A2 Biology Notes

8/12/2019 A2 Biology Notes

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Note: Edexcel Unit 4 and 5 encompasses material found in both Unit 1 and Unit 2 (synoptic). This pack does not cover the

material found in the latter two units and so should be used in conjunction with the packs that were designed for those

units. Do not forget that a scientific article is published every year which you will be required to read and answer questions

on in the exam.

Difference in muscle fibres 3.7.2

Slow-twitch Fast-twitch

Colour Dark red/ brown Pale white

Myoglobin More Less

Mitochondria More Less

Capillaries More Less

Kerb cycle enzyme

content

High Low

Glycogen content Low High

Resistance to fatigue High Low

Type of respiration

involved in

Aerobic Anaerobic

Creatine phosphate Low High

Sarcoplasmic

reticulum

Little Extensive

Muscle is made up of myofibrils lying parallel to each other.

Each myofibril is made up of sarcomeres.

Actin and myosin are proteins that make up a large part of the sarcomeres.

The cytoplasm of myofibrils is called the sarcoplasm.

There are a lot of mitochondria to provide energy.

The sarcoplasmic reticulum is a network of membranes that stores and releases calcium

ions.


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Slow muscle fibres in athletes

Endurance athletes exercise for long periods of time;

Respire / release energy aerobically;

Or too much lactate would accumulate;

Slow twitch fibres adapted to aerobic metabolism;

As have many mitochondria;

Site of Krebs cycle;

And electron transport chain;

Much ATP formed;

Also are resistant to fatigue;

Short-distance athletes (fast muscle)

ref to fast twitch muscle fibres;

ref creatine phosphate / eq in these fibres;

ref to storage of ATP;

idea that energy obtained from ATP (from creatine phosphate);

idea that athlete would need to respire anaerobically;

ref to glycolysis producing atp rapidly;

idea that glycolysis occurs in the cytoplasm;

idea of some aerobic respiration because of oxygen present;

description of lactate fermentation:

(-Pyruvate obtained from glycolysis is reduced to lactate)

(-This regenerates NAD by oxidising the reduced NAD formed in glycolysis)

(-The NAD can then be re-used so glycolysis can continue and form ATP)

The contraction of muscle with its components3.7.3

Function of Calcium ions

reference to {calcium ions / eq} released by sarcoplasmic reticulum ;

activate myosin / cause myosin to hydrolyse ATP ;

bind to / cause change in shape of troponin ;

cause tropomyosin to detach from actin filament / expose binding site (in actin) ;

Function of actin

reference to shape producing myosin binding sites ;

myosin globular heads fit / eq ;

Function of myosin

reference to globular heads (on the polypeptide chains) ;

reference to ATP on these heads ;

reference to phosphates ;

idea that head may be able to act as ATPase enzyme ;


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Troponin

attached to tropomyosin ;

ref to 3 sub units ;

ref to binding properties;

(one binds to actin, one to tropomyosin, one to calcium ions)

Tropomyosin

Long chain / eq ;

covers the myosin binding sites ;

on actin ;

Muscle contraction*

Calcium ions bind to troponin;

Remove blocking action of tropomyosin / exposes actin binding sites;

ATP allows myosin to join / bind to actin / form cross-bridge;

allows sliding of the actin & myosin filaments;

Re-cocks myosin cross bridge / allows detachment from actin (due to binding of ATP);

Enables calcium ions to be pumped back in;

hydrolysis of ATP releases energy / eq;

which changes configuration of myosin head / eq;

Phosphocreatine allows regeneration of ATP without respiration;

Phosphocreatine releases Pi to join ADP;

Sliding filament hypothesis tropomyosin covers binding site on actin;

calcium ions remove tropomyosin;

myosin binding sites exposed on actin;

binding / calcium ions increase ATPase activity;

breaks down ATP yielding energy;

used to re-cock myosin head;

ATP attachment cause actomyosin head to break /ATP required for separation of actin and

myosin/cross bridges;

#2

Myosin & actin filaments slide over one another;

idea that sarcomere contracts;

ref to simultaneous contraction;

idea that myofibrils / muscle fibres contract;

idea that sarcomere returns to original length at relaxation;

A-bands stay the same length, I-band gets shorter, H-zones get shorter, sarcomere overall

gets shorter (at contraction)


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Structure of a sarcomere

A / dark band is mainly due to myosin (thick) filaments;

H zone only myosin filaments;

darker band has both types of filament;

light band has only actin (thin) filaments;

Visual representation of muscle sarcomere

Muscle movement3.7.4

Antagonistic muscle action

motor area of brain involved;

impulses from brain;

idea of antagonistic muscles;

One muscle contracts; (accept correct reference to extensor muscles)

reference to sliding filaments in correct context;

Other muscle relaxes; (accept correct reference to flexor muscles)

action of lever explained in terms of muscle, joint and bone

Tendons

Attach muscles to bones

Ligaments

(connective tissue that) attaches bones to bones

Joints

ref to contraction/relaxation of muscles to move bones


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Skeleton It is made up of bone. It is strong and hard. Made up of bone cells embedded in a matrix

of collagen and calcium salts. It is strong under compression but not dense. This is to reduce the

weight moved about

Tendon vs ligaments

tendons join muscle to {bone / another muscle} whereas ligaments join bone to bone ;

at the joint, tendons move bone whereas ligaments restrict movement of bones / eq ;

tendons are less elastic than ligaments / eq ;

Inelasticity of tendons

(tendons) attach muscle to bone / eq ;

do not stretch when muscle contracts / all force transferred to bone / eq ;

so { bone / skeleton } is moved / eq

Why muscles need to be antagonistic

idea that muscles cannot extend themselves ;

need opposing muscle to extend / eq ;

antagonistic muscle allows control (of movement) / eq

Respiration reaction3.7.5

C6H12O6 + 6O2 6CO2 +6H2O + ATP

Glucose + oxygen carbon dioxide + water + energy

Phosphorylation of ATP3.7.6

ref to energy from glucose;

ref to ADP + the inorganic phosphate / eq ;

energy (from glucose) stored phosphate bonds ;

ref to ATP synthase ;

idea that ATP diffuses to cells needing energy;

ref to hydrolysis breaking down ATP;

energy released from bonds;

ref to ATPase;

