Structure of a Cell and Its Types
Despite the great diversity in the structure of cells, the basic structural plan is similar in all the cells.
It can be expressed as generalized cells. A small speck of protoplasm that constitutes one cell is called protoplast.
Protoplast or protoplasm of a cell can be differentiated in three essential parts, i.e.
1. Plasma membrane or Cell membrane
1. Plasma Membrane:
Each cell is delimited by a thin, elastic but firm lipoproteinaceous, selectively or differentially permeable plasma membrane. It maintains the shape of the cell and also protects its internal vital structures. It diffuses harmful substances, and in this way, it regulates the flow of materials into and out of the cell.
In case of plant cells, outer to the plasma membrane there is a non-living, rigid cell wall made up of cellulose. The cell wall is freely permeable. It gives shape and provides rigidity as well as elasticity to bear stress and strain.
The cell is made up of a dense, viscous, colloidal mass called cytoplasm, present between the plasma membrane and nuclear membrane. It is the ‘physical basis’ of all metabolic activities that take place in the cell. It is differentiated into cytoplasmic vacuolar system and hyaloplasm.
Cytoplasmic vacuolar system is formed by membrane bound spaces and consists of endoplasmic reticulum, golgi bodies and nuclear membrane. The hyaloplasm forms homogeneous ground substances.
There are various living and non-living structures, collectively called cytoplasmic inclusions. The living cytoplasmic inclusions are called cell organelles or protoplasmic inclusions and the non-living structures are called deutoplasmic or ergastic bodies.
A. Cell Organelles (the ‘little organs’):
They are the living cytoplasmic inclusions which are capable of growth and divisions. They perform specific functions and enjoy the same status in a cell like the organs in the organism.
(a) Endoplasmic Reticulum (ER):
The interconnected network of membrane is called endoplasmic reticulum. It is visible only under the electron microscope. Two types of ER are present in the cell. I) Rough Endoplasmic Reticulum it is beaded and rough in appearance because of the presence of small granular structures called ribosome. II) Smooth Endoplasmic Reticulum-they are mostly found in the outer part of cytoplasm. It is smooth in appearance with no ribosome on its surface.
They are small granular structures visible only under the electron microscope. They are found attached on the outer surface of endoplasmic reticulum and freely in the cytoplasm. They are called protein factories of a cell.
They are popularly known as Power houses of a cell. The inner membrane of the mitochondria is produced into finger like processes called cristae. In the mitochondria, respiratory enzymes are present that help in the oxidation of food materials and help in the release of energy. The energy thus released is stored in the form of energy-rich bonds called ATP (Adenosine Triphosphate).
They are small spherical bodies bound by a single unit membrane. The most important Microbodies are lysosomes. They contain about 40 different types of enzymes which are hydrolyses. If these enzymes are released into the cytoplasm because of rupturing of their limiting membrane, they can digest all the components of the cell. Hence, they are known as Suicide bags of a cell. Other types of Microbodies are peroxisomes, spherosomes, etc.
They are found in animal cells whereas absent in the plant cell. They are present near the nucleus. They are concerned with the process of cell division, formation of basal bodies and flagella.
They are present only in plant cells and seldom found in animal cells. They are double walled vesicles of different shapes. They are capable of division.
Depending on the colour, they are categorized into two types: (i) Leucoplasts (ii) Chromoplasts.
They are colourless plastids as they do not contain any pigment. They help in the storage of food.
They are coloured plastids because of the presence of coloured substances called pigments. They are of two types, i.e., I) photosynthetically inactive II) photosynthetically active.
(I) Photosynthetically inactive plastids:
They are found in petals of flowers and ripe fruits. They contain coloured pigments such as carotene (red- orange), xanthophill (yellow).
(II) Photosynthetically active plastids:
They also contain coloured pigments. They also help in trapping solar energy and photosynthesis. Depending on the colour of pigments, they are divided into chloroplasts (green-coloured), rhodoplasts (red coloured) and pheoplasts (brown coloured).
They are very microscopic tube-like structures which may be present independently as parts of some organelles such as centrioles, basal bodies, cilia, flagella, etc. They help to provide strength.
