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IGCSE Biology All TopicsTotal Flash Cards » 38
Cells
All animals and plants aremade up of cells. Most cells
have:A nucleus – controlling theactivity of the cell. All cellshave these at one point.
Cytoplasm – wherechemical reactions occur.Inside the cytoplasm areenzymes which speed up
these reactions. Cytoplasmalso contains mitochondriawhich is where energy is
released.A cell membrane – to
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control the passage ofsubstances in and out of
the cell.
Plants also haveCell walls – to strengthen
the cell.Chloroplasts – to absorbsunlight energy to makefood by photosynthesis.Vacuole –full of cell sap.
Cells, tissues and organs
Cells act together to formtissues. For instance, the cellson the surface of a leaf form"pallisade tissue". A group ofcells with similar structuresand a particular function arecalled a tissue. Tissues aregrouped together to form an
organ. A leaf is an organ.Organs are grouped togetherto form an organism such as a
whole plant or animal. Cells ----> Tissue ----> Organ -
---> Organism Tissues are usually formedfrom specialised cells. The
cells in the pallisade tissue arespecialised to perform
photosynthesis and containmany chloroplasts. Some
other specialised cells are:Guard cellsEgg cells
Sperm cellsRed blood cells
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Diffusion
Diffusion is the net movementof particles from an area of
high concentration to an areaof a lower concentration. The
steeper the concentrationgradient, the more rapid the
rate of diffusion. 2 examples of diffusion are:
Oxygen (required forrespiration) passes through
cell membranes and gasexchange surfaces (e.g.
alveoli in lungs) by diffusionCarbon dioxide entersleaves and leaf cells by
diffusion
Osmosis
Osmosis is the movement ofwater from an area of highwater concentration (moredilute) to an area of a low
water concentration (stronger)through a partially permeable
membrane. A partially permeable
membrane allows watermolecules to pass through (as
they are smaller) but notsolute molecules (they are too
big). It acts like a sieve.Visking tubing is a partiallypermeable membrane. It isused in dialysis machines.Diffusion and osmosis can
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Active Transport
only work if the concentrationgradient is right. Sometimes
an organism needs totransport something against aconcentration gradient. Theonly way this can be done is
through active transport, usingenergy produced by
respiration. In Active transport,the particles move across a
cell membrane from a lower toa higher concentration.
Examples of active transport In plants: Plants need mineralsalts (e.g. nitrates) for makingproteins and growth. Nitratesare at a higher concentrationinside the root cells than they
are when dissolved in thewater around the soil particles.If the plant relied on diffusionalone, the vital nitrate saltswould drain out of the cellsinto the soil. So energy isdeployed by the cells to
actively transport nitratesacross the cell membrane into
the root cells, against theconcentration gradient. Inhumans Active transport
takes place during digestion offood in the small intestine.
After food has been absorbedby the villi for some time, the
concentration of food
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molecules inside the villiincreases, making it
impossible for more food todiffuse into the villi. So simplesugars, amino acids, minerals
and vitamins are activelyabsorbed into the villi, from an
area of lower to an area ofhigher concentration
Shareen Saqlain
The Cell Cycle happens in cells
Prophase
The first and longest phase ofmitosis, prophase, can take asmuch as 50-60 percent of thetotal time to complete mitosis.
During prophase, thechromosomes become visible.
The centrioles, two tinystructures located in the
cytoplasm near the nuclearenvelope, separate and take
up positions on opposite sidesof the nucleus. The centriols
lie in a region called thecentrosome that helps to
organise the spindle, a fanliekmicrotubule structure that helpseparate the chromosomes.
During prophase, thecondensed chromosomes
become attached to fibers in
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the spindle at a point near thecentromere of each chromatid.
Metaphase
The second phase of mitosis,metaphase, often only lasts a
few minutes. Duringmetaphase, the chromosomes
line up across the center ofthe cell. Microtubules connect
the centromere of eachchromosome to the poles of
the spindle.
Anaphase
Anaphase is the third phase ofmitosis. During anaphase, the
centromeres that join thesister chromatids separate,
allowing the sister chromatidsto separate and become
individual chromosomes. Thechromosomes continue to
move until they haveseparated into two groups
near the poles of the spindle.Anaphase ends when the
chromosomes stop moving.
