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Cell: The Fundamental Unit of Life

Updated on November 24, 2017
Human Body Cells
Human Body Cells

Life begins with a cell. All living organisms are made up of cell. A cell performs all the basic functions of life. A cell can be defined as the basic, smallest, structural and functional unit of life. A group of similar cells combine to form tissues, specialised tissues combine to form organs and organs that work together to perform a specific function constitute an organ system, for example, oesophagus (food pipe), stomach, small intestine, large intestine, pancreas, liver, all these organs work together to digest the food and hence, form digestive system which is an organ system. Similarly other organ systems, such as excretory system, reproductive system etc., have different sets of organs that work together to perform various life activities. Thus, different organ systems work together to carry out life processes that make up a living organism.

Cells Tissues → Organs → Organ systems → Organisms

Living Organisms are classified into two categories:

  • Unicellular organisms: In unicellular organisms, such as amoeba, paramecium, Euglena, bacteria, yeast etc. all life processes or functions i.e. ingestion, digestion, absorption, assimilation, and egestion are performed by a single cell.
  • Multi-cellular Organisms: In multicellular organisms such as plants, trees, insects, animals, human beings etc., each and every function of a body is performed by specialised organs composed of similar tissues (group of similar cells). They have great capacity to survive. They live longer life.

Size of a cell: Cell size is measured in micrometer and millimetre. They are, generally, not visible to naked eyes. Microscope is needed to see them. The size of all living organisms varies. Size of bacterial cell lies between 0.2 to 100 micrometer. Size of ostrich's egg cell is about 170 mm × 130 mm. however, the size of a cell is related to the function not to the size of an organism.

1 micrometre (μm) = 10-3 milimetre

Difference between multicellular and unicellular organisms

S. No.
Unicellular Organisms
Multicellular Organisms
1
Their body is composed of a single cell.
Their body is composed of various cells.
2
All life activities are performed by a single cell. Each cell organelle performs its specific task.
All life activities are not performed by a single cell. There is division of labour i.e each organ and organ system performs specialised function.
3
Since there is a single cell to perform all activies, operational efficiency is low.
Operational efficiency is high because iof specialised cells.
4
An injury to a cell can cause death of a unicellular organism.
Injury to few cells does not affect the organisms. The new cells are reproduced again.
5
They are reproduced through cell division.
All cells can't reproduce through cell division.
6
Single parent is involved in reproduction.
Sexual reproduction occurs i.e. two parents are involved in reproduction.
 Eukaryotes and Prokaryotes
Eukaryotes and Prokaryotes
Animal Cell
Animal Cell
Plant Cell
Plant Cell

Prokaryotic cells: Nuclear region is not well-organised e.g. bacteria, blue-green algae.

Eukaryotic cell: Nuclear region is well-organised, have nuclear membrane with nucleus. Examples: Plants, animals, fungi, protozoa, etc.

Components of a cell

Major components of a cell membrane:

  • Plasma Membrane
  • Cytoplasm
  • Nucleus

Plasma membrane: Every cell is bound by a thin and a delicate membrane called the plasma membrane, it is the outer covering or layer of a cell that acts as a protective shield and It separates cells from one another and also from the surrounding medium. All basic life processes goes on inside a cell. Plasma membrane is also called cell membrane. Cell membrane is porous i.e. it contains tiny pores on its surface which allows entry and exit of only selected substances and prevents the movement of some substances across it. Hence it is also called selectively permeable membrane.

In a plant cell and also in a bacterial cell, there is again an outer covering besides the cell membrane, this is called cell wall. This cell wall provides further protection against temperature variations, moisture etc.

Cytoplasm: A thick jelly like fluid inside the cell membrane is called cytoplasm. Cytoplasm covers the space between nucleus and cell membrane. Cytoplasm contains various structures or bodies that performs specific tasks. These structures are called cell organelles. These cell organelles are as under:

  • Mitochondria: Provides energy to the cell, also called powerhouse of the cell.
  • Vacuoles: Stores soluble food, wastes and secretions of cell in water i.e.they store cell sap
  • Ribosomes: Small granular structures and responsible for protein synthesis
  • ER (Endoplasmic reticulum): It is a network of tubules and channels, interconnected and continuous and joining the nucleus membrane. It's function is to store, synthesise, and transport cell products. There are two types of ER - Rough ER and Smooth ER, Rough ER contains ribosomes, manufacture proteins while SER do not contain ribosomes, manufacture fats and steroid hormones.
  • Golgi bodies or Golgi apparatus: Secretes chemical substances like enzymes, hormones, and proteins.
  • Plastids: Present only in plant cell, they are of three types:
  1. Chloroplasts: contains green pigment called chlorophyll which helps in photosynthesis.
  2. Chromoplasts: They contain non-green pigments responsible for giving colours to fruits and flowers
  3. Leucoplasts: They are colourless, helps in the storage of food.
  • Centrosome: present only in animal cell. it is located near nucleus and helps in cell division.

Nucleus
Nucleus

Nucleus: Nucleus is the brain centre of a cell, controls all the metabolic activities of the cell. It's a large spherical structure present inside the cell. In plant cell it lies at the periphery while in animal cell it is located at the centre. Nucleus has four main parts:

Nuclear Membrane: It has double membrane called nuclear envelope. The membrane separates its content from nucleoplasm. It is porous and allows movement of certain materials between cytoplasm and the nucleus.

Nucleoplasm: A thick fluid inside the nucleus.

Nucleolus: Small spherical body present inside the nucleus, it contains Ribonucleic acid (RNA).

Chromosomes: Fine thread-like structures composed of DNA (Deoxyribonucleic acid) are present in the nucleus, during cell division this fibrous structures or chromatin fibres condense to form chromosomes which carry genes. These genes are responsible for passing genetic characteristics from parents to their offspring.

Plant cell and Animal cell: a Comparison

S. No
Parametres
Plant Cell
Animal Cell
1
Cell Wall
Present
Absent
2
Size
Larger in comparison to animal cell
Smaller in size.
3
Plastids
Present
Absent
4
Centrosome
Absent
Present
5
Golgi bodies
Dictyosome scatteredin the cytoplasm
Present near nucleus and well developed
6
Vacuoles
vacuoles are large and few, covers most of the space in cell.
Vacuoles are small in size and sometimes absent too.
7
Lysosomes
Mostly absent
Present, also called suicidal bags, responsible for breaking down of waste and harmful substances.

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© 2017 sonal

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    • sonal shrivastava profile imageAUTHOR

      sonal 

      9 months ago from INDIA

      Thanks for the nice advice, I will try my best to cover articles on different aspects of cells.

    • profile image

      suyash 

      11 months ago

      Try to write articles on different cells like blood cells, neuronal cells. It will be more informative for STUDENTS.

    • profile image

      suyash 

      17 months ago

      very informative

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