O-Level Biology notes

Must know

• Magnification = image size ÷ actual size
• 1 mm = 1000 µm, so convert lengths to the same unit before dividing
• Only plant cells have a cellulose cell wall, chloroplasts and a large central vacuole
• Cells form tissues, tissues form organs, and organs form organ systems

What the syllabus asks you to do

• Identify the cell wall, cell membrane, cytoplasm, nucleus, vacuoles and chloroplasts in diagrams and light micrographs, and state their functions
• Identify the endoplasmic reticulum, Golgi body, mitochondria and ribosomes in diagrams and electron micrographs, and state their functions
• Compare the structure of typical animal and plant cells
• Explain how the structures of specialised cells, such as red blood cells, muscle cells and root hair cells, are adapted to their functions
• Describe the organisation of life from cells to tissues, organs, systems and the organism
• Calculate magnification and actual size for drawings and micrographs, as in practical work

Common mistakes

• Writing that the cell wall is partially permeable.Link 'partially permeable' only to the cell membrane. The cellulose cell wall is fully permeable and gives shape.
• Drawing chloroplasts in a root hair cell because 'all plant cells have chloroplasts'.Ask whether the cell receives light. Only photosynthesising cells, such as palisade cells, have chloroplasts.
• Dividing 45 mm by 1500 and writing 0.03 µm as the actual size.Convert first: 45 mm = 45 000 µm, then 45 000 ÷ 1500 = 30 µm.
• Writing that mitochondria 'produce energy'.Write that mitochondria release energy from glucose by aerobic respiration.
• Calling the stomach a tissue, or muscle tissue an organ.A tissue is similar cells doing one job. An organ is several different tissues working together, such as the stomach.

Exam tips

• When asked to compare plant and animal cells, write paired statements such as 'plant cells have a cellulose cell wall, animal cells do not'. A list of plant cell parts on its own does not show a comparison.
• In magnification questions, write the formula, show the unit conversion on its own line, and give the answer with the right unit: µm for actual size and a times sign for magnification.
• For specialised cells, link each structure to its function in one sentence using 'so' or 'which', for example 'a long root hair, which increases the surface area for absorbing water'.

Full guide to cell Structure and Organisation, with a quiz and more practice questions.

Must know

• Diffusion: down a concentration gradient, no energy from the cell needed
• Osmosis: water only, from higher to lower water potential, across a partially permeable membrane
• Active transport: against a concentration gradient, needs carrier proteins and energy from respiration
• Percentage change in mass = (final − initial) ÷ initial × 100%

What the syllabus asks you to do

• Define diffusion and describe its role in nutrient uptake and gas exchange in plants and humans
• Define osmosis and explain the movement of water in terms of water potential
• Investigate and describe the effects of osmosis on plant and animal tissues, including percentage change in mass
• Define active transport and discuss its importance as an energy-consuming process that moves substances against a concentration gradient, as in ion uptake by root hairs and glucose uptake in the villi

Common mistakes

• Defining osmosis as 'the movement of particles from high to low concentration'.Say osmosis is the net movement of water molecules from higher to lower water potential through a partially permeable membrane.
• Writing that water moves from a concentrated solution to a dilute one.Think in water potential: a dilute solution has more free water, so water moves from dilute to concentrated.
• Saying an onion cell in pure water swells and bursts.Check for a cell wall. Plant cells become turgid because the cellulose wall resists bursting. Only animal cells burst.
• Leaving 'partially permeable membrane' out of the osmosis definition.Learn the definition as one fixed sentence and check it names water, water potential and a partially permeable membrane.
• Saying active transport moves substances down a gradient without energy.Tie active transport to two phrases: against the concentration gradient, and energy from respiration.
• Writing 8% instead of −8% for a strip that fell from 5.0 g to 4.6 g.Always work out final minus initial, so a loss automatically comes out negative.

