Biology Unit 5

Stimuli, nerves and reflex arcs.


Stimulus - detectable change in environment (internal/external) to produce a response

Receptors - cells/organs that detect stimuli.

Effectors - cells, tissues, organs that carry out a response

Co-ordinator - acts as a 'switch board,' connecting information from each receptor with appropriate effector.

Taxes/taxis - response where direction is determined by direction of stimulus. 

Kinesis - response is an increase of random movements used to get organism into favourable conditions.

Tropism - growth movement of plant in response to directional stimulus. (photo/hydro/geo)

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Neurones and the reflex arc.

Receptor > Sensory Neurone > Intermediate Neurone > Motor Neurone > Effector 

  • The reflex arc is involuntary and does not require brain power, leaving the brain to carry out more complex responses and so is not overloaded.
  • does not have to be learnt
  • protects body from harmful stimuli
  • fast (important in withdrawal reflex)
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Control of heart rate

  • SAN is stimulated - releasing wave of electrical activity and the atria contract
  • Activity reaches the AVN - helped by non-conductive collagen which stops impulse going further
  • Impulse travels down bundle of His and to the purkinje fibres at the apex
  • The fibres release all electrical activity from the apex upwards, causing contraction of the ventricles.

Sympathetic SPEEDS UP. Parasympathetic SLOWS DOWN. 

Chemoreceptors - found in wall of carotid arteries and detect change in pH due to CO2 conc. When pH is lowered, they send impulses to medulla oblangata to increase heart rate, via SAN. 

Pressure receptors - found in wall of carotid arteries and aorta. When BP is too high, impulse is sent to the centre of medulla oblangata via the parasympathetic pathway to decrease heart rate.

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The role of receptors in the PACINIAN CORPUSCLE AN

Pacinian Corpuscle

  • Pressure is applied and membrane around its neurone becomes stretched.
  • Sodium ions diffuse into neurone.
  • Potential of membrane changes and produces a generator potential.
  • Generator potential reaches threshold for action potential to be created.

The Eye

RODS - black and white, 120mil in each eye, share a sensory neurone, response to low light, threshold for generator potential is lower, low light causes rhodopsin to break down, cannot distinguish colour.

CONES - full colour, 3 types (respond to different wavelengths), 6mil in each eye, connected to one bipolar cells each, greater threshold, respond to high light, found in fovea as it recieves high light intensity.

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Co-ordination and plants

2 main forms: nervous and hormonal. 

Chemical mediators are typically released by infected/injured cells and cause arterioles to dilate. e.g. histamine and prostaglandins.


Plants respond to light, gravity and water, they do this using growth hormones.

IAA is produced at the top of the stem only. IAA diffuses AWAY from the light. The plant grows on the side where IAA is present. As a result, plants grow TOWARDS THE LIGHT.

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Neurones are adapted to carry electromechanical charges called NERVE IMPULSES.

Each neurone comprises a cell body that contains a NUCLEUS and large amounts of CYTOPLASM and RER, which is used in the production of proteins and neurotransmitters.

Extending from the cell body is a single long fibre called an AXON and smaller branched fibres called DENDRONS

Axons are surrounded by SCHWANN CELLS, which protect and provide INSULATION because their membranes are rich in a lipid known as MYELIN.

The three types of neurones are: MOTOR, SENSORY and INTERMEDIATE.


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Membrane Potentials


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Membrane Potentials

Stimulus excited membrane. Sodium channels open.

If potential differences reaches threshold (about -55mV), more sodium channels open.

DEPOLARISATION - membrane becomes more positive due to sodium ions going in.

Sodium channels close, potassium channels open. Potassium floods OUT.

REPOLARISATION - membrane begins to get back to resting potential.

There is a delay in closure of potassium channels. Too many potassium ions flood out.

HYPERPOLARISATION - potential difference is more negative than resting potential.

Sodium-potassium pumps returns membrane to its resting potential.

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  • At the end of a pre-synaptic neurone - when action potential reaches the synapse, voltage-gated calcium channels open. Calcium ions diffuse into the cell.
  • Ca ions cause synaptic vesicles to fuse with cell membrane and release their contents (ACh) by exocytosis.
  • ACh diffuses across synaptic cleft.
  • ACh binds to neuroreceptors in the post-synaptic membrane, causing Na+ channels to open. Na ions diffuse in.
  • Depolarisation of the post-synaptic membrane (Post-synaptic potential) which may initiate action potential.
  • ACh must be removed from synaptic cleft to stop synapse being permanently on. Cholinesterase breaks down the ACh.
  • Breakdown products of ACh are absorbed by the pre-synaptic neurone by endocytosis and used to re-synthesise more neurotransmitter, using energy from mitochondria.

INHIBITION - Cl ions diffuse in, resulting in hyperpolarisation and no action potential.


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Action potentials

SALTATORY CONDUCTION - jumping from one node to the next.

