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Edexcel AS Biology Revision Notes Written by Tim Filtness
Unit 1: Lifestyle, Transport, GenesUnit 1: Lifestyle, Transport,
GenesUnit 1: Lifestyle, Transport, GenesUnit 1: Lifestyle,
Transport, Genes & & & &
HealthHealthHealthHealth Topic 1: Lifestyle, health & Risk
1.1.2
Water molecules are polar
H = Positively charged (+) O = Negatively charged (-) This
allows them to form Hydrogen Bonds with other water molecules. This
gives water some useful properties;
Property Explanation
Less dense as a solid Arctic ecosystems float, ice insulates
water beneath it etc
High SHC Cells do not heat up or cool down easily, therefore can
hold a fairly stable temp. (cf enzymes)
Present naturally in all three states
Allows the water cycle to function
Transparent Allows photosynthesis underwater
Cohesion Generates surface tension, capillary uptake,
transpiration etc
Good solvent Essential role in transport in biological
systems
Immiscible with hydrophobic molecules
Allows membranes to form and, therefore, control movement in /
out of cells
High latent heat of evaporation
Evaporation of water has a strong cooling effect and
comparatively little water is required to lose a lot of heat
Buffer Water is capable of accepting and donating protons,
therefore acts as a buffer
Edexcel AS Revision
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.1.3 Saccharides are made from sugar molecules, which are made
from combinations of the elements Carbon, Hydrogen and Oxygen
only
Saccharides are used for;
1. Fuels for respiration (e.g. glucose) 2. Energy storage
molecules (e.g. starch and glycogen) 3. Structural molecules (e.g.
cellulose)
Monosaccharides one sugar molecule only Disaccharides two sugar
molecules joined together Oligosaccharides a few sugar molecules
joined together Polysaccharides many sugar molecules joined
together You need to know the different structures of glucose. You
should be able to draw this out if requested.
Disaccharide Name Component monosaccharides
Maltose Glucose + Glucose
Sucrose Glucose + Fructose
Lactose Glucose + Galactose
Glucose Glucose
H
OH
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Edexcel AS Biology Revision Notes Written by Tim Filtness
There are three polysaccharides specifically mentioned on your
syllabus (starch, glycogen and cellulose). Cellulose is in Topic 4
(2.4.3) but is included here for reference.
Polysaccharide Structure and Function
Glycogen
1. Made from Poly ( Glucose).
2. Found in muscle and liver cells for energy storage
3. Insoluble, so no osmotic effect in tissues
4. Lots of branches (i.e. 1-6 glycosidic bonds present), which
allows quick access to glucose
5. Compact shape, so good for storage
Starch
1. Actually made from two molecules in combination; Amylose and
Amylopectin
2. Both are made from Poly ( Glucose).
3. Found in Amyloplasts (starch grains) inside plant cells for
energy storage
4. Insoluble, so no osmotic effect in tissues
5. Amylose has no branches (i.e. 1-4 glycosidic bonds only), so
access to glucose is slow
6. Amylopectin has some branches (i.e. both 1-4 & 1-6
glycosidic bonds)
Cellulose
1. Made from Poly ( Glucose).
2. Main component of cell walls as it is a very strong
structural molecule
3. Insoluble for obvious reasons!
4. Cellulose has no branches (i.e. 1-4 glycosidic bonds only),
so adjacent cellulose chains line up close
5. Hydrogen bonds form between adjacent chains, creating very
strong cellulose fibrils
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Edexcel AS Biology Revision Notes Written by Tim Filtness
Saccharides join together in condensation reactions, which
produce water. A glycosidic bond forms between the saccharide
molecules. The opposite of a condensation reaction is a hydrolysis.
This requires;
1. Heat + HCL 2. OR an enzyme (e.g. Amylase)
1.1.4
Tests for Sacharides:
- Iodine solution turns brown blue/black in the presence of
starch
- Benedicts solution turns blue brick red in the presence of a
reducing sugar
- Non reducing sugars (most disaccharides and all
polysaccharides) will give a positive result to Benedicts if heated
in acid first.
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.1.5
Triglycerides are either fats or oils. They are made from the
elements C, H & O only.
Triglycerides are used for;
1. Long term energy storage molecules 2. Insulation 3.
Protection (e.g. pericardium) 4. Buoyancy 5. Synthesis of specific
hormones (e.g. steroids)
The C=C bonds form kinks in the fatty acid chains, which push
adjacent triglycerides away from each other. This lowers the effect
of intermolecular forces (e.g. van der vaals forces), which lowers
the boiling and melting temp.
Triglycerides are formed in condensation reactions between;
1 x glycerol 3 x fatty acid An ester bond forms between the
fatty acid and the glycerol
Saturated triglycerides have no C=C bonds in them. They form
fats. Unsaturated triglycerides DO have C=C bonds in them. They
form oils.
