6A: Stimuli and response Flashcards
- Tropism in plants (IAA) - Taxis and kinesis - Receptors (Pacinian corpuscle & eye) - Control of heart rate
What is a tropism?
The response of a plant to a directional stimulus
What is a positive/negative tropism?
Positive - growth towards the stimulus
negative - growth away from the stimulus
What is phototropism?
Growth of a plant in response to light
What type of phototropism do shoots have?
Shoots have positive phototropism - they grow towards light
What type of phototropism do roots have?
Roots have negative phototropism - they grow away from light
What is gravitropism?
Growth of a plant in response to gravity
What type of gravitropism do shoots have?
Shoots have negative gravitropism - they grow upwards
What type of gravitropism do roots have?
Roots have positive gravitropism - they grow downwards
What is IAA and where is it produced?
Indoleacetic acid (IAA) is a type of auxin is produced in the meristem tissue in flowering plants.
How is IAA transported around the plant?
Via diffusion and active transport over short distances, via the phloem over long distances
What does IAA do to growth in shoots?
Promotes growth
What does IAA do to growth in roots?
Inhibits growth
Where does IAA move to during phototropism?
IAA is actively transported to the more shaded side of the root/shoot so there’s uneven growth
Describe what IAA does during phototropism in shoots
IAA conc. increases on the shaded side:
Increases plasticity of cell walls by actively transporting hydrogen ions from the cytoplasm to the spaces in the cell wall, causing the cells to elongate
= shoot grows towards the light
Describe what IAA does during phototropism in roots
IAA conc. increases on the shaded side:
Growth is inhibited so the root bends away from the light
Where does IAA move to during gravitropism?
IAA is actively transported to the underside of shoots and roots, so there’s uneven growth
Describe what IAA does during gravitropism in roots
IAA conc. increases on the lower side:
Growth is inhibited so the root grows downwards
Describe what IAA does during gravitropism in shoots
IAA conc. increases on the lower side:
Growth is promoted, cells elongate so the shoot grows upwards
What is a tactic response (taxes)?
A simple response to stimuli whose direction is determined by a directional stimulus
e.g a spider moving away from light 🕷️☀️
What is a kinetic response (kineses)?
Give an example
A simple response where speed + rate of change of direction will change in response to intensity of a non-directional stimulus
e.g in high humidity a woodlouse would move slowly and turn less (so that they stay in favourable conditions for longer)
Define positive taxis
Organism moves towards a favourable stimulus
Define negative taxis
Organism moves away from a favourable stimulus
What is a reflex action?
Automatic, rapid responses that do NOT involve conscious areas of the brain
What 3 neurons are used in a reflex reaction?
Sensory –> Relay –> Motor
Why are reflex actions faster than normal voluntary actions?
Because they do not involve conscious areas of the brain
What 2 branches can the ‘nervous system’ be split into?
PNS (peripheral) and CNS (brain and spinal cord)
What are the two systems within the PNS?
Autonomic and Somatic
What are the two branches of the autonomic system?
Sympathetic and parasympathetic
Describe the autonomic nervous system
Controls self regulated action of internal organs and glands
Describe the somatic nervous system
Controls voluntary movements of skeletal muscles
Describe the sympathetic nervous system
Fight or flight responses
- Stimulate effectors and speeds up activities
- Controls responses to stressful situations
- heightens awareness
Describe the parasympathetic nervous system
Rest and Digest
- Inhibits effectors and so slows down activities
- Controls activities in normal restful conditions
- Conserving energy and replenishing the body’s reserves
How is heart rate controlled?
The sinoatrial node sends out waves of electrical excitation
Describe the (5) stages of the control of heart rate
- A wave of electrical excitation spreads out from the SA node causing both atria to contract
- The wave of excitation reaches the atrioventricular (AV) node
- There is a short delay at the AV node
- The signal then passes down the bundle of His to the Purkinje fibres
- The wave of excitation is released from the Purkinje tissues causing the ventricles to contract from the base upwards
What is the medulla oblongata
A region of the brain that controls changes to heart rate
Describe the cardioacceleratory centre
Centre in the medulla oblongata that increases heart rate - linked to the SA node by the sympathetic nervous system
Describe the cardioinhibitory centre
Centre in the medulla oblongata that decreases heart rate - linked to SA node by the parasympathetic
Describe how chemoreceptors modify heart rate during exercise
(9 steps)
- Increase in rate of respiration
- Increase in CO₂ production
- Decrease pH of blood
- Detected by chemoreceptors
- Increases frequency of impulses sent to cardioacceleratory centre
- Increases frequency of impulses sent from the cardioacceleratory centre to the SA node via the sympathetic nervous system
- SA node increases heart rate
- Increased removal of CO₂
- Blood pH increases and returns to normal
What are chemoreceptors and where are they found?