Glycolysis3.7.7

Phosphorylation of {substrate / glucose} / eq ;

To activate the glucose / make the glucose reactive ;

ATP then phosphorylates glucose;

Formation of fructose {biphosphate / diphosphate / 6C sugar} ;

(phosphorylated sugar) split into two {three-carbon sugar phosphates / triose phosphates /

GALP} ;

(then) converted to pyruvate by {oxidation / dehydrogenase / dehydrogenation / removal of

hydrogen} ;

Produces reduced NAD ; (net) 2 ATP (formed for each sugar phosphate) ;


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Occurs in the cytoplasm;

Reduced NAD goes to oxidative phosphorylation;*

Pyruvate goes to link reaction then the krebs cycle in oxygen;

Pyruvate undergoes lactate fermentation without oxygen;

*

1. Transfers electrons / H+ / hydrogen (ions), to electron transfer

chain / FAD ;

2. Reference to inner membrane of mitochondrion / cristae ;

3. Oxidative phosphorylation ;

4. Generation of ATP (from ADP) ;

Krebs cycle 3.7.9 (simple mark scheme)

reference to involvement of 6C, (5C) and 4C compounds ;

CO2 formed / reference to decarboxylation / eq ;

ATP formed / eq ;

reference to dehydrogenation ;

reduced {NAD / FAD / coenzyme} produced ;

reference to substrate level phosphorylation ;

idea of 4C compound used to regenerate 6C compound (which allows cycle to continue) ;

CGP mark scheme

Acetyl coA (from link reaction) combines with oxaloacetate to form citrate

Coenzyme returns to link reaction for re-use

6C citrate converted to 5C Decarboxylation and dehydrogenation occurs

Hydro used to produce NAD from FAD

5C molecule converted to 4C

Decarboxylation and dehydrogenation occur to produce 1 FAD and 2 NADH

ATP made when phosphate directly transferred to the constituent ADP (substrate-level

phosphorylation)

The citrate is then converted to oxaloacetate (regeneration of 6C from 4C)

Link reaction

pyruvate {decarboxylated / eq} ;

hydrogen joins (from pyruvate) to NAD to reduce it ;

pyruvateacetate ;

acetate {combines / joins with / eq} coenzyme A;

ref to formation of acetyl coenzyme A;

ref to products (two acetyl coenzyme A, two CO2 molecules, two reduced NAD);

no ATP made ;


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Oxidative phosphorylation3.7.10

(H+ ions) from reduced NAD / eq ;

H+ ions pumped into inter membrane space / eq ;

reference to energy needed (for pump) (from electron carrierss) / eq ;

reference to movement of electrons along ETC /eq;

(ETC on) inner membrane / cristae;

H+ ions follow diffusion gradient / eq ;

ref to ATP synthase;

idea that this causes an energy change or makes energy available ;

ATP is formed when inorganic phosphate joins ADP/ eq ;

idea that this occurs on stalked particles ;

ref to chemiosmosis (movement of H+ ions across a membrane to generate ATP) ;

Electron transport chain electrons/hydrogen combine with NADH or FADH // hydrogen atoms/electrons released by

NADH/FADH as theyre oxidised to form NAD/FAD

electrons passed through series of carriers;

idea of oxygen being final acceptor;

H+ / protons passed into intermembrane space;

H+ / protons flow back through stalked particles/enzyme;

energy released in transfer;

energyused to make ATP;

from ADP and inorganic phosphate/ using ATPase

Chemiosmosis

NADH and FADH2 contain stored chemical energy

Energy is used to pump H+ ions into the mitochondrial membrane, against a concentration

gradient

The inner membrane is impermeable to protons creating a concentration, electrochemical

and pH gradient

H+ leaves the envelope through ATPase proteins. The energy generated is used tophosphorylate ATP


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Why oxygen is needed in ETC/Oxidative phosphorylation

ATP formed as electrons pass along transport chain;

oxygen is terminal electron acceptor / accepts electrons from electron transport chain;

electrons cannot be passed along electron transport chain if no O2 to accept them;

forms H2O / accepts H+ from reduced NAD/FAD / oxidises reduced NAD/FAD;

How mitochondria supply energy for muscles

the final product of glycolysis enters mitochondrion

ATP is the energy source (for muscle contraction) / eq ;

correct ref to role of {mitochondrial membranes / eq} ;

ref to ATP produced in {(mitochondrial) matrix / Krebs cycle} ;

{NADH / reduced NAD/ FADH/reduced FAD} transfers {H+ / electrons} to the ETC

hydrogen splits into H+ and electrons ;

correct ref to {proton gradient / chemiosmosis} ;

electrons are passed from one carrier to the next from higher to lower energy levels / eq ;

ATP produced from ADP and Pi / ref oxidative phosphorylation ;

mitochondria only work in aerobic conditions

Fate of lactate3.7.11

reference to lactate in the blood / eq ;

{transported to / broken down in / eq} liver ;

lactate is {converted to pyruvate / eq} ;

this involves {oxidation / production of reduced NAD / eq} ; pyruvate is then {oxidised / eq} ;

reference to Krebs cycle ;

{this requires extra oxygen / reference oxygen debt} / eq ;

idea that carbon dioxide and water are produced ;

Heart3.7.11

Muscle contraction

reference to {Sinoatrial node / SAN} ;

initiates heart beat & depolarisation / eq ;

passes through (wall of) atria / eq ;

causes atrial {systole / eq} ;

impulse sent to AVN & conducts to ventricles / eq ;

AVN allows atria to fully allow blood to be emptied;

Impulse sent through {Purkyne fibres / bundle of His} ;

ventricular {systole / eq} follows (from apex) / eq ;

atrioventricular valves closed (and prevent flow to atria) ;

semilunar valves opened by pressure / eq ;

blood forced into arteries / eq ;

changed pressure in {diastole / eq} closes semilunar valves ;


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Myogenic

idea that stimulation generated from within (muscle) e.g. no external stimulation ;

idea of brings about depolarisation ;

ECG

The peak at p represents an atrial systole. A wave of depolarisation has been sent from theSAN causing the atria to contract.

