Download SNAB Unit 1 Subject Information File

2012
Nobel School AS Biology
Teachers
Mrs Bryant
Mr Jarvis
Mr Shibli
Mrs Starkey
Topic 1 - Lifestyle, health & risk
1 Demonstrate knowledge and understanding of the practical and investigative skills identified
in numbers 4 and 5 in the table of How Science Works on page 13 of this specification.
6 Explain why many animals have a heart and circulation (mass transport to overcome
limitations of diffusion in meeting the requirements of organisms).
2 Explain the importance of water as a solvent in transport, including its dipole nature.
8 Explain how the structures of blood vessels (capillaries, arteries and veins) relate to their
functions.
7 Describe the cardiac cycle (atrial systole, ventricular systole and diastole) and relate the
structure and operation of the mammalian heart to its function, including the major blood
vessels.
11 Explain the course of events that leads to atherosclerosis (endothelial damage,
inflammatory response, plaque formation, raised blood pressure).
10 Describe the blood clotting process (thromboplastin release, conversion of prothrombin to
thrombin and fibrinogen to fibrin) and its role in cardiovascular disease (CVD).
12 Describe the factors that increase the risk of CVD (genetic, diet, age, gender, high blood
pressure, smoking and inactivity).
18 Analyse and interpret quantitative data on illness and mortality rates to determine health
risks (including distinguishing between correlation and causation and recognizing conflicting
evidence).
19 Evaluate design of studies used to determine health risk factors (including sample selection
and sample size used to collect data that is both valid and reliable).
20 Explain why people’s perceptions of risks are often different from the actual risks (including
underestimating and overestimating the risks due to diet and other lifestyle factors
in the development of heart disease).
17 Analyse data on energy budgets and diet so as to be able to discuss the consequences of
energy imbalance, including weight loss, weight gain, and development of obesity.
3 Distinguish between monosaccharides, disaccharides and polysaccharides (glycogen and
starch – amylose and amylopectin) and relate their structures to their roles in
providing and storing energy (β-glucose and cellulose are not required in this topic).
4 Describe how monosaccharides join to form disaccharides (sucrose, lactose and maltose) and
polysaccharides (glycogen and amylose) through condensation reactions
forming glycosidic bonds, and how these can be split through hydrolysis reactions.
5 Describe the synthesis of a triglyceride by the formation of ester bonds during condensation
reactions between glycerol and three fatty acids and recognise differences between
saturated and unsaturated lipids.
14 Analyse and interpret data on the possible significance for health of blood cholesterol levels
and levels of high-density lipoproteins (HDLs) and low-density lipoproteins (LDLs). Describe the
evidence for a causal relationship between blood cholesterol levels (total cholesterol and LDL
cholesterol) and CVD.
9 Describe how the effect of caffeine on heart rate in Daphnia can be investigated practically,
and discuss whether there are ethical issues in the use of invertebrates.
16 Describe how to investigate the vitamin C content of food and drink.
1
Revision
Notes
Unit 1 – Lifestyle, Transport, Genes and Health
  
15 Discuss how people use scientific knowledge about the effects of diet (including obesity
indicators), exercise and smoking to reduce their risk of coronary heart disease.
13 Describe the benefits and risks of treatments for CVD (antihypertensives, plant statins,
anticoagulants and platelet inhibitory drugs).
Unit 1 – Lifestyle, Transport, Genes and Health
Topic 2 – Genes and Health
1 Demonstrate knowledge and understanding of the practical and investigative skills identified
in numbers 4 and 5 in the table of How Science Works on page 13 of this specification.
6 Describe the properties of gas exchange surfaces in living organisms (large surface area to
volume ratio, thickness of surface, difference in concentration) and explain how the
structure of the mammalian lung is adapted for rapid gaseous exchange.
2 Explain how models such as the fluid mosaic model of cell membranes are interpretations of
data used to develop scientific explanations of the structure and properties of cell
membranes.
5 Describe how membrane structure can be investigated practically, eg by the effect of
alcohol concentration or temperature on membrane permeability.
3 Explain what is meant by osmosis in terms of the movement of free water molecules through
a partially permeable membrane (consideration of water potential is not required).
4 Explain what is meant by passive transport (diffusion, facilitated diffusion), active transport
(including the role of ATP), endocytosis and exocytosis and describe the involvement of carrier
and channel proteins in membrane transport.