Exam tips

• Learn the three definitions word for word. Examiners look for the key phrases: 'net movement', 'concentration gradient', 'water potential', 'partially permeable membrane' and 'energy from respiration'.
• In osmosis explanations, always compare the water potential of the solution with that of the cell sap or cytoplasm, then state the direction of water movement and its effect on the cell.
• If a graph of percentage change in mass crosses zero, the concentration at that point has the same water potential as the cell contents. Read it off the graph and say so.

Full guide to movement of Substances, with a quiz and more practice questions.

Must know

• Enzymes are biological catalysts made of protein, each specific to its substrate
• Rate of reaction = 1 ÷ time taken
• Above the optimum temperature, the active site changes shape and the enzyme is denatured
• Pepsin works best at about pH 2, salivary amylase at about pH 7

What the syllabus asks you to do

• List the chemical elements that make up carbohydrates, fats and proteins
• State the main roles of carbohydrates, fats and proteins in living organisms
• Describe and carry out the tests for starch, reducing sugars, protein and fats
• State that large molecules are built from smaller units: starch, glycogen and cellulose from glucose, proteins from amino acids, and fats from glycerol and fatty acids
• Explain enzyme action in terms of the active site, enzyme-substrate complex, lowering of activation energy and specificity, using the lock and key hypothesis
• Investigate and explain the effects of temperature and pH on the rate of enzyme-catalysed reactions

Common mistakes

• Writing that amylase is 'killed' at 60 °C.Write 'denatured', then explain that the active site changes shape so the substrate no longer fits.
• Saying an enzyme at 5 °C is denatured.At low temperature molecules collide less often, so the rate is low, but the active site is unchanged and works again when warmed.
• Adding Benedict's solution and reading the colour without heating.Heat in a boiling water bath for 2 to 3 minutes before recording the colour.
• Expecting sucrose to turn Benedict's solution brick-red.Remember sucrose is a non-reducing sugar, so the solution stays blue.
• Writing that the substrate has the 'same shape' as the active site.Use 'complementary shape', which explains why each enzyme fits only one substrate.

Exam tips

• In any temperature or pH explanation, use the chain: shape of active site, fit of substrate, number of enzyme-substrate complexes, rate. Missing a link often costs a mark.
• For food tests, state the reagent, any heating, the starting colour and the final colour. 'Turns brick-red' alone is not a full answer.
• When a graph shows the rate falling above the optimum, use the word 'denatured' and mention the active site, not just 'the enzyme stops working'.

Full guide to biological Molecules and Enzymes, with a quiz and more practice questions.

Must know

• Starch to maltose by amylase, maltose to glucose by maltase
• Proteins to polypeptides by pepsin and trypsin, polypeptides to amino acids by peptidases
• Fats to fatty acids and glycerol by lipase, after bile emulsifies them
• Absorbed glucose and amino acids travel to the liver in the hepatic portal vein

What the syllabus asks you to do

• Describe the functions of the parts of the digestive system, from the mouth and salivary glands to the rectum and anus, in ingestion, digestion, absorption, assimilation and egestion
• Describe peristalsis as rhythmic, wave-like muscle contractions that mix and propel food along the alimentary canal
• Describe the functions of digestive enzymes such as amylase, maltase, protease and lipase, listing their substrates and end-products
• Explain how the structure of a villus, including its capillaries and lacteal, suits it for absorption
• State that the hepatic portal vein carries blood rich in absorbed nutrients from the small intestine to the liver
• State the role of the liver in glycogen storage, fat digestion, amino acid metabolism and urea formation, and the breakdown of alcohol and hormones

Common mistakes

• Writing 'bile digests fat into fatty acids and glycerol'.Say bile emulsifies fat into small droplets, and lipase then digests the droplets.
• Saying starch digestion carries on in the stomach.Remember the acid at about pH 2 denatures salivary amylase. Starch digestion restarts in the duodenum.
• Ending a starch answer with 'amylase turns starch into glucose'.Write two steps: amylase digests starch into maltose, then maltase digests maltose into glucose.
• Saying digested food is absorbed in the large intestine.Link absorption of digested food to the villi of the ileum. The large intestine mainly absorbs water.
• Saying the liver stores excess amino acids.Write that excess amino acids are deaminated in the liver and the nitrogen becomes urea.

