Biology For class IX Cell Theory

Biology

Lets start

What are Prokaryotes?

• These types of organisms have primitive and incomplete cells.They do not have cell membrane around their genetic substance(DNA).
• It contains single chromosome
• Usually the cell size is very small.
• Membrane bound cell organelles are absent eg mitochondria.
• Cell divisions by fission  i.e. no mitosis.
• eg archaebacteria,bacteria and cynobacteria.

What are Eukaryotes?

These are  advance and complete cells.They have nuclear membrane around their genetic material.

• They are large cells.
• They have well defined nuclear membrane around their genetic material(DNA)
• Membrane bound cell organelles are present.
• Cell division by mitosis.

What is a cell?

'Cell' is a Latin word meaning ' a little room'.

Who was the first person to discover a cell?

Robert Hooke  first observed the  bark of a cork under Primitive Microscope.He observed the Honeycombed like structure  and he named them Cells.This was around 1665.

Who observed the living cell for the first time?

Leeuwenhoek(1674) with an advance Microscope observed the pond water under Microscope and discovered free living organisms for the first time.

What is a Cell Theory?

Cell Theory states that all the plants and animals are made up of cells and cells are the basic units of life. Two biologist Schleiden and Schwann gave the cell theory. further Virchow  expanded this by saying that a cell comes from preexisting cells only.

Living organisms can be divided into  Unicellular   Organisms and Multicellular organisms.
Ex. of unicellular organisms are Amoeba,Chlamydomonas,Paramecium and Bacteria.
Ex of Multicellular organisma  are Fungi,Plants and animals.

In Multicellualr Organisms we see Division of Labour that means different parts of the  body perform different functions.like the function of the  heart is to pump blood, the function the stomach is to digest food.

Even in unicellular organism we find cell organelles. each cell organelle perform specific function like making new material,cleaning up the waste. A cell is able to perform all its function just because of these cell organelles.

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Structural organisation of the cell

Now the question arises how such a small unit organises itself to perform all the function that a large multicellular organism can perform.  when a cell is observed under a microscope  we can see  three   features common in all cells.

• Plasma Membrane
• Nucleus
• Cytoplasm

Lets see these features one by one.

                Plasma Membrane    or Cell Membrane

It controls the  movements of material in and out of the cell.. It  is also called  selectively permeable membrane. selectively because it allows some material to enter or exit and disallow others to enter or exit the cell.

The movement of o2 and co2  in a cell takes place by  the process of diffusion.Diffusion is the process in which a material flows from  a region of high concentration to a  region where its  concentration is low. thus diffusion plays a vital role in gaseous exchange in a cell.

water also obeys the law of diffusion.When movement of water takes place through selectively permeable  membrane , it is called Osmosis. Absoption of water by roots is an example of Osmosis.


Now there are three type of solutions

• Hypo tonic Solutions

• If the medium surrounding the cell has more water concentration than inside the cell, then it is called Hypotonic solutions and  more water will enter the cell as a result a cell is likely to swell up.

• Isotonic  Solutions

• If the medium surrounding the cell has same concentration as inside the cell in that case solution is called Isotonic Solution and as a result   the amount of water that enters the cell and the amount of water that leaves the cell remains same, so the size of the cell does not change.

• Hypertonic Solutions

• If the medium surrounding the cell has a lower concentration of water it is called Hypertonic solution, the cell will lose water and as a result will shrink.

• The Plasma Membrane is flexible and is made up of organic molecules called Lipids and Proteins.
• The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment.This process is called Endocytosis.eg Amoeba takes its food by this process only.
• Plasma membrane is selectively Permeable membrane

                                                                 CELL WALL

Plant cell in addition to cell membrane have another outer rigid covering called Cell Wall.This cell wall is made up of Cellulose.This  is a very strong substance and provide rigidity to the cell.the plant cell wall consists of tiny cellulose fibres called microfibrils.

Functions:

• It gives strength to the cell.
• It is freely permeable to water and other soluble substances.
• It allows the cell to become Turgid.

Plasmolysis :When a living cell loses water from the cell through osmosis the cell membrane get shrinked or the content of the cell away from the cell wall.This phenomenon is called Plasmolysis.

                                                  

                                                                 NUCLEUS

Nucleus is centrally located, spherical structure enclosed by nuclear membrane.It contain the nucleolus and chromatin material inside it. Nucleolus is related with Ribosomes formation.Nuclear membrane contain many tiny pores called nuclear pore in it.These pores allows the exchange of material between the nucleus and cytoplasm.

Chromatin Material is a thin, thread-like mass of chromosomes material and composed of genetic material DNA and protein.

                                                                  CYTOPLASM

Cytoplasm is that part which occur between nuclear membrane and plasma membrane.
various cell organelles are found in cytoplasm.

Centrosomes:  These are tubules like structure.They are bounded by any membrane but consists of two granules like centrioles.These are found only in animal cells.

It helps in cell division.

Endoplasmic Reticulum(ER): It is a  membraneous network enclosing a fluid filled lumen.It occurs in three forms Cisternae,vesicles and tubules.

Ribosomes:   These are dense ,spherical and granular particles.These are basically ribonucleic acid and protein.

Golgi Apparatus:   These are membrane bound sac like structure.These sacs are stacked together in parallel rows.

Lysosomes:   These are simple sac  spherical structure.These are single membrane bound structure and contain powerful enzymes.These enzymes are capable  of digesting or breaking down all organic material.These are also called Digestive Bags.

Mitochondria:  These are double bounded structure of varying shapes.Outer membrane is porous and  inner membrane has many folds which increases the surface area of the mitochondria.
These  are the site of cellular respiration.They are often called Power House of the cell.

Plastids: They are plant cells only.They are self replicating organelles.They are of three type

• Chromoplasts:    It gives colours to flowers.
• Chloroplasts:   They trap solar energy and utilise it for preparing food for plant.
• Leucoplasts:    It stores food

Vacuoles: They are a kind of storage sac.In animal cell they are very small and sometime absent.In plant cell they are very prominent, and large in size.