Maharashtra Board Class 7 Science Chapter 11 Cell Structure and Micro organisms PDF Download

11. Cell Structure And Micro-organisms

What is the name of the minute components of which the body of a living organism is made? Is the number of these smallest units the same in the bodies of all living organisms?

We have learnt that cellular organization is the primary characteristic of all living organisms and that the cell is the fundamental structural and functional unit of living organisms.

Complete the flow chart:

Words → Sentences → Paragraphs → Chapters → Book

We see the structural organization of a book in the above flow chart. Similarly, there are organizational levels in living organisms, too. These are: cells, tissues, organs, organ systems, organism. Cells form the basis of the structure and function of all living organisms. It is only with the help of cells that living organisms carry out all their different life processes.

Where do the life processes of unicellular organisms take place?

Teacher's Note

Cells are the basic building blocks of all living things. Like how bricks make a house, cells make our body. You can see cells only under a microscope.

Exam Trick

Remember: Cell = Unit of life. Just like a classroom is made of many desks, your body is made of many cells working together.

Points to Remember

Cells are the smallest units of life.
All living things are made of cells.
The cell is the basic unit of structure and function in living organisms.
Cells work together to form tissues, organs and systems.
We need a microscope to see most cells.

Great Scientists

In 1665, a scientist, Robert Hooke observed a thin section of cork under a microscope. In this section, he saw hexagonal compartments like those of a bee-hive. He named those compartments 'cells'. This term is derived from the Latin word 'cella' which means 'small room'.

In 1838, the two scientists M. J. Schleiden and Theodore Schwann formulated a theory about cellular structure which stated, 'All the living organisms are made up of cells and the cell is the fundamental component of living organisms'. In 1885, Rudolph Virchow stated that all cells are formed from pre-existing cells.

Measurement And Observation Of Cells

In 1673, Anton van Leeuwenhoek assembled various lenses to construct a microscope. He was the first to observe live bacterial and protozoan cells under the microscope.

Cells are extremely minute in size. We cannot see cells with the naked eye. Micrometre and nanometre are the units used for measuring their sizes. The compound microscope is used for observing cells. An object on a glass slide magnified many times by the lenses of this microscope.

1 centimetre = ....... millimetres, 1 millimetre = 1000 micrometres, 1 micrometre (or micron) = 1000 nanometres

Take a piece/segment of onion and carefully separate the thin skin from its concave surface with the help of forceps. Place the membrane on a glass slide and put a drop of water on it. (Take care that the membrane does not get folded while placing on the slide). Put a drop of a dilute solution of iodine or eosin over it and observe under the 10X objective of the compound microscope. Don't forget to put a cover-slip over the onion skin on the slide before placing it under the microscope.

In the same way, observe the cells from various parts of plants like leaves, bark, root tips, etc. Do you remember that last year you had observed the amoeba and paramoecium that are found in water?

Are the cells that you observed, all alike? What is their structure? What are their shapes?

Teacher's Note

When you look at onion skin under a microscope, you will see small boxes. These boxes are cells. Each cell has a wall around it, just like rooms in a building.

Exam Trick

Remember: Onion cells = Square boxes under microscope. The cell wall is like the boundary wall of your school.

Points to Remember

The compound microscope helps us see tiny cells.
Cells can be observed on glass slides.
Different plant cells look like small boxes.
We use iodine or eosin to color the cells for viewing.
A cover-slip protects the cells under the microscope.

Cell Size

There is great variation in the shapes of cells. Their shapes are mainly related to their function. Observe the cells of different shapes shown below.

Cells are of many different shapes e.g. circular, rod-shaped, columnar, spiral, oval, rectangular, etc.

Each cell contains various components for carrying out the life-processes of the living organism. These components are called organelles. These organelles are studied in detail with the help of the electron microscope which can magnify images up to (2 x 10⁵) two billion times their actual size.

There are two main types of cells - animal cells and plant cells. These cells consist of various types of membrane-bound cell organelles. Plant cells have a definite shape due to the presence of the cell wall around them. Besides, unlike animal cells, plant cells contain single large vacuole. All these cells are known as eukaryotic cells.

The cell has four main parts - the cell wall, cell membrane, cytoplasm and cell organelles.

(a) Cell wall: The cell wall is the outermost covering of a cell. It is present only in plant cells.

(b) Plasma membrane: The plasma membrane, also called the cell membrane, is a kind of thin covering. It is extremely delicate and flexible. It is the outermost covering of animal cells.

(c) Cytoplasm: The liquid part in the cell, present around the nucleus is called cytoplasm. It occupies the space between the plasma membrane and nucleus. Cell organelles are scattered in the cytoplasm.

(d) Cell organelles: These mainly include the nucleus, endoplasmic reticulum, Golgi bodies, lysosomes, mitochondria, vacuoles, plastids, etc. Plant cells contain chloroplasts.

Which are the common components of plant and animal cells? Which are the different ones?

The nucleus is the most important organelle of the cell. There is a porous double membrane around it. The nucleus controls all functions of the cell. The endoplasmic reticulum is a sprawling net-like organelle. Its function is to make necessary changes in the proteins produced by ribosomes and send them to the Golgi bodies. Golgi bodies are made up of several flat sacs. Their function is the proper distribution of proteins. Mitochondria and plastids are organelles with double outer coverings. As mitochondria produce energy, they are called the powerhouses of the cell. The chloroplasts in plant cells carry out the function of photosynthesis. Vacuoles help to throw out waste products of the cell. Vacuoles in animal cells are small whereas there is only one large vacuole in a plant cell.

Teacher's Note

Plant cells have a hard cell wall, but animal cells don't. This is like how a school building has strong walls, but a tent doesn't have hard walls.

Exam Trick

Remember: Plant cell = Has a wall (like a house). Animal cell = No wall (like you without clothes). Mitochondria = Powerhouse, gives energy to cells.

Points to Remember

Plant cells have a cell wall, animal cells do not.
The nucleus controls all cell activities.
Mitochondria are the powerhouses of the cell and make energy.
Chloroplasts are found only in plant cells and make food.
Vacuoles store water and waste materials in cells.

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