Exam tips

• Learn a table of enzyme, where it is made, where it acts, substrate and product. Most digestion questions can be answered straight from it.
• When a question says 'trace' or 'describe the digestion of', move through the organs in order and name an enzyme and product at every stage.
• For the liver, give specific functions such as deamination, glycogen storage and bile production rather than 'cleans the blood'.

Full guide to nutrition in Humans, with a quiz and more practice questions.

Must know

• Red cells carry oxygen, phagocytes engulf pathogens, lymphocytes make antibodies, platelets start clotting
• Heart rate = beats counted × (60 ÷ seconds counted), in beats per minute
• The pulmonary artery carries deoxygenated blood, the pulmonary vein carries oxygenated blood
• The left ventricle wall is thicker because it pumps blood to the whole body

What the syllabus asks you to do

• Identify the main blood vessels to and from the heart, lungs, liver and kidneys
• Relate the structures of arteries, veins and capillaries to their functions
• Describe the transfer of materials between capillaries and tissue fluid
• State the components of blood and their roles in transport and defence, including phagocytosis, antibody formation and clotting
• Describe the structure and function of the heart in terms of muscular contraction and the working of valves
• Describe coronary heart disease as the blocking of coronary arteries, list its possible causes and state preventive measures

Common mistakes

• Defining arteries as 'vessels that carry oxygenated blood'.Define arteries as carrying blood away from the heart. The pulmonary artery carries deoxygenated blood.
• Labelling the left ventricle on the left of a heart diagram.Remember the diagram faces you, so the heart's left side is on the right of the page.
• Writing 'capillary walls are thin so blood cells can pass through'.Say substances such as oxygen and glucose diffuse across the one-cell-thick wall. Red blood cells stay inside.
• Saying platelets form the clot on their own.Give the chain: platelets trigger fibrinogen to become fibrin, and the fibrin mesh traps blood cells to form the clot.
• Saying phagocytes produce antibodies.Phagocytes engulf pathogens. Lymphocytes produce antibodies.

Exam tips

• When tracing blood through the heart, name every chamber, valve and vessel in order. Skipping a valve or writing just 'a vein' instead of naming the vena cava loses marks.
• For pulse or heart rate calculations, show the conversion to one minute clearly and give the unit, beats per minute.
• Structure and function answers need both parts: 'thick elastic wall' alone is not enough, add 'to withstand high pressure'.

Full guide to transport in Humans, with a quiz and more practice questions.

Must know

• Aerobic: glucose + oxygen gives carbon dioxide + water + a large amount of energy
• Anaerobic in muscle: glucose gives lactic acid + a small amount of energy
• Alveoli: a huge surface area, walls one cell thick, a moist lining and a dense capillary network
• Breathing in: thorax volume increases, pressure falls, air flows in

What the syllabus asks you to do

• Identify the larynx, trachea, bronchi, bronchioles, alveoli and associated capillaries and state their functions in gas exchange
• Describe breathing and the roles of the cilia, diaphragm, ribs and internal and external intercostal muscles
• Explain how the structure of an alveolus is suited to gas exchange
• State the toxic components of tobacco smoke, nicotine, tar and carbon monoxide, and describe their effects on health
• Define aerobic respiration in human cells and state its equation in words and symbols
• Define anaerobic respiration in human cells and state its word equation
• Explain why cells respire anaerobically during vigorous exercise, causing an oxygen debt that is removed by rapid, deep breathing afterwards

Common mistakes

• Writing 'the lungs carry out respiration'.Respiration is a chemical process in every cell. Breathing is the movement of air into and out of the lungs.
• Writing 'glucose gives ethanol and carbon dioxide' for muscle cells.For human muscle, write glucose gives lactic acid, with a small amount of energy.
• Writing that respiration 'produces energy'.Write that respiration releases energy from glucose.
• Saying the inrushing air raises the pressure, which then fills the lungs.Use the chain: muscles contract, volume increases, pressure falls below atmospheric, then air flows in.
• Saying carbon monoxide damages the alveoli.Carbon monoxide combines with haemoglobin and cuts oxygen transport. Tar damages the lungs and causes cancer.

