Response to the Environment (Edexcel A Level Biology (A) SNAB): Flashcards
Exam code: 9BN0
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Define stimulus.
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Define stimulus.
A stimulus is a change in the internal or external environment that is detected by a receptor.
State the pathway of a nervous response from stimulus to response.
Stimulus → receptor → sensory neurone → CNS → motor neurone → effector.
What are the two parts of the human nervous system?
The central nervous system (CNS) — brain and spinal cord.
The peripheral nervous system (PNS) — all the nerves.
What can an effector be?
A muscle or a gland.
Define hormone.
A hormone is a chemical messenger produced by an endocrine gland and carried in the blood to target organs.
How do hormones bring about a response in a target organ?
They bind to specific receptors on the target cells' surface membranes.
This alters the activity of the target organ.
Give two differences between nervous and hormonal control.
Nervous control is fast and short-lived; hormonal control is slower and longer-lasting.
Nerve impulses are electrical; hormones are chemical and carried in the blood.
What is the role of the CNS in coordinating a response?
It acts as the coordinating centre.
It processes incoming impulses and sends out impulses to the correct effectors.
The pathway of hormone action is: stimulus → receptor → hormone → .
The pathway of hormone action is: stimulus → receptor → hormone → effector.
True or False?
Hormonal responses are generally faster than nervous responses.
False.
Hormonal responses are slower — they are used for processes that don't need instant responses.
Where are receptor cells found?
In the sense organs (such as the eyes and nose).
Also inside the body, e.g. pressure receptors in blood vessels.
Give an example of a hormonal response pathway.
High blood sugar → cells in the pancreas → insulin → liver cells.
The liver cells convert glucose into glycogen.
Define neurone.
A neurone is a specialised cell that carries electrical impulses around the body.
Name the three main types of neurone.
Sensory, relay and motor neurones.
What is the function of a sensory neurone?
It carries impulses from receptors to the brain and spinal cord (the CNS).
What is the function of a motor neurone?
It carries impulses from the CNS to effectors (muscles or glands).
What is the function of a relay neurone?
It is found entirely within the CNS.
It connects sensory and motor neurones.
Define myelin sheath.
The myelin sheath is an insulating fatty layer of Schwann cells around an axon that speeds up impulse transmission.
Define nodes of Ranvier.
The nodes of Ranvier are the uninsulated gaps between the Schwann cells along a myelinated axon.
Why do impulses travel faster along a myelinated neurone?
The impulse jumps between the nodes of Ranvier rather than moving along the whole axon.
This is faster than in an unmyelinated axon.
What is the role of the axon in a neurone?
It is the long fibre that carries the electrical impulse away from the cell body.
The myelin sheath is made of specialised cells called cells.
The myelin sheath is made of specialised cells called Schwann cells.
True or False?
A bundle of neurones is called a nerve.
True.
Many neurones grouped together form a nerve.
How does the structure of a motor neurone differ from a sensory neurone?
A motor neurone has a large cell body at one end with many dendrites.
A sensory neurone has its cell body branching off the middle of the axon.
What is the role of receptor cells?
To detect changes in the environment (stimuli).
They send impulses along sensory neurones to the CNS.
State the reflex pathway from stimulus to response.
Stimulus → receptor → sensory neurone → CNS → motor neurone → effector.
What is the role of the CNS in a response to a stimulus?
It acts as the coordinating centre.
It decides which part of the body responds and sends impulses along motor neurones.
Which two sets of muscles control the diameter of the pupil?
The circular muscles and the radial muscles of the iris.
What happens to the pupil in bright light, and which muscles cause it?
The pupil constricts.
The circular muscles of the iris contract.
What happens to the pupil in dim light, and which muscles cause it?
The pupil dilates.
The radial muscles of the iris contract.
Why does the pupil constrict in bright light?
To limit the amount of light entering the eye.
This prevents damage to the retina.
Why are the iris muscles described as antagonistic?
When one set contracts, the other relaxes.
So the circular and radial muscles work in opposition.
In bright light, the muscles of the iris contract to constrict the pupil.
In bright light, the circular muscles of the iris contract to constrict the pupil.
True or False?
In dim light the pupil constricts to let in more light.
False.
In dim light the pupil dilates to let in more light.
What is the purpose of changing pupil diameter?
