Most asked questions Biology igcse

• Species


  Group of organisms that can reproduce to produce fertile offspring
• Sensitivity


  The ability to detect or sense stimuli in the internal or external environment and to make appropriate responses
• Growth


  Increase in size/length/mass/volume; increase in cell number
• Organ


  Structure made up of a group of tissues, working together to perform a specific function(s)
• Sense organ


  Groups of receptor cells responding to specific stimuli
• Bacteria


  • Cell wall, peptidoglycan/murein
  • No nucleus/no nuclear membrane, have nucleoid
  • Loop of DNA
  • No mitochondria
  • No chloroplast
  • No vacuoles
  • Smaller ribosomes
  • Have pili
  • Have capsule
  • Small
• Features all prokaryotes have

  • Cytoplasm
  • Cell membrane
  • Single celled/unicellular
  • No (true) nucleus/no nuclear membrane
  • Loop of DNA/chromosome/naked DNA
  • No, (membrane-bound) organelles/mitochondria/chloroplasts
  • (Peptidoglycan/murein) cell wall
  • Plasmids
• Protoctista cells


  • Nucleus/nuclear membrane/nuclear envelope
  • (Named) organelle(s)/internal membranes
  • Cell walls (if present) have different composition
  • Linear chromosomes
• Other names for protists

  • Protoctists
  • Protoctista
  • Protists
  • Protista
• Nucleus


  • Stores/contains chromosomes/genes/alleles/genetic information/DNA
  • Controls the activity/reactions of the cell
  • Controls how cells develop/divide/reproduce/grow
  • Stores instructions for making proteins/protein synthesis/making RNA
  • Codes for protein
• Cell membrane


  • Forms a barrier between the cell and its surroundings
  • Keeps contents of cell inside
  • Allows/controls/movement of substances, into/out, of the cell/across membrane
• Tissue


  A group of cells with similar structures working together to perform a shared function
• Roles of DNA in a cell

  • Store of genetic information/genetic make-up/genes
  • DNA/genes/alleles/genetic information/base sequence, codes for/makes (named) proteins
  • Sequence of bases in DNA codes for sequence of amino acids
  • Transfer of information to daughter cells (during mitosis)
  • DNA copied as mRNA
• Cell of a leaf causes wilting


  • Water lost from the cells down water potential gradient
  • Pressure of water on the cell wall decreases
  • Turgor pressure decreases
  • Plants/cells rely on water for support/to prevent wilting
  • Water in cells not being replaced as quickly (as it is being lost)
• Plasmolysis


  • Cytoplasm/vacuole, decreases in size/volume
  • (Some) cell membrane/cytoplasm, pulls away/AW, from cell wall
  • Plasmolysis/cells are plasmolyzed
  • Cells, are flaccid/not turgid/lose turgor
  • Cell walls no longer, pushed outward/withstand pressure
• Plant cells don't burst because the cell wall offers resistance
• Features of diffusion that do not apply to active transport

  • Passive/does not require energy
  • Substances move down a concentration gradient
  • Does not have to occur across a membrane
  • Occurs with gases
  • No need for protein, carrier/channels/pumps
• Less fat in diet benefits by reducing cholesterol, risk of atherosclerosis/blockage of arteries/atheroma/stroke/CHD, risk of heart attack/heart diseases, risk of obesity, calorie intake
• More fiber in diet benefits by reducing risk of constipation, reducing risk of colon/bowel cancer, fiber absorbs and removes toxins
• Similarities between marasmus and kwashiorkor

  • Both caused by protein (energy) deficiency
  • Both types of malnutrition/deficiency disease/caused by lack of a balanced diet
  • Primarily affects children
  • Diarrhoea
  • Low body mass/weight loss
  • Poor growth
  • Irritability/tiredness
  • Wastage of muscles
  • Dry skin/brittle nails
  • Brittle hair/change of hair colour
  • Fatty liver
  • Anaemia
• Hydrogen carbonate changes colour in different pH
• Enzyme shape


  • Enzyme must be complementary shape to substrate to make enzyme-substrate complex/to allow substrate to bind to enzyme
  • Enzyme is specific to one substrate
• How enzymes function


