becomes more permeable to sodium
So more sodium diffuse into the neuron down the sodium ion electrochemical gradient
Inside of the neuron becomes less negative
45
What happens during depolarisation during action potential
If potential difference reaches threshold (around -55 mV) an action potential is stimulated
More voltage gated sodium ion channels in the membrane open
More sodium ions diffuse into the axon down the electrochemical gradient
The inside of the axon becomes less negative thereby reducing the potential difference across the axon
Depolarisation triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation
This is positive feedback
The action potential that is generated will reach a potential of around +30mV
46
What happens during repolarisation during action potential
About 1ms after the potential difference has reached +30mV, all the sodium ion voltage-gated channel proteins in this section close, stopping any further sodium ions diffusing into the axon
Potassium ion voltage-gated channel proteins in this section of axon membrane now open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient
This returns the potential difference to normal/resting potential (about -70mV) (repolarisation)
This is example of negative feedback
47
What happens during hyperpolarisation during action potential
Potassium ion channels are slow to close so there is a slight “overshoot” where too many potassium ions diffuse out of the neuron
This means that the potential difference across this section of axon membrane briefly becomes more negative than the normal resting potential (less than -70mV)
This refractory period acts as a time delay between one action potential and the next
It makes sure action potential don’t overlap but instead pass along discrete separate impulses
48
What happens during the return to resting potential during action potential
Once the potassium proteins are closed the sodium-potassium pump restores the resting potential
By pumping 3 sodium ions out and 2 potassium ions in (SOPI)
The sodium ion channel proteins in this section of membrane become responsive to depolarisation again
This maintains the resting potential until the membrane is excited by another stimulus
49
Each stage of action potential with concentrations of ions, membrane potential, whether sodium potassium pump is working, open or closed voltage gated sodium channels and open or closed voltage gated potassium channels
50
Why is there a refractory period
Acts as a time delay between one action potential and the next
Makes sure action potentials don’t overlap but instead pass along discrete separate impulses
Makes sure they are unidirectional
Means there is a limit to the frequency at which nerve impulses can be transmitted
51
How does action potential travel along an axon
An action potential triggered in the neuron causes depolarisation of that section of the axon
The current causes the opening of sodium ion channels a little further up the axon
This causes an influx of sodium ions in this section of the axon generating an action potential in this direction
The previous section of the axon is the repolarisation stage (the sodium channels are closed and potassium channels are open) and is unresponsive
This makes the action potentials discrete events and means the impulses can only travel in one direction
52
What is a synapse
This is a junction between a neuron and another neuron or between a neuron and an effector cell
E.g. a muscle or gland cell
53
What is a synaptic cleft
This is the small gap between the cells at a synapse
54
Process of synaptic transmission
Presynaptic neuron has a swelling called a synaptic knob
This contains synaptic vesicles filled with neurotransmitters
When an action potential reaches the end of a neuron it causes the neurotransmitters to be released into the synaptic cleft
They diffuse across to the postsynaptic membrane and bind to specific receptors
When neurotransmitters bind to receptors they might trigger an action potential (in neuron), cause muscle contraction, or cause a hormone to be secreted (from gland cell)
Neurotransmitters are removed from the cleft so the response doesn’t repeat
E.g. they’re taken back into presynaptic neuron or broken down by enzymes (and products taken back into the neuron)
55
About synapses that use Acetylcholine
These are called cholinergic synapses
They use acetylcholine (ACh)
These are broken down by an enzyme called acetylcholinesterase
56
Stages of how neurotransmitters transmit impulses between neuron
An action potential triggers calcium influx
Calcium influx causes neurotransmitter release
The neurotransmitter triggers an action potential in the postsynaptic neuron
57
What happens when an action potential triggers a calcium influx
Action potential arrives at the synaptic knob of the presynaptic neuron
Action potential stimulates voltage-gated calcium ion channels in the presynaptic neuron to open
Calcium ions diffuse into the synaptic knob (pumped out afterwards by active transport)
58
What happens when calcium influx causes neurotransmitter release
The influx of calcium ions into the presynaptic knob causes the synaptic vesicles to move to the presynaptic membrane
They must fuse with the presynaptic membrane
The vesicles release the neurotransmitter into the synaptic cleft by exocytosis
59
What happens when the neurotransmitter triggers an action potential in the postsynaptic neuron
The neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane
This causes sodium ion channels in the postsynaptic neuron to open
