Parts Of The Cell

Parts Of The Cell

The basic components of a cell are cell membrane cytoplasm and the nucleus. The entire content of the living cell is called the protoplasm (proto-first; plasma – liquid). It includes the cytoplasm and the nucleus. Protoplasm is the living substance of the cell. To understand about the parts of a cell, let us examine both plant (onion peel) and animal (cheek) cells by performing some activities.

ACTIVITY I:

To study cell structure in plants using onion peel.

Take an onion bulb and prepare a peel from the fleshy white layers of the bulb. Place a piece of this peel in a drop of water on a glass slide. Put a drop of iodine or safranine stain on the peel and cover it with a coverslip. Observe the slide under the microscope.

 

ACTIVITY II:

To study cell structure in cheek (animal) cells.

Take a clean slide and put a drop of water on it. Now open your mouth and lightly scrape the inner part of your cheek with a clean cotton bud. Put the scrapping on the slide in a drop of water. Add a drop of iodine or methylene blue solution.

Put a cover slip and observe it under a microscope.

Human cheek cells

The above diagrams of plant and animal cell as obtained from the onion peel and cheek respectively clearly show the basic components of the cell.

CELL MEMBRANE (also called plasma membrane):

It is a very thin covering of the cell enclosing the cytoplasm and the nucleus. This membrane separates cells from one another and also the cell from the surrounding medium. The plasma membrane is porous and chemically made up of protein and phospholipids.

Functions of cell membrane:

The plasma membrane performs the following functions:

• It protects the cell.
• It provides shape to the cell.
• It regulates the movements of substances to enter and leave the cell. Hence plasma membrane is also called selectively permeable membrane.

CELL WALL:

In addition to the plasma membrane, plant cells have an additional layer outside the plasma membrane called the cell wall. Cell wall is a thick layer made up of cellulose. Cell wall is absent in animal cell.

Functions:

• Cell wall provides shape and support to the plant cell as plants do not have skeleton for support.
• Cell wall provides protection against variations in temperature, high wind speed, atmospheric moisture, etc. Plants are exposed to these variations because they cannot move.

CYTOPLASM (Kytos – hollow; plasma – liquid)

It is the jelly-like substance present between the cell membrane and the nucleus. The cytoplasm occupies most of the space inside the cell and contains many important tiny components or structures called organelles. These organelles perform various life functions.

ENDOPLASMIC RETICULUS

It is only found in eukaryotic cells. It is also known as Ergastoplasm. Ordinary microscope does not reveal the clear structure of it. Electron microscope gives its clear vision. In 1945 K.R. Porter revealed the structure of endoplasmic reticulum which is branched and irregular net work structure. It’s an extensive membrane - bound network of channels which acts as intracellular transport system ER.

It is continuous between the outer nuclear membrane upto plasma membrane.

Depending upon the presence or absence of ribosome on the surface of ER there are two types of E.R in the cells.

Smooth ER— It is devoid of ribosomes on its surface.

Rough ER— It is studded with ribosomes and called Rough ER (RER) or Granular ER (GER).

E.R is present in all the cell except prokaryotes and mammalian erythrocytes (RBC).

According to the shape, the endoplasmic reticulum is of the following types.

• Vesicles : They are round or oval tubes.
• Cisternae :They are long, flat and unbranched structure.
• Tubules :They are small branched tubes.

FUNCTIONS:

• It helps to distribute the soluble matters.
• Protein synthesis is done by the ribosomes.
• Synthesized proteins are stored in the tubules.
• They provide extra surface for the enzymatic reactions.
• They give rigidity to the protoplasm.
• The nuclear membrane of nucleus is formed by them in the new cells.
• Some enzymes present in endoplasmic reticulum help in the synthesis of fats, steroids and cholesterol.
• SER in liver cells takes part in detoxifying many poisons and drugs.

Structure of Endoplasmic Reticulum

GOLGI COMPLEX

Golgi complex was discovered and first described under the name - internal reticular apparatus by Camello Golgi.

Alternative names of Golgi complex are – Golgi bodies, Golgi apparatus, Golgisome, Lipochondria, Dalton complex, Idiosome, Baker’s body, etc.

The apparatus lies near the nucleus. It is found in all eukaryotic cells excepts mammalian RBCS.

Units of Golgi complex are called Dictyosomes.

They are composed of

• Cisternae : They are flattened, plate like tubules.
• Vesicles : It is spherical tubules.
• Vacuoles : They are the large spherical peripherally occurring vesicles.

Cisternae occur in small stacks. Tubules and vesicles occurs on their periphery.

They are composed of lipids and fats mainly and thats why they are known as Lipochondria.