Test for a triglyceride (Emulsion test):
1. Add ethanol (dissolves fat) 2. Add water 3. White precipitate
indicates a positive result
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.1.6 Ficks law: Rate of Diffusion = Surface Area x Conc
Gradient
Distance If we apply this to a cube, the rate at which O2
reaches the centre of the cube is a product of the ratio of the
Surface Area compared to the Volume (i.e. SA:Vol) In humans the
mass transport system is the circulatory system and the heart. The
specialized exchange organs include the lungs and the digestive
system.
Amoeba Large SA:Vol ratio Can rely on diffusion through its
surface.
Human Small SA:Vol ratio Diffusion through surface is too slow
to supply O2. Therefore require a mass transport system and
specialized exchange organs
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.1.7
You need to know;
1. the names of the 4 chambers of the heart
2. the names of the 2 arteries and 2 veins attached to the
heart
3. The names of the two sets of valves in the heart
4. The cardiac cycle
5. The initiation and conduction pathways of the heartbeat
Contraction in the heart:
Remember, the atria contract first. The L & R atria contract
at
the same time. The ventricles contract second. The L & R
Ventricles contract at the same time.
Aorta
Pulmonary Artery
Vena Cava
Vena Cava
Cuspid Valve
Semi-lunar Valve
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Edexcel AS Biology Revision Notes Written by Tim Filtness
0 0.2s Atrial Systole The atria contract, atrial pressure rises
and blood is pushed from atria ventricles
0.2 0.3s Ventricular Systole
The ventricles contract, ventricular pressure rises above atrial
pressure and the cuspid valves shut (1)
Ventricular pressure rises, but no blood leaves the heart
yet!
When ventricular pressure rises above pressure in the arteries
the semi-lunar valves open (2)
Blood leaves the heart
0.3 0.4s Diastole The ventricles relax. Ventricular pressure
falls and when pressure in the arteries > ventricular pressure
the semi-lunar valves shut (3).
0.4 0.7s Diastole The entire heart is relaxed. The cuspid valves
open (4) and both atria and ventricles fill with blood.
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1. SAN sends a wave of electrical activity (depolarization)
around the walls of the atria.
2. A ring of insulating tissue blocks the wave from passing into
the ventricles.
3. The AVN conducts the wave into the Ventricles slowly, which
gives the ventricles time to fill.
4. The Purkinje fibres are fast-conducting and take the wave to
the apex of the heart first, so the ventricles contract bottom
upwards.
1.1.8
Artery: Arteries carry high pressure blood away from the
heart.
Key Points:
collagen & connective tissue
smooth muscle& elastic tissue
lumen (blood)
0.1-10mm
SAN: Sino-Atrial Node AVN: Atro-Ventricular Node Purkinje Fibres
(in bundle of His)
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1. Thick muscle layer to withstand high pressure blood 2.
Elastic tissue allows artery to stretch when blood is forced
into it. The elastic layer recoils during diastole, converting
pulsatile into laminar (continuous) blood flow.
3. Protective collagen layer 4. Round shape 5. Relatively small
lumen
Vein: Veins carry low pressure blood towards the heart.
Key Points:
1. Thin muscle layer (low pressure blood) 2. Valve to stop
backflow 3. Protective collagen layer 4. Not a round shape (wall
not thick enough to hold shape) 5. Large lumen (decreases effect of
friction)
Capillary: Capillaries are adapted for exchange they are not
connected directly to the heart.
basement membrane(collagen)
endothelium cell
red blood cell
8 m
collagen & connective tissue
smooth muscle& elastic tissue
lumen (blood)
semilunar valve
0.1-20mm
Small hole
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Edexcel AS Biology Revision Notes Written by Tim Filtness
Key Points:
1. Walls are one cell thick (cells are called endothelial cells)
2. Lumen is the same width as one RBC (therefore more of RBC
in contact with wall, therefore smaller diffusion distance) 3.
No muscle or elastic tissue 4. Tiny (compare the scales and remind
yourself what a m is)
1.1.9
Dig up your Daphnia Core Practical notes in the Practical
Handbook
1.1.10 & 1.1.11 Atherosclerosis is a disease in which the
wall of arteries becomes furred up with fatty deposits called
plaques or atheromas. The sequence of atherosclerosis is as
follows;
1. Endothelial layer on the inside of an artery is damaged
2. Inflammation (an A2 topic) of the artery wall occurs
3. White blood cells move into the artery wall
4. Cholesterol begins to accumulate at the site of damage
5. Atheroma forms
6. Lumen narrows
7. Pressure increases After atherosclerosis has developed there
is a chance that a blood clot might form in the damaged area. This
makes the problem much worse!
As hypertension speeds atheroma formation these steps are a
vicious cycle!