The time between P and the QRS complex is known as the PR interval. During this time, the

impulse is at the atrioventricular node. This delay allows for atria to completely empty blood

into the ventricles

The electrical impulse travels from the bundle of his to the Purkyne tissue. The Purkyne

tissue passes the impulse to ventricles, and the ventricles contract base up. This is known as

the ventricular systole. The QRS complex represents the ventricular systole.

The T wave represents the rapid repolarisation of the Purkyne tissue in the ventricles. This is

also known as diastole.

How ECGs can aid the diagnosis of CVDs and other heart conditions

An ECG is used to investigate the rhythms of the heart by producing a record of the electrical

activity of the heart. The depolarisation in the heart causes tiny little electrical changes on

the surface of the skin. Electrodes attached to the skin measure these changes.

Usually, when an ECG is taken, the patient is lying down. However, some heart conditionsare only shown when the patient is exercising. Therefore, during a stress test, the patient is

exercising.

Tachycardia

Heart rates beats too fast

Heart cant pump blood efficiently

AVN problems

The ventricles will not contract (although the atria will)

Impulses do not travel from atria to ventricles


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Fibrillation

Irregular heart beat

Contraction of atria and ventricles not correct

Ventilation etc.3.7.12

Tidal VolumeThe volume of air that enters and leaves the lings at each natural resting breath

Ventilation RateTidal volume X frequency of inspirationIt is a measure of the volume of air

breathed in per minute

Calculating mean tidal volume and mean breathing rate from graph

For tidal volume, take the peak of a wave and subtract the bottom from it (so for points marked

above, 4.7-2.5 = 2.2). For the breathing rate, take a specified amount. For example, in questions they

may ask to find out the mean breathing rate during 30 secondscount the number of breaths (so 10

or 10.5). Divide that by 30 and multiply by 60 to get the breathing rate (20 or 21). Else, count the

entire breaths given in the diagram for 60 secs.


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Physiological effects of exercise

faster and deeper breathing, which increases the rate at which oxygen enters the blood in

the lungs and carbon dioxide leaves it

faster and stronger heart beat, which increases the rate at which blood moves through the

blood vessels, delivering oxygen to muscle tissues and removing carbon dioxide and lactate

from them

The control of heart rate

Adrenaline causes SAN to increase its rate of contraction

Action potential sent across motor neurone (part of sympathetic nervous system) from the

cardiovascular centre to the brain (medulla) to the SAN

The neurone releases neurotransmitter (norandrenaline) which further raises SAN

contraction rate

Reasoning: CO2 concentration in blood rises; this decreases pH; the low pH stimulates

cardiac centre to generate higher frequency of action potentials along sympathetic nerve

Further information

The cardiovascular centre in the medulla controls the heart rate.

Chemical and stretch receptors in the lining of blood vessels and chambers of the heart sendimpulses to the cardiovascular centre

Nervous control of the heart is autonomic and is divided into the sympathetic and

parasympathetic nervous systems. Sympathetic is excitatory and parasympathetic is

inhibitory.

Impulses travelling down the sympathetic nerve increase the frequency of impulses from theSAN. The parasympathetic does the opposite.

Heart response to low oxygen content in muscles

Blood vessels in muscles secret nitric oxide

Arteriole walls relax and dilated so will carry more blood

Blood returns to right atrium faster

Muscle in atrial walls stretch

So heart contracts with more force and SAN contracts faster

The control of ventilation rate

Nerve impulses sent from ventilation centre to medulla oblongata in brain to muscles in

diaphragm and intercostal muscles

They contract rhythmically

Stretch receptors in lungs (stimulated in inhalation) send impulses to ventilation centre

This information is used to regulate breathing rate

Chemoreceptors in ventilation centre and aortic body sense pH fall, which is caused by more

CO2 in the blood

Another set of chemoreceptors in the walls of the carotid arteries, called carotid bodies,sense oxygen concentration, as well as carbon dioxide concentration, in the blood


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Nerve impulses are sent from the aortic body and carotid bodies to the ventilation centre,

and this then sends impulses to the breathing muscles, causing them to contract harder and

faster.

This increases the rate and depth of breathing.

Breathing rate rising (mark scheme)

concentration of carbon dioxide in the {alveoli / lungs / eq} is higher / eq ;

concentration of carbon dioxide in the blood is higher / eq ;

idea of pH of blood falls (due to increased CO2)) ;

(CO2 / pH levels in blood) detected by chemoreceptors ;

in {carotid body / carotid artery / aortic body / aorta} ;

reference to {ventilation / respiratory} centre ;

idea of control is in medulla ;

sends {more / eq} impulses along {neurones / nerves} ;

to intercostals muscles / diaphragm ;

Negative feedback -3.7.15

A negative feedback system is a system enabling the body to maintain a condition within a narrow

range. For example if one factor goes up, system instigates change to bring it back down again.

Change in normal level of a factor

Receptor detects change

Receptor sends a communication hormonally or via the nervous syste,

The effector carries out a response to bring about corrective change Return to normal level of factor

Negative Feedback system in the heart -1

Baroreceptors are sensitive to pressure. They detect changes in pressure in the carotid

arteries.