10 Describe the basic structure of mononucleotides (as a deoxyribose or ribose linked to a
phosphate and a base, ie thymine, uracil, cytosine, adenine or guanine) and the structures of
DNA and RNA (as polynucleotides composed of mononucleotides linked through condensation
reactions) and describe how complementary base pairing and the hydrogen bonding between
two complementary strands are involved in the formation of the DNA double helix.
14 Outline the process of protein synthesis, including the role of transcription, translation,
messenger RNA, transfer RNA and the template (antisense) DNA strand (details of
the mechanism of protein synthesis on ribosomes are not required at AS).
12 Explain the nature of the genetic code (triplet code only; nonoverlapping and degenerate
not required at AS).
13 Describe a gene as being a sequence of bases on a DNA molecule coding for a sequence of
amino acids in a polypeptide chain.
7 Describe the basic structure of an amino acid (structures of specific amino acids are not
required) and the formation of polypeptides and proteins (as amino acid monomers linked
by peptide bonds in condensation reactions) and explain the significance of a protein’s primary
structure in determining its three-dimensional structure and properties (globular and fibrous
proteins and types of bonds involved in three dimensional structure).
8 Explain the mechanism of action and specificity of enzymes in terms of their threedimensional structure and explain that enzymes are biological catalysts that reduce activation
energy, catalysing a wide range of intracellular and extracellular reactions.
9 Describe how enzyme concentrations can affect the rates of reactions and how this can be
investigated practically by measuring the initial rate of reaction.
11 Describe DNA replication (including the role of DNA polymerase), and explain how Meselson
and Stahl’s classic experiment provided new data that supported the accepted theory of
replication of DNA and refuted competing theories.
15 Explain how errors in DNA replication can give rise to mutations and explain how cystic
fibrosis results from one of a number of possible gene mutations.
2
16 Explain the terms: gene, allele, genotype, phenotype, recessive, dominant, homozygote and
heterozygote; and explain monohybrid inheritance, including the interpretation of genetic
pedigree diagrams, in the context of traits such as cystic fibrosis, albinism, thalassaemia,
garden pea height and seed morphology.
17 Explain how the expression of a gene mutation in people with cystic fibrosis impairs the
functioning of the gaseous exchange, digestive and reproductive systems.
18 Describe the principles of gene therapy and distinguish between somatic and germ line
therapy.
19 Explain the uses of genetic screening: identification of carriers, preimplantation genetic
diagnosis and prenatal testing (amniocentesis and chorionic villus sampling) and discuss the
implications of prenatal genetic screening.
20 Identify and discuss the social and ethical issues related to
genetic screening from a range of ethical viewpoints.
Unit 2 – Development, plants and the environment
Topic 3 – Voice of the genome
1 Demonstrate knowledge and understanding of the practical and investigative skills identified
in numbers 4 and 5 in the table of How Science Works on page 13 of this specification.
3 Describe the ultrastructure of an animal (eukaryotic) cell (nucleus, nucleolus, ribosomes,
rough and smooth endoplasmic reticulum, mitochondria, centrioles, lysosomes,
and Golgi apparatus) and recognise these organelles from EM images.
4 Explain the role of the rough endoplasmic reticulum (rER) and the Golgi apparatus in protein
transport within cells and including its role in formation of extracellular enzymes.
2 Distinguish between eukaryotic and prokaryotic cells in terms of their structure and
ultrastructure.
6 Explain the role of mitosis and the cell cycle for growth and asexual reproduction.
7 Describe the stages of mitosis and how to prepare and stain a root tip squash in order to
observe them practically.
9 Explain how mammalian gametes are specialised for their functions.
10 Describe the process of fertilisation in mammals and flowering plants (starting with the
acrosome reaction in mammals and pollen tube growth in plants and ending with the fusion of
the nuclei) and explain the importance of fertilisation in sexual reproduction.
8 Explain the role of meiosis in the production of gametes and genetic variation through
recombination of alleles and genes including independent assortment and crossing over
(details of the stages of meiosis are not required).
