Topic 6A - Stimuli And Response Flashcards
How can animals increase their chances of survival?
Responding to changes in their external environment, e.g by avoiding harmful environments such as places that are too hot or cold.
They also respond to changes in their internal environment to make sure that the conditions are always optimal for their metabolism. Plants also increase their chances of survival by responding to changes in their environment. Any change in the internal or external environment is called a stimulus.
What do receptors do?
Receptors detect stimuli - they can be cells or proteins on cell surface membranes. There are loads of different types of receptors that detect different stimuli. Effectors are cells that bring about a response to a stimulus, to produce an effect. Receptors communicate with effectors via the nervous system or the hormonal system, or sometimes using both.
How does the nervious system send information?
The nervous system is made up of a complex network of cells called neurones. There are 3 main types:
Sensory neurones transmit electrical impulses from receptors to the CNS - the brain and spinal cord. Motor neurones transmit electrical impulses from the CNS - the brain and spinal cord. Motor neurones transmit electrical impulses from the CNS to effectors. Relay neurones transmit electrical impulses between sensory neurones and motor neurones. A stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neurone. When an electrical impulse reaches the end of a neurone, chemicals called neurotransmitters take the information to the next neurone, which then sends an electrical impulse. The CNS (the coordinator) processes the information and sends impulses along motor neurones to an effector.
Stimulus -> receptor -> CNS -> Effectors -> Response
What are the two different systems that the nervous system is made up of?
1)The central nervous system - made up of the brain and spinal cord.
2)The peripheral nervous system - made up of the neurones that connect the CNS to the rest of the body. It also has two different systems:
a) the somatic nervous system controls conscious activites
b) the autonomic nervous system controls unconscious activities, it’s got two divisions
i) the sympathetic nervous system gets the body ready for action
ii) the parasympathetic nervous system calms the body down. It’s the “rest and digest” system.
What are reflexes?
Body responds to a stimulus without making a conscious decision to respond. Information really fast from receptors to effectors. So simple reflexes help organisms to protect the body because they’re rapid. The pathway of neurones linking receptors to effectors in a reflex is called a reflex arc. You need to learn a simple reflex arc involving three neurones - a sensory, a relay, and a motor neurone.
Example: hand-withdrawal response to heat
Thermoreceptors in the skin detect the heat stimulus. The sensory neurone carries impulses to the relay neurone. The relay neurone connects to the motor neurone. The motor neurone sends impulses to the effector. Your muscle contracts to withdraw your hand and stop it being damaged. If there’s a relay neurone involves in the simple reflex arc then it’s possible to override the reflex e.g in the example above your brain could tell your hand to withstand the heat.
Describe nervous system communication.
When an electrical impulse reaches h the end of a neurone, neurotransmitters are secretly directed onto target cells - so the nervous response is localised. Neurotransmitters are quickly removed once they’ve done their job, so the response is short-lived. Electrical impulses are really fast, so the response is rapid.
How do plants respond to stimuli?
They respond to changes in their environment. They sense the direction of light and grow towards it to maximise light absorption for photosynthesis. They can sense gravity, so their roots and shoots grow in the right direction. Climbing plants have a sense of touch, so they can find thing to climb up and reach the sunlight.
What is a tropism?
The response of a plant to a directional stimulus. Plants respond to stimuli by regulating their growth. A positive tropism is growth towards the stimulus. A negative tropism is away. Phototropism is the growth of a plant in response to light. Shoots are positively phototropism and grow towards light. Roots are negatively phototropic and grow away from light. Gravitons is the growth of a plant in response to gravity. Shoots are negatively gravitropic and grow upwards. Roots are positively gravitropic and grow downwards.
How do plants respond to stimuli?
Specific growth factors - these are hormone-like chemicals that speed up or slow down plant growth. Growth factors are produced in the growing regions of the plant (e.g shoot tips, leaves) and they move to where they’re needed in the other parts of the plant. Growth factors capped auxins stimulate the growth of shoots by cell elongation - this is where cell walls become loose and stretchy, so the cells get longer. High concentrations of auxins inhibit
What is Indoleacetic acid
IAA is an important auxin that’s produced in the tips of shoots in flowering plants. IAA is moved around the plant to control tropism - it moves by diffusion and active transport over short distances, and via the phloem over long distances. This results in different parts of the plant having different concentrations of IAA. The uneven distribution of IAA means there’s uneven growth of the plant.
