Unit 6 - Response to Stimuli Flashcards
Stimulus
A detectable change in the internal or external change in the environment of an organism
Receptor
Detects the stimulus
Coordinator
- Intermediate Neurone
- Coordinates a suitable response to a stimulus
Effector
- Produces a response to a stimulus
- Response carried out by an organ, tissue or cell
Response
Action produced by the effector
Why is responding to stimuli important?
Increases chance of survival by:
- Moving towards beneficial resources
- Evading predators
Taxes
- A response to a stimuli in which the direction of movement is determined by the direction of the stimuli
- Directional
Positive Taxis
Movement towards a favourable stimulus
Negative Taxis
Movement away from an adverse stimulus
Phototaxis
Movement towards or away from a light stimulus
Chemotaxis
Movement towards or away from a certain concentration of a chemical
Kineses
- When an organism adjusts their movement speed and turning frequency based on the favourability of their environment
- Non-directional
How does favourability of conditions impact kineses?
- Increased turning in unfavourable conditions
- Accelerated movement in favourable conditions
Tropism
The growth of a part of an organism like a plant in response to a directional stimulus
Phototropism
Growth towards/away from light
Gravitropism (AKA Geotropism)
Growth towards or away from gravity
Hydrotropism
Growth towards or away from moisture/water
What tropisms do plant shoots display?
- Positive Phototropism and Negative Gravitropism
- Optimises light absorption for photosynthesis
What tropisms do plant roots display?
- Positive Gravitropism and Negative Phototropism
- Increases absorption of minerals and water in the soil
Why is Tropism important to Plants?
Increases chance of survival by:
- Optimising photosynthesis
- Anchoring the plant for support
- Efficient access to nutrients, minerals and water
Plant Growth Factors
Chemicals that influence plant development mainly by regulating cell elongation and division
Difference between Plant Growth Factors and Animal Hormones
- PGFs act locally at the site where they are produced whereas animal hormones are usually secreted and travel in the bloodstream to act on an organ
- PGFs are produced in a variety of tissues throughout the plant whereas animal hormones are produced in specialised cells within glands
IAA
- Type of Auxin
- Stimulates/inhibits cell elongation
- Results in growth of roots and shoots
- Synthesised in growing tips (meristems) of shoots and roots (where mitosis occurs)
Control of Positive Phototropism in Shoots using IAA
- IAA is produced in cells in the tip of the plant shoot and transported down the shoot by diffusion
- IAA is initially distributed evenly across all regions as it travels down the shoot
- Light stimulates IAA to move from the light side of the shoot to the shaded side
- IAA becomes concentrated on the shaded side
- IAA causes cell elongation of shoot cells so the shaded side grows longer
- The shoot tip bends towards the light
Control of Negative Phototropism/Positive Gravitropism in Shoots using IAA
- IAA is produced in cells in the tip of the plant shoot and transported along the root by diffusion
- IAA is initially distributed evenly across all regions as it travels down the shoot
- Any available light stimulates IAA to move to the shaded side of the root
- Gravity also pulls IAA to the lower side of the root
IAA becomes concentrated on the shaded side - IAA inhibits cell elongation of root cells so the shaded side doesn’t grow
- The root bends away from any available light and downwards towards the pull of gravity
Acid Growth Hypothesis
- Auxin binds to the cell-surface membrane
- Hydrogen ions are actively transported from the cytoplasm into the cell wall
- The cell wall becomes more plastic
- Cells elongate and the plant grows
Central Nervous System (CNS)
- Consists of brain and spinal cord
- Serves as the primary command centre for the body
Periphery Nervous System (PNS)
- Consists of all the nerves that connect the CNS to the rest of the body
- Facilitates bi-directional communication
- Has 2 further divisions
2 Divisions of the PNS
- Sensory Nervous System (Neurones): Carry impulses from receptors to the CNS
- Motor Nervous System (Neurones): Carry impulses from the CNS to effectors
- Motor Nervous System also has 2 divisions
2 Divisions of the Motor Nervous System
- Somatic: Carries nerve impulses to body muscles and is under conscious control
- Autonomic: Carries impulses to glands, smooth muscle and cardiac muscles and is under subconscious control (2 divisions)
2 Divisions of the Autonomic Nervous System
- Sympathetic: ‘Fight or Flight’, increases activity levels (heart rate, breathing rate, digestion slows down, etc)
- Parasympathetic: ‘Rest and Digest’, reduces activity levels (heart rate, breathing rate, digestion increases)
Reflex
- An involuntary response to a sensory stimuli
- Rapid, short-lived, localised response
- The pathway of neurones that lead to a reflex is known as a reflex arc
Reflex Arc
- Stimulus (heat or sharp object for a reflex arc)
- Receptor (temperature receptors in the skin on the back of the hand generate nerve impulse in the sensory neurone
- Sensory Neurone: passes nerve impulse to spinal cord
- Coordinator: Intermediate neurone IN THE SPINAL CORD links the sensory and motor neurone
- Motor Neurone: Carries nerve impulse form spinal cord to muscle in the upper arm
- Effector: Muscle is stimulated to contract
- Response: Move hand away from heat/sharp object
Why does a Reflex Arc not travel through the Brain?
