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8. Cell Structure and Functions

This chapter talks about cell, the building block of life

The cell is the basic structural unit of a living organism. The cells of living organisms are complex living structures themselves.

8.1      Discovery of cell

Cell was discovered by Robert Hooke in 1665. He took a thin layer of cork, which is part of the bark of the cork tree and observed it under magnification. He noticed partitioned boxes / compartments in the slice of cork.

Cells of a living organism can only be seen under a microscope.

8.2      Cells – shape, number and size

8.2.1      Number

Different organisms have different cellular composition.

  1. Organisms that are made of more than one cell are called Multi-cellular organisms. For example
    1. Human Beings
  2. Organisms that are made up of just a single cell are called Uni-cellular organisms. For examples
    1. Amoeba

In multi-cellular organisms, different functions of the body are performed by tissues and organs made from different cells. Whereas, in unicellular organisms, all the functions like capturing & digesting food, excretion, reproduction etc are carried out by the single cell itself.

8.2.2      Shape

The cells are of different shapes and sizes depending upon the function they have to perform. For example, the function of a nerve cell is to receive & transfer messages, to help to control and coordinate the working of different parts of the body, they are very long.

The cell membrane provides shape to the cell in plants & animals. Cell Membrane is further covered by a cell wall which gives additional covering in plant cells. It also gives shape & rigidity to the cell.

8.2.3      Size

The size of a cell can be very small (few microns) as in bacteria which can not be seen with an unaided eye or can be as big as 170 mm x 130 mm in case of the egg of an ostrich.

Also, it is important to remember that the size of the cell has no relation with the size of the body of the plant or the animal. The size of the cell is determined by the function it has to perform.

8.3      Cell Structure & Function

A group of cells in multi-cellular organisms like plants and animals form Tissues. A group of tissues form an Organ. Organs perform specific function for the organism. For example – Roots of a plant help in absorption of water & minerals from soil.

8.3.1      Parts of the Cell

The basic components of a cell are

  1. Cell Membrane – The container in which cell material resides is called cell membrane. It is also called as plasma membrane. It forms the outermost layer of the cell and encloses cytoplasm & the nucleus. It separates cells from each other as well as from the surrounding medium. The cell membrane is porous and allows the movement of substances in both inward & outward direction
  2. Cytoplasm – It is a jelly-like substance present between membrane and nucleus. Various other cell components are also present in the cytoplasm. These are called as Cell Organelles. For example – Mitochondria, Golgi bodies and Ribosomes.
  3. Nucleus – It is a very important component of cell. It is located in the centre of cell and is usually spherical. Nucleus is the control centre of the activities of the cell. It is separated from the cytoplasm by its membrane called the nuclear membrane. This membrane is porous and allows the movement of substances both inward & outward. Nucleus consists of the following components
    1. A smaller spherical body called as Nucleolus
    2. Thread-like structures called Chromosomes which carry genes and help in transfer of characters from the parents to the offspring

The entire content of a living cell is known as protoplasm. Protoplasm includes the cytoplasm as well as the nucleus.

A Gene is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic from parents to the offspring. For example – children inherit the physical features of their parents like colour of the eyes and hair.

Distinction of cells based on nuclear membrane

  1. The cells where the nuclear material is present in the cytoplasm without any nuclear membrane are called Prokaryotic cells. The organisms having such cells are called as Prokaryotes. For example – Bacteria & Blue Green Algae
  2. The cells where the nuclear material is present inside the nuclear membrane are called Eukaryotic Cells. The organisms having such cells are called as Eukaryotes. For example – Human beings

Some cells have blank structures present in the cytoplasm. These structures are called Vacuole. They appear as empty spaces under the microscope. In animal cells, vacuoles are temporary & small in size, whereas in plant cells, they are permanent & are bigger in size. The various functions of vacuole are

  1. They store excretory waste in plant cells
  2. In unicellular organisms like amoeba, digestion of food takes place inside a vacuole which is called as Food Vacuole.

8.3.2      Plants and Animal Cells

Plant and Animal cells are different and have some specific components, which are unique to them.

The cell organelles which contain pigments and are only found in plant cells are called Plastids. They are of the following types

  1. Chloroplasts – These plastids are green in colour and contain a green pigment called Chlorophyll which is used to synthesize food during photosynthesis
  2. Chromoplasts – These plastids are coloured and give colour to the flowers & attract insects for pollination
  3. Leucoplasts – These plastids are colourless and help the cells in storing the food like glucose, starch etc.

Comparison of Plant and Animal Cells

Cell Component Plant Cell Animal Cell
Cell Membrane Present Present
Cell Wall Present. It is made up of cellulose Absent
Nucleus Nucleus is shifted towards the periphery due to large vacuole Nucleus is present in the center
Cytoplasm Present Present
Plastids Plastids are present Plastids are absent

 

Vacuoles Vacuoles are permanent and large Vacuoles are temporary & small in size or they may be absent.

Pseudopodia – Pseudopodia are the finger like projections that are protruding out of the body of Amoeba. Pseudopodia appear & disappear as amoeba moves and feeds. Also, pseudopodia help amoeba in movements & capturing food.