To control the amount of light reaching the retina.
What carries the impulse from light receptors in the eye to the CNS?
A sensory neurone.
Define resting potential.
The resting potential is the potential difference across the membrane of a neurone at rest, about −70 mV (inside negative).
How does the sodium-potassium pump help establish the resting potential?
It uses ATP to pump 3 Na⁺ out and 2 K⁺ in per cycle.
This creates ion concentration gradients across the membrane.
Define action potential.
An action potential is a rapid reversal of the membrane potential from about −70 mV to +30 mV, caused by depolarisation.
What causes depolarisation during an action potential?
Voltage-gated sodium ion channels open.
Na⁺ floods into the axon, making the inside less negative and reaching +30 mV.
Define threshold potential.
The threshold potential (about −55 mV) is the level the membrane must reach for an action potential to be triggered.
What causes repolarisation of the membrane?
Voltage-gated potassium channels open and Na⁺ channels close.
K⁺ diffuses out, making the inside negative again.
Define the refractory period.
The refractory period is the time after an action potential when the membrane cannot be stimulated again.
Why is the refractory period important?
It makes action potentials discrete events.
It ensures impulses travel in one direction only.
Define the all-or-nothing principle.
An action potential is only generated if the threshold is reached, and it is always the same size regardless of stimulus strength.
How is the strength of a stimulus represented if action potentials are all the same size?
By the frequency of action potentials.
A stronger stimulus produces more action potentials per second.
Define saltatory conduction.
Saltatory conduction is the way an impulse 'jumps' between the nodes of Ranvier in a myelinated axon, speeding up transmission.
In a myelinated axon, action potentials can only occur at the of Ranvier.
In a myelinated axon, action potentials can only occur at the nodes of Ranvier.
True or False?
A stronger stimulus produces a larger action potential.
False.
By the all-or-nothing principle, all action potentials are the same size; a stronger stimulus increases their frequency.
Define synapse.
A synapse is the junction between two neurones (or a neurone and an effector), across which impulses are transmitted by neurotransmitters.
Define synaptic cleft.
The synaptic cleft is the small gap between the presynaptic and postsynaptic neurones.
Define neurotransmitter.
A neurotransmitter is a chemical, such as acetylcholine, that carries an impulse across a synapse by diffusion.
What happens when an action potential arrives at the presynaptic knob?
The membrane depolarises, opening voltage-gated calcium channels.
Calcium ions diffuse in, triggering the next steps.
What do calcium ions cause in the synaptic knob?
They make vesicles fuse with the presynaptic membrane.
The vesicles release neurotransmitter into the cleft by exocytosis.
What happens when neurotransmitter binds to receptors on the postsynaptic membrane?
Sodium ion channels open.
Sodium ions diffuse in, and if threshold is reached an action potential is generated.
Why must acetylcholine be broken down after transmission?
To stop continued stimulation of the postsynaptic neurone.
The enzyme acetylcholinesterase breaks it down.
Why do synapses ensure impulses travel in only one direction?
Neurotransmitter is released only from the presynaptic side.
Receptors are only on the postsynaptic side, so transmission cannot occur in reverse.
Define summation.
Summation is when the effects of several impulses are added together to reach the threshold and trigger an action potential.
How can summation be achieved?
By several presynaptic neurones converging on one postsynaptic neurone.
Or by many action potentials arriving in quick succession.
The influx of ions into the synaptic knob triggers vesicles to release neurotransmitter.
The influx of calcium ions into the synaptic knob triggers vesicles to release neurotransmitter.
True or False?
An electrical impulse jumps directly across the synaptic cleft.
False.
The impulse is carried across by the diffusion of neurotransmitter, not electrically.
What are the two types of photoreceptor in the retina?
Rod cells and cone cells.
What are rod cells sensitive to, and what type of image do they give?
They are sensitive to light intensity (brightness).
They produce a black and white image.
What are cone cells sensitive to, and what type of image do they give?
They are sensitive to different wavelengths of light.
They allow colour vision.
Where in the retina are cone cells concentrated?
In the fovea.
Define rhodopsin.
Rhodopsin is the light-sensitive pigment in rod cells that breaks apart (bleaches) into retinal and opsin when light hits it.
Define bleaching of a photoreceptor pigment.
