HC5 Structure of the prokaryotic
cell (BOOK)
Chapters 1.6 and 2.1 – 2.13
CH1 The Microbial World
1.6 Improving Contrast in Light Microscopy
Dyes can be used to stain cells and increase their contrast so that they can be more easily seen in the
bright-field microscope. Many dyes are positively charged and are therefore called basic dyes. Basic
dyes bind strongly to negatively charged cell components (nucleid acids and acidic polysaccharides)
and therefore also stain the surfaces of cells.
Stains that render different kinds of cells different colours are called differential stains. An important
example is the gram stain which can be used to identify gram negative and gram positive bacteria;
gram negative bacteria appear pink after staining, and gram positive bacteria appear purple-violet.
Afterwards, when treated with ethanol, gram negative cells decolorizes but gram positive cells won’t.
After this, they are stained with a counter stain and visible under the microscope by different
colours:
Phase-contrast and dark-field microscopy
A disadvantage of staining is that staining often kills cells and can distort their features. Phase-
contrast microscopy is based on the principle that cells differ in refractive index from their
surroundings.
In dark-field microscopy, light does not pass through the specimen, instead light is directed from the
sides of the specimen and only light that is scattered when it hits the specimen can reach the lens.
Fluorescence microscopy visualises specimens that fluoresce.
, CH2 Microbial Cell Structure and Function
2.1 Cell Morphology
Morphology means cell shape. Prokaryotic cells have common morphologies:
2.2 The Small World
Metabolic rate of a cell varies inversely with the square of its size, for very large cells nutrient uptake
would eventually limit metabolism to the point that they would no longer be competitive with
smaller cells.
For a cell there are advantages to being small. Small cells have more
surface area relative to cell volume than do large cells and thus have
a higher surface-to-volume ratio.
When cell size increases, its S/V ratio decreases (see image).
The S/V ratio of a cell controls many of its properties including its
growth rate and evolution. Because how fast a cell can grow depends
in part on the rate at which it can exchange nutrients and waste
products with its environment. As a result, free-living smaller cels
tend to grow faster than free-living larger cells.
However there are also lower limits to cell size; there is a minimum
volume needed to house the essential components of a free-living cell.
2.3 The Cytosplamic Membrane
The cytoplasmic membrane surrounds the cytoplasm. However there are differences between
bacterial and archaeal membranes:
Structure of cytoplasmic membranes
- BACTERIAL cytoplasmic membrane
o Surrounds cytoplasm
o Separates it from environment
o Are selectively permeable (transport systems)
o Structure:
Phospholipid bilayer (phospholipids consists of fatty acids and glycerol +
phosphate —> see image (KNOW IMAGE)
Fatty acids point inward to form hydrophobic environment, hydrophilic part
is exposed to external environment/cytoplasm
Bilayer can be anionic or neutral (opposing charges)
cell (BOOK)
Chapters 1.6 and 2.1 – 2.13
CH1 The Microbial World
1.6 Improving Contrast in Light Microscopy
Dyes can be used to stain cells and increase their contrast so that they can be more easily seen in the
bright-field microscope. Many dyes are positively charged and are therefore called basic dyes. Basic
dyes bind strongly to negatively charged cell components (nucleid acids and acidic polysaccharides)
and therefore also stain the surfaces of cells.
Stains that render different kinds of cells different colours are called differential stains. An important
example is the gram stain which can be used to identify gram negative and gram positive bacteria;
gram negative bacteria appear pink after staining, and gram positive bacteria appear purple-violet.
Afterwards, when treated with ethanol, gram negative cells decolorizes but gram positive cells won’t.
After this, they are stained with a counter stain and visible under the microscope by different
colours:
Phase-contrast and dark-field microscopy
A disadvantage of staining is that staining often kills cells and can distort their features. Phase-
contrast microscopy is based on the principle that cells differ in refractive index from their
surroundings.
In dark-field microscopy, light does not pass through the specimen, instead light is directed from the
sides of the specimen and only light that is scattered when it hits the specimen can reach the lens.
Fluorescence microscopy visualises specimens that fluoresce.
, CH2 Microbial Cell Structure and Function
2.1 Cell Morphology
Morphology means cell shape. Prokaryotic cells have common morphologies:
2.2 The Small World
Metabolic rate of a cell varies inversely with the square of its size, for very large cells nutrient uptake
would eventually limit metabolism to the point that they would no longer be competitive with
smaller cells.
For a cell there are advantages to being small. Small cells have more
surface area relative to cell volume than do large cells and thus have
a higher surface-to-volume ratio.
When cell size increases, its S/V ratio decreases (see image).
The S/V ratio of a cell controls many of its properties including its
growth rate and evolution. Because how fast a cell can grow depends
in part on the rate at which it can exchange nutrients and waste
products with its environment. As a result, free-living smaller cels
tend to grow faster than free-living larger cells.
However there are also lower limits to cell size; there is a minimum
volume needed to house the essential components of a free-living cell.
2.3 The Cytosplamic Membrane
The cytoplasmic membrane surrounds the cytoplasm. However there are differences between
bacterial and archaeal membranes:
Structure of cytoplasmic membranes
- BACTERIAL cytoplasmic membrane
o Surrounds cytoplasm
o Separates it from environment
o Are selectively permeable (transport systems)
o Structure:
Phospholipid bilayer (phospholipids consists of fatty acids and glycerol +
phosphate —> see image (KNOW IMAGE)
Fatty acids point inward to form hydrophobic environment, hydrophilic part
is exposed to external environment/cytoplasm
Bilayer can be anionic or neutral (opposing charges)