11 Explain what is meant by the terms stem cell, pluripotency and totipotency and discuss the
way society uses scientific knowledge to make decisions about the use of stem cells
in medical therapies (eg regulatory authorities relating to human embryo research, ability of
stem cells to develop into specialised tissues, potential sources of stem cells, who could benefit
from the therapies, procedures to obtain stem cells and their risks).
13 Explain how cells become specialised through differential gene expression, producing active
mRNA leading to synthesis of proteins. which in turn control cell processes or determine cell
structure in animals and plants (details of transcription factors are not required at AS).
5 Describe how the cells of multicellular organisms can be organised into tissues, tissues into
organs and organs into systems.
14 Explain how phenotype is the result of an interaction between genotype and the
environment (eg animal hair colour, human height, monoamine oxidase A (MAOA) and
cancers), but the data on the relative contributions of genes and environment is often difficult
to interpret.
15 Explain how some phenotypes are affected by alleles at many loci (polygenic inheritance) as
well as the environment (eg height) and how this can give rise to phenotypes that show
3
continuous variation.
12 Describe how totipotency can be demonstrated practically using plant tissue culture
techniques.
Topic 4 – Biodiversity and natural resources
1 Demonstrate knowledge and understanding of the practical and investigative skills identified
in numbers 4 and 5 in the table of How Science Works on page 13 of this specification.
13 Explain the terms biodiversity and endemism and describe how biodiversity can be
measured within a habitat using species richness and within a species using genetic diversity,
eg variety of alleles in a gene pool.
14 Describe the concept of niche and discuss examples of adaptation of organisms to their
environment (behavioural, physiological and anatomical).
15 Describe how natural selection can lead to adaptation and evolution.
2 Compare the ultrastructure of plant cells (cell wall, chloroplasts, amyloplasts, vacuole,
tonoplast, plasmodesmata, pits and middle lamella) with that of animal cells.
3 Compare the structure and function of the polysaccharides starch and cellulose including the
role of hydrogen bonds between β-glucose molecules in the formation of cellulose microfibrils.
5 Compare the structures, position in the stem and function of sclerenchyma fibres (support)
and xylem vessels (support and transport of water and mineral ions).
4 Explain how the arrangement of cellulose microfibrils in plant cell walls and secondary
thickening contribute to the physical properties of plant fibres, which can be exploited by
humans.
7 Identify sclerenchyma fibres and xylem vessels as seen through a light microscope.
8 Describe how to determine the tensile strength of plant fibres practically.
9 Explain the importance of water and inorganic ions (nitrate, calcium ions and magnesium
ions) to plants.
10 Describe how to investigate plant mineral deficiencies practically.
6 Describe how the uses of plant fibres and starch may contribute to sustainability, eg plantbased products to replace oil-based plastics.
12 Compare historic drug testing with contemporary drug testing protocols, eg William
Withering’s digitalis soup; double blind trials; placebo; three-phased testing.
11 Describe how to investigate the antimicrobial properties of plants.
16 Discuss the process and importance of critical evaluation of new data by the scientific
community, which leads to new taxonomic groupings (ie three domains based on molecular
phylogeny).
17 Discuss and evaluate the methods used by zoos and seedbanks in the conservation of
endangered species and their genetic diversity (eg scientific research, captive breeding
programmes, reintroduction programmes and education).
4
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:
heart
arteries
arterioles
veins
venules
capillaries
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.
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Arteries
Veins
 narrow lumen
 wide lumen
 thicker walls
 thinner walls
 more collagen, elastic fibres and
 less collagen, elastic fibres and smooth
smooth muscle
muscle
 no valves
 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
Atrial systole
DETAIL
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.
6
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:
Inflammation occurs – white blood cells
move into the artery wall. They accumulate
cholesterol. A deposit (atheroma) builds up.
Lining (endothelial) cells damaged eg by
high blood pressure or cigarette smoke
toxins.
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.
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.
7
1.1.6
Symptoms of cardiovascular disease:
Coronary heart disease
Early symptoms




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
shortness of breath
angina – chest pain on exertion
irregular heartbeat
no symptoms, but changes on ECG
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
8
The following factors increase a person’s 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.
9
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.
See Figure 1.30 on page 28 for an explanation of how tissue fluid is formed.
10
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.
Risk factors increase the chance of the harmful outcome.
11
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
People’s 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.
Eating fewer kilojoules than you use results in weight loss.
