-
1.1.1
In small unicellular organisms, substances move around slowly by
diffusion.
Diffusion is too slow to move substances round the larger bodies
of multicellular organisms. They have a circulatory system:
substances are carried in blood pumped by a heart.
In a closed circulatory system (eg in vertebrates) blood is
enclosed in narrow blood vessels. This increases efficiency: blood
travels faster as a higher pressure is generated.
Valves ensure blood flows in one direction:
Fish have a single circulation: heart pumps blood to gills for
gas exchange, then to tissues and back to the heart.
Birds and mammals have a double circulation: right ventricle
pumps blood to lungs. Blood returns to the left atrium and then the
left ventricle pumps it to the rest of the body. Blood travels
round the body faster, delivering nutrients faster, so the animals
have a higher metabolic rate.
1.1.2
Arteries and veins contain collagen: a tough, fibrous protein to
make them tough and durable.
The artery wall stretches as blood is pumped in and then recoils
as the heart relaxes. Blood flow is continual and there is a
pulse.
Contracting muscles and low pressure in the chest when breathing
in assist blood flow in veins. Valves prevent backflow. There is no
pulse and pressure is low.
See diagrams and photomicrographs: Figure 1.10 on page 8 of the
textbook.
1
heart arteries arterioles
veins venules capillaries
-
Arteries Veins
narrow lumen
thicker walls
more collagen, elastic fibres and
smooth muscle
no valves
wide lumen
thinner walls
less collagen, elastic fibres and smooth
muscle
valves
1.1.3
Figure 1.9 on page 8 of the textbook: make sure you know the
structure of the heart.
The chambers of the heart (atria and ventricles) fill with blood
when they relax (diastole) and pump blood out when they contract
(systole).
The cardiac muscle making up the atria and ventricles is
supplied with blood by the coronary arteries.
PHASE OF CARDIAC CYCLE DETAIL
Atrial systole Pressure in the atria increases as they fill with
blood returning from the veins.
Increased pressure opens the atrioventricular valves allowing
blood to enter the ventricles.
The atria contract to force remaining blood into ventricles.
Ventricular systole Ventricles contract from the base up,
increasing the pressure and closing the atrioventricular
valves.
The semilunar valves open and blood is forced into the
arteries.
Diastole As the atria and ventricles relax, pressure falls.
In the ventricle, this causes closure of the semilunar
valves.
In the atria blood is drawn into the heart from the veins.
2
-
1.1.4
Atherosclerosis: a disease process where fatty deposits block an
artery or increase its chances of being blocked by a blood clot
(thrombosis)
How atherosclerosis (hardening of the arteries) occurs:
In the arteries supplying the heart, this causes a heart attack
(myocardial infarction).
In the arteries supplying the brain, it causes a stroke.
An infarction is when tissue dies due to a lack of oxygen.
This is usually the result of a lack of blood ischaemia.
1.1.5
Blood clots when it flows very slowly, or when blood vessel
walls are damaged.
A blood clot consists of cells trapped in a mesh of insoluble
fibrin protein.
When platelets come into contact with the vessel wall, they
become spiky they stick to each other and the collagen in the wall:
a platelet plug is formed.
See Figure 1.14 on page 13 and make sure you understand the
roles of thromboplastin, prothrombin, thrombin, fibrinogen and
fibrin in the blood clotting process.
1.1.6
3
Lining (endothelial) cells damaged eg by high blood pressure or
cigarette smoke toxins.
Inflammation occurs white blood cells move into the artery wall.
They accumulate cholesterol. A deposit (atheroma) builds up.
Calcium salts and fibrous tissue build up in the atheroma, now
called a plaque. Artery is less elastic it has hardened.
Blood pressure increases in narrowed artery. Positive feedback
causes more damage to endothelial cells.
-
Symptoms of cardiovascular disease:
Coronary heart disease
Early symptoms shortness of breath angina chest pain on exertion
irregular heartbeat no symptoms, but changes on ECG
Heart attack crushing pain in chest which may spread around the
body eg into arms or back
indigestion-type pain with dizziness no detectable symptoms
Stroke
Full stroke numbness or paralysis on opposite side of body
(slurred speech, dribbling mouth, drooping eyelid or mouth)
dizziness, blurred or loss of vision confusion
Mini-stroke (transient ischaemic attack)
same as for full stroke, but only temporary
4
-
The following factors increase a persons risk of developing
cardiovascular disease:
GENETIC
This is not straightforward, but risk is increased if your
parents have CVD.
DIET
some vitamins act as antioxidants, reducing the damaging effects
of free radicals
high salt levels cause the kidneys to retain water, increasing
blood pressure
AGE
More likely as you get older.
GENDER
Incidence is much higher for men than women.
HIGH BLOOD PRESSURE
SMOKING
carbon monoxide prevents haemoglobin from carrying sufficient O2
heart rate increases
nicotine stimulates adrenaline release, increasing heart rate
and blood pressure
chemicals damage endothelium triggering atherosclerosis
decreased levels of HDLs
INACTIVITY
most common risk factor
exercise can halve the risk of developing CHD
reduces blood pressure
STRESS
Leads to increased blood pressure, poor diet and increased
alcohol consumption.
ALCOHOL
Heavy drinkers have an increased risk of CHD as alcohol raises
blood pressure, contributes to obesity and causes irregular
heartbeat. It also increases levels of LDLs.Moderate amounts of
alcohol may increase HDL levels.
5
-
1.1.7
Blood pressure is a measure of the hydrostatic force of the
blood on the walls of a blood vessel.
It is higher in arteries and capillaries than in veins.
Systolic blood pressure is highest and occurs when the
ventricles contract.
Pressure is at its lowest in the arteries when the ventricles
relax: diastolic blood pressure.
Both are measured, using a sphygmomanometer, in mmHg eg
120/80.
Any factor which causes arteries or arterioles to constrict will
lead to high blood pressure or hypertension.
These include:
loss of elasticity with age atherosclerosis adrenaline high salt
diet.
High blood pressure caused by atherosclerosis leads to a
worsening of the condition!
Tissue fluid
At the arterial end of a capillary, the blood pressure forces
tissue fluid (water + small molecules dissolved in it) out through
the capillary wall.
At the venous end, blood pressure is lower and fluid is no
longer forced out.
As the blood is more concentrated here (because of water loss
and the presence of plasma proteins) fluid moves back in by
osmosis.
20% of the tissue fluid returns to the circulation via the lymph
system.
Hypertension causes more fluid to be forced out. The fluid
accumulates in the tissues causing oedema.
6
-
See Figure 1.30 on page 28 for an explanation of how tissue
fluid is formed.
1.1.8
Cardiac muscle contracts without being stimulated by a nerve
impulse.
The electrical charge in the heart muscle cells changes
depolarisation. This spreads from cell to cell (like a wave)
causing them to contract.
Depolarisation starts in the sinoatrial node or SAN (pacemaker)
in the right atrium and spreads across the left and right atria
causing them to contract.
The atria are electrically insulated from the ventricles so the
wave of depolarisation converges on the atrioventricular node
(AVN).
