Organisms Respond To Internal And External Environments Flashcards
What is a stimulus?
change in an organisms internal or external environment.
Why is it important that organisms can respond to stimuli?
Organisms increase their chance of survival by responding to stimuli
What is a tropism (+ve and -ve)
Growth of a plant in response to a directional stimulus
Positive tropism = towards a stimulus;
negative tropism = away from stimulus
Summarise the role of growth factors in flowering plants
● Specific growth factors (hormone-like growth substances) eg. Auxins (such as IAA) move
(via phloem or diffusion) from growing regions eg. shoot / root tips where they’re produced
● To other tissues where they regulate growth in response to directional stimuli (tropisms)
Describe how indoleacetic acid (IAA) affects cells in roots and shoots
● In shoots, high concentrations of IAA stimulates cell elongation
● In roots, high concentrations of IAA inhibits cell elongation
Explain gravitropism in flowering plants
*hint - are 5 steps
- Cells in tip of shoot / root produce IAA
- IAA diffuses down shoot / root (evenly initially)
- IAA moves to lower side of shoot / root (so concentration increases)
- In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
- So shoots bend away from gravity whereas roots bend towards gravity
Explain phototropism in flowering plants
- Cells in tip of shoot / root produce IAA
- IAA diffuses down shoot / root (evenly initially)
- IAA moves to shaded side of shoot / root (so conc. ↑)
- In shoots this stimulates cell elongation whereas in
roots this inhibits cell elongation - So shoots bend towards light
whereas roots bend away from ligh
List the 2 simple responses that can maintain a mobile organism in a
favourable environment
Taxes
Kinesis
Describe taxes
Taxes (tactic response)
○ Directional response
○ Movement towards or away from a directional stimulus
Describe kinesis
Kinesis (kinetic responses)
○ Non-directional response
○ Speed of movement or rate of direction change
changes in response to a non-directional stimulus
○ Depending on intensity of stimulus
Example of taxis and kinesis in woodlice in response to light
Examples: taxis- woodlice moving away from light to avoid predators;
kinesis- woodlice moving faster in drier
environments to increase their chance of moving to an area with higher humidity to prevent drying out.
Explain the protective effect of a simple (eg. 3 neurone) reflex
● Rapid as only 3 neurones and few synapses (synaptic transmission is slow)
● Autonomic (doesn’t involve conscious regions of brain) so doesn’t have to be learnt
● Protects from harmful stimuli eg. escape predators / prevents damage to body tissues
Describe the basic structure of a Pacinian corpuscle
Lamellae (layers of connective tissue)
Stretch mediated sodium ion channel (closed)
Sensory neurone ending
Sensory neurone axon
Myelin sheath (Schwann cells)
Describe how a generator potential is established in a Pacinian corpuscle
- Mechanical stimulus eg. pressure deforms lamellae and stretch- mediated sodium ion
(Na+) channels - So Na+ channels in membrane open and Na+ diffuse into 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
Explain what the Pacinian corpuscle illustrates
● Receptors respond only to specific stimuli
○ Pacinian corpuscle only responds to mechanical pressure
● Stimulation of a receptor leads to the establishment of a generator potential
○ When threshold is reached, action potential sent (all-or-nothing principle)
Explain the differences in sensitivity to light for rods & cones in the retina
Rods are more sensitive to light
● Several rods connected to a single neurone
● Spatial summation to reach / overcome threshold (as enough
neurotransmitter released) to generate an action potential
Cones are less sensitive to light
● Each cone connected to a
single neurone
● No spatial summation
Explain
Explain the differences in visual acuity for rods & cones in the retina
Rods give lower visual acuity
● Several rods connected to a single neurone
● So several rods send a single set of impulses
to brain (so can’t distinguish between
separate sources of light)
Cones give higher visual acuity
● Each cone connected to a single neurone
● Cones send separate (sets of) impulses to brain
(so can distinguish between 2 separate sources
of light)
Explain the differences in sensitivity to colour for rods & cones in the retina
Rods allow
monochromatic vision
● 1 type of rod / 1
pigment
Cones allow colour vision
● 3 types of cones - red-
, green- and blue-sensitive
● With different optical pigments → absorb different wavelengths
● Stimulating different combinations of cones gives range of colour perception
Cardiac muscle is myogenic. What does this mean?
It
It can contract and relax without receiving electrical impulses from nerves.
