Unit 6 -responses to internal and external environment

Define: stimulus/receptor/coordinator/effector/response

Stimulus- change in internal/external environment of an organism that leads to a response

Receptor- detects stimulus, specific to one type of stimulus

Coordinator- formulates a suitable response to a stimulus ( nervous system)

Effector- muscle/gland that produces response to stimulus

Response- change brought due to stimulus

What is tropism give examples

Growth movement of a plant in response to directional stimulus ( away or towards)

• phototropism/ gravitropism/hydrotropism

Describe how IAA results in phototropism in shoots

1. IAA produced in meristems of shoot tips/ root tips

2. IAA detects light due to receptors

3. IAA diffuses to the shaded region so there’s a higher concentration of IAA

4. Causes more cell elongation

5. Shoot bends towards the light

Outline the acid growth hypothesis

Auxin promotes the active transport of H+ ions

• H+ ions lower the PH

• Optimum conditions for expansin enzymes to break the H bonds in cellulose

• Reduces rigidity

Describe how IAA results in gravitropism in roots

1. IAA produced in meristems in shoot tips

2. IAA accumulates on the lower side

3. Inhibits cell elongation so cells elongates faster on the upper side

4. Root curves downwards towards gravity

Define Taxes and Kinesis

• Taxes: Directional response by organisms who move towards a favourable stimulus or away from an unfavourable one

• Kinesis: Non-directional response by organisms who change the speed of movement or the rate of direction change in response to a non-directional stimulus

Why is taxis important

• survival from harmful stimuli + favourable environment

• Find food

• Mating

Why is positive photo taxis in algae useful

• increases rate of light-dependent reaction/

• Increases rate of independent reaction as more ATP/NADPH

• More glucose produced

• More respiration

What statistical test is used for choice hammer practical

Chi-squared,to see if there’s a significant difference between the observed and expected number of maggots in each chamber

How can we keep maggots similiar

• same previous treatment ( environment/feeding)

• Same size/age

• Same species

How are neurones different from other cells

1. Dendrites bring information to cell body + axon carries information away from cell body

2. Communicate with other cells via electrochemical processes

3. Specialised structures (NT

What is a reflex

Involuntary actions that are fast and automatic, don’t involve the brain

• protect us from harmful stimuli

• Effective from birth, not learned

• Fast as neurone pathway is short few synapses

What is an action potential + resting potential

• Resting potential: difference in electrical charge maintained across the membrane of the axon of a neurone when not stimulated

- (-70mv)

• Action potential; changed that occur in the electrical charge across the membrane of an axon when its stimulated and a nerve impulse passes

-(+40mv)

Define depolarisation + generator potential

Depolarisation: temporary reversal of charges on the cell-surface membrane of a neurone that takes place when a nerve impulse is transmitted

Generator potential: depolarisation of the membrane of a receptor cell as a result of a stimulus which changes the PD

Describe how a resting potential is established

1. Active transport of Na+ out of axon and K+ into the axon by sodium-potassium pump

2. 3 Na+ out and 2 K+ in

3. Membrane more permeable to K+ ions (K+ channels open) k+ diffuse out

4. Membrane less permeable to Na+ (Na+ channels closed)

How is the Resting potential maintained

1. Higher concentration of K+ inside axon + Higher concentration of Na+ outside neurone

2. Membrane less permeable to Na+ (Na+ channels closed)

3. Na+ actively transported out axon 3Na+ out 2K+ in by sodium-potassium pump

4. Inside axon more negative compared to outside

5. Diffusion of K+ ion out of neurone- maintains electrochemical gradient

Name each stage of the generation of action potential

1. Stimulus

2. Depolarisation

3. Repolarisation

4. Hyperpolarisation

5. Resign potential

What happens during depolarisation

1. Stimulus causes the sodium ion channels in axon membrane to open

2. Na+ diffuses into the axon down the electrochemical gradient

3. Inside the axon is less negative as p.d reduced

4. If threshold of -55mv is reached, more sodium channels open so more Na+ enter

5. Axon reaches action potential +40mv

What happens during repolarisation

1. Sodium ion voltage-gated channels close

2. Potassium ion voltage-gated channels open K+ diffuse or of axon down concentration gradient

3. Sodium potassium pump actively transports 3 Na+ out for every 2 K+ in (original distribution of ions)

4. Hyerpolarisation period axon becomes more negative than R.P

5. Potassium ion voltage-gated channels close returning to R.P -70mv

What is a refractory period and why is it important

Period where the cell is recovering, no action potential generated/ restores axon to R.P ( sodium ion channels not activated)