It then travels down the Bundle of His in the septum and into
the Purkyne fibres which then make the ventricles contract from the
bottom upwards pushing blood into the aorta and pulmonary
artery.
When the cells are depolarised, there is a small electrical
current detectable on the skin.
This is measured in an electrocardiogram or ECG, which can be
used to diagnose cardiovascular disease, problems with the
conducting system or irregular heartbeat rhythms (arrhythmias).
P wave depolarisation of the atria causing atrial systole
PR interval time taken for impulses to travel from SAN, through
AVN to ventricles.
QRS complex depolarisation of the ventricles causing ventricular
systole
T wave repolarisation of the ventricles leading to ventricular
diastole
1.1.9
Risk is the probability of occurrence of some unwanted event or
outcome.
A time period is always quoted eg children in a class having a 1
in 5 (0.2 or 20%) risk of catching head lice in a year.
Not all individuals are at risk to the same degree.
7
-
Risk factors increase the chance of the harmful outcome.
Factors that contribute to health risks include:
heredity
physical environment
social environment
lifestyle and behaviour choices
Two factors are positively correlated if an increase in one is
accompanied by an increase in the other eg the number of people
suffering sunburn and the amount of ice cream sold.
A positive correlation does not necessarily mean that the two
are causally linked!
1.1.10
Peoples behaviour is affected by the perception of risk.
They overestimate the risk of something happening if the risk is
not under their control, unnatural, unfamiliar, dreaded, unfair or
very small.
There is a tendency to underestimate the risk if it has an
effect in the long-term future eg health risks associated with
smoking.
When data is lacking to estimate the risk, the outcome is
uncertain.
1.1.11 Carbohydrates, proteins, lipids and alcohol all contain
energy: used to be measured in calories; the SI unit is the Joule.
Average person requires 8000-10000 kiloJoules per day.
The Department of Health issues Dietary Reference Values to
encourage balanced & healthy diets and to indicate the amount
of energy which should be derived from different foods.
The basal metabolic rate is the energy required to maintain life
processes and varies between individuals.
BMR is higher in males and people who are younger, heavier or
more active.
8
-
Eating fewer kilojoules than you use results in weight
loss.Eating more kilojoules than you use results in a gain in
weight.
1.1.12
Carbohydrates are a large family of compounds with the general
formula Cx(H20)n
monosaccharides(monomers)
single sugar units glucose used in respiration
fructose found in fruit & honey
galactose found in lactose
(all the above are hexose sugars: C6H12O6)
disaccharides 2 single sugar units combinedmaltose(2 glucose
molecules)
found in germinating seeds eg barley
sucrose(glucose and fructose)
crystals used in cooking
lactose(glucose and galactose)
sugar found in milk
oligosaccharides 3-10 sugar units found in vegetables eg leeks,
lentils, beans
polysaccharides(polymers)
long chains of glucose molecules starch
25% amylose(unbranched & spiral)
starch is found stored in plants: compact and insoluble with
little osmotic effect.
75% amylopectin(branched)
glycogen branched stored in animals and bacteria
Cellulose is also a polysaccharide long chains of a slightly
different form of glucose.9
-
Make sure you can recognise the structural formulae for glucose,
maltose, fructose and galactose molecules see pages 32 and 33.
1.1.13
When monosaccharides join together, they are linked by a
glycosidic bond.
This is formed by a condensation reaction during which water is
given off.
Glycosidic bonds are broken in hydrolysis. Water is required for
the reaction to take place.
1.1.14
Lipids contain the elements carbon, hydrogen and oxygen. They
are insoluble in water.
They provide twice as much energy as carbohydrates and supply
the body with essential fatty acids. Vitamins are often found
dissolved in lipids.
The most common type are triglycerides: made up of 3 fatty acids
joined to 1 glycerol:
When the molecules join together, a condensation reaction takes
place.
Ester bonds are formed.
Saturated fatty acids contain the maximum number of hydrogen
atoms and no carbon-carbon double bonds. Found in animal fats and
dairy products.
Monounsaturated fats contain 1 double bond eg in olive oil.
Polyunsaturated fats contain a larger number of double bonds eg
vegetable and fish oils.
If one of the fatty acids in a triglyceride is replaced with a
phosphate group, a phospholipid is formed. These molecules make up
part of the cell membrane.
10
GLYCEROL
fatty acid
fatty acid
fatty acid
-
Cholesterol is a short lipid molecule with a structure very
different to a triglyceride. Important for cell membranes, sex
hormones and bile salts. Found in food, associated with saturated
fats.
1.1.15
Body mass index (BMI) is a method of classifying body weight
relative to height.
body mass / kgBMI =
height2 / m2
Normal range is around 20. Less than this is underweight and
over 30, obese.
20% of the population are obese excess dietary fat and
inactivity are the likely causes.
Obesity increases the risk of cardiovascular disease and Type II
diabetes.
1.1.16
It is estimated that around 46% of deaths from coronary heart
disease in the UK are due to blood cholesterol levels of more than
5.2 mmol per litre.
Insoluble cholesterol is transported combined with proteins to
form soluble lipoproteins.
high-density lipoproteins or HDLs
contain more protein and transport unsaturated fats to the liver
where they are broken down
reduce blood cholesterol deposition
low-density lipoproteins or LDLs (the main blood cholesterol
carriers)
associated with saturated fats overload membrane receptors and
reduce cholesterol absorption from the blood
associated with the formation of atherosclerotic plaques
Saturated fats also reduce the activity of LDL membrane
receptors and therefore increase blood cholesterol levels.
11
-
Eating both monounsaturated and polyunsaturated fats reduces the
level of LDLs in the blood.
1.1.17
Practical on the effect of caffeine on heart rate in
Daphnia.
1.1.18
A persons risk of developing coronary heart disease can be
reduced by:
DIET
should be energy balanced
reduced cholesterol, saturated fats and salt
more polyunsaturated fats, including omega-3 fatty acids found
in oily fish
more fruit and vegetables containing soluble fibre and
antioxidants
include food with added sterols and stanols (plant compounds
which reduce cholesterol)
EXERCISE
A person who is physically active is much more likely to survive
a heart attack or stroke.
STOP SMOKING
After stopping, the risk of CHD is almost halved after one
year.
CONTROLLING BLOOD PRESSURE
Can be achieved by changes in lifestyle and diet, but drugs such
as antihypertensives and blockers can be used.
12
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1.2.1
In larger organisms, there is a reduced surface area to volume
ratio, which presents a problem for the exchange of substances
between the organism and its environment.
The respiratory system provides a large surface area to volume
ratio to ensure efficient gas exchange.
Ficks law explains that:
surface area x difference in concentrationRate of diffusion
thickness of the gas exchange surface
In the respiratory system:
the alveoli provide a large surface area
circulation of blood through numerous capillaries and efficient
ventilation of the lungs maintains an effective concentration
gradient
flattened epithelial cells making up the walls of the alveoli
and capillaries (which are very close together) reduce the distance
gases travel between air and blood
Look at Figure 2.2A on page 52 to revise the structure of the
respiratory system.
1.2.2.
The cell membrane is made up of a phospholipid bilayer.
