Cell Structure and Function

Types of Cells - Prokaryotic Cell and Eukaryotic Cell

Types of Cells

Cells are categorized into two types – prokaryotic cell and eukaryotic cell depending on the presence or absence of a true nucleus in the cell.



A usual cell contains cytoplasm which is surrounded by a thin membrane known as the cell membrane. The key function of a cell membrane is to protect the constituents of the cell from the outside environment. Selective materials are only permitted to enter the cell through the cell membrane. It involves other cell organelles like mitochondria, nucleus etc.

Cells are joined together to form tissues, tissues form organs, organs form organ systems such as circulatory system, digestive system, central nervous system, etc. They combine together to form different forms of life in different shapes and sizes. Most of the organisms are multicellular like humans. There are unicellular organisms like bacteria and protozoa and multicellular organisms like human beings.

Cell Structure:

There are several cells in an individual, and the different kinds of cells include - prokaryotic cell, plant and animal cell. The size and the shape of the cell vary from millimeter to microns, which are usually based on the type of function that it performs. Cells usually vary in their shapes. They could either be rod-shaped, flat, curved, concave, spherical, rectangular, oval etc. These cells will only be visible under a microscope.

Prokaryotic Cell Structure

They are the first organisms to be existing on our planet earth. Organisms with this cell type, are called prokaryotic organisms (or) prokaryotes. Prokaryotic cells are single-celled organisms, with the deficiency of nucleus and comprise of a capsule, cytoplasm, cell wall, cell membrane, ribosome, nucleoid, plasmids, pili, and flagella.




Prokaryotic Cell General Features

  • • The size of a cell varies from 1-10 microns. Few prokaryotic cells vary in their size.

  • • They are unicellular, which forms a colony.

  • • The shape of the cell contains rod spherical and flat shaped organisms.

  • • Mode of nutrition - A few organisms are photosynthetic, and getnourished on living things and dead things.

  • • They reproduce asexually by binary fission, transformation, conjugation, transduction.


  • Structure and Functions of a Prokaryotic Cell:

    Capsule: It is the greasy outer coating of the cell wall. The key function of the capsule is to protect the cell from getting dry and also helps in protecting cells from outside pressures.

    Cell wall: It is the stronger and a rigid structure, which provides the shape and protects the inside organelles of a cell. It is the middle layer, which lies between the capsule and the cell membrane.

    Cell membrane: It is the inner delicate structure, which plays an important role in regulating the entry and exit of any materials in the cell. It acts as a permeable membrane. It is of about 5-10nm in width, which helps in the oozing of proteins and elimination of waste products.

    Cytoplasm: It is the viscous membrane, which is lying in between the cell membrane and nucleoid.

    Nucleoid: It is the cytoplasm region covering genetic material. The DNA of a prokaryotic organism is one big loop, which is situated inside the nucleoid. It plays an important role in cell division.

    Ribosome: It consists of both RNA and proteins. It supports protein synthesis in the cell. They are tiniest membrane present inside the cytoplasm.

    Plasmids: They are the minute membrane of a cell with double-stranded DNA. Plasmids are hardly present in prokaryotic organisms.

    Pili: It is the thinnest tissue of a prokaryotic cell. They consist of a protein complex named pilin and are mostly involved in sticking to the objects particularly during sexual reproduction.

    Flagella: It is a membrane which is in a helical shape, and its sizes vary from 19-20nm in diameter and plays an important role in mobility of an organism from one place to another place.

    Eukaryotic Cell Structure

    They are the cells with a true nucleus. Organisms, with this cell kind, are identified by the term eukaryotes. Animals, plants and other organisms apart from bacteria, blue-green algae and E. coli have been grouped into this group. Eukaryotic cells are more complex in comparison to prokaryotic cells. These organisms contain a membrane-bound nucleus with many cell organelles to make several cellular functions within the system.




    Eukaryotic Cell General Features

    The size of a eukaryotic cell varies from 10-100 microns.

    • They are multicellular, and have membrane-bound organelles.
    • They reproduce either sexually or asexually.
    • Type of nutrition - Autotrophic and heterotrophic
    • KingdomsProtozoa, Fungi, Algae, Plantae and Animalia are eukaryotic organisms.

