Edexcel AS Biology Unit 1 Exam Revision Notes

Explain the importance of water as a solvent in transport, including its dipole nature.

Less dense as a solid- Arctic ecosystems float, ice insulates water beneath it etc. High Specific Heat Capacity- Cells do not heat up or cool down easily, therefore can hold a fairly stable temp. Present naturally in all three states- Allows the water cycle to function.

Transparent - Allows photosynthesis underwater Cohesion - Generates surface tension, capillary uptake, transpiration etc.

Good solvent- Essential role in transport in biological systems.

Immiscible with hydrophobic molecules- Allows membranes to form and, therefore, control movement in / out of cells.

High latent heat of evaporation- Evaporation of water has a strong cooling effect and comparatively little water is required to lose a lot of heat.

Buffer- Water is capable of accepting and donating protons. Therefore acts as a buffer.

Water is a polar covalent compound. Due to difference in electro-negativities of oxygen and hydrogen, one end of the water molecule bears a slight negative charge, while the other end bears a slight positive charge. This is called the dipolar nature of water. The polar nature of water molecule makes it a good solvent.Distinguish between monosaccharides, disaccharides and polysaccharides (glycogen and starch amylose and amylopectin) and relate their structures to their roles in providing and storing energy (-glucose and cellulose are not required in this topic).Glycogen structure and function

Glycogen consists of a () glucose joined by 1, 4 and 1, 6 glycosidic bonds. Glycogen has lots of branches which allows quick access to glucose. Also its a very compact molecule so its good for storage and is insoluble, so no osmotic effect and does not diffuse out of cells also can easily be hydrolysed by enzymes into glucose and used for respiration.Starch structure and function Starch is a mixture of two polysaccharides of alpha- glucose- amylose and amylopectin.

Amylose is straight chain of glucose joined together with 1,4 glycosidic bonds. The angles of the glycosidic bonds give it a coiled structure. This makes it compact, so its really good for storage because you can fit more glucose into a small space in a cell.

Amylopectin is a long branched chain of glucose joined together by 1,4 and 1,6 glycosidic bonds. Its side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily. This means that the glucose can be released quickly. Starch is insoluble, so no osmotic effect.

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.Glucose + Galactose ------ Lactose

Glucose + Glucose ------- Maltose

Glucose + Fructose ------- Sucrose

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.Triglycerides are the most common form of lipids. It consists of 1 glycerol molecule and

three fatty acid chains. The OH groups of glycerol and the -COOH group of each fatty

acid are linked by an ester bond formed by condensation reactions.Saturated lipids dont have any double bonds between the carbon atoms in their hydrocarbon tail. Unsaturated lipids do have double bonds between the carbon atoms in their hydrocarbon tails. These double bonds cause the chain to kink. Explain why many animals have a heart and circulation (mass transport to overcome limitations of diffusion in meeting the requirements of organisms).

The heart is needed to pump blood around the body Many animals have a small surface area to volume ratio and many animals have a high metabolic rate. A circulatory system is needed to overcome limitations of diffusion. The mass transport system in the circulatory stem carry raw materials from specialised exchange organs to the body cells, and to remove metabolic waste.

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.Atrial systole

Pressure in the atria increases as they fill with blood returning from the veins. Increased pressure opens the atrioventricular valaves allowing blood to enter the ventricles. The atria contract to force remaning blood into ventricles.

Ventricular systole

Ventricles contract from the base up, increasing the pressure and closing the atriventricular valaves. The semilunar vales open and blood is forced into the ateries

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.

The structure and operation of the mammalian heartThe cavity of the heart is divided into four chambers. The two upper chambers are thin walled atria. There receive blood into the heart

The two lower chambers are thick walled ventricles. The ventricles pump blood out of the heart, with the muscular wall of the left ventricle much thicker than that of the right ventricle. Also the walls of the heart muscle are supplied with oxygenated blood via coronary arteries.

The atrio-ventricular valves are large valves, positioned to prevent backflow from the ventricles to atria. The edges of these valves are supported by tendons anchored to the muscle walls of the ventricles below.

