Unit 6- AQA ALEVEL biology

Stimulus

Detectable change in the internal/external environment of an organism that leads to a response/ change in the organism

Tropism

When plants respond to a stimuli via growth

Can be positive (towards)

Can be negative (away)

Process of positive phototropism in shoots

1) cells in the tip of the shoot make IAA, which is transported evenly down the shoot

2) light causes IAA to build up on the shaded side of the shoot

3) high concentration of IAA causes elongation of shoot cells on the shaded side

4) shaded side grows faster causing shoot to bend towards the light

Process of positive gravitropism in roots

1) cells in the tip of the root make IAA which in transported evenly across the root

2) gravity causes IAA to move from upper side to lower side of root so IAA builds up on lower side

3) high concentration of IAA inhibits elongation of root cells so cells on the lower side elongate less

4) upper side grows faster which causes shoot to bend downwards towards gravity (anchors the plant in)

IAA

Indoleacetic acid (growth factor)

type of auxin which can control cell elongation in shoots and inhibit growth of cells in the roots

Made in the tip of roots/shoots and can diffuse

Reflex

Rapid, automatic response to an external or internal stimuli

Taxis

organism will move its entire body towards a favourable stimulus or away from an unfavourable stimulus

Kinesis

Organism changes the speed of movement and frequency of direction changes in order to randomly move towards a favourable condition/ away from an unfavourable one

2 main nervous systems

1) the central nervous system (brain and spinal chord)

2) the peripheral nervous system (sensory and motor neurones)

Voluntary nervous system

Carries nerve impulses to body muscles and is under voluntary control

Autonomic nervous system

Carries nerve impulses to glands, smooth muscle an cardiac muscle and is ‘subconscious’

Structure of a motor neurone

• soma (main cell and nucleus)

• axon

• myelin sheath (myelination over Swann cells)

• nodes of ranvier (saltatory conduction between them )

• Dendrites

The spinal chord

• Column of nervous tissue that runs along the back inside the vertebral column for protection (emerging at intervals are pairs of nerves)

Example of a reflex arc (hot stimulus)

• Stimulus- hot pan

• Receptor- temperature receptors in skin

• SEnsory neurone- passes impulse to spinal chord

• Intermediate neurone- in spinal cord links the neurones together

• Motor neurone passes impulse from spinal cord to muscle in upper arm

• Effector- the muscle in upper arm is stimulated to contract

• Response- drop saucepan

Importance of reflex arcs

• protect the body from harmful stimuli

they are fast, because the neurone pathway is short with very few synapses

• Brain is not overloaded with situations in which the response is the same

are involuntary and therefore do not require the decision making powers of the brain the leaving it free to carry out more complex responses

Receptors

Cells which detect specific stimuli to establish a generator potential to cause a response

1) Pacinian corpuscle

2) Rods

3) cones

Pacinian corpuscle

Receptor located deep in skin that responds to mechanical pressure (transducers the mechanical energy of the stimulus into a generator potential

• has a capsule/lamellae

• Axon membrane which travels into the lamellae

• Shwann cells/ myelination

• Has stretch-mediated Na+ channels

Method of action in pacinian corpuscle

1) in its resting state, the stretch-mediated Na+ channels are too narrow to allow Na+ through

2) when mechanical pressure is applied the corpuscle changes shape which stretches/deforms the membrane/ lamellae the neurone

3) this widens the channel and the Na+ can now diffuse into the neurone

4) this influx of Na+ changes the potential of the membrane causing depolarisation

5) this creates a generator potential which creates an action potential

Features of rod cells

• high in number

• Most highly concentrated in periphery of retina (not present at fovea)

• See in black and white

• Contain rhodopsin which is very sensitive to light (only requires low intensities to be stimulated)

• High retinal convergence (multiple rods connected to one bipolar neurone

• Low visual acuity

Features of cone cells

• fewer in number

• Highest concentration at the fovea (fewer in the periphery of retina)

• 3 different types of cones (RGB) each of which responds to a different wave length of light- allows us to see in colour

• Contain iodopsin which is less sensitive to light (therefore need higher light intensities to be stimulated)

• No retinal convergence (each cone cell connected to one bipolar neurone)

