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Cell: Structure and Functions

 

Table of Content


Cell is the structural and functional unit of life. Cell structure and functions includes following topics:

  1. Cell: The Unit of Life
  2. Biomolecules
  3. Cell Cycle and Cell Division
     

1. Cell: The Unit of Life

All organisms are made of cells. Cells vary in their shape, size as well as functions. Based on the presence or absence of a membrane bound nucleus and other organelles, organisms are classified as Eukaryotes or Prokaryotes.

Prokaryotes includes Bacteria, Blue Green Algae, Mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). These cells are small and multiply at a faster rate.

Eukaryotes includes Protists, Plants, Fungi and Animals. They have cell organelles separated by a membrane. They have well developed nucleus separated by a nuclear membrane.

Prokaryotes

Eukaryotes

No membrane bound nucleus

Membrane bound nucleus

Cell walls made of peptidoglycan (Thickness of wall depends on whether the cell is Gram +ve or -ve)

Cell walls, if present, made of cellulose (Chitin in fungi)

Have pili & fimbriae (for adhesion) and flagella (for propulsion)

Have cilia or flagella (for movement)

Mucilaginous capsule

No mucilaginous capsule present (numerous internal structures present including microtubules, ER, Golgi, secretary)

Cell size ranges from 0.5 µm to 100 µm

Cell size ranges from 10 – 150 µm

Fig.1. Difference Between Prokaryotes and Eukaryotes

A typical Eukaryotic cell consists of a Cell Membrane, Nucleus, Mitochondria, Endoplasmic Reticulum, Golgi Body, Chloroplast in Plant Cells, etc. Plant cells have a cell wall outside the cell membrane. These cell organelles are different from each other in structure and function.

In Eukaryotes (Higher Organisms), cell is surrounded by a membrane known as Plasma Membrane or Cell Membrane. Plasma membrane is made up of lipids and protein. Plant cell is surrounded by cell wall. Cell wall is made up of Polysaccharide known as Cellulose. Cellulose provides strength and rigidity to the cell.
 

What is a Cell in a Human Body?

Cell is the Structural and Functional unit of Human Body. There are different types of cells present in Human Body. The largest cell in the human body is ovum in Females. And the smallest cell in the Human Body is Sperm.

Apart from these, brain contain nerve cell or neurons, liver cells are known as Hepatocytes, kidney cells are known as Nephron etc.


What is the Structure of the Cell?

A typical Cell Structure is described as follows:

Ribosomes are cell organelles involve in protein synthesis. They are composed of large and small subunits. A network of tubules spread in a cytoplasm is known as Endoplasmic Reticulum or ER. There are two types of ER- Rough ER and Smooth ER. Rough ER is covered by ribosomes whereas smooth ER do not contain ribosomes. Rough ER is involved in protein synthesis and smooth ER is involved in fatty acid synthesis and detoxification.

Different cell organelles

Fig.2. Different cell organelles

Golgi Body is involved in secretion process, that is, it is involved in packaging of materials that are needed to be transported.

Lysosomes help in digestion of lipids, proteins and carbohydrates. Mitochondria is a double membrane structure- the outer membrane and inner membrane. The inner membrane is folded to form a structure known as Cristae. It is a site for ATP synthesis. 

   Plant cell structure

Fig.3. Plant cell structure

Plant Cell contain a cell organelle known as Chloroplast which is absent in animal cell.  It contains a green pigment known as Chlorophyll. The leaves appear green due to the presence of chlorophyll in it. Chlorophyll is essential for Photosynthesis. Nucleus is a double membrane structure. It contains a genetic material known as DNA. DNA Transfers Genetic Information from parents to the offsprings.


What is the Process by which Materials are taken into the Cell Membrane?

The most common process by which materials are taken into the cell membrane is phagocytosis. It is a process by which materials are taken in 

Note: For detailed study, kindly refer to the content “Cell - The Unit of Life.
 

2. Biomolecules

Types of biomoleculesBiomolecule is any molecule present in living organisms. It includes Proteins, Lipids, Carbohydrates and Nucleic Acids. Most of the biomolecules are composed of Oxygen, Hydrogen, Nitrogen and Carbon. Biomolecule is any molecule present in living organisms. Proteins are biomolecules that are essential for the survival of the living organisms. Amino Acids are the building blocks of the proteins. 

Proteins exists in different forms such as primary structure, secondary structure, tertiary structure and quaternary structure.

Lipids are usually insoluble in water. They are fatty acid esters. They are the principal component of Cell Membranes.

Carbohydrates are made up of Carbon, Hydrogen and Oxygen Atoms. It includes Sugars, Cellulose and Starch.

Note: For detailed study, kindly refer to the content Biomolecules”.
 

3. Cell Cycle and Cell Division

Cell Division is the process by which a cell divides to form two nearly equal daughter cells. The sequence of events that occur during cell growth and cell division are known as Cell Cycle. It is divided into two phases:

  • Interphase

  • M phase.

Interphase is the period between the cell divisions. It is the resting phase. It comprises of:

  • G1 phase is the phase in which a cell prepares for DNA replication.

  • S phase is the phase where DNA replicates.

  • G2 phase is the phase when the cell prepares itself for M phase or mitosis.

   Cell cycle and cell division

Fig.5. Cell cycle and cell division

M phase or the Mitotic phase is the process of formation of two daughter cells from the parent cell. The most important characteristics of M phase is, the DNA content remains same. It is also divided into different phases-  Prophase, Metaphase, Anaphase, Telophase and Lastly Cytokinesis in which cell divides.

Meiosis also known as Reductional division is responsible for the formation of gametes required for reproduction. It reduces the DNA content to half as compared to the parent cell. 

Mitosis

Meiosis

Occurs in somatic cells during growth, repair and replacement of cells

Occurs in sex organs during gamete formation

Two daughter cells are formed

Four daughter cells are formed

Both parent and daughter cells have diploid number of chromosomes

Parent cell has diploid number of chromosomes and daughter cells have haploid number of chromosomes

Daughter cells are identical to parent cell

Daughter cells are not identical to parent cell

Fig.5. Difference between mitosis and meiosis.

Note: For detailed study, kindly refer to the content Cell cycle and cell division”.
 

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