Edexcel AS Biology Unit 2: Topic 4

The whole of Edexcel Biolgy's unit : topic 4. Everything you need to know!

  • Created by: Lu
  • Created on: 30-05-13 14:10

Habitats and Niches

  • A species is a group of organisms with similar morphology, physiology and behaviour, which can interbreed to produce viable, fertile offspring, and which are reproductively isolated (In place, time or behaviour) from other species.
  • habitat is the place where an organism lives
    • It has a particular set of conditions which supports a distinct combination of organisms.
    • Within a habitat there may be many populations - A population is a group of interbreeding individuals of the same species, found in an area.
    • The various populations in an area make up a community
  • Species occupy different niches
    • A niche is "the way an organism exploits its environment"
    • All the species occupying a habitat have different niches
    • If two species in the same habitat occupy the same niche, they will compete with each other. The better-adapted organisms will out-compete the other and exclude it from the habitat.
  • Orchid niches - Orchids mimic female insects to trick their pollinators into psedocopulation
  • Woodpecker niches - All woodpeckers have a powerful beak which they use for probing into rotting wood, and a very long tongue to pick up insects. Woodpeckers haven't reached Madagascar or the Galapagos Islands where other organisms fill their niche. In Madagascar the Aye-aye uses its long bony finger in a similar way to woodpeckers beaks.
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Adapting to the Environment

  • Being adapted means being speicalised for the environment.
  • There are three categories of adaptations:
    • Behavioural - Actions helping an organism survive or reproduce
      • Plants turning leaves towards the sun
      • Rodents burying nuts for future consumption
    • Physiological - Internal features of organisms helping them survive or reproduce
      • Insects developing distasteful blood to keep away predators
      • Danish scurvy grass adapting to tolerate high salt concentrations to live on icy roads with salt on them, or very near to the sea
    • Anatomical - Observable structures (Such as body parts) that have adapted to help them survive or reproduce
      • Bumblebees developing long tongues to reach down the corolla of flowers
      • Elephants developing large, thin ears for heat loss
  • Co-adaptation is where two organisms become dependant on eachother and evolve in tandem
    • Brazil nuts developed hard shells to avoid being eaten. The agouti is the only animal that developed strong enough teeth to eat them, but they also bury some for later consumption. Those that are buried and not eaten grow into new trees. The brazil nut relies on the agouti for seed dispersal and the agouti relies on the brazil nut for food.
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How Adaptations Come About

  • Natural Selection - Organisms change over time corresponding to their changing environment
    • Survival of the fittest - Organisms possessing phenotypes that put them at an advantage over other organisms in a species are more likely to survive and pass on their genes
    • Evolution is "a change in allele frequency in a population over time"
      • For natural selection to lead to evolution, there must be genetic variation
      • A selectively neutral allele can become advantageous when the environment changes
    • 1) A population naturally has genetic variation with new alleles created through mutations
    • 2) A change in the environment causes a change in selective pressures on the population
    • 3) An allele that had no paticular advantage becomes favourable
    • 4) Organisms with the allele are more likely to survive and reproduce
    • 5) Offspring are more likely to have this allele. It becomes more common in the population
  • Natural selection is happening constantly
    • If environment stays constant, organsisms become better adapted to their existing niches
    • Common example of changing environment creating resistence is in human head lice
  • The ability of a population to adapt depends on: the strength of the selection pressure, the size of the gene pool, and the reproductive rate of the organism
  • A gene pool consists of all the genes present in a population
  • Being perfectly adapted to a specialised niche means an organism can avoid competition, but is vunerable to the environment changing
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Classifying Organisms

  • Biodiversity is the variety of different organisms due to evolution by natural selection
  • Scientists have different ways of identifying species
    • Binomial system of names: part 1 is the genus, part 2 defines the individual species
    • Dichotomous key allows species identification through a series of 'two answer' questions
    • Taxonomy places organisms into groups based on features into a hierarchical system:
      • Kingdom - Animalia, Plantae, Fungi, Protoctista, Prokaryotae
      • Phylum
      • Class
      • Order
      • Family
      • Genus
      • Species
    • Phylogenic trees show the evolutionary relationships of prokaryotes from RNA sequencing
    • Wose discovered there were three Domaines: 
      • Archaea
      • Bacteria 
      • Eukarya
    • Using DNA classification is the most accurate way of classing animals as closely related animals may have very different phenotypes and vice versa
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Quantifying Biodiversity

