An Overview of Cell

Introduction:

• Every cell acts as a compartment because it is enclosed by a plasma membrane.
• Eukaryotic cells have well-defined nuclei whereas they are absent in the case of prokaryotes.
• Eukaryotic cells may also possess some internal compartmentalization due to the presence of membrane-lined organelles like mitochondria, endoplasmic reticulum, Golgi body, etc.
• membraneless organelles, ribosomes are found in both prokaryotes and eukaryotes.
• The cytoplasm is the main arena of cellular activities in both plant and animal cells in which various chemical reactions occur to keep the cell in the living state.
• The cells can be of undifferentiated, differentiated, or dedifferentiated types.
• The undifferentiated cells are cells that divide rapidly and are not mature to perform a specialized function. For example- root and shoot apical meristem, germinal epithelium, etc.
• While the differentiated cells are the cells which have lost the property of division and are mature enough to perform a specific function.For example- Parenchyma, collenchyma, etc.
• Dedifferentiated cells are the differentiated cells that revert to an undifferentiated state to take over the function of the division. For example- healing of wounds, regeneration in animals, vegetative propagation in plants, secondary xylem, secondary phloem, etc.
   

Topics covered:
 

Cell number, size, shape, and activities
Prokaryotic cell vs Eukaryotic cell
Plant cell vs Animal cell

Cell number, size, shape, and activities
 

Introduction:

• The size, shape, and activities of the cells differs greatly.
• For example, the smallest cells are of Mycoplasmas while the largest isolated single cell is the egg of an ostrich.
• These cells also show a wide range of variations in their shape.
• The shape of the cell is related to their position, e.g., flat in surface cells, polygonal in the cortical region; and function, e.g., Biconcave in RBCs to pass through capillaries and carry oxygen.

Detailed explanation:

Size:

• There are a few examples that help us to observe the varying range of size of the cells.
• The smallest cell available in the environment is of Mycoplasma with a size range of 0.1-0.3 m.
• The bacterial cell range is 3-5 m.
• The viruses are smaller in size and range (0.02-0.2 mm).
• Unicellular eukaryotes have a size of 1-1000 m.
• The largest cell of the plant kingdom is of unicellular green algae (Acetabularia) which is upto 10 cm in length.
• Nerve cells are some of the longest cells.
• Among multicellular organisms, human red blood cells are about 7.0 µm in diameter.
• The largest isolated single cell of the animal kingdom is the egg of an ostrich.
• The surface area to volume ratio of the cell is inversely related to the size of the cell.
• Hence, the smaller cells having high surface area to volume ratio are metabolically more active than larger cells with low surface area to volume ratio.

Shape:

• The cells vary in their shapes as they are disc-like, polygonal, columnar, cuboid, amoeboid, thread-like, or irregular.
• The shape of a cell is related to its position such as a flat in surface cells, polygonal in the cortex; and functions such as nerve cells are branched and long in structure to conduct impulses, Sperms are the male reproductive cells that have tails for motility.
   

Various Functions of different cells:
 

1. Red blood cells (RBCs)

• The red blood cells are biconcave in shape to pass through capillaries. These cells are responsible for oxygen transfer.

2. White blood cells (WBCs):

• These cells are irregular in shape and can perform phagocytosis.

3. Columnar epithelial cells:

• These cells form the lining of the stomach and intestines.
• These cells maximize the surface area for absorption.

4. Nerve cells:

• Some nerve cells of human beings have a meter-long 'tail' or axon.
• A nerve cell is an electrically excitable cell that forms the nervous system of the organism’s body.
• These cells communicate with each other and the other cells of the body to transfer nerve impulses.

5. Tracheid:

• This supports the upward conduction of water along with the dissolved minerals.
• This is an important tissue of all vascular plants.

6. Mesophyll cells:

• These cells are present in the interior region of the leaves.
• These cells are helpful in photosynthesis due to the presence of chloroplasts.
   