B. Deutoplasmic or Ergastic Bodies:
These are non-living cytoplasmic inclusions which are not capable of growth or division. They include vacuoles reserve food, excretory granules, secretory granules and crystals of minerals.
The nucleus controls all the metabolic activities, directly or indirectly. It is surrounded by a double membrane, porous nuclear envelope or karyotheca. It is surrounded by a clear jelly like envelope called nucleoplasm.
At least one nucleolus and a network of thread-like structures called chromatin reticulum are found in nucleoplasm. The nucleolus is related with the formation of ribosomes.
The chromatin reticulum condenses into small and thick rodlike structures called chromosomes during cell division. These chromosomes contain the hereditary units called genes.
The genes are made up of complex chemical substances called DNA (deoxyribonucleic acid). The number of chromosomes is definite in the cells of each species, e.g., every human body cell contains 23 pairs of chromosomes.
The complexity and structure of an organism are not determined by the number of chromosomes. Rather, these are genes which determine the characteristics and functioning of all species.
Types of Cell:
Depending upon the structure, cells are of two types: (a) Prokaryotic cells (b) Eukaryotic cells.
(a) Prokaryotic Cells:
(Pro-primitive, karyon- nucleus) – These are cells with primitive nucleus called nucleoid or genophore. They are without any nuclear envelope. They are found in all bacteria and blue-green algae.
1. Single-membrane system in which the cell is surrounded by a cell membrane. No other membrane or membrane-bound organelle is present.
2. Comparatively small, 1-10 p in size.
3. Most of the prokaryotic cells are covered by a slimy capsule made of polysaccharides.
4. Cell wall made of carbohydrates, proteins, lipids, muramic acid, and diaminopimelic acid is present in most of the cells. Cell membrane present is associated with respiratory enzymes.
5. Cell membrane may be folded so as to form mesosomes and photosynthetic lamellae.
6. Sap-vacuoles are absent in the cytoplasm.
7. All the membrane-bound organelles such as endoplasmic reticulum, mitochondria, lysosomes, Centrosomes, Golgi complex, Microbodies, microfilaments are absent.
8. Free-occurring ribosome of 70S type are present.
9. Cytoplasm does not exhibit streaming movements.
10. There is no endocytosis (pinocytosis and phagocytosis) or exocytosis in these cells.
11. No true nucleus. The genetic material called prochromosomes occur freely in the cytoplasm and are known as nucleoid.
12. DNA material is naked, i.e., without any protein covering, double stranded and circular.
13. They are always haploid.
14. Nucleolus absent.
15. During cell division, no mitotic apparatus is formed.
16. Being haploid no meiotic division takes place.
17. There is no gamete formation or true fertilization.
18. Cilia/flagella when present are single stranded.
19. Found in bacteria and blue-green algae.
(b) Eukaryotic Cells:
These cells contain an advanced nucleus encircled by own nuclear envelope.
Major differences between prokaryotic and eukaryotic cells are tabulated below:
1. Double-membrane system, having one cell membrane and other nuclear membranes Moreover, membrane-bound organelles are present in the cytoplasm.
2. They are larger, 5-15 p in size. No such capsules are present.
3. Cell wall, found in plant cell only, is made of cellulose. Cell membrane does not contain any respiratory enzymes.
4. Such structures are not formed.
5. Very large sap-vacuoles are present in plant cells, but generally absent in animal cells.
6. All the membrane-bound organelles are present.
7. Free-occurring or attached on the surface of ER ribosome are present which are of 80S type. 70S ribosome are found in some organelles.
8. Streaming movements are exhibited by the cytoplasm.
9. Endocytosis and exocytosis take place in most of the animal cells.
10. Nucleus delimited by a double nuclear envelope is present.
11. DNA molecules are covered by histone proteins and are linear.
12. Cells are typically diploid, but haploid cells also occur.
13. 1-4 nucleoli occur in the nucleus.
14. Mitotic apparatus is formed during cell division.
15. Meiotic division takes place during gametogenesis.
16. Gamete formation and fertilization take place during sexual reproduction.
17. Cilia/flagella when present are eleven (9 + 2) stranded.
18. Found in all the organisms other than bacteria and blue-green algae.