• High visual acuity

Establishing resting potential

1) 3 Na+ ions actively transported out of axon by Na+/K+ATP-ase

2) 2 K+ ions actively transported in to axon by Na+/K+ATP-ase

3) creates an electrochemical gradient (Na+ wants to move in and K+ wants to move out)

4) membrane is more permeable to K+ ions so some K+ ion allowed to diffuse out of axon

Resting = -65mV

Nervous impulse

Self-propagating wave of electrical activity that travels along the atonal membrane (reversal of potential difference)

Process of action potential

1) stimulus causes voltage-gated Na+ channels in the axon membrane (at -55mv/ threshold)

2) causes an influx of Na+ down the electrochemical gradient causing depolarisation

3) neighbouring voltage-gated Na+ channels open ( more Na+ influx up to + 4o mv of depolarisation)

4) at +40mv, voltage-gated channels close and voltage gated K+ channels open

5) K+ ions diffuse out of axon, down their electrochemical gradient, depolarising the axon membrane

6) hyperpolarisation which causes axon to becom more negative than resting potential

How is size of a stimulus perceived?

• by the frequency of the action potentials generated

Absolute refractory period

NO more action potential can be generated regardless of how strong the stimulus (during depolarisation)

Relative refractory period

Possible for further action potentials but requires a stronger stimulus (durging hyperpolarisation)

Importance of refractory periods

• make sure action potentials are only propagated in one direction

• Produces discrete impulses (seperate action potentials)

• Limits the frequency of action potentials

How to increase the speed of nervous impulses?

• myelination insulates the axon, saltatory conduction between nodes of ranvier allows impulse to hop between nodes

• Larger axon diameter= faster speed of conductance

• Higher temperature increases diffusion of ions

Explain why. Ions can only be exchanged between nodes of ranvier

Because the remainder of the axon is covered by a myelin sheath that prevents ions being exchanged/ prevents a potential difference being set up

Neurotransmitters

Chemicals used to transmit information between neurones/ neurone and effector

E.g. acetylcholine

Transmission across an excitatory synapse

• action potential depolarises pre-synaptic bulb,opening voltage-gated Ca2+ channels

• Influx of Ca2+ ions triggers exocytosis of vesicles containing acetylcholine, which is released into the synaptic cleft

• Acetylcholine diffuses across the synaptic cleft and binds to receptors on post synaptic membrane

• ligand-gated Na+ ion channel open, allowing an influx of Na+ ions,

• Once threshold (-55mv) reached, neighbouring voltage-dated Na+ ion channels open. Which generates an action potential in the post synaptic membrane

• Acetylcholinesterase hydrolyses acetylcholine

• Products are sent back to the pre-synaptic bulb where they undergo endocytosis

• ATP resynthesises the products back into acetylcholine to be released again

Inhibitory synapse

Prevents depolarisation

E.g. Causes an influx of Cl- ions so threshold cannot be reached

How are synapses unidirectional?

• presynaptic neurone releases the neurotransmitter

• Receptors for the neurotransmitter are on the post-synaptic neurone

Spatial summation

• multiple pre-synaptic neurones release neurotransmitters into the synaptic cleft to one post-synaptic membrane, to reach threshold and generate an action potential

Temporal summation

One pre-synaptic neurone releases neurotransmitter at a higher frequency (enough to exceed threshold to generate action potential in post-synaptic neurone)

Why are their multiple neuromuscular junctions along the muscle?

To ensure the wave of contraction is stimulated across the full muscle (otherwise the fibres would not contract simultaneously and response would be slow)

Features of a neurone-neurone synapse

• inhibitory or exhibitors

• Links neurones to neurones to other effectors

• Can occur between sensory, intermediate and motor neurones

• New action potentials

• Multiple different neurotransmitters

Features of a neuromuscular junction

• only excitatory

• Links neurones to muscles

• Only occurs between motor neurones and the muscle

• End of action potential pathway

• Acetylcholine binds to receptors on membrane of muscle fibre

structure of skeletal muscle

• Made up of many fibres share a sarcoplasm and a nucleus

• Sarcoplasm contains mitochondria and sarcoplasm if reticulum

• Bibles are made up of myofibrils

• Myofibrils are made up of two myofilaments (actin and myosin

Structure of actin

• thin protein filamanets made up of two twisted strands

• Has a troponin complex attached to the protein filament (moved to expose binding sites on tropomyosin)

Structure of myosin

• thick protein filament that has two proteins heads which stick out of each molecule