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Edexcel AS Biology Revision Notes Written by Tim Filtness
Clot formation:
1. Platelets are activated by substances released by the damaged
artery wall
2. Platelets become sticky and form a platelet plug on the
surface of the atheroma
3. Platelet plus releases chemicals which activate
thromboplastin
4. Thromboplastin initiates the clotting cascade
There is a real danger of the blood clot becoming dislodged from
the site of formation. It could be carried around the bloodstream
and deposited elsewhere. If this occurs; - in the brain a stroke
occurs - in the coronary arteries, CHD or even an infarction might
occur - anywhere else, ischaemia and even gangrene are possible
1.1.12 Risk factors for CVD. There are lots, but these 7 are
specifically mentioned on your syllabus
Thromboplastin
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Edexcel AS Biology Revision Notes Written by Tim Filtness
Risk Factor Explanation
Age Atherosclerosis occurs naturally as our arteries become less
elastic with age. Less elastic = higher pressure during systole,
hypertension, atherosclerosis bummer.
Gender Girls have less atherosclerosis: fact. Two
explanations;
1. Girls make oestrogen, which has a protective effect against
atherosclerosis. Evidence to support this theory is that incidence
of atherosclerosis in post-menopausal women rises to that of
men.
2. Women tend to have less stressful jobs / be at home more less
stress less hypertension, etc
Hypertension Speeds up atheroma formation AND causes endothelial
damage (which is the 1st step in atherosclerosis)
Smoking Nicotine is very, very good at damaging the endothelium.
Remember that next time youre tempted to dally behind the bike
shed
Inactivity Allsorts of factors here; - lower BMI = less
hypertension - fitter heart = less hypertension - exercise
decreases LDL levels - exercise increases metabolic rate lowering
BMI - Possibly some indirect contributing factors as well
if you exercise regularly you probably put stock in looking
after yourself are you likely to be smoking or drinking as
well?
Genetic predisposition Some alleles give you less protection
from / greater risk of developing atherosclerosis. To an extent, a
higher chance of getting atherosclerosis does run in families
Diet Millions of contributing factors here; - High salt intake
causes hypertension - Eating saturated fats decreases HDL level -
Eating more calories than you need causes BMI to
increase. High BMI is associated with atherosclerosis
- Alcohol causes hypertension directly
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.1.13
Drug treatments for atherosclerosis and their side effects;
Antihypertensives Diuretics The Loop of Henle is the part of the
nephron (in the kidney) that regulates water reabsorption.
Essentially, it puts Na+ back into the blood by active transport.
This lowers the water potential in the blood, so water follows the
Na+ by osmosis. Most diuretics block the protein that actively
transports the Na+, so less water is returned to the blood, thus
reducing the pressure. Three problems with this, however;
1. The blood gets more viscous, which makes the heart beat
harder
2. Dehydration can occur 3. Only treating the symptom
Blockers block the adrenaline receptor in the heart. This stops
the heart from beating harder in response to stress and, therefore,
reduces hypertension. There are some side effects in some cases
(e.g. sleep disturbance, depression, vasoconstriction of the
extremities) but generally theyre pretty good. One of the main
problems is bradycardia, which can become serious if you have CHD.
Can you explain why? Ca2+ channel blockers stop the heart muscle
from contracting too hard. You dont need to know why, but if youre
interested look up Starlings Law of the heart Major side effect is
arrhythmia, which can develop into fibrillation and infarction.
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Edexcel AS Biology Revision Notes Written by Tim Filtness
ACE Inhibitors are REALLY complicated, but I dont know how much
of this youre supposed to know, so here is the full version of
things Basically, our kidneys make Angiotensinogen all the time,
but it doesnt do anything itself (its not a hormone) it just
circulates in the blood. However, when we are hyoptensive (i.e.
have low blood pressure) the kidneys start to make Renin enzyme,
which turns Antiotensinogen into Angiotensin I. After this, ACE
enzymes (found in the endothelial cells lining arteries) quickly
turn the Antiotensin I into Angiotensin II, which is a powerful
hormone. It has the following effects;
1. General vasoconstriction 2. Causes the hypothalamus to
release ADH (look it up from
GCSE, it was in Unit 3), which increases water reabsorption by
the kidney
3. Stimulates the brain to release aldosterone, which causes the
kidneys to increase salt reabsorption, which in turn increases
water reabsorption.
All of these effects increase blood pressure, so ACE inhibitors
will, therefore, do the opposite. The major side effect is kidney
failure. Vasodilators dilate blood vessels, reducing blood
pressure. If this occurs too much you get hypotension, which can
cause heart attacks (not enough blood returns to the heart to fill
it properly)
Angiotensinogen Angiotensin I Angiotensin II Renin Enzyme ACE
Enzyme
A protein made by the kidneys, which circulates in the blood
An intermediate, also circulating in the blood
The important one! This is the hormone that increases blood
pressure!