The valves on the right hand side are called the tricuspid valve and the valve on the left side is called the bicuspid valve or mitral valve. Semilunar valves separate the ventricles from pulmonary artery and aorta. Explain how the structures of blood vessels (capillaries, arteries and veins) relate to their functions.Artery

Arteries carry blood from the heart to the rest of the body. Theyre thick-walled, muscular and have elastic tissue in the walls to cope with the high blood pressure caused by the heartbeat.

The inner lining (endothelium) is folded, allowing the artery to expand, this also helps it to cope with high blood pressure.

Veins

Veins carry low-pressure blood towards the heart. Veins contain valves to stop blood flowing backwards. Veins have thinner muscles connective layer which carry blood with oxygen to the lungs. Wide lumen offers less resistance to blood flow.

Capillaries

Capillary walls are only one cell thick, which speeds up diffusion of substances e.g. glucose and oxygen into and out of cells. There are networks of capillaries in tissues called capillary beds, which increase the surface area for exchange. Blood Clotting Formation1. Platelets are activated by substances released by the damaged artery wall.

2. Platelets become sticky and form a platelet plug on the surface of the atheroma.

3. Platelets release a clotting factor called thromboplastin.

4. In the presence of calcium ions and vitamin K, thromboplastin converts inactive prothrombin into active thrombin.

5. This in turn converts the soluble fibrinogen into insoluble fibrin, which forms a network of fibres, trapping cells and debris to make a clot.

Atherosclerosis Formation

1. Atherosclerosis is a disease in which the wall of arteries becomes furred up with fatty deposits called plaques or atheromas.

2. There is damage to the endothelial lining of the artery. This could be due to high blood pressure. This is then an inflammatory response, and white blood cells move into the artery wall.

3. Cholesterol then builds up, and this leads to the formation of an atheroma.

4. Next calcium salts and fibres also begin to build up, this results in a hard swelling called a plaque being formed.

5. The artery then becomes narrower. This then increases the blood pressure in the artery even more, making it more likely that the whole process will happen again. Factors that increase the risk of CVDGenetic: Some alleles give you less protection from / greater risk of developing atherosclerosis. To an extent, a higher chance of getting atherosclerosis does run in families.Diet: High in saturated fats increases blood cholesterol level leads to atheroma formation, which leads to blood clots and therefore heart attacks or stroke.

Age: Atherosclerosis occurs naturally as arteries become less elastic with age. Also more likely of getting CVD as you get older.

Gender: Incidence is much higher for men than women. As women tend to have less stressful jobs and be at home more so less hypertension.

High Blood Pressure: increases risk of damage to the artery walls, which increases risk of atheroma formation leads to CVD.

Smoking: Chemicals damage endothelium triggering atherosclerosis. Inactivity: Exercise can reduce the risk of developing CHD and reduces blood pressure

Benefits and Risks of treatment for CVD

Antihypertensive: DiureticsBenefits: reduce blood pressure by decreasing blood volume. Risks: dizziness, nausea and muscle cramps.

ACE inhibitors

Benefits: reduce blood pressure by enhancing vasodilation.

Risks: dizziness, impaired kidney function.

Beta Blockers

Benefits: Reduces blood pressure by reducing heart rate

Risks: diabetes

Calcium channel blockers

Block calcium channels in the muscles lining the arteries. The blood pressure lowers because muscles cant contract.

Risks: constipation, headaches and dizziness.

Statins: lower cholesterol level in the blood by blocking the liver enzyme that makes cholesterol.Risks: nausea, constipation and diarrhoea.

Anticoagulants e.g. warfarin: reduces risk of clot formation

Risks: uncontrolled bleeding, dosage control is essential.

Platelets inhibitory drugs: e.g. aspirin, clopidofgrel: makes platelets less sticky

Risks: stomach bleeding.

High density lipoproteins are mainly protein. They transport cholesterol from body tissues to the liver where its recycled or excreted. Their function is to reduce total blood cholesterol when the level is too high.

Low density lipoproteins are mainly lipid. They transport cholesterol from the liver to the blood. Their function is to increase total blood cholesterol when the level is too low.

Lifestyle advice to reduce the risk of CVD

Diet

Scientific research has linked a diet high in saturated fat to an increased risk of CVD. This information can be used to educate people about the risk of certain diets and to encourage them to reduce their saturated fat intake.