Telophase
Following anaphase istelophase, the fourth and finalphase of mitosis. In telophase,the chromosomes, which weredistinct and condensed, begin
to disperse into a tangle ofdense material. A nuclearenvelope re-forms around
each cluster of chromosomes.The spindle begins to break
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apart, and a nucleolusbecomes bisible in each
daughter nucleus. Mitosis iscomplete
Plant structure
Plants are divided into flowers,stems, leaves and roots with
root hairs. A generalised plantis shown in the illustration.
The stem provides support forthe leaves and flowers. It alsoallows water and food to travelboth up and down the plant. The leaves make the food for
the plant. Photosynthesistakes place in the leaves.
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The roots anchor the plant inthe soil and take up water andsalts (mineral ions) from the
soil. The root hairs provide alarge surface area for water
and salt uptake. The flowersare reproductive organs. Theyattract insects that carry pollenfrom one plant to another. Thisprocess of transferring pollen
from plant to plant is known aspollination.
The structure of the leaf
Leaves produce the food forthe plant. The structure of theleaf is shown in the illustration.The leaf has prominent veins
that contain two types oftubes, the xylem tubes andthe phloem tubes. The leafhas the following parts (from
top to bottom):Waxy cuticle
Upper EpidermisPallisade layerSpongy layer
VeinsLower epidermis
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Guard cells that formstomata
Leaves are green becausethey contain the green
pigment called chlorophyll.Chlorophyll is used in
photosynthesis.
The structure of flowers
lowers are composed of:Sepals - these are arrangedunderneath the flower and
are typically green.Petals - often brightly
coloured to attract insects.Stamens - stalk-like
filaments that have anthersat the top which produce
pollen. Pollen contains themale gametes.
Pistil - contains one orseveral carpels that containthe ovaries with ovules, the
female gametes.Sometimes the carpels aremerged. A stalk called thestyle leads upwards from
each pistil and is topped bya sticky stigma thatreceives the pollen.
The pistil is the bottle shaped
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structure. A pistil can becomposed of one or many
carpels and a flower can haveseveral pistils.
Plant growth
Plant growth requires glucoseproduced by photosynthesis
and energy produced byrespiration. It also requiresminerals obtained from the
soil. Plant growth is controlledby plant hormones calledauxins. Auxins Mineralsneeded for plant growth
There are three minerals thatare essential for plant growth:
phosphates, nitrates andpotassium. Small quantitiesof iron and magnesium are
also needed, especially for theproduction of chlorophyll.
Phosphates: used inphotosynthesis and
respiration. Phosphatedeficiency: purple leaves and
small roots. Nitrates: used in the
production of aminno acids.Amino acids are combined to
make proteins.
Nitrate deficiency: yellowingof leaves and poor, stunted
growth.
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Potassium: maintainselectrical potentials and helps
enzyme action.Potassium deficiency:
leaves become yellow withspotty, brown, dead areas.
Photosynthesis
Like all living things, plantsneed food to live. This food isused for energy and to makenew materials when plants
grow. Plants are able to taketwo inorganic chemicals,
carbon dioxide gas and water,to make an organic chemical,glucose. This simple food canbe used as an energy sourceor converted into other useful
organic molecules. The process requires an inputof energy. Plants have found a
way to capture the energyfrom sunlight using a pigmentcalled chlorophyll. Once this
light energy has beencaptured it can be used to
create glucose, converting thelight energy into chemical
energy. Oxygen gas isreleased as a waste chemical.
As light energy is used tocreate organic materials the
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process is namedPhotosynthesis. The formula for
photosynthesis is:
Respiration in plants
Respiration is the productionof energy from glucose andoxygen with the release of
carbon dioxide and water aswaste products. This is theopposite of photosynthesiswhich is the production of
glucose and oxygen from theenergy in sunlight, carbon
dioxide and water. Respiration: C H O + 6O→ 6CO + 6H O + Energy
released Glucose + Oxygen→ Carbon dioxide + water +
Energy released Photosynthesis: 6CO +
6H O + Energy → C H O +6O Carbon dioxide + water +Energy → Glucose + Oxygen
Plants use the energy fromrespiration to power the
processes involved in growth.
