OCR Gateway B Module B6: Beyond the Microscope

Bacteria:

• A typical bacterial cell is just a few microns (thousandths of a mm) across
• Bacterial cells usually have one or more flagellum for movement, a cell wall to maintain shape, and bacterial DNA
• They can be categorised by shape: e.g. spherical, spiral, rod or curved rod shape
• Bacteria reproduce asexually, with binary fission (splitting into two)
• Aseptic technique must be used when handling bacteria; tools must be sterilised to prevent contamination from other microbes
• They can live in a range of habitats and survive on a variety of energy sources
• Some bacteria make their own food
• Large containers called fermenters can be used to grow large amounts of bacteria

Yeast:

• Yeast is a type of single-celled fungus, containing a nucleus, cytoplasm, cell wall and a bud - this bud is used for reproduction: it is a new cell which eventually breaks away from the parent cell
• Its growth rate depends on pH, temperature, food availability and the removal of waste products
• Growth rate doubles for every 10 °C rise in temperature until the optimum is reached

Viruses:

• Viruses are small structures (smaller than bacteria/fungi) made from a strand of genetic material surrounded by a protein coat. They're not living cells
• They reproduce in living cells and only attack specific cells
• The reproduction process of a virus:
  - The virus attaches itself to the host cell
  - It injects its genetic material
  - It uses the cell to make new viruses and causes is to die, releasing new viruses

Disease transmission:

• Microorganisms which cause disease are called pathogens
• Cholera and food poisoning are caused by bacteria
• Influenza and chickenpox are caused by viruses
• Athlete's foot is caused by a fungus
• Some pathogens spread in food (e.g. salmonella)
• Some spread in water, prevented with water treatment (e.g. vibrio cholera)
• Barrier methods prevent spreading by direct contact
• The correct use of paper tissues and isolation of patients prevents the spread of airborne droplets containing microbes like those causing influenza
• When a harmful microbe enters the body, is reproduces with very few symptoms (the incubation period). They then start producing toxins which cause symptoms
• There is an increased risk in areas of natural disasters due to damage to sewage systems, shortages in medical staff/damaged hospitals and lack of electricity (and therefore refrigeration)

Antibiotics, antiseptics and treatment:

• Important discoveries:
  - Louis Pasteur realised that microbes in the air can make food go bad
  - Joseph Lister invented the first antiseptic, using carbolic acid to prevent wound infection
  - Sir Alexander Fleming discovered the first antibiotic: penicillin
• Antiseptics are used outside the body to prevent the entry of microbes
• Antibiotics are used inside the body and have no effect on viruses
• Some strains of bacteria are developing a resistance to antibiotics due to natural selection. Therefore, doctors only prescribe antibiotics when necessary and ensure that the dose is completed so no partially resistant bacteria are killed

Useful bacteria:

• Some bacteria are useful in: yoghurt making, cheese production, vinegar production, silage production, composting

Making yoghurt:

• Equipment is sterilised
• Milk is pasteurised by heating to 78 °C, cooled and then incubated with lactobacillus bacteria, which breaks down lactose in milk into lactic acid. This is left at around 46 °C for four hours
• Flavours and colours are added after it is sampled and checked for quality and safety. The yoghurt is finally packaged

Fermentation (the production of ethanol):

• Fermentation in yeast involves anaerobic respiration in yeast:
  glucose (sugar) → ethanol (alcohol) + carbon dioxide, (C6H12O6 → 2C2H5OH + 2CO2)
• Making beer/wine:
  - Sugar is extracted from barley grains (beer) or from crushed grapes (wine)
  - Yeast is added and kept warm for fermentation. Air is kept out so that it anaerobically respires
  - It is left to clear, and then this clear liquid is drawn off
  - The remaining beer/wine is finally pasteurised
• Distillation is used instead, if the alcohol needs to be in high concentration (e.g. for whiskey). This is because yeasts can only tolerate certain levels of alcohol. However, different strains of yeast can tolerate different levels
• Some products of fermentation can be further treated to increase the alcohol concentration and produce spirits
• Yeast breaks down sugar at different rates in different conditions, e.g. temperature and oxygen availability
• Pasteurisation is used to kill harmful microbes; the temperature and duration for this depends on the drink

Biofuels:

• Biofuels use energy trapped in biomass: for example, fast-growing trees' wood can be burnt for energy, or sugar can be fermented and the product used as a fuel
• Biofuels are carbon neutral because biofuels are burnt at the same rate as it is being produced. However, if the land has been cleared for land to grow the biomass in, habitats may be lost, and it's no longer carbon neutral due to the deforestation releasing carbon dioxide into the atmosphere

Biogas:

• Biogas is mainly methane, with some carbon dioxide, hydrogen, nitrogen and hydrogen sulfate
• The rotting of organic material such as dead plants and animal waste produces a mixture of gases including methane, caused by the action of bacteria
• Methane is being released from landfill sites for this reason, but this is dangerous - it can burn or explode preventing use of the site for many years
• It is produced in a digester, where organic waste is constantly added as a gas and the remaining solids are constantly removed
• Production increases as temperature increases, up to around 45 °C, when enzymes start to denature and bacteria die
• Biogas with over 50% methane is safe to burn, but with less than this it can be dangerous; at 10% it's explosive
• Biogas is cleaner than petrol and diesel, but it releases less energy than natural gas

Gasohol:

• Gasohol is a mix of alcohol and petrol, used to fuel cars in Brazil where there is lots of sugar cane (which can be fermented to form alcohol) but little oil

Soil:

