Cell: Structure and Functions

CELL-STRUCTURE AND FUNCTIONS

CELL

All organisms are made of smaller parts called organs. Organs are made of still smaller parts.The smallest living part of an organism is a cell,Cells are the basic building blocks of all living things. Cell is a structural and functional unit of living organisms.Cells may be compared to bricks. Bricks are assembled to make a building. Similarly, cells are assembled to make the body of every organism. All organisms are made up of cells.All basic chemical and physiological functions of living beings – for example, repair, growth, movement, immunity, communication, and digestion etc are carried out inside the cells.

Discovery of the Cell:
(i) In 1665, the English scientist Robert Hooke observed slices of cork which is part of bark of the tress, under a simple magnifying device. He noticed partitioned boxes or compartments in the cork slice.

(ii) These boxes looked like a honeycomb. He also noticed that these boxes were separated from the other by a wall or partition. Hooke named them ‘cell’ for each box. Hooke observed as boxes or cells in the cork were actually dead cells. These boxes appeared like a honeycomb.

Over the next 170 years, research led to the formation of the cell theory, first proposed by the German botanist Matthias Jacob Schleiden and the German physiologist Theodore Schwann in 1838 and formalized by the German researcher Rudolf Virchow in 1858.

Types of cell:
1. Prokaryotic Cells:
 Prokaryotic cells are cells that do not have a true nucleus or membrane-bound organelles. Organisms within the domains Bacteria and Archaea have prokaryotic cells, while other forms of life are eukaryotic.

2. Eukaryotic cells:
   
   Eukaryotic cells are cells that contain a nucleus and organelles, and are enclosed by a plasma membrane. Organisms that have eukaryotic cells include protozoa, fungi, plants and animals.
   Organisms Show Variety in Cell Number, Shape and Size:

Millions of living organisms have cells with different shapes and sizes. Their organs also vary in shape, size and number of cells.

Human body has trillions of cells which vary in different size and shapes. Different groups of cells perform a variety of functions. Organisms made of more than one cell are called multicellularorganisms. An organism with billions of cells Starts life as a single fertilized egg cell. The fertilised egg cell multiplies by dividing process and the number of cells increases for development.
(ii) The single-celled organisms are made up of single cell called unicellular organisms. A single celled organism performs all the necessary functions that multicellular organisms perfor

A single-celled organism, like amoeba, captures and digests food, respires, excretes, grows and reproduces. Similar functions in multi-cellular organisms are carried out by groups of specialised cells forming different tissues. Tissues, in turn, form organs.

1. Shape of Cells:
   (i) Generally, cell’s shapes are rounding, spherical or elongated. Some types of cells are long and pointed at both ends. They exhibit spindle shape. Some types of cells are branched like the nerve cell or a neuron, which receives and transfers messages for helping to control and coordinate the working of different parts of the body.
2. ii) Components of the cell are enclosed in a membrane. This membrane provides shape to the cells of plants and animals. Cell wall is an additional covering over the cell membrane in plant cells. It gives shape and rigidity to these cells.
3. Size of Cells:
   (i) The cell’s size in living organisms may be as small as a millionth of a metre (micrometre or micron) or may be as large as a few centimetres.
   (ii) Most of the cells are microscopic in size and are not visible to the naked eye. They need to be enlarged or magnified by a microscope for seeing.
   (iii) The smallest cell is 0.1 to 0.5 micrometre in bacteria. The largest cell measuring 170 mm ×130 mm, which is the egg of an ostrich.
   (iv) The size of the cells has no relation with the size of the body of the animal or plant.

Cell Structure and Function:
(i) Each organ in the living organisms performs different functions such as digestion, assimilation and absorption. Similarly, different organs of a plant perform particular/ specialized functions. Example: roots help in the absorption of water and minerals.
(ii) Each organ is further made up of smaller parts called tissues. A tissue is a group of similar type cells performing a particular function. 

Parts of the Cell:
Cell membrane, cytoplasm and nucleus are the basic components of a cell.

1. Cell Membrane:
   (i) The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane.
   (ii) It is living part of the cell, thin, delicate and elastic.
   (iii) This membrane separates cells from one another and also the cell from the surrounding medium.
   (iv) It is selectively permeable. It allows the flow of limited substances in and out of the cell.
   (v) This gives shape to the cell.
   (vi) In addition to the cell membrane, an outer thick layer in cells of plants, called cell wall.
   (vii) This additional cell wall surrounding the cell membrane is required by the plants for protection against variations in temperature, high wind speed, atmospheric moisture, etc. They are exposed to these variations because they cannot move.
   (viii) Cells can be observed in the leaf peel of Tradescantia, Elodea or Rhoeo.
2. Cytoplasm:
   (i) Cytoplasm is the jelly-like substance present between the cell membrane and the nucleus.
   (ii) Various other components or organelles of cells are present in the cytoplasm. Like mitochondria, golgi bodies, ribosomes, etc.
3. Nucleus:
   (i) It is very important component of the living cell. It is generally dense and spherical organelle and located in the centre of the cell. It can be seen easily with the help of a microscope. Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus.
   (ii) It is consist of nucleolus, thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.
   (iii) Genes contains genetic codes which are responsible for the unique physical character of an animal or a plant.
   (iv) Nucleus acts as control centre of the activities of the cell. The entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus. It controls all the metabolic activities of cell.
   (v) Nucleus is the storehouse of genes. Without nucleus, cell can neither survive nor shows specialized activities.

Comparison of Plant and Animal Cells:

EXERCISES

1. a)True b) Falsec) Falsed) True
2. Nerve cells are the primary cells in the nervous system. They are responsible for relaying electrical messages to cells and tissues in other organ systems. 

3.(a) Cytoplasm:

The cytoplasm (also known as cytosol) is the protoplasm of a cell outside the cell nucleus. It is the gel-like material plus the organelles outside the nucleus, and inside the cell membrane. … It includes dissolved molecules, and water that fills much of the cell.

4.Various cell organelles are present in the cytoplasm. It is a clear and colorless fluid that contains organelles like Mitochondria, Ribosomes, Golgi bodies.

5.

6.

7. Chromosomes are thread-like structures present in the nucleus that carry genes. All the necessary information required for the transfer of characteristics from the parents to the offspring are stored in the genes. Inheritance of characteristics is possible only because of chromosomes.

8.Cells constitute various components of plants and animals. A cell is the smallest unit of life and is capable of all living functions. Cells are the building blocks of life. This is the reason why cells are referred to as ‘the basic structural and functional unit of life’.

9. Chloroplasts are plastids required for the food making process, called photosynthesis, and thus they are only present in plant cells.
10. Across

1. Chlorophyll
2. Organelle
3. Protoplasm
4. Genes

 Down

1. Chloroplasts
2. Organ
3. Membrane
4. Vacuole
5. Tissue