class 8th Notes for Cells

  class 8th science

ICSE/ CBSE Notes-

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cells

 1- A cells is considered to be the fundamental unit of life

 2- A cell is defined as the basic smallest structural and functional unit of an organism.

3- They are complex units that are microscopic in nature.

4- It can carry out the life process like reproduction, nutrition respiration

5- A cell is capable of independent existence 

6-A cell forms a basic structural  and fundamental unit of life.

cells

cells structure

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Discovery of the Cell

Robert Hooke in 1665 observed slices of cork under a simple magnifying device. Cork is a part of the bark of a tree.

• The term ‘cell’ was first invention by Robert Hooke in the year 1665 when he noticed slice of cork by a microscope

Shape and size of Cells

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1- According to the functions performed by the cells they are different shapes and sizes

2-The neurons or nerve cells are long so that they can carry messages from one part of the body to another.

3- The largest cell in the human body is the -OVUM

fig Ovum

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4- The longest cell in the human body is Neurons or Nerve Cells

 fig-Nerve cell

5- The smallest cell of the human body is Sperms

fig- Sperms

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According to number of cell organisms are -

1-unicellular organisms

An organism can be made up of a single cell unit and they are called unicellular organisms like an amoeba, paramecium, etc. 

fig- amoeba

fig

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2-multicellular organism

An organism made up of many cells is called a multicellular organism like human beings, plants etc

multicellular

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Types of Cells

There are two types are cells are given below

1-Prokaryotic cells

2-Eukaryotic cells

Prokaryotic – Size: generally small ( 1-10 µm) 

• The nucleus is not well defined and known as a  Nucleoid 
• Single, circular chromosome.
• Membrane-bound organelles absent. Eg: Bacterial cell

Eukaryotic – Size: generally large ( 5-100 µm).

• The nucleus is well defined and surrounded by a nuclear membrane.
• Possess more than one and linear chromosome.
• Membrane-bound cell organelles are present. Eg: Human cell.

              Where 1 µm = 10 $-6$m.

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Cell Structure and Function

1-Each living organism has many organs

2-Each organ in the system performs different functions such as digestion, assimilation and  absorption.

3-Similarly, different organs of a plant perform specific functions. For example, roots help in the

absorption of water and minerals.

4-Each organ is further made up of smaller parts called tissues.

5-A tissue is  a group of similar cells performing a  specific function.

6-The cell in a living organism  is the basic structural unit.

 Parts of the Cell 

  The basic components of a cell are 

1-Cell  membrane

2-Nucleus

3-Cytoplasm 

1-Cell  membrane

1-  The membrane separates cells from one another and also the cell from the surrounding  medium.

2- The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane

3-The plasma  membrane is porous and allows the movement of substances or materials

both inward and outward.

4-The boundary of the onion cell is the cell membrane covered by another thick  covering 

called the cell wall.

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2-Nucleus

• Nucleus  is the most important part of cell.
• It control and also gives the direction of cellular activities.
• It control every cells of the body so we can say that nucleus is control room of the cells
• Shape of the nucleus is spherical
• Nucleus is established at the center of cell ,which is surrounded by cytoplasm

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Properties of  Nucleus

• Nucleus is apart from the cytoplasm by a membrane called the Nuclear membrane
• Nuclear membrane is  POROUS  in nature , it  allow entry and exit of some selected materials
•  Nucleus is filled with transparent granular substance named  NUCLEOPLASM

Chromosomes 

• Chromosomes are present in nucleus.
• Its structure is like a thread. 
• It carry  GENES.
• Genes help to transfer of character from one generation to another generation
• The whole system which contain Cytoplasm and nucleus is called PROTOPLASM

 
3-Cytoplasm 

• Cytoplasm is the space between the cell membrane and nucleus.
• This space is filled by translucent jelly like fluid
• Cytoplasm includes all  cell organelles ,in which Nucleus not include.

These organelles are given bellow-

1. Mitochondria
2. Plastids
3. Endoplasmic reticulum 
4. Golgi complex
5. Lysosomes
6. Ribosomes
7. Vacuole
8. Centriole
9. Cilia and flagella
10. Micro Tubular

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1- Mitochondria

• Mitochondria  is the Powerhouse of the cell.
• Its shape  are cylindrical or spherical.
• Mitochondria enclosed by a double Membrane.
• It provide energy to  the cells by the process of cellular respiration.
• This energy is stored in the form of  ATP

 where  ATP means-  adenosine triphosphate (ATP), energy-carrying molecule found in the cells of all living things.

2-Plastids

• Plastids is large organelles found scattered in plant cells.
• Plastids are mainly three types
• chromoplast- It found in flower ,it is coloured  plastids
• chloroplast- it contain green colour  pigment called chlorophyll,which is involved in photosynthesis.
• Leucoplast- it is colourless plastids , it store fat ,proteins,carbohydrate

3-Endoplasmic reticulum

It is the the Network of inter connecting membrane filled with fluid.it  is found in to types

1-Smooth Endoplasmic reticulum- It have NO ribosome and synthesize lipids

2-Rough Endoplasmic reticulum - It have ribosome and synthesize protein

Golgi complex

• It discovered by C.Golgi in 1898.
• The function of Golgi body is secretion.

Lysosomes

• It make an interacellular design 
• It is filled with digestive enzymes
• It contain suicidal bags

Ribosome

• It consist RNA (ribonucleic acd) and protien
• It help in protein synthesis

Vacuole

• Mostly it found in plant cell.
• It is single membrane bound organelle filled with fluid.
• It help in osmoregulation  to maintain the osmotic pressure of food

Centriole

• They are two cylindrical rod shaped structure.
• It is close to nucleus.
• it help in the formation of spindles during the cell division

Cilia and Flagella

• It is microscopic  hair like  structure.
• it is projecting on the cell surface
• it help in moment in unicellular organisms.

Micro Tubular

• it maintain the cell shape.
• it isin the form of structure

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click for  Divisibility rules of  number 

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