Structures in the heart
Sinoatrial node (SAN)
Atrioventricular node (AVN)
Bundle of his
Purkyne tissue
Describe the myogenic stimulation of the heart and transmission of a
subsequent wave of electrical activity
- Sinoatrial node (SAN) acts as pacemaker → sends regular waves of electrical activity across atria
○ Causing atria to contract simultaneously
2 . Non-conducting tissue between atria / ventricles prevents impulse passing directly to ventricles
○ Preventing immediate contraction of ventricles - Waves of electrical activity reach atrioventricular node (AVN) which delays impulse
○ Allowing atria to fully contract and empty before ventricles contract - AVN sends wave of electrical activity down bundle of His, conducting wave between ventricles to apex where it branches into Purkyne tissue
○ Causing ventricles to contract simultaneously from the base up
Where are chemoreceptors and pressure receptors located?
Chemoreceptors and pressure receptors are located in the aorta and carotid arteries.
Describe the roles of chemoreceptors, pressure (baro) receptors, the autonomic
nervous system and effectors in controlling heart rate
- Baroreceptors detect [fall / rise] in blood pressure and / or chemoreceptors detector [fall/ rise] in blood pH
- Send impulses to medulla
- Which send more frequent impulses to SAN along [sympathetic / parasympathetic] neurones
- So [more / less] frequent impulses sent from SAN and to / from AVN
- So cardiac muscle contracts [more / less] frequently
- So heart rate [increases / decreases]
List the structures in a myelinated motor neurone
Dendrite
Cell body/soma
Axon
Myelin sheath made of Schwann cells
Node of ranvier
Axon terminal
Describe resting potential
Inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside).
Explain how a resting potential is established across the axon membrane in
a neurone
● Na +/K+ pump actively transports:
○ (3) Na+ out of axon AND (2) K+ into axon
● Creating an electrochemical gradient:
○ Higher K+ conc. inside AND higher Na+
conc. outside
● Differential membrane permeability:
○ More permeable to K+ → move out by facilitated diffusion
○ Less permeable to Na+ (closed channels)
Explain how changes in membrane permeability lead to depolarisation and
the generation of an action potential
- Stimulus
● Na+ channels open; membrane permeability to Na+ increases
● Na+ diffuse into axon down electrochemical gradient (causing depolarisation) - Depolarisation ● If threshold potential reached, an action potential is generated
● As more voltage-gated Na+ channels open (positive feedback effect)
● So more Na
+ diffuse in rapidly - Repolarisation ● Voltage-gated Na
+
channels close
● Voltage-gated K+ channels open; K+ diffuse out of axon - Hyperpolarisation ● K+
channels slow to close so there’s a slight overshoot – too many K+ diffuse
out - Resting potential ● Restored by Na
+/K+
pump
Describe the all-or-nothing principle
● For an action potential to be produced, depolarisation must exceed threshold potential
● Action potentials produced are always same magnitude / size / peak at same potential
○ Bigger stimuli instead increase frequency of action potentials
Explain how the passage of an action potential along non-myelinated axons results in nerve impulses
● Action potential passes as a wave of
depolarisation
● Influx of Na
+ in one region increases
permeability of adjoining region to Na+ by
causing voltage-gated Na+ channels to open
so adjoining region depolarises
Explain how the passage of an action potential along non-myelinated and
myelinated axons results in nerve impulses
● Myelination provides electrical insulation
● Depolarisation of axon at nodes of Ranvier only
● Resulting in saltatory conduction (local
currents circuits)
● So there is no need for depolarisation along
whole length of axon
Suggest how damage to the myelin sheath can lead to slow responses and /
or jerky movement
● Less / no saltatory conduction; depolarisation occurs along whole length of axon
○ So nerve impulses take longer to reach neuromuscular junction; delay in muscle contraction
● Ions / depolarisation may pass / leak to other neurones
○ Causing wrong muscle fibres to contract
Describe the nature of the refractory period
● Time taken to restore axon to resting potential when no further action potential can be generated
● As Na+ channels are closed / inactive / will not open
Explain the importance of the refractory period
●
● Ensures discrete impulses are produced (action potentials don’t overlap)
● Limits frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus)
○ Higher intensity stimulus causes higher frequency of action potentials
○ But only up to certain intensity
● Also ensures action potentials travel in one direction– can’t be propagated in a refractory region
In the second half of the refractory period an action potential can be produced but requires greater stimulation
to reach threshold.