FUNCTIONS:

Membranes of golgi apparatus may develop connections with membranes of ER to form complex cellular membrane system.

The main function of Golgi complex - Secretion (enzymes). Golgi complex is abundant in glandular cells. It is mainly concerned with the formation and packing of material for export from the cell across the plasma membrane by a process of reverse pinocytosis. They form the plasma membrane of the cell. They are responsible for the development and growth of the reproductive cells in mammals. Anterior part of sperms i.e., Acrosome is formed by them. They help in the formation of cell wall.

Golgi apparatus

LYSOSOME

They are 0.3 µ in a diameter thick, spherical or irregular shaped bodies surrounded by a single, thin membrane of lipoproteins which contain digestive enzymes for intracellular digestion and waste disposal.

This liquid mainly contains enzymes which help in the digestion of fat, starch and nucleic acids. As it has function in intra cellular digestions lysosomes are called digestive bags.

Lysosomes were discovered by Christinan de Duve (1955) in rat liver cells.

Lysosomes are called enzyme packets since they store several (50) types of hydrolysing enzymes.

Lysosomes are abundant in digestive glands.

The most important enzyme in lysosome is acid phosphatase.

FUNCTIONS:

Lysosomes carry on intracellular and extra cellular digestion. Digestion of food, foreign bodies, etc., by lysosomes is called heterophagy. Digestion of the own cell components by lysosomes is called autophagy /autolysis. Lysosomes are called suicidal bags or atom bombs of the cell.

Structure of Lysosome

MITOCHONDRIA

The mitochondria has been derived from two separate words (Gr. Mitos=tissue) and (chondros=granule) meaning granular cytoplasmic organelles.

It is known as the “Power houses of the cell” because it produces the energy (ATP) for the vital activities of the cell. Mitochondria are also considered as - bioplasts, cellular furnaces.

ATP is known as the ‘‘energy currency of the cell’’. The body uses energy stored in ATP for making new chemical compounds and for mechanical work. It is a sausage shaped cell organelles of aerobic eukaryotes which take part in aerobic respiration called krebs cycle.

• Mammalian RBCS are devoid of mitochondria.
• Mitochondria were first described by- Kolliker.
• The term ‘Mitochondrion’ was coined by Benda (1897).
• Altmann described them under the name Bioplasts.

Internal structure of mitochondria

SOME OTHER CELL ORGANELLES

PLASTIDS:

If you observe cells of Tradescantia leaf under the microscope, you may notice several small, coloured bodies in the cytoplasm. These coloured bodies scattered in the cytoplasm of leaf cells are called plastids. They are of different types.

• Chromoplast: Contain orange, red or yellow coloured pigments. They provide colours to flowers, fruits, leaves, etc.
• Chloroplast: Contain green pigment called chlorophyll. They provide green colour to leaves and other plant parts. Chlorophyll in the chloroplast of leaves is essential for photosynthesis.

 

Internal Structure of Chloroplast

• Leucoplasts: Contain no pigments. They are found in storage organs like potato, groundnut, etc.

VACUOLES:

While observing the onion cells under the microscope, you must have noticed blank-looking structures in the cytoplasm.These clear spaces surrounded by a membrane are called vacuoles. In case of a plant cell, vacuoles are larger and generally a single large vacuole is present. Vacuoles are generally absent in animal cells, and if present they are smaller in size.

Structure of Vacuole

NUCLEUS (PLURAL – NUCLEI; NUCLEUS - KERNEL)

The nucleus is an important component of the living cell. It is generally single, spherical and located in the centre of the cell surrounded by the cytoplasm. It can be stained and seen easily with the help of a microscope. The various components of the nucleus are.

• Nuclear membrane
• Nuclear sap or nucleoplasm
• Nucleolus (plural nucleoli)
• Chromosomes

Structure of a nucleus

• Nuclear membrane: It is the outermost covering layer of the nucleus. It separates the nucleus from the cytoplasm. This membrane, like the cell membrane is porous and allows the exchange of materials between the cytoplasm and the nuleoplasm.
• Nucleoplasm: The jelly-like substance inside the nucleus is called the nucleoplasm. Chromosomes and nucleolus is present in the nucleoplasm.
• Nucleolus: With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus use in  ribosome synthesis.
• Chromosomes: They are thread-like structures present in the nucleus. Chemically chromosomes are made up of protein and DNA (deoxyribonucleic acid). The chromosomes can be seen only when the cell divides.

Chromosomes carry genes which are unit of inheritance in living organisms. They control the transfer of a hereditary characteristic from parents to offspring.

A chromosome

FUNCTIONS OF NUCLEUS:

The nucleus is the master of the cell. It has the following functions.

• Transmission of characters from one generation to another.
• Control all the life functions taking place inside the cell.