Scientific studies have also shown that obese people are more likely to develop CVD. Obesity indicators, like body mass index, can be used to assess if people are overweight or obese. Obesity indicators can be used to monitor the effects any changes in lifestyle have on the persons weight.

Smoking

Scientific research has linked smoking to an increased risk of CVD. This research had led to TV adverts and warnings on cigarette packets about the risks of smoking. The NHS encourages people to give up by giving free advice and prescribing nicotine patches. All of this encourages people to stop smoking and so reduce their risk of CVD.

Exercise

Scientific research has linked inactivity to an increased risk of CVD. This research has led to campaigns that encourage people to exercise more frequently to reduce their risk of CVD.

Evaluating design of studies to determine health risk factors

Things to look out for:

Sample size- the greater the number of people used in a study, the more reliable the results.

Variable- the more variables that have been controlled in a study, the more reliable the results.

Data collection- think about all the problems with the method and see if bias has slipped in. the less bias involved in collecting the data, the more reliable the result. Talk about where the data was collected from.

Controls- the presence of controls increases the reliability of the results.

Repetition by other scientists- if other scientists produce the same results, then the results are more reliable.

Properties of gas exchange surfaces in living organisms Large surface area to volume ratio (large surface area of alveoli)

Theyre thin- this provides a short diffusion pathway across the gas exchange surface. (Thin alveolus walls which also ensure rapid diffusion).

The organism also maintains a steep concentration gradient of gases across the exchange surface.

Explain how the structure of the mammalian lung is adapted for rapid gaseous exchange.

The mammalian lung has small and millions of alveoli which increases the surface area in which diffusion takes place. The alveoli is one cell thick, which reduces the diffusion distance and also makes the gases diffuse much more faster and efficient and the walls of the capillaries are also only cell thick . Alveoli are covered with capillaries. Concentration gradient is maintained by ventilation and by blood flow which increases the rate of diffusion.Cell membrane

Osmosis is the net movement of water molecules from a region of high water concentration to a region of low water concentration, through a partially permeable membrane.

Diffusion is the passive movement of molecules from a region of high concentration to a region of low concentration. Particles diffuse down a concentration gradient.

Diffusion, facilitated diffusion and osmosis are passive they do not require energy.The involvement of carrier and channel proteins in membrane transport.

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.

Facilitated diffusionFacilitated diffusion is the movement of molecules across cell membranes though protein channels and carrier proteins. Large, polar molecules (e.g. sodium ions) or water-soluble molecule cannot pass through the hydrophobic central zone of the phospholipids layer. The process does not require the use of ATP and therefore can only move substances down their concentration gradient.

Active transport

Active transport is an active method of movement through a membrane and therefore requires the cell to expand energy in the form of ATP in order to carry out the process.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.

There are two types of Cytosis:

Endocytosis, where substances are taken into the cell.

Exocytosis, where substances are released from the cell.

Endocytosis involves the engulfing of solid particles e.g. bacteria by monocyte white blood cells. This is called phagocytosis. Endocytosis could also involve taking in small volumes of liquid called pinocytosis e.g. by cells lining the small intestine.Exocytosis is involved in secretion, where a substance such as mucus or enzyme is released from or secreted by the cell.

Structure of the cell membrane

The cell membrane is made up of a phospholipid bilayer. The phosphate head of the phospholipid is polar and attracts water it is hydrophilic. The fatty acid tails are hydrophobic. In the cell membrane, the hydrophobic tails face inwards to avoid water, while the hydrophilic heads point outwards.Amino acids & proteins

An amino acid consists of a central carbon atom attached to an amino group, NH2, a carboxylic group, COOH, a hydrogen atom and a variable side group called the R group. The R group represents one of 20 different side chains.

Amino acids are linked together by peptide bonds to form dipeptides and polypeptides.

Proteins 4 structural levels

Primary structure

A linear sequence of amino acids joined by peptide bonds

Secondary structure

Folding and coiling of the primary structure. There is a negative charge on the CO of the carboxylic group and there is a positive charge on the NH of the amino group. Hydrogen bonding occurs between amino acids. Many hydrogen bonds can hold together a helix structure and a beta pleated sheet made up of polypeptides laid parallel to each other.

Tertiary structure

Further coiling and folding into a complex shape. Tertiary structure is held together by disulphide bridges, ionic bonds, hydrogen bonds and hydrophilic & hydrophobic interactions.