An increase in blood pressure in the arteries stretches the baroreceptors

Baroreceptors send impulse to the cardiovascular centre

Cardiovascular centre sends impulses through the parasympathetic nerves to slow down theheart rate and widens the blood vessels, thereby lowering the blood pressure

Negative feedback system in the heart -2

During exercise, adrenaline dilates the blood vessels causing the blood pressure to fall

The baroreceptors stop sending impulses

The cardiovascular centre detects this and sends impulses down the sympathetic nerve to

stimulate the heart and increase blood pressure.

Negative feedback system in the lungs

Chemoreceptors detect fall in pH of the blood

Impulse sent to respiratory centre

Respiratory centre sends impulse sent to diaphragm and intercostal muscles to contractharder and more rapidly to increase breathing rate


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Homeostasis etc.3.7.16

Homeostasis

The maintaining of a constant internal environment

Importance of homeostasis

temperature / pH; optimum for enzymes / effect of pH /

temperature on enzyme activity;

OR

water potential / blood glucose;

effect of osmotic / blood glucose imbalance on cells;

How body returns to normal temperature if too high

thermoreceptors in hypothalamus / eq ;

detect the increase in (core) blood temperature /eq ;

reference to heat loss centre activated ;

reference to autonomic nervous system ;

reference to impulses down motor neurones ;

to {effectors / named effector} / eq ;

detail of method of heat loss / eq ;*

*vasodilation of blood vessels, sweat released, heat loss from blood

through radiation

If too low

hypothalamus (contains the thermoregulatory centre);

has receptors which detect temperature changes of blood;

receives impulses from receptors in skin;

nerve impulses transmitted (from hypothalamus / brain);

ref to heat gain centre activated ;

results in vasoconstriction / constriction of arterioles / dilation of shunt vessels;

diversion of blood to core / specified organ / less blood to skin;

muscular contraction /shivering generates heat via respiration;

release of thyroxine / adrenaline;

increase in metabolic rate / respiration;

correct reference to negative feedback mechanisms


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The transcription factors

Thyroxine affects protein synthesis in a cell by binding to transcription factors in the nucleus

of a cell. The activated transcription factors bind to specific regions of DNA (genes), and

either increase or decrease the ability of RNA polymerase to attach to the DNA and catalyse

the production of a complementary strand of mRNA from that gene.

This may increase the transcription of a particular gene, called up-regulating, or it may

decrease transcription, called down-regulating. Most steroid hormones, such as oestrogen

and testosterone, act in this way. Transcription factors may bind with a large number of different areas of DNA, so they can

switch many different genes on or off. Thyroxine, for example, is known to affect the

expression of at least 20 genes.

Anabolic steroids

Steroid hormone enters the cell and binds to receptor

Hormone receptor complex enters nucleus and acts as a transcription factor, switching a

gene linked to enzyme synthesis on or off

Synthetic forms of testosterone stimulate more protein synthesis, this leads to more muscle

being made


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Disadvantages of using erythropoietin

Leads to serious health problems and death

Excess red blood cells thicken the blood and can lead to stroke and heart attacks.

Ethical issues

Agree:

can no longer compare weight-lifters/competition is no longer fair;

illegal;

uninformed decision taking;

possibility of death;

health risks;

Disagree:

individual has right to make own decision re health risks;

drug free sport is not fair anyway; due to differences in training resources;

pressures from coach / sponsors / public;

financial rewards;


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Plants and photoreceptors4.4.2

Phytochromes are proteins that are sensitive to light with wavelengths between 600 nm and 800 nm

that is, red light. They exist in two forms, which are changed from one form to the other when

they absorb light. Phytochrome red, Pr, absorbs red light (wavelength about 660 nm) and is changed

to phytochrome far-red, Pfr, which absorbs far-red light (wavelength about 730 nm). This changes it

back to Pr.

Location

Leaves / seeds/ roots/ stems/ petioles/ flowers/ fruits/ shoots/ coleoptiles / buds ;

Conversion

(in dark) slow conversion in darkness (PFR) to PR ;

(Infar red light) fast conversion (PFR) to PR;

(in red light)fast conversion PFR to PR

Season/Weather

Daylight: More red light than far-red so more PRPFR than PFRPR

Many plants require PFR to germinate

Season doesnt matter

Short-day / long-day plants

Short-day plants need a lot of Pr in their tissues in order to flower

only happens when they have long, uninterrupted nights.

If white or red light is shone on them even very briefly during the night, they will not flower,

because this converts the Pr to Pfr.

Long-day plants need a lot of Pfr in their tissues in order to flower, which only happens when

they have short nights not long enough for all the Pfr to be converted to Pr.

Importance of plants adapting to changes in day length

idea of {flowering / development /eq} happens at the right time ;

therefore flowers when insects available / leaf fall in autumn / same species flower at the

same time / seeds germinate at the right time / eq ;

idea that day length changes to a set pattern e.g. always {short days in winter / long days in

summer} ;

comparison with other less regular stimuli e.g. temperature ;


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Neurones etc.4.4.3

Sensory vs relay vs motor neurone

sensory neurone {stimulated / eq} by {receptor / sense organ / eq} ;

sensory neurones carries {impulses / eq} to {relay / motor / CNS / eq} neurone / {relay /

motor / eq} neurones receive impulse from sensory neurone ;

relay neurone {synapses / connects / eq} with sensory and {motor neurones / lots of

neurones} /

idea of allowing coordination of reflex ;

motor neurone {carries impulse / stimulates / eq} {effector / muscle / gland / eq}

Longer, more detailed mark scheme of the above

Sensory

1. transmits {impulses / action potentials} from sense organ to CNS / eq ;

2. pseudo-unipolar cells / cell body in centre of {cell / axon} / single dendrite ;3. myelinated ;

Relay

4. correct reference to (relay neurone transmitting impulses) {between sensory and motor neurone /

to other neurones} ;