The energy obtained fromrespiration is used to turnglucose into many other
substances. Typical uses ofglucose are:
Storage products
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Uses of glucose
- glucose is used to makestarch which can be
converted back to glucose asrequired. Potatoes and rice
are examples of parts ofplants that contain starch.-glucose is converted into
lipids, especially in seeds.Sunflower oil and rapeseed
oil, which are used in cooking,come from sunflower and
oilseed rape seeds.Structural products
- glucose is converted tocellulose to make cell walls.
Other products- glucose and nitrates are
used to make amino acidswhich are used to make
proteins.- glucose is also abasic raw material for making
chlorophyll.
The Human Digestive System Yummy in my TUMMY
The Mouth
Digestion begins inside themouth, where chewing does 2things - makes the food easierto swallow and increases the
surface area (this helps tospeed up digestion). Also
inside the mouth, amylase (fordigesting starch) can be
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found, produced by thesalivary glands. The gulletthen carries food from the
mouth to the stomach by itsmuscular squeezing action
called peristalsis.
Stomach
The stomach does a numberof things, including:
pummels and churns thefood
produces proteaseenzymes to digest proteinproduces hydrochloric acid
which kills bacteria andgives the ideal pH for
protease to work.
Liver
The liver produces bile whichemulsifies fats i.e. breaks
them down into small dropletsfor a larger surface area. Thiswill increase the rate at whichthe fat is digested by lipase.Bile also neutralizes the acidproduced by the stomach to
provide ideal alkalineconditions for enzymes in the
small intestine. The gallbladder stores the bile madeby the liver until it is needed.The small intestine produces
amylase, lipase and protease.The pancreas, a pistol shapedorgan, produces the enzymesamylase, lipase and protease
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Small Intestine
and releases them into thesmall intestine when needed.The enzymes are used to fullydigest the food molecules, sothat they are small enough todiffuse into the bloodstream.Food molecules diffuse in thesmall intestine, which is idealfor this purpose. The small
intestine walls are very thin sothat there is a short diffusionpathway. It is long and foldedwith villi to increase surfacearea. There is a rich bloodsupply to maintain a steepconcentration gradient for
diffusion.
Large Intestine & Anus
When all the useful productshave diffused into the blood,the remaining waste reachesthe large intestine where the
majority of the remainingwater is absorbed into theblood stream. Finally the
waste products leave the bodyin the form of faeces through
the anus.
Breathing and Respiration The Basics
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Breathing
The zone of the bodybetween the neck and the
bottom of the ribs is knownas the thorax. The major
organs in the thorax are theheart and lungs. The lungs
and associated airwaysallow us to breathe.
In the head the airwaysconsist of the mouth and nasalpassages. Air and food has a
common passage in thethroat.
Larynx or voicebox. This iswhere there is speech and
sound generation.Trachea or windpipe.
Two tubes that are each
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known as a bronchus,plural bronchi.
Bronchioles which aresubdivisions of each
bronchus.Alveoli which are sacks atthe end of the airways thatallow oxygenation of the
blood.Pleural cavity"
pleural membrane.