List the factors that affect speed of conductance
Myelination
Axon diameter
Temperature
Describe how myelination affects
● Depolarisation at Nodes of Ranvier only → saltatory conduction
● Impulse doesn’t travel / depolarise whole length of axon
Describe how axon diameter affects the speed of conductance
● Bigger diameter means less resistance to flow of ions in cytoplasm
Describe how temperature affects the speed of conductance
● Increases rate of diffusion of Na+ and K+ as more kinetic energy
● But proteins / enzymes could denature at a certain temperature
Describe the structure of a synapse
Pre synaptic neurone
Axon
Vesicle containing neurotransmitter
Voltage gated calcium ion Channel
Axon terminal
Synaptic cleft
Receptor and sodium ion Channel
What are cholinergic synapses
Synapses that use the neurotransmitter acetylcholine (ACh).
Describe transmission across a chlolingergic synapse
- Depolarisation of the pre synaptic membrane causes opening of voltage gated ca2+ channels
- Ca2+ diffuse into pre synaptic neurone/knob - Causing vesicles containing ACH to move and fuse with pre synaptic membrane
- releasing ACh into synaptic cleft (by exocytosis) - ACh diffuses across the synaptic cleft to bind to receptors on post synaptic membrane
- Causing Na+ channels to open
- Na+ diffuse into post synaptic knob chasing depolarisation
- if threshold is met an action potential is generated
Explain what happens to acetylcholine after synaptic transmission
Explain what happens to acetylcholine after synaptic transmission
● It is hydrolysed by acetylcholinesterase
● Products are reabsorbed by the presynaptic neurone
● To stop overstimulation- if not removed it would keep binding to receptors, causing depolarisation
Explain how synapses result in unidirectional nerve impulses
● Neurotransmitter only made in / released from pre-synaptic neurone
● Receptors only on post-synaptic membrane
Explain summation by synapses
● Addition of a number of impulses converging on a single post-synaptic neurone
● Causing rapid buildup of neurotransmitter (NT)
● So threshold more likely to be reached to generate an action potential
Importance- low frequency action potentials release insufficient neurotransmitter to exceed threshold.
Describe spatial summation
● Many pre-synaptic neurones
share one synaptic cleft /
post-synaptic neurone
● Collectively release sufficient NT
to reach threshold to trigger an
action potential
Describe temporal summation
● One pre-synaptic neurone
releases neurotransmitter many
times over a short time
● Sufficient NT to reach threshold
to trigger an action potential
Describe inhibition by inhibitory synapses
● Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as:
○ Cl-
channels open → Cl- diffuse in
○ K+ channels open → K+ diffuse out
● More Na+ required for depolarisation
● Reduces likelihood of threshold being met / action potential formation
at post-synaptic membranes
Importance- both excitatory and inhibitory neurones forming synapses with the same post-synaptic membrane
gives control of whether it ‘fires’ an action potential.
Describe the structure of a neuromuscular junction
Very similar to a synapse except:
● Receptors are on muscle fibre instead of postsynaptic membrane and there are more
● Muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter
Compare transmission across cholinergic synapses and neuromuscular
junctions
- In both, transmission is unidirectional.
In a cholergernic synapse transmission occurs from neurone to neuron (or effectors or glands)
In neuromuscular junction transmission occurs from motor neurone to muscle - in cholinergic synapse neurotransmitters can be excitatory or inhibitory, in neuromuscular junction its always excitaotrory
In cholenergic synapse Action potential may be initiated in postsynaptic
neurone, in neuromuscular junction Action potential propagates along sarcolemma
down T tubules
Use examples to explain the effect of drugs on a synapse
● Some drugs stimulate the nervous system, leading to more action potentials, eg.:
○ Similar shape to neurotransmitter
○ Stimulate release of more neurotransmitter
○ Inhibit enzyme that breaks down neurotransmitter → Na
+
continues to enter
● Some drugs inhibit the nervous system, leading to fewer action potentials, eg.:
○ Inhibit release of neurotransmitter eg. prevent opening of calcium ion channels
○ Block receptors by mimicking shape of neurotransmitter
Describe how muscles work
● Work in antagonistic pairs → pull in opposite directions eg. biceps / triceps
○ One muscle contracts (agonist), pulling on bone / producing force
○ One muscle relaxes (antagonist)
● Skeleton is incompressible so muscle can transmit force to bone
Advantage of muscles working in an
the second muscle required to reverse movement caused by the first
(muscles can only pull) and contraction of both muscles helps maintain posture.
