Updated date:

May 10, 2021

Cell: The Fundamental Unit of Life

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Life begins with a cell. All living organisms are made up of cells. 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 combines 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, esophagus (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 the 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 specialized organs composed of similar tissues (a group of similar cells). They have great capacity to survive. They live a longer life.

Size of a cell: Cell size is measured in micrometer and millimeter. They are, generally, not visible to naked eyes. A microscope is needed to see them. The size of all living organisms varies. Size of the bacterial cell lies between 0.2 to 100 micrometer. The size of an 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^-3millimeter

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.

Classification of cells:

Prokaryotic cells: The nuclear region is not well-organized e.g. bacteria, blue-green algae.

Eukaryotic cell: The nuclear region is well-organized, it has a nuclear membrane with the nucleus. Examples: Plants, animals, fungi, protozoa, etc.

Classification of cells

Prokaryotic cell: The nuclear region is not well-organized e.g. bacteria, blue-green algae.

Eukaryotic cell: The nuclear region is well-organized, it has a nuclear membrane with the 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 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 going on inside a cell. The plasma membrane is also called a cell membrane. The 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 a 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 a 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 the nucleus and the cell membrane. The cytoplasm contains various structures or bodies that perform specific tasks. These structures are called cell organelles. These cell organelles are as under:

Mitochondria: Provides energy to the cell, also called the 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. Its function is to store, synthesize, and transport cell products. There are two types of ER - Rough ER and Smooth ER, Rough ER contains ribosomes, manufacture proteins while SER does 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 the plant cell, they are of three types:

Chloroplasts: contains a green pigment called chlorophyll which helps in photosynthesis.
Chromoplasts: They contain non-green pigments responsible for giving colors to fruits and flowers
Leucoplasts: They are colorless, helps in the storage of food.

Centrosome: present only in the animal cells. it is located near the nucleus and helps in cell division.

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

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

Take your test

For each question, choose the best answer. The answer key is below.

Q.1 Who discovered the cell?
- Matthias Jakob Schleiden
- Theodor Schwann
- Robert Virchow
- Robert Hooke
Q.2 Cells _______
- have no colour
- have colour
- can be transparent or have colour
- None of these
Q.3 These cells do not have regular shape:___________
- Amoeba and Paramecium
- Amoeba and Euglena
- Amoeba and White Blood Cells
- White Blood Cells and Hydra
Q.4 The smallest cell in human body _________
- White Blood Cells (WBCs)
- Red Blood Cells (RBCs)
- Nerve Cells
- Bone Cell
Q.5 The largest cell in human body ___________
- WBCs
- RBCs
- Neurons
- Epithelial Cell
Q.6 Plastids are ____________
- Present in animal cell only
- Present in plant cell only
- Present in both plant and animal cell
- absent in both the plant cell and animal cell
Q.7 In a cell, which of the following is not a part of cytoplasm?
- golgi apparatus
- nucleoplasm
- cell membrane
- mitichondria
Q.8 Nucleolus is rich source of_______
- DNA
- RNA
- ribosomes
- plastid
Q.9 All metabolic activities of a cell is controlled by ___________
- Cytoplasm
- Cell membrane
- Nucleus
- Golgi body
Q.10 The sites of protein synthesis inside the cell __________
- Mitochondria
- Vacuoles
- Ribosomes
- Golgi bodies
Q.11 Plant cell does not contain ___________
- cell wall
- plastid
- centrosome
- cell membrane
Q.12 __________ is called as power house of the cell
- ribosomes
- nucleus
- mitochondria
- plastid
Q.13 _________ provides colour to flowers and fruits.
- Chloroplasts
- Chromoplasts
- Leucoplasts
- None of these
Q.14 Which of the following are Eukaryotes ?
- bacteria, blue-green algae
- fungi, bacteria, plants, animals
- protozoa and fungi
- fungi, protozoa, plants, animals
Q.15 Which of the following statement is true?
- cell wall is present in both - the plant cell and the animal cell
- centrosome is present in plant cell
- centrosome is present in animal cell
- centrosome is present in both - the plant cell and animal cell
Q.16 Which of the following statement is true?
- Leucoplasts are present in animal cell which helps in the storage of food
- Leucoplasts are present in plant cell which helps in the storage of food
- Leucoplasts are present in animal cell and plant cell both
- Leucoplasts have colour
Q.17 _________contain chlorophyll
- chromoplasst
- leucoplasts
- chloroplasts
- none of these
Q.18_____ store cell sap (soluble food, secretions of cell, wastes etc.)
- Vacuoles
- mitochondria
- golgi apparatus
- ribosomes
Q.20 Nucleus and cytoplasm together constitute ________ which is living content of the cell
- nucleoplasm
- protoplasm
- cell fluid
- none
Q.19 Which of the following is responsible for blood clotting?
- RBC
- WBC
- platelets
- None

Answer Key

Robert Hooke
have no colour
Amoeba and White Blood Cells
Red Blood Cells (RBCs)
Neurons
Present in plant cell only
cell membrane
DNA
Nucleus
Ribosomes
centrosome
mitochondria
Chromoplasts
fungi, protozoa, plants, animals
centrosome is present in animal cell
Leucoplasts are present in plant cell which helps in the storage of food
chloroplasts
Vacuoles
protoplasm
platelets

Interpreting Your Score

If you got between 0 and 6 correct answers: Poor

If you got between 7 and 12 correct answers: Below Average Performance!

If you got between 13 and 16 correct answers: Average Performance!

If you got between 17 and 18 correct answers: Better!

If you got between 19 and 20 correct answers: Excellent!

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.