The key features of breathingare that when we breathe in
the intercostal musclesbetween the ribs and thediaphragm both contract;
when we breathe out both ofthese muscles relax. When we
breathe in the contraction ofthe intercostals pulls the
sternum up and away from thebody and the descent of the
diaphragm increases thevolume of the thoracic cavity.Notice that in the resting state(breathing out) the diaphragmbulges up under the lungs, thelungs themselves are slightlyelastic and pull the diaphragm
back to this position
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Oxygenation of the blood
The two main functions of thelungs are to oxygenate theblood and to remove waste
carbon dioxide. The blood is oxygenated in the
alveoli. The alveoli are thinwalled and surrounded by
capillaries. The blood entersthe capillary network around
the alveoli from thepulmonary artery and leavesthe capillary network via the
pulmonary vein.Oxygen diffuses into the blood
through the alveolar andcapillary walls and carbondioxide diffuses out of theblood. The alveoli have asurface area of about 70
square metres to make thisgas exchange as fast as
possible. Carbon dioxide dissolves inwater and can easily and
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reversibly form compoundssuch as carbonic acid and
bicarbonates. Oxygen does not dissolve
much in water, to overcomethis problem the oxygen in theblood is stored in red blood
cells. These containhaemoglobin which can
combine with oxygen to formoxyhaemoglobin. The red
blood cells contain the oxygenin the blood. The blood
transports oxygen from thelungs to the rest of the body. Oxy-haemoglobin is bright redand haemoglobin is dark red,
this is why veins look dark andwhy all the diagrams showveins in blue and arteries in
red. The exception is thepulmonary artery which carriesdark red, de-oxygenated blood
to the lungs and thepulmonary vein which carriesbright red oxygenated blood
away from the lungs.Thespecial adaptions of the alveoli
for gas exchange are:Thin walls
Huge surface areaCovered in capillaries to
provide bloodA wet lining to dissolve
gases
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Aerobic respiration
Respiration is the process inwhich the chemical bonds of
energy-rich molecules such asglucose are converted into
energy usable for lifeprocesses. Aerobic
respiration uses oxygen tooxidise glucose and produceenergy. The equation for the
oxidation of glucose is:C H O + 6O → 6CO +6H O + Energy released
Glucose + Oxygen → Carbondioxide + water + Energy
released In a fire there is a massive
uncontrolled release of energyas light and heat. Respiration
is a similar process but itoccurs in gradual steps.
Most animals and plants useaerobic respiration as a
primary source of energy. glucose + oxygen = carbondioxide + water = energy
When a person is doing veryheavy exercise and the bloodcannot supply enough oxygen
another sort of respirationoccurs. This converts glucoseinto energy without the needfor oxygen and is known asanaerobic respiration. The
reaction is: Glucose → Energy
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Anaerobic respiration
released + lactic acid Anaerobic respiration releases
less energy than aerobicrespiration. Unfortunately theinsufficient blood supply that
leads to anaerobic respirationalso means that the lactic acidbuilds up in the muscles. Highlactic acid concentrations are
painful and felt as cramp.When exercise stops, the
blood supply is able to provideenough oxygen to convert thelactic acid to carbon dioxideand water but this takes time
and the muscle pain maycontinue after exercise until
the lactic acid has beenconverted.
The delay in the removal oflactic acid is known as the
oxygen debt. Carbon dioxideand lactic acid both causeincreases in breathing rateand heart rate to allow thebody to repay the oxygen
debt. The oxygen debt is thereason why we continue to be
out of breath even afterexercise. If athletes are very fit
their circulation can provideextra oxygen more rapidly andtheir recovery time, the time
required to restore normalbreathing and pulse, will be
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shorter than in people who arenot fit.
Yeasts and anaerobicrespiration
The direct conversion ofglucose to energy without theuse of oxygen occurs in manyyeasts and fungi. The ethanolthat is used in alcoholic drinks
is a result of anaerobicrespiration in yeast, the
reaction is: Glucose → Energyreleased + ethanol + carbon
dioxide Brewers use various types of
brewers yeast to producealcohol. In fizzy alcoholic
drinks such as champagne thebottles are tightly stoppered to
prevent the carbon dioxidefrom escaping.
The Heart a Muscle
The blood transports food,proteins, blood cells, gases,water, minerals and wasteproducts around the body.
Blood contains:Plasma - a straw colouredliquid in which the bloodcells are suspended and
the other components of theblood are dissolved.
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Blood
Red blood cells - these arered and doughnut shapedfor a large surface area.
They carry oxygen from thelungs to the tissues as oxy-
haemoglobin (theoxygenated form of
haemoglobin). They do nothave a cell nucleus.
White blood cells - theseare colourless cells with alarge cell nucleus. Theydefend the body against
disease by engulfing micro-organisms (bacteria andviruses) and producing
antitoxins and antibodies.Platelets - these are
fragments of cells and verysmall. They do not have a
cell nucleus. They formblood clots at the site ofinjuries. Clots protect the
body from further infectionand blood loss.