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Edexcel AS Biology Revision Notes Written by Tim Filtness
Statins Two effects;
1. Block an enzyme in the liver that makes cholesterol. 2.
Remove LDL from the circulation
Associated with liver failure. Anticoagulants As the second
stage of atherosclerosis is associated with blood clotting
(thrombosis), anticoagulants block the clotting process. There are
many, many different ways of doing this. Blood clots slowly.
Platelet inhibitory drugs These work in the same way as
anticoagulants but target platelets, which are required to activate
the clotting process. They, therefore, have the same
side-effects.
1.1.14 Cholesterol is the major component in atheromas. High
blood cholesterol level is, therefore, a bad thing. We get
cholesterol from two sources;
1. Diet 2. It is made by the liver
Lipoproteins (also made by the liver) ferry cholesterol around
in the bloodstream and play a role in pushing the liver towards
making more cholesterol, or excreting more cholesterol. There are
two types of lipoprotein;
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Edexcel AS Biology Revision Notes Written by Tim Filtness
High Density Lipoproteins (HDLs) take cholesterol out of the
circulation to the liver, where it is converted into bile salts and
(ultimately) excreted. HDLs lower cholesterol levels. Low Density
Lipoproteins (LDLs) take cholesterol from the liver and put it into
the circulation to the liver. LDLs increase cholesterol levels.
Crudely
High HDL = good High LDL = bad High cholesterol = bad
1.1.15 You need to understand that scientists use their
scientific knowledge of the effects of diet, exercise and smoking
to try and predict risk of CVD and, therefore, to give people
advice about how to reduce their risk.
1.1.16
Dig up your Vitamin C Core Practical notes in the Practical
Handbook
1.1.17 Body Mass Index = Mass
(Height)2 Your energy budget balances the number of calories you
require with those that you consume. Ideally, they ought to be the
same.
Energy consumed > Energy expended mass gain Energy consumed
< Energy expended mass lost
BMI < 18.5 Underweight BMI between 18.5 and 25 Normal BMI
between 25 and 30 Overweight BMI > 40 Obese
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.1.18 & 1.1.19 You need to be able to analyze data on
mortality rates to determine health risk. Be careful! If two sets
of data follow the same pattern they are correlated
If two sets of data follow the same pattern because one factor
directly affects the other they are causal In order to assess
whether data is correlated or causal scientists experiment, the
idea being to try and falsify the Null Hypothesis that one factor
does not affect the other. However, be aware that the design of the
experiment often affects the results. Things to watch out for;
1. People selected were not representative of the population
(e.g. all students, all female, etc) i.e. not accurate
2. Only a few people were involved in the experiment (i.e. not
very reliable)
3. Not all the variables were controlled i.e. a systematic error
in the experiment (i.e. smokers included with non-smokers)
If you get a question on this section of the syllabus always
ask yourself WHERE HAS THE DATA COME FROM?
1.1.20 Why might peoples perception of risk be different from
the actual risk?
1. They dont understand the risk fully and underestimate it
(e.g. if you smoke your risk of CVD is X and if you are obese your
risk of CVD is Y. BUT if you are both your risk is not X + Y but XY
much greater!)
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Edexcel AS Biology Revision Notes Written by Tim Filtness
2. They dont understand the risk fully and overestimate it (e.g.
the person who thinks they actually might win the lottery this
week)
Broadly speaking, risk factors for CVD tend to be underestimated
because people dont realise that risk factors tend to be associated
with other i.e. if you smoke and drink and are obese, chances are
you also eat a diet high in saturated fat and salt. Quite quickly
the risks stack up
Oxygen Dissociation Curve This is not mentioned on the syllabus,
but it is in the text book. The prudent man learns it anyway
Remember, each Haemoglobin (Hb) can bind up to 4 O2 molecules. The
affinity of Hb for O2 changes depending on how many O2 are being
carried. A: The haemoglobin is in the lung and is O2 loading.
Affinity of Hb is high, therefore it fills up with O2 easily.
A B C
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Edexcel AS Biology Revision Notes Written by Tim Filtness
B: The haemoglobin is in the respiring tissues. Initially
affinity is high, so Hb does not give O2 away easily to tissues
that already have enough. However, when Hb gives up its 1st O2 the
affinity suddenly drops, so Hb tends to unload 3 O2 just where it
is required! C: With 3 O2 removed the affinity is high again, so
the last O2 is kept as an emergency. It is only given up if the Hb
passes through tissues with very low PO2 When the line shifts
position
1. Foetal Hb has a higher affinity than adult Hb. This is so the
foetus will load with O2 from the maternal Hb. Foetal ends with L,
therefore shifts to the LEFT
2. Llamas (starts with L) live at altitude and need to have Hb
with higher affinity to load O2 in the thin air.
