As Biology Revision Notes 591g

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. 1

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.

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:

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 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 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

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 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.

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 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. 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

α 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 α 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 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 ed to 1 glycerol: G L Y C E R O L

When the molecules 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.

10

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 / 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. 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 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 β blockers can be used.

12

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

α

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. 13

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. 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 ive – they do not require energy. 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.

15

The bases pair in a particular way, based on their shape and chemical structure: adenine

thymine

guanine

A & T pair forming 2 hydrogen bonds

cytosine

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



attached to the tRNA molecule is a specific amino acid

 the amino acids, arranged in the order dictated by the mRNA codons, are ed 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: residual or R group – different in each amino acid

17

R H amine group

O N

C

C

H

carboxylic acid group

OH H

20 different amino acids are found commonly in the proteins of living organisms. The amino acid monomers 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 α helix or a β 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

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

1.2.13 19

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 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. 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 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 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

 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.

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

26

division

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

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

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

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.

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 ed 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

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 β galactosidase and lactose in prokaryotes. See core practical on this topic.

2.4.14 Differences in phenotype between 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. 30

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.  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

 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

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 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

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 ing 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. 34

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. 35

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 α and β glucose. Starch and cellulose are two important polysaccharides in plants.

Starch

Cellulose

Made up of α glucose monomer.

Made up of β 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.

36

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 ed  they must be able to transport water and inorganic ions up to the leaves Xylem vessels do both; sclerenchyma fibres assist with . 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 37

elongated cells

• • •

•

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 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. 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 38

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.

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

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

    

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

40

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.

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

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. 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

Increased herbicide use to control resistant

Transgenic plants or plants to which resistance genes have been transferred could prove very difficult to manage and keep under control.

42

have the choice of eating GM products, or not.

crops. 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.

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 43

 anaerobic decomposition eg in bogs, paddy fields, landfill sites  digestive system of cattle  incomplete 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.

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

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. 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 45

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 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 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 and predict the interaction between them. 46

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. 47

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