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Doc Brown's Edexcel GCSE Science-Biology Revision Notes

EDEXCEL GCSE Science BIOLOGY UNIT B1 Influences on life

BIOLOGY UNIT B1 Topic 1 Classification, variation and inheritance

  • 1.1 You must be able to demonstrate an understanding of how biologists classify organisms according to how closely they are related to one another including:
    • The Five Kingdoms of Life are sub-divided into ...
    • a) Species – groups of organisms that have many features in common
    • b) Genus – contains several species with similar characteristics
    • c) Family – comprising of several genera
    • d) Order – comprising of several families
    • e) Class – comprising of several orders
    • f) Phylum – comprising of several classes
    • g) The Five Kingdoms of life are ...
      • animalia - all animals
      • plantae - all plants
      • fungi -
      • protoctista -
      • prokaryotes -  
  • 1.2 Be able to describe the main characteristics of the five kingdoms including:
    • a) Animalia – animals are multicellular, do not have cell walls, do not have chlorophyll, feed heterotrophically (heterotrophs can't make their own food)
    • b) Plantae – are multicellular, have cell walls, have chlorophyll, feed autotroprically (autotrophs can make their own food)
    • c) Fungi – multicellular, have cell walls, do not have chlorophyll, feed saprophytically (saprophytes feed off dead organisms and decaying material)
    • d) Protoctista – unicellular (single celled), have a nucleus, protoctista include algae
    • e) Prokaryotes – unicellular (single celled), have no nucleus e.g. bacteria
  • 1.3 Be able to explain why scientists do not classify viruses in any of the five kingdoms and regard them as non-living.
    • Viruses, which are smaller than bacteria, cannot reproduce themselves, have protein coat containing a few genes, they invade cells and make them reproduce the invading virus.
  • 1.4 Be able to describe the main characteristics of the phylum Chordata as animals with a supporting rod running the length of the body, an example of this being the backbone in vertebrates.
    • Vertebrates are divided into five classes, groups of amphibians, birds, fish, mammals and reptiles
  • 1.5 Be able to explain how scientists place vertebrates into the five groups based on:
    • a) Oxygen absorption methods – lungs, gills and skin
    • b) Reproduction – internal or external fertilisation, oviparous (lay eggs) or viviparous (give birth to live young)
    • c) Thermoregulation – homeotherms ('warm blooded' - kept at a constant temperature) and poikilotherms ('cold blooded' - body temperature varies with external temperature).
  • 1.6 Be able to demonstrate an understanding of the problems associated with assigning vertebrates to a specific group based on their anatomy and reproduction methods and why many vertebrates are difficult to classify.
    • The duck-billed platypus has a bill like a duck, tail like a beaver, its homeothermic, lays eggs but suckles its young. Not an easy one to classify! but its closer to a mammal than any of the other four vertebrate groups.
  • 1.7 Be able to discuss why the definition of a species as organisms that produce fertile offspring may have limitations:
    • Some organisms do not always reproduce sexually and some hybrids are fertile.
    • Some organisms can reproduce asexually but are still classed as the same species.
    • Many closely related species can interbreed producing viable offspring and technically classed as a different species.
  • 1.8 HT only: Be able to explain why binomial classification is needed to identify, study and conserve species, and can be used to target conservation efforts.
    • The binomial name of species consists of a two part Latin name (handy for use any country with its own language!).
      • The Latin name cannot be confused linguistically with 'local' or country names.
      • Study and identification produces a common data base of information on species-organisms with a universal name.
      • From the database, species at threat can be identified and preservation strategies put in place.
  • 1.9 Be able to explain how accurate classification may be complicated by:
    • a) variation within a species
    • b) HT only: hybridisation in ducks produces viable new species
    • c) HT only: ring species - a group of related populations that live near each other, neighbouring populations may interbreed but those well separated geographically may not. Sorting out which are genuinely different species is not easy.
  • 1.10 Be able to construct and use keys to show how species can be identified.
    • Does the organism do this or that? Structural features? etc. etc. working your way through an identification key.
  • 1.11 Be able to explain how organisms are adapted to their environment and how some organisms have characteristics that enable them to survive in extreme environments, including deep-sea hydrothermal vents and polar regions
    • In studying these examples know and understand that organisms, including microorganisms have features (adaptations) that enable them to survive in the conditions in which they normally live and some cases understand that some organisms have adapted to live in environments that are very extreme.
      • Know that so-called extremophiles may be tolerant to high levels of salt, high temperatures or high pressures.