Describe the gross and microscopic structure of skeletal muscle
● Made of many bundles of muscle fibres (cells) packaged together
● Attached to bones by tendons
● Muscle fibres contain:
○ Sarcolemma (cell membrane) which folds inwards
(invagination) to form transverse (T) tubules
○ Sarcoplasm (cytoplasm)
○ Multiple nuclei
○ Many myofibrils
○ Sarcoplasmic reticulum (endoplasmic reticulum)
○ Many mitochondria
Describe the structure of a mmyofibril
● Made of two types of long protein filaments, arranged in parallel
○ Myosin- thick filament
○ Actin- thin filament
● Arranged in functional units called sarcomeres
○ Ends – Z-line / disc
○ Middle – M-line
○ H zone– contains only myosin
Explain
Explain the banding pattern to be seen in myofibrils
● I-bands- light bands containing only thin actin filaments
● A-bands- dark bands containing thick myosin filaments
(and some actin filaments)
○ H zone contains only myosin
○ Darkest region contains overlapping actin and
myosin
Give an overview of muscle contraction
● Myosin heads slide actin along myosin causing the sarcomere to contract
● Simultaneous contraction of many sarcomeres causes myofibrils and muscle fibres to contract
● When sarcomeres contract (shorten)…
○ H zones get shorter
○ I band get shorter
○ A band stays the same
○ Z lines get closer
Describe the role of actin myosin calcium ions tropmyosin and atp in muscle contraction
1 Depolarisation spreads down sarcolemma via T tubules causing Ca2+
release from sarcoplasmic reticulum, which diffuse to myofibrils
2 Calcium ions bind to tropomyosin, causing it to move → exposing
binding sites on actin
3 Allowing myosin head, with ADP attached, to bind to binding sites on
actin → forming an actinomyosin crossbridge
4 Myosin heads change angle, pulling actin along myosin, (ADP
released), using energy from ATP hydrolysis
5 New ATP binds to myosin head causing it to detach from binding site
6 Hydrolysis of ATP by ATP(hydrol)ase (activated by Ca2+) releases
energy for myosin heads to return to original position
7 Myosin reattaches to a different binding site further along actin
Process is repeated as long as calcium ion conc. is high
During muscle relaxation:
- Ca2+ actively transported back into the endoplasmic reticulum using energy from ATP
- Tropomyosin moves back to block myosin binding site on actin again → no actinomyosin cross bridges
Describe the role of phosphocreatine in muscle contraction
● A source of inorganic phosphate (Pi)→ rapidly phosphorylates ADP to regenerate ATP
○ ADP + phosphocreatine → ATP + creatine
● Runs out after a few seconds → used in short bursts of vigorous exercise
● Anaerobic and alactic
General properties of slow vs fast switch
Slow twitch is ● Specialised for slow, sustained
contractions (eg. posture, long
distance running)
● Obtain ATP mostly from aerobic
respiration → release energy slowly
● Fatigues slowly
Fast twitch is ● Specialised for brief, intensive
contractions (eg. sprinting)
● Obtain ATP mostly from anaerobic
respiration → release energy quickly
● Fatigues quickly due to high lactate conc.
Compare the, location of slow and fast
skeletal muscle fibres
Slow twitch are ● High proportion in muscles used for
posture eg. back, calves
● Legs of long distance runners
Fast twitch are ● High proportion in muscles used for fast
movement eg. biceps, eyelids
● Legs of sprinters
Compare the structure of slow and fast
skeletal muscle fibres
For slow twitch ● High conc. of myoglobin → stores
oxygen for aerobic respiration
● Many mitochondria → high rate of
aerobic respiration
● Many capillaries → supply high conc. of
oxygen / glucose for aerobic
respiration and to prevent build-up of
lactic acid causing muscle fatigue
For fast twitch ● Low levels of myoglobin
● Lots of glycogen → hydrolysed to provide
glucose for glycolysis / anaerobic
respiration which is inefficient so large
quantities of glucose required
● High conc. of enzymes involved in
anaerobic respiration (in cytoplasm)
● Store phosphocreatine