5. short axons ;

6. no myelination / eq ;

Effector (motor)

7. transmits {impulses / action potentials} from CNS to {effector / named effector} / eq ;

8. multipolar cells / short dendrites / many dendrites from cell body / cell body at end of cell ;

9. long axon ;

10. myelinated ;

General

11. reference to {Schwann cells / nodes of Ranvier} ;

12. reference to myelin causing faster impulse / eq ;

13. reference to synapses (between neurones) ;

14. reference to secretion of {neurotransmitter / namedneurotransmitter} ;

15. credit structural detail of synapse e.g. mitochondria in presynaptic knob / receptor molecules on

postsynaptic membrane / sodium channels in postsynaptic membrane ;


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Structure of myelin sheath

Schwann cells ;

Wrap / fold / roll, around axon ;

Fatty / lipid / phospholipid, nature ;

Reference to nodes as, gaps in the sheath, / naked /exposed / uncovered, axon ;

Myelination

1.{neurone (cell) surface membrane exposed / no myelination / eq} at nodes of Ranvier ;

2. Nodes are the site of clusters of {sodium-gated channel proteins / potassium channels} ;

3. Which {open / close} when impulse arrives / eq ;

4. Allowing depolarisation at nodes / eq ; 5. idea that myelin/eq acts as an (electrical) insulator (on neurone surface between nodes) ;

6. reference to Schwann cell ;

7. idea that impulse/depolarisation jumps to next node ;

8. Reference to this being saltatory conduction ;

9. idea that this happens between the myelin layers of the Schwann cell ;

Action potentials etc.4.4.4

Events:

Before action potential

Stimulus

Sodium channels open making membrane more permeable

Sodium ions diffuse into neurone down electrochemical gradient

Inside of neurone less negative

Potential difference is -70mV

Depolarisation

Potential difference reaches threshold (around -55Mv)

More sodium channels open

More sodium ions diffuse


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Repolarisation

Potential difference reaches +30Mv

The sodium channels close

Potassium ions channels open

Mebrane becomes more permeable to potassium

More potassium ions diffuse out of neurone down the electrochemical gradient Membrane approaches resting potential

Hyperpolarisation

Potassium channels slow to close

Overshoot / to many potassium ions diffuse out of neurone

Potential difference becomes more negative than resting potential

Resting potential

active transport/pumping of sodium (ions across membrane);

out of neurone/higher concentration outside;

differential permeability to K+ and Na+;

Refractory period

no (new) action potential/nerve impulse be produced in this time;

impulse/action potential can only move in one direction

channels are recovering / Na+ closed at repolarisation & K+ closed during hyperpolarisation;

Role of nodes of ranvier

Gap in the myelin sheath /eq ;

Enables {depolarisation/action potential/eq} (of axon) ;

Causes impulse to jump from node to node / saltatory conduction ;

Speeds up (transmission of )impulse ;

Synapses4.4.5


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Synapses and synaptic transmission

(impulse causes) calcium ions/Ca++ to enter axon;

vesicles move to/fuse with (presynaptic) membrane;

acetylcholine (released);

(acetylcholine) diffuses across synaptic cleft/synapse;

binds with receptors on (postsynaptic) membrane; (reject active site disqualify point)

sodium ions/Na+ enter (postsynaptic) neurone;

depolarisation of (postsynaptic) membrane;

if above threshold nerve impulse/action potential produced;

A more advanced mark scheme

diffuses across {gap /eq} ;

binds to (receptors on) post-synaptic membrane / eq ;

idea of gated-channels opening or Na+ travels through post-synaptic membrane ;

causing a depolarisation / eq ; (if sufficient present) an action potential is set up in {post-synaptic membrane/adjacent cell /

eq} ;

details such as temporal or spatial summation ;

idea that allows coordination / one way flow of information ;

idea that it allows integration in post-synaptic cell ;

neurotransmitter broken down (by enzyme) / eq ;

so that do not get {prolonged /eq} action potential in post-

synaptic membrane / make receptors available again ;

credit reference to fate of products e.g. reabsorbed through pre-synaptic membrane OR to

be re-synthesised into neurotransmitter substance

Role in nervous system

When one neurone connect to many neurones, information can be dispersed to different areas in

the body (synaptic divergence)

When many connect to one neurone the information can be amplify (synaptic convergence)

Summation

When a stimulus is weak, only a small bit of neurotransmitter is released. This may not be enough to

stimulate an action potential.

Spatial summation occurs when excitatory potentials from many different presynaptic neurons cause

the postsynaptic neuron to reach its threshold and fire.

Temporal summation occurs when a single presynaptic neuron fires many times in succession,causing the postsynaptic neuron to reach its threshold and fire.

Rods, cones, etc.4.4.6

Rod cells

In the dark Na+ flows into the outer segment through specific cation channels

Na+ move into inner segment and the pumps continue to transport them back out of the cell

Influx of Na+ produces a slight depolarisation of cell (to-40mV) which triggers the release of

an NT from the rod cells

The NT binds to the bipolar cell and stops it depolarizing


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In the light rhodopsin absorbs light, changes shape & breaks down into retinal and opsin

Opsin causes a series of membrane bound reactions and ends in the hydrolysis of the

molecule attached to the cation channel in the outer segment.

This closes the cation(+ve) channel

The Na+ into the rod decreases and the inner segment continues to pump Na+ out The inside of the cell becomes hyper polarised (more -ve)

the release of the glutamate NT stops

The bipolar cell depolarises

The neurones of the optic nerve are depolarised and this sets up the AP

The cone cells give information in colour. They are found in the fovea, whereas cones are found in

the peripheral parts of the retina.