  • Genetic diversity is the amount of variation between organisms in a species.
  • Sources of genetic variation are:
    • Chromosome and gene point mutations - A source of new genetic material
    • Meiosis - Independent assortement and Crossing over
    • Mate selection - Different combinations of alleles come together. Conservationalists can choose which animals or plants mate to maximise genetic diversity
    • Random fertilisation -Different combinations of alleles depending on the gametes involved
  • Species richness - The number of different species in a given habitat
  • Species evenness - The proportions of different species in a given habitat
  • A very common species in a habitat is called the dominant organism
  • If a species becomes dominant at the expense of the others, it becomes a pest
  • An endemic species is a species found only in a particular area
  • A Biodiversity hotspot is an area with one of the greatest amounts of species richness and evenness
  • Genetic diversity in a species can be measured directly through DNA sequencing
    • This determines the bases in a segment of DNA and thus determines which alleles are present
    • This can be used to work out the heterozygosity index - the proportion of genes that are present in heterozygous form
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General Plant Structure

  • Plants must grow tall to maximise photsyntheitic activity. They therefore must be very strong to hold their own weight and withstand strong winds, they do this by:
    • Producing strong cell walls out of cellulose (a polymer made of sugar molecules)
    • They build columns and tubes from specialised cells
    • They stiffen these cells with lignin (another polymer)
  • Trees add another ring of lignified tissue each year, producing a strong but flexible structure
  • Generalised structure of a plant cell:
    • Ridgid cell wall
    • Cell surface membrane (with pits and plasmodesma between cells)
    • Mitochondria
    • Nucleus (with nuclear envelope, nucleolus and chromatin)
    • Rough endoplasmic reticulum
    • Smooth endoplasmic reticulum
    • Chloroplast
    • Vacuole (with vacuolar membrane [tonoplast])
    • Ribosomes
    • Golgi apparatus
    • Amyloplasts (containing starch grains)
  • There are also parechyma tissues between specialised tissues throughout the plant. In some places they also have specialised functions, eg. storage in roots
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Cell Walls

  • Plants' strength comes partly from cellulose cell walls and "glue" holding them together
  • Cellulose is made up of β-glucose molecules, bonded together through condensation reactions
    • Every alternate molecule is inverted
    • This allows the 1,4 glycosidic links to form
    • It is a long, unbranched molecule (there are no 1,6 glycosidic links)
    • The chains remain straight and hydrogen bonds form between chains forming bundles called microfibrils.These make up the cell walls.
    • Microfibrils are wound in a helical arrangement & stuck together with polysaccheride 'glue'
      • The polysaccheride glue is made of hemicelluloses and pectins
      • These bind to the microfibril surfaces and to eachother to hold them together
    • The microfibrils are layed at different angles making the structure stong and flexible
  • Cell walls don't completely separate plant cells, there are narrow fluid filled channels between them called plasmodesmata crossing cell walls making the cytoplasm continuous throught he cells
    • Cell walls are fully permeable to water and solutes
  • At some places, called pits, the cell wall is thin because only the first layer of cellulite is deposited. Plasmodesmata are often located in these pits, aiding the moement of substances between cells
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Plant Stems

  • To grow a tall plant, some cells in the stem must be stiffened to provide support, and some cells must be able to transport water and other minerals from the roots to the leaves.
    • There are two types of cell specialised for these function:
      • Xylem vessels - forming tubes for transport and stiffened cell walls for stem support
      • Sclerenchyma fibres - columns of these with stiffened cell walls also provide support
  • There are three basic types of tissue found in plants: 
    • Dermal tissue (epidermis)
    • Vascular tissue
    • Ground tissue
  • The stem is made up mainly of parenchyma tissue, surrounded by a thin layer of collenchyma tissue and then the epidermis. 
    • The vascular tissue is found in bundles towards the outside of the stem. It is made up of:
      • The xylem vessels (for transport of water and inorganic ions)
      • The phloem sieve tubes (transports sugar from photosynthesis up and down the stem)
      • The sclerenchyma fibres (on the outside of the bundles, for structure)
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Xylem Vessels