Quaternary structure

More than 1 polypeptide chain collided or twisted around each other.

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.

Explain the importance of the primary structure of an enzyme to its function.Primary structure determines its three-dimensional folding. The sequence of amino acids will determine which R groups appear in certain places this will cause the protein to coil and held up forming specific bonds e.g. hydrogen or disulphide bridges in certain places due to the R groups this causes a specific shape which is important for enzymes as their active site will only suit a specific moleculeIf it doesnt fit the enzyme is useless so the primary structure (sequence of amino acid) is important in causing this correct shape for the enzyme job. Describe the three- dimensional (tertiary structure of an enzyme)

Enzymes are proteins. The tertiary structure of an enzyme is determined by the amino acid sequence in the polypeptide structure(the bonds that form between these when they start to find more in the tertiary structure determine the 3D shape- these bonds include ionic, disulphides and hydrogen bonds,e.g. lots of cysteine amino acids will form disulphide bonds) enzymes have an active site, which is what attaches to the substrate, and this active site is what gives enzymes their high specificity. The rest of the enzyme is the supporting structure. Enzymes are globular proteins; they are soluble, roughly round in shape. Explain how the primary structure of an enzyme determines its three- dimensional (tertiary) structure and its properties.

The primary structure is the basic sequence of amino acids in a poly peptide chain. The structure of the R- group determines how the chain is folded. The hydrophobic R- groups point inwards and are protected by the hydrophilic side chains. This makes the enzyme soluble which is useful as enzymes speeds up metabolic reactions. The chain of amino acids also decides the depression on the enzyme called an active sight which is specific for a certain substrate. 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.

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.

The kinetic energy in the system is increasing. Therefore more molecules will have sufficient energy to meet the activation energy for the reaction.Optimum temperature for the enzyme has been reached. At this point concentration of enzyme/ substrate is the new limiting factor for the reaction.

Enzymes are starting to denature. This causes in its tertiary structure, likely resulting substrate molecules will no longer being able to bind with the enzymes active site, reducing the chance of a enzyme- substrate complex forming therefore lowering the rate of reaction

Mononucleotides structure

A mononucleotide consists of a deoxyribose or ribose linked to a phosphate and a base.

DNA contains 4 bases. The purines are Adenine and Guanine. The pyrimidines are Cytosine and Thymine.

In RNA the purines are Adenine and Guanine. The pyrimidines are Cytosine and Uracil.

DNA is made of two polynucleotide strands and RNA has one polynucleotide strand. RNA molecules are relatively short in length, compared with DNA.

DNA Double-Helix

Two complementary DNA strands join together by hydrogen bonding between the bases. Each base can only join with one particular partner, this is called complementary base pairing.

DNA Replication

1. The DNA helix unzips to form two single strands. Each original single strand acts as a template for a new strand.

2. Free- floating mononucleotides join to each original template strand by complementary base pairing- A with T, G with C.

3. The mononucleotides on the new strand are joined together by the enzyme DNA polymerase. Hydrogen bonds form between the bases on the original and new strand.

4. Each new DNA molecule contains one strand from the original DNA molecule and one new strand.

Describe how a molecule of mRNA is made during transcription.

DNA molecule unzips as hydrogen bonds between strands break. Free nucleotides line up along one strand of DNA- the antisense strand.

The nucleotides follow the complementary base pair pattern

RNA polymerase, an enzyme, bonds the nucleotides together, forming phosphodiester bonds in a condensation reaction forming the mRNA strand. Explain the nature of the genetic code.The genetic code is a triplet code. The genetic code is linear and non-overlapping, and the genetic code can be degenerated, more than one codon can code for one amino acid. Protein synthesis

Transcription

RNA copy of a gene is made in the nucleus. The hydrogen bonds between the two DNA strands in a gene break, separating the strands and the DNA molecule uncoils at that point.

One of the strands is then used as a template to make an RNA copy, called messenger RNA. The template strand is called the antisense strand.

Free RNA mononucleotides line up alongside the template strand. Once the RNA mononucleotides have paired up with their complementary bases on the DNA strand theyre joined together, forming an mRNA molecule.

The mRNA moves out of the nucleus through a nuclear pore, and attached to a ribosome in the cytoplasm.