Rods and cones in nocturnal mammal vs human

more rods and no / fewer cones present;

rods at the fovea / rods not mainly at periphery; rods have high sensitivity / show retinal convergence /converse for cones;

rhodopsin bleached at low light intensities / iodopsin bleached;

at high light intensities

The contraction/dilation of pupils4.4.7

Antagonistic musclesthe pupil reflex

circular muscles contract (and radial muscles relax) to {constrict / eq} pupil ;

radial muscles contract (and circular muscles relax) to {dilate / eq} pupil ;

need for fine control of aperture to allow pupil to be reset to a different size / allow

changing

to take account of varying light intensity ;

(these) muscles can only shorten / eq ;

antagonistic muscles have opposite effects / eq ;

idea that contraction of one muscle set stretches the other ;

Pupil size / light

details of impulse e.g. depolarisation;

ref to light hitting photoreceptors / eq;

send impulses via bipolar cells;

to optic nerve;

ref to sensory neurone;

reference to {motor / eq} neurone connected to (radial) muscles ;

reference to contraction of radial muscle


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#2

(light hits) photoreceptors (on the retina);

impulses pass to the brain ;

ref. to sensory neurone ;

ref. to innate / inborn / autonomic response ;

impulses along parasympathetic nerve ;

ref to motor neurone ;

circular muscles contract / radial muscles relax ;

pupil {contracts /constricts / becomes smaller} ;

Dim Light

Light falling on cones causes impulses to travel along neurones

Dim light means the frequency of action potentials decrease

This is detected by a control centre in the midbrain Nerve impulses synapse with the sympathetic cranial nerve

The muscles in the iris are stimulated

The circular muscles relax and the radial muscles contract

Pupils are wider, increasing the amount of light entering the eye

This allows as much light as possible to fall on the rods and cones to maximise what you can see

Phototropism etc.4.4.8

Hormonal vs nervous response

hormonal coordination is {chemical / via blood} whereas nervous coordination is electrical /

along axons ;

hormonal slow whereas nervous fast / eq ;

hormonal longer term action whereas nervous short term / eq ;

idea that hormone action is widespread whereas nervous action is localised ;

Phototropism

IAA is made in the growing point at the tip of the shoot and then moves down the shoot.

When blue-light receptors pick up unidirectional light, they make the IAA accumulate on the shady

side.

The IAA switches on genes for expansins, which make cell walls stretchy so the cells can get longer.

This makes the cells on the shady side get longer than the cells on the light side, so the shoot bends

towards the light.


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Brain4.4.9


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The account of the location and functions of each of the following areas of the mammalian brain:

cerebral hemispheres, cerebellum and medulla.

1. Receives information from sensory organs / reference to sensory input ;

2. Idea of {interpretation / coordination} of information ;

3. {Initiates / transmits} impulses (to effector) ;

Cerebral hemispheres / cerebrum

4. At the front of the brain / frontal lobes / fore brain ;

5. Reference to voluntary action ;

6. Reference to a named sense ;

7. Reference to {thought / learning / intelligence / memory / personality / emotion

/ speech / language} ;

Cerebellum

8. At the back of the brain / hind brain ;

9. Idea of (controls) {skeletal / muscular} movement ;

10. Reference to {modification of movement / named example e.g. talking} ;

11. Balance / posture / muscle tone ;

Medulla

12. At top of spinal cord / hind brain / below pons ;

13. Reference to involuntary action ;

14. Involved in homeostatic control / correct named example ;

16. Reference to {simple reflex centre / named example} ;


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Brain scans4.4.10

fMRI

idea that fMRI can allow brain activity to be seen in real time ;

idea that fMRI uses {radio waves / magnetic field} ;

increase supply of oxygenated blood in active areas / eq ;

that {reflects/does not absorb} fMRI signals / eq ;

idea of seen as a white area

CT scan

Thousands of tiny X-ray beams are passed through the area to be observed

The strength of the beam is reduced as it passes through tissue. Denser tissue reduces the strength

of the beam more.

The X-rays that make it through are detected and measured The data is put into a computer to produce a cross-sectional image of a thin slice of the body

Occasionally, special dyes are injected into the blood to make areas X-ray opaque so they show up

more clearly

Positives: able to identify major structures in the brain; can identify bleeding in the brain

Downside: cantpick up fine structural details; images are frozen in time so cant detect changes in

brain with changing or other activities

Example: blood has lighter density from brain tissue; revelation of extent and location of bleeding;

work out damaged blood vessels and which affected

Magnetic Resonance Imaging (MRI) Uses magnetic fields and radio waves

Hydrogen atoms are imaged as they have a strong MRI signal

The signals produced are used to produce an image by a computer.

Different tissues respond differently to the magnetic field due to the amount of water in the

structure.

Thin slices are imaged to produce 2D images. However, these can be put together to form 3D

images

Positives: Reduces risk of damage from X-rays; produces detailed images; can be used to diagnoses

brain injuries, strokes, tumours and infections of the brain or spine; enables doctors and scientists to

make links between structures in the brains and patterns of behaviour seen in their patients

Downside: Doesnt show how the brain works; noisy so can be distressing for the patient


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4.4.11 to 4.4.13

Theres far too much information to put here and it essentially will be from the textbook. However,

here are a few mark schemes/basic notes.

How the study of animals has helped humans

all animals have common evolutionary origin / share genes / eq / OR human and mammal

brains are similar ;

{nervous system / neurones / synapses / neurotransmitters / eq} work in a similar way in allanimals ;

ref. to studies of {Aplysia / sea slug / chimpanzees (Kohler)} ;

details of {Aplysia / sea slug / chimpanzees (Kohler)} studies;

ref to limitations of animals in explaining learning in humans

Visual cortex Axons grow from the retina to the thalamus and synapse with the neurons in the thalamus

in a particular arrangement.

The axons from the thalamus then grow towards the visual cortex in the occipital lobe. They

synapse onto the column cells of the visual cortex.

The adjacent columns of cells receive stimulation from the same area of the retina in the left

and right eye and the pattern repeats across the visual cortex.

