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

What is meant by myogenic and neurogenic

Cardiac muscles are myogenic → contractions initiated from within as opposed to to by nerve cells (neurogenic)

SAN sinoatrial nodes

Describe how the cardiac cycle is controlled by the SAN and the AVN

1. SAN conducts an electrical impulse

2. atria contract at the same time

3. AVN passes electrical activity after a short delay to allow atria to fully contract

4. Via purkyne tissue and bundle of His

5. Ventricles contract from the base upwards

What receptors are involved in controlling heart rate

1. Chemoreceptors detect rise in CO2/ H+/ carbonic acid

2. Baroreceptors detect rise in bp → arteries stretch

Found in carotid artery

• changes to HR controlled by medulla oblangata in cardioregulatory centre

Explain how increased exercise leads to decreased heart rate

1. High levels of CO2 dissolve in blood form carbonic acid to lower PH

2. Chemoreceptors in the aorta detect, sends more impulse to CV centre in medulla

3. Medulla end more impulses to SAN via sympathetic nervous system

4. SAN send more impulses so heart rate increases → CO2 removed

5. Signals medulla to send more impulses to SAN via parasympathetic NS to slow down HR

Explain how a rise in bp results in decrease in rate of a heartbeat

1. Increased HR increases muscle contraction so SV increases

2. Carotid arteries stretch which stimulates baroreceptors

3. send more impulses to medulla oblongata

4. sends more impulses to SAN via parasympathetic NS

5. SAN send less impulses so less heart contractions to decrease bp

What are the three types of muscle

cardiac muscle

Skeletal muscle → many nuclei

Smooth muscle cells → involuntary movements

advantage of antagonistic muscles

• muscles can only pull

• When one muscle contracts the other muscle is pulled out

• Maintains posture

describe the structure of skeletal muscles

• Divided into muscle fibres (myofibrils) → thin and thick filaments

• Repeated units of of muscle fibres → sarcomere

• Muscle fibres covered with cell surface membrane → sarcolemma

• Cytoplasm → sarcoplasm

• Sarcoplasmic reticulum → (SER) store and release calcium ions

Describe structure of thin and thick filaments

• contains 2 actin strands that twist around each other

• Troponin + tropomyosin proteins

• Myosin: myosin tails form a central stalk

• Globular heads attach to specific sites on actin + have ATpase

label different regions of a sarcomere

• I-band: only actin

• H-band: only myosin

• A-band: actin and myosin so appears darker

• M-line → middle line

• Z-line → start and end of one sarcomere

what happens during contraction of sarcomere

• I band shortens

• H band shortens

• A band the same as myosin does not move

• Z lines move closer together

Myosin+ actin filaments slide over each other

what is a neuromuscular junction and what are differences of NMJ compare to cholinergic synapses

where motor neurone meets the muscle fibre stimulated by action potential

• Only excitatory

• A.P ends here

• Ach binds to receptors of muscle fibres not post-synaptic neurone

• Only links neurones to muscles

• Only involves motor neurones no sensory/relay

describe the roles of calcium ions and ATP in the contraction of a myofibril

• A.p reaches muscles via T tubules that branch throughput the sarcoplasm, tubules are in contact with sarcoplasmic reticulum

1. A.P opens Ca2+ ion channels and Ca2+ diffuses into myofibrils from sarcoplasmic reticulum

2. Ca2+ cause movement of tropomyosin on actin

3. Exposes binding sites of actin

4. Myosin heads attach to binding sites on actin → cross-bridge

5. Hydrolysis of ATP

6. Myosin heads bend to perform power stroke

What’s the role of phosphocreatine in providing energy for muscle contraction

• phosphocreatine provides phosphate/ phosphorylates ADP to make ATP

• Generates ATP quickly, anaerobic + alactic

ADP + CP→ ATP + Cr

Differences between fast and slow twitch muscle fibres

• Slow twitch→ aerobic respiration

Dense network of capillaries + large store of Ca²+

• fast twitch → anaerobic (glycolysis)

High rate of ATP hydrolysis in myosin head

What is homeostasis

Maintenance of constant internal conditions despite fluctuations in both the body’s activities and external environment

Controlled by nervous system + endocrine system or a combination

Importance of maintaining core body temperature, pH, blood glucose and water potential

• Temperature: low→ fewer collisions/ high → bonds in tertiary structure break → active site denatures (less E-S complex)

• PH: Bonds in tertiary structure Hb- active site changes denature

• Blood glucose: Low water potential→ water moves out of cells→ cell shrinks no metabolic reactions

Respiratory substrate

• Water potential: cell bursts/shrink

Water is a metabolite + solvent for reactions

What’s meant by negative and positive feedback

• negative feedback: change is detected by receptors and effectors return the system to its original state

• Positive feedback: corrective mechanism stays on, system deviates even more from the original level

What are ectotherms and what are some advantages/disadantages

Animals that cant control body temperature, rely o external sources of heat

• survive longer periods without food

• More energy used for growth

• Slower metabolic reactions

• Less active in cooler temperature→ risk of predation

What are hormones

Chemicals that are released by glands and travel in the blood to certain target cells

• target cells have specific receptors on the cell surface + complementary shape to the hormone

