Unit 2: Section 4

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Microorganisms and Food

Microorganisms can be used to make food:

  • Bread is made by mixing yeast (a fungus), sugar, flour and water into a dough. The yeast turn the sugar into ethanol and carbon dioxide - it's the carbon dioxide that makes the bread rise.
  • Wine is made by adding yeast to grape juice. The yeast turn the sugar in the grape juice into ethanol (alcohol) and carbon dioxide.
  • Cheese is made by adding bacteria to milk. The bacteria turn the sugar in the milk into lactic acid, which causes the milk to curdle. An enzyme is then used to turn the curdled milk into curds and whey. The curds are separated off and left to ripen into cheese.
  • Yoghurt is made by adding bacteria to milk. The bacteria turn the sugar in the milk into lactic acid, causing the milk to clot and thicken into yoghurt.
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Microorganisms and Food

Advantages of using microorganisms to make food:

  • Populations of microorganisms grow rapidly under the right conditions, so food can be produced quickly.
  • Microorganisms can grow on a range of inexpensive materials.
  • The environment can be artificially controlled - so you can potentially grow food anywhere and at any time of year.
  • Conditions for growth are easy to create.
  • Some of the food made using microorganisms often lasts longer in storage than the raw product they're made from.

Disadvantages of using microorganisms to make food:

  • There's a high risk of food contamination. The conditions created to grow the desirable microorganisms are also favourable to harmful microorganisms. This could cause the foods produced to spoil, or, if eaten, cause illnesses such as food poisoning.
  • The conditions required to grow microorganisms can be simple to create, but small changes in temperature of pH can easily kill the microorganisms.
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Microorganisms and Food

Prevention of food spoilage by microorganisms can be achieved by killing the microorganisms or depriving them of the conditions they need to grow, in the following ways:

  • Salting - prevents microorganisms taking in water by interfering with their ability to absorb water. Salting is simply adding salt to foods.
  • Adding sugar - also interferes with the microorganism's ability to absorb water.
  • Freezing - slows down reactions taking place in microorganisms and freezes the water in the food, sothe microorganisms can't use it.
  • Pickling - vinegar has a low pH, which reduces enzyme activiry in microorganisms. This means they can't function properly, inhibiting their growth.
  • Heat - heat treatment involves heating food to a high temperature, which kills any organisms present. Pasteurisation is one form of heat treatment - it involves raising liquids such as milk to a high temperature.
  • Irradiation - exposes foods to radiation. This treatment kills any microorganisms present and can extend shelf life considerably.
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Balanced Diet

  • Carbohydrates - Provide energy
  • Fats (lipids) - Act as an energy store, provide insulation, make up cell membranes, physically protect organs.
  • Proteins - Needed for growth, the repair of tissues and to make enzymes.
  • Vitamins - Different vitamins have different functions, e.g. vitamin D is needed for calcium absorption, vitamin K is needed for blood clotting.
  • Mineral salts - Different mineral salts have different functions, e.g. iron is needed to make haemoglobin in the blood, calcium is needed for bone formation.
  • Fibre - Aids movement of food through gut.
  • Water - It is used for chemical reactions. We need a concstant supply to replace water lost through urinating, breathing and sweating.
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Balanced Diet

Causes of malnutrition

  • Not having enough food - you get too little of every nutrient.
  • Having an unbalanced diet:
    - Getting too little of a nutrient can lead to all kinds of deficiency illnesses
    - Getting too many carbohydrates or fats can lead to obesity
  • Not being able to absorb the nutrients from digestion into your bloodstream properlly.


  • Obesity is defined as being 20% (or more) over the recommended body weight.
  • Too much sugary or fatty food and too little exercise are the main causes of obesity.
  • People can also be obese due to an underactive thyroid gland, but this is uncommon.
  • Obesity can increase the rish of diabetes, arthritis, high blood pressure, CHD and even some forms of cancer.
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Balanced Diet

Coronary Heart Disease

  • A diet high in saturated fat raises blood cholesterol level. This increases the build up of fatty deposits in the arteries (atheromas), which causes atherosclerosis.
  • A diet high in salt can cause high blood pressure. This can damage artery walls, which causes atherosclerosis.
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Balanced Diet


  • Cholesterol is a lipid made in the body.
  • Some is needed for the body to function normally.
  • Cholesterol needs to be attached to a protein to be moved around, so the body forms lipoproteins - substances composed of both protein and lipid. There are two types of lipoprotein:
    - High Density Lipoproteins (HDLs): Mainly protein. They transport cholesterol from body tissues to the liver where it's recycled or excreted. Their function is to reduce blood cholesterol when the level is too high.
    - Low Density Lipoproteins (LDLs): Mainly lipid. They transport cholesterol from the liver to the blood, where it circulates until needed by cells. Their function is to increase blood cholesterol when the level is too low.
  • A diet high in saturated fat raises LDL level - so more cholesterol is transported to the blood, increasing total blood cholesterol and increasing the risk of CHD.
  • A diet high in polyunsaturated far raises HDL level - so more cholesterol is transported from the blood to the liver, decreasing total blood cholesterol and decreasing the risk of CHD. 
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Food Production

  • Humans rely on plants for food because plants are at the start of all food chains.
  • Plants use the energy from sunlight to convert carbon dioxide and water into complex organic compounds (such as carbohydrates).
  • Humans, and other animals, eat, digest and absorb the compounds, which they use for energy and to grow.
  • We grow plants for direct consumption and to feed animals, which we then eat.
  • Many modern farming methods aim to maximise productivity by increasing plant and animal growth.


  • Fertilisers are chemicals that increase crop yields by providing minerals (such as nitrate, phosphate and potassium) that plants need to grow.
  • Minerals in the soil are used up during crop growth. Fertilisers replace these minerals, so that a lack of minerals doesn't limit growth of the next crop.
  • Fertilisers can be natural - made by natural processes, or atificial - made by humans.
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Food Production


  • Pesticides are chemicals that increase crop yields by killing pests that feed on the crops. This means fewer plants are damaged or destroyed.
  • Pests include microorganisms, insects or mammals.
  • Pesticides may be specific and kill only one pest species, or broad, and kill a range of different species - this could mean that some non-pest species aare also harmed.

Animals and antibiotics

  • Animals farmed for food are sometimes given antibiotics.
  • Antibiotics help to treat or prevent diseases caused by bacteria.
  • Animals normally use energy fighting diseases, which reduces the amount of energy available for growth. Giving them antibiotics means animals can use more energy to grow, increasing food production.
  • Antibiotics also help to promote the growth of animals.
  • This is thought to be because the antibiotics influence bacteria in the animals' gut, allowing the animals to digest food more efficiently.
  • This can increase both the growth rate of the animal and its size when mature.
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Food Production

Selective breeding

  • Selective breeding involves selecting plants with good characteristics to reproduce together in order to increase productivity.
  • Selective breeding can also be used to increase the productivity of animals. 
  • Useful characterisics such as fast growth rate and high meat, milk or egg yields can be developed.
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