Prokaryotic cell vs Eukaryotic cell
 

Introduction:

• Prokaryotes (with primitive nuclei) are described as organisms that lack membrane-bound organelles and a well-defined nucleus.
• Eukaryotes (with well-developed nuclei) are described as organisms that contain a well-defined nucleus and show the presence of membrane-bound organelles.
• Mesokaryotes are the organisms that were used to be considered as intermediates between the prokaryotes and eukaryotes.
• As the mesokaryotes show the presence of a well-defined nucleus but the absence of histone proteins. For example, Dinoflagellates.

Definition and characteristics features:
 

The basis of the organization of the cell's genetic material (DNA) allows the cells to be categorized into prokaryotic and eukaryotic cells.
 

Prokaryotic cell :

• Prokaryotic cells are smaller in size.
• They consist of a single envelope system.
• They lack a well-defined nucleus, also known as genophore or nucleoid.
• The genetic material is ds circular DNA that lies free in the cytoplasm.
• These cells are mostly unicellular on the basis of cell structure.
• Prokaryotes include bacteria, blue-green algae, chlamydiae, Archaebacteria, and Mycoplasma or PPLO.

Eukaryotic cell :

• Eukaryotic cells are comparatively larger in size.
• They consist of two envelope system.
• Eukaryotic cells contain a well defined nucleus and other membrane bound cellular organelles.
• The genetic material is ds linear DNA present in the well-defined nucleus.
• These cells are mostly multicellular on the basis of cell structure with a few exceptions of unicellular cells.
• The eukaryotes include protists, plants, fungi and animals.

Detailed explanation:
 

Prokaryotic cells:
 

• The prokaryotic cells show the absence of a well-defined nucleus.
• The genetic material is ds circular DNA which is naked and lies free in the cytoplasm.
• Histone proteins are absent in prokaryotes but coiling of DNA occurs around basic polyamines.
• It shows the presence of the single naked chromosome which is also known as the prochromosome.
• Many prokaryotes also have additional small circular DNA entities called plasmids. Plasmids carry additional specific factors like resistance, fertility, etc.
• Bacteria and cyanobacteria are prokaryotic organisms that show the presence of a cell wall made of peptidoglycan. However, the cell wall is absent in Mycoplasma.
• Few of the prokaryotic cells such as bacterial cells or cyanobacteria show the presence of non-motile appendages such as pili and fimbriae.
• The flagella are the thin filamentous extensions from their cell wall which helps in motility. It lacks 9+2 organization and is composed of flagellin protein.
• The fimbriae is responsible for attachment to the substratum and pilli aids in conjugation in the bacterial cells.
• The prokaryotic cells have a one envelope system as they are surrounded by a cell membrane and lack membrane bound organelles.
• Few prokaryotic cells such as purple and green sulphur bacteria are photosynthetic and they contain photosynthetic pigments in chromatophores.
• Photosynthetic bacteria also show the presence of gas vacuoles that provide buoyancy to the cells.
• The reserve food material is stored in the form of (glycogen, phosphate granules, cyanophycean granules) cell inclusions within the cell.
• The prokaryotic cells show the absence of membrane-lined organelles such as endoplasmic reticulum, Golgi apparatus, lysosomes, microtubules, microfilaments, and centrioles.
• Ribosomes lie independently in the cytoplasm of the cells and are of the 70S type as compared to the 80S type present in the eukaryotic cells.
• The process of transcription and translation occurs in the cytoplasm of the prokaryotic cells.
• The cytoskeleton is not found in prokaryotes.
• They do not show the process of formation and fusion of gametes(sexual reproduction). Instead, they show sexual recombination by the process of transformation, transduction, and conjugation. In favorable conditions, they reproduce by binary fission or amitosis.
• Spindle apparatus is not found in these cells.
• They lack centriole.
• Examples: Archaebacteria, Eubacteria, Cyanobacteria.