Factors affecting speed of nerve impulse:

  • Myelin sheath (inc)
  • Axon diameter (pos. correlation due to less leakage of ions)
  • Temperature (pos. correlation up to a certain point due to denaturing of enzymes)

REFRACTORY PERIOD - ensures action potential goes in one directions (acting to resting regions) and ensures action potentials are separated, and limits number of action potentials.

ALL OR NOTHING PRINCIPLE - if generator potential reaches threshold value, action potential is created, if not then nothing will happen.

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SMOOTH - unstriped, strong, involuntary (autonomic NS), rarely fatigues, found in walls of blood vessels.

CARDIAC - strong, involuntary, rarely fatigued, found on heart.

SKELETAL - striped, voluntary (somatic NS), depends on respiration, used for movement.

  • Made up of myofibrils: actin (thin, twisted) and myosin (thick with bulbous heads)


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Muscle contraction

Binding site on actin is blocked by tropomyosin so myosin head cannot attach (relaxed)
Ca2+ ions attach to troponin, causing tropomyosin to change position and binding site is available.
Myosin head attaches to actin binding site, forming actomyosin cross bridges.
Myosin heads change angle, pulling actin over the myosin towards centre of sarcomere.
Molecule of ATP attaches to each myosin head Hydrolysis of ATP releases energy which is used to detach myosin head from the actin and reposition head further along actin.
Myosin head changes angle  again, pulling the actin. Cycle repeats.
When contraction stops, Ca2+ ions are pumped by active transport out of myofibrils into the sarcoplasmic recticulum. (requires energy from hydrolysis of ATP)
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Sliding Filament Mechanism

Action potential arrives at the end of a motor neurone, at the neuromuscular junction.

Acetlycholine is released.

Action potential in muscle cell membrane is initiated.

Action potential is carried quickly throughout the large muscle cell by invaginations in cell membrane called Tubules.

Action potential causes sarcoplasmic reticulum (large vesicle) to release its store of calcium ions into the myofibrils.

Calcium ions bind to troponin on the thin filament, which changes shape, moving tropomyosin into the groove in the process.

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Principles of homeostasis & feedback mechanisms


  • e.g. reptiles
  • control body temperature by behaviour e.g. basking in the sun.
  • variable metabolic rate.


  • control body temperate using homeostasis.
  • constantly high metabolic rate and generate a lot of heat from metabolic reactions.

Body temperature detected by THERMORECEPTORS.

HYPOTHALAMUS sends signals to the effectors.

Body reacts e.g. vasodilation/vasoconstriction

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Control of blood glucose

Insulin and glucagon produced in Islets of Langerhans.

INSULIN - binds to receptors on membranes to open glucose channels. Activates enzymes to convert glucose into glycogen/fat. Inc rate of respiration esp. in muscle cells.

GLUCAGON - binds to receptors on membranes of liver cells and break down glycogen into glucose to promote formation of glucose of fatty acids and amino acids. Dec rate of respiration.

GLYCOGENOLYSIS - produce glucose from glycogen

GLYCOGENESIS - produce glycogen from glucose

GLUCONEOGENESIS - produce glucose from fatty acids.

ADRENALIN - produced in adrenal glands which binds to receptors of liver cells to break down glycogen into glucose. ATP transforms into Cyclic AMP, activates enzymes.

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Diabetes, treatment and control

Type 1 - insulin dependent, body is unable to produce insulin. Treated using insulin injections 2-4 times a day, glucose levels monitores using biosensors.

Type 2 - insulin independent, linked to obesity, insulin is no longer effective or not enough is produced. Controlled by regulating carbohydrate intake in the diet and matching it with exercise. Drugs/injections may be used as supplements.

Modern methods:

  • injections - discreet, cheap, site and technique is specific, needle phobia could be problematic.
  • pump - better control, uses less insulin, needs to be worn at all times, expensive.
  • Inhaled - less painful, simple, timing is difficult, expensive, lung cancer?
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The Menstrual Cycle


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The Menstrual Cycle

  • Pituitary gland releases FSH - follicle ripens.
  • Stimulates production of OESTROGEN (positive feedback)
  • Oestrogen causes repair of lining on uterus wall and inhibits FSH production (negative feedback)
  • Stimulates production of LH which causes follicle to release egg (OVULATION) and development of corpus luteum.
  • Stimulates production of PROGESTERONE which maintains lining of uterus wall in readiness for the implantation of fertilised egg and inhibits production of LH and FSH.


The pill increases levels of oestrogen and progesterone, therefore inhibiting FSH and LH production so the follicle will not ripen and ovulation will not occur. Uterus wall will remain thick.

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Structures of DNA and RNA


  • phosphate group
  • deoxyribose sugar
  • organic base


  • single helix
  • ribose sugar
  • AUCG
  • unstable


  • clover shape, fairly stable.
  • anticodon and amino acid attached.
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Transcription and Splicing

  • DNA HELICASE breaks hydrogen bonds between bases in specific region of DNA molecule.
  • Strands separate and expose bases in region.
  • RNA POLYMERASE moves along one of the two DNA strands. (template strand)
  • Nucleotides on this strand join with complementary nucleotides from the pool in nucleus.
  • As pre-mRNA is produced, DNA strands rejoin behind it.
  • Only about 12 base pairs are exposed at any one time.
  • Production of pre-mRNA is complete once RNA POLYMERASE reaches a stop codon.