3. Myoglobin (has an L in it) is an O2 store in muscles. It has
very, very high affinity for O2 so only gives off O2 when in the
emergency section of the graph. Whales and diving mammals have vast
quantities of myoglobin in their muscles.
4. Bohr (ends in R) shift occurs when Hb is exposed to acid. The
affinity drops and O2 is unloaded more easily. Acids tend to be
- carbonic acid (made from CO2) - lactic acid (made in anaerobic
respiration)
Both acids are produced when O2 is in short supply, so it makes
sense for Hb to give up more O2 in these circumstances.
End of Topic 1End of Topic 1End of Topic 1End of Topic 1
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Edexcel AS Biology Revision Notes Written by Tim Filtness
Unit 1: LifestyleUnit 1: LifestyleUnit 1: LifestyleUnit 1:
Lifestyle, Transport, Genes & , Transport, Genes & ,
Transport, Genes & , Transport, Genes &
HealthHealthHealthHealth
Topic 2: Genes & Health 1.2.2 Cell membranes are made from a
double layer (bilayer) of phospholipids, which align heads inwards
and tails outward because of their attraction / repulsion from
water. Sat in teh membrane are transmembrane proteins. The proteins
have a number of roles;
- channels into / out of the cell (see 1.2.4)
- receptors for hormones (tend to be glycoproteins)
- cellular glue joining adjacent cells together (look up
desmosomes if youre interested)
- anchors for the cytoskeleton
Edexcel AS Revision
Notes
Charged phosphate head hydrophilic
Uncharged fatty acid tails hydrophobic
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.2.3 Osmosis is the movement of water molecules from high
concentration to low concentration through a partially
permeable
membrane.
Water molecules cannot pass through the bilayer itself because
they are charged and are repulsed by the fatty acid tails. There
are a few theories about how the water actually gets through, but
these are the best so far;
1. Passes through special channels called aquaporins 2. Moves
through ion channel as ligands on ion complexes (e.g.
with Na+ or Mg2+)
1.2.4 How do molecules move in / out of the membrane?
1. Uncharged hydrophobic molecules (e.g. steroid hormones,
cholesterol, ethanol) pass freely between fatty acid tails by
diffusion
2. Large hydrophilic molecules (e.g. enzymes) move in by
endocytosis and out by exocytosis
3. Small hydrophilic molecules (e.g. glucose, mineral ions,
water) move in and out via proteins in the membrane. There are 3
types of these;
Channel Proteins
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Edexcel AS Biology Revision Notes Written by Tim Filtness
Movement is governed by molecules diffusing freely through the
channel. Sometimes the channel will only open under specific
circumstances (i.e. if a certain hormone is present, or under
certain environmental conditions e.g. temp, pressure etc). These
are gated channel proteins Facilitated Diffusion proteins Protein
channel has an active site specific to a particular hydrophilic
molecule. It attaches to the molecule, spins around in the membrane
and deposits it on the other side. Movement is governed by the
concentration gradient. Active Transport proteins As above, but the
movement is against the concentration gradient. Energy (in the form
of ATP) is required to get movement to occur.
1.2.5
Dig up your Beetroot Core Practical notes in the Practical
Handbook
1.2.6 Ficks law: Rate of Diffusion = Surface Area x Conc
Gradient
Distance
1111 3333 2222
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Edexcel AS Biology Revision Notes Written by Tim Filtness
You should be able to explain breathing in terms of volume
and
pressure changes in the Thoracic Cavity (GCSE idea)
Adaptations for rapid gas exchange (all related to Ficks
Law)
Remind yourself why humans need lungs in the first place,
why
cant we just breathe through our skin like Amoeba do?
Larynx (voicebox)
Bronchiole
Bronchus
Intercostal Muscle
Thoracic Cavity contained within pleural membranes
Diaphragm
Thachea
Ribs
Alveolus
Human Respiratory System
Element of Ficks Law Adaptation
Surface Area Each alveolus has a small SA, but there are
millions, which produces a huge total SA
Distance Each alveolus is one cell thick, as are the capillaries
which surround them. Therefore, the total diffusion distance is
only two cells!
Conc Gradient Ventilation maintains a constantly high PO2 in the
alveoli. Additionally, as soon as O2 has been collected by
haemoglobin the circulation removes it, therefore maintaining a low
PO2 in the blood. This keeps the concentration gradient high!
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.2.7
Amino acids are connected by peptide bonds. They are formed in
condensation reactions and broken up in hydrolysis reactions.
