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TNPSC Science Notes - 6th Standard - Cell Structure

Cell Structure 
6th Std Science Series - 1

The human body is made up of several small units called cells. The Cell is the basic structural and functional unit of all living organisms. Not only human beings, but other organisms like plants and animals are also composed of cells.

Cells can be observed only through a scientific instrument called Microscope. The cell is like a small factory with nearly twelve to thirteen cell organelles, which are involved in various specified tasks. 

Robert Hooke Named the Cell


Robert Hooke named these chambers 'cells' in 1665. He became a famous scientist by showing the cell magic through his lens. In Latin, the word 'cellula' means "a small chamber". 

Classification of Cells


The cells of plants and animals are not similar. Bacteria and some algae are made up of a single cell. They lack membrane-bound organelles.

1. Prokaryotic cell - a Simple Cell

A cell that does not contain membrane-bound organelles and a well defined nucleus is called a Prokaryotic cell i.e. a simple cell. e.g. Bacteria.

2. Eukaryotic cell - a Complete Cell

A cell that contains a well defined nucleus,a nuclear membrane and membrane-bound cell organelles is called a Eukaryotic cell, i.e. a complete cell. e.g. cells of plants an animals.

An Animal Cell

The plant and animal cells are not alike. They vary in their size and structure according to their functions, but both the cells are similar in their basic organisation.

Plasma Membrane 

The plasma membrane enveloping the cell. It give shape to the cell. I act as a guard. It control the entry and exit of materials.

Protoplasm

A colloid, found inside the plasma membrane. It have two components of the cell namely the cytoplasm and the nucleus. J.E. Purkinjee coined the term protoplasm. 'Proto' means 'first' and 'plasma' means 'colloid'.

Cytoplasm 

Cytoplasm, it located in between the plasma membrane and the nucleus. It made up of carbohydrates and proteins. Organelles and lipid droplets are present.

Nucleus 

Nucleus is the control centre of the cell. spherical in shape. It have the nucleoplasm, the nucleolus and the chromatin reticulum. enclosed by the nuclear membrane. It carry "the genetic characters from generation to generation".

Mitochondria - (singular - Mitochondrion) - the Powerhouses of the cell

This involved in cell respiration, and help in the oxidation of food materials that you eat and provide energy. It do not rest.It also known as the Powerhouses of the cell.

Golgi Bodies

Tubular structures, involved in the secretion of digestive enzymes and the formation of lysosomes.It separate proteins from the ingested food and give strength to the cells and the body. In plant cells, it known as Dictyosomes.

Endoplasmic Reticulum 

It helps in transportation of materials from one part of the cell to another.

Ribosomes - Protein factories of the cell:

Granular structures. It called the Protein factories of the cell.It helps in protein synthesis.

Lysosomes - the Suicidal bags of the cell:

Spherical yellow coloured bodies. It helps in cell protection. It destroy the pathogens that enter the cell. Lysosomes called the Suicidal bags of the cell. In addition to It helps in cell digestion.

Centrosome 

It can be seen only in the animal cell. It look like a stick and It is a microtubule. It found near the nucleus. It have centrioles. Centrosome plays an important role in cell division i.e., formation of new cells.

Vacuoles 

Light blue in colour and appear like bubbles. It stores cell sap. Vacuoles maintain intracellular pressure. 

The Plant cell

The centrosome is absent in the plant cell. Plants are more rigid than animals due to the presence of the cell wall. They have plastids. They have larger vacuoles.

Cell wall

It is an outer layer, which gives shape to the cell. It is made up of cellulose. Its function is to protect the inner organelles and to give shape to the cell. 

Plastids 

These organelles are found only in plant cells. They contain pigments. Based on the pigments, they are classified into three types.

The Three Types of Plastids:

The differences between a plant cell and an animal cell:

All activities like eating, drinking, jumping, playing, breathing, thinking and even sleeping are due to the functioning of the cells. Each cell is a small factory. The brain has several million cells. When the cells, the so called small factories get affected and injured, diseases are caused and we visit a doctor. e.g. cancer, hereditary diseases, diabetes, etc.

The Exam. Key Points to Remember

1. There are about 6,50,00,000 cells in the human body.

2. Bones are made up of a special type of dry cells.

3. Anton Van Leeuwenhoek (1675) discovered that blood contains RBC (Red Blood Cells).

4. In animals, the nerve cell is the longest cell.

5. In animals, the bone cell is the toughest cell.

6. Mature Red Blood Cells of mammals do not contain a nucleus.

Cell organelles and their functions

1. Plasma membrane
  • It gives shape to the cell.
  • It selects the substances required by the cell and transports them in and out.
  • It controls the movements of substances in and out of the cell.
  • It protects the cell.
2. Cytoplasm
  • It distributes the nutrients within the cell.
3. Nucleus
  • It controls all the activities of the cell.
  • It carries the hereditary characters from one generation to another.
4. Mitochondria
  • They provide energy to the cell.
5. Golgi bodies
  • They secrete enzymes and hormones.
  • They store protein.
  • They help in the formation of Lysosome.
6. Endoplasmic reticulum
  • It helps in transportation within the cell.
  • It helps in protein synthesis.
7. Ribosomes
  • They synthesize protein.
8. Lysosome 
  • It destroys the germs that enter the cell.
  • It helps in intracellular and extracellular digestion.
9. Centrosome
  • It helps in cell division
10. Vacuoles
  • They control intracellular pressure.
  • They store cell sap.
11. Plastids
  • They help in photosynthesis.
  • They give colour to flowers and fruits
12. Cell wall
  • It gives shape and protection to the plant cell.
Important Questions with Answers in Cell Structure Lesson:

1. The structural and functional unit of the living organism is ________.

a) nucleus 
b) cell 
c) mitochondria 
d) ribosome

Answer: b) cell 

2. The instrument used to magnify things placed on a slide is______.

a) telescope 
b) microscope 
c) binocular 
d) periscope

Answer: b) microscope

3. Select the prokaryotic cell from the given cells.









Answer: (b)

4. The ________ is the powerhouse of the cell.

a) mitochondria 
b) ribosome 
c) lysosome 
d) nucleus

Answer: a) mitochondria 

5. The organelle which is known as ‘suicidal bag’ is the ________.

a) dictyosome 
b) ribosome 
c) centrosome 
d) lysosome

Answer: d) lysosome

6. The spherical organelle which controls the activities of the cell is ________.

a) golgi bodies 
b) ribosome 
c) nucleus 
d) lysosome

Answer: c) nucleus 

7. The organelle that involves in destroying the germs which enters the cell is ______.

a) dictyosome 
b) ribosome 
c) centrosome 
d) lysosome

Answer: d) lysosome 

8. The organelle which is found only in the animal cell is _______.

a) mitochondria 
b) centrosome 
c) plasma membrane 
d) chloroplast

Answer: b) centrosome

9. The cell of a /an _______contains a large vacuole.

a) onion peel 
b) bacteria 
c) nerve 
d) muscle

Answer:a) onion peel 

10. The longest cell is the ________.

a) bone cell 
b) nerve cell 
c) cell of a muscle 
d) blood cell 

Answer: b) nerve cell 

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