Crowley and Katz injected labelled tracers into ferrets. They found the visual cortex formsbefore the critical periodfor visual development. The column cells are also seen in new born

monkeys. Therefore the formation of the visual cortex must be genetically determined not

due to the environmental stimuli. There are postnatal periods when the NS needs specific

experiences to develop properly. These are critical windows or sensitive periods.

What happens during the critical window?

Visual stimulation is needed for the refinement of columns and the full development of the

visual cortex.

Each time a neurone fires into a target cell in the visual cortex, the synapses of the other

neurones sharing the target cell are weakened and release less NT If this competition

continues then only the synapses on axons receiving light stimulation will fire until

stimulations from the light deprived eye will be lost.

More neurones are produced than are required in the nervous system. In the retina 80% of

the original neurones are pruned and die.

There is a critical window for the development of the mature visual cortex

The eyes of new-born kittens and monkeys were sutured. The age at which the animals weredeprived of vision and the length of time the eye remained closed was varied

One eye of a kitten was sutured before its eyes opened. The open eye developed normally,

however, the kittens were blind in the closed eye; very few cells in the closed eye were firing

One eye of a four month old kitten that could see normally was sutured. There was no effecton the vision of the sutured eye

Closing the eye of a kitten for a short period during the critical period for visualdevelopment has as much effect as closing the eye from birth.

Similar results were achieved when monkeys were used.


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Human Genome Project

What is it?

To map (human) chromosomes / to find where each gene is located on (human)

chromosomes ;

To determine base sequence ;

International project (about human genes) ;

Benefits of identifying people in tests to determine diseases

To warn people at risk to take precautions / make life-style changes ;

To plan medical provision (for the individual) ;

To determine NHS priorities / eq ;

To warn people when there is a risk that if they have children they may have genetic

disorders ;

Will make it easier to develop {ways of treating genetic deficiencies / gene therapy}

Ethical issues

May mean people put under (undue) pressure to {have abortions / not have children} ;

May lead to discrimination over {jobs / insurance premiums} ;

May lead to (other ethically questionable) developments such as designer babies / eugenics

/ immigration ;

{Stress / anxiety}knowing something might happen may cause psychological stress even if

it never happens in your life-time / people may not believe test is reliable / people may notwant to know ;

{Civil rights / personal freedom}who should decide who should have genetic tests? / who

decides who deserves very expensive treatment on the NHS? ;

Data protection issueswho will have access to genetic information about individuals ; [this

marking point could be a development of marking point 2]

Serotonin etc.

Why L-dopa is used as a treatment

Dopamine cannot enter the brain/L-dopa can be converted to dopamine/L-dopa is{easier/cheaper} to make/converse

Parkinsons

ref to non-functioning of synapses;

idea that {substantia nigra/basal ganglion/mid brain} cells not active;

idea that these release dopamine;

idea that neruones release dopamine in {cerebellum/motor cortex/parietal lobe}

ref to loss of motor function / muscle movement / eq;


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Depression

The NT serotonin is secreted by cells in the brain stem, whose axons extend into the cortex,

cerebellum and spinal cord. This is a huge area of the brain.

A lack of serotonin leads to depression - feelings of sadness, anxiety, hopelessness, a loss of

interest in pleasurable activities, decreased energy levels, insomnia, thoughts of death.

The cause is multifactorial - several genes and environmental factors are involved eg stress.

The 5-HTT gene codes for a transporter protein that controls serotonin reuptake into the

presynaptic neurones. Those with the short version of 5-HTT are susceptible to depression if

an environment trigger occurs.

Other NTs may be involved and the lack of serotonin may be the cause OR the result of

depression. In depressed people there are fewer nerve impulses transmitted around the

brain. The pathways involved in in serotonin synthesis are often present in low

concentrations but the serotonin binding sites are more numerous than normal

Treating Depression

Selective Serotonin reuptake inhibitors (SRIs) inhibit the uptake of serotonin.This leaves

more serotonin in the synaptic cleft. Therefore, more impulses travel along the post synaptic

axon. This results in the relief of the symptoms

Tricylic antidepressants (TCAs) increase the levels of serotonin and noradrenalin in the brain.

Monoaminooxidase inhibitors inhibit the enzymes that break down serotonin

Developing new treatments for Parkinsons -- Gene Therapy

Results from the Human Genome Project help to develop gene therapies

Scientists are investigating inserting healthy genes into the affected cells

The two main approaches are: adding genes to prevent cells from dying; adding genes to

enhance dopamineproduction

The problems are that safety is of prime concern, so gene therapy like this is still years away

Habituation

Less calcium (ions) enter into neurone /fewer calcium channels open

Fewer neurotransmitter vesicles {move to/fuse with}

(presynaptic/neurone 1) membrane/less neurotransmitter released (into synaptic cleft)

Less neurotransmitter diffuses (across the synaptic cleft)

Less neurotransmitter binds to {receptor/sodium channel} on (postsynaptic/neurone 2)

membrane

Fewer sodium (ion) channels open/less sodium (ions) enter

Less membrane depolarisation/{fewer/no} action potentials initiated (in neurone 2)


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Animal testing

For

Clinical trials stage 1 involves animals. Without animals we would be unable to discover new

drugs

Animal testing is better than nothing and does, in some cases avert potential loss of human

life

Animal testing is for the greater good

Machines like MRI were tested using animals Animal testing has advanced our understanding of human physiology

Against

Computer simulations can be used in clinical trials

Animal physiology is different to human physiology. Therefore animal testing is unhelpful

Animals have rights too

Animals have no informed consent

Testing on animals when potential side effects are unknown is immoral

Animals cant tell you if they are suffering Animals are often treated poorly in labs


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Genetic modification

How to modify bacteria

{Restriction enzyme / endonuclease} to cut out (cellulose) {gene / section of DNA} (from

fungus) ;

