INTRODUCTION:

The unicellular eukaryotes are deferent from prokaryotes with several characteristics like as motility, thickness of cell, intracellular organelles etc. The eukaryotes have separated transcription from translation, while prokaryotes have not. It would be easy to look at each of these features as something that eukaryotes gained during the course of evolution, but we would like to suggest that each of these features is actually the result of a process of loss.^1,2 In order to understand our argument, it is necessary to think of evolution not as the elaboration of individual traits, but as selection for more and more efficient ecologies. Thus, we propose that eukaryotic species did not evolve from elaboration or complexification of individual prokaryotic species, but rather that individual eukaryotic species evolved by integration and simplification from communities of diverse prokaryotes. By integrating a community of prokaryotes into a single cell, eukaryotes integrated many of the best features of that community, thereby localizing their functions.³

Integration of partial duplications, however, would have resulted in incomplete overlapping of some features, resulting in some functions being distributed within eukaryotic cells, including features such as multiple internal membranes; multiple (partial) genomes; both nuclear and cytoplasmic (mitochondrial) DNA; and genes only partially integrated and therefore characterized by having axons punctuated by intones.⁴

Introduction to Prokaryotes and Eukaryotes

· Microorganism and all other living organism are classified as prokaryotes or eukaryotes.

· Prokaryotes: pro- before and Karyone- kernel.

· Eukaryotes: Eu- true and Karyone- kernel

· Organism whose cells are organized into complex structures by internal membranes and a cytockeleton.⁵

Eukaryotic cell:

· Cell is the basic structural and functional unit of life in all organisms.

· Eukaryotic cell are those cells which have nucleus inside the cell and comprise of proper organelles and boundaries. Ex: Plant Cell, animal cells etc.⁶

Plant cells are eukaryotic cells with a true nucleus along with specialized structures called organelles that carry out certain specific functions.

Plant Cell Structure

· It includes various components known as cell organelles that perform different functions to sustain itself.⁷

· Cell may be divided in to 3 parts:

· Plasma membrane

· Cytoplasm

· Nucleus

Structure and Function of cell organelles

Fig: Plant Cell

Plasma membrane:

Cell wall:

· It is located outside the cell membrane. It is a rigid layer which is comprises proteins, polysaccharides, and cellulose.

· The primary function of the cell wall is to protect and provide structural support to the cell.⁷

· The plant cell wall is also involved in protecting the cell against mechanical stress and to provide form and structure to the cell. It also filters the molecules passing in and out of the cell.

· The formation of the cell wall is guided by microtubules. It consists of three layers, namely, primary, secondary and the middle lamella.^8,9

Cell membrane:

· It is the semi-permeable membrane that is present within the cell wall. It is composed of a thin layer of protein and fat.¹⁰

· The cell membrane plays an important role in regulating the entry and exit of specific substances within the cell.

· For instance, cell membrane keeps toxins from entering inside, while nutrients and essential minerals are transported across.¹¹

Cytoplasm:

· It consists of the entire cellular component inside the plasma membrane and the nucleus.

• It has 2 components:

• Cytosol: liquid portion of cytoplasm, contain water, mineral and dissolved solutes.¹²

• Organelles: Ex- Mitochondria, Ribosomes, Endoplasmic reticulum, Golgi complex, Lysosomes, Peroxisomes.¹³

Golgi apparatus:

· They are stacked flat sacs found in all eukaryotic cells.

· Function: receives proteins from the rER and distributes them to other organelles or out of the cell (receiving, processing, packaging, and shipping).^14,15

Ribosomes:

· The ribosome is consisting of two subunits made of protein and RNA.

· Function: location of protein synthesis.¹⁶

Endoplasmic Reticulum:

· Structure: a system of membranous tubules and sacs

· Function: intercellular highway (a path along which molecules move from one part of the cell to another).¹⁷

· Two types: – Rough Endoplasmic Reticulum

– Smooth Endoplasmic Reticulum

Rough Endoplasmic Reticulum:

· Rough Endoplasmic Reticulum (rER): prominent in cells that make large amounts of proteins to be exported from the cell or inserted into the cell membrane. It covered with ribosomes.^18,19

Smooth Endoplasmic Reticulum:

· Smooth Endoplasmic Reticulum (sER): involved in the synthesis of lipids and breakdown of toxic substances.

· Not covered with ribosomes.²⁰

Mitochondria:

Structure:

They are the double-membrane organelles found in the cytoplasm of all eukaryotic cells.

Function:

They provide energy by breaking down carbohydrate and sugar molecules; hence they are also referred to as the “Powerhouse of the cell.”^21,22

Lysosome:

Structure:

Lysosomes are spherical organelles, called as suicidal bags as they hold digestive enzymes in an enclosed membrane.

Function:

They perform the function of cellular waste disposal by digesting worn-out organelles, food particles and foreign bodies in the cell.^23,37

Peroxisomes:

Structure:

Spherical organelles that contain enzymes within single membranes.

Function:

Degrade hydrogen peroxide, a toxic compound that can be produced during metabolism.²⁴

Vacuoles:

Structure:

It occupies around 30% of the cell’s volume in a mature plant cell. Tonoplast is a membrane that surrounds central vacuole.

Function:

Used for temporary storage of wastes, nutrients, and water.²⁵

Plastids:

· There are three types of plastids in plant cells:

1. Leucoplasts: They are found in non-photosynthetic tissues of plants.

• Function: They are used for the storage of protein, lipid, and starch.²⁶

2. Chromoplasts:

They are heterogeneous, coloured plastid which is responsible for pigment synthesis and for storage in photosynthetic eukaryotic organisms.²⁷

Chromoplasts have red, orange and yellow coloured pigments which provide colour to all ripe fruits and flowers.²⁸

3. Chloroplast:

Structure:

The chloroplast is shaped like a disc that contain chlorophyll surrounded by a double membrane. Each chloroplast contains a green coloured pigment called chlorophyll required for the process of photosynthesis.²⁹

Function:

Photosynthesis (conversion of light energy to chemical energy stored in the bonds of glucose).

Nucleus:

The nucleus is a membrane-bound structure that is present only in eukaryotic cells. The vital function of a nucleus is to store DNA or hereditary information required for cell division, metabolism, and growth.[36]

Nucleolus:

It manufactures cell’s protein-producing structures and ribosomes.

Nucleopore: Nuclear membrane is perforated with holes called nucleopore that allows proteins and nucleic acids to pass through.^30,31

Prokaryotic cell

Prokaryotic are single celled organisms that are most primitive forms of life on earth.³²

Do not have a nucleus, mitochondria, Endoplasmic Reticulum, Golgi apparatus, Lysosomes etc. Ex: Bacteria

Cell structure:

Capsule:

Found in some bacterial cells, this additional outer covering protects the cell when it is engulfed by other organisms.

Cell wall:

Outer covering of most cells that protects the cell and gives it shape.³³

Cell Membrane:

Surround the cell cytoplasm and regulates the flow of substance in and out of the cell.

Cytoplasm: a gel like substance.

Pili: Hair like structure on the surface of the cell that attach to other bacterial cell, help bacteria attach to surface.

Flagella:

Long, that aids in cellular locomotion.³⁵

Ribosomes:

Responsible for protein production.

Plasmids:

Gene carrying, circular DNA structure that are not involved in reproduction.³⁴

Nucleoid Region

Fig: Prokaryotic cell

CONFLICT OF INTEREST:

The author declares no conflict of interest to this review.

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Received on 24.03.2021 Modified on 19.04.2021

Asian J. Pharm. Res. 2021; 11(3):202-205.

DOI: 10.52711/2231-5691.2021.00036