      • Flamingos filter-feed on brine shrimp and blue-green algae and their pink or reddish color comes from carotenoid proteins in their diet of animal and plant plankton which can survive in the very salty lakes the flamingos fly to for feeding.

      • There are certain microorganisms, eg bacteria colonies, that live by hot volcanic vents of water on land (eg geysers) or on the seabed (where the vents are called 'black smokers'). The bacteria cannot rely on photosynthesis so they make there own food by using chemical energy derived from the minerals on and around the vent. These bacteria then become the producers for a food chain that can support several animal species - so we still have food chains and food webs in these extreme conditions.

      • There are creatures that happily live on the deep ocean beds where the pressure from the water above is enormous. Deep sea fish often have large mouths to collect scraps of food and/or large eyes to cope with dim light conditions.

    • Know and understand animals and plants may be adapted for survival in the conditions where they normally live, eg deserts, the Arctic.

      • Know and understand that animals may be adapted for survival in dry and arctic environments by means of:

        • Changes to surface area - heat/water transfer factor

          • Desert animals eg in Africa, tend to have a large surface area/volume ratio to allow excess body heat to be readily lost. This helps overheating, particularly as they do not sweat much and produce smaller volumes of concentrated urine, both helping to reduce water loss.

          • Animals living in very cold climates eg the arctic regions and northern Europe and Russia, tend to have a smaller surface area/volume ratio to minimise heat loss. Their bodies need to compact with a minimum volume - 'roundish' to minimise the surface area through which heat is lost. The arctic fox and wolves have short ears and a short snout to minimise surface area, hence minimise heat loss.

        • Thickness of insulating coat

          • Desert animals have thinner coats than animals in colder climates, which aids heat loss.

          • Animals living very cold climates have thick hairy coats to minimise heat loss, but the fur must be in good condition to trap insulating air and keep cold water away from the skin. The fur of animals like the arctic fox is an extremely good insulator and can survive at temperatures as low as -50oC. It has a long winter coat with thick dense underfur. Bears, similarly, have thick fur coats.

        • Amount of body fat

          • Desert animals have thin layers of body fat compared to animals in colder climates, which aids heat loss.

          • Animals in arctic regions have thick layers of insulating fat or blubber AND these also act as an important energy store - fat/blubber has a very high calorific value, useful in lean times and scarcity of food. eg seals, penguins, polar bears, whales

        • Camouflage

          • Desert animals have sand coloured coats which give good camouflage to minimise being seen and attacked by predators, it also the enables animal to a predator itself, prey becomes the hunter!

          • Arctic animals like polar bears have white fair to blend in with the icy/snowy background to increase the chances of a kill. Smaller white coated animals are less likely to seen and caught. The white arctic fox is a mean hunter! Birds like the ptarmigan stand a better chance of survival from predators turning white in colour in winter, and brown in the summer, thereby blending into the landscape with the change in seasons

      • Know and understand that plants may be adapted to survive in dry environments by means of:

        • changes to surface area, particularly of the leaves - through which water is naturally lost by transpiration

          • To reduce the surface area, to reduce water loss, plants like cacti have thin spines instead of broader leaves.

        • water-storage tissues

          • Plants like cacti have relatively thick fleshy stems which contain groups of specialised cells that store water. Some giant cacti like the saguaro cactus in the deserts of Arizona (USA) can be 20m high and hold in storage several tonnes of water - more than enough to see it through the driest of dry seasons.

