Factors limiting the size of cells:
1. Firstly, the surface area to volume ratio limits the size of a cell. The smaller the size of an
object, the larger its surface area to volume ratio. A large surface area to volume ratio is
beneficial because nutrients required to sustain the cell enter via the surface of the cell and
are needed in quantities related to the cell volume.
2. Secondly, the nucleo-cytoplasmic ratio limits the size of a cell. Each nucleus can only
control a certain volume of cytoplasm. Some cells overcome this limitation by having more
than one nucleus, some special types of cells have multiple nuclei. Cells that contain
multiple nuclei are known as multinucleate cells and are also known as multinucleated cells
and as polynuclear cells.
3. Thirdly, the fragility of the cell membrane limits the size of a cell. All cells have and need
a cell membrane, even if the cell has a cell wall. The structure of cell membranes include
phospholipids, cholesterol and various proteins. It must be flexible in order to enable
important functions of cell membranes, such as exocytosis, the movement of the content
of secretory vesicles out of the cell, endocytosis, the movement of the content of secretory
vesicles into the cell, etc. As the size of cells increases, the risk of damage to the cell
membrane increases. This limits the maximum size of cells especially animal cells because
they do not have cell walls.
4. Lastly, the structures that hold the cell together limit the size of a cell. In order for cells to
survive, they must remain intact and so sufficient mechanical structures must hold the cell
contents together. Cells need sufficient structural support, which is provided by
support from outside the cell membrane, provided by the extracellular matrix
support from within the cell membrane, provided by the cytoskeleton.
This support becomes almost impossible with large cells
Shapes of Cells:
A description of the shape of a cell is called the cell morphology.
Shapes of Bacterial Cells:
The three basic bacterial shapes are coccus (spherical), bacillus (rod-shaped), and spiral (twisted),
however pleomorphic bacteria can assume several shapes.
9
, Cocci (or coccus for a single cell) are round cells, sometimes slightly flattened when they
are adjacent to one another.
Bacilli (or bacillus for a single cell) are rod-shaped bacteria.
Spirilla (or spirillum for a single cell) are curved bacteria which can range from a gently
curved shape to a corkscrew-like spiral. Many spirilla are rigid and capable of movement.
A special group of spirilla known as spirochetes are long, slender, and flexible.
Arrangement of Cocci
1. Diplococci
The cocci are arranged in pairs.
Example: Streptococcus pneumoniae,
2. Streptococci
The cocci are arranged in chains, as the cells divide in one plane.
Examples: Streptococcus pyogenes, Streptococcus agalactiae
3. Staphylococci
The cocci are arranged in grape-like clusters formed by irregular cell divisions in three plains.
Examples: Staphylococcus aureus
Arrangement of Spiral Bacteria
Spirilla (or spirillum for a single cell) are curved bacteria which can range from a gently curved
shape to a corkscrew-like spiral. Many spirilla are rigid and capable of movement. A special group
of spirilla known as spirochetes are long, slender, and flexible.
10
1. Firstly, the surface area to volume ratio limits the size of a cell. The smaller the size of an
object, the larger its surface area to volume ratio. A large surface area to volume ratio is
beneficial because nutrients required to sustain the cell enter via the surface of the cell and
are needed in quantities related to the cell volume.
2. Secondly, the nucleo-cytoplasmic ratio limits the size of a cell. Each nucleus can only
control a certain volume of cytoplasm. Some cells overcome this limitation by having more
than one nucleus, some special types of cells have multiple nuclei. Cells that contain
multiple nuclei are known as multinucleate cells and are also known as multinucleated cells
and as polynuclear cells.
3. Thirdly, the fragility of the cell membrane limits the size of a cell. All cells have and need
a cell membrane, even if the cell has a cell wall. The structure of cell membranes include
phospholipids, cholesterol and various proteins. It must be flexible in order to enable
important functions of cell membranes, such as exocytosis, the movement of the content
of secretory vesicles out of the cell, endocytosis, the movement of the content of secretory
vesicles into the cell, etc. As the size of cells increases, the risk of damage to the cell
membrane increases. This limits the maximum size of cells especially animal cells because
they do not have cell walls.
4. Lastly, the structures that hold the cell together limit the size of a cell. In order for cells to
survive, they must remain intact and so sufficient mechanical structures must hold the cell
contents together. Cells need sufficient structural support, which is provided by
support from outside the cell membrane, provided by the extracellular matrix
support from within the cell membrane, provided by the cytoskeleton.
This support becomes almost impossible with large cells
Shapes of Cells:
A description of the shape of a cell is called the cell morphology.
Shapes of Bacterial Cells:
The three basic bacterial shapes are coccus (spherical), bacillus (rod-shaped), and spiral (twisted),
however pleomorphic bacteria can assume several shapes.
9
, Cocci (or coccus for a single cell) are round cells, sometimes slightly flattened when they
are adjacent to one another.
Bacilli (or bacillus for a single cell) are rod-shaped bacteria.
Spirilla (or spirillum for a single cell) are curved bacteria which can range from a gently
curved shape to a corkscrew-like spiral. Many spirilla are rigid and capable of movement.
A special group of spirilla known as spirochetes are long, slender, and flexible.
Arrangement of Cocci
1. Diplococci
The cocci are arranged in pairs.
Example: Streptococcus pneumoniae,
2. Streptococci
The cocci are arranged in chains, as the cells divide in one plane.
Examples: Streptococcus pyogenes, Streptococcus agalactiae
3. Staphylococci
The cocci are arranged in grape-like clusters formed by irregular cell divisions in three plains.
Examples: Staphylococcus aureus
Arrangement of Spiral Bacteria
Spirilla (or spirillum for a single cell) are curved bacteria which can range from a gently curved
shape to a corkscrew-like spiral. Many spirilla are rigid and capable of movement. A special group
of spirilla known as spirochetes are long, slender, and flexible.
10