Eukaryotic cells:
 

• The eukaryotic cells contain the well defined membrane-bound nucleus which consists of the genetic material of the organism in the form of cell chromosomes.
• The eukaryotic cells are made up of two envelope systems as they are surrounded by the plasma membrane and also consist of membrane-bound cell organelles.
• Membrane-bound organelles are responsible for the different functions inside the cells like endoplasmic reticulum, Golgi apparatus, lysosomes, microtubules, microfilaments, and centrioles.
• Plants and fungi possess cell walls made of cellulose and chitin respectively.
• However, animal cells do not possess cell walls.
• The reserved stored material in plants is starch and in animals is glycogen.
• Eukaryotic flagella show a 9+2 organization and are composed of microtubules.
• Ribosomes can be attached to the endoplasmic reticulum or lie free in the cytoplasm and are 80S type as compared to 70S type in the prokaryotes.
• The 70S ribosomes are also present in the cell organelles like chloroplast and the mitochondria present in the eukaryotes.
• The process of transcription takes place in the nucleus whereas the process of translation occurs in the cytoplasm.
• Cytoskeleton, a network of protein filamentous structures, is present in eukaryotes.
• Generally, they show the process of formation and fusion of gametes (sexual reproduction).
• Spindle apparatus (a subcellular structure that helps in distribution of duplicated chromosomes to daughter cells during cell division) is found in eukaryotes.
• Centrioles are found in animals but absent in higher plants.
• Examples: Protists, Plants, fungi and animals.

Tabular difference between the prokaryotic and eukaryotic cells:
 

Plant cell vs Animal cell
 

Introduction:

• The organisms comprising prokaryotic cells are called prokaryotes whereas the organisms comprising eukaryotic cells are called eukaryotes.
• The eukaryotes consist of both plant and animal cells that form the respective plant and animal bodies.
• Both the plant and the animal cells contain a well-defined nucleus.
• These cells also contain the membrane-bound organelles which are suspended in the cytoplasm of the cell.

Detailed Explanation:

Some common eukaryotic features shared by both plant and animal cells are as follows-

• The eukaryotic cells contain the membrane-bound nuclei which consist of the genetic material of the organism in the form of cell chromosomes.
• They are made up of two envelope systems as they are surrounded by the plasma membrane and also consist of membrane bound cell organelles.
• Membrane-bound organelles which are responsible for the different functions inside the cells.
• Ribosomes can be attached to the endoplasmic reticulum or found freely and are the 80S type.
• The 70S ribosomes are also present in the cell organelles like chloroplast and the mitochondria found in the eukaryotes.
• The process of transcription takes place in the nucleus whereas the process of translation occurs in the cytoplasm.
• They both show the presence of microfilaments and microtubules.
• Peroxisomes are found in both plant and animal cells.
• In plants the reserve food material is in the form of starch while in animal cells it is glycogen.
• In plants due to absence of centriole the type of spindle formed during cell division is of anastral type while in animals the spindle formed is of amphiastral type.
• In plants the adjacent cells are connected by plasmodesmatal connections while in animals cell junctions are found.
• In plants cytokinesis during cell division occurs by cell plate formation while in animals it occurs by cell furrow method.
• Presence of cell walls, plastids and large vacuoles is characteristic of plant cells.
• Animal cells are demarcated by the presence of centrioles, lysosomes and many small vacuoles. Whereas centriole is absent in higher plants and the function of the lysosomes is taken over by the vacuoles and spherosomes in plants.
  
  
  
  
  
  
   

Difference between plant cell and animal cell:
 

Frequently asked questions (FAQ)

Q1. Where does transcription and translation occur in a prokaryotic cell?
Ans:

• The cytosol region of the prokaryotic cells is the locatory site of the process of transcription and translation.
• Transcription is the process of utilizing the DNA region into mRNA (messenger RNA) synthesis whereas translation is the process of forming proteins from mRNA.

Q2. What are gas vacuoles and where are they found?
Ans:

• The gas vacuoles are the cell inclusions found in photosynthetic bacteria like green and purple sulphur bacteria.
• It provides buoyancy and also helps the photosynthetic bacteria in getting optimal light and oxygen.

Q3. Define nucleoid in prokaryotes.
Ans:

• Prokaryotes do not have a well-defined nucleus but the genetic material (DNA) is not scattered throughout the cell.
• The DNA is held together by the positively charged histone proteins in the region known as nucleoids.
• The DNA in a nucleoid region is organized in large loops held by proteins.