Exam tips

• Learn the definitions exactly: aerobic respiration is the release of energy by the breakdown of glucose in the presence of oxygen, and anaerobic respiration is the same process in the absence of oxygen.
• For breathing, write the answer as a chain: muscles, ribs and diaphragm, volume, pressure, air movement. Each link is usually a mark.
• For alveoli, give the feature and the reason together, for example 'walls one cell thick, so the diffusion distance is short'.

Full guide to respiration in Humans, with a quiz and more practice questions.

Must know

• Excretion removes metabolic waste. Egestion removes undigested food (faeces), which was never part of metabolism
• Ultra-filtration: high blood pressure in the glomerulus forces water, glucose, amino acids, salts and urea into the Bowman's capsule. Blood cells and plasma proteins are too large and stay in the blood
• Selective reabsorption: all the glucose and amino acids, and much of the water and salts, return to the blood from the tubule
• Urine is water with urea and excess salts dissolved in it

What the syllabus asks you to do

• Define excretion and explain why nitrogenous and other waste must be removed from the body
• Identify the kidneys, ureters, bladder and urethra and state what each does in excretion
• Outline how a nephron makes urine by ultra-filtration and selective reabsorption
• Outline how dialysis works when the kidneys fail

Common mistakes

• Writing that faeces are excreted.Faeces are egested. They are mostly undigested food that never entered the body's cells, so removing them is not excretion.
• Saying red blood cells or proteins pass into the filtrate.Only small molecules are filtered. Blood cells and plasma proteins are too large and stay in the glomerulus.
• Saying glucose is not filtered because it is useful.Glucose is filtered, because it is small. It is then reabsorbed. Filtration is by size, reabsorption is by usefulness.
• Mixing up the ureter and the urethra.The ureter runs from kidney to bladder (one from each kidney). The urethra runs from bladder to the outside.
• Writing that dialysis fluid contains no glucose.Dialysis fluid has glucose at the same concentration as healthy plasma. That is why the patient does not lose glucose.

Exam tips

• For ultra-filtration, name the cause (high blood pressure in the glomerulus), what passes (small molecules, named) and what stays (cells and plasma proteins).
• For dialysis questions, compare the fluid with the blood for each substance, then say which way it diffuses and why.

Test yourself

1. Name the organ that makes urea and the organ that removes it from the blood. Answer: The liver makes urea. The kidneys remove it.
2. Why does the filtrate contain urea but no plasma protein? Answer: Urea is a small molecule and is forced out in ultra-filtration. Plasma proteins are too large to pass through the capillary walls.
3. Why must dialysis fluid flow in the opposite direction to the blood and be replaced often? Answer: To keep the urea concentration in the fluid lower than in the blood along the whole tubing, so urea keeps diffusing out.

Must know

• Hormone: a chemical substance, produced by a gland, carried by the blood, which alters the activity of one or more specific target organs
• High blood glucose: more insulin, so the liver and muscles turn glucose into glycogen. Low blood glucose: more glucagon, so the liver turns glycogen back into glucose
• Reflex arc: receptor, sensory neurone, relay neurone, motor neurone, effector
• Near object: ciliary muscles contract, suspensory ligaments slacken, lens becomes thicker

What the syllabus asks you to do

• Define homeostasis and explain it as a stimulus, a corrective mechanism and negative feedback
• Describe how body temperature is kept constant: temperature receptors in the skin, sweating, shivering, changes in blood flow near the skin, and the co-ordinating role of the hypothalamus
• Define a hormone and an endocrine gland, using the islets of Langerhans in the pancreas as the example
• Explain how insulin and glucagon regulate blood glucose. Describe type 2 diabetes, its risk factors and how it is managed
• Outline the role of anti-diuretic hormone (ADH) in osmoregulation
• State that the brain, spinal cord and nerves co-ordinate the body, and outline a reflex action from receptor through sensory, relay and motor neurones to effector
• Describe the eye in front view and in horizontal section, the job of each part in focusing near and distant objects, and the pupil reflex in bright and dim light