Phototropism - IAA moves to the more shaded parts of the shoots and roots, so there’s uneven growth. Cells elongate and the shoot bends towards the light in the shoots. In the roots, IAA concentration increases on the shaded side, so growth is inhibited and the root bends away from the light. Gravitopism - IAA moves to the underside of shoots and roots, so there’s uneven growth. IAA concentration increases on the lower side - cells elongate so the shoot grows upwards. Concentration in roots increases on the lower side - growth is inhibited so the root grows downwards.
What responses keep simple organisms in a favourable environment?
Simple mobile organisms have simple responses to keep them in a favourable environment. Their response can either be tactic or kinetic:
Tactic responses (taxes) - the organisms move towards or away from a directional stimulus e.g. light
Woodlice show a tactic response to light (phototaxis) - they move away from a light source. This helps them survive as it keeps them concealed under stones during the day (where they’re safe from predators) and keeps them in damp conditions (which reduces water loss)
Kinetic responses (kineses) - the organisms’ movement is affected by a non-directional stimulus e.g. humidity
For example, woodlice show a kinetic response to humidity. In high humidity they move slowly and turn less often, so that they stay where they are. As the air gets drier, they move faster and turn more often, so that they stay where they are. As the air gets drier, they move faster and turn more often, so that they move into a new area. This response increases the chance that a woodlouse will move to an area with higher humidity. This improves the survival chances of the organism - it reduces their water loss and it helps to keep them concealed.
How can you use choice chambers?
It is a container with different compartments, in which you can create different environmental conditions. It can be used to investigate how animals, such as woodlice, respond to conditions like light intensity or humidity in the laboratory. Here’s how you can use a choice chamber:
Construct a choice chamber using the equipment shown in the diagram. Petri dish base, divider, base is divided into two compartments. Fine mesh. Petri dish lid. Woodlice placed on mesh.
To investigate the effect of light intensity on Woodlice movement, cover one half of the lid including the sides with black paper. This will make one side of the chamber dark. Put damp filter paper in both sides of the base. Place 10 woodlice on the mesh in the centre of the chamber and cover the chamber with the lid. After 10 minutes, take off the lid and record the number of woodlice on each slide of the chamber. Repeat the experiment after gently moving the woodlice back to the centre. You should find that most woodlice end up on the dark side (a tactic response). To investigate humidity, place some damp filter paper in one side of the base and a desiccating (drying) agent in the other side. You can also use a maze.
What are receptor specific to?
Receptors are specific - they only detect one particular stimulus e.g light, pressure or glucose. 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 membrane of some pancreatic cells.
When a nervous system receptor is in its resting state, there’s a difference in charge between the inside and outside of the cell - this is generated by ion pumps and ion channels. This means that there’s a voltage across the membrane. Voltage is also potential difference. The pd when a cell is at rest is called its resting potential. When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell - altering the potential difference. The change in potential difference due to a stimulus is called the generator potential. A bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger change in pd - so a bigger generator potential is produced. If the generator potential is big enough it’ll trigger an action potential - an electrical impulse along a neurone. An actions potential is only triggered if the generator reaches the threshold level. Action potentials are all one size, so the strength of the stimulus is measured by the frequency of action potentials. If the stimulus is too weak the generator potential won’t reach the threshold, so there’s no action potential.
What are pacinian corpuscles?
They’re pressure receptors in the skin. They are mechanoreceptors - they detect mechanical stimuli e.g pressure and vibrations. They’re found in your skin. Pacinian corpuscles contain the end of a sensory neurone, called a sensory nerve ending. The sensory nerve ending is wrapped in loads of layers of connective tissue called lamellae. When a pacinian copuscle is stimulated, e.g by a tap on the arm, the lamellae are deformed and press on the sensory nerve ending. This causes the sensory neurone’s cell membrane to stretch, deforming the stretch-mediated sodium ion channels. The channels open and sodium ion diffuse into the cell, creating a generator potential. If the generator potential reaches the threshold, it triggers an action potential.
What are photoreceptors?
Light receptors in your eye. Light enters the eye through the pupil. The amount of light that enters is controlled by the muscles of the iris. Light rays are focused by the lens onto the retina, which lines the inside of the eye. The retina contains photoreceptor cells which detect light. The fovea is an area of the retina where there are lots of photoreceptors. Nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve, which is a bundle of neurones. Where the optic nerve leaves the eye is called the blind spot - there aren’t any photoreceptor cells, so it’s not sensitive to light.