Obtain plasmids from bacteria ;

Open plasmid using {same restriction enzyme / endonuclease} ;

Insert (cellulose) gene into plasmid ;

Reference to sticky ends ;

(Ends joined by) ligase ;

Insert plasmid into bacteria ;

Reference to method such as {electroporation / treatment with calcium chloride / electrical

shocking / salt shocking} ;

How to modify plants

1. Idea: transferring {gene / DNA} from another species to the rape plant ;

2. {identify / find / isolate} gene in the donor plant ;

3. Reference to cutting DNA / restriction enzymes ;

4. Splicing DNA / reference to sticky ends ;

5. insert {gene / DNA} into plasmid / ligase enzymes ;

6. Plasmid is a circle of DNA from bacteria ;

7. {The bacteria / Agrobacterium / a vector} is used to transfer the {gene /DNA} into the (rape) cells ;

8. Correct reference to antibiotic resistant marker gene ;

9. Correct reference to use if protoplasts allowing easier entry of vector

Benefits and risks of GM crops

positive:

less crops lost to insect damage/ diseases spread by insects;

can spray herbicide with no loss to crop/reduce competition from weeds;

more saleable product;

less use of insecticide;

possibly cheaper food; max 3

negative:

gene transfer to non-crop species;

consumer resistance to un-natural products; transfer of genes into food chains/effect of food chains/examples;

creation of plague weeds/uneconomic plants;

excessive use of herbicides;

Reject disadvantages of selective breeding

How to identify the resistant gene

expose cells to the fungus/named example;

non-resistant ones die, resistant ones survive;

OR identify by adding marker gene/gene probe/(qualified) marker probe; description of positive result e.g. radioactivity/fluorescence / complementary base pairing;


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How to modify animal cells

The gene for protein is injected into the nucleus of a fertilised animal egg cell

Egg cell implanted into an adult animal cellsit grows into a whole animal

So these contains a copy of the gene in every cells

The protein made from the gene is purified from milk of animal

Ways genes are inserted into host cell

MicroinjectionDNA injected into cell with a fine micropipette

MicroprojectileDNA is shot into the cell at high speed carried on a minute gold, tungsten pellet

VirusInfects the cells with the desired gene

Liposome wrappinggene is wrapped in liposomes which fuse to membranes and

Ethical issues of GM crops

An issue about antibiotic resistance ;

Antibiotic markers could lead to antibiotic resistance in (human) {pathogens /microbes / bacteria} ;

May cause allergy;

Possible example of such an allergy / attempt to describe what is meant by an allergic response ;

Could make crop harmful to consumers ;

Reference to presence of possible toxic substances ;

May {affect / spread to} {other (non-GM) crops wild plants} ;

Pollen may spread to {non-GM / organic farms} / may transfer herbicide resistance to other

{weeds / wild plants} ;

Loss of genetic diversity / increased reliance on fewer and fewer strains ;

(Worlds human food supply) vulnerable to {climate / environmental} change/newly emerged

{disease / pathogen} of the crop plants ;

International biotech companies cannot be trusted ;

Could make developing world / the world in general too dependent on a small number of

suppliers / issues too complex for public to understand therefore easily misled ;

Inadequate testing of GM products ;

Until the recent concern many GM products were released onto the market untested / adequate

international protocols for testing do not exist ;

GM may have unforeseen {environmental I medical} consequences ;

Reference to the precautionary principle / (the risk may be small) but the dangers could be too

catastrophic to take the risk / when we find a problem it could be too late ;

{Ethical / religious} objections to tampering with nature / GM difficult to control / current research

might be harmless but where will it all lead? ;

Economic factors may force people to adopt GM to compete (even if they are opposed to it) / (in the

wrong hands) risks of promoting {biological warfare / eugenetics / designer babies}


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Experiment-based questions

Spirometer idea of calibration for volume ;

idea of calibration for time ;

description of how to calculate tidal volume (from trace) / eq ;

idea that one peak = one breath ;

reference to breathing rate is number of peaks per minute ;

idea of standardised group of males and females e.g. same age, non-smokers ;

idea that traces taken at rest ;

reference to replicates ;

description of how to calculate the mean from the trace;

Rate of respiration

ref to {soda lime / potassium hydroxide} to absorb {CO2 / carbon dioxide} ;

ref to control and reasoning e.g. results only due to {woodlice / named organism} respiring ;

named example of control e.g. {beads / stones / eq} which have same mass ;

syringe set the fluid in the manometer to known level / eq ;

idea of time e.g. 30 minutes ;

ref to volume of air decreasing / woodlice/named organism consuming oxygen ;


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idea that pressure reduced in in tube;

idea that the pressure will be reduced / liquid in manometer moves towards test tube;

distance moved by liquid in a given time is measured to calculate the volume of oxygen in by

the woodlice per minute;

ref to controls e.g. temperature ;

#2

reference to constant temperature ;

use of water bath / eq ;

reference to {suitable / stated / fixed time / eq} ;

Reference to measuring {volume / distance} ;

description of how to obtain volume ;

calculation of rate described / eq ;

reference to replicates ;

description of control e.g. no woodlice ;

idea of welfare of animals important ;

reference to {mass / eq} of woodlice ;

Habituation

1 Collect one giant African land snail, and place it on a clean, firm surface. Allow the snail

to get used to its new surroundings for a few minutes until it has fully emerged from its

shell.

2 Dampen a cotton wool bud with water.

3 Firmly touch the snail between the eye stalks with the dampened cotton wool bud andimmediately start the stopwatch. Measure the length of time between the touch and the


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snail being fully emerged from its shell once again, with its eye stalks fully extended.

4 Repeat the procedure in step 3 for a total of 10 touches, timing how long the snail takes

to re-emerge each time.

5 Record your results in a suitable table.

6 Present your results in an appropriate graph


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Documents

A2 Biology Notes