AS Biology Unit 2

variation
1. sampling
  1. sampling bias- when unrepresentative choices are made either deilberately or unknowingly by investigators.
  2. chance-even if sampling bias is avoided, the individuals chosen may not be representative.
  3. random sampling is the best way to prevent sampling bias and a method of doing this is to measure out the area studying and divide into a grid of numbered lines. use a random number generator and use the number given as a coordinate and then tae samples from each coordinate given.
  4. using a lagre sample size prevents the results from becoming invalid as it reduces the likeliness that chance will affect the results.
2. causes of variation
  1. genetic differences such as mutations in genes which may or may not be passed on through each generation.
  2. meiosis-forms gametes which mix up genetic material before its passed to offspring to produce differences
  3. fusion of gametes- in sexual reproduction, offspring inherit characteristics from both parents so causes variation.
DNA and meiosis
1. 4 bases... Guanine, adenine, thymine and cytosine, each paired together by hydrogen bonds
2. nucleotides are made up of a deoxyribose sugar, a phosphate group and an organic base.
3. DNA shaped in a double helix with deoxyribose and phosphate providing a backbone around the pairs of bases.
4. DNA is the material responsible for passing genetic information from cell to cell and generation through generation.
5. the triplet code- when trying to find how DNA bases code for amino acid, scientists found that there must be a minimum of 3 bases that code for each amino acid. they found this as only 20 amino acids occur in proteins and there are 4 bases so 20/4=3
6. homologous chromosomes- a pair of chromosomes, one maternal and one paternal that have the same gene loci therefore produce the same features.
7. allele- a form of a gene
8. meiosis- 1st division- the homologous chromosomes pair up and their chromatids wrap around each other. one chromosome from each pair goes into one of the two daughter cells produced.
9. meiosis 2nd division- the chromatids move apart, four cells are formed and in humans, each one of these cells contains 23 chromatids.
10. meiosis produces variation due to independent segregation and recombination by crossing over.
11. in crossing over, the chromatids of each pair become twisted around one another causing tensions which causes part of the chromatid to break off. the broken bits then rejoin with the chromatids of it homologous partner.
genetic diversity
1. selective breeding- known as artificial selection which involves choosing which individuals characteristics are wanted and using them to parent the next generation.
2. the founder effect- this occurs when just a few individuals colonise a new region. these few people will carry only a few alleles from the population so these alleles will then be passed on through generations showing no genetic diversity
3. genetic bottlenecks- populations sometimes suffer from a dramatic drop in population numbers . this could be due to a chance event e.g a volcano/earthquake. the few survivors will possess a small variety of alleles which will be passed on through generations so will have a low genetic diversity.
variety of life
1. haemoglobin-a grop of chemically similar molecules which consist of four polypeptide chains. they each have a quaternary structure in which all four polypeptides are linked together to form a spherical molecule. each Hb has a haem group which can combine with a single oxygen molecule.
2. each haemoglobin can carry four oxygen molecules maximum
3. the role of Hb is to transport oxygen and to do this it must readily associate with oxygen at the surface where gas exchange takes places and also readily dissociate from oxygen at tissues that require it.
4. haemoglobins with a high affinity for oxygen take up oxygen easily but release it less readily. Hb's with a low affinity for oxygen struggle to take up oxygen but release it more readily.
5. in an area of little oxygen, a haemoglobin with a high affinity for oxygen is required but an organism with a high metabolic rate needs to release oxygen readily into its tissues so needs HB with a low affinity for oxygen.
the cell cycle
1. nuclear division- the process by which the nucleus divides (mitosis/meiosis)
2. cell division- follows nuclear division and is the process in which the whole cell divides.
3. mitosis produces two daughter nuclei that have the same number of chromosomes as the parent cell and each other.
4. stages of mitosis...interphase, prophase, metaphase. anaphase, telophase.
5. interphase- when DNA replicates, the chromosomes are invisible and the cell is synthesising proteins.
6. prophase- chromosome become visible and the nuclear envelope disintegrates.
7. metaphase- chromosomes line up in the middle of the cell and spindle fibres form.
8. anaphase- spindle fibres attach to chromatids and pull chromatids apart towards the poles.
9. telophase- nuclear envelope reforms and spindle fibres disintegrate. two daughter nuclei are formed.
10. cell cycle has 3 stages... interphase, nuclear division and cell division.
cellular organisation
1. epithelial cells- found in animals and consist of a sheet of cells that line the surface of organs and often have a protective function.they can also be found where diffusion takes place e.g the alveoli of the lungs.
2. xylem- occurs in plants and is used to transport water and mineral ions throughout the plant and also gives mechanical support
3. an organ is a combination of tissues that are coordinated to perform a variety of functions. e.g the digestive system which is an organ system
exchange and transport
1. things which need to be interchanged between an organism and its environment... respiratory gases, nutrients, excretory products and heat
2. exchange surfaces to allow effective transfer of materials...a large surface area to volume ratio, very thin so that the diffusion distance is short, partially permeable to allow selected materials to cross without obstruction, movement of the internal medium e.g blood to maintain a diffusion gradient.a
3. gas exchange in insects... to reduce water loss they have waterproof coverings on their body surfaces and a small surface area to volume ratio to minimise the area over which water is lost.
4. insects use tracheae to diffuse respiratory gases by an internal network of small tubes called tracheoles.this allows air to be brought directly to respiring tissues.
