The Cell, Biology tutorial

The Cell: The Unit of Living Organisms


The cell is the unit of living organism, whether it is plant or animal. They have the essential common structure though they could vary in formation and function by specialization.

The Cell Concept:

It was German physiologist Theordor Schwann who initial published generalization that every organisms are made up of fundamentally parts, known cells. Every organism is either made of one cell or many cells. According to him cell concept has come to concept of life, origin, nature and continuity.

Cell Size, Shape and Organization:

a) Cell Size:

The organism is composed of several cells. The cell is generally very small to be observed by naked eye. In body of the adult man, it is evaluated that there are approx 30,000,000,000,000 cells. This can be more closely symbolized as 30x1012. This is approximately equal to number of seconds in 950,000years.

b) Shape:

When the cell has nothing firm inside it interfering with the plastic nature, it supposes the spherical shape. It also presumes this figure in water. Where cells touch each other, walls of the group of cells turn out to be flat, where as their face surfaces are convex. As single cells, every cell flattens out on a hard surface. Into water where it is freely suspended, shape is spherical. Where there are the collection of cells, they are flat at sides they touch each other, while, on free surfaces they are in convex shape.

c) Organization:

There are 3 major parts in the cell, central nucleus, outermost membrane and region between cell and central nucleus called endoplasm.


Every cell has the nucleus though in some (red blood cell) it vanishes, whereas in others (bacteria) nuclear material is not surrounded within the nuclear membrane. Nucleus is essential organelle of cell. It guides production of proteins and other cell constituents. It is also liable for moving genetic materials. When it is separating, it doesn't have membrane and its contents are more dissimilar and intensely staining.

The Cell Membrane:  

This is layer of protoplasm existing outermost surface of cell. It manages to direct movement in and out of cell. This procedure can be known as controlled diffusion. It is by this procedure that all things needed by cell for all life metabolisms are obtained and all that should be passed out are excluded. When the immense number of substances tries to pass by cell membrane at same time, there is jam. Tiny molecules or ions with electric charge may suffer complexity charged. Several cells take in substances from outer surface by expanding cell membrane by pinocystosis.

The Endoplasmic Reticulum:

The cell membrane frequently has invaginations and evaginations which at times go very deep in cell. These connect to the interlocking network of double membranes and vesicles expanding through cytoplasm. This is endoplasmic reticulum. It is in continuity with nuclear membrane. Enormous structural surface of endoplasmic reticulum could be covered with many granules known as ribosomes liable for manufacture of proteins. Reticulum breaks down and reforms every time cell divides.

The Golgi Bodies are complicated membranous components of cytoplasm. It comprises of the double folding and branching system of membranes and vesicles roughly similar to endoplasmic reticulum except that it doesn't have ribosomes.

Cytoplasmic Matrix:

The rest of cytoplasm and uneven compartments of separated by reticular membrane is cytoplamic matrix. It is composed of mixture of salts, water, small organic compounds and macromolecules of several kinds. It can flow like the liquid and solidify as it occurs with elastic deformation. It tends to be solid (gel) on surface and liquid (sol) within. It has the variety of structures known as organelles:

Mitochondria: the complex enlarged double membrane structures related with respiration and energy release.

Lysosome: almost spherical bodies that have digestive enzymes. These ingest and digest all food substances.

Chloroplasts: Present in green plant cells. These are extremely organized structures liable for photosynthesis.

Cilia and Flagella: are other kinds of organelles which could be found in cells. They are related with locomotion

Difference between Plant and Animal Cells:

Every cell includes similar basic structures in common. But mainly plant cells vary from animal cells. The fundamental dissimilarities between animal and plant cells are given below.

 i) Animal cells encompass denser protoplasm and have several small vacuoles, whereas in plant cells vacuoles are larger and fewer.

ii) The animal cells include the thin restrictive membrane where as plant cells include thick cellulose cell-wall.

iii) Animal cell never include chloroplasts but plant cells have.

Kinds of Cells:

There are several specialized cells. Cells turn out to be specialized to do specific functions. Usually cells are not mainly specialized. Though, few are specialized in some different ways. Protoplasm for instance is usually sensitive in different ways. This sensitivity is conveyed to other parts of cell as the result they contract or enlarge. Such essential traits of transmission, sensitivity and reactiveness are necessary traits of living substances, whether are of plants or animals. Though, presence of green pigments and big vacuoles in plant cells are the type of specialization of plant cells for particular functions. Green pigment chlorophyll is for photosynthesis whereas large vacuoles have aqueous solution of mineral salts that control turgidity of bordering cytoplasm.

Many plant cells have the firm outer cell wall of polysaccharides and lignin. These give firm mechanical support to plant plus prevent any action within cytoplasm of liquefied cells. Few animal cells contain their outer membranes nearly bare. Cell surfaces are extremely active and always have structures, and with which they get stimuli. They as well frequently have very contractile protein with which they reply to apparent stimuli.

a) Nerve Cells:

Nerve cells are elongated cells which transmit a stimulus from one place to another. It is therefore an example of a specialized cell. Electrochemical disturbances go from point to point within cell cortex and travel willingly from one nerve cell to another.

b) Muscle Cells:

These are group of specialized cells. They are also lengthened, but in their own case they have such protein materials which contract are relax.

c) Secretory Cell:

Few cells like those in pancreas contain well enhanced endoplasmic reticulum prepared with zymogene granules. Even among secretory cells, there are different types. While some are concerned with secreting digestive enzymes, others secrete supporting substances and some even nectar or toxic substances.

Cell specialization is the tremendous growth of the specific cell to perform the function. This is generally along with suppression of other functions. Cells as living units of organisms usually perform all living functions to the extent to maintain cell as living. When cell is dedicated to do one of life functions to the far greater extent, other functions are comparatively suppressed. For instance cytoplasm of cells has secretory properties. Though cytoplasm of secretory cells have complicated secretory skills. Likewise, all cells usually have respiratory pigment cytochrome. From this dedicated respiratory pigment, hemoglobin may have developed in red blood cells. Conduction of impulses is the universal property of cell surfaces. Conduction of nerve impulses along nerve fibers is the expansion of the characteristic. Light sensitivity is the features of protoplasm generally, but light sensitivity of cells of eye is the great growth of feature.

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