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What is a Cell And How is the Structure of a Cell?

by May 29, 2019Biology0 comments

The term ‘cell’ is derived from a Latin word cella which means a small room. Scientifically cell is a fundamental structural and functional unit of any living organism. It is the smallest unit of any living being. A cell cannot be seen with a naked eye. A branch of biology that deals with studying of any cell is called Cellular Biology.

History of cells

Cells are not visible to naked eyes. In 17th-century, a compound microscope was discovered. Robert Hooke was the very first man to view cells using this microscope. He saw a cork under the microscope and named those segments it is made up of as a cell. The plant scientist Matthias Schleiden and the animal scientist Theodor Schwann were the ones who stated that all plants and animals are made up of numerous cells.

Unicellular and multicellular organisms

Any living being can be either unicellular or multicellular. All organisms that are made up of a single cell are called unicellular. For example, various microorganisms like algae, bacteria, etc are called unicellular organism. They cannot be viewed with naked eyes. They can be only seen under a powerful microscope.

Every organism that is made up of more than one cell is called a multicellular organism. Every living being that can be seen with naked eyes is said to be multicellular. All human beings, animals, plants, birds, etc are made up of millions of cells. A total number of cells varies from one organism to another.

Types of cells

Cells can be classified into two types: Eukaryotic and Prokaryotic.

Eukaryotic cells-

all types of plants and animals are made up of eukaryotic cells. They are wider and bigger in size. These cells have sections within. Each of these sections is room for certain cell activities. All cells also have small organs within which are called organelles. The most important organelle of a cell is called a Nucleus. It contains the DNA of any cell.

Eukaryotic cells may or may not have cell walls. They contain chromosomes inside the nucleus. They contain mitochondria and chloroplasts. An organism with eukaryotic cells can be unicellular or multicellular.

Prokaryotic cells-

these cells were the first cells on earth. These cells are smaller and less complex in comparison to the eukaryotic cells. They do not contain a nucleus. The nuclear region is called nucleoid. They are covered with a cell envelope which comprises a plasma membrane enclosed by a cell wall. Some bacteria also have another layer called the capsule.

All organisms made up of Prokaryotic cells are also unicellular organisms. The outer membrane of these cells act as a protective layer and gives rigidity to every cell. The interior of these cells is filled with cytoplasm and contain the DNA of the cell. The exterior of these cells have projections like flagella and pili. These projections are made up of protein and they help in the movements of these prokaryotic organisms. Division of all cell takes place in the form of binary fission.

The cell organelles

Every living cell has got some organelles. They are:

  • Nucleus:

Nucleus is a cell organelle that can only be found in a eukaryotic cell. It is the controlling center of the cell. It is larger than other organelles in size and contains the DNA of the cell. The nucleus of the cell is responsible for all the important activities inside the cell. It controls the functions of growth, metabolism, etc.

Within a nucleus is another small body called nucleolus. It stores the RNA of the cell. The RNA (Ribonucleic Acid) helps in the production of protein within the cells.

  • Nucleolus:

This is the largest structure that can be found in the nucleus of any eukaryotic cell. It is the place where the ribosome biogenesis takes place. They are made up of DNA and RNA.

  • Ribosome:

This cell organelle is the contributor of the protein synthesis within a cell. It consists of two smaller units known as small ribosomal subunits and large ribosomal subunits. All the subunits contain one or more than one Ribosomal RNA molecule and various ribosomal proteins.

The subunits can be found floating in the cytoplasm of the cell or implanted within the Endoplasmic Reticulum. With the help of the RNAs, the ribosome produces a useful protein for the cell that is necessary for survival.

  • Endoplasmic Reticulum:

This is a cell organelle that is found in eukaryotic cells. It forms an interconnecting network of flattened or tube-like structures called Cisternae. Endoplasmic reticulum can be found in all types of eukaryotic cells except red blood cells (RBC) and spermatozoa. Some part of endoplasmic reticulum helps in the protein synthesis and the others help in the production of useful fats.

  • Golgi complex:

The Golgi complex is also known as a Golgi apparatus or Golgi body. They can be found in the cytoplasm of a cell. They are located at the intersection of the Secretory, Lysosomal, and endocytic pathways. They collect and dispatch proteins produced in the endoplasmic reticulum.

  • Cytoskeleton:

It is present in the cytoplasm in the cell. It can also be found in various cells of Bacteria and Achaea.  It is a complicated network of protein filaments that expands from the cell nuclease to the cell membrane. A cytoskeleton of various organisms is composed of similar protein. The structure, function and the behavior of every cytoskeleton are different for different organisms.

Cytoskeleton can perform different functions. They give a rigid shape to every cell. They help the cells to migrate. They also help in cell signaling pathways.

  • Mitochondrion:

This organelle is made up of two membranes. They are found in most of the eukaryotic cells. A red blood cell does not have a mitochondrion. They are called the powerhouse of the cells. They produce the energy for the cells which helps the cells to carry out various functions. They help the cells in the process of respiration and metabolism.