5. how gases move in and out of insects... along a diffusion gradient- oxygen is used up and so the concentration towards the end of the tracheoles falls creating a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere to the cells.
6. gases enter and leave tracheae through tiny pores called spiracles. these spiracles can be opened and closed by a valve. this means that when spiracles are open, water can evaporate from the insect but most of the time spiracles are closed to prevent water loss.
7. plant leaves and gas exchange- most occurs in the leaves which have adaptations for rapid diffusion... a thin flat shape produces a large surface area, many small pores (stomata) and numerous interconnnecting air spaces that occur throughout mesophyll.
8. stomata- minute pores on the underside of leaves which are surrounded by guard cells which can open and close to control the rate of gaseous exchange which is important to prevent water loss by evaporation.
classification
1. a species is similiar to one another but different to members form other species.
2. a species is capeable of breeding to produce living fertile offspring and therefore when reproducing sexually, any of the genes of individuals can be combined with any other so belong to the same gene pool.
3. the bionomial system is used to name species and the first (generic) name says which genus the organism belongs and the second (specific) name says which species the species belongs to.
4. both names are printed in italics or if handwritten are underlined to indicate the scientific names
5. classification- the grouping together of organisms
6. taxonomy is the practice of biological classification.
7. artificial classification divides organisms due to differences such as colour, size or number of legs etc.
8. natural classification is based upon evolutionary relationships between organisms and their ancestors.
relationships between organisms
1. DNA determines the the proteins of an organism and proteins determine the features of an organism. by comparing DNA and proteins of different species helps scientists to determine evolutionary relationships between them.
2. DNA hybridisation depends upon particular properties of of DNA double helix. when DNA is heated it splits into 2 complementary single strands.
3. when cooled the complementary strand bases on each strand recombine, using this property, DNA hybridisation can be used to compare the DNA of two species.
4. hybridisation... DNA from two species is extracted and cut into short pieces. the DNA from one species is then labelled by fluorescent raker and mixed with the other species. the mixtures of both species DNA is heated to separate strands and then cooled to allow strands to reform with complementary bases.
5. hybrid strands once cooled can then be seperated out and the temperature be increased in stages.the higher the temperature, the more hydrogen bonds are formed so the more hydrogen bnds formed, the stronger the hybrid strand will be showing that the species are closely related if a high temperature is reached.
6. courtship behaviour is the behaviours shown when two members of the same species want to breed. it helps to achieve finding a mate as it allows animals to recognise members of their own species, to identify a mate that is capable of breeding and it allows them to form a pair bond.
adaptation and selection
1. mutations- are changes in DNA that result in different characteristics.
2. conjugation- occurs when one bacterial cell transfers DNA to another.
3. one cell produces a thin projection that meets another cell and forms a conjugation tube between the cells. the donor cell then replicates one of its smaller plasmids. the plasmid is then broken to make it linear before it passes through the tube into the recipient cell. contact between cells is brief so only a portion of the donors DNA is transferred. the recipient cell acquires new characteristics from the donor cell.
4. antibiotics- are substances produced by living organisms that can destroy or inhibit the growth of microorganisms.
5. antibiotics work by preventing bacteria from making normal cell walls.they inhibit the synthesis and assembly of peptide cross linkages between bacterial cell walls. this weakens the walls making them unable to withstand pressure so are unable to prevent water from entering and so osmotic lysis occurs killing the bacterium.
biodiversity
1. species diversity-the number of different species and the number of individuals of each species within any one community
2. genetic diversity- the variety of genes possessed by the individuals that make up any one species
3. ecosystem diversity- refers to the range of different habitats within a particular area.
4. agriculture- can cause species diversity to drop due to farers selecting crops for specific characteristics so only a few features are grown whereas a large range of species needs to be available to be economic.
5. deforestation- many different species are adapted to living in forest/ woodland environments and since deforestation is the clearing of forests for grazing/housing/reservioirs, many of these habitats are being destroyed causing a drop in the biodiversity
starch, glycogen and cellulose
1. starch
  1. a polysaccharide that is found in many parts of a plant in the form of small grains
  2. it is an important component of food and is a major energy source in most diets.
  3. made up of chains of alpha glucose monosaccharides linked by glycosidic bonds that are fromed by consentration reactions.
  4. the unbranched chain is tightly coiled making the molecule very compact.
2. glycogen
  1. similar structure to starch but has shorter chains, is highly branched and is found in animals
  2. it is stored as granules in the muscles and liver and due to the shorter chains it is more readily hydrolysed to alpha chains.
3. cellulose
  1. differs from starch and glycogen as it is made up of monomers of beta glucose. to form glycosidic links, this means that each beta glucose molecule has to rotate 180 degrees to its neighbour.
  2. unlike starch, cellulose has straight unbranched chains that run parallel to one another allowing hydrogen bonds to form cross linkages between adjacent chains.
  3. due to a large number of hydrogen bonds, cellulose is fairly strong making it a valuable structural material. due to this cellulose is a major component in plant cell walls and provides the plant with rigidity.
  4. the cellulose cell wall also prevents the cell from bursting as water enters by osmosis. this occurs by it exerting an inward pressure to stop any further influx of water.

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AS Biology Unit 2

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