The influx of sodium ions into the postsynaptic membrane causes depolarisation
An action potential on the postsynaptic membrane is generated if the threshold is reached
The neurotransmitter is removed from the synaptic cleft so the response doesn’t keep happening
60
What happens at an excitatory synapse
Neurotransmitters depolarise the postsynaptic membrane
This makes it fire an action potential if the threshold is reached
61
What happens at an inhibitory synapse
This is when neurotransmitters bind to receptors on the postsynaptic membrane
They hyperpolarise the membrane (make the potential difference more negative)
This prevents an action potential from being fired
62
How can information be dispersed or amplified at a synapse
Synaptic divergence is where one neuron is connected to many neurons and information is dispersed to different parts of the body
Synaptic convergence is where many neurons connect to one neuron so information is amplified
63
What are the two types of summation
Spatial summation
Temporal summation
64
What happens is a stimulus is weak and how is this overcome
Only a small amount of neurotransmitter will be released from a neuron into the synaptic cleft
This may not be enough to excite the postsynaptic membrane to the threshold level and stimulate an action potential
Summation is where the effect of neurotransmitters can be combined
65
What happens in spatial summation
When neurons converge, the small amount of neurotransmitter released from each neuron can be enough altogether to reach the threshold in the postsynaptic neuron and trigger an action potential
If some neurons release an inhibitory neurotransmitter then the total effect of all the neurotransmitters might be no action potential
Stimuli might arrive from different sources but spatial summation allows signals from multiple stimuli to be coordinated into a single response
66
What happens in temporal summation
Temporal summation is where two or more nerve impulses arrive in quick succession from the same presynaptic neuron
This makes an action potential more likely, because more neurotransmitter is released into the synaptic cleft
67
What does summation generally allow
Both types of summation means synapses accurately process information.
This finely tunes the response
68
How does synapses make sure the impulses are only transmitted one way
Receptors for neurotransmitters are only on the postsynaptic membranes.
This means synapses make sure impulses can only travel in one direction
69
Label the synapse
70
Graph of how spatial summation works
Excitatory potentials from many neurons trigger threshold point
71
Graph of how temporal summation works
Many excitatory potentials from one neuron trigger action potential
72
What is a hormonal system made up of
Made up of glands (endocrine glands) and hormones
73
What are endocrine glands
Groups of cells that are specialised to secrete hormones
E.g. the pancreas secretes insulin
74
What are hormones
Chemical messengers
Many hormones are proteins or peptides e.g. insulin
Some are steroids e.g. progesterone
75
How do hormones travel around the body
Diffuse into the blood and are taken around the body by the circulatory system
76
What are exocrine glands
E.g. in digestive system
Secrete chemicals through ducts into organs or to the surface of the body
77
How do hormones activate a response in a cell
Diffuse out of blood
Bind to specific receptors for that hormones on target cells
These are found on membrane or in cytoplasm of cells in the target organs
78
Flow chart for how hormones trigger a response in the target cells
Stimulus: low blood glucose conc
Receptor: receptors on pancreas detect low blood glucose conc
Hormone: pancreas releases hormone glucagon into blood
Effectors: target cells in liver detect glucagon and convert glycogen into glucose
Response: glucose released into the blood, so glucose conc increases
79
Why is a hormone the first messenger
A hormone is first messenger because it carries the chemical message the first part of the way from the endocrine gland to the receptor on the target cells
Rather than directly moving into cell to produce response
80
Process of what happens inside a cell when hormone binds
Hormone (first messenger) binds to its receptor on target cell it activates an enzyme in the cell membrane
The enzyme catalyses the production of a molecule inside the cell called a signalling molecule (secondary messenger)
Signalling molecule carries the chemical message from the receptor to other parts of the cell
Signalling molecule activates a cascade (chain) or reactions which results in a response to the stimulus
81
Example of how adrenaline activates response from cells
Adrenaline is first messenger
Binds to specific receptors in cell membranes of many cells (e.g. liver cells)
When adrenaline binds it activates an enzyme in the membrane called adenylyl cyclase
Activated adenylyl cyclase catalyses the production of secondary messenger called cAMP from ATP
cAMP activates a cascade
E.g. cascade of enzyme reactions to make more glucose available to the cell by catalysing the breakdown of glycogen into glucose
82
What are the adrenaline glands
Endocrine glands that are above the kidneys
Each gland has an outer part (cortex) and an inner part (medulla)
Cortex and medulla have different functions and produce different responses
83
Function of the adrenal cortex
Secretes steroid hormones (e.g. cortisol and aldosterone when you’re stressed)
These hormones have a role in both the short-term and long-term responses to stress
