Organisms respond to changes in their internal and external environment Flashcards
Describe phototropism in plants
- Cells in the tip of the shoot/root produce IAA
- IAA diffuses down the shoot/root
- IAA moves to shaded side of the shoot/root
- In shoots, this stimulates cell elongation, whereas in roots, it inhibits cell elongation
- So shoots bend towards light whereas roots bend away from light
Describe how heart rate is controlled
- Baroreceptors detect a [fall / rise] in blood pressure and/or chemoreceptors detect a [rise / fall] in blood CO2 concentration or [fall / rise] in blood pH
- This sends impulses to the medulla / cardiac control centre which send more frequent impulses to SAN along the [sympathetic (increases heart rate) / parasympathetic (decreases)] neurones
- So [more / less] frequent impulses are sent from the SAN to / from the AVN so the cardiac muscle contracts [more / less] frequently
- So heart rate [increases / decreases]
Describe the locations of chemoreceptors and pressure receptors
Chemoreceptors and pressure receptors are located in the aorta and carotid arteries
Describe the Myogenic stimulation of the heart
- Sinoatrial node (SAN) acts as pacemaker - sends regular waves of electrical activity across the atria causing it to contract simultaneously
- Non-conducting tissue between atria/ventricles prevents impulse passing directly to the ventricles preventing immediate contraction of ventricles
- Waves of electrical activity reach the atrioventricular node (AVN) which delays the impulse, allowing the atria to fully contract and empty before ventricles contract
- AVN sends a wave of electrical activity down the bundle of His which branches into Purkyne tissue and causes the ventricles to contract simultaneously from the base up
Describe the differences in colour vision for rods and cones
- Rods only allow monochromatic vision (black and white)
There is only one type of rod that only contains one pigment - Cones allow colour vision
3 types of cones; red, blue and green
With different optical pigments → absorb different wavelengths
Stimulation of different combinations/proportions of cones gives a range of colour perception
Describe the differences in visual acuity for rods and cones
- Rods give lower visual acuity
Several rods are connected to a single neurone
Several rods send a single set of impulses to the brain (therefore, it cannot distinguish between separate sources of light)
Cones give higher visual acuity
Each cone is connected to a single neurone - Cones send separate (sets of) impulses to the brain (so can distinguish between 2 separate sources of light)
Describe the differences in sensitivity to light for rods and cones
- Rods are more sensitive to light
Several rods are connected to a single neurone Spatial summation is required to reach/overcome threshold to generate an action impulse - Cones are less sensitive to light
Each cone connected to a single neurone
No spatial summation
Describe how a generator potential is established in a pacinian corpuscle
- Mechanical stimulus (eg. pressure) deforms the lamellae and stretch-mediated sodium (Na+) channels
Na+ channels in membrane open and Na+ diffuse into the sensory neurone - Greater pressure causes more Na+ channels to open and more Na+ to enter
- This causes depolarisation, leading to a generator potential - If generator potential reaches threshold it triggers an action potential (see section 6.2)
Describe the importance of the protective effect
Rapid - only 3 neurones and a few synapses involved
Automatic - so it doesn’t have to be learnt
Protects from you from harmful stimuli
Describe Kineses (kinetic responses)
- Non-directional response
- Speed of movement or rate of direction changes in response to a non-directional stimulus (a stimulus that doesn’t relate to the direction an insect is in e.g. conditions of environment)
- Eg. woodlice move faster when in drier environment to increase chances of moving to an environment with higher humidity to prevent drying out
Describe Taxes (tactic responses)
- Directional response (response by movement in a direction)
- Movement towards (positive taxis) or away from (negative taxis) a stimulus
- Eg. woodlice move away from light to avoid predators
Describe gravitropism in plants
Cells in the tip of the shoot/root produce IAA
IAA diffuses down the shoot/root
IAA moves to lower side of shoot/root
In shoots, this stimulates cell elongation, whereas in roots, it inhibits cell elongation
So shoots bend away from gravity and roots bend towards gravity
Describe the role of growth factors in flowering plants
Specific growth factors (hormone-like growth substances) eg. Auxins (such as IAA) move from growing regions eg. shoot / root tips (where produced) to other tissues where they regulate growth in response to directional stimuli
Describe the inhibition by inhibitory neurotransmitters feature of synapses
- Inhibitory neurotransmitters hyperpolarise the postsynaptic membrane so chloride channels will open and chloride ions will diffuse in, potassium channels will also open so potassium ions will diffuse out
- Therefore, more sodium ions are needed for depolarisation
So reduces chance of threshold being met and action potential formation at post-synaptic membranes
Describe the summation feature of synpases
- Addition of a number of impulses converging on a single post-synaptic neurone
- Causing rapid buildup of neurotransmitter
- So threshold more likely to be reached to generate an action potential
Describe the unidirectionality of synapses
- Neurotransmitter only made in/released from the pre-synaptic neurone
- Receptors only present on the post-synaptic membrane
Describe the factors that affect speed of conductance
- Myelination
- Depolarisation at Nodes of Ranvier only - this is called saltatory conduction
Impulse doesn’t travel / depolarise the whole length of the axon
Axon diameter - a larger diameter means less resistance to the flow of ions in cytoplasm - Temperature
Increases the rate of diffusion of ions as there is more kinetic energy - However, proteins/enzymes could denature at certain temperatures
Describe the nature and importance of the refractory period
Nature: Refractory period is the time needed to restore the axon to the resting potential when no further action potential can be generated because Na+ channels are closed / inactive / will not open
Importance:
Ensures discrete impulses are produced (action potentials don’t overlap)
Limits frequency of impulse transmission at a certain intensity (prevents over reaction to a stimulus)
Ensures action potentials travel in one direction
Describe the all-or-nothing principle
For an action potential to be produced, depolarisation must exceed the threshold potential
Action potentials produced are always the same magnitude / size / peak at the same potential
Describe the generation of action potential
Stimulus: Na+ channels open and membrane permeability to Na+ increases, which causes Na+ to diffuse into the axon down an electrochemical gradient (causing depolarisation)
Depolarisation: If threshold potential is reached, an action potential is generated - this is because more voltage-gated Na+ channels open and therefore, more Na+ diffuses in rapidly
Repolarisation: voltage-gated Na+ channels close and voltage-gated K+ channels open causing K+ diffuse out of axon
Hyperpolarisation - K+ channels are slow to close so there’s a slight overshoot as too many K+ diffuse out
Resting potential is then restored by the Na+ / K+ pump
Describe the establishment of resting potential
Na+ / K+ pump actively transports 3 Na+ out of the axon and 2K+ into the axon
This creates an electrochemical gradient as there is a higher concentration of K+ inside the axon and a higher concentration of Na+ outside the axon
This creates differential membrane permeability as the axon is more permeable to K+, which moves out by facilitated diffusion and becomes less permeable to Na+ due to the closed channels
Describe the role of phosphocreatine in muscle contraction
Releases high-energy phosphates
To recycle ATP (as reformation of ATP requires phosphate molecules)
Describe the role of calcium ions and tropomyosin in the cycle of actinomyosin bridge formation
Before contraction, they cover the myosin binding sites on actin molecules, preventing myosin heads from binding
Calcium ions cause tropomyosin proteins to change position on actin filaments
Describe the role of ATP in myofibril contraction
Binds to myosin heads and is hydrolysed
Releases energy for the myosin head bending
Provides energy to detach myosin head and reattach further along
Also provides energy for calcium ions to be actively transported back into the sarcoplasmic reticulum
