Basic biology

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Basic biology

Post by rajathadri on Wed Jul 07, 2010 6:49 am

Cell Organelles: Their Structure and Functions

We know that the cell is the basic structural and functional unit of life. But what is present inside a cell? How does it perform its various functions?

A cell contains various cell organelles which help perform its functions. Let us explore what all cell organelles are present inside the cell

Some organelles are visible under the light microscope while some are visible only under the electron microscope.

Now let us study about the organelles that are visible under the light microscope

Cell Membrane

Take a peel of onion by separating it from the fleshy portion. Add a drop of methylene blue on a slide containing the peel, put cover slips, and observe it under a microscope. What do you observe? Note your observations and draw a diagram of the structure you have observed.

You will observe brick-like cells placed one over the other with a round ball like nucleus in the centre. The boundary of the onion cells is known as the cell membrane, and it is covered by another thick layer called the cell wall.

Hence, thecell membrane is the outer boundary of the cell, which separates one cell from the other. It also separates the contents of the cell from its surrounding medium.

The cell membrane is porous in nature and helps in the inward and outward movement of substances.

Cell Wall

The cell wall is an additional protective, rigid structure present outside the cell membrane. It is present only in plant cells. It protects them from heat, humidity, pressure etc.


The jelly-like substance present between the cell membrane and the nucleus is called the cytoplasm. It is an important component of the cell as various cell organelles like mitochondria; ribosomes etc. are present in it.


The nucleus is a spherical structure which is generally present in the centre of the cell. Let us study the components of nucleus.

The cell membrane, the cytoplasm, and the nucleus form the basic components of the cell.

Do You Know?

Do you know that the red blood cells of the human body do not have a nucleus?

Paramecium is a unicellular organism having two nuclei.

Some muscle cells in humans have a large number of nuclei.


When you observe an onion peel under the microscope, you will observe a large empty space in the cells. Do you know what these spaces are? These empty spaces are called vacuoles. These vacuoles are larger in plant cells than in animal cells.

The organelles visible under an electron microscope are

Endoplasmic Reticulum

Endoplasmic reticulum, or ER, is an interconnected network of membranous structureslike tubules, vesicles, and cisternae. Cisternae are the flattened disk-like membranous structures. Tubules are tubular in shape, while vesicles are sac-like structures. The function of the ER is intra-cellular transport

Golgi Apparatus

They are the membrane-bound, sac-like structures called cisternae. They are arranged parallel to each other in stacks. They were discovered by Camillo Golgi in 1898. Golgi body is usually composed of five-eight cisternae in stacks. Vesicles leave the Golgi body from one end known as the cis face. The other end is known as the trans face. The table given below lists some functions of the Golgi apparatus.


Mitochondrionis a membrane-enclosed organelle found in eukaryotic cells.

Mitochondria are responsible for the production of most of the energy (or ATP) in cells. Therefore, mitochondria are also known as the power house of cells. A mitochondrion is composed of two lipid membranes, enclosing the matrix. The inner membrane gets folded to form numerous cristae. Cristae are the main site for ATP production. Mitochondrial matrix contains mitochondrial DNA and ribosomes.


They are the membrane-bound vesicles, which contain digestive (hydrolytic) enzymes like acid hydrolase. They digest a variety of substances including worn out organelles, food particles, viruses, and bacteria. They are also known as ‘suicide-bags’ of cells as they burst out and release hydrolytic enzymes in the cytosol, where cells are heavily injured.


Plastids are major organelles found in plant cells and algae

Chloroplasts are plastids containing the pigment called chlorophyll. A chloroplast is enclosed by two lipid membranes.


They are found either freely floating in the cytoplasm or attached to the outer surface of the endoplasmic reticulum. They help in protein synthesis.

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