These can stimulate the breakdown of proteins and fats into glucose which increases the amount of energy available so the brain and muscles can respond to the situation
Increasing blood volume and pressure by increasing the uptake of Na+ and water by the kidneys
Suppressing the immune system
84
Function of adrenal medulla
Secretes catecholamine hormones (modified amino acids)
E.g. secretes adrenaline and noradrenaline
These act to make more energy available in the short term by:
Increasing heart and breathing rate
Causing cells to break down glycogen and glucose
Constricting some blood vessels so that blood is diverted to the brain and muscles
85
Endocrine function and histology of the pancreas
Endocrine tissue in the pancreas are contained in the islets of langerhans
Found in clusters around blood capillaries
The islets of langerhans secrete hormones directly into the blood
Made up of alpha cells and beta cells
86
Function of alpha and beta cells in the islets of langerhans
Alpha cells: secrete glucagon
Beta cells: secrete insulin
These help to control blood glucose concentration
87
Why does glucose concentration need to be controlled
All cells need constant energy to work
Glucose conc in the blood is normally around 90mg / 100cm3
88
When may glucose concentration rise and fall
Can rise after eating food containing carbohydrates
Can fall after exercise as more glucose is used in respiration to release energy
89
How is glucose levels monitored
Monitored by cells in the pancreas
90
What is glycogenesis
Making glycogen from converting glucose to glycogen
91
Gluconeogenesis
Making glucose from converting amino acids & glycerol to glucose
92
What is glycogenolysis
Converting glycogen back into glucose
93
How does insulin lower high blood glucose levels
1. Pancreas gland detects that blood glucose levels are too high
2. β cells of the pancreas releases the hormone Insulin
3. Insulin travels in the bloodstream and binds to specific receptors on the cell membranes of liver cells and muscle cells (by increasing the permeability of these membranes)
4. Insulin also activates enzymes that convert glucose into glycogen in the liver and body cells. Glycogen is stored in the cytoplasm of these cells. Forming glycogen is called glycogenesis.
5. Insulin also increases the rate of respiration of glucose, especially in muscle cells.
6. Blood glucose levels are lowered and return to normal homeostatic levels
94
How does glucagon increase blood glucose levels
1. Pancreas gland detects that blood glucose levels are too low
2. α cells of the pancreas releases the hormone glucagon
3. Glucagon travels in the bloodstream and binds to specific receptors on the cell membranes of liver cells. This activates a secondary messenger within the liver (cyclic AMP)
4. Cyclic AMP stimulates an enzyme cascade that results in the hydrolysis of glycogen into glucose. The process of breaking down glycogen is called glycogenolysis.
5. Glucagon also promotes the formation of glucose from glycerol and amino acids (called gluconeogenesis)
6. Glucagon decreases the rate of respiration of glucose in cells.
7. Blood glucose levels rise and return to normal homeostatic levels
95
How is insulin secreted
Glucose transporter proteins (on the surface on the B- cells) absorb glucose by facilitated diffusion
Higher levels of glucose increase the rate of respiration and therefore the production of APT molecules
High ATP levels triggerthe potassium ion channels in the B cell membrane to close.
K+ ions build up inside the cell making it less negative.
B-cell membrane becomes depolarized
Depolarisation causes voltage gated calcium ion channels to open.
Calcium ions diffuse into the beta cell
Calcium ions bind to and move the vesicles to the plasma membrane of the B cell.
Vesicles bind to plasma membrane.
Insulin is released into the bloodstream via exocytosis
96
What is type 1 diabetes
This is an autoimmune disease where the body attacks and destroys the beta cells in the islets of langerhans
Means that you can’t produce any insulin
After eating the blood glucose concentration rises and stays high which can result in death if untreated
Kidneys can’t reabsorb all this glucose so some is excreted in urine
97
When does diabetes type 1 usually develop
Usually develops in children or young adults
Risk of developing type 1 diabetes if there’s a close family history of the disease
98
How can insulin therapy be used to treat type 1 diabetes
Most people use regular insulin injections throughout the day
Can use an insulin pump which is a machine that continuously delivers insulin into the body via a tube inserted beneath the skin
99
Other way to treat type 1 diabetes
Can be treated by islet cell transplantation
Receive healthy islet cells from a donor so their pancreas can produce some insulin
May still require some additional insulin therapy
100
How do people usually try to monitor their blood glucose concentration
Diet: eating a balanced diet reduces the amount of insulin that needs to be injected, people stick to a carefully planned diet to manage the amount of glucose they are taking in
Activity: doing regular exercise reduces the amount of insulin that needs to be injected by using up blood glucose
101
How does type 2 diabetes occur
Occurs when beta cells don’t produce enough insulin or when the body’s cells don’t respond properly to insulin
Cells don’t respond properly because the insulin receptors on their membrane don’t work properly, so the cells don’t take up enough glucose
Means that blood glucose concentration is higher than normal
102
When does type 2 diabetes effect people
Usually acquired later in life than type 1