What groups of tissues are involved in maintaining blood-glucose concentrations

• group of cells Islets of Langerhans

• Alpha cells → glucagon

• Beta cells → insulin

• Good capillary blood supply enables them to be secreted directly into the blood

How does insulin work to reduce blood glucose concentration

• Glucose is absorbed by the beta cells via carrier proteins

• Vesicles containing insulin move towards cell-surface membrane and release insulin into capillaries

• Insulin binds to receptor molecules( liver/muscle/adipose tissues)

• More carrier proteins join to the membranes

• Increases permeability to glucose → uptake

• Enzymes convert glucose to glycogen

What happens when there’s a fall in blood glucose concentration

• detected by alpha cells

• Alpha cells secrete glucagon

• Glucagon binds to receptors

• Increased Glycogenolysis glycogen → glucose

• Increased gluconeogenesis fats + A.A → glucose

(Cells respire less)

Describe the livers role in carbohydrate metabolism

• glycogenesis: glucose → glycogen

• Glycogenolysis: glycogen → glucose

• Gluconeognesis → production of glucose from sources other than carbohydrates (fats/amino acids)

Describe the role of adrenaline in control of glucose

• Amino acid derived hormone produced in adrenal glands

• Stimulate glycogenolysis glycogen → glucose

• Inhibits insulin+ action of gut/ increase HR + SV

• Lipid-insoluble proteins → cant diffuse through phospholipid bilayer

Explain the second messenger model

1. Adrenaline binds to receptors on liver cell-surface membrane

2. Causes conformational change in shape of receptor

3. Activates adenylate cyclase enzyme converts ATP → cAMP

4. cAMP activates protein kinase converts glycogen → glucose

5. Increased facilitated diffusion of glucose into the blood

Function of kidneys

• ultrafiltration of blood

• Selective reabsorption of all glucose some ions and water

• Excretes toxic urea, excess ions, water as urine

A.A → ammonia → urea

How does blood enter the glomerulus

Blood enters into glomerulus via Afferent arteriole

Smaller diameter of efferent arteriole causes higher hydrostatic pressure

Describe the three filters of ultrafiltration

• endothelium: narrow gaps allows small molecules to pass through

• Basement membrane: fine mesh of collagen fibres + glycoproteins, stops larger molecules getting through (proteins + RBC)

• Podocytes: epithelial cells with finger like projections (microvilli) allows fluid to pass into lumen of bowman’s capsule

Describe process of ultrafiltration (4 marks)

Blood enters Bowman’s capsule (mass of capillary) via afferent arteriole

1. High hydrostatic pressure caused by smaller diameter of efferent arteriole

2. Small molecules pass through basement membrane acts as a filter (glucose, ions, water)

3. RBC + proteins too large so remain in capillaries and carried by efferent arteriole

4. Pores between podocytes allow substances to dissolve in blood plasma to pass into lumen of bowman’s capsule

Describe the reabsorption of water by the proximal convoluted tube (5 marks)

1. Na+ actively transported into capillary from epithelial cell

2. Lowers concentration of Na+ in cell + maintains diffusion gradient of Na+

3. Na+ moves into epithelial cell from Proximal convoluted tubule via f.d

4. Co-transport of glucose/ amino acids/ Cl- against its concentration gradient

5. Glucose/ amino acids/Cl- move into capillary by f.d

6. Lower water potential in the capillary so water moves into capillary by osmosis (reabsorbed)

Why might a person with diabetes have glucose present in their urine

• Higher concentration of glucose in filtrate

• Carrier proteins are saturated

• Not all lactose is reabsorbed into blood

Explain the role of the loop of henle in the absorption of water from the filtrate (6 marks)

1. Na+ actively transported out of the ascending limb into medulla

2. Ascending limb is impermeable to water

3. Lowers water potential in the medulla

4. Water moves out of descending limb by osmosis into medulla

5. Na+ diffuses into descending limb to recycle in loop of henle

6. Longer loop= lower water potential in the henle

→ counter current: filtrate in ascending + descending limb flow in opposite directions

How does the length of loop of henle affect water absorption ?

longer loop of henle = more water reabsorbed

• Lower water potential in medulla

• More water moves into descending loop via osmosis

• Concentration gradient maintained for a longer period of time

What is osmoregulation and osmoreceptors

Process by which organisms regulate the water content of the body

• osmoreceptors in hypothalamus monitor the water potential of blood

• Posterior pituitary gland secretes ADH

How does ADH cause more water or be reabsorbed

• ADH binds to receptors on cell-surface membrane of distal convoluted tubule

• Activates phosphorylase which converts ATP → CAMP

• Vesicles fuse with cell-surface membrane

• Contains aquaporin + fuses to cell-surface membrane

• Increases permeability to water so more water reabsorbed

→ decreases volume of urine + increases concentration

Describe the process i which concentrated urine is produced in kidneys

1. Osmoreceptors in hypothalamus detect the low water potential of blood

→ change shape/ impulses to posterior pituitary gland

2. Posterior pituitary gland secretes more ADH into blood

3. More aquaporin channels on cell surface membrane

4. Increases permeability to water So more water reabsorbed by osmosis down water potential gradient