        • extensive root systems

          • Cacti generally have one of two kinds of root system. (i) Some have relatively few roots, but roots that can burrow deep into the ground to seek out underground water. (ii) Other cacti have many shallow spread out roots that can rapidly absorb water eg if it rains, which may be very infrequent in desert regions.

  • 1.12 Be able to demonstrate an understanding of Darwin’s theory of evolution by natural selection including:
    • a) variation – most populations of organisms contain individuals which vary slightly from one to another, those with superior characteristics are more likely to survive,
    • b) over-production – most organisms produce more young than will survive to adulthood ensuring some will survive,
    • c) struggle for existence – because populations do not generally increase rapidly in size there must therefore be considerable competition for survival between the organisms,
    • d) survival - those with advantageous characteristics are more likely to survive this struggle,
    • e) advantageous characteristics inherited – better adapted organisms are more likely to reproduce successfully passing on the advantageous characteristics to their offspring
    • f) gradual change – over a period of time the proportion of individuals with the advantageous characteristics in the population will increase compared with the proportion of individuals with poorly adapted characteristics, and the poorly adapted characteristics may eventually be lost.
  • 1.13 Be able to describe and understand that variation within a species can be continuous or discontinuous.
    • The differences between species are usually pretty obvious and expected, but there is also variation within the same species eg different sizes, different shades of skin/hair/eye colour.
    • The causes of variation within a species are twofold - genetic and environmental.
    • Continuous variation is when a particular characteristic of an individual lies within a range with no distinct category.
    • Discontinuous variation occurs when a particular characteristic of the species fits into a few particular and specific categories with no range of variation.
  • 1.14 Appreciate the variations within a species to illustrate continuous variation and discontinuous variation.
    • Examples of continuous variations in humans include height, weight, length of hand
      • When data from a large sample are plotted in the form of a distribution graph of number of individuals (y axis) versus the value or short value range of the characteristic on (x axis), the graph is usually a 'bell shape' which statistically is described as a 'normal distribution' graph curve.
        • The peak of the graph occurs at the mean value for a given statistical population.
        • The most common values will occur around the average , with a low probability of a value of being very low or high in the character data range.
    • Examples of discontinuous variation in humans include eye colour, blood grouping,
      • Any graph of the number of individuals versus the characteristic will not show any systematic curve that you see with continuous variation graphs.
  • 1.15 Be able to interpret information on variation using normal distribution curves
  • 1.16 Be able to demonstrate an understanding of the causes of variation, including:
    • a) genetic variation – different characteristics as a result of ...
      • (i) mutation - mutations that are inherited may change the characteristics of the species
      • (ii) reproduction - the 'controlled randomness' of the possible gene combinations of the offspring inherited from their parents ensures that no offspring can be identical to either parent.
    • b) environmental variation – different characteristics caused by an organism’s environment (acquired characteristics) eg
      • sun tan caused by extra melanin pigment on exposure to lots of sunlight,
      • withering unhealthy plants drying to grow in dry soil, or too shaded light conditions
  • 1.17 HT only: Demonstrate an understanding of how speciation occurs as a result of geographic isolation.
    • A species is group of similar organisms that can interbreed to give fertile offspring.

    • Speciation is the development of a new species and can happen when populations of the same original species becomes so different (genetically) that they can no longer interbreed to give fertile offspring.

    • Speciation can occur via isolation – two populations of a species become separated, eg geographically,

    • In the two geographical regions, the climate might be different, the other plants and animals may be different.

    • However, if each population can survive, by the process of natural selection, two distinct species can evolve (or perhaps one population remains the same, but the other has to adapt to a different environment).

