Types of cells,Cell Size

 


Types of Cells:

The cells are of three types: 

undifferentiated  ( stem cells), 

differentiated ( post _ mitotic cells) and 

dedifferentiated. 

(a) Stem Cells 

They are unspecialised cells which possess the power of  division , e.g., stem apical meristem, root apical meristem, vascular  cambium, cork cambium, stratum getmjnativum of skin, germinal epithelium,  bone marrow, etc. Zygote is also an undifferentiated cells ( Stem cell). 

(b) Post __ mitotic Cells:

These cells are specialized to perform specific functions,  Differentiation occurs in shape, size, structure  and function through an orderly switching  on and off of some particular   genes  of the cells by  means of chemicals named as inducers and repressors.Differentiation leads to better organisation, division of labour and higher efficiency. 

(c) Dedifferentiated Cells

They are differentiated cells which revert to undifferentiated state to take over the function of division by reactivation  of certain genes  that prevent differentiation,  allow limited growth and induce division. Cork cambium of plants is always produced through  dedifferentiation. Dedifferentiation helps in healing of wounds, regeneration  in animals, or vegetative propagation  in plants . Cell culture  experiments   are based on this dedifferentiation of cells


Cell Size

There is a wide variation in the size.of cells.The smallest cells  are those of Mycoplasma. They  have a size of 0.1__ 0.5 ųm
( average 0.3ųm). The smallest mycoplasma has a volume of 1.0× 10-³ųm³ while the smallest bacterium possesses  a volume of 2.0. × 10-² ųm³ ( average  length  of bacterium is 3 to 5 ųm). Viruses  are still smaller. They do not have a cellular structure. The smallest virus has a volume of 7.0× 10-⁷ ųm³. Unicellular eucaryotesbhave a size of 1_ 1000 ųm. Sporozoite of Plasmodium is only 2ųm long. Cells of multicellular euaryotes have a size range of 5__ 100 ųm.Erythrocytes are 7_ 8 ųm in diameter. ' Small lymphocytes ' are still smaller ( 6ųm). Cells of kidney, liver, skin and intestine are 20__ 30 ųm in diameter. Muscle and nerve cells are comparatively very large. A striated muscle cell can be 1_ 40 ųm long and 30_ 80 ųm in thickness. Longest cells of human body are the nerve cells which may reach a length  of 90cm. Smallest human cells are RBC's. Platelets are even smaller  but are cell fragments. 

Amongst  plants, many algae have large cells.Internodal cells of Chara  are 1_ 10 cm in length. Acetabularia, a unicellular  alga, is upto 10 cm in length  which is differentiated  into rhizoid ,stalk and cap. Plant fibres are still longer, e.g., 4 cm in Cotton, 55 cm  in Ramie, 30_ 90 cm in Jute and over a metre in Hemp.

   Eggs are large sized cells because  they store food for the partial or complete development of the embryo. Human egg is slightly  over 0.1 ųm or 100 ųm in diameter having  a volume  1.4 × 10⁶ ųm³ or 0.1 ųm with  a volume  of 5.0 × 10¹³ ųm³ while the egg of Ostrich is 170× 150 mm with a volume of 1.1 × 10¹⁵ ųm³.




Differences Between  Prokaryotic  and Eukaryotic Cells 

Prokaryotic Cell:

1):   Size of the cell is usually  small ( 0.1_ 5.0 ųm).

2):   It has one envelope organization. 

3): The flagella,  if present  are single stranded  and are without  differentiation of axonema  and  sheath.

4): In prokaryotic cell an organized nucleus is absent.Instead a nucleoid is  found Nueolus is absent.

5): Cell wall possesses  muramic acid.

6): In prokaryotic cell DNA is  naked, that is, without an association  with histones and it is usually  circular. 

7): The amount  of DNA is comparatively  low in prokaryotic cells and it remains same throughout  life.


8): DNA lies free in the cytoplasm and is not associated  with any organelle. 

9):  Transcription and translation noccur on the cytoplasm .

10): Protein synthesis occurs only in  cytoplasm.

11):  Respiratory  enzymes are associated with plasma membrane.

12):  Endocytosis and exocytosis  are rare.


13): Prokaryotic  cells do not possess  endoplasmic reticulum,  mitochondria,  Golgi apparatus, centrioles and lysosomes and other microbodies.

14): Cytoplasm  does not show streaming  movements. 

15): Ribosomes are of 70S type.

16): True or sap vacuoles are usually  absent instead, gas vacuoles may be found.

17): Microtubules and microfilaments are commonly  absent.

18):  Thylakoids, if present, lie freely in the cytoplasm.

19): Sexual reproduction is absent, so gametes are not formed.


20):  A spindle apparatus is not formed during division. 

21): Cell membrane may be infolded  to form a complex structure  called mesosome.

22): Cell membrane takes part in separating replication products. 

23): As DNA do not have any superfluous parts therefore , RNA does not require any  processing. 

24): Nucleoid is equivalent  to a single chromosomes or prochromosome. 