Common mistakes

• Writing that insulin turns glucose into glycogen.Insulin is a hormone, not an enzyme. Write that insulin makes the liver and muscles convert glucose into glycogen.
• Confusing glucagon with glycogen.Glucagon is the hormone. Glycogen is the stored carbohydrate in the liver and muscles.
• Saying blood vessels move closer to the skin when hot.Vessels do not move. Arterioles near the skin dilate, so more blood flows near the surface.
• Saying the lens gets thicker for distant objects.Distant: ciliary muscles relax, lens thinner. Near: ciliary muscles contract, lens thicker.
• Writing that sweat cools the body by itself.It is the evaporation of water in the sweat that removes heat from the skin.

Exam tips

• For any homeostasis question, write the chain: change detected, by what, which response, how it corrects, and that the correction stops once the level is normal.
• For a reflex arc, name all five parts in order and say what the effector does. Say where the relay neurone is (spinal cord or brain).
• In the pupil reflex, name the muscle that contracts as well as the one that relaxes.

Test yourself

1. State the definition of a hormone. Answer: A chemical substance, produced by a gland, carried by the blood, which alters the activity of one or more specific target organs.
2. A person has not eaten for several hours. Which hormone rises, and what does it do? Answer: Glucagon. It makes the liver convert glycogen into glucose, so blood glucose rises back to normal.
3. On a hot day with little to drink, what happens to ADH and the urine? Answer: More ADH is released, the collecting ducts reabsorb more water, and a small volume of concentrated urine is made.

Must know

• A typical virus: genetic material inside a protein coat, and it reproduces only in living host cells
• A typical bacterium: cell wall and DNA but no nucleus. Some are pathogenic, some are not
• Vaccines contain an agent that resembles a pathogen, so white blood cells quickly make antibodies when the real pathogen invades
• Antibiotics stop bacteria making their cell structures. They do not work on viruses

What the syllabus asks you to do

• Tell infectious diseases (spread from person to person) from non-infectious diseases, with examples of each
• Explain that pathogens such as bacteria and viruses spread through body fluids, food and water
• State that a virus is genetic material inside a protein coat that reproduces only in living host cells, and that a bacterium has a cell wall and DNA but no nucleus
• State the signs and symptoms of influenza (caused by the influenza virus) and pneumococcal disease (caused by pneumococcus), how each spreads and how to reduce the spread
• State how vaccines prevent disease by preparing white blood cells to make antibodies quickly
• Explain why antibiotics act on bacteria but not on viruses, and discuss how misuse and overuse speed up antibiotic resistance

Common mistakes

• Writing that antibiotics kill viruses.Antibiotics act on bacteria only. Viral diseases such as influenza are not cured by antibiotics.
• Saying bacteria become resistant because they get used to the antibiotic.Resistant bacteria were already present by chance. The antibiotic kills the others, so the resistant ones survive and multiply.
• Saying a vaccine contains antibodies.A vaccine contains an agent that resembles the pathogen. The body's own white blood cells make the antibodies.
• Calling all bacteria harmful.Some bacteria are pathogenic and some are not.

Exam tips

• When asked how a disease spreads, name the route (droplets, body fluids, food or water) and one way to block it.
• For antibiotic resistance, use the natural selection chain: variation, selection by the antibiotic, survival, reproduction, more resistant bacteria.

Test yourself

1. Give one infectious and one non-infectious disease. Answer: Infectious: influenza (or pneumococcal disease or HIV). Non-infectious: type 2 diabetes (or coronary heart disease).
2. State two ways a bacterium differs from a virus. Answer: A bacterium has a cell wall and a cell membrane and cytoplasm, and can divide on its own. A virus is genetic material in a protein coat that reproduces only inside host cells.
3. How does a vaccine protect a person? Answer: It contains an agent that resembles the pathogen, so if the pathogen invades later, white blood cells quickly produce antibodies and destroy it before illness develops.

