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Edexcel Biology Unit 4 - Microbiology Notes
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This document contains detailed notes and explanations for Microbiology, a section of Edexcel Unit 4 Biology.
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UNIT 4 TOPIC 66A – MICROBIOLOGY
Bacteria - These are a type of microorganism that play important roles in our environment such as
decomposers. However, some bacteria are harmful as they can cause disease in animals and plants. Such
microorganisms that cause diseases are known as pathogens.
Here are some of the common features that all bacteria consist of:
• a cell wall containing peptidoglycan
• a circular loop of DNA which contains genetic material of the bacterium
• a cell surface membrane (phospholipid bilayer)
• 70S Ribosomes
These are the features that only SOME bacteria contain:
• Flagella: rotating whip-like structures which can move the bacteria about.
• Capsule/Slime Layer: Thick substance around the outside of the cell wall, which prevents
dehydration of the cell under extreme conditions.
• Plasmids: small circles of DNA that code for specific characteristics.
Bacteria can be classified by using 2 main characteristics; the shape of the bacteria and the contents of it's
cell wall. In terms of cell wall, bacteria can either be Gram-positive or Gram-negative.
Gram Positive - has a thick layer of peptidoglycan containing substances such as teichoic acid.
The crystal violet complex from the stain is trapped in the thick peptidoglycan layer and resists decoloring
when the bacteria are dehydrated using alcohol. (The bacteria appear purple/blue under a light
microscope).
Gram Negative – has a thin peptidoglycan layer with no teichoic acid, between 2 phospholipid bilayers.
Lipopolysaccharides which are located on the outer membrane, dissolved once the bacteria are
dehydrated with alcohol. This exposes the peptidoglycan layer which takes up the red safranin
counterstain. (The bacteria appear red under a light microscope).
, Bacterial Reproduction
• The most common process is asexual reproduction. Through this process the bacterium divides into
two, known as binary fission.
- Enzymes break open the circular piece of DNA which unwinds and is replicated.
- New cross walls are laid between the 2 new daughter cells (mesosome plays a role here)
- New cell membrane and cell wall extend inward forming a septum
- Plasmids divide at the same time
The daughter cells usually each contain both a copy of the original genome and any plasmids present in
the parent cell.
There are 3 ways in which genetic material from one bacterium can be taken in and used as part of the
DNA of another bacterium.
1. Transformation: a short piece of DNA is released by the donor and actively taken up by the
recipient.
2. Transduction: transferred by means of a vector like a bacteriophage
3. Conjugation: (sex pilus) a plasmid that contains the formation of sex pilus.
Measuring Bacterial Colony Growth: Log Scales
• LAG PHASE: here the bacteria acclimatizing to their new environment and aren’t reproducing at
their maximum rate.
• LOG PHASE: also known as the exponential phase, this is where bacterial growth rate is at its
theoretical maximum. There is an optimum amount of nutrients and space available for growth.
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