The phosphate head of the phospholipid is polar and attracts
water it is hydrophilic.
The fatty acid tails are hydrophobic.
13
GLYCEROL
fatty acid
fatty acid
phosphategroup
-
In the cell membrane, the hydrophobic tails face inwards to
avoid water, while the hydrophilic heads point outwards.
In the phospholipid bilayer are other molecules:
proteins: some are fixed, while others move around. May be
enzymes, carriers or
channels.
cholesterol: reduces the fluidity of membrane by preventing
movement of
phospholipids.
glycoproteins: (polysaccharide + protein) cell recognition and
receptors
glycolipids: (polysaccharide + lipid) cell recognition and
receptors
1.2.3
Practical on the effect of temperature on membrane
structure.
1.2.4
Osmosis is the movement of water molecules from an area where
they are in high concentration to an area of lower concentration
through a partially permeable membrane.
Water molecules form hydrogen bonds with solutes, reducing the
movement of the water molecules.
1.2.5
Diffusion is the movement of molecules or ions from an area of
their high concentration to an area of their low concentration.
It will continue until the substance is evenly distributed
throughout the whole volume.
Small uncharged molecules eg oxygen and carbon dioxide can
diffuse across the cell membrane.
Hydrophilic molecules and ions cannot penetrate the hydrophobic
phospholipid tails.
Diffusion is made easier, or facilitated, by proteins:
channel proteins span the membrane and have a specific shape to
transport specific particles. Some are gated they can be open or
closed.
14
-
carrier proteins bind with the molecule or ion, change shape and
transport the particle across the membrane. Movement can occur in
either direction, depending on the concentration gradient.
Diffusion, facilitated diffusion and osmosis are passive they do
not require energy.
In active transport, ATP supplies energy to change the shape of
a carrier protein moleculewhen substances are moved against the
concentration gradient ie from low to high concentration.
Exocytosis involves the bulk transport of substances out of the
cell eg insulin into the blood.
Vesicles (little membrane sacs) fuse with the cell surface
membrane and the contents are released.
Endocytosis is the reverse: substances are taken into a cell by
the creation of a vesicle.
1.2.6
DNA is a type of nucleic acid called deoxyribonucleic acid.It is
a long chain molecule made up of nucleotides.
One nucleotide is made up of:
-a 5 carbon sugar
-a phosphate group
-an organic base
Nucleotides link together by condensation reactions between the
sugar of one and the phosphate group of the other.
Each nucleotide in DNA has 1 of 4 different bases: Adenine,
Guanine, Cytosine or Thymine.
Two long polynucleotide strands, running in opposite directions,
are held together by hydrogen bonds between the bases.
This ladder-like structure, with alternating sugar and phosphate
molecules forming the uprights and pairs of bases forming the
rungs, is then twisted in a helix.
15
-
The bases pair in a particular way, based on their shape and
chemical structure:
A & T pair forming 2 hydrogen bonds C & G pair forming 3
hydrogen bonds
RNA (ribonucleic acid) is made up a single strand of
nucleotides. In these the sugar is called ribose and the bases are
adenine, guanine, cytosine and uracil (not thymine).
There are 3 types of RNA:
messenger RNA (mRNA)
transfer RNA (tRNA)
ribosomal RNA (rRNA)
1.2.7
The sequence of bases in the DNA of the chromosomes acts as a
coded recipe for making proteins.
TRANSCRIPTION
occurs in the nucleus, catalysed by RNA polymerase
DNA helix unwinds, hydrogen bonds break and RNA nucleotides pair
with the exposed bases on the template strand of the DNA
3 bases on the DNA (triplet) are transcribed into 3 bases on the
RNA (codon)
the messenger RNA (mRNA) molecule formed enters the cytoplasm
through a nuclear pore
TRANSLATION
occurs on the ribosomes of the rough endoplasmic reticulum
the beginning of the sequence is always marked with the start
codon AUG which codes for the amino acid methionine
a transfer RNA molecule (tRNA) with 3 bases exposed (an
anticodon) pairs with a specific codon on the mRNA
16
adenine thymine guanine cytosine
-
attached to the tRNA molecule is a specific amino acid
the amino acids, arranged in the order dictated by the mRNA
codons, are joined with peptide bonds to form a polypeptide
a stop codon signals the last amino acid in the polypeptide
chain
base triplets in DNA
transcription (in the nucleus)
codons in mRNA
translation (on the ribosomes)
amino acid sequence in polypeptide chain
See Figure 2.36 on page 79 for a more detailed explanation.
1.2.8
The genetic code in the DNA making up the chromosomes acts as a
code for protein synthesis.
It dictates the amino acids required to make the protein and the
order in which they should be bonded together.
3 bases code for 1 amino acid and these base triplets are
non-overlapping.
The code is degenerate: there is more than 1 triplet for each
amino acid.
A gene is a sequence of bases on a DNA molecule (ie a short
section of a chromosome) coding for a sequence of amino acids in a
polypeptide chain.
1.2.9
Structure of an amino acid:
17
residual or R group different in each
amino acid
-
RH O
N C CH OH
H
20 different amino acids are found commonly in the proteins of
living organisms.
The amino acid monomers join together in a condensation reaction
to form peptide bonds.The polymer formed is called a
polypeptide.
Proteins are made up of one or more polypeptides.
Primary structure the sequence of amino acids in the polypeptide
chain
Secondary structure the shape the molecule folds into as a
result of hydrogen bonding between the C=O of one amino acid and
the N-H of the amine group of another an helix or a pleated
sheet
Tertiary structure the final 3D shape of the molecule, held
together by ionic bonds, interactions between hydrophilic R groups
and strong disulphide bridges between R groups containing
sulphur
Quaternary structure if the protein contains more than one
polypeptide chain
Fibrous proteins remain as long chains, often with several
polypeptides cross-linked for extra strength.
They are insoluble and are important structural molecules eg
keratin, collagen.
Globular proteins are folded into a compact spherical shape.
They are soluble and are important metabolic molecules eg
enzymes, antibodies and some hormones.
1.2.10
Enzymes are globular proteins which act as catalysts. They speed
up chemical reactions by lowering the activation energy, and remain
unchanged at the end of the reaction.
18
amine groupcarboxylic acid
group
-
Part of the molecule is a specifically shaped active site, into
which a substrate fits to form an enzyme-substrate complex.
The lock and key hypothesis suggested an exact match between the
shapes of the substrate and active site.
The induced fit hypothesis describes the active site moulding
around the substrate once it is in place.
1.2.11
An increase in temperature (and therefore an increase in the
kinetic energy of the molecules) increases the likelihood of a
collision between enzyme and substrate molecules.
The rate of reaction increases.
Beyond the optimum temperature, the increased vibration of the
atoms in the protein molecule break the bonds maintaining the
tertiary structure.
The active site of the enzyme is irreversibly destroyed or
denatured.
pH changes around the enzymes optimum pH, alter the charge
distribution in the active site, reducing the compatibility of
enzyme and substrate.
Tertiary structure bonds are again affected and extreme changes
will denature the enzyme.
An increase in either substrate or enzyme concentration will
increase the rate of reaction until the other acts as a limiting
factor.