  • Lysosome:

A lysosome is a cell organelle that can be found in the animal cells. It has a membrane on the outer side. They are spherical in shape. The lysosome performs various activities like secretion, repairing of the plasma membrane, energy metabolism, and cell signaling.

It helps the cells to dispose of the wastes. It digests those materials present in the cytoplasm that are no longer useful. The lysosome contains more than 60 types of different enzymes and more than 50 types of membrane proteins.

  • Centrosome:

This organelle is the primary microtubule organizing center (MTOC) of animal cells. It also regulates the progress of the cell cycle. Fungi and plant cells do not contain centrosome. They play a major role in the mitosis of the cells.

  • Vacuole:

It is an organelle that is bound with a membrane and can be found in all the plant cells. They are the bound structures that are filled with water that contains various organic and inorganic molecules including various enzymes.

Most of the plant cells contain one vacuole that is large in size. The membrane surrounding the vacuole is called tonoplast. In animal cells, the vacuole is smaller in size but more in number. Some animal cells even lack a single vacuole.

  • Cell membrane:

It is also called a plasma membrane or a cytoplasmic membrane. It is that outer layer of the cells that is helpful in protecting the interior of a cell from the external environment. It controls the movement of various materials in or out of the cell or an organelle.

A cell membrane borders the cytoplasm of living cells. It separates the intercellular substances from the extracellular substances. It helps in containing the various components of a cell. It also provides the shape and rigidity of every cell.

The cell membrane is semi-permeable. It can control the substances that enter and leave each cell. Therefore it is helpful in the transporting of various substances that are necessary for the cells to carry out various life functions. They also help in the processes of osmosis and diffusion.

The Cytoplasm

It is the fluid that is contained within a cell that is enclosed by the cell membrane. The nucleus lies in this cytoplasm. The nucleus also consists of a fluid called the nucleoplasm.

The cytoplasm contains three main components. They are- cytosol, the organelles, and several cytoplasmic additions. The cytosol is a gel-like substance. The organelles are the different small functional structures within a cell. The various cytoplasmic additions are small bodies that are insoluble and float in the cytoplasm. The cytoplasm is mostly made up of water and does not exhibit any color.

Most of the functions of a cell take place within the cytoplasm. Functions such as cell division, glycolysis, metabolism, excretion, etc take place in its cytoplasm. The inner area of the cytoplasm is called the endoplasm and the outer area is called the ectoplasm.

Plant cell and animal cell

A plant cell and an animal cell have got many differences. They exhibit a good amount of differences. There are many organelles that are absent or present in either one. They both have a lot of common organelles as both of them are Eukaryotic cells. But there are some major dissimilarities.

In plants, a cell has a definite edge and is either square or rectangular in shape. But for animal cells, they are irregular in shape. Plant cells have a cell wall but it is absent in an animal cell. Both of them have a plasma membrane and endoplasmic reticulum.

In a plant cell, Nucleus is located on a particular side of the cell whereas in animal cells it is present in the center of a cell. Plant cells have very less amount of lysosomes in comparison to animal cells. Centrosomes are absent in plant cells but present in animal cells. In plant cells, plastids can be found but not in animal cells. A plant cell contains a large vacuole in the center but animal cells contain numerous small vacuoles.

Outer projections called cilia is not present in plant cells but animal cells have cilia. There are a few numbers of mitochondria present in plant cells but an animal cell contains many mitochondria.

Growth of cells and the process of cell metabolism

A cell grows in between two consecutive cell divisions through cellular metabolism. In the process of cell metabolism, cells process various nutrients. The process of metabolism can be further divided into catabolism and anabolism. In catabolism, a cell breaks down every complex molecule to generate energy. In anabolism, a cell uses the energy to form complex molecules to carry out various functions.

The complex sugar molecules that are consumed by any living organisms are broken down into simple molecules of sugar called monosaccharide. Glucose is a very common example of a monosaccharide. Then this glucose is further broken down into adenosine triphosphate (ATP). ATP molecules are suppliers of energy.

Different cellular processes

There are many other processes that a cell performs. They are:

  • Active and passive movement of various substances inside and outside of the cells.
  • Autophagy- in this process every cell consume their own components.
  • Holding together the surrounding cells and the tissues.
  • Movement of cells with the help of contraction, flagella, and cilia.
  • Regulating the various functions of cells through signaling.
  • Cell division by the process of mitosis and meiosis.
  • Repairing of the DNA.
  • The processes of photosynthesis, chemosynthesis, respiration, and glycolysis.

Cell biology is a vast section. It comprises of a lot of information about the various types of cells and their functions. Each type of cells exhibit a different function and they vary from one another.

Author Bio:

The writer of this informative piece is Professor Michelle Johnson. She is a fantastic online tutor and has been guiding her students for a long time. She is an MBA graduate and has six years of working experience at Harvard University. She has a detailed knowledge of many subjects and her guidance has always proved useful for her students. You can contact her if you need any guidance.