Linked to obesity
Risk of developing type 2 is also increased in people from certain ethnic groups
E.g. African or Asian
Also people with a close family history of the disease
103
How is type 2 diabetes usually treated
Can be managed through lifestyle changes (e.g. healthy diet, exercise and losing weight)
Medication can be prescribed such as: metformins, sulfonylureas, thiazolidinediones
Insulin therapy can be used in addition or instead
104
What does metformin do
Usually first medicine to be prescribed
Acts on liver cells to reduce the amount of glucose that they release into the blood
Can increase the sensitivity of cells to insulin so more glucose can be taken up with the same amount of insulin
105
What does sulfonylureas
E.g. glicazide
Stimulates the pancreas to produce more insulin
106
What do thiazolidinediones
E.g. pioglitazone
These make the body cells more sensitive to insulin
107
Where can we obtain insulin from for insulin therapy
Insulin used to be extracted from animal pancreases (e.g. pigs and cattle)
Insulin can be made from genetically modified (GM) bacteria
108
Why is using GM bacteria for insulin better
Cheaper than extracting from animal pancreases
Larger quantities can be produced
GM bacteria make human insulin which is more effective than using pig or cattle as it’s less likely to trigger allergic response or be rejected by immune system
Less ethical or religious problems
E.g. vegetarians maybe object to the use of animals or religious people object to the use of pigs
109
How could stem cells be used to cure diabetes
Stem cells can be grown into beta cells
Beta cells would be implanted into the pancreas of a person with type 1 diabetes
This means the person would be able to make insulin as normal
This is still being developed but, if effective, it’ll cure people with type 1 diabetes
110
How does responding to the internal and external environment help an organism to survive
Responding to changes in their external environment allow animals increase their chances of survival
Responding to changes in their internal environment to make sure that the conditions are always optimal for their metabolism
Also occurs in plants
111
What are receptors
Receptors detect stimuli
Receptors are specific - they only detect one particular stimulus
112
What are the types of receptors
There are many different types of receptor that each detect a different type of stimulus.
Some receptors are cells
e.g, photoreceptors are receptor cells that connect to the nervous system.
Some receptors are proteins on cell surface membranes
e.g. glucose receptors are proteins found in the cell membranes of some pancreatic cells
113
What are effector cells
Effectors are cells that bring about a response to a stimulus, to produce an effect
Effectors include muscle cells and cells found in glands, e.g. the pancreas
114
How do receptors produce a response from effectors
To produce a response, receptors need to communicate with effectors and effectors may need to communicate with other cells
Happens via cell signalling
Cell signalling can occur between adjacent cells or between distant cells.
E.g. nervous impulses or hormones
115
What do cell surface receptors do
Cell-surface receptors allow cells to recognise the chemicals involved in cell signalling
116
How can the internal environment be changed
Changes in your external environment can affect your internal environment - the blood and tissue fluid that surrounds your cells.
117
What does homeostasis involve
Homeostasis involves control systems that keep your internal environment roughly constant (within certain limits)
118
Why is homeostasis important
Keeping your internal environment constant is vital for cells to function normally and to stop them being damaged.
It's particularly important to maintain the right core body temperature.
This is because temperature affects enzyme activity, and enzymes control the rate of metabolic reactions
119
What can happen if body temperature is too high
If body temperature is too high (e.g. 40 °C) enzymes may become denatured.
The enzvme's molecules vibrate too much, which breaks the hydrogen bonds that hold them in their 3D shape.
The shape of the enzyme's active site is changed and it no longer works as a catalyst.
This means metabolic reactions are less efficient.
120
What does a homeostatic system involve
Homeostatic systems involve receptors, a communication system and effectors
Receptors detect when a level is too high or too low, informations communicated via nervous system or hormonal system to effectors
The effectors respond to counteract the change bringing the level back to normal.
The mechanism that restores the level to normal is called a negative feedback mechanism
121
What does negative feedback do
Negative feedback keeps things around the normal level
e.g. body temperature is usually kept within 0.5°C above or below 37 °C
122
What does a negative feedback system need to work
Negative feedback only works within certain limits
If the change is too big then the effectors may not be able to counteract it
e.g. a huge drop in body temperature caused by prolonged exposure to cold weather may be too large to counteract
123
What does a positive feedback system do
These amplify a change from the normal level
124
How can positive feedback systems be useful
Positive feedback is useful to rapidly activate something
e.g. a blood clot after an injury:
Platelets become activated and release a chemical - this triggers more platelets to be activated, and so on.
Platelets very quickly form a blood clot at the injury site.