  • 1.18 Be able to explain how new evidence from DNA research and the emergence of resistant organisms supports Darwin’s theory.
    • DNA research suggests that all life has common origins, we all have a line of ancestors going back hundreds of thousands or millions of years.
    • DNA analysis shows a close relationship between species that have relatively recently diverged from a common ancestor (a high percentage of our DNA is the same as the DNA of apes!).
    • Evolution has been driven by small changes in DNA over many generations and this gradually changes the nature of the species and due to speciation, can lead to new species.
    • Today we can see evolution in action and the survival of the 'fittest genes' eg
      • The deadly bacteria MSRA is a strain of microorganism that has survived and prospered by having genetic characteristics making it resistant to most antibiotics.
      • Bacteria (and viruses) can mutate quite quickly and those most resistant (and carried by us!) will tend to multiply at the expense of bacteria killed by antibiotics (less carried by us!).
      • Certain strains of rats have become resistant to the poison warfarin.
  • 1.19 Be able to explain the role of the scientific community in validating new evidence, including the use of:
    • a) scientific journals - enable new findings to be communicated to other scientists working in the same areas of science, so ideas and knowledge are widely spread AND other scientists can check whether the research is valid eg do other scientists get the same results? do other scientists draw the same conclusions? do other scientists agree with, and find the theory valid?
    • b) the peer review process - a sort of refereeing system, research papers are read and checked by people competent to understand the contents of research papers (their peers) - this ensures standards are high in terms of 'good scientific practice'.
    • c) scientific conferences enable scientists to meet and present and discuss their findings, compare their work, listen to new ideas, get ideas to take back to their own research project. Its also a forum for other scientists to hear about research which isn't necessarily exactly their own specialist field, but broadens their own knowledge of related fields of science.
  • 1.20 Be able to describe the structure of the nucleus of the cell as containing chromosomes, on which genes are located.
    • Human cell nuclei contain 23 pairs of chromosomes.
    • Chromosomes carry the genes which control the development and subsequent characteristics of an organism.
  • 1.21 You must understand that genes exist in alternative forms called alleles which give rise to differences in inherited characteristics.
    • A gene is a shorter section of the huge DNA coiled up molecules that make up chromosomes.
    • Alleles are essentially different versions of the same gene.
      • Therefore eg in humans, between the two copies of the chromosomes you can have two alleles the same (homozygous) or different (heterozygous) for a particular gene.
    • Individual alleles can be 'dominant' or 'recessive' in character - see section 1.22.
    • In section 1.21 remember alleles are different versions of the same gene.
  • 1.22 Know the meaning of, and use appropriately, the terms:
    • genotype - a 'bit of genetic code' pairs of or individual alleles eg XX, XY, X, Y (and it is the genotype pairs that give rise to the phenotype you observe in the organism).
    • dominant - if two alleles for a characteristic are different (heterozygous) then only one of the alleles can determine the nature of the characteristic - know as the dominant allele (usually shown as a capital/upper case letter) eg a gene for height might be H, so HH or Hh genotypes will give a tall organism. A dominant allele will override a recessive allele.
    • recessive - if an allele is not dominant, it is described as recessive (small/lower case letter), and, in order for the recessive allele to be expressed in the phenotype observed, you must have a double recessive allele eg homozygous genotype hh will give rise to phenotype short.
    • homozygous - if a pair alleles for a characteristic are the same on a gene eg genotype XX (phenotype female)
    • heterozygous - if a pair of alleles for a characteristic are different on a gene eg genotype XY (phenotype female)
    • phenotype - the result of 'gene expression' - the nature of the characteristic you see eg tall, blue eyes, male etc.
  • 1.23 Be able to analyse and interpret patterns of monohybrid inheritance using a genetic diagram, Punnett squares and family pedigrees.
    • Just an example, consider some of the results of Mendel’s work which preceded the work by other scientists which links Mendel’s ‘inherited factors’ with the chromosomes of the humble pea.
    • Punnett square genetic table for crossing tall pea plants with dwarf pea plants
      Parent genotypes: TT x tt
      Gametes: T and T
      Genotypes of children T T
      t Tt Tt
      t Tt Tt
    • This gives 100% tall plants (genotype Tt), but they all carry the allele t for dwarf pea plants
    • The diagrams above and below give a modern genetic interpretation of Mendel's results from initially crossing a pure line of tall pea plants with a pure line of dwarf pea plants (F1) and then cross-breeding their offspring to give F2.