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How the heart works
Due the continuous cardiaccycle which takes an
approximate second, the heartis the fastest and the strongest
muscle in the body. Thestrongest external muscle is
the tongue, however the heartis much stronger.
The heart works in threestages:
1. Blood flows into the atria.2. Both atria contract at the
same time, forcing bloodinto the ventricles.
3. Both ventricles contract atthe same time, forcing
blood through thepulmonary artery and aorta.
The heart valves ensure thatthe blood goes in the correctdirection, they stop backflow.When the atria contract the
valves between the atria andthe ventricles open passivelyand the high pressure in the
arteries keeps the valvesbetween the ventricles and the
arteries closed. When theventricles contract the valves
between the ventricles and theatria shut and the valves
between the ventricles and thearteries open.
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The net effect of the action ofthe heart is as follows.
Deoxygenated blood from thebody enters the right atrium
and flows into the rightventricle where it is pumped
through the lungs. In the lungsthe blood is oxygenated. It
then flows into the left atriumand on into the left ventriclewhere it is pumped out at
sufficient pressure to reachevery part of the body. Theatria have thin walls and actas reservoirs, the ventricles
have thick, muscular walls andact as pumps. The valves inthe heart prevent backflow.
You need to know thefollowing names of arteries
and veins:Aorta - the big artery that
comes out of the leftventricle.
Pulmonary artery - the bigartery that comes out of the
right ventricle.Vena cava - this refers to
the big vein that connects tothe right atrium. It has two
sections the "superior" venacava that drains blood fromthe top part of the body andthe "inferior" vena cava thatdrains blood from the bodybelow the heart. To make
the sections clear it is oftensaid that there are two
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"vena cavae", the superiorvena cava and the inferiorvena cava. Either learn thenames of the two sections
or, when you draw the heartput the arrow to the venacava as close to the heart
as possible.Pulmonary vein - the veinthat is directly attached to
the left atrium.
The menstrual cycle is acycle of events that occurs in
the womb (uterus) andovaries of female mammals. It
is associated with theproduction of eggs and
preparing the uterus for theimplantation of fertilised eggs.
The menstrual cycle occursover a period of about 28
days. The changes during thecycle are due to four
hormones, progesterone,oestrogen, FSH (follicle
stimulating hormone) and LH(luteinising hormone).
Progesterone and oestrogenhave wide ranging effects onthe body but in the context of
the menstrual cycleprogesterone is mainly
involved in maintaining thelining of the uterus and
oestrogen is mainly concernedwith building up the lining of
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The menstrual cycle
the uterus. FSH stimulates theproduction of eggs and LH
stimulates the release of theegg. FSH and LH are
produced by the pituitarygland in the brain.
There are four stages in themenstrual cycle.
Stage 1. Days 1-4.Menstruation (bleeding)occurs. The lining of the
uterus disintegrates and isshed. This is due to low levels
of progesterone. Stage 2. Days 4-14. The
uterine lining grows back. Thisis due to high levels of
oestrogen. Stage 3. Day 14. The egg
(called an ovum) is released.This is due to LH.
Stage 4. Days 14-28. Thelining of the uterus is
maintained in case the eggbecomes fertilised andimplanted in the uterus.
Maintenance of the lining isdue to high levels of
progesterone. The four hormones interact
with each other. FSH causesOestrogen release and
oestrogen inhibits FSH. LHstimulates both oestrogen and
progesterone production.
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Before ovulation LH release isstimulated by oestrogen but
after ovulation it is inhibited byboth oestrogen and
progesterone. In summary, the hormoneshave the following effects:
Oestrogen: causes growth ofthe uterine lining. Inhibits FSH.Stimulates release of LH and
hence release of the egg.Inhibits LH after ovulation.
Progesterone: maintains theuterine lining. Inhibits LH afterovulation. LH: Stimulates the
release of the egg (calledovulation). Stimulates
oestrogen and progesteroneproduction. FSH: Stimulates
egg development and therelease of oestrogen.
Birth control tablets containhigh levels of progesterone
and oestrogen. The oestrogeninhibits FSH production so that
eggs cease to develop. FSH is used to treat infertility
because it stimulates theproduction of eggs.