• Multiple myosin molecules makes up the thick filament

Fast-twitch muscle fibres

Contract rapidly and powerfully for a short period

• have thick myosin filaments

• Lots of glycogen

  • Enzymes involved in glycolysis and phosphocreatine ( ADP to ATP in anaerobic conditions)

Slow twitch muscle fibres

Contracts more slowly and less powerfully but for a longer period of time

• lots of myoglobin, which stores o2 for aerobic respiration

• Lots of vasculature (delivers glucose and o2)

• Lots of mitochondria (aerobic respiration)

Sliding filament theory

1) Action potential from neuromuscular junction travels deep into muscle fibre through T-tubule system

2) Sarcoplasmic reticulum has actively transported Ca+ ions from the sarcoplasm, so con. Of Ca2+ in the sarcoplasm was low

3) Arrival of the action potential opens voltage-gated Ca2+ channels in the sarcoplasmic reticulum, so Ca2+ diffuses into sarcoplasm down their chemical gradient

4) Ca2+ ions bind to troponin and causes tropomyosin to move, exposing actin binding sites

5) ADP molecules attach to the myosin heads, allowing the heads to bind to the actin filaments and form cross-bridges

6) Myosin head change angle, pulling the actin filament along the myosin filament, ADP is released

7) ATP attaches to each myosin head, which detaches from the actin filament

8) ATPase hydrolyses ATP-> ADP. Provides energy to return myosin head to its original position

9) Myosin heads attach further along the actin filament via ADP and the process repeats

Relaxation of muscles

• Ca2+ ions actively transported back into the sarcoplasmic reticulum

• Allows tropomyosin to block the binding sites so myosin cannot bind

Atrial systole

Contraction of atria

Ventricular systole

Contraction of ventricles

Diastole

Relaxation of the heart

Sinoatrial node

• located in the wall of the right atrium

• Known as the pacemaker of the heart as sets the rhythm of the heart

• Ensure left and right atria contract simultaneously

Atrioventricular node

• located at the bottom of the right atrium

• Receives electrical impulses (signals) from the SA nods

• Delays the impulses to allow all the blood to be squeezed into the ventricles from the atria

Bundle of His

Carries impulses from the AV node down to the ventricles through the septum

Purkinje fibres

• smaller fibres that branch off from the bundles of his

• Carry the impulses to the lower walls of the ventricle

• Stimulate the contraction of the right and left ventricles to contract at the same time

Control of heart rate by chemoreceptors

• blood CO2 levels means blood pH is low

• Chemoreceptors in carotid body detect this and increase the frequency of impulses sent to the centre of the medulla oblong at a that increases heart rate

• Medulla increases frequency of impulses sent to the SA nods via the sympathetic nervous system, increasing heart rate

• Increased blood flow allows CO2 to diffuse out more at the lungs, so pH returns to normal

  (Vice versa for low CO2 levels)

Control of heart rate by baroreceptors

• high blood pressure detected by baroreceptors, which increases the frequency of impulses sent to centre of medulla oblong at a that decreases heart rate

• Medulla increases frequency of impulses sent to the SA node via the parasympathetic nervous system, decreasing heart rate

• Lead to reduction in blood pressure to normal

  (Vice versa for low blood pressure)

Where are alpha and beta cells located?

In islets of Langerhans in the endocrine pancreas

What do Alpha cell secrete, and what does it do?

Release glucagon

• increases blood glucose concentration

What doBeta cells secrete, and what does it do?

Secrete Insulin

• decreases Blood glucose concentration

How does insulin decrease blood glucose?

• insulin binds to its receptor Tyrosine Kinase on cell surface membrane

• Receptor phosphorylase’s proteins and causes downstream effects that result in:

  • GLUT-4 carrier protein that are normally kept in vesicles in the cytoplasm are inserted into membrane so more glucose can be taken up by facilitated diffusion

  • Activation of enzymes in glycogenesis

  • Inhibition of glycogensolysis and gluconeogenesis

How does glucagon increase blood glucose?

