Fundamental unit of life-Cell


Objects having characteristics of cellular organization, growth, reproduction, ability to sense environment and give response are living organisms.

There are some important features of living organisms:

  • It should grow, which means its structure changes as time goes by in an advantageous manner.
  • It should show adaptation to the environment.
  • It should maintain some balanced conditions in its inner structure. This is called Homeostasis.
  • Its structure is highly organized.
  • It should be able to break down or build up nutrients to release or store energy based on need. This is called Metabolism.
  • It should be able to reproduce itself.

Classification of living world

The practice of classifying organisms is called taxonomy. Linneaeus developed a hierarchy of groups for taxonomy. To distinguish different levels of similarity, each classifying group, called taxon (pl. taxa) is subdivided into other groups. To remember the order, it is helpful to use a mnemonic device. The taxa in hierarchical order:

Domain – Archea, Eubacteria, Eukaryote

Archea (Archeabacteria) consists of archeabacteria, bacteria which live in extreme environments. The kingdom Archaea belongs to this domain.

Eubacteria consists of more typical bacteria found in everyday life. The kingdom Eubacteria belongs to this domain.

Eukaryote encompasses most of the world’s visible living things. The kingdoms Protists, Fungi, Plantae, and Animalia fall under this category.

Kingdom – Plantae, Animalia, Fungi, Protists, Eubacteria (Monera), Archaebacteria






Species : smallest classification


Cell structure and its functions

Cell is the smallest structural and functional unit of an organism, which is typically microscopic and consists of cytoplasm and a nucleus enclosed in a membrane.

Structure of cell

The cell is a basic unit for life forms. As well as enabling sophisticated control of biochemical processes by providing compartments and regulating chemical fluxes between them, cells also have structural integrity and can exert forces. In the case of multicellular organisms (animals and plants), each cell contributes some mechanical property to the tissue it forms together with other cells. Furthermore, many cells are eliminated during the life of a complex organism (e.g. skin layers in animals), which entails cell division and restructuring of the organisation with neighbours. Some types of cell are actually very motile, moving through tissues (e.g. various immune system cells and some cancer cells). This dynamic aspect is even more obvious during the development of multicellular organisms, when many stages of cell division and migration take place.


A first division of organisms is between those whose cells have within them a nucleus, the structure containing most of the genetic material in the form of DNA, and those whose cells don’t. The nucleated cells are called eukaryotic and are found in animals, plants, fungi, protozoa and algae. In contrast, bacteria (and the less common archaea) do not have a nucleus and their DNA is spread throughout the cell. These cells are called prokaryotic. Eukaryotic organisms can be unicellular or multicellular while all prokaryotes are unicellular.

Some of the important cell oragenells are as follows:

The Nucleus

A cell nucleus is the part of the cell which contains the genetic code, the DNA. The nucleus is small and round, and it works as the cell’s control center. It contains chromosomes which house the DNA. The human body contains billions of cells, most of which have a nucleus. All eukaryote organisms have nuclei in their cells, even the many eukaryotes that are single-celled. Bacteria and Archaea, which are prokaryotes, are single-celled organisms of quite a different type and do not have nuclei. Cell nuclei were first found by Antonie van Leeuwenhoek in the 17th century.

The nucleus has a membrane around it but the things inside it do not. Inside it are many proteins, RNA molecules, chromosomes and the nucleolus. In the nucleolus ribosomes are put together. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA into proteins. When a cell is dividing or preparing to divide, the chromosomes become visible with a light microscope. At other times when the chromosomes are not visible, the nucleolus will be visible.

Endoplasmic reticulum

The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs that begin at the nucleus and ramble through the cytoplasm. There are two types of endoplasmic reticulum distinguished by the presence or absence of ribosomes.

Rough ER consists of stacked, flattened sacs with many ribosomes attached; oligosaccharide groups are attached to polypeptides as they pass through on their way to other organelles or to secretory vesicles.

Smooth ER has no ribosomes; it is the area from which vesicles carrying proteins and lipids are budded; it also inactivates harmful chemicals.

Golgi bodies

The Golgi apparatus or Golgi complex is found in most cells. It is another packaging organelle like the endoplasmic reticulum (ER). It was named after Camillo Golgi, an Italian biologist. It is pronounced GOL-JI in the same way you would say squee-gie, as soft a “G” sound. While layers of membranes may look like the rough ER, they have a very different function.

The Golgi apparatus gathers simple molecules and combines them to make molecules that are more complex. It then takes those big molecules, packages them in vesicles, and either stores them for later use or sends them out of the cell. It is also the organelle that builds lysosomes (cell digestion machines). Golgi complexes in the plant may also create complex sugars and send them off in secretory vesicles. The vesicles are created in the same way the ER does it. The vesicles are pinched off the membranes and float through the cell.