- Increases the rate of coordination as it means fewer neurones are involved
- Brain isn’t overloaded with responses that are the same every time and it can then carry out more complex responses that require decision making
Importance of Reflexes
- Protect the body from harm which increases chance of survival
- Fast because of short neural pathway with only 1-2 synapses (which are the slowest part of a neural pathway) - very important with withdrawal reflexes
- Absence of decision making process means response is rapid
Features of Sensory Receptors
- Specific to a single type of stimulus (e.g. pressure, light, temperature)
- Produces a generator potential by acting as a transducer (converts one form of energy into another, in case converting energy to electrical energy)
Mechanoreceptor: Pacinian Corpuscle
- Pressure is applied to the receptor, lamellae are deformed and the membrane is stretched
- Stretch-mediated Na+ channels open, causing an influx of Na+
- Membrane is depolarised causing a generator potential
- If threshold is reached an action potential occurs
- Impulse is sent down sensory neurone towards the brain
Photoreceptors: Conversion of Light to an Electrical Impulse
- Light hits photopigments (rhodopsin and iodopsin)
- Light bleaches and breaks down photopigments
- This increases the permeability of the photoreceptor membranes, causing an influx of Na+
- Membrane is depolarised causing a generator potential
- If threshold is reached then an action potential occurs
- An impulse is sent down the sensory neurone towards the brain
Photoreceptors: Rods
- Contain rhodopsin (pigment)
- Provide black and white vision
- Found in peripheral retina
- Many rods converge to one sensory neurone (retinal convergence) so spatial summation to reach threshold
- Very light sensitive to detect light at low intensity
- Low visual acuity as signals from multiple rods result in only one impulse regardless of how many neurones are stimulated
Photoreceptors: Cones
- Contain iodopsin (pigment)
- 3 different cone cells provide different colour vison (RGB)
- Densely packed in the fovea
- 1 cone per sensory neurone so more light is needed to create generator potential, temporal summation
- Less light sensitive, only detects light at high intensity
- High visual acuity as 1-to1 connection allows brain to distinguish between stimulation of individual cells
Cardiac Muscle
- Myogenic
- Means that its contraction is reliant on impulses generated in the muscle rather than receiving impulses from neurons in the nervous system (neurogenic)
Electrical Excitation in the Heart
- Wave of excitation spreads out from the sinoatrial node (SAN) that spreads across both atria, causing them to contract
- Layer of non-conductive tissue prevents the wave from crossing to the ventricles
- Wave of excitation enters the atrioventricular node AVN)
- After short delay the AVN sends wave of excitation to bundle of His (made up of purkinye tissue)
- Bundle of His conducts wave to atrioventricular septum where Bundle of His branches into purkinye tissue
- Purkinye tissue release wave of excitation causing both ventricles to contract from apex (bottom) up
Change in Heart Rate using Chemoreceptors
- Increased exercise increases metabolic rate
- More CO₂ in the blood which lowers blood pH
- Lower pH detected by chemoreceptors in the carotid arteries and aorta walls
- Chemoreceptors send signal to the medulla oblongata
- Medulla oblongata increases frequency of impulses sent to the SAN via sympathetic nervous system
- Heart Rate increases
- Opposite is true for decrease in heart rate (parasympathetic nervous system, low CO₂ concentration, less impulses)
Change in Heart Rate using Baroreceptors
- High blood pressure is detected by baroreceptors in the carotid arteries and aorta walls
- Baroreceptors send signals to the medulla oblongata
- Medulla oblongata decreases frequency of impulses sent to SAN via parasympathetic nervous system
- Heart rate decreases
- Opposite is true for increase in heart rate (sympathetic nervous system, low blood pressure, increased impulses)