Primary Structure a long chain of amino acids connected by
peptide bonds. Most proteins do not function in their primary form
Secondary Structure the long chain of amino acids is folded into
two types of structure;
- Alpha helix - Beta sheet
Both are held together by hydrogen bonds
Test for Protein:
- Biuret solution turns blue purple halo in the presence of
protein
Proteins are polymers of amino acids. There are ~20 amino acids,
each of which has the same basic structure with a different
variable group (R group)
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Edexcel AS Biology Revision Notes Written by Tim Filtness
As the shape of a protein determines its function (think about
the active site of an enzyme, for example) it is really important
that all the bonds holding the shape together form in the right
places. The most important bonds are those that hold the 3o and 4o
structure together and these all form between R groups of specific
amino acids. Therefore;
The specific sequence of specific amino acids determines
the shape of the protein and, therefore, its function.
1.2.8 In the Lock and Key hypothesis, the active site and the
substrate are completely complementary.
1. Substrate diffuses into the active site 2. Substrate binds to
the active site 3. Bonds in the substrate are broken as a result 4.
Products form and unbind from the active site 5. Products diffuse
out of the active site
Tertiary Structure sections of secondary structure are folded up
further, forming a protein with a 3D shape. The shape is held
together by covalent bonds (e.g. disulphide bridges) between R
groups of specific amino acids. Quarternary Structure formed when
two or more tertiary proteins are combined e.g. haemoglobin is made
from 4 x haem proteins
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Edexcel AS Biology Revision Notes Written by Tim Filtness
In the Induced Fit hypothesis the mechanism of action is the
same except that the active site changes shape to fit the substrate
once the substrate has bound. The shape change causes bonds in the
substrate to break, forming the products. All enzymes work by
reducing the Activation Energy (Ea). This is the energy required to
get the reaction to start. Enzymes provide an alternate reaction
pathway (i.e. a different way for the reaction to happen in the
active site), which requires less energy to start.
1.2.9
Dig up your Enzyme Core Practical notes in the Practical
Handbook
1.2.10
DNA is made from many nucleotides joined together. It is,
therefore, a polynucleotide Each nucleotide contains 3 things;
(i) Sugar molecule, (ii) Nitrogenous base (iii) Phosphate group
(negatively charged)
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Edexcel AS Biology Revision Notes Written by Tim Filtness
There are 2 types of nucleotide;
(i) Ribose nucleotides - make RNA (ii) Deoxyribone nucltodies -
make DNA
DNA nucleotides have 2H atoms on the C2 carbon atom
RNA nucleotides have an H and an OH on the C2 carbon Other
differences: RNA is single stranded, DNA is double stranded RNA has
different bases RNA used to make 3 different things (mRNA, tRNA
& rRNA),
DNA only used to determine genetic code DNA only found in
nucleus, RNA in nucleus & cytoplasm
Polynucleotides are formed by connecting the phosphate group of
one nucleotide with the 3rd carbon atom of another, forming the
Sugar-Phosphate Backbone
DNA is made from 2 strands of DNA polynucleotides, held together
by hydrogen bonds between the bases. Because the strands face in
opposite directions DNA is an antiparallel molecule.
H / OH
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.2.11 DNA Synthesis: The proof that DNA Replication is
semi-conservative (rather than conservative, or dispersive the
other theories) was provided by Meselson & Stahl. Their
experiment is shown on the next page (you need to know what they
did), but you should be able to interpret their results as
follows;
RNA
Adenine (A) pairs with Uracil (T) Guanine (G) pairs with
Cytosine (C)
DNA
Adenine (A) pairs with Thymine (T) Guanine (G) pairs with
Cytosine (C)
DNA synthesis is semi-conservative (i.e. half of each new strand
is old DNA & half is new DNA)
1. Helicase unwinds the DNA forming the replication fork.
2. New nucleotides diffuse into the fork and hydrogen bind with
their complementary partners
3. DNA Polymerase joins the nucleotides together forming the new
sugar phosphate backbone
You can make this more complicated by looking closely at what
happens on the lagging strand, but you dont need to know it (if
youre interested look up Okazaki fragments)
Original DNA, all heavy DNA band at bottom of centrifuge
1st generation DNA, old, new DNA band in middle of
centrifuge
2nd generation DNA, some and (forms one band at top) & some
all new second band in the middle of centrifuge. No other theory
predicts formation of TWO BANDS
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.2.12
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.2.12 & 1.1.13 The genetic code is read from the sequence
of bases in the DNA. Each of the ~30,000 instructions in the code
is a gene and tells the body how to make one specific protein.
Genes, therefore, code for proteins. The genetic code is read in
sequences of 3 bases, called codons each codon represents one
specific amino acid. e.g. in this gene the code is ATG CCA CTA GCA
CGC, which corresponds to the following amino acids
1.2.14
Protein Synthesis occurs in two stages;
(i) Transcription
(ii) Translation
A Gene
A Protein
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Edexcel AS Biology Revision Notes Written by Tim Filtness
Transcription: Takes place in nucleus A complementary copy of
the gene is made using RNA 1. Gene opens up. Hydrogen bonds break
between bases
2. RNA nucleotides attracted to complementary bases and form
hydrogen bonds.