The nervous system consistsof the brain, spinal cord and
relay neurones (Centralnervous system) and
peripheral nervous system
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The Nervous System (sensory neurones and motorneurones). The nervous
system lets the organism reactto the environment and
surroundings and coordinatetheir behaviour.
The senses
Sense organs containreceptors that are sensitive to
stimuli. Typical stimuli aredue to changes in chemical
composition, mechanicaleffects, heat, sound and light.Stimuli are the changes that
are detected. Receptorsdetect the change.
There are five sense organsthat are studied at this stage,the nose, tongue, eyes, ears
and skin. The nose has smell receptors
that are sensitive to a widerange of chemical stimuli.
Much of what is called 'taste'is actually smell; the range offlavours in food is very limited
if the nose is blocked. The tongue has taste
receptors that are sensitive tochemical stimuli. There arefour basic tastes: bitter, salt,
sweet and sour. The eyes have receptors thatare sensitive to light, the ears
have receptors that are
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sensitive to sound and theskin has receptors that aresensitive to temperature,
touch, pressure and stretch.
Light from the surroundingsenters the eye through thepupil. It is focussed by thecornea and lens so that it
forms an image on the retina.The cornea performs crude
focussing and the lensperforms active, fine
focussing. The eye is filledwith transparent liquid (the
humours). The iris controls the size of
the pupil and so controls howmuch light gets into the eye. In
bright light the circularmuscles in the iris contract,this makes the pupil smallerand allows less light into theeye. In dim light the radial
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The eye
muscles contract, this makesthe pupil larger and allows
more light into the eye. The lens changes shape toallow the eye to focus on
things at various distances.The shape of the lens is
changed by the action of theciliary muscles. The ciliarymuscles form a circle that isattached to the lens by thesuspensory ligaments. Tofocus on distant objects theciliary muscles relax which
pulls the suspensoryligaments tight and makes the
lens thinner. To focus onnearby objects the ciliary
muscles contract which allowsthe suspensory ligaments torelax so that the lens forms a
more spherical shape. The retina contains cells thatare sensitive to light. These
cells are called rods andcones.
The rods are more sensitivethan the cones but do not
provide any colourinformation. In dim light ourview of the world is largely
provided by rods and appearsas a black and white image. There are three types of coneand these are each sensitive
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to different colours of light.The cones are especially
densely packed in the part ofthe retina called the fovea;
this dense packing means thatthe fovea is most sensitive to
fine detail in the image. The part of the eye where theoptic nerve enters is called
the blind spot. The blind spotdoes not contain any light
receptors (it has no rods orcones).
Neurones
Neurones are cells that arespecialised to transmit
electrical impulses aroundthe body. They consist of three
principle components: thedendrites, the cell body andthe axon. Each neurone has
its own nucleus. (The USspelling of neurone is
"neuron").
There are three types ofneurone: sensory neurones,relay neurones and motor
neurones. Neurones connect to other
neurones by means ofsynapses. Nerve impulsestravel down the axon to one
side of the synapse where the
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electrical signal causeschemicals to be released.These chemicals diffuse
across the gap and generatean electrical impulse in the
neurone on the other side ofthe synapse.
Reflex arcs
The nervous system has manyreflexes. These are automatic
responses to stimuli. Veryquick responses are managed
by Reflex arcs. A reflex arcconsists of the following
components:StimulusReceptor
Sensory neuroneRelay neuroneMotor neurone
EffectorResponse
Pain reflex: a painful stimuluscauses a muscle to contract
automatically
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A typical reflex arc called the"pain reflex" is illustrated.
In the pain reflex the stimulusis mechanical damage, the
receptor is a "pain receptor",the pain receptor generates anelectrical impulse in a sensory
neurone which creates anelectrical impulse in a relay
neurone, which in turn createsan electrical impulse in a
motor neurone. The impulse inthe motor neurone stimulatesa muscle, which is an effector,
and this creates movementaway from the painful stimulus
as a response. Another reflex is the "patellar"or "knee jerk reflex" where ablow beneath the knee-cap
makes the leg straighten. Thecontraction of the pupil in
response to bright light is yetanother reflex.