  • Xylem vessels are made up of large cells, with thick cell walls
  • They form a column acting as tubes for transport of water and mineral ions
  • The cell walls are waterproofed with lignin
    • This impregnates the cellulose walls and as the cell becomes lignified, the entry of water and solutes is restricted. Simultaneously, the tonoplast breaks down and there is autolysis of the cell contents: the organelles, cytoplasm and membrane breaks down, leaving a hollow tube
  • There are pits for movement between vessels and the end walls are lost or hightly perforated
  • Water is lost from a plant by diffusing down the stomata and evaporating from all surfaces
    • This evaporation provides the force needed to draw water up a plant;
      • The xylem vessels produce a massive pull on the water behind them, through capilary action, this is enough to draw water up the whole stem of the plant
      • This stream of water passing through the plant is known as the transpiration stream
      • Energy for water movement comes from the sun which heats & evaporates the water
      • The water is under a lot of tension, but the xylem vessels don't break because of the cohesive forces between water molecules
      • This is known as the cohesion-tension theory
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Strength in Plants

  • Water is polar, which leads to many of the properties of it that are vital to plants 
    • Hydrogen bonds between water molecules give it cohesion 
    • Hydrogen bonds between water molecules and cell walls give it adhesion 
    • It has solvent properties so can carry substances around the plant 
    • It takes a lot of energy to heat a little bit of it so it keeps a constant internal temperature
    •  It expands as it freezes and therefore floats so plants can survive in water underneath ice
  • The movement of water through xylem vessels provides a mass flow system
  • The inorganic ions required by all plants are:
    • Nitrates - for making amino acids, chlorophyll, nucleic acids and ATP
    • Magnesium - a component of chloryphyll 
      • (Deficiency of nitrates and magnesium makes leaves yellow)
    • Calcium - for structure of the cell wall and permeability of cell membranes
    • Potassium - needed for photosynthesis and respiration enzymes
    • Phosphates - a component of cell membranes
  • Both xylem and sclerenchyma fibres are lignified for strenght. The taller a plant must grow, the more cells become lignified
  • Turgid cells completely full cells. Plants rely on fully turgid parenchyma cells for strength.
    • If a cell loses water, turgor is lost. This causes a plant to wilt.
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Uses of Plants

  • Plant fibres can be obtained manually by pulling them out or by 'retting' (digesting the polysaccherides that hold them together using caustic alkali, enzymes or fungi and bacteria)
  • These are used in textiles, but also biocomposites and biomass to make other things
  • Plants sometimes store toxic compounds in hairs on their leaves, eg. stinging nettles
  • Some have antimicrobial properties (ability to destroy or inhibit the growth of microorganisms):
    • Mint - this is why it is used in toothpastes
    • Garlic can destroy Campylobacter and Helicobacter (which cause intestinal infections)
      • Some strains of these have become resistant to penicillin
      • Garlic contains allicin which interferes with lipid synthesis and RNA production
      • Typically the seed coat, fruit coat, bulb and roots have greater antibacterial properties
  • Many plants contain poisons that at low dosage, kill pathogenic microbes without killing humans. We extract these and use them as medicines (eg. morphine and aspirin)
  • Case investigation: in the 1700's, foxgloves (which are poisonous to humans) were found to cure a disease called dropsy (oedema) when taken in small does in a tea. Withering discovered how to achieve the correct does by slowly increasing the does on patients until they began showing negative side effects. Just below this dose would be the most effective
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Drug Testing

  • Pre-clinical testing
    • Studies are done on animals and isolated cells or tissue cultures in laboratories
    • This determines whether the compound is both effective and safe for human testing
    • This is authorised by the Medicines and Healthcare Products Regulatory Agency
  • Clinical trials - Phase 1
    • The drug is tested on a small group of volunteers
    • Determines if the drug is absorbed, distributed, metabolised and excreted as predicted
    • The affects of different doeses are monitered
  • Clinical trials - Phase 2
    • Small groups of volenteers (10-300 people) with the disease are treated with the drug to see its effectiveness
  • Clinical trials - Phase 3
    • Double blind randomised controlled trial
    • A lrage group of volenteers with the disease are split into two groups - One recieves the treatement and the other is given a placebo
    • Neither the doctor nor the patient knows which group they are in
    • They montitor both groups for improvements and adverse reactions
  • Licensing
    • It is marketed but trials continue to assess effectiveness and safety
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Plant Seeds