Must know

• Carbon dioxide + water gives glucose + oxygen, with light energy and chlorophyll
• Balanced: 6CO₂ + 6H₂O gives C₆H₁₂O₆ + 6O₂
• Limiting factors: light intensity, carbon dioxide concentration and temperature
• Iodine turns blue-black where a leaf contains starch and stays brown where it does not

What the syllabus asks you to do

• Identify the tissues of a leaf in transverse section and relate chloroplast distribution, stomata, mesophyll and vascular bundles to their functions
• State that chlorophyll absorbs light energy and converts it into chemical energy to form carbohydrates, and describe their uses
• State the word and symbol equations for photosynthesis
• Describe how carbon dioxide reaches the mesophyll cells of a leaf
• Investigate and discuss the effects of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis
• Discuss light intensity, carbon dioxide concentration and temperature as limiting factors

Common mistakes

• Writing that photosynthesis 'produces energy'.Write that light energy is converted into chemical energy stored in glucose.
• Saying a blue-black iodine result shows glucose.Iodine detects starch. Explain that the glucose made in photosynthesis is stored as starch.
• Leaving out why the leaf goes into ethanol, or heating ethanol over a flame.Learn each step with its reason: ethanol removes chlorophyll, and a hot water bath is used because ethanol is flammable.
• Explaining a plateau with 'the plant has enough light'.Name another factor, such as carbon dioxide concentration or temperature, as now limiting the rate.
• Writing that plants stop respiring during the day.State that plants respire all the time, and that in light photosynthesis is usually faster.

Exam tips

• In variegated leaf or covered leaf questions, state what each part had or lacked (light, chlorophyll), then link to photosynthesis, glucose and starch, then the iodine colour.
• For limiting factor graphs, name the limiting factor on each part of the curve: the rising part is limited by the factor on the x-axis, the plateau by something else.
• Learn the steps of the starch test with the reason for each step. Questions often give one step and ask why it is done.

Full guide to nutrition in Plants, with a quiz and more practice questions.

Must know

• Energy flows one way: sun, producers, consumers. It is lost as heat and is not recycled
• Only a small fraction of the energy at one trophic level, often about 10%, reaches the next
• Efficiency of transfer = energy at the higher level ÷ energy at the lower level × 100%
• Biomagnification: a persistent insecticide becomes more concentrated at each level, so top carnivores get the highest dose

What the syllabus asks you to do

• Describe energy flow through an ecosystem as non-cyclical, and the roles of producers, consumers and decomposers in food chains and food webs
• Explain how energy is lost along a food chain and discuss how efficient transfer is between trophic levels
• Describe and interpret pyramids of numbers and pyramids of biomass
• Describe how carbon is cycled in an ecosystem and the role of forests and oceans as carbon sinks
• Describe how deforestation and burning fossil fuels raise carbon dioxide levels and lead to global warming, and discuss how human action can reduce it
• Describe the effects of sewage in water, plastic waste in the sea, and insecticides that build up along food chains (biomagnification)
• Discuss how conservation and the sustainable use of resources keep ecosystems such as coral reefs, tropical rainforests and mangroves balanced

Common mistakes

• Writing that energy is recycled by decomposers.Decomposers recycle matter, such as carbon and mineral salts. Energy is lost as heat and is not recycled.
• Saying energy is used up or destroyed along a food chain.Energy is not destroyed. It is lost to the surroundings, mostly as heat from respiration.
• Drawing food chain arrows from eater to eaten.Arrows show the direction of energy flow, from the organism eaten to the organism that eats it.
• Saying algae use up the oxygen in a polluted river.It is the bacteria decomposing the dead algae and plants that use up the dissolved oxygen.
• Explaining biomagnification as each animal making more insecticide.The insecticide is not broken down or excreted, and each consumer eats many organisms from the level below, so it becomes more concentrated.