1.2.12
DNA copying or replication must occur before a cell divides to
ensure that daughter cells receive a copy of the genetic code.
DNA double helix unwinds
hydrogen bonds between the base pairs break
free DNA nucleotides line up along side each strand
hydrogen bonds form between complementary bases
DNA polymerase links adjacent nucleotides
2 identical DNA double helices are formed by this
semi-conservative replication
1.2.1319
-
Sometimes, the DNA replication does not work perfectly an
incorrect base may slip into place.This is called a gene
mutation.
If this occurs in a sperm or ovum which ultimately forms a
zygote, every cell in the new organism will carry the mutation.
If the mutation occurs in non-coding DNA, it will have no
effect.
In a gene, it will cause an error in the mRNA and an incorrect
amino acid may be included in the polypeptide chain causing a
genetic disorder eg sickle cell anaemia.
A number of different mutations can affect the gene coding for
the cystic fibrosis transmembrane regulatory (CFTR) protein
channels, which allow chloride ions to pass through the
membrane.
The most common mutation is a deletion of 3 nucleotides
resulting in the loss of the 508th amino acid in the protein.
The altered protein may not open, or may reduce the flow of
chloride ions through the channel.
1.2.14
Human cells contain 23 pairs of homologous chromosomes. At a
particular position or locus on each of the pair is found a gene
for a particular characteristic.
Different forms of the same gene are called alleles. If a cell
contains two copies of an allele, their genotype is described as
homozygous. Different alleles at a locus result in a heterozygous
condition.
The characteristic resulting from the genotype is the organisms
phenotype.
A recessive allele (represented by a small case letter eg f) is
only expressed in the homozygous condition.
A dominant allele (represented by the same letter in the upper
case eg F) will be expressed in the phenotype in either the
homozygous or heterozygous condition.
See page 85 on how to set out a monohybrid genetic cross.
In the 19th century, Gregor Mendel initiated the study of
genetics using the garden pea. He established patterns of
inheritance of a number of phenotypes including height and the
morphology of seeds.
20
-
In humans, recessive mutations of single genes result in:
cystic fibrosis: mucus which is too viscous
thalassaemia: abnormal haemoglobin formation
albinism: lack of pigment production
1.2.15
In the respiratory system, the amount of water in the mucus
produced must be regulated:
too runny and it floods the airway too viscous (sticky) and it
cant be cleared by the cilia
This is controlled by the transport of sodium and chloride ions
across the epithelial cells.Water follows the ions because of
osmosis.
See Figure 2.19 on page 67 for a full explanation of why in
cystic fibrosis, the mucus is too viscous.
Summary:
the CFTR channel is non-functional, so chloride ions cannot pass
out of the cell towards the lumen
the sodium ion channels are open and sodium ions are continually
absorbed from the mucus
water is drawn out of the mucus by osmosis and it becomes much
too viscous
The cilia cannot move the viscous mucus it builds up in the
airway and becomes infected.
Because of low oxygen levels in the mucus, anaerobic bacteria
thrive.
White blood cells invade the mucus, then die and release DNA
making it even more viscous.
Mucus blocks the bronchioles, reducing the number of ventilated
alveoli. This reduces the efficiency of gas exchange.
In the digestive system, the viscous mucus blocks the pancreatic
duct.
21
-
Enzymes are not released into the small intestine and food is
therefore not digested effectively. Undigested food cannot be
absorbed and energy is lost in the faeces (malabsorption
syndrome).
CF also affects the reproductive system:
in females, a mucus plug blocks the cervix in males, the vas
deferens leading from the testes is either blocked or missing
1.2.16
Gene therapy attempts to alter the genotype and phenotype of
target cells:
normal alleles inserted into target cell using viruses or
liposomes (see below)
normal form of gene transcribed and translated
functioning protein produced by target cell
Using viruses: Viral DNA for replication is deleted and replaced
with normal allele.A gene promoter is required to initiate
transcription and translation.Produces side effects eg headache,
fever, increased heart rate.
Using liposomes: Normal allele inserted into a plasmid, which is
then combined with the liposome (a spherical phospholipid
bilayer).
Patient breathes in aerosol containing the liposomes and the DNA
is carried into the target cells.
In CF trials, chloride transport in respiratory epithelial cells
has been restored to 25% of normal. Treatment is temporary as
epithelial cells are constantly lost.
Altering specific somatic cells (body cells) like this is
permitted in the UK.
Altering germ cells (sperm and eggs) is known as germ line
therapy and is not legal.
1.2.17
Practical on using gel electrophoresis to separate DNA
fragments.
22
-
Electrophoresis is a technique which can separate DNA fragments
of different lengths:
restriction endonucleases cut the DNA into fragments at specific
base sequences
fragments placed on a gel connected to electrodes
fragments separate according to their size and charge
fragments are transferred to a nylon filter (Southern
blotting)
strands of the DNA helix are separated by an alkaline buffer
the desired sequence is identified using a gene probe
image obtained by placing the radioactive probe next to X-ray
film
A gene probe is a short, radioactive base sequence,
complementary to the base sequence of the gene.
See Figure 2.44 on page 91 for a full explanation of this
technique.
There is a large number of mutations responsible for the
abnormal CFTR protein in cystic fibrosis. A gene probe identifies
one specific base sequence. While a positive result will confirm a
diagnosis, a negative result must be treated with caution!
1.2.19
Uses of genetic screening:
identifying carriers: heterozygotes with normal phenotypes. This
can be followed up with counselling to help potential parents make
a decision.
embryo testing: a sample of cells from a developing fetus can be
analysed. The sample is obtained either by amniocentesis
(withdrawing amniotic fluid around 15-17 weeks of pregnancy) or by
chorionic villus sampling (cells removed from the placenta at 8-12
weeks).
Both techniques carry a risk of miscarriage.
pre-implantation genetic diagnosis: used to test an embryo
created by IVF.
23
-
1.2.20
Genetic screening has obvious advantages, but is a contentious
business! You need to consider the social, ethical, moral and
cultural issues related to the process.
2.3.1
Organelle Structure and function
nucleus enclosed in double membrane with pores contains
chromosomes with genes made of DNA to control
protein synthesis
ribosomes made of RNA and protein free in cytoplasm or attached
to RER site of protein synthesis
rough endoplasmic reticulum
interconnected sacs with ribosomes attached transport proteins
to other parts of cell
smooth endoplasmic reticulum
synthesis of lipids and steroids
mitochondria double membrane inner folded into cristae site of
later stages of aerobic respiration
24
-
centrioles one pair found in animal cells made of protein
microtubules involved in spindle formation and cellular
transport
lysosomes digestive enzymes wrapped in membrane breakdown of
unwanted structures or old cells
nucleolus dense body in nucleus synthesis of ribosomes
2.3.2
Proteins synthesised on the ribosomes of the RER are moved to
other parts of the cell through the cavities of the endoplasmic
reticulum.
The Golgi apparatus is a stack of membrane-bound sacs formed
from fused vesicles from the ER.
Proteins are modified here and packaged in vesicles. Some eg
enzymes and hormones are released from the cell.