• ensures action potential is propagated in one direction

• Limits the frequency of A.P (prevents overstimulation)

• Produces separate impulses

Describe the All or nothing principle

• if threshold p.d (-55mv) is reached an action potential will fire → ALL

• If threshold p.d not reached, no action potential generated → NOTHING

→ bigger stimulus causes more frequent action potentials not bigger A.P (all +40mv)

Describe the propagation of an action potential

• As one region of axon produces A.P becomes depolarised, acts as stimulus for next region to be depolarised

• Previous region becomes repolarised + returns to R.P

Describe the role of Schwann cells

• forms multilayered lipoprotein coat (forming myelin sheath) with node of ranvier at either end

• Provides electrical insulation → carry out phagocytosis + plays a role in nerve regeneration

• Myelin sheath is an electrical conductor, prevents A.P forming in and on myelination

What is meant by saltatory conduction, is conduction faster in myelinated or non-myelinated sheaths

• A.P can only occur at node of ranvier so A.P jumps from node to node

• Faster speed of conductance in myelinated sheath

• A.P in non-myelinated sheath travels the entire length of axon

What factors affect the speed of conductance

• presence of myelin sheath

• Diameter of axon

→ greater diameter→ less resistance → less collisions → increase speed

• Temperature

→ faster rate of ion diffusion/ increase Ke beyond optimum → sodium potassium ion channels denature (proteins)

Describe the sequence of events that allows information to pass from one neurone to the ext neurone across a cholinergic synapse

1. Action potential reaches the presynaptic knob and calcium channels open

2. Ca²+ ions diffuse into pre-synaptic neurone

3. Vesicles fuse with presynaptic membrane

4. Acetylcholine released into the synaptic cleft and diffuses across the synapse

5. Ach binds to receptors on post synaptic membrane

6. Sodium ions enter the postsynaptic neurone

7. Depolarisation of postsynaptic membrane

8. If above threshold, an A.P is produced

Explain why an A.P is less likely to be generated when GABA is released

• NT (GABA) cause chloride ion channels to open

• Chloride ions move into axon by diffusion

• More K+ ions move out of the axon

• Inside of the neurone more negative than usual

• Hyperpolarisation so A.P cant be generated

What’s the structure of the Pacinian Corpuscle?

Mechanoreceptors found deep in the skin, detect strong pressures (not light touch)

Contain stretch mediated sodium ion channels

How does the Pacinian Corpsucle cause an A.P

1. At rest, more sodium ions on the outside than the inside

2. Pressure distorts the neurone cell membrane, opens the stretch mediated sodium ion channels

3. Stronger pressure → more Na+ channels open

4. Na+ diffuses in causing depolarisation allowing generator potential to be established

5. If threshold is reached, A.P is generated

What does the P.C illustrate

• Receptors respond to specific stimuli

• P.C responds to mechanical pressure

• When a receptor is stimulated leads to generator potential being reached

• All or nothing principle, when threshold is reached A.P generated

Describe the two types of summation

Temporal summation: two impulses/ NT are sent in quick succession from the same presynaptic neurone → cone cells

Spatial summation: impulses from different pre synaptic neurones that act on synapses on the same postsynaptic neurone → rod cells

Describe the structure of the retina

Retina contains photoreceptors

Blind spot where optic nerve attaches → no receptors

Fovea → greater density of photoreceptors

How does the breakdown of rhodopsin lead to a generator potential being reached

Rhodopsin (opsin + retianal)

1. light energy causes retinal to change shape no longer biding to opsin so breaks down

2. Breakdown causes membrane to be more permeable to Na+ ions

3. Change in distribution of Na+, change in p.d across rod membrane → generator potential

4. Rhodopsin resynthesised using energy from hydrolysis of ATP by mitochondria n inner segments

Define visual acuity and explain why rod cells have low visual acuity but high sensitivity

Ability of the eye to distinguish between different shapes + details at a given distance

• many rod cells joined to the same bipolar neurone, only a single impulse stimulated

→ cant distinguish separate sources of light that stimulate them

• high sensitivity as enough NT to reach the threshold

Why do cone cells have high visual acuity and low sensitivity to light

Has blue/green/red light sensing cells

• Each cone cell connected to one bipolar neurone, sends separate sets of impulses to the brain

• Requires temporal summation not enough NT released so threshold not reached

• Stimulation f different combinations of cones gives range of colour perception