    • Punnett square genetic table for crossing tall pea plants
      Parent genotypes: Tt x Tt
      Gametes: T and T
      Genotypes of children T t
      T TT Tt
      t Tt tt
    • The first resulting offspring (F1) were all tall pea plants, and these were then crossed with each other, to give the second set of offspring (F2) shown above.

    • This gave approximately 75% tall plants (genotype TT or Tt) and 25% dwarf pea plants (genotype tt)
    • Mendel found that the second cross produced tall : dwarf pea plants in the approximate ratio of 3 : 1.

  • 1.24 Be able to calculate and analyse outcomes (using probabilities, ratios and percentages) from monohybrid crosses.
  • 1.25 Be able to describe the symptoms of the genetic disorders:
    • a) sickle cell disease
      • Sickle cell anaemia is a genetic (inherited) blood disorder in which red blood cells (the carriers of oxygen around the body), develop abnormally. Instead of being round and flexible, the sickle red blood cells become shaped like a crescent (hence the name 'sickle'). These abnormal red blood cells can then clog sections of blood vessels (especially the narrow capillaries) leading to pain. These painful effects can last from a few minutes to several months. The abnormal blood cells have a shorter life-span and are not replaced as quickly as normal healthy red blood cells leading to a shortage of red blood cells, called anaemia. Symptoms of sickle cell anaemia include tiredness, painful joints and muscles and breathlessness, especially after exercise ie any extra physical exertion.
    • b) cystic fibrosis
      • Cystic fibrosis is a genetic disorder disease passed down through families. Cystic fibrosis causes thick, sticky mucus to build up in the lungs, digestive tract, and other areas of the body and is one of the most common chronic lung diseases in children and young adults. Sadly, it is a life-threatening disorder caused by a defective gene which causes the body to produce abnormally thick and sticky fluid, called mucus. The thick mucus builds up in the breathing passages of the lungs (causing lung infections) and in the pancreas, the organ that helps to break down and absorb food (causing digestion problems).

  • 1.26 HT only: Be able to evaluate the outcomes of pedigree analysis when screening for genetic disorders:
    • a) sickle cell disease
    • For sickle cell anaemia to occur in a child, both parents must carry the recessive allele for sickle cell disease, but neither is affected by it.
    • However, there is a 1 in 4 chance that one of their children will be affected by this genetic disorder - refer to table and diagram below, which shows a double recessive allele is needed for the offspring to be affected (genotype aa).
    • Punnett square genetic table for sickle cell anaemia
      Genotypes of parents: Aa x Aa

      normal but both carriers

      Gametes: F,f plus F,f
      Genotypes of children A a
      A AA Aa
      a Aa aa
    • b) cystic fibrosis
    • The parents may be carriers of the cystic fibrosis disorder without actually having the disorder themselves.

    • It is caused by a recessive allele of a gene and can therefore be passed on by parents, neither of whom has the disorder.

    • Punnett square genetic table for cystic fibrosis
      Genotypes of parents: Ff x Ff

      normal but both carriers

      Gametes: F,f plus F,f
      Genotypes of children F f
      F FF Ff
      f Ff ff
    • Cystic fibrosis is caused by a recessive allele f (so it needs genotype ff, a double recessive allele).

    • The genetic diagram shows that both parents must be carriers of the recessive allele and there is a 3/4 chance of having a normal child (FF non-carrier or Ff carrier) and a 1/4 chance of having a child with cystic fibrosis (ff sufferer and carrier).

 

 

 

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