Cell: Types and Characteristics, Cell Theory

• A cell can replicate itself on its own. Therefore, they are referred to as the building blocks of life. 
• All organisms are built up from cells – either containing a single cell (unicellular) or multiple cells (multicellular).
• The smallest cells that are known to the scientific world are mycoplasmas.
• They are responsible for providing a definite structure to the body and for converting nutrients into energy.
• They can be found in different shapes and sizes in different organisms.
• Cell organelles present in each cell perform various functions required for the proper working of the body of the organism.
• The hereditary material that determines every aspect of a living being is also present in a cell.

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Discovery of Cells

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Robert Hooke is credited with the discovery of cells in 1665. While observing a piece of a bottle cork under a compound microscope, he discovered minuscule particles that resembled small rooms. Thus, he named them cells.

• Later, Anton Van Leeuwenhoek observed these cells under a compound microscope with higher magnification. He was able to find that the cells had some sort of movement or motility.
• Therefore, he decided that these entities are living entities. Eventually, after more observations, they were renamed as animalcules. 
• In 1883, another botanist from Scotland, Robert Brown, provided more details into the structure of the cells.

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Cell Structure

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Specific components present in each cell of the body of the living organism are responsible for the proper functioning and growth of the body. The major components present in a cell are

• Cell wall
• Cytoplasm
• Cell organelles
• Nucleus
• Cell membrane

Cell Wall

This is the most prominent part of the cell that is made up of cellulose, pectin and hemicellulose.

• This surrounds the cell membrane and is highly rigid and stiff.
• Cells are provided with shape and support and are rescued from possible mechanical injuries.
• The cell wall is only found in plant cells.
• It serves as a protection for the plasma membrane and other components.
• It is the outermost layer in plant cells.

Cytoplasm

The cytoplasm is a thick, jelly-like, clear substance that can be found inside the cell membrane.

• The endoplasmic reticulum, mitochondria, ribosomes and vacuoles are freely suspended in the cytoplasm.
• The cytoplasm is the centre of most of the chemical reactions that take place inside a cell.

Nucleus

The nucleus is the storehouse of the DNA or the genetic material of the cell.

• It is separated from the rest of the cell by a nuclear envelope.
• It protects the DNA and thus becomes a crucial part of a cell.
• The nucleus is involved in sending appropriate signals to the cells in order to grow, mature, divide and finally die.

Cell Membrane

This serves as the support system and as the protection of a cell.

• It is a porous membrane that is in charge of the movement of materials in and out of the cell.
• The cell membrane separates the cell from the outer environment and is present in all cells.
• It protects the cellular components from damage and other leakages.
• As plants are immobile, their cell membranes are highly developed to protect them from external threats.
• This is facilitated by the presence of the cell wall.
• It can be pictured as a wall-like structure between two cells as well as between a cell and its external environment.
• It is also known as the plasma membrane.

Cell Organelles

Cell organelles are the cellular components.

• These cell organelles comprise both membrane and non-membrane bound organelles that exist within cells and have distinct shapes and functions.
• They coordinate and work effectively in order for the cell to function normally.
• Some of them provide shape and support, while others are engaged in cell motility and reproduction.

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Types of Cells

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There are two types of cells based on their cellular structure: Prokaryotes & Eukaryotes.

Prokaryotes

Prokaryotic cells do not contain a nucleus. Instead, certain prokaryotes, such as bacteria, have a region, called nucleoid, within the cell where genetic material is freely suspended.

• All of them are single-celled microorganisms, such as Archaea, bacteria, and cyanobacteria. 
• The cell size ranges from 0.1 to 0.5 µm.
• DNA or RNA can be used as hereditary material.
• Prokaryotes reproduce mostly by binary fission, a type of asexual reproduction.
• They are also known to employ conjugation, which is commonly regarded as the prokaryotic equivalent of sexual reproduction (it is NOT sexual reproduction).