See Figure 3.9 on page 101.
2.3.3
The cells described above, with membrane-bound organelles are
eukaryotic.
Organisms with eukaryotic cells are classified into 4 kingdoms:
Animals, Plants, Fungi and Protoctists.
The 5th kingdom is the Prokaryotes, with prokaryotic cells
which:
are smaller than eukaryotic cells
have no membrane-bound organelles
25
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have no nucleus
have circular DNA, not associated with protein
have small rings of DNA, called plasmids
always have a cell wall
To compare prokaryotic & eukaryotic cells, see Figures 3.4
and 3.8 on pages 98 & 100.
2.3.4
Mitosis is a type of cell division, which retains the full or
diploid number (2n) of chromosomes.
In humans, a cell with 46 chromosomes divides to form 2
identical daughter cells, each with 46 chromosomes.
Before nuclear division, a copy of each chromosome is made by
semi-conservative replication of the DNA. Each double helix is
called a chromatid.
These stages are part of the cell cycle:
inte
rpha
se
G1 (first gap phase) synthesis of cellular proteins and
organelles
S (synthesis phase) replication of DNA
G2 (second gap phase) synthesis of spindle proteins
26
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divi
sion
mitosis (nuclear division) separation of the 2 DNA helices
making up the chromosome
cytoplasmic division cleavage of a single cell into two daughter
cells
2.3.5
Mitosis, with identical daughter cells, ensures genetic
stability - important for:
growth: development from a single cell to a multicellular
organism
repair: regeneration of lost or damaged parts or replacement of
old or damaged cells
asexual reproduction eg budding in Hydra, vegetative
reproduction in plants
2.3.6
Cell division is a continuous process, but 4 stages of mitosis
(nuclear division) can be described:
prophase chromosomes condense (get shorter and thicker)
microtubules are organised into a spindle by the centrioles nuclear
membrane breaks down
metaphase the centromeres of the chromosomes attach to the
spindle at the equator
anaphase centromeres split spindle fibres pull chromatids to
opposite poles spindle breaks down
telophase chromosomes unravel two nuclear envelopes form
27
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Make sure you are familiar with the details of the core
practical in which you observed the stages of mitosis.
2.3.7
The sex cells or gametes are adapted for sexual
reproduction.
OVUM large cell, incapable of independent movement wafted along
oviducts by cilia and muscular contractions of the tubes cytoplasm
contains protein and lipid food reserves surrounded by a jelly-like
coat the zona pellucida which hardens after
one sperm penetrates ovum preventing any others entering
SPERM smaller than the ovum and motile (it can move) long tail
for swimming, powered by energy released by mitochondria head
contains acrosome (package of digestive enzymes) to break down
the
zona pellucida
2.3.8
At fertilisation (in the oviducts) the sperm nucleus enters the
ovum and fuses with its nucleus forming a zygote.
The diploid number is restored and the cell contains genetic
information from both parents.
2.3.9
Gametes are produced in the ovaries and testes of animals by
meiosis which:
produces haploid cells (contain half the number of chromosomes
found in a body cell: one of each homologous pair)
creates genetic variation among offspring
During meiosis, pairs of homologous chromosomes line up at the
equator.
As either of the pair can end up at either pole (random
assortment), genetically variable gametes are produced.
28
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2.3.10
Fuelled by nutrients from the ovum, the zygote divides rapidly
to form smaller cells the embryo remains the same size.
After 3 divisions, there are 8 totipotent stem cells each could
form a total human being.
After 5 days, a blastocyst (a hollow ball of cells) is
formed:
the outer cell layer forms the placenta
the inner are pluripotent embryonic stem cells (each can form
most, but not all cell types)
As the embryo develops, cells differentiate and become more
specialised.
Most lose the ability to develop into a wide range of cell
types, but some dont: they are multipotent stem cells.
2.3.11
Stem cells, isolated from embryos could provide new cells,
tissues or organs for transplantation.
Opinion varies according to the status accorded to a human
embryo.
A significant number of people consider the use of an embryo for
research purposes morally and ethically unacceptable.
UK research is regulated by the Human Fertilisation and
Embryology Authority (HFEA)
Bills passed in 2001 and 2002 allow spare embryos from IVF
treatment to be used as a source of stem cells for research into
serious diseases.
2.3.12
The specialised function of a cell depends upon the proteins it
synthesises ie which genes are expressed.
29
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Transcription of a gene is initiated by RNA polymerase and
transcription factors binding to a promoter region (section of DNA
adjacent to gene).
RNA polymerase + transcription factors = transcription
initiation complex
Some transcription factors are always present in all cells.
Others are only synthesised in certain cells at a particular
stage of development, often in an inactive form, which is later
activated by signal proteins.
Signal proteins may act directly by entering the cell or
indirectly through a second messenger.
See Figure 3.33 on page 122.
The gene remains switched off until all the transcription
factors, in their active form, are present.
Transcription of a gene can be prevented by protein repressor
molecules, which prevent attachment of the transcription initiation
complex.
2.3.13
Sometimes the gene for an enzyme required for the metabolism of
a particular substrate can be expressed only when that substrate is
present (induction) eg galactosidase and lactose in
prokaryotes.
See core practical on this topic.
2.4.14
Differences in phenotype between members of a population are
caused by:
genetic make-up (genotype)
the environment in which the individual develops
Some are due completely to genotype eg blood groups and show
discontinuous variation: they fall into discrete categories with no
overlap.
Others are influenced by both genotype and environment and show
continuous variation eg human height, skin and hair colour,
cancer.
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See Figure 3.38 on page 127.
Human height
average height has increased in the past 150 years for various
reasons.
a person may have genes for being tall, but not achieve their
potential height because of malnutrition.
Skin andhair colour
the pigment is called melanin and is made from tyrosine in a
reaction catalysed by the enzyme tyrosinase.
melanin is made by melanocytes activated by
melanocyte-stimulating hormone (MSH).
UV light increases the amount of MSH and the number of MSH
receptors on the melanocytes.
melanin (packaged as melanosomes) transferred to neighbouring
skin cells and surrounds the nucleus, protecting the DNA from
harmful UV light.
variation is skin colour is affected not only by exposure to UV
light, but also by the number of MSH receptors in skin cells.
albinos have a gene mutation preventing the production of
melanin they have white skin, white hair and no pigment in their
iris and retina.
some animals have mutant alleles for tyrosinase so that the
unstable enzymes only works in cooler areas: extremities are
darker.
2.3.15
Cancer occurs when the rate of cell multiplication is faster
than the rate of cell death.This causes the growth of a tumour.
Cancer is caused by environmental damage to DNA from
physical factors such as UV light and asbestos
31
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chemical carcinogens such as those in the tar in cigarette
smoke
viruses may trigger cancer by altering the DNA
Chemicals called radicals are produced by the cell metabolism
and can damage DNA. Fresh fruit and vegetables contain antioxidants
to destroy radicals.
The cause may also be genetic. About 5% of cancers are due to an
inherited gene.