  • INTRONS are removed from pre-mRNA in eukaryotic cells.
  • EXONS can be rejoined in a variety of combinations.
  • A gene can code for up to a dozen different proteins depending on the order of exons.
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  • RIBOSOME attaches to the mRNA and encloses two codons.
  • first tRNA molecule with an amino acid attached diffuses into the ribosome and the ANTICODON attaches to the first mRNA codon by complementary base pairing.
  • the next amino acid tRNA attaches to the adjacent mRNA codon.
  • The bond between the amino acid and tRNA is cut and a PEPTIDE BOND is formed.
  • RIBOZYMES catalyse this process.
  • the tRNA molecules detach and goes to collect another amino acid.
  • The chain peels away from the ribosome and carries on until a 'stop' codon is reached.
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IN VIVO Cloning

A cell that produces the wanted protein is selected and the mRNA removed.

REVERSE TRANSCRIPTASE is used to make complementary DNA. 

DNA POLYMERASE is used to add the complementary bases to the cDNA. 

The DNA and the plasmid are cut with the same RESTRICTION ENZYME. 

The fragments of DNA and the plasmid are mixed with DNA LIGASE - producing a plasmid containing the DNA. 

The bacteria is heated up with Ca2+ ions to make it more permeable to plasmids. 

To identify the bacteria with plasmids in, 3 markers can be used: 

  • ENZYME MARKERS which uses lactase that turns a colourless substance blue. 

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IN VITRO Cloning

DNA is mixed with bases and DNA POLYMERASE and heated up to 95 degrees c. 

Hydrogen bonds break and strands separate. 

Mixture cooled down to 40 degrees c and PRIMERS are added.

Primers form short double-stranded sections as hydrogen bonds form. (ANNEALING)

Mixture is heated up to 72 degrees c and mixed with DNA POLYMERASE. 

New nucleotides binds to strands, indicated by the primers. 


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DNA Sequencing (Chain Termination Sequencing)

Label 4 test tubes A, C, T and G. Add to them:

  • Sample of DNA 
  • Labelled primer
  • 4 nucleotides
  • Modified dideoxy bases (1% conc) which stops further addition of DNA. 

In each test tube, the fragments will stop at the correct base (A in test tube A).

The contents of the test tubes are run side by side on AGAR GEL and visualised by autoradiography.

The sequence can be read from the smallest (lightest - at the bottom) to the biggest (at the top)

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Restriction Mapping

A piece of DNA is cut with two different RESTRICTION ENZYMES on their own and together.

This gives 3 different mixtures of restriction sites, which are run on an ELECTROPHORESIS GEL (labelled E1, E2 and E1+E2) 

The first lane on the gel contains a DNA LADDER which is a mixture of known fragments of DNA - used to calibrate the gel.

How to work out the fragments lengths: 

  • E1+E2 is the total fragment length. 
  • Number of bands in E1 - 1 is the number of recognition sites for that enzyme. 
  • Number of bands in E2 - 1 is the number of recognition sites for that enzyme. 
  • Using logic, the restriction map can be constructed.  
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Genetic Fingerprinting / Electrophoresis

DNA sample is cloned using PCR

DNA is heated to a high temperature to break HYDROGEN BONDS between base pairs.

DNA POLYMERASE and free nucleotides are added.

DNA is cooled and PRIMERS are added. DNA POLYMERASE joins free nucleotides together. 

Nucleotides pair up to single strands of DNA by complementary base pairing. 

Cycle is repeated until multiple copies of DNA are cloned.

Cloned DNA is cut into fragments of different lengths using different RESTRICTION ENZYMES

Fragments are placed into wells and electrophoresis takes place

Fragments separate according to size and move towards anode.

Position of fragments is compared to other samples.

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Gene Mutations

SUBSTITUTION - swapping one base for another 

NON-SENSE - base change produces a stop codon resulting in a very different protein 

MIS-SENSE - base change produces a single different amino acid, this could affect the protein if the amino acid is involved in forming bonds. 

SILENT - the base change results in the same amino acid being produced, 

DELETION - a base is removed from the primary sequence.

INSERTION - a base is added to the primary sequence of bases.

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Genetic Screening

The order of nucleotides on the mutated gene is determined by DNA sequencing. 

Fragment of DNA with complementary bases to the mutated portion of the gene is produced.

DNA probe is formed by radioactively labelling the DNA fragment. 

PCR techniques are used to produce multiple copies of the DNA probe.

Probe is added to single stranded DNA fragments from the person being screened. 

If the donor has the mutated gene, the probe will bind to its complementary bases on the donor DNA.

The DNA fragments will be labelled with the probe and can be distinguished using X-ray film. 

If complementary fragments are present, the DNA probe will be taken up and X-ray film will be exposes.

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