25): A distinction of interphase and M_  phase is absent.


26): Additional  small circular DNA segments  or plasmids may occur. 

Eukaryotic Cells:

1): The cell size is comparatively  larger ( 5__ 100 ųm).

2): It has two envelope  organization. 

3): It flagella,  if  present ,are 11_ stranded, and show differentiation  of axoneme and sheath. 

4): They have an organized nucleus  which is differentiated into nuclear envelope, chromatin,one or more nucleoli and nucleoplasm.

5): Cell wall, if present, does not contain muramic acid.


6): In these cells nuclear  DNA is associated with histone proteins. Nuclear DNA is linear while extra nuclear DNA is commonly circular.


7): The amount of DNA is comparatively  very high in these cells and shows a regular alternation between  diploid and haploid stages.

8): Most of the cell DNA lies in the nucleus and a small quantity  is also found in the plastids  and mitochondria. 


9):  Transcription occurs in the nucleus while translation  takes place in the cytoplasm. 

10): Protein synthesis  takes place  in  cytoplasm, mitochondria  and plastids. 


11): Respiratory  enzymes  are present in both Cytoplasm as well as mitochondria. 

12): They are quite common.

13): In Eukaryotic  cell cytoplasm  is differentiated into cytoplasmic  matrix and endoplasmic reticulum. Mitochondria, golgi apparatus  and lysosomes are present. Centrioles are present in cells of organisms which show motile stages during their life cycle.


14): Cytoplasm usually  shows streaming movements. 

15): Ribosomes are  of 80S type. 70S  ribosomes, however, occur in pastids and mitochondria. 


16): True or sap vacuoles are commonly  found.

17): Microtubules and microfilaments are important constituents of Eukaryotic cells. 

18): Thylakoids, if present, are grouped inside the chloroplasts. 

19): Gametes are formed either directly  or through meiosis,  as sexual  reproduction is found in the life cycle.

20): A spindle apparatus  is produced  during nucleus division. 

21): Mesosome _ like structures  are absent. 

22): Spindle apparatus  separates replication products.

23): RNA  requires  processing  as DNA possesses  superfluous parts.

24): Nucleus contains more than one chromosome.

25): There is a distinction  of I_ phase and M_ phase.

26): Plasmids are usually  absent.


The euaryotic cells of animals  and plants are slightly  different. The pplant cells possess  cell wall, plastids, glyoxisomes, large vacuoles and crystals. They generally  lack centrioles and lysosomes. Golgi bodies  occur as separate  units called dictyosomes. 


Differences Between  plant and Animal Cells 




Plant Cell:

1): A plant cell possesses  a rigid wall on the outside.

2): Plant cell is usually  larger in size and cannot change its shape.

3): Plastids  are found in plant cells.

4): Plant cells exposed to sunlight possess  chlorophyll  containing plastids called chloroplasts. 

5): A mature plant cell contains a large central vacuole. 

6): Nucleus lies on one side  in the peripheral cytoplasm. 

7): Plants cells  possess  few mitochondria which have tubular cristae.

8): Plant cells do not burst if placed in hypotonic  solution  due to the presence  of cell wall.

9): Centrioles are usually  absent except in lower plants.

10): Spindle formed during  nuclear division is anastral.

11): Golgi apparatus occurs as number of distinct or unconnected units called dictyosomes. 

12): Cytoskeleton in plant cell does not contain intermediate fibres.

13): Lysosomes  are rare, their activity being performed  by specialized  vacuoles. 

14): Glyoxysomes may be present. 

15): Crystals of inorganic  substances  may occur  inside the cells.

16): Reserve food in plant cell is generally  starch and fat.

17): Plant cells can synthesise all the amino acids, vitamins and coenzymes required by them.

18): Cytokinesis occurs by cell plate method.

19): Adjacent  cells may be connected  through  plasmodesmata.

Animal Cells:

1): A cell wall is absent in animals  cell.

2): An animal cell is comparatively  smaller in size and can change its shape.

3): Plastids  are usually  absent.

4): Chlorophyll is absent. 


5): An animal ce often possesses many small vacuoles. 

6): Nucleus  usually lies in the centre.

7): Animal cells  have numerous mitochondria having plate like cristae. 

8): Animal cells usually  burst if  placed in hypotonic  solution  unless and unit they possess contractile  vacuoles. 

9): Centrioles are found  in animals  cells.

10): Spindle  formed during  nuclear division  is amphiastral.


11): Golgi apparatus  is either  localized or consists of a well_ connected  single complex.

12): Cytoskeleton contains  intermediate  fibers  in these cells.


13): Typical lysosomes occur in animal cells.


14): They are absent.

15): Crystals usually do not occur in animal cells.

16): Reserve food in animal cell is usually  glycogen and fat.


17): Animal cells cannot synthesis all the amino acids, vitamins and coenzymes required by them.


18): Cytokinesis occurs by constriction. 

19): Adjacent cells are connected  through number of Junctions. 











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