Edexcel Biology B is very scarce when it comes to finding information on it online. I've taken it upon my self to publish my own notes to help fellow students that have a hard time understanding the subject. The information in the text book is very large and can be hard to narrow down to a particu...
i Understand the basic aseptic techniques used in culturing organisms
Aseptic techniques:
● All equipment should be sterilised
● Flaming equipment in a bunsen flame ensures sterility
● Inoculation should be done with a flamed instrument
● Lids should be replaced as quickly as possible
ii Understand the principles and techniques involved in culturing microorganisms.
Problems with culturing microorganisms:
-Pathogens may enter the culture and grow, risking the spread of disease
-Safe organisms may mutate to dangerous forms
-contamination with unwanted organisms can spoil an investigation
Precautions to take
-adequate O2 supply as anaerobic conditions encourage the growth of pathogens
-culture temperature 20℃ or below as human body temperature encourages growth of
human pathogens
iii Understand the use of different media (broth cultures, agar and selective media
Microorganisms can be grown in a laboratory culture using a variety of media which provide
them with the nutrients they need.
Agar plate- Contains nutrient agar, a solid medium in which bacteria can grow. Molten agar
is usually mixed with nutrient broth and is poured into a petri and allowed to set. It acts as a
sponge to hold the nutrients in a solid block.
Selective Media-Can be used to grow specific types of organisms by using a recipe which
discourages the growth of unwanted organisms. It could rely on the addition or omission of a
certain nutrient or include an antibiotic if the desired bacteria is resistant
iv Understand the different methods of measuring the growth of a bacterial culture as
illustrated by cell counts, dilution plating, mass and optical methods (turbidity)
, Bacteria growth can be measured in various ways:
Cell counts:
● Cells can be counted using a haemocytometer. This is a microscopic grid engraved
(with a grid) with a known depth of sample which allows counts in a known volume
using a microscope.
● It is useful as it only counts viable cells so it is accurate, however the process is slow
and the equipment involved is expensive
Dilution plating:
● A culture is diluted until the number of cultures can be coined when spread on a plate
● It is useful as equipment is not expensive,only counts viable cells and obtains a direct
cell count. However it is slow as incubations periods and serial dilutions are required
Dry mass:
● A known volume of culture can be dried and weighed to find the dry mass of bacterial
cells.
Optical methods:
● Turbidimetry is a specialised form of calorimetry.As turbidimetry increases
transmission decreases and absorption increases.
● It is useful as it is quick and can be conducted in the field. However equipment is
expensive and a collaboration curve is required
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