Bleaching is the breaking apart of a light-sensitive pigment (e.g. rhodopsin into retinal and opsin) when light falls on it.
What is the state of a rod cell in the dark?
It is depolarised.
Sodium ions flow in, and it releases an inhibitory neurotransmitter that stops the bipolar neurone firing.
What happens in a rod cell when light hits it?
Rhodopsin bleaches, closing the sodium channels.
The cell becomes hyperpolarised and stops releasing inhibitory neurotransmitter.
How does hyperpolarisation of a rod cell lead to a nerve impulse?
It stops releasing inhibitory neurotransmitter.
So the bipolar neurone is no longer inhibited and generates an action potential to the optic nerve.
What carries impulses from the eye to the brain?
The optic nerve.
The light-sensitive pigment found in rod cells is called .
The light-sensitive pigment found in rod cells is called rhodopsin.
True or False?
Rod cells allow us to see in colour.
False.
Cone cells detect colour; rod cells detect light intensity and give black-and-white vision.
Define tropism.
A tropism is a directional growth response of a plant to a stimulus.
What is the difference between phototropism and geotropism?
Phototropism is a growth response to light.
Geotropism is a growth response to gravity.
What is the difference between a positive and negative tropism?
A positive tropism is growth towards the stimulus.
A negative tropism is growth away from the stimulus.
Define IAA.
IAA (indoleacetic acid) is a type of auxin — a plant growth factor that affects cell elongation.
What effect does IAA have on cells in shoots?
It stimulates cell elongation.
A higher IAA concentration increases the rate of elongation.
Explain how IAA causes a shoot to bend towards light.
IAA moves to the shaded side of the shoot.
The higher IAA there increases cell elongation, so that side grows faster and the shoot bends towards the light.
What effect does IAA have on cells in roots?
It inhibits cell elongation.
This is the opposite of its effect in shoots.
Explain how IAA causes a root to bend downwards.
IAA moves to the lower side of the root.
The high IAA there inhibits elongation, so the lower side grows slower and the root bends down.
How is it thought that IAA brings about plant responses?
By altering the transcription of genes.
This changes the production of proteins involved in cell growth.
IAA is a type of , a group of plant growth factors that affect cell elongation.
IAA is a type of auxin, a group of plant growth factors that affect cell elongation.
True or False?
IAA has the same effect on cell elongation in both roots and shoots.
False.
It stimulates elongation in shoots but inhibits it in roots.
Why is growing towards light advantageous for a plant?
It maximises the rate of photosynthesis.
This increases glucose production.
Define phytochrome.
Phytochrome is a plant pigment that exists in two interconvertible forms and detects night length to control flowering.
What are the two forms of phytochrome and which is active?
Pᴿ is the inactive form (absorbs red light).
Pꜰᴿ is the active form (absorbs far-red light).
What happens to phytochrome during the day?
Pᴿ absorbs red light and is converted to Pꜰᴿ.
So levels of the active form (Pꜰᴿ) rise during the day.
What happens to phytochrome during the night?
In darkness, unstable Pꜰᴿ slowly converts back to Pᴿ.
So levels of Pᴿ rise during the night.
What stimulus controls flowering in plants?
Night length.
This is detected via the relative amounts of the two phytochrome forms.
When do long-day plants flower?
When nights are short (days are long), e.g. in summer.
Explain how high Pꜰᴿ levels trigger flowering in a long-day plant.
Short nights mean Pꜰᴿ stays high.
Pꜰᴿ activates the genes that stimulate flowering, so flowers are produced.
How does Pꜰᴿ activate flowering at the molecular level?
It activates the expression of genes that stimulate flowering.
The genes are transcribed and translated into proteins that cause flowers to form.
Which wavelength of light does Pᴿ absorb, and what does it convert into?
It absorbs red light (660 nm).
This converts it into Pꜰᴿ.
The active form of phytochrome, which builds up during the day, is .
The active form of phytochrome, which builds up during the day, is Pꜰᴿ.
True or False?
The conversion between Pᴿ and Pꜰᴿ is reversible.
True.
Pᴿ converts to Pꜰᴿ in red light, and Pꜰᴿ converts back to Pᴿ in far-red light or darkness.
What does it mean for a plant if a flowering gene is 'switched on'?
The gene is expressed — transcribed and translated.
The protein it codes for is produced, leading to flowering.
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