The progression through the cell cycle (G1, S, G2, M) is
controlled by:
oncogenes which stimulate the cycle. Mutations can result in the
cycle being continually active and lead to excessive cell division
and tumour formation
tumour suppressor genes which stop the cycle. Mutations mean
there is no brake on the cycle and control is lost.
If tumours are not removed, cancer cells can spread to other
parts of the body through the blood and lymphatic systems. This is
called metastasis.
2.3.16
A genome is all the DNA of an organism or species.
In 2001, the Human Genome Project published a working draft of
the sequence of bases in human cells. Work continues to identify
specific genes and establish their function.
Detailed information about the genome
30 000 40 000 genes average human gene contains 3000 bases
non-coding sequences (junk DNA) makes of
50% 1.4 millions locations of single nucleotide
polymorphisms
Identification of new genes breast cancer gene total colour
blindness gene genes analysed for mutations causing disease
32
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Identification of new drug targets a molecule that a drug
interacts with identification of genes allows identification of
drug targets
Preventative medicine and improved drug treatment
variation in base sequences may account for why some people
experience side effects from drug therapies
identification of mutations associated with a particular disease
allows patient to make lifestyle changes or adopt preventative drug
therapy
Understanding basic biology receptor proteins in the sense of
taste post-production processing of proteins
Investigating evolution repeat sequences replicate and insert
themselves into the DNA modifying, reshuffling and creating new
genes
comparisons with the genome of other organisms establishes
evolutionary pathways
Part of the budget for the HGP has been set aside to address the
ethical, legal and social issues which may arise from the
project:
should health insurance companies have access to information
about genetic
predisposition of potential clients to particular
conditions?
when, and on whom should predisposition tests be carried
out?
who keeps this information confidential?
should scientists have the right to patent particular
sequences?
how will treatment made possible by the project be paid for?
is it acceptable to destroy embryos found to contain mutant
genes?
is it acceptable to select embryos on the basis of desirable
characteristics?
inserting genes into embryos (germ line gene therapy) presents
many risks
should genes be transferred between species for transplantation
purposes?33
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2.4.1
Water is a polar molecule: the hydrogen end is slightly positive
and the oxygen end is slightly negative. The positive end of one
molecule is attracted to the negative end of another - hydrogen
bonding.
This cohesion (attraction between like molecules) is important
in transporting water through plants. It also creates surface
tension useful for supporting organisms eg pondweed, pond
skaters.
Hydrogen bonding affects the properties of water eg it explains
why water is liquid at normal biological temperatures.
It also means that the amount of energy required to raise the
temperature of water is high. This avoids large changes of
temperature inside living organisms.
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Ionic substances eg NaCl and polar molecules eg sugars dissolve
in water. This is vital for chemical reactions to occur and for the
transport of substances in living organisms.
Water is often a reactant eg in hydrolysis reactions and
photosynthesis.
Water expands as it freezes. The density of ice is less than
liquid water, so ice floats enabling organisms to live in liquid
water under ice in frozen ponds and lakes.
Plants also require inorganic ions, absorbed through the roots
and transported in the xylem:
Nitrates Used by cells to manufacture amino acids/proteins,
nucleic acids, ATP and growth substances.
Calcium Important constituent of cells walls and affects the
permeability of the cell membrane.
Magnesium Required for chlorophyll production a deficiency
results in yellowing of older leaves.
2.4.2
See Figure 4.5 on page 148 the ultrastructure of a generalised
plant cell.
Compare this with Figure 3.8 on page 100 the ultrastructure of a
generalised animal cell.
Organelle Comments
Cell wall Rigid structure composed mostly of the polysaccharide
cellulose.Fully permeable to salts and water.
Chloroplasts Contain mixture of pigments (chlorophyll). Site of
photosynthesis, where solar energy is converted into chemical
energy.
Amyloplasts Storage vacuoles containing insoluble starch
grains.
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Tonoplast The membrane surrounding the large, central
vacuole.
Vacuole Contains cell sap: a concentrated solutions of salts,
sugars, pigments. Important in determining osmotic properties of
the cell.
Plasmodesmata Fine thread of cytoplasm linking neighbouring
cells.
Pits Points in the cell wall with only a thin layer of cellulose
where plasmodesmata are found.
Middle lamellae The region between cell walls of neighbouring
cells which cements them together. Contains pectins eg calcium and
magnesium pectates.
2.4.3
See Figure 4.7 on page 149 showing the structure of and
glucose.
Starch and cellulose are two important polysaccharides in
plants.
Starch Cellulose
Made up of glucose monomer. Made up of glucose monomers.
Contains 1,4 and 1,6 glycosidic bonds ie there is
side-branching.
Contains 1,4 glycosidic bonds only ie no side-branching.
Used as a storage carbohydrate. Used as a structural
carbohydrate to form the cell wall.
Winds into a spiral shape. Remains as a long, straight
chain.
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Hydrogen bonds form between the OH groups of adjacent cellulose
chains. A bundle of about 70 cellulose molecules linked in this way
creates a microfibril.
The microfibrils are wound around the cell at different angles
and stuck together with a polysaccharide glue made of
hemicelluloses and pectins.
This composite structure makes the cell wall strong and
flexible.
2.4.4
To compete effectively for light, plants must grow tall. This
presents two problems:
they must be mechanically supported they must be able to
transport water and inorganic ions up to the leaves
Xylem vessels do both; sclerenchyma fibres assist with
support.
Xylem vessels (together with phloem sieve tubes) form vascular
bundles.
The sclerenchyma fibres are found on the outside of the
bundle.
Look at Figure 4.13 on page 154 and know the location of the
vascular bundles in the stem.
The polymer lignin gives strength to the structures and renders
them waterproof.
Because plants fibres are long and thin, flexible and strong,
they have been used by humans for thousands of years eg for
clothing, rope, floor coverings, paper.
Extracting fibres is called retting bacteria/fungi, enzymes and
in some cases caustic alkali breaks down the polysaccharides
holding the fibres together, leaving the more resistant fibres
intact.
Plant fibres are used to absorb heavy metals and oil spillages.
They can be combined with plastic to form biocomposites.
2.4.5
Xylem vessels Sclerenchyma fibres
made up of large cells with thick cell elongated cells
37
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walls form a column of cells to transport
water and inorganic ions waterproofed and strengthened by
the
polymer lignin laid down in spirals or rings
dead tissue formed from previously living cells
sole function to provide support and mechanical strength
cell wall heavily thickened with lignin which provides great
tensile and compressional strength*
dead tissue formed from previously living cells
*tensile strength means it doesnt break easily on stretching;
compressional strength means it doesnt buckle easily.
2.4.6
Water evaporates from the surface of the spongy mesophyll cells
and diffuses down the diffusion gradient through the stomata of the
leaves. This is called transpiration.
Water in these cells is replaced from the xylem, lowering the
hydrostatic pressure at the top of the vessel. This results in
water being drawn up from below: the transpiration stream.
Because of hydrogen bonding causing cohesion between water
molecules, water moves up the stem in a continuous column: the
cohesion-tension theory. Thick xylem walls prevent them from
collapsing.
There is adhesion (attraction between unlike molecules) between
the water and the xylem walls. The narrow xylem vessels have a high
surface area to volume ratio so that the high adhesive forces hold
the column of water within the tube.
