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An Overview of Cell: Classification, Structure, Functions, Practice Problems and FAQs

In this world, there are a variety of living things. Wherever you go you find them. It may be flowers, trees, animals, birds etc. Do you know why they are called living? Here comes the hero, the cell. 

A house is made up of a structural unit called a building brick (structural unit). In the same way organisms are made up of the structural unit called the cell. Despite being different in size and shape, all living beings have the commonality of being made up of cells.

Can we see cells with naked eyes?

No, cells are so tiny that we cannot see them with our naked eyes.

Basic structure of a cell

Table of Contents

Classification of Cells

Cells can be classified in the following ways:

Classification on the Basis of Cell Shape

Cells vary a lot in shape not only in different organisms but also in different tissues of the same organism. The shapes of cells can vary depending on the function. For example, Amoeba has irregular shape due to the formation of pseudopodia which is essential for its locomotion and feeding.

Commonly cells are of the following types based on shape

Cell shape

Occurrence

Figure

Elongated

Tracheids

Tracheds

Round and Oval

Mesophyll cells

Mesophyll cells

Disc-shaped, biconcave

Red blood cells

Red blood cells

Polygonal 

Skin cells

Skin cells

Columnar 

Cells of large intestine

Cells of large intestine

Cuboidal

Cells lining the salivary ducts

Cells lining the salivary ducts

Thread-like

Neuron

Neuron

Irregular

Amoeba

Amoeba

Amoeboid

White blood cell

White blood cell

Classification on the Basis of Presence or Absence of Nuclear Membrane

Cells can be classified into two types based on the presence or absence of a nucleus:

  • Prokaryotic 

  • Eukaryotic 

Types of cells

Difference Between Prokaryotic and Eukaryotic Cells

Prokaryotic cell

Eukaryotic cell

Single membrane system is present

Double membrane system is present

Genetic material is not enclosed in a nuclear envelope and is present suspended in the cytoplasm in a region called nucleoid

Genetic material is enclosed in a nuclear envelope and is not present in direct contact with cytoplasm

Cell wall is made up of peptidoglycan

Cell wall is made up of chitin in fungi and cellulose in plants

Ribosomes are of the 70S type

Ribosomes are of the 80S type

Nucleolus is absent

Nucleolus is present

Transcription and translation occurs in cytoplasm

Transcription happens in the nucleus and translation done in cytoplasm

Cytoplasm lacks membrane-bound cell organelles

Cytoplasm contains all membrane-bound cell organelles

Examples include bacteria, cyanobacteria etc.

Examples include all plant and animal cells

typical prokaryotic cell

tyical eukaryotic cell

Classification on the Basis of Cell Size

Cells can be of different sizes. Some are microscopic while others are macroscopic. The sizes of microscopic cells also vary. Some of the possible sizes are listed below:

Type of cell

Organism

Size

Figure

 

Virus

0.02 to 0.2 μm

Virus

Prokaryotic cell

Mycoplasma

0.1 - 0.3 μm

Mycoplasma

Bacterium

1 to 2 μm

Bacterium

Eukaryotic cell

Animal cell

10 to 20 μm

Animal cell

Plant cell

Plant cell

mycoplasma

Basic Structure of a Cell

A cell is composed of a large number of biomolecules. These biomolecules are enclosed in small subcellular compartments known as cell organelles. Various cell organelles and their functions are as follows:

Cytoplasm

Cytoplasm is the semi-solid, jelly-like matrix present inside the cell. It consists of cell organelles, enzymes, storage bodies etc.

components of cytoplasm

Cell Membrane or Plasma Membrane

Every cell (prokaryotic or eukaryotic) is surrounded by a thin membranous covering made up of lipids and proteins, known as the plasma membrane. It helps to communicate with the external environment in a regulated manner.

Cell Wall

It is present outside the plasma membrane of plant cells, fungal cells and bacteria for structural support. It has diverse composition as follows:

  • Plants - Cellulose and pectin

  • Fungi - Chitin 

  • Eubacteria - Peptidoglycan

Nucleus

It is the most important cell organelle which is the house of genetic material. It is surrounded by a nuclear membrane which protects it by forming a boundary between the nucleus and cytoplasm.

nucleus of an eukaryotic cell

Endoplasmic reticulum

It is involved in the synthesis of certain substances in the cell. It is found in eukaryotic cells and is involved in the synthesis of proteins, lipids, steroids etc.

types of endoplasmic reticulum

Golgi Apparatus

It is a feature of eukaryotic cells. It is involved in the transportation of materials within the cell. It is extensively present in secretory cells. The plant cells usually have several small Golgi complexes known as ‘dictyosomes’.

golgi apparatus

Ribosome

They are the protein factories of the cell because they are the sites of protein synthesis in any cell. These are common to prokaryotic and eukaryotic cells and are of two types as follows:

  • 70S - Found in prokaryotes. 