Exam tips

• In efficiency calculations, always put the higher trophic level on top and give the answer as a percentage.
• For eutrophication, write the full sequence: nutrients, algal bloom, light blocked, plants die, decomposers multiply and use oxygen, fish die.
• For a pyramid question, say which kind it is before explaining its shape.

Test yourself

1. Why is energy flow called non-cyclical? Answer: Energy enters as light, passes along food chains and leaves as heat. It does not return to producers, so it must keep coming from the sun.
2. Why can a pyramid of numbers be inverted when a pyramid of biomass is not? Answer: A pyramid of numbers counts organisms whatever their size, so one large tree can feed many insects. Biomass is lost between levels, so biomass is usually smaller at each higher level.
3. Name two carbon sinks. Answer: Forests and oceans.

Must know

• A nucleotide is a sugar, a phosphate group and one of four bases
• Complementary base pairing: adenine with thymine, cytosine with guanine
• A gene codes for one polypeptide and is a unit of inheritance
• Human insulin gene, cut with a restriction enzyme, is joined into a bacterial plasmid with DNA ligase

What the syllabus asks you to do

• Outline how DNA, genes and chromosomes are related
• State that DNA is a double helix of two strands of nucleotides, each made of a sugar, a phosphate group and one of four bases, and state the rule of complementary base pairing
• State that a gene is a sequence of nucleotides in DNA that codes for one polypeptide and is a unit of inheritance
• State that DNA carries the genetic code used to make specific polypeptides (no detail of transcription or translation is needed)
• State that genes can be moved from one organism to another to make transgenic organisms, and explain how the gene for human insulin is put into bacteria to make insulin
• Discuss the benefits and ethical issues of genetic engineering in medicine, crops and farm animals

Common mistakes

• Pairing A with G or C with T.Learn the pairs: A with T, C with G. A useful check is that A and T are equal, and C and G are equal.
• Saying a gene is a chromosome.A gene is a short section of DNA. A chromosome is a whole DNA molecule carrying many genes.
• Using different restriction enzymes for the gene and the plasmid.Use the same restriction enzyme, so the sticky ends of the gene and the plasmid match.
• Saying DNA ligase cuts DNA.Restriction enzymes cut. DNA ligase joins.

Exam tips

• For the insulin method, name both enzymes and the plasmid, and finish with the bacteria multiplying and making insulin.
• In a 'discuss' question on genetic engineering, give at least one benefit and one ethical concern, each tied to an example.

Test yourself

1. What three parts make up a nucleotide? Answer: A sugar, a phosphate group and a base.
2. What is a transgenic organism? Answer: An organism whose cells contain a gene transferred from another organism.
3. Why must the same restriction enzyme cut the gene and the plasmid? Answer: So their sticky ends are complementary and can be joined by DNA ligase.

Must know

• Mitosis: genetically identical cells, chromosome number kept. Used in growth, repair and asexual reproduction
• Meiosis: genetically dissimilar cells, chromosome number halved. Used to make gametes
• Haploid: one set of chromosomes (human gametes, 23). Diploid: two sets (human body cells, 46)
• Pollination: transfer of pollen grains from an anther to a stigma

What the syllabus asks you to do

• Define asexual and sexual reproduction. State that mitosis gives genetically identical cells and is used in growth, repair and asexual reproduction
• Define haploid, diploid and homologous pairs, and state that meiosis halves the chromosome number, gives genetically different cells and is used to make gametes
• Identify the parts of insect-pollinated and wind-pollinated flowers, state their functions and explain how the two types differ
• Outline pollination, self-pollination and cross-pollination, and the growth of the pollen tube to the ovule before fertilisation
• Identify the parts of the male and female reproductive systems and state what each does
• Outline the menstrual cycle with reference to oestrogen and progesterone only, then describe fertilisation, implantation and the roles of the amniotic sac, amniotic fluid, placenta and umbilical cord
• Discuss how HIV is transmitted and how to reduce transmission