The rate of transpiration increases as:
temperature increases windspeed increases humidity decreases
surface area and number of stomata in leaf increases when stomata
are open ie in sunlight
2.4.7
Practical on extracting fibres from nettles and testing their
strength (Activity 4.6)
2.4.8
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Plants contain many antibacterial compounds eg allicin in
garlic. Many medicines are derived from plants eg aspirin from
willow bark, morphine from poppies.
In 1775, Dr William Withering published A Treatise on the
Foxglove. He bought the recipe for a herbal cure for oedema
(accumulation of fluid in the tissues) from a patient and used it
on an unpredictable hit and miss basis as a treatment for the
condition.
He began with a low dose and increased it until the patient
suffered side effects. An amount slightly less than this was
considered the ideal dose.
The extract from the foxglove plant, Digitalis purpurea, is now
marketed as a drug called digitalin and is used to treat heart
disease.
New drugs are now tested extensively before marketing it can
take over 10 years.
Pre-clinical testing Laboratory and animal testing
Clinical testing I Small group of healthy volunteers assess how
the body deals with the drug
Clinical testing II Small group of volunteer patients are
treated to assess effectiveness.
Clinical testing III Large group of patients divided into two
for double-blind trial ie neither doctor nor patient knows if
theyre given the drug or an inactive placebo.
2.4.9
Practical on the antibacterial properties of plants. (Activity
4.7)
2.4.10
A seed contains an embryonic plant with its own food supply,
inside a protective coat.
When conditions are suitable (water, oxygen, warmth), they
re-start growth: germination. They absorb water through the
micropyle causing the cells to expand and rupture the seed
coat.
Water triggers metabolic changes: growth substances are
activated and enzymes (amylase, maltase, lipase and protease) are
released to digest stored food.
39
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Seeds are vital to the survival of a plant as they:
protect the embryo by means of a lignified seed coat (testa)
aid dispersal to avoid competition with the parent plant
provide nutrition for the new plant
When the ovule in a flowering plant is fertilised by the nucleus
in a pollen grain, it develops into a seed. This happens inside the
ovary, which develops into a fruit.
The embryo plant consists of three parts:
a young root (radicle) a young shoot (plumule) one or two seed
leaves (cotyledons)
Some seeds store food in endosperm tissue rather than in the
cotyledons.
Some seeds germinate as soon as conditions are suitable. Others
are dormant and must be activated by eg:
an extended period of chilling intense heat mechanical abrasion
or microbial degradation of the seed coat a minimum period of light
chemical action in an animals gut
Seeds are adapted for dispersal:
Method Adaptation Example
Wind Small light seeds with wings or parachutes sycamore,
dandelion
Animal Hooked fruits, succulent fruits burdock, blackberry
Water Fibrous seeds coats with lots of air coconut,
waterlily
Self Explosive rupture of seed coat (dehiscence) peas,
laburnum
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2.4.11
Seeds (particularly of cereal crops) are useful in animal and
human diets. Carbohydrate polymers and oils also have major
industrial uses.
Uses of starch Uses of oils
Thickening agent: when heated, starch granules absorb water and
thicken the liquid (gelatinisation) eg custard, wallpaper
paste.
With little water and high temperature and pressure, starch
puffs into an expanded structure eg cereals, corn snacks,
packaging.
Dried, cross-linked starch is a super-absorbent used in nappies
and tampons.
Other uses include glues, plaster, hair mousses and
antiperspirants.
Widely used in cooking.
Can be used as a fuel eg castor oil & peanut oil were both
used to power the first diesel engine.
Hydrolysis of oils with alkali produces fatty acid salts (soaps)
and glycerol (used in paint manufacture.)
Sustainability means we can keep using the resources in the long
term without harming the environment.
The use of oil-based plastics and fuels is not sustainable
as:
they release carbon dioxide and contribute to global warming oil
reserves will eventually run out they generate non-biodegradable
waste
Burning plant-based fuels also produces carbon dioxide, but it
was recently absorbed when the plants grew. However, there are
still problems eg:
paper bags are less strong than plastic bags and disintegrate
when wet degradation of waste requires aerobic organisms, so little
happens in deep landfill sites closer to the surface, methane (a
greenhouse gas) is often produced
41
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2.4.12
Artificial selection involves choosing plants with advantageous
features and then breeding them eg by self-pollinating or saving
seeds from one year to plant during the following year. This has
gone on for thousands of years, but is a very slow process.
New plants are produced by spontaneous mutation, which may be
induced by chemicals or radiation. Many die, but some are fertile
and useful.
If a plant doesnt normally self-pollinate, inbreeding depression
can occur: a loss of size, yield and fertility.
Two inbred lines can be crossed resulting in hybrid vigour:
plants more vigorous than either parent.
Hybridising two different species of plants is possible: wheat
currently used in bread making was produced in this way.
In the 1980s, genetic modification was developed, allowing
specific characteristics to be rapidly introduced to a species a
faster and more efficient method of artificial selection.
A plant is genetically modified by introducing a new gene using
an infective bacterium or virus, or by shooting into the plant
minute pellets covered in DNA.
Antibiotic resistance marker genes are used to identify
successfully modified cells, which are then micropropagated to
produce parent plants.
See Figure 4.40 on page 176.
2.4.13 & 2.4.14
Arguments for Arguments against
Improved plant quality eg tomatoes with PG inhibited, which stay
firmer for longer.
Increased yield of crops eg by reducing competition with weeds
in Roundup Ready crops. These are plants which have been modified
to contain a resistance gene to glyphosate, so that competitors are
destroyed, but they remain.
So long as food is clearly labelled, people
Creation of antibiotic resistant microbes by using marker
genes.
Altered genes creating toxic or allergenic substances in the
plant.
Transgenic plants or plants to which resistance genes have been
transferred could prove very difficult to manage and keep under
control.
Increased herbicide use to control resistant 42
-
have the choice of eating GM products, or not.
crops.
Companies hold patents for the GM crops and developing countries
cant afford them.
2.4.15
The atmosphere is a thin layer of gases extending 100km above
the Earths surface. It keeps the Earths average temperature stable
and suitable for living organisms.
1. The Sun radiates energy (mostly visible light) and the Earth
absorbs some of it.
2. Earth warms up and radiates infra-red back into space.
3. Some is absorbed by greenhouse gases and the atmosphere (and
the Earth) is warmed.
The main greenhouse gases are:
water vapour carbon dioxide methane nitrous oxide CFCs.
Although methane absorbs more infrared radiation than carbon
dioxide does, it breaks down quicker and there is less of it.
Carbon dioxide Methane
Relative abundance 3.7 x 10-2 1.8 x 10-4
Greenhouse factor 1 20
Sources respiration in plants, animals and decomposers
increased combustion of fossil fuels
anaerobic decomposition eg in bogs, paddy fields, landfill
sites
digestive system of cattle incomplete combustion of
fossil fuels
43
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How levels might be controlled
Reduction in deforestation and burning of trees
Reduced combustion of fossil fuels eg in aircraft, oil and coal
based power stations, cars and public transport.
better waste recycling
using methane as a biofuel (burns to produce two less serious
greenhouse gases)
2.4.16
See Figure 4.68 on page 206 for a full diagram of the carbon
cycle.