Translation

MRNA leaves then nucleus, through the nuclear poresMRNA travels to the ribosomes, where it is read

TRNA molecule will bind to a complementary reaction on the MRNA chain.

When two TRNA molecules are bound to the MRNA their amino acids will react forming a peptide bond.

TRNA molecule will break away from the amino acid and MRNA chain and go into the cytoplasm to bond another specific amino acid for its triplet code.

Then eventually forming a polypeptide (primary structure) Gene

A sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain.

Allele

Alternative forms of a gene.

Dominant

An allele that is expressed when present in the genotypeRecessive

An allele that is only expressed in the absence of another dominant allele.

Homozygous

Where a diploid individual has 2 copies of the same allele present in the genotype.

Heterozygous

Where a diploid individual has 2 different alleles present in the genotype.

Genotype

The alleles present in an organism for a particular characteristic.

Phenotype

The physical appearance of an organism with regard to a particular characteristicMutations is a change in a base sequence on the DNA.Effect of cystic fibrosis: the gas exchange

Mucus accumulates in the lungs, bacteria trapped in mucus increase the possibility of infections

Mucus can block bronchioles, which reduces the number of alveoli in contact with fresh air so reduces the surface area for gas exchange. Effect of cystic fibrosis: the digestive system

Mucus blocks the pancreatic duct, so digestive enzymes cant reach the small intestine and food is not properly digested. This leads to tiredness and difficulty in gaining weight.

Effect of cystic fibrosis: the reproductive system

In women, mucus can block the cervix preventing entry of sperm.

In men, the sperm duct is either missing or blocked with mucus, so sperm cannot leave the testes 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.

Gene therapy

Somatic therapyThis involves changing the alleles in body cells, particularly the cells that are most affected by the disorder. Somatic therapy doesnt affect the individuals sex cells though, so any offspring could still inherit the disease.

Germline therapyThis involves changing the alleles in the sex cells. This means that every cell of any offspring produced from these cells will be affected by the gene therapy and they wont suffer from the disease.

Genetic screening

Identification of carriers

Carrier testing shows whether people without a disorder carry an allele that can cause a disorder (e.g. CF) .Carrier testing allows people to make informed decisions about things like whether to have children and whether to carry out prenatal testing if the women is pregnant.

Carrier testing social issues

Finding out the person is a carrier may cause emotional stress or affect the persons ability to find a partner.

Other genetic abnormalities may be found, which could cause further stress.

Carrier testing ethical issues

The tests arent always 100% accurate- they could give a false result. This means decisions could be based on incorrect information.

There are concerns that the results of genetic tests could be used by employers or life insurance companies, resulting in genetic discrimination.

Preimplantation genetic diagnosis (PGD)

PGD is carried out on embryos produced by in vitro fertilisation (IVF). It involves screening embryos for genetic disorders before theyre implanted into the woman.

The advantages of PGD are it reduces the chance of having a baby with a genetic disorder only embryos without the genetic disorders tested will be implanted.

PGD Social issues

It can be used to find out other characteristics (e.g. gender, eye colour) leading to concerns that in the future, embryos may be selected for other characteristics designer babies)

PGD Ethical issues

False results could provide incorrect information.

Prenatal testing

Prenatal testing involves screening unborn babies (foetuses) for genetic disorders. Theyre offered to pregnant women with a family history of genetic disease. Prenatal testing allows parents to make informed decisions. If the test is positive, the parent may decide to have the child or to have an abortion.

There are two types of test:

1. AmniocentesisThis is carried out at 15-16 weeks of pregnancy. A sample of amniotic fluid (the fluid that surrounds the fetus) is obtained using a very fine needle. This fluid contains fetal cells. The cell contains DNA, which can be analysed.

2. Chorionic villus sampling (CVS)

This is carried out at 8-12 weeks of pregnancy. A sample of cells is taken from the chorionic villi (part of the fetus that connects it to its mother) using a fine needle or a catheter (a thin flexible tube). The cells contain fetal DNA, which can be analysed.

Prenatal testing social issues

Prenatal tests slightly increase the risk of miscarriage by around 1%.

Prenatal testing ethical issues

False results could provide incorrect information and some people consider it unethical to abort a fetus because it has a genetic disorder.