  • 80S - Found in eukaryotes. 

ribosomes

Mitochondria

It is called the ‘powerhouse of the cell’ because it is the site of ATP synthesis - the cell’s energy currency. It is found in all aerobic eukaryotes. It is absent in mature RBCs and prokaryotic cells.

mitochondria

Lysosomes

Lysosomes in a eukaryotic cell protects the cell by engulfing the foreign bodies entering the cell and helps in cell renewal. These are known as the cell’s suicidal bags since they contain hydrolytic enzymes that can destroy the cell components and degrade the cell under certain conditions.

lysosome

Chloroplasts

They are the primary organelles for photosynthesis. They contain the green pigment, chlorophyll, which helps in trapping the sunlight, required for photosynthesis. These are only present in photosynthetic plants and algae.

chloroplast

Practice Problems of Cells

  1.  The cell organelle which is responsible for extracting energy from carbohydrates to form ATP?
  1. Lysosome
  2. Ribosome
  3. Chloroplast
  4. Mitochondrion

Solution : Mitochondria is called the ‘powerhouse of the cell’ because it is the site of ATP synthesis - the cell’s energy currency. It is found in all aerobic eukaryotes.

  1. The plasma membrane is composed of _______. 
  1. phospholipids and integral proteins only
  2. phospholipids, extrinsic proteins, and intrinsic proteins
  3. phospholipids and hemicellulose
  4. phospholipids and oligosaccharides only

Solution : The plasma membrane is composed of phospholipids, extrinsic proteins, and intrinsic proteins. This is proposed by the fluid mosaic model. 

  1. This ribosome is engaged in protein synthesis in animal cells.
  1. Ribosomes of only cytosol
  2. Ribosomes which occur on nuclear membrane and ER
  3. Ribosomes of only mitochondria and cytosol
  4. Ribosomes of only nucleolus and cytosol

Solution : Ribosomes are the protein factories of the cell because they are the sites of protein synthesis in any cell. Ribosomes which occur on the nuclear membrane and endoplasmic reticulum are mainly engaged in protein synthesis. 

  1. Lysosomes are known as ‘suicidal bags’ because of the ________. 
  1. parasitic activity
  2. presence of food vacuole
  3. hydrolytic activity
  4. catalytic activity

Solution - Lysosomes in a eukaryotic cell protects the cell by engulfing the foreign bodies entering the cell and helps in cell renewal. These are known as the cell’s suicidal bags since they contain hydrolytic enzymes that can destroy the cell components and degrade the cell under certain conditions.

FAQs of Cells

Question 1. How do you explain a cell?

Answer. Cell is the structural and functional unit of life that makes up all living organisms and the tissues of the body. Any cell has three main parts: the cell membrane, the nucleus, and the cytoplasm. 

Question 2. What are the common cell structures or cell organelles?

Answer. A cell is composed of a large number of biomolecules. These biomolecules are enclosed in small subcellular compartments known as cell organelles. Parts common to all cells are the plasma membrane, the cytoplasm, ribosomes, and genetic material.

Question 3. Why are cells important in our life?

Answer. Cells are structural and functional units of living beings from microorganisms to humans. Cells are the smallest form of life. It is a complex biological machinery where the proteins, chemicals, and signals responsible for everything that happens inside our bodies is present.

Question 4. What is the most interesting fact about cell membranes?

Answer. Every cell (prokaryotic or eukaryotic) is surrounded by a thin membranous covering made up of lipids and proteins, known as the plasma membrane. It helps to communicate with the external environment in a regulated manner.

Related Topics to Cell in Biology

 

NCERT Class 11 Biology Chapters

The Living World Biological Classification Plant Kingdom
Animal Kingdom Morphology of Flowering Plants Anatomy of Flowering Plants
Structural Organization in Animals Cells: The Unit of Life Biomolecules
Cell Cycle and Division Transport in Plants Mineral Nutrition
Photosynthesis in Higher Plants Respiration in Plants Plant Growth and Development
Digestion and Absorption Breathing and Exchange of Gases Body Fluids and Circulation
Excretory Products and their Elimination Locomotion and Movement Neural Control and Coordination
Chemical Coordination and Integration

 

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