A cell can be defined as the protoplasm surrounded with cell membrane, which works as structural and functional unit of life. Each cell is capable of performing the basic functions of life such as respiration, reproduction, excretion, growth etc.

The cell theory

The theory of cell is summed up in the following points:

1. All the living organisms are composed of small living units called cells.
2. All the cells are fundamentally similar in chemical compositions and metabolic activities.
3. The entire cell arises from pre-existing cell.
4. The cells are structural and functional units of life.
5. The growth of multicellular organism is occurred by cell division and that of unicellular organism is occurred by cellular growth.

Shape and size of the cell

Due to different functions, cells are found in different shapes oval, round, rectangular, cylindrical etc. The shape of cells is influenced by factors like a cell wall, pressure, location etc.

The cells may be prokaryotic or eukaryotic. In prokaryotic cells, nuclear materials and organelles are not enclosed by membranes but in eukaryotic cells, nuclear materials and organelles are covered with membranes. A bacteria has the smallest cell and the largest cell is found inside an ostrich egg. The size of prokaryotic cells range from 10²nm to 2 \(\times\) 10³nm and size of eukaryotic cell ranges from 10⁴nm to 10⁵nm.

Prokaryotic and Eukaryotic Cells

Prokaryotic Cells: Prokaryotic cells lack nuclear membrane. It also lacks membrane bound organelles. There is the absence of mitotic apparatus and nucleoli in a prokaryotic cell. Mitotic cell division is absent in prokaryotes. The cell organelles present in the prokaryotic cells are ribosomes which are of 70s type.

Eukaryotic Cells: the Eukaryotic cell is a type of cell that consists membrane bound organized nucleus and all other types of cell organelles. It contains all membrane bound organelles such as mitochondria, endoplasmic reticulum, golgi complex, centrosome, vacuoles, etc. The ribosomes found in eukaryotic cells are of the 80stype.

Parts of Eukaryotic Cells

Eukaryotic cells can be divided into following three major parts:

• Cell wall and cell membrane
• Cytoplasm
• Nucleus

Cell wall and cell membrane

The outermost, rigid, protective layer found in cells is known as the cell wall. It is absent in animal cells.

The main functions of cell wall are as follows:

1. It provides fixed shape and mechanical strength.
2. The cell wall of root caps absorbs water and minerals from the soil.
3. It protects the inner content of the cell.

The animal’s cells have fine cell coat at their surface, called as a cell membrane. It is made up protein and lipid.

The main functions of cell membrane are as follows:

1. It helps in feeding and locomotion in a unicellular organism.
2. It helps in excretion in a unicellular organism.
3. It helps in transportation by the process of diffusion.

Cytoplasm

The non-crystalline, translucent substance found in between the plasma membrane and nucleus is known as cytoplasm. Cell organelles and cell inclusions are found in it, which are jointly known as cytoplasmic structures. Some major cell organelles are described below:

Mitochondria

They are made up of protein, lipids and a small amount of DNA. They contain different types of enzymes in which intracellular respiration occurs. The energy produced by this process is stored here in the form of ATP. Due to this reason, mitochondria are known as the powerhouse of a cell.

Endoplasmic reticulum and ribosome

It is a doubled layered membrane. The major functions of endoplasmic reticulum are as follows:

1. It increases the surface area in the cell for metabolism.
2. It provides mechanical support to the cell.
3. It provides the pathway for transportation of materials.

Ribosomes are small round structures found in the cytoplasm and consider as protein factories as they synthesize protein.

Golgi bodies

The major functions of Golgi bodies are as follows:

1. It stores, condenses and packages the materials like protein, lipids etc.
2. It is involved in cell secretion.
3. It forms lysosomes, hormones etc.

Lysosomes

They are sac-like structures bound by a membrane made of lipid and protein and contain digestive enzymes in them. They carry the following functions:

1. They help in digestion of unnecessary material inside a cell.
2. They also help in extracellular digestion.

Centrosome

They are found only in animal cells.

The major functions of centrosome are as follows:

1. They form asters during cell division.
2. It controls the movement of cilia and flagella.

Plastids

They are found only in a plant cell. They are oval, round and in other shapes. On the basis of pigments found in them, they are divided into three types. They are chloroplast, chromoplasts, and leucoplast. Chloroplasts contain green pigment and help in preparation of food. Chromoplasts are found in coloured parts of plants and help in pollination. Leucoplasts are non-pigmented plastids which are concerned with storage of protein, starch, oil etc. in plant bodies.

Vacuole

They are fluid filled sacs bounded by a single membrane called tonoplast.

The major functions of vacuole are as follows:

1. They balance amount of glucose and water in cells.
2. They store waste products and helps in excretion.

Nucleus

It is very important part of a cell. It is a spherical structure that consists of four major components of it. They are:

1. Nuclear membrane: It maintains the shape of the nucleus and regulates the nucleocytoplasmic interactions.
2. Nucleoplasm: It provides the site of enzyme activities .
3. Chromatin fibers: They shorten and thicken during cell division and divide in particular members into the cells of particular organisms called chromosomes.
4. Nucleolus: It controls all cellular activities of the cells and forms ribosomes.

Differences between plant cell and animal cell