3. RNA nucleotides joined together by enzyme RNA Polymerase.
4. Complementary RNA copy of gene now made. It is called mRNA
(messenger RNA)
5. Single stranded mRNA molecule diffuses out of gene
6. mRNA molecule leaves nucleus through nuclear pore (large
holes in nuclear envelope)
7. Many mRNA strands are made before gene closes.
MRNA is complementary, not a copy!
DNA TAC GAA TCT GAG CAC GGC TAT ATC
mRNA. AUG CUU AGA CUC GUG CCG AUA UAG
Translation: Takes place in cytoplasm MRNA code read by ribosome
and amino acids assembled in
correct order to make protein 1. mRNA strand binds to cleft in
ribosome. Start AUG codon fits
into bottom of P site
2. tRNA diffuses into P site and recognises the mRNA codon using
its specific anticodon
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Edexcel AS Biology Revision Notes Written by Tim Filtness
3. A second tRNA diffuses into the A site and recognises the
mRNA codon there.
4. The amino acids between the two tRNAs join together forming a
peptide bond
5. The tRNA in the P site diffuses into the cytoplasm and binds
to another specific amino acid.
6. The ribosome moves one codon down the mRNA chain so that the
P site is filled with the tRNA from the A site and the A site is
empty
7. When the ribosome reaches the stop codon it releases the mRNA
and the amino acid chain.
Most ribosomes translate whilst attached to the rER. The
completed primary protein is inserted into the rER, where enzymes
fold it into its secondary and tertiary shape. Many ribosomes can
translate the same piece of mRNA at the same time. A polysome
forms
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Edexcel AS Biology Revision Notes Written by Tim Filtness
A Mutation = a change in the genetic code. By changing the
genetic code mutations ultimately change the sequence of amino
acids in primary proteins. This changes the sequence of R groups in
the protein and, therefore, the way in which the protein folds up.
This affects on the function of the protein
Any mutation in the CFTR gene that stops / impairs the function
of the CFTR protein causes Cystic Fibrosis. To date over 2000
different mutations have been catalogued, each of which causes
CF.
Mutation Explanation
Neutral mutations
The function of the protein is unchanged after the mutation
(i.e. the protein still does its job as well as it did before the
mutation). There are 2 possible causes;
One codon is altered. However, the codon still codes for the
same amino acid. Therefore, the protein is the same
A codon is changed, leading to a different amino acid in the
primary protein. However, this protein is not in a place crucial
for folding, so the protein is still the same shape and functions
the same.
Deletion mutations A nucleotide is deleted from the DNA code,
which changes all the codons after the deletion. This causes
frame-shift.
Insertion mutations A nucleotide is inserted into the DNA code,
which changes all the codons after the insertion. This causes
frame-shift.
Frame-shift
mutations
All the codons in the sequence are altered, meaning that every
amino acid after the addition / deletion is different. Normally,
this has a huge impact on the function of the protein.
Non-sense mutations
One of the altered codons in the frame-shifted sequence changes
to become a stop codon. Protein synthesis stops half-way through
the gene, resulting in only half of the protein being made. Almost
always the protein does not function.
1.2.15
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.2.16
Key Definitions:
Gene: a sequence of DNA coding for a specific protein Allele: an
alternative version of a gene Genotype: the pair of alleles an
individual possesses Phenotype: the physical appearance Recessive:
allele does not affect the phenotype in the presence of a Dominant
allele Dominant: always affects the phenotype Homozygote:
individual possesses two copies of the same allele Heterozygote:
individual possesses two different alleles
A Genetic Diagram
Parents Phenotype: Brown eyes Brown eyes
Parents Genotype: B b B b
Gametes: F1 Genotype: B B B b B b b b F1 Phenotype: 3 : 1 Brown
eyes : blue eyes
Note the gametes are always put in circles
B b B b
B b
B
b
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Edexcel AS Biology Revision Notes Written by Tim Filtness
You also need to interpret inheritance problems involving seed
morphology (shape) and plant height.
1.2.17
Goblet cells secrete mucus onto the surface of the epithelium,
which lines the lungs. Epithelial cells regulate the water content
of mucus. In the alveoli mucus is very watery to allow it to be
wafted easily by cilia. However, higher up the lungs water is drawn
out of mucus to reduce its volume: one cannot fit the mucus from 6
small bronchioles into one larger one, so water is removed. In
Cystic Fibrosis the mechanism controlling the water content of the
mucus does not work properly and the water removal process is
constantly switched on in all parts of the lung. This means the
mucus is too sticky in the alveoli and cannot be wafted.
Disease Heritability Effect
Sickle Cell Anaemia Recessive
A mutation in the haemoglobin genes Cause haemoglobin molecules
to stick together inside red blood cells. RBCs become distorted
into a sickle shape. They can become stuck inside capillaries
leading to clots and stroke. RBCs have limited oxygen carrying
capacity.