Eukaryotes

A true nucleus characterizes eukaryotic cells. The diameter of the cells extends from 10 to 100 µm. Plants, fungi, protozoans, and mammals are all included in this broad category. The plasma membrane controls the transport of nutrients and electrolytes into and out of cells. It is also responsible for cell-to-cell communication.

• They reproduce both sexually and asexually. 
• Plant and animal cells have various distinct characteristics.
• Plant cells, for example, have chloroplasts, central vacuoles, and other plastids, but animal cells do not.

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Functions of Cell Organelles

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There are a number of cell organelles that carry out different functions for the proper growth and development of the body. They are:

• Nucleolus: Ribosome synthesis is the major activity going on in a nucleolus. It also controls activities related to the cell as well as its reproduction.
• Nuclear Membrane: This is the protection of the nucleus as it forms a clear boundary between the nucleus and other cell organelles.
• Chromosomes: These are the important role-players in determining the sex of an individual. Each cell is composed of 23 pairs of chromosomes.
• Golgi Bodies: They are involved in the transportation of materials within the cell and so are rightly called the post office of the cell.
• Mitochondria: Mitochondria, also known as the powerhouse of the cell, produces ATP or the energy currency of the cell.
• Chloroplast: Chloroplast are the primary organelles that take part in photosynthesis. It is the storehouse of the pigment chlorophyll.
• Vacuoles: They store food, waste materials and water inside the cell.
• Lysosomes: These work for the protection of the cell by engulfing foreign bodies and helping in the renewal of the cell. They are known as the suicide bags of the cell.
• Ribosomes: These help in synthesising protein in a cell.
• Endoplasmic Reticulum – This helps in the transportation of substances throughout the cells. The metabolism of carbohydrates, and the synthesis of lipids, proteins and steroids are facilitated by the endoplasmic reticulum.

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Characteristics of a Cell

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A cell performs a lot of functions that are vital for the proper growth and development of an organism. They are as follows:

Provides support and structure

The cells form the basic structure of all living beings.

• The cell wall and cell membrane present in each cell provide the necessary structural support to each organism.
• The skin is built up of millions of tiny cells.
• The xylem that is present in vascular plants is also made up of a large number of cells in order to provide structure and support to the plants.

Allows transport of substances

All the chemical processes that take place inside a cell are facilitated by the transport of nutrients within a cell.

• Similarly, the wastes produced by these chemical reactions are transported outside the cell actively as well as passively.
• Oxygen, ethanol, carbon dioxide and other small molecules diffuse across a cell according to the concentration gradient, known as passive transport.
• Active transport uses a lot of energy to transport large molecules in and out of the cell.

Aids in reproduction

Reproduction is affected by cells through the processes of mitosis and meiosis.

• Mitosis is a type of asexual reproduction where a single parent cell divides to form two daughter cells.
• Meiosis is the type of sexual reproduction where the final daughter cells formed are genetically different from the parent cell.

Energy production

Energy is required by the cells to carry out a lot of chemical processes. This energy is produced by the process of photosynthesis in plants and by the process of respiration in animals.

Facilitates growth

Mitosis is the process through which growth occurs in a living being. Through the division of the parent cell into daughter cells, the cells multiply and thus facilitate growth in an organism.

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Cell Theory

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German scientists Theodor Schwann, Matthias Schleiden, and Rudolf Virchow proposed the Cell Theory.

According to cell theory:

• Cells make up all living things on Earth.
• A cell is the fundamental unit of life.
• All cells arise from pre-existing cells.

A modern version of cell theory was subsequently developed, which includes the following postulates:

• Energy flows inside the cells.
• Genetic information is passed from one cell to cell.
• All cells have the same chemical composition.

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Things to Remember

• A cell is the basic structural and fundamental unit of life.
• Robert Hooke discovered cells in 1665.
• Cells are responsible for providing proper structure and support to an organism.
• There are two types of cells – prokaryotic cells and eukaryotic cells.
• Cell wall, nucleus, cytoplasm and cell membrane are the major components in the structure of a cell.
• Cells provide support and structure, aid in reproduction, facilitate growth, produce energy and allow transportation of substances.

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