Common mistakes

• Defining pollination as the fusion of male and female gametes.Pollination is the transfer of pollen from anther to stigma. Fertilisation is the fusion of the nuclei.
• Saying the blood of the mother and fetus mix in the placenta.They come close but do not mix. Substances diffuse across.
• Saying mitosis makes gametes.Gametes are made by meiosis, which halves the chromosome number.
• Writing that the pollen grain travels down the style.The pollen grain stays on the stigma. It grows a pollen tube down the style, and the male nucleus moves down the tube.
• Naming hormones other than oestrogen and progesterone in the menstrual cycle.The syllabus asks for oestrogen and progesterone only. Say what each does to the uterus lining.

Exam tips

• For flower comparisons, write paired statements for the same feature, such as 'stigma sticky and inside the flower, compared with feathery and hanging outside'.
• In placenta questions, name the substance and the direction: oxygen and glucose to the fetus, carbon dioxide and urea to the mother.

Test yourself

1. State two uses of mitosis. Answer: Any two of growth, repair of damaged tissue and asexual reproduction.
2. Give two features of a wind-pollinated flower. Answer: Any two: small dull petals or none, anthers hanging outside on long filaments, large feathery stigmas, many small light smooth pollen grains.
3. What does progesterone do in the menstrual cycle? Answer: It keeps the lining of the uterus thick and ready for implantation. When it falls, the lining breaks down.

Must know

• Tt × Tt gives genotypes 1 TT : 2 Tt : 1 tt and phenotypes 3 : 1
• Tt × tt gives 1 Tt : 1 tt, a 1 : 1 ratio
• Test cross: cross the unknown dominant phenotype with a homozygous recessive
• Females are XX and males XY, so each child has a 1/2 chance of being a boy

What the syllabus asks you to do

• Distinguish between the terms gene and allele
• Explain the terms dominant, recessive, codominant, homozygous, heterozygous, phenotype and genotype
• Predict the results of simple crosses with expected ratios of 3 : 1 and 1 : 1, and use genetic diagrams to solve monohybrid inheritance problems
• Explain why observed ratios often differ from expected ratios, especially with small numbers of offspring
• Explain codominance and multiple alleles using the inheritance of the ABO blood groups (A, B, AB and O) and the alleles IA, IB and IO
• Describe the determination of sex in humans by the XX and XY chromosomes
• Describe mutation as a change in a gene, as in sickle cell anaemia, or in chromosome number, as in Down syndrome, and name ionising radiation and chemical mutagens as factors that increase it
• Distinguish between continuous and discontinuous variation and give examples of each

Common mistakes

• Using T for tall and S for short in the same cross.Use one letter per gene: capital for the dominant allele and lower case for the recessive, such as T and t.
• Writing a gamete as Tt.Each gamete carries one allele of each gene, so a Tt parent makes T and t gametes, each written in a circle.
• Calling 172 tall to 68 short 'wrong' because it is not exactly 3 : 1.Treat ratios as probabilities. Fertilisation is random, so observed numbers vary, especially in small samples.
• Saying a family with three boys is more likely to have a girl next.Each child has a 1/2 chance of being a girl, whatever the earlier children are.
• Writing 'Tt' when the question asks for the phenotype.Genotype is the alleles, such as Tt. Phenotype is the observable feature, such as tall.
• Treating IA as dominant to IB, or calling group AB 'a blend'.Say IA and IB are codominant, so both are fully expressed in IAIB, and both are dominant to IO.

Exam tips

• Always label each line of a genetic diagram: parental phenotypes, parental genotypes, gametes, offspring genotypes, offspring phenotypes, ratio. The labels themselves usually carry marks.
• State the key at the start, for example 'T = allele for tall, t = allele for short', especially if the question did not give one.
• When a question gives numbers of offspring, convert the ratio to expected numbers and comment on whether the observed results are close to them.

Full guide to inheritance, with a quiz and more practice questions.