They are receptors that detect change in blood pH, they are found in the wall of carotid arteries
Describe how chemoreceptors modify heart rate to decrease blood pH
- Decrease in rate of respiration, so decrease in CO₂ production = Increase pH of blood
- Detected by chemoreceptors
- Increases frequency of impulses sent to cardioinhibitory centre
- Increases frequency of impulses sent from cardioinhibitory centre to SA node via the parasympathetic nervous system
- SA node decreases heart rate
- Decrease removal of CO₂
- Blood pH returns to normal
What happens to blood pressure if you reduce cardiac output?
Blood pressure decreases
What happens to blood pressure if you reduce heart rate?
Blood pressure decreases
What are baroreceptors and where are they found?
They detect changes in pressure and they are found in the carotid arteries and aorta
Describe how baroreceptors modify heart rate when blood pressure is higher than normal.
- Baroreceptors transmit more impulses to the cardioinhibitory centre
- More impulses are sent from the cardioinhibitory centre to the SA node via the parasympathetic nervous system
- SA node decreases heart rate
Describe how baroreceptors modify heart rate when blood pressure is lower than normal.
- Baroreceptors transmit more nervous impulses to the cardioacceleratory centre
- More impulses are sent to the SA node via the sympathetic nervous system
- SA node increases heart rate.
Explain how the contraction of the heart is myogenic
The signal for cardiac compression arises within the heart tissue itself (rather than the brain)
Why are receptors described as specific?
Because they only detect one type of stimulus e.g light, pressure or glucose conc.
What type of stimulus is detected by the pacinian corpuscle?
Mechanical e.g pressure
Where is the pacinian corpuscle found?
Skin, joints, ligaments and tendons
Describe how a generator potential is created in a pacinian corpuscle
(4)
- Pressure causes pacinian corpuscle to be stretched (stretch mediated Na+ channels open)
- Na+ diffuses into the neuron
- Neuron becomes depolarised creating a generator potential
- The generator potential leads to an action potential
What is a transducer?
Converts one form of energy into another (convert the energy of the stimulus into the energy of an electrical impulse)
What does light enter the eye through and how is this controlled?
Light enters the eye through the pupil, this is controlled by the muscles of the iris
What type of cells does the retina contain?
Photoreceptor cells
What focuses light onto the retina?
The lens
What is the fovea?
An area of the retina where there are lots of photoreceptors
What is the optic nerve?
A bundle of neurons that carries nerve impulses from the photoreceptor cells in the retina to the brain.
What are the 2 types of photoreceptors in the human eye?
Rods and cones
Describe how photoreceptors in the eye convert light into an electrical impulse
- Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive optical pigments
- Light bleaches the pigments, causing chemical change
What stimulus is detected by rods?
Low intensity light
Where are rods found?
The retina
What is rhodopsin formed from?
Opsin and retinal
What pigment do rods contain?
Rhodopsin
Describe how rod cells produce an action potential
- Light strikes rod cell
- Rhodopsin breaks down to form retinal and opsin
- This causes the rod cells to depolarise
- A generator potential occurs
- This leads to an action potential in the bipolar neuron
What stimulus is detected by cones?
High intensity (bright) light
Where, specifically, are cones found?
Retina (fovea)
What pigment do cones cells contain?
Iodopsin
How many types of iodopsin are there? Describe them
3 types of iodopsin:
Red, Green and blue
Describe how cone cells produce an action potential
- Light strikes cone cell
- Iodopsin breaks down
- This causes the cone cells to depolarise
- A generator potential occurs
- This leads to an action potential in the bipolar neuron
What type of summation do rod cells carry out? what effect does this have on visual acuity
Rods carry out spatial summation (more than one rod synapses with one bipolar neuron). It results in low visual acuity
What type of vision do rod cells produce?
Black and white
Which is more sensitive to light, rhodopsin or iodopsin?
Rhodopsin is more sensitive
What type of vision do cone cells produce?
Different cones for different wavelengths of light = colour vision
Why do cone cells produce high visual acuity?
Because only one cone cell synapses with one bipolar neuron