Eating more kilojoules than you use results in a gain in weight.
12
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
combined
maltose
(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.
Make sure you can recognise the structural formulae for glucose, maltose, fructose and
galactose molecules – see pages 32 and 33.
13
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:
G
L
Y
C
E
R
O
L
When the molecules join together,
a condensation reaction takes place.
Ester bonds are formed.
fatty acid
fatty acid
fatty acid
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.
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.
14
1.1.15
Body mass index (BMI) is a method of classifying body weight relative to height.
body mass / kg
BMI =
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.
Eating both monounsaturated and polyunsaturated fats reduces the level of LDLs in the blood.
15
1.1.17
Practical on the effect of caffeine on heart rate in Daphnia.
1.1.18
A person’s 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.
16
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.
Fick’s law explains that:
surface area x difference in concentration
Rate 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.
phosphate
group
The phosphate head of the phospholipid
is polar and attracts water – it is hydrophilic.
The fatty acid tails are hydrophobic.
G
L
Y
C
E
R
O
L
fatty acid
fatty acid
In the cell membrane, the hydrophobic tails face inwards
to avoid water, while the hydrophilic heads point outwards.
17
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.
 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.
18
In active transport, ATP supplies energy to change the shape of a carrier protein molecule
when 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.
The bases pair in a particular way, based on their shape and chemical structure:
adenine
guanine
thymine
A & T pair forming 2 hydrogen bonds
cytosine
C & G pair forming 3 hydrogen bonds
19
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
 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
20
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:
residual or
R group – different in each
amino acid
R
H
amine group
O
N
C
H
C
carboxylic acid
group
OH
H
20 different amino acids are found commonly in the proteins of living organisms.
21
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.
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.
22
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
1.2.13
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.
23
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 organism’s 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.
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
24
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 can’t 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.
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
25
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.
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
26
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.
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.
27
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
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
28
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
 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.
29
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.
division
interphase
These stages are part of the cell cycle:
G1 (first gap phase)
synthesis of cellular proteins and organelles
S (synthesis phase)
replication of DNA
G2 (second gap phase)
synthesis of spindle proteins
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
30
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
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




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
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
31
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.
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 don’t: they are
multipotent stem cells.
32
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.
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.
33
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.
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
and
hair
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.
34
 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
 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.
35
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
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
36
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?
37
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.
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.
38
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.
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.
39
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.
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.
40
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
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


elongated 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 doesn’t break easily on stretching; compressional strength means it doesn’t 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.
41
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
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 they’re given the drug or an inactive placebo.
42
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.
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 animal’s gut
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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
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
Thickening agent: when heated, starch
granules absorb water and thicken the liquid
(gelatinisation) eg custard, wallpaper paste.
Uses of oils
Widely used in cooking.
Can be used as a fuel eg castor oil & peanut
oil were both used to power the first diesel
engine.
With little water and high temperature and
pressure, starch ‘puffs’ into an expanded
structure eg cereals, corn snacks, packaging. Hydrolysis of oils with alkali produces fatty
acid salts (soaps) and glycerol (used in paint
Dried, cross-linked starch is a supermanufacture.)
absorbent used in nappies and tampons.
Other uses include glues, plaster, hair
mousses and antiperspirants.
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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
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 doesn’t 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.
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2.4.13 & 2.4.14
Arguments for
Arguments against
Improved plant quality eg tomatoes with PG Creation of antibiotic resistant microbes by
inhibited, which stay firmer for longer.
using marker genes.
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.
Altered genes creating toxic or allergenic
substances in the plant.
So long as food is clearly labelled, people
have the choice of eating GM products, or
not.
Increased herbicide use to control resistant
crops.
Transgenic plants or plants to which
resistance genes have been transferred
could prove very difficult to manage and
keep under control.
Companies hold patents for the GM crops
and developing countries can’t afford them.
2.4.15
The atmosphere is a thin layer of gases extending 100km above the Earth’s surface. It keeps the
Earth’s 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.
46
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
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.
 anaerobic decomposition
eg in bogs, paddy fields,
landfill sites
 digestive system of cattle
 incomplete combustion of
fossil fuels
 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.
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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
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.
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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.
Pollen
Vast 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 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 don’t 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.
49
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.
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.
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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.
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.
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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.
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.
52