    Structure and Functions of a Eukaryotic Cell:

    Plasma membrane: They are semi-permeable membranes that behave as a boundary of a cell, which protects and splits up the cell from the outside environment.

    Nucleus: It is a double layered wall of Phospholipid bilayer. They are the stockroom for the cell’s genetic materials in the form of DNA and keep all the essential information, which are vital for a cell to control.
    Nuclear membrane: It is the double membrane layer that surrounds the nucleus and it plays a part in the entry and exit of resources within the nucleus.

    Nucleolus: It is the non-membrane bound organelle, which is located within the nucleus. Mitochondria: They are the double smooth membrane, which is in all eukaryotic cells. They are the powerhouse of the cell. It plays a vital part in the synthesis of ATP and transforms glucose into ATP.

    Endoplasmic reticulum: Double membrane organelle, which splits the cell into sections. It is linked to the nuclear membrane of the cell. It plays an important role in protein synthesis, biosynthesis of lipids and steroids, stores and controls calcium and digestion of carbohydrates. The endoplasmic reticulum is of two type’s rough and smooth Endoplasmic reticula.

    Ribosome: It is in the cytoplasm. They are the spot for cells protein synthesis, which is made of ribosomal RNA and proteins.

    Golgi Bodies: It is the compacted membrane, which is mostly used to preserve the materials made by the cell. This membrane also helps in storing, carrying materials within the cell. Therefore, it is also named as the post office of a cell.

    Lysosomes: They are the membrane-bound organelles, which have digestive enzymes to break down macromolecules. Lysosome plays a vital part in protecting the cell by destroying foreign bodies entering the cell.

    Cytoplasm: Jelly types of organelles, which are in the inner region of a cell. It plays a vital part in keeping a cell in a stable and keeps the cell organelles distinct from each other.

    Chromosomes: The rod-shaped structures, which are made of proteins and DNA. Chromosomes also play a vital part in determining the sex of an individual.

    Plant Cell Structure:

    Plant cells are eukaryotic cells, with a true nucleus, multicellular large and progressive membrane-bound organelles. These plant cells are relatively different from animal cells like in shape and other several organelles which are only found in animal cells but are absent in plant cells. Depending on structure and functions, plant cells comprise of:



    Cell wall: It is the outer layer of a plant cell, which helps in providing the form and strength to the complete plant. A cell wall is made of cellulose that protects and helps the plant to grow.

    Cell membrane: A biological membrane that divides living cell organelles from nonliving structures. This membrane plays a vital part in helping a cell to communicate with the outside environment and in carrying proteins and other molecules throughout the cell.

    Chloroplasts: Green colored oval shaped double membrane organelles, which are the spots of photosynthesis. The green colored pigment (chlorophyll) located in the leaves helps plants in absorbing solar energy to make food.

    Golgi Bodies (or) Golgi complex: The sac-like structures, which are in a cell to manufacture, store, packing and shipping the substance throughout the cell.

    Mitochondria:Mitochondria play an important part in liberating energy and they are the powerhouse of a cell.

    Vacuole: They are the fluid bags, which are in great numbers in plant cells. The chief function is this membrane is to preserve food and other waste substances. Vacuoles are the biggest organelles present in the plant cells.

    Animal Cell Structure:

    Animal cells are eukaryotic cells, with the existence of a true nucleus; multicellular big and advanced membrane-bound organelles. Similarly, like plant cells, animal cells have similar organelles except for the cell wall, chloroplasts, number of vacuoles and many more. Due to the deficiency of the cell wall, the shape of an animal cell is uneven. Depending on structure and functions, animal cells comprise of:



    Cell membrane: They are semi-permeable membrane covering the cell. It supports holding the cell together and controls the entry and exits of nutrients into the cell.

    Vacuole: They are the fluid bags, which are in fewer numbers in animal cells as compared to plant cells. The key function is this membrane is to preserve food and other waste substance.

    Golgi Bodies (or) Golgi complex: The bag like assemblies, which are existing in a cell to manufacturing, storage, packing and transporting the selected particles throughout the cell.

    Ribosome: It is situated in the cytoplasm. They are the place for protein synthesis, which is made of ribosomal RNA and proteins.

    Mitochondria: They are bar-shaped organelles, plays an important part in liberating energy and they are known as the powerhouse of a cell.