• Glycogen secreted, attaching to receptors on the surface of target cells

• When glucagon binds it causes a protein to be activated into adenylate cyclise and to convert ATP into Cyclic AMP (cAMP)

• CAMP activates an enzyme, protein kinase that can hydrolyse glycogen into glucose

• Activates enzymes involved in gluconeogenesis And glycogenolysis

• Inhibits enzymes involved in Glycogenesis

Type I Diabetes

• 5-10% of cases

• Autoimmune disease

• Complete lack of insulin

• Usually childhood onset

• Genetic factor (30-40% concordance in MZ twins)

  Treatment:

  • Insulin replacement (injection/insulin pump)

  • Correct dose is very important- must match glucose intake

Type II diabetes

• 85-95% of cases

• Caused by insulin resistance- beta cells still releasing it but it is havingg no effect

  • leads to overcompensation by pancreas leading to beta cell failure

• Strongly associated to obesity (80% of cases)

• Interestingly, higher genetic factor (60-90% in MZ twins)

  Treatment:

  • Regulate carb in take and match to ammount of exercise

  • Drugs that stimulate insulin secretion or slow rate of glucose absorption in gut

Functional unit of the kidney

Nephron

Structure of the nephron

• cortex (upper level)

  • Afferent arterioles

  • Efferent arteriole

  • Glomerelus

  • Bowmans’s capsule

  • Proximal convoluted tube

  • Distal convoluted tube

  • Collecting duct

• Medulla (lower, salty level)

  • Ascending loop of Henle

  • Descending loop of Henle

  • Efferent arteriole

  • Collecting duct

Ultrafiltration in glomerulus

• afferent arteriole is wider then efferent arteriole

• Creates hydrostatic pressure in glomerulus to force small molecules out into Bowman’s capsule against gradient

• Endothelium of glomerular capillaries has small spaces between cells, podocytes have foot processes with small slits between them

  • Allow substances to be filtered out into Bowman’s Capsule

Function of Proximal convoluted Tubule

• epithelial cells that line the tubule have

  • Microvilli- large surface area

  • Lots of mitochondria- ATP for active transport

• Selective reabsorption of: water, glucose, amino acids, Na+, K+, Cl-, HCO3

  • Na+ ions actively transported out of epithelial cells of tubule into the blood

  • Na+ ions diffuse down concentration gradient from tubule lumen into epithelial cells through special carrier proteins

  • Bring other molecules/ions with them

  • Water moves through aquaporins by itself

Function loop of Henle

Responsible for counter-current multiplication

• Descending loop of Henle

  • Thin

  • Very permeable to water- lots of water moves out of tubule back into blood via osmosis

• Ascending loop of Henle (thick)

  • Impermeable to water

  • Na+ ions are actively transported out of Ascending limb

  • This lowers water potential in interstitial space of the medulla- hence water moves out of descending limb by osmosis

  • Means water will move out of collecting duct into interstitial space

Function of Distal convoluted tubule

• starts to make final adjustments to filtrate

  • Na+ ions and Cl- ions reabsorbed

Function of Collecting duct

• final adjustments made

• Water reabsorbed from collecting duct (moves out via osmosis due to Na+ ions in the interstitial space

  • Reabsorption depends on action of ADH

Hormones and Osmoregulation

• osmoreceptors in Hypothalamus detect changes in blood water potential levels

  • When level is low, water leaves the receptor cell by osmosis, causing them to shrink, which is detected b the hypothalamus- secretes hormones

What does the hypothalamus secrete in response to low water levels?

• anti-diuretic hormone (ADH)

• ADH is made in the hypothalamus and taken to the pituitary gland, where it is released into blood

• ADH travels in the blood to the Kidney

Action of ADH in the Kidney

• binds to a receptor on epithelial cells in the collecting ducts (and distal convoluted tubule)

  • Leads to increased transcription/translation of aquaporins (water channel proteins) and insertion into ell membrane

  • Increases permeability to water

  • More water reabsorbed into blood

    • Also causes upregulation of urea transporters

    • Increases permeability to urea

    • Further lowers water potential around the duct in interstitial space

    • More water leaves the collecting duct and is reabsorbed Ito the blood

Homeostasis

Maintenance of constant internal conditions in spite of internal or external changes

• Temperature, water, glucose and pH important

Negative feedback

Reversing a change in order to return the condition to the optimum

• stimulus detected by receptor delivered to co-ordination which sends as signal to an effector to produce a response (e.g. lowering or raising blood pressure)

Positive feedback

Deviation from the optimum which causes changes resulting in an even greater deviation from th norm.

• This is usually harmful due to the large, unstable change in the body