The process ends with negative feedback, when the body detects the blood clot has been formed
125
Why aren’t positive feedback systems involved in homeostasis
Positive feedback isn't involved in homeostasis because it doesn't keep your internal environment constant
126
Examples of ectotherms
Reptiles, fish
127
What are ectotherms
Ectotherms can't control their body temperature internally - they control their temperature by changing their behaviour
e.g. reptiles gain heat by basking in the sun
Their internal temperature depends on the external temperature (their surroundings)
128
What do the activity level of ectotherms depend on
Their activity level depends on the external temperature
They're more active at higher temperatures and less active at lower temperatures
They have a variable metabolic rate and they generate very little heat themselves
129
Examples of endotherms
Mammals and birds
130
How do endotherms control their body temperature
Endotherms control their body temperature internally by homeostasis.
They can also control their temperature by behaviour
e.g. by finding shade
Their internal temperature is less affected by the external temperature (within certain limits)
131
What do the activity level of endotherms depend on
Their activity level is largely independent of the external temperature - they can be active at any temperature (within certain limits)
They have a constantly high metabolic rate and they generate a lot of heat from metabolic reactions
132
Mechanisms to reduce body temperature
Sweating
Hairs lie flat
Vasodilation
133
Mechanisms to increase body temperature
Shivering
Much less sweat
Hairs stand up
Vasoconstriction
Hormones
134
How does sweating reduce body temperature
More sweat is secreted from sweat glands when the body's too hot.
The water in sweat evaporates from the surface of the skin and takes heat from the body.
The skin is cooled.
135
How do hairs lying flat reduce body temperature
Mammals have a layer of hair that provides insulation by trapping air (air is a poor conductor of heat).
When it's hot, erector pili muscles relax so the hairs lie flat.
Less air is trapped, so the skin is less insulated and heat can be lost more easily.
136
How does vasodilation reduce body temperature
When it's hot, arterioles near the surface of the skin dilate (this is called vasodilation).
More blood flows through the capillaries in the surface layers of the dermis.
This means more heat is lost from the skin by radiation and the temperature is lowered
137
How does shivering increase body temperature
When it's cold, muscles contract in spasms.
This makes the body shiver and more heat is produced from increased respiration
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How does having much less sweat increase body temperature
Less sweat is secreted from sweat glands when it's cold, reducing the amount of heat loss.
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How does hairs standing up increase body temperature
Erector pili muscles contract when it's cold, which makes the hairs stand up.
This traps more air and so prevents heat loss.
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How does vasoconstriction increase body temperature
When it's cold, arterioles near the surface of the skin constrict (this is called vasoconstriction) so less blood flows through the capillaries in the surface layers of the dermis.
This reduces heat loss.
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How does hormones increase body temperature
The body releases adrenaline and thyroxine.
These increase metabolism and so more heat is produced.
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How is body temperature controlled
Body temperature in mammals is maintained at a constant level by a part of the brain called the hypothalamus
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How does the hypothalamus control body temperature
Body temperature in mammals is maintained at a constant level by a part of the brain called the hypothalamus.
The hypothalamus receives information about temperature from thermoreceptors (temperature receptors):
Thermoreceptors in the hypothalamus detect internal temperature (the temperature of the blood).
Thermoreceptors in the skin (called peripheral temperature receptors) detect external temperature (the temperature of the skin).
Thermoreceptors send impulses along sensory neurones to the hypothalamus, which sends impulses along motor neurones to effectors (e.g. skeletal muscles, or sweat glands and erector pili muscles in the skin).
The effectors respond to restore the body temperature back to normal.
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Pathways of changes in the body to decrease temperature back to normal
Thermoreceptors detect temperature is too high
Hypothalamus sends signals to effectors
Changes:
• vasodilation
• sweating
• hairs lie flat
• no shivering
• no adrenaline or thyroxine released
More heat's lost and less heat's produced by the body
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Pathways of changes in the body to increase temperature back to normal
Thermoreceptors detect temperature is too low
Hypothalamus sends signals to effectors
Changes:
• vasoconstriction
• much less sweating
• hairs stand upright
• shivering
• adrenaline and thyroxine released
More heat's produced and conserved by the body
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What can happen if body temperature becomes too low
If body temperature is too low enzyme activity is reduced, slowing the rate of metabolic reactions.
The highest rate of enzyme activity happens at their optimum temperature (about 37 °C in humans).
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How can the release of hormones be stimulated
Can be stimulated by a change in concentration of a particular substance, as a result of another hormone or by electrical nerve impulses
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What is the unequal distribution of ions on different sides of the membrane called
Membrane potential which creates a charge difference