Q4. What are mesokaryotes?
Ans:

• Mesokaryotes are the organisms that were used to be considered as intermediates between the prokaryotes and eukaryotes.
• As the mesokaryotes show the presence of a well-defined nucleus but the absence of histone proteins.
• For example, Dinoflagellates.

Q5. What do you mean by a well-defined nucleus?
Ans:

• The structure of the nucleus of a eukaryotic cell is well defined and is made up of a nuclear envelope, nucleoplasm, nucleolus, and chromatin network.
• A nuclear envelope is a double-walled delimiting membrane of the nucleus.
• The nucleoplasm or karyolymph contains various substances like nucleic acids, protein molecules, minerals, and salts.
• It contains a chromatin network and nucleolus.
• The nucleolus is made up of rRNA (ribosomal RNA) and ribosomal proteins

Q6. What are the similarities between the plant and animal cells?
Ans:

• The eukaryotic cells contain the membrane-bound nuclei which consist of the genetic material of the organism in the form of cell chromosomes.
• They are made up of two envelope systems as they are surrounded by the plasma membrane and also consist of membrane-bound cell organelles.
• Membrane-bound organelles which are responsible for the different functions inside the cells.
• Ribosomes can be attached to the endoplasmic reticulum or found freely and are the 80S type.
• The 70S ribosomes are also present in the cell organelles like chloroplast and the mitochondria found in the eukaryotes.
• The process of transcription takes place in the nucleus whereas the process of translation occurs in the cytoplasm.
• They both show the presence of microfilaments and microtubules.
• Peroxisomes are found in both plant and animal cells.

Q7. Name the smallest and the largest cell.
Ans:

• The smallest cell is PPLO and the largest isolated cell is that of an ostrich egg in the animal kingdom.
• The largest cell in the plant kingdom is Acetabularia which is a unicellular algae.
• PPLO stands for pleuropneumonia-like organisms (Mycoplasma) that causes pneumonia in humans and is resistant to antibiotics like penicillin.
• The ostrich egg is the largest and unicellular cell currently known on earth.

Q8. Name the longest cell in the human body.
Ans:

• The nerve cells are the longest cells present in the human body.
• The nerve cells constituting the nervous system are responsible for the transmission of nerve impulses or messages to the brain and also to take away messages from the brain to the receptor organs.

Q9. Why are smaller cells metabolically more active than larger cells?
Ans:

• The surface area to volume ratio of the cell is inversely related to the size of the cell.
• The smaller cells have a higher surface area to volume ratio and are metabolically more active than the larger cells.

Q10. What is the shape of RBCs and what function do they perform?
Ans:

• The red blood cells are round and biconcave in shape, supporting the cell to pass through the capillaries.
• These cells are responsible for oxygen transfer.

Q11. What is a spindle apparatus?
Ans: Spindle apparatus is a subcellular structure which helps in distribution of duplicated chromosomes to daughter cells during cell division and is found in eukaryotes.

Q12. Differentiate between cell plate formation and cell furrow.
Ans:

• The presence of the solid cell wall in the plant cell is known as Plant cytokinesis.
• It takes place by two methods which are the cleavage method and cell plate formation method.

(i) Cell furrow or cell cleavage method:

• A viscous fibrous dense mid body is formed in the centre.
• Constriction of peripheral microfilaments takes place toward the centre (Centripetally) and finally both sides meet and two daughter cells are formed e.g. Animal cell.

(ii) Cell plate method:

• In the case of plant cells, some spindle fibers exist that form phragmoplast.
• Golgi vesicles and some ER vesicles deposit in the central part of phragmoplast and their deposition proceeds towards the peripheral part (centrifugally) and finally a cell plate is formed followed by the formation of middle lamella.
• Now primary wall deposits at each side of middle lamella and ultimately both cells get separated to form two daughter plant cells.

Q13. How can we differentiate an anastral spindle from that of an amphiastral spindle?
Ans:

• Amphiastral spindle arrangement is found in animals which consists of the spindle with two asters, one at each pole.
• Anastral spindle arrangement is found in plants in which the spindle has no aster due to the absence of centrioles in them.