Thalassaemia
Recessive
A mutation in (usually) the gene coding for alpha haem causes
very slow haemoglobin production. This results in anaemia and
reduced haemocrit (% RBCs per unit volume of blood). Regular
transfusions are required.
Achondroplasia
Dominant
Caused by a mutation in one of genes controlling collagen
production in bones. As a result bone growth plates fuse too early,
leading to shortening of the long bones. Homozygous dominant
genotype is fatal.
Albinism
Recessive
A mutation in the gene coding for melanin protein stops
melanocytes from producing melanin. Melanin colours hair, skin etc
and provides protection from UV rays.
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Edexcel AS Biology Revision Notes Written by Tim Filtness
Normally:
Cystic Fibrosis:
The sticky mucus causes the effects of CF and affects;
a) Lungs, b) Digestive system c) Reproductive system
Cl-
Na+
Mucus
Tissue Fluid
Water
Water
Cl-
Na+
Mucus
Tissue Fluid
Water
Water
X
Cl- moves out of epithelial cells into mucus via the CFTR
protein. Na+ is drawn into mucus to balance the charge. Na+ passes
between epithelial cells. The combined effect of Na+ and Cl-
reduces water concentration making hypertonic mucus. Water is drawn
into mucus by osmosis and mucus is dilute.
CFTR is blocked or absent, so Cl- stays inside epithelial cells.
Na+ does not move into mucus as there is no charge to balance.
Mucus is hypotonic. Water is drawn into epithelial cells by osmosis
and mucus is sticky.
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.2.18
The problem with genetic diseases is that they are caused by a
mutation that is present in every cell of the body. In order to
cure the disease you need to change the DNA in every cell of the
body, which is impossible. However, in the case of CF because the
CFTR protein is only transcribed by epithelial cells (the cells
lining the lungs, digestive system and reproductive tracts) only
these cells need to be targeted. So how can you change DNA inside
living cells? Answer: use gene therapy, which attempts to add a
normal copy of the CFTR gene to the DNA inside epithelial cells.
Gene Therapy (in humans no plasmids are used) Step 1: cut out a
working copy of the gene from normal DNA using a restriction
enzyme. OR use reverse transcriptase enzyme to make a copy of the
gene from CFTR mRNA
Step 2: add the gene to a vector, which will insert the new gene
into the DNA of the target cell
Step 3: hope the gene is successfully incorporated in the DNA in
the nucleus
Tissue Effect of CF
Lungs
Mucus produced is too sticky and blocks the alveoli. This makes
the person breathless. The mucus also provides ideal conditions for
bacteria, so chest infections are common.
Reproductive system
Mucus blocks the vas deferens in boys and the fallopian tubes in
girls, making the individual infertile
Digestion
Mucus blocks the bile duct and the pancreatic duct. Enzymes do
not reach the small intestine and food is not digested
properly.
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Edexcel AS Biology Revision Notes Written by Tim Filtness
Vectors are used to get the working gene into the epithelial
cells. Somatic Cells = Body cells. Somatic Cell Gene Therapy
therefore only affects the targeted cells
Germ Cells = Gametes. Germ Cell Gene Therapy therefore affects
the entire organism that is produced when the gamete is fertilised.
NB: Genetic engineering of bacteria is different and involves
plasmids and DNA ligase enzyme as well (look it up)
1.2.19
Genetic screening is used to determine whether a person has a
genetic disease or not.
Vector Explanation
Liposome
An artificial vesicle. A little bubble of membrane in which the
CFTR gene is placed. When the liposome is inhaled the gene can
enter the epithelial cell by endocytosis.
Virus
Viruses naturally insert their own DNA into host cells DNA. So,
if we remove the viral DNA and replace it with the CFTR gene that
ought to be inserted instead.
Method Summary
Amniocentesis
A long needle is inserted through the mothers abdomen into the
amniotic fluid of the developing embryo. As this is produced by the
embryo it will contain embryionic cells and, therefore, embryo
DNA
Chorionic villus
sampling
As above, but cells are taken from the placenta, which is also
made by the embryo.
Pre-implantation
genetic diagnosis
(PIGD)
Gametes are fertilized in vitro (outside of the body) and the
resultant embryos are then tested. Only embryos known NOT to have
the disease are implanted in the uterus.
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Edexcel AS Biology Revision Notes Written by Tim Filtness
1.2.20
End of Topic 2End of Topic 2End of Topic 2End of Topic 2
Advantages of genetic testing Can opt for termination Can get
counselling Can buy special medical
equipment / care in preparation for birth
Can opt not to have children (if parents are tested)
Utilitarian argument
Disdvantages of genetic testing Abortion is morally wrong Tests
can be inaccurate Small chance of test
resulting in miscarriage Unnatural procedure Embryos right to
life Embryos cannot give
informed consent