Two factors are likely to be mainly responsible for the
imbalance in the carbon cycle and the increased levels in carbon
dioxide concentration:
combustion of fossil fuels: coal is formed over millions of
years from plants which photosynthesised converting CO2 into
carbohydrate. It remains as a carbon sink until the CO2 is released
back into the atmosphere through combustion.
deforestation: mature forests are stable releasing the same
amount of CO2 through respiration (and decay) as they absorb in
photosynthesis. Cutting the trees down and either burning them or
leaving them to decay adds CO2 to the air.
Other minor factors affecting CO2 levels:
Increase in acid rain eroding limestone Incorporation of into
calcium carbonate shells in marine organisms Volcanoes producing
carbon dioxide
Carbon dioxide levels are not rising as fast as calculations
predict. This may be because:
increased levels stimulate photosynthesis more is dissolving in
the ocean more is stored as organic compounds in the soil
44
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A biofuel eg wood, straw, dried chicken litter is any source of
energy produced, directly in plants or indirectly in animals, by
recent photosynthesis.
This provides a renewable energy source and is carbon dioxide
neutral. When combusted, there is no net increase in CO2 levels,
unless transporting the biofuel involves combustion of fossil
fuels.
In Brazil, alcohol produced from the refining of sugar cane is
added to petrol to make gasohol.
Methane produced from anaerobic fermentation of human sewage or
animal slurry is an effective biogas.
Reafforestation involved planting young trees which, because of
rapid growth, absorb a lot of CO2 for photosynthesis. As the forest
matures, in will no longer be a net absorber.
However, as higher temperatures and increased CO2 levels
stimulate photosynthesis:
there will be more food and therefore more animals respiring
more respiring microbes develop
2.4.17
Practical on investigating how carbon dioxide may affect global
warming (Activity 4.23)
2.4.18
Evidence for global warming comes from a range of sources. Look
carefully at the graphs and tables of data on pages 190-195 you
need to be able to describe and analyse them.
TEMPERATURE RECORDS
Some have been kept since the 17th century they are useful
although not as accurate as current data.
PEAT BOGS
Climate information for up to 12000 years ago can be obtained by
studying plant and insect remains, the decay of which has been
slowed or stopped by the anaerobic or acidic conditions in a peat
bog.
PollenVast amounts of pollen, protected by a tough outer layer
can be carbon dated to indicate the species of trees which grew in
a particular period of time. Since different
45
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species flourish in different environmental conditions,
information on the climate at that time can be discerned.
Beetles On the same basis, climatic information can be obtained
from the exoskeletons of preserved bog beetles, which responded to
climate change faster than plants.
DENDROCHRONOLOGY
The study of tree rings gives a clue to past climatic
conditions. Each year a new layer of xylem is laid down: wide
vessels in spring, narrower vessels in summer. The wider the ring,
the more the tree grew more than likely because the conditions were
warmer or wetter.
ICE SAMPLES
Bubbles of air trapped in ice can be analysed to estimate carbon
dioxide levels. The ratio of different oxygen isotopes gives an
indication of the temperature at that time.
A combination of information from these sources helps provide
evidence to support the various theories on global warming which
have been proposed.
2.4.19
Actual data gathered can be extrapolated to predict future
changes. While a straight line graph is easy to extrapolate, a
curve is best dealt with by a computer.
These predictions dont account for change in the future period
of time eg reductions in carbon dioxide emissions, or increased
levels due to better living conditions in developing nations.
Global warming due to increased carbon dioxide levels is only
one factor which may affect climate change.
Other include:
other greenhouse gases eg methane, CFCs and nitrous oxide
aerosols extremely small liquid particles in the atmosphere cloud
cover the fraction of the earth covered with ice and snow and the
consequent reflection
Modelling climate change is done by computer on programmes which
take all these factors into account and predict the interaction
between them.
46
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Several major climate models are in use, but they differ (one
predicts a fall of 50C for the UK and another a rise of 50C) and
have limitations due to:
limited data limited knowledge of how the climate system works
limited computer resources failure to consider all factors
affecting climate change changing trends in snow/ice cover and CO2
emissions
2.4.20
The major aspects to climate change are:
changing temperatures
changing rainfall patterns
changing seasonal cycles
These affect living organisms in the following ways:
DISTRIBUTION OF SPECIES
A community is a group of species found in the same place at the
same time. Climate change affects the balance between species: some
flourish and become dominant in the new conditions, others die. If
they are mobile, or have good seed dispersal the distribution of
the species may change. Pests and diseases may also spread to new
areas.
Examples are described on pages 184 and 185.
ALTERED DEVELOPMENT and LIFE CYCLES
Plant growth is determined mainly by the rate of photosynthesis.
This is affected by the interaction of a number of factors such as
temperature, carbon dioxide concentration and light intensity
according to the law of limiting factors.
Overall, crop production in cooler climates will benefit from
climate change, whereas warmer tropical regions may suffer from
poorer yields.
Spawning, hatching and growth rates in animals eg trout are
often cued by temperature. In these fish, growth ceases when a
critical temperature is reached, so global warming could result in
underweight organisms.
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In reptiles, the male:female ratio could be affected, as this is
determined by temperature.
Phenology is the study of natural events in the lives of animals
and plants eg time of flowering, fruiting, egg laying, hatching,
migration. These events are frequently related to seasonal
change.
Life cycles of organisms are intricately related eg hatching of
marine worm eggs to coincide with a high level of phytoplankton.
The eggs hatch in response to day length (photoperiodism) while the
phytoplankton grow in response to temperature. A mismatch in timing
could result in a lack of food supply for the worms.
2.4.21
The rate of metabolic reactions is controlled by enzymes, which
are temperature dependent.
A reaction occurs when the substrate binds with the active site
of the enzyme forming an enzyme-substrate complex.
The likelihood of this happening depends on a collision
occurring between the two molecules and this is determined by their
kinetic energy ie how fast they are moving.
Up to a certain point, increasing temperature increases the rate
of reaction. The kinetic energy of substrate and enzyme molecules
is increased, they collide more frequently and more ES complexes
are formed.
After the optimum temperature (at which the rate of reaction is
highest)increasing temperature causes the atoms in the enzyme to
vibrate. Bonds holding its 3D structure in place break. The active
site changes shape so that the substrate no longer fits in the
enzyme is denatured.
2.4.22
Practical on the effects of temperature on the development of
brine shrimps. (Activity 4.18)
2.4.23
Climate change is a controversial issue with major political and
economic implications.
Major decisions on eg reducing CO2 emissions need to be
determined at governmental level and with international agreement
eg the Kyoto Protocol.
48
-
While scientific method aims to be objective, the evidence in
this case is limited, arguably imprecise and open to differing
interpretation by different people.
Business interests, political manoeuvring, cultural and ethical
issues all influence the way conclusions are drawn and action
implemented.
49
TRANSCRIPTIONTRANSLATIONSee Figure 2.36 on page 79 for a more
detailed explanation.