[Y2] Organisms Respond To Changes In Their Internal and External Environment Flashcards
What is a stimulus?
A detectable change in the internal or external environment of an organism that leads to a response in the organism.
What is the benefit of being able to respond to a stimulus?
- 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.
How are stimuli detected
By receptors specific to the type of stimuli.
What is a coordinator?
Something that formulates a suitable response to a stimulus.
How are responses produced to stimuli?
By an effector.
What is the sequence of events in the nervous system?
stimulus → receptor → coordinator → effector → response
What is taxes?
A simple response whose direction is determined by the direction of the stimulus
How might a motile organism respond to a stimulus via taxis?
Moving its whole body:
- towards a favourable stimulus.
- away from an unfavourable one.
Give an example of a positive taxis.
- Single-celled algae will move towards light.
- This increases their chances of survival since, being photosynthetic, they require light to manufacture their food.
Give an example of a negative taxis.
- 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.
What are kineses?
A response where organisms change its speed at which it moves and the rate of change of direction.
How might a kineses stimuli be different from a taxes one?
- 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.
Give an example of kinesis.
- 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.
What are tropisms
A growth of part of a plant in response to a directional stimulus.
What is:
- positive phototropism?
- negative phototropism?
- positive gravitropism?
- negative gravitropism?
- 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.
What do plant growth factors do?
- 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.
Give an example of a plant growth factor.
Indoleacetin acid (IAA)
- belongs to the subgroup called auxins.
- among other things, IAA controls plant cell elongation.
What is unilateral light?
A light that is detected from one side.
What takes place in the response of shoots of flowering plants to unilateral light?
- 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
How does IAA differ in roots to its effect in shoots?
phototropism
- Promotes cell elongationín shoots
- Inhibits cell elongation in roots: therefore roots are negatively phototrophic.
What takes place in the response of a horizontally-growing root to gravity?
- 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.
How does IAA differ in shoots to its effect in roots?
gravitropism
- 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.
What effect does IAA have on plant cells?
Increases their plasticity (ability to streach).
What type of cells/part of cell does IAA work in?
- 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.
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.
What is a reflex arc?
The pathway of neurons involved in a simple nervous response to stimuli.
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…
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.
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.
What is the spinal cord?
A column of nervous tissue that runs along the back and lies inside the vertebral column fro protection.
What are the characteristics of a reflex?
- Rapid.
- Short-lived.
- Localised.
- Involuntary.
Why is a reflex sometimes a spinal reflex?
One of the neurones involved is in the spinal cord.
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.
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.
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.
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.
What is a generator potential?
When receptors in the nervous system convert the energy of the stimulus into a nervous impules.
What does the Pacinian corpuscle respond to?
Mechanical stimuli, such as pressure.
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.)
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.
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).
(In mammals) where are light receptors located?
The retina.
How many light receptors are there?
Millions.
How many types of light receptor are there? Name them.
Two.
Rods and cones.
How are rods and cones transducers?
They convert energy carried by light into electrical energy.
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.
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
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).
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.
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).
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.
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.
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.
What is the autonomic nervous system responsible for?
Involuntary (subconscious) activities of the internal muscles and glands.
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.
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.
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.
What is the opposite of myogenic?
Neurogenic.
What about the heart makes it myogenic?
It has a sinoatrial node (SAN) where the initial stimulus for contraction originates.
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.
What is the resting heart rate of a typical adult human?
70 bpm
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.
What type of stimuli may cause a change in heart rate?
- Chemical (e.g. O₂/CO₂ concentration)
- Pressure (e.g. blood pressure)
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.
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.
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.
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.
What are neurotransmitters?
- Chemicals secreted in target cells for the nervous system.
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.
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).
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.
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.
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.
What is the charge of an axon relative to its outside at resting potential?
Negative.
In humans what is the resting potential?
-65mV
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.
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.
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.
Can depolarisation occur across the whole axon membrane?
No, only a particular point on the axon membrane.
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.
What is a travelling wave of depolarisation?
An action potential.
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.