Mitochondria are rod-shaped organelles that can be considered the power generators of the cell, converting oxygen and nutrients into adenosine triphosphate (ATP). ATP is the chemical energy “currency” of the cell that powers the cell’s metabolic activities. This process is called aerobic respiration and is the reason animals breathe oxygen. Without mitochondria (singular, mitochondrion), higher animals would likely not exist because their cells would only be able to obtain energy from anaerobic respiration (in the absence of oxygen), a process much less efficient than aerobic respiration. In fact, mitochondria enable cells to produce 15 times more ATP than they could otherwise, and complex animals, like humans, need large amounts of energy in order to survive.

The number of mitochondria present in a cell depends upon the metabolic requirements of that cell, and may range from a single large mitochondrion to thousands of the organelles. Mitochondria, which are found in nearly all eukaryotes, including plants, animals, fungi, and protists, are large enough to be observed with a light microscope and were first discovered in the 1800s. The name of the organelles was coined to reflect the way they looked to the first scientists to observe them, stemming from the Greek words for “thread” and “granule.” For many years after their discovery, mitochondria were commonly believed to transmit hereditary information. It was not until the mid-1950s when a method for isolating the organelles intact was developed that the modern understanding of mitochondrial function was worked out.

Vesicles: There are two types of vesicles: lysosomes and Peroxisomes

Lysosomes: Lysosomes are membrane-enclosed organelles that contain an array of enzymes capable of breaking down all types of biological polymers—proteins, nucleic acids, carbohydrates, and lipids. Lysosomes function as the digestive system of the cell, serving both to degrade material taken up from outside the cell and to digest obsolete components of the cell itself. In their simplest form, lysosomes are visualized as dense spherical vacuoles, but they can display considerable variation in size and shape as a result of differences in the materials that have been taken up for digestion . Lysosomes thus represent morphologically diverse organelles defined by the common function of degrading intracellular material.


Peroxisomes are small, membrane-enclosed organelles that contain enzymes involved in a variety of metabolic reactions, including several aspects of energy metabolism. Although peroxisomes are morphologically similar to lysosomes, they are assembled, like mitochondria and chloroplasts, from proteins that are synthesized on free ribosomes and then imported into peroxisomes as completed polypeptide chains. Although peroxisomes do not contain their own genomes, they are similar to mitochondria and chloroplasts in that they replicate by division.

Some specialized Plant Organelles

Plastids: There are three types of plastids: Chloroplasts, Chromoplasts, Amyloplasts

Chloroplasts are oval or disk shaped, bounded by a double membrane, and critical to the process of photosynthesis.Chromoplasts have carotenoids, which impart red-to-yellow colors to plant parts, but no chlorophyll. Amyloplasts have no pigments; they store starch grains in plant parts such as potato tubers.

Central Vacuole

Vacuoles are essentially membrane-bound sacs found in the cytoplasm. In animal cells, vacuoles are relatively small, and are used as temporary storage areas for materials and for transport purposes. In plant cells, however, there is generally a large single central vacuole. Comprising approximately 90% of an mature plant cell, the central vacuole provides structure and support to the cell by maintaining turgor pressure, which is essentially fluid pressure that keeps the cells rigid. They are necessary to cell functions in many different ways such as maintaining cell structure and storing nutrients, waste products, and many other substances. Below is a picture of a central vacuole in a cell.


Functions of cell

Structure and support: Like a classroom is made of bricks, every organism is made of cells. While some cells such as the collenchyma and sclerenchyma are specifically meant for structural support, all cells generally provide the structural basis of all organisms. For instance, skin is made up of a number of skin cells. Vascular plants have evolved a special tissue called xylem, which is made of cells that provide structural support.

Growth: In complex organisms, tissues grow by simple multiplication of cells. This takes place through the process of mitosis in which the parent cell breaks down to form two daughter cells identical to it. Mitosis is also the process through which simpler organisms reproduce and give rise to new organisms.

Transport: Cells import nutrients to use in the various chemical processes that go on inside them. These processes produce waste a cell needs to get rid of. Small molecules such as oxygen, carbon dioxide and ethanol get across the cell membrane through the process of simple diffusion, which is regulated with a concentration gradient across the cell membrane. This is known as passive transport. However, larger molecules, such as proteins and polysaccharides, go in and out of a cell through the process of active transport in which the cell uses vesicles to excrete or absorb larger molecules.

Energy production: An organism’s survival depends upon the thousands of chemical reactions that cells carry out relentlessly. For these reactions, cells require energy. Most plants get this energy through the process of photosynthesis whereas respiration is the mechanism that provides energy to animal cells.

Metabolism: Metabolism includes all the chemical reactions that take place inside an organism to keep it alive. These reactions can be catabolic or anabolic. The process of energy production by breaking down molecules (glucose) is known as catabolism. Anabolic reactions, on the other hand, use energy to make bigger substances from simpler ones.

Reproduction: Reproduction is vital for the survival of a species. A cell helps in reproduction through the processes of mitosis (in more evolved organisms) and meiosis. In mitosis cells simply divide to form new cells. This is termed as asexual reproduction.  Meiosis takes place in gametes or reproductive cells in which there is a mixing of genetic information. This causes daughter cells to be genetically different from the parent cells. Meiosis is a part of sexual reproduction.

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