Final topic for the paper 3 Biology exam.

Pages 83-88 AQA GCSE Biology Revision Guide

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  • Created by: jenny100
  • Created on: 17-05-12 11:17

The Theory of Biogenesis

Back in the day people used to think that life could spontaneously generate from non-living material. This theory couldn't be proved so people moved on to believe that living things are created by other living organisms - this was the theory of biogenesis.

  • Before 1765 it was believed that substances in food were changed into microbes and this caused food to go off. A scientist called Lazzaro Spallanzani boiled two sets of broth, sealing one and leaving one open. Only the open one went off. This showed that microbes got into the broth from the air but opponents thought it meant air was necessary to start the change
  • The theory that 'fresh' air cause food to to change into microbes was disproved by Theodor Schwann in 1837. He showed that meat doesn't go off if you heat the air around it first to kill the microbes
  • Louis Pasteur (1859) heated broth in two flasks, one was open topped and the other was curved (swan neck flask) so the microbes would settle in the loop and not enter the broth. The broth in the curved flask stayed fresh proving that microbes caused food to go off not the air
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Food and Drink from Microorganisms


  • Culture of bacteria is added to milk
  • The bacteria produce solid curds in the milk
  • The curds are separated from the liquid whey
  • More bacteria is sometimes added to the curds then left to ripen
  • (Moulds are added to give blue cheese its colour and taste)

Yoghurt (bacteria are used to clot milk during the manufactur of yoghurt)

  • Milk is often heat treated to kill any bacteria then cooled
  • A starter culture of bacteria is added. The bacteria added ferment the lactose sugar into lactic acid
  • The acid causes the milk to clot and solidify as yoghurt
  • (Sterilised flavours, e.g. fruit, are sometimes added)
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Using Yeast

  • Yeast is a single-celled fungus and a microorganism
  • Yeast can respire with or without oxygen
  • Equation of anaerobic respiration of glucose by yeast (fermentation): Glucose --> ethanol + carbon dioxide + energy
  • Can also respire aerobically, this produces more energy to grow and reproduce: glucose + oxygen --> water + carbon dioxide + energy

Yeast is used to make BREAD:

  • Yeast converts sugars into carbon dioxide which makes the bread rise and ethanol
  • As the CO2 expands it gets trapped in the dough making it lighter
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Using Yeast 2

Alcoholic Drinks (Beer)

  • Beer's made from grain using barley - usually barley
  • The barley grains germinate for a few days during which the starch is broken down into sugar by enzymes. Then the grains are dried in a kiln, this process is called malting
  • Malted grain is mashed up and water is added to produce a sugary solution with lots of bits in. This is sieved to remove the bits
  • Hops are added to the mixture to give the beer its bitter flavour
  • The sugary solution is then fermented by yeast turning the sugar into alcohol
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Microorganisms in Industry

Fermenters - Large Scale

  • A fermenter is a big container of liquid culture medium where microorganisms can grow and produce their useful product 
  • Food is pumped in to provide glucose for the microorganisms to respire as well as oxygen, this causes them to respire and produce carbon dioxide and heat. The culture needs to be kept at a constant temperature, this is done with water flowing through a water-cooled jacket
  • The pH must be kept at the correct level by using instruments 
  • Sterile conditions are needed to prevent contamination from other microorganisms 
  • A stirrer (motorized) is used to keep the food and microorganisms in suspension and not sinking to the bottom of the fermenter 
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Microorganisms in Industry 2

Mycoprotein - Food from fermenters

  • Mycoprotein means protein from fungi. It's a single-cell protein
  • It's used to make meat substitutes for veggies e.g. Quorn 
  • Fusarium is the main source of mycoprotein
  • The fungus is grown in fermenters using glucose syrup as food. This is obtained by digesting maize starch with enzymes 
  • The fungus respires aerobically so oxygen is supplied with nitrogen and other materials
  • The fermenter is sterilised using steam and the incoming nutrients are heat sterilised and the air supply filtered

Penicillin is made by growing mold in fermenters

  • Penicillin is a antibiotic made by growing the mold Penicillium chrysogenum in a fermenter
  • The mold is grown in a liquid culture medium containing sugar and other nutrients
  • As the mold grows the sugar is used up, the mold only starts to make penicillin after most of the nutrients are used up for growth
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Fuels From Microorganisms

Fermentation is when bacteria or yeast break sugars down by anaerobic respiration. Fuels can be made by the fermentation of natural products or waste

Ethanol is made by anaerobic fermentation of sugar

  • Yeast make ethanol when they break down glucose by anaerobic respiration
  • Sugar cane juices can be used or glucose from digesting maize starch from enzymes (carbohydrase)
  • The ethanol is distilled to separate it from the glucose and yeast
  • Cars can run on a mixture of ethanol and petrol called gasohol
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Fuels From Microorganisms 2 - Biogas

  • Biogas is usually 70% methane and 30% carbon dioxide 
  • Lots of different microorganisms are used to produce biogas. They ferment plant and animal waste which contains carbohydrates. For large scale production sludge waste is used
  • It's made in a simple fermenter called a digester/generator 
  • Biogas generators need to be kept at a constant temp to keep the microorganisms constantly respiring
  • Biogas cannot be stored as a liquid (it needs too high a pressure) so it must be used straight away
  • The by-products of biogas are used as a fertiliser
  • There's 2 types of biogas generators - batch and continuous generators

Batch generators make biogas in small batches. They're manually loaded up with waste, which is left to digest and the by-products are cleared away at the end of each session.

Continuous generators make biogas all the time. Waste is continuously fed in and biogas is produced at a steady rate <-- more suited to large scale biogas production

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Fuels From Microorganisms 3 - Biogas Generator

The diagram shows a simple biogas generator which needs: an inlet for waste material to be put in, an outlet for digested material to be removed, an outlet for the biogas so it can be piped to where it is needed


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Fuels From Microorganisms 4 - Biogas The Issues

The economic and environmental effects of using biofuels:

  • Greener alternative to fossil fuels: the carbon dioxide released into the atmosphere was taken in by plants so they're carbon neutral 
  • The use of biofuels doesn't produce significant amounts of sulfur dioxide or nitrogen oxides which causes acid rain 
  • Burning methane as a biogas means it's not released into the atmosphere and doesn't add to global warming 
  • The raw material is cheap and readily available
  • Digested material is a good fertiliser so people can grow more crops
  • Biogas stops women having to collect wood for fuel in developing countries
  • Biogas generators act as a waste disposal system, normally it would lie  around causing disease and polluting other water supplies
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Using Microorganisms Safely

  • Microorganisms are grown (cultured) in a culture medium
  • They need carbohydrates as an energy source, plus mineral ions and sometimes supplementary proteins and vitamins
  • These nutrients are usually added to the agar jelly
  • Agar jelly can be poured when hot and set when cold in Petri dishes

Equipment is sterilised to prevent contamination

  • Petri dishes and growth medium must be sterilised before use
  • Petri dishes must have a lid to stop any microorganisms in the air contaminating the culture

In labs at school cultures must be kept at about 25C because harmful pathogens aren't likely to grow at this temperature.

Pathogen = A bacterium, virus or other microorganism that can cause disease 

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