Maharashtra Board Class 11 Biology Chapter 5 Cell Structure And Organization PDF Download

5. Cell Structure And Organization

Can you recall?

1. Who observed cells under the microscope for the first time?

2. Who made the first microscope?

3. How do onion peel cells and our body cells differ?

4. Why bacterial nucleus is said to be primitive?

Find out

1. How do a combination of lenses helps in higher magnification?

2. When do we use plane and concave mirror and diaphragm?

3. What is the difference between magnification and resolution?

5.1 Cell

Cell is called a structural and functional unit of life of all living organisms capable of independent existence and can perform all functions of life.

To see cells clearly we need a microscope. Larger cells can be seen through simple microscope but to see smaller cells we require compound microscope. Simple microscope can magnify image 50 to 100 times but a compound microscope can do so 1000 times or more. In the microscope we use in the laboratory, a beam of light is used to make things visible hence it is light microscope. To see interior of cell we need electron microscope. It can magnify image 500000 times.

There is no typical shape of a cell. Cells may be spherical, rectangular, flattened, polygonal, oval, triangular, conical, columnar, etc.

Cell size varies greatly in various plants and animals. Some of them are not visible to naked eye. Some are barely visible while some are macroscopic. The smallest cell size can be seen in mycoplasma (0.3 µm in length), bacterial cell size is 3 to 5 µm, while the largest size of cell is seen in Ostrich egg (nearly 15cms). Longest cells are nerve cells.

You already know that cell theory was proposed by Schwann and Schleiden. However, in this theory, there was no explanation about formation of new cells. It was Rudolf Virchow (1855) who explained for the first time that new cells are formed by cell division from pre existing cells (Omnis cellula-e-cellulla).

In later years, advanced research in cytology led to modification in cell theory, which is now known as Modern Cell Theory.

Teacher's Note

Cells are the basic building blocks of all living things. Just like bricks make a house, cells make our body. You can see plant cells in an onion or your cheek cells under a microscope.

Exam Trick

Remember: Prokaryotes = bacteria with no nucleus. Eukaryotes = plants and animals with nucleus. Think "pro" means "simple" and "eu" means "true nucleus".

Points to Remember

Cell is the smallest unit of life.
Simple microscope magnifies 50 to 100 times.
Compound microscope magnifies 1000 times or more.
Cell sizes range from 0.3 µm to 15 cms.
New cells form from existing cells.

5.2 Kinds of Cells

Living organisms are grouped into two main categories the Prokaryotes and Eukaryotes. The prokaryotes have simple cellular organization while eukaryotes exhibit high degree of organization.

A. Prokaryotic Cells

The cell in prokaryotes show following main features. It has chemically complex protective cell envelop. However, it does not have well-defined nucleus and other membrane bound cell organelles.

Cell envelop is a three-layered structure with outer glycocalyx, middle cell wall and inner plasma membrane. Glycocalyx is present as either slime layer (loose sheath) or capsule (tough). Bacteria are better observed when stained.

The most followed staining method is 'Gram staining' developed by Danish bacteriologist Hans Christian Gram. The cell wall is made up of peptidoglycan (in Gram positive bacteria) and murein (in Gram negative bacteria). It gives mechanical strength to the cell. Cell membrane is a phospholipid bilayer. All these structures give protection to the cell and also help in inter-cellular transport. In motile bacteria either cilia or flagella are found. Both are driven by rotatory movement produced by basal body (which works as motor). Other parts are filament and hook.

Some other surface projections are the tubular pili (which help in inter-cellular communication) and fimbriae (for clinging to support).

The cell membrane shows infoldings called mesosomes, which help in cell wall formation and DNA replication. Some bacteria especially photosynthetic cyanobacteria show more longer extensions called chromatophores. They carry photosynthetic pigments. The cytoplasm contains dense particles called ribosomes helping in protein synthesis. Ribosomes are described by their sedimentation rate in Svedberg units. Bacterial ribosome are 70S (composed of a larger subunit 50S + smaller subunit 30S).

Teacher's Note

Bacteria are very small organisms. They do not have a nucleus like we do. Their DNA is floating free inside like a cloud.

Exam Trick

Remember: Prokaryotic = "pro" means before, "karyo" means nucleus. So no nucleus! Bacteria and cyanobacteria have this type of cell.

Points to Remember

Prokaryotic cells have no true nucleus.
Cell wall is made of peptidoglycan or murein.
Ribosomes in bacteria are 70S type.
Flagella helps bacteria move around.
Gram staining helps identify bacteria.

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