  • Seeds are adapted to: 
    • Protect the embryo - through lingification of the outer lyer (testa)
    • Aid dispersal - through hooks, wings, fruits, dehiscence, lightness, fibrous seed coats
    • Provide nutrition for plant - With the endosperm and stored in the cotyledon or hypocotyl
  • Seeds remain dormant until conditions are correct for germination
    • When conditions are correct, water is absorbed through a small pore in the seed coat.
    • This triggers metabolic changes, & production of plant growth substances is switched on
    • This causes the secretion of maltase, amylase, proteases and lipases to break down starch into glucose, proteins into amino acids, and lipids into glycerol and fatty acids
    • Glucose is converted to sucrose for transport to the radicle and plumule
  • Starch from seeds can be used for:
    • Thickening glues and custard - starch swells & thickens water when heated (gelatinised)
    • Stiffening fabrics and paper - Using water and heat, starch can stiffen and soften again
    • Super-absorbants - Chemically crosslinked, gelatinised & dried strach is very absorbant
    • Starch foam - Starch gelatinised at high temperatures and pressures expands into foam pastels which are used for packaging
  • Vegetable oils from seeds can be used in cooking and fuels
    • Biodiesel produces less sulphur dioxide and carbon dioxode than regular diesel
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  • The use of oil-based plastics and fuels is not sustainable because:
    • Burning oil-based fuels releases CO2 into the atmosphere (contributing to global warming)
    • Oils reserves will eventully run out
    • Plastics are non-biodegradable causing waste-disposal problems
  • Using plants reduces these problems as they use as much CO2 as they produce, more can be grown and they are biodegradable
  • However, plant based products are not always suitable replacements for plastic, and are heavier so cost more and produce more pollution to transport
  • Biodegradability also does not always work as bacteria involved require oxygen so it cannot happen deep underground. At the surface where it is effective it can still produce methane
  • Also, the planet may not be big enough:
    • New plants grown for products take up much room, so rainforrests are being cut down
    • Many uncultivated areas are unsuitable, or needed for wildlife and water conservation
    • Increasing agriculture is very energy-demanding
    • The human population is rapidly growing
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The Role of Zoos I

  • Centres for scientific research
    • Enabling us to understand how to conserve particular speices
    • Captive breeding programmes
      • Increasing the number of individuals in the species (If they are endagered)
      • Maintaining genetic diversity within the captive population
      • Reintroducing animals into the wild
    • Genetic diversity is lost through:
      • Genetic drift - The change in allele frequency. In a small population, some alleles may not get passed onto offspring by chance, due to random fertilisation.
      • Inbreeding depression - Inbred offspring are less fit for their environment (they may be smaller, not live as long, and the females may produce less eggs. In a small population, the likelyhood of closely related individuals breeding increases. This increses the frequecy of homozygous genotypes, and therefore increases the inheritance of recessive alleles from both parents. Many recessive alleles have harmful effects, so interbreeding depression results.
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The Role of Zoos II

  • Conserving genertic diversity can be improved by keeping studbooks 
    • these show the history and location of all the captive animals of a speices. 
    • Breeding plans are based on these, ensuring genes from founder memebers of the group are equally represented in a population 
      • 'bad' breeders are encouraged whilst overly-successful breeders are held back
    • Cytogenics and molecular biology support these studbooks by giving information on the nature of the genes themselves
      • These can reveal which individuals are best matched to maximise genetic diversity, or if individuals are more closely related than previously thought
  • Reintroducing animals into the wild
    • Animals must be taught skills (eg.hunting) to be able to survive in the wild
    • Keeping animals in enclosures that best replicate their native habitats is beneficial
    • Reintroduced animals often require helpfrom humans at first to 'encourage' them in their new habitats
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Seed Banks

  • Plants are threatened by habitat distruction, climate change and over-harvesting
  • Most plants produce very large numbers of seeds, so collecting small samples will not damage the species
  • Seeds survive for much loger if they are kept dry and cool
    • For every 1% reduction in seed moisture content, the seed's life span doubles
    • For every 5°C reduction in temperature, seed life span doubles
    • Seeds are dried and stored at -20°C
  • Soon after storage, a sample is taken to test germination ability
    • This is done around every 10 years
    • If germination falls below 75%, the seeds are grown to collect a new seed sample which is then placed into storage
  • Seed banks aim to preserve seeds that are endangered and potentially useful to humans in future
  • The can also be used for: Research, Habitat Restoration and Species reintroduction
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