• Soil contains minerals, the particles are smaller in a clay based soil
• Loam contains clay, sand, humus (partly decomposed animal/plant waste)
• The mass of humus in a soil can be measured by burning the soil in a Bunsen burner - the humus will burn and the mass reduction shows the humus content
• The air content in a soil sample is found by measuring how much water is needed to fill the spaces
• Water content is found by heating the soil lightly to evaporate the moisture, and measuring the mass change
• More humus allows for the soil to hold more water and air
• If soil has larger particles, air content and permeability (ease of water passing through the soil) is higher
• Organisms in soil require oxygen for respiration; and water
• Humus decomposes to release minerals and increase the air content of the soil

Life in soil:

• A typical food web in a soil:
  - Herbivores (e.g. slugs, snails and wire worms)
  - Detritivores (e.g. earthworms, millipedes and springtails)
  - Carnivores (e.g. centipedes, spiders and ground beetles)
• Bacteria and fungi are important as they can be decomposers
• Earthworms bury organic material for decomposition; aerate and drain soil to allow for organisms to respire aerobically; and mix soil layers and neutralise soil so plants can grow and so dead material is decomposed
• These important earthworm functions were first understood by Darwin

Living in water:

• In water, there is no risk of dehydration for marine organisms, less temperature variation, the water provides physical support and water products are easily decomposed
• However, water is dense so it restricts movement
• In freshwater, organisms may take in too much water by osmosis. If it is salt water, too much water may be lost to the surroundings
• Some organisms (e.g. amoeba) have contractile vacuoles that can store excess water and empty it by fusing with the cell membrane

Plankton: phytoplankton (microscopic plants) and zooplankton (microscopic animals):

• Plankton have limited movement and rely on currents
• Phytoplankton are capable of photosynthesis and are therefore producers; food webs that start with them are examples of grazing food webs which are common in oceans. Some food chains in water rely on 'marine snow' (when dead organic organisms and waste is eaten by zooplankton), and bacteria which act as producers
• Aquatic microorganisms can be affected by pollutants (e.g. oil, sewage, PCBs, fertilisers, pesticides, detergents) and acid rain
• Their numbers vary at different depths and seasons:
  - Phytoplankton photosynthesis varies based on the amount of light in the water and temperature which varies by season
  - Mineral availability (such as iron) also affect phytoplankton numbers

Uses of enzymes outside the body:

• Biological washing powders and stain removers
• Cheese making and juice extraction
• Preparation of medical products (e.g. reagent sticks)
• Altering flavour of food products

Biological washing powders:

• Biological washing powders do not work at extreme pH levels or high temperatures
• Enzymes in these washing powders include:
  - Amylase to digest starch (a carbohydrate)
  - Lipases to digest fat and remove fatty stains
  - Proteases to digest protein and remove protein stains

Sweeteners using enzymes:

• Sucrose can be broken down with invertase, which converts sucrose into glucose and fructose, which are much sweeter than the sucrose, therefore sweetening the food
• With this method, foods can be sweetened without adding so much sugar (e.g. in low calorie foods as energy content is lower with this method)

Immobilising enzymes:

• Enzymes can be immobilised in gel beads by mixing it with alginate and dropping the mixture in calcium chloride solution
• Immobilised enzymes can be useful in reactions because the mixture does not become contaminated with the enzyme and they can be used in continuous flow processing
• People with diabetes can test their urine using reagent strip sticks for the presence of glucose, immobilised enzymes on reagent sticks can be used to measure these glucose levels

Lactose intolerance:

• Some people/animals are lactose intolerant - meaning they can't produce the lactase enzyme
• Therefore, the bacteria in the gut ferment lactose producing diarrhoea and wind
• Milk can be treated for these people by using immobilised lactase to convert lactose in milk into glucose and galactose, which can be absorbed from the milk by the body with no side effects
• People with lactose intolerance can eat yoghurt because bacteria have converted the lactose in milk into lactic acid

Genetic engineering:

• Genetic engineering is the altering of genetic code of an organism by inserting genes
• Genes from one organism can work in another
• The organism which receives the new gene is called a transgenic organism
• The stages in genetic engineering:
  - Removing a desired gene from one organism
  - Cutting open DNA in another organism and inserting the new gene into this
  - Now this gene should work in the transgenic organism
• Enzymes do the cutting and inserting of DNA
  - Restriction enzymes are used to cut open DNA. They leave some unpaired bases on the cut end which acts as a 'sticky end'
  - Ligase enzymes are used to join DNA strands because the 'sticky ends' on each cut section can join by complimentary base pairing
• A person's DNA can be used to make a DNA 'fingerprint' which is unique. They are made as follows:
  - DNA is extracted from a sample (e.g. from blood)
  - Is it then cut open or fragmented using restriction enzymes
  - The fragments are separated using electrophoresis
  - The fragments are made visible using a radioactive probe
• People are worried as these DNA 'fingerprints' could be used to withhold life insurance if a person is likely to develop a disease, for example

Genetically engineering bacteria:

• Bacteria can be used to produce the human insulin hormone for diabetics to use:
  - The gene for insulin production is cut from a person's DNA
  - A bacterial DNA loop is cut open
  - The insulin gene is inserted into this
  - This DNA loop is then inserted into a bacterium, which will reproduce
  - Now large amounts of insulin will be made and can be harvested
• The loops of DNA used here are called plasmids which are found in bacteria's cytoplasms and can therefore be used as vectors for the genes
• Assaying techniques are used to check that the new gene has been correctly transferred:
  - Scientists add genes that make the bacteria resistant to antibiotics
  - The bacteria are grown on agar containing the antibiotic
  - The bacteria that survive have successfully taken up the plasmid