  • Enzymes are proteins
  • Enzymes can be reused/are unchanged in a reaction
  • Enzymes are specific
  • Enzymes are catalysts/speed up reaction
  • Lowers (activation) energy needed for the reaction
  • Successful collisions
  • Active site
  • Enzyme and substrate fit together
  • Enzyme and substrate have complementary shape
  • Enzyme-substrate complex
  • Ref. to optimum temperature/pH
  • Too much heat results in denatured enzymes
  • Too little kinetic energy/heat, less (successful) reactions
  • Incorrect pH results in denatured enzymes
• Catalyst


  • Substance that increases the rate of (chemical) reactions
  • Not changed, during/by, the reaction
• Uses of carbohydrates in plants

  • Cellulose for cell walls
  • Starch for energy/respiration
  • To attract insects to flowers/nectar/fruits
• Uses of amino acids in plants

  • To make (named) proteins
  • For enzymes
  • For growth
• Gravitropism enables a plant to survive


  • Roots/plants growing down anchor the plant
  • Roots/plants growing down are more likely to reach water/(named) minerals
  • Shoots/plants growing up are more likely to reach light
  • Shoot or plant has better access to (named) pollinators
• Palisade mesophyll cells


  • Contain many chloroplasts/lots of chloroplasts
  • Are tightly packed
  • Are located near the top of the leaf
  • Arranged 'on end'/vertically/lengthways/columnar
• Parts of a plant can act as both a source and a sink


  • Source when it is (moving sucrose from) a region of production/photosynthesising
  • Sink when it is, growing/storing/respiring/a region of utilisation
• Roots adapted to absorb ions

  • Root hair (cells)
  • Through carrier molecules/AW
  • Large/increased, (surface) area (for absorption)
  • Roots grow continually (to find new sources of ions)
• Increase in temperature


  Increases rate of photosynthesis
• Increasing temperature above optimum

  Decreases rate of photosynthesis
• Bile


  • Emulsifies fats
  • Increases surface area of fat globules
  • For lipase to speed up/faster digestion of fats
  • Neutralises stomach acid
  • Alters/increases, pH for pancreatic/intestinal enzymes
  • Denatures pepsin
• Symptoms of anaemia


  • Tired/lethargic/'no energy'/weakness
  • Shortness of breath
  • Chest pain
  • Fast heartbeat
  • Frequent infections
  • Headache/dizziness/light headedness
  • Cold, hands/feet
  • Inflammation/soreness of tongue
  • Brittle nails
  • Unusual craving for non-nutritive substances, such as ice, dirt or starch
  • Poor appetite
  • Tingling or crawling feeling in legs
• Sickle cell anaemia


  • Mutation
  • Change in base sequence in gene for haemoglobin
  • Different sequence of amino acids
  • Inherit the allele
  • Having the recessive allele produce, abnormal haemoglobin
  • Red blood cells have sickle shape
• How people with sickle-cell anaemia inherit the disease

  1. Both parents carry the, recessive allele/allele for (sickle cell) anaemia

  2. Both parents are heterozygous

  3. Half the gametes of both parents have the recessive allele

  4. People/children, who are homozygous recessive have (sickle cell) anaemia

  5. There is is 25% chance of being homozygous recessive/having (sickle cell) anaemia
• Distribution of sickle-cell anaemia

  • People with sickle cell anaemia/heterozygous/carriers are resistant to malaria
  • People with (homozygous) sickle cell anaemia are, less likely to survive/die of sickle cell disease
  • People who are heterozygous are more likely to survive/have selective advantage
  • They are more likely to breed
  • Pass on allele for sickle cell
  • So increase in frequency of sickle cell allele (in population)
  • Selective advantage for sickle cell only exists where, (mosquitoes carrying) malaria are present
• Sickle-cell anaemia reduces oxygen delivery to tissues


  • Haemoglobin is, abnormal/rigid/AW
  • Abnormal haemoglobin carries less oxygen (than normal haemoglobin)
  • Red blood cells are, sickle shaped/AW
  • (Sickle cells) stick together/clot (in blood vessels)
  • Fewer red blood cells
• Effects of diarrhoea on the body

  • Watery faeces
  • Dehydration
  • Loss of, salts/ions/electrolytes
  • Cramps/stomach pain
  • Death
• Double circulation


  • One loop to lungs/pulmonary circulation, and one loop to rest of the body/systemic circulation
  • Blood flows through heart twice, for one (complete) circuit/to get back to the same point