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Cell biology is defined as the branch of biology which deals with the study of cells. This branch comprises the study regarding shape, size, types, composition, division, and growth of the cells. A cell is the basic and fundamental unit of life, which acts as a building block of the organisms, hence it is crucial to study all the concepts related to cell biology thoroughly.
| Table of Content |
Keyterms: Cell, fundamental unit of life, Unicellular organism, Multicellular organisms, nucleus, organisms, organ
What is a Cell?
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All the organisms are composed of micrometre-sized blocks. These building blocks are termed as cells. Cells are the living building blocks that differentiate living organisms from non-living things. An organism can be made up of a single cell (a Unicellular organism) or many cells (Multicellular organisms). Cells are the fundamental and structural units of life.
Cells
Anton Von Leeuwenhoek was the first scientist who noticed and termed the unit as cell. The discovery of microscopes and other instruments then further helped to determine the structural components of the cell. Robert Hook later coined the term ‘nucleus’.
Levels of organisation
Cells combine together to form a tissue, tissues combine together to form an organ, organs together form an organ system, organ systems and rest of the components then form an organism. Hence, a cell is the basic unit of life.
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|---|---|---|
| Cell: Definition, Structure, Functions & Diagram | Parts of Cell | Endoplasmic Reticulum |
| Cell Organelles Types and Functions | What Are Tissues? | Discovery of Cells |
Components of a cell and their functions
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The cell comprises several smaller components termed the organelles and a fluid component called the cytoplasm. The cell comprises three major components, cell membrane, nucleus, and in between the two is the cytoplasm, embedded with the organelles.
Components of a cell
Plasma membrane/Cell membrane
The major action of the plasma membrane is to protect the cell. It prevents the entry of harmful foreign substances into the cell. The cell membrane is made up of lipids. They facilitate the transport of elements across the membrane, like ions, water molecules, etc.
Endoplasmic reticulum
Endoplasmic reticulum is of two types, rough endoplasmic reticulum, and smooth endoplasmic reticulum. The rough endoplasmic reticulum plays a crucial role in protein synthesis, and the smooth endoplasmic reticulum plays a crucial role in lipid and steroid synthesis.
Endoplasmic reticulum
Ribosomes
Ribosomes play a crucial role in the final step of protein synthesis.
Golgi apparatus
Golgi apparatus stores all the secretions modifies the secretions and packages them.
Mitochondria
It is defined as the powerhouse of the cell, it plays a crucial role in the final steps of aerobic respiration and ATP generation. This organelle also synthesises polysaccharides.
Mitochondria
Pinocytic Vesicles
They uptake the dissolved material from the environment.
Lysosomes
They are called the scavengers of the cell. They have enzymes to dissolve and dispose of the waste material.
Centrioles
They help in the division of the cells.
Microtubules
They help in the movement of the cells and provides physical support. They help in cell division and the movement of the chromosomes during cell division.
Microtubules
Microfilaments
They also help in the movement and support of the cells.
Nucleus
Comprises of two parts, nuclear membrane, and nucleolus. The nuclear membrane is a thin lipid membrane characterised by the presence of pores that act as channels to transport substances. The nucleus is the organelle that stores the genetic material in the form of chromatic fibres.
Nucleus
Cell wall
They are present in only the plant cells.
Types of cells
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Cells are of various types and can be classified on the basis of several categories. They can be eukaryotes, prokaryotes, animal cells, plant cells, different cells present in the body like smooth muscle cells, lymphocytes, red blood cells, macrophages, nerve cells, etc.
On the basis of evolution: Prokaryotes and Eukaryotes
Prokaryotes are the first cell that was originated, and eukaryotes have evolved from the prokaryotes. Prokaryotes are bacteria and archaea, eukaryotes are animals, plants, fungi, etc. There are several differences and similarities amongst them.
Prokaryotic cell and Eukaryotic cell
Key Differences
| Prokaryotes | Eukaryotes |
|---|---|
| Prokaryotes do not have a membrane-bound nucleus. | Eukaryotic cells have a membrane-bound nucleus. |
| The genetic material i.e. the DNA is bundled together in the nucleoid region, but it is not stored within any membrane-bound nucleus. | In the nucleus, eukaryotes store their genetic information |
| Prokaryotes have no membrane-bound organelles | Eukaryotes have membrane-bound organelles. |
| Prokaryotic DNA consists of molecules of double-stranded and circular DNA. | Eukaryotic DNA consists of several molecules of double-stranded linear DNA. |
Similarities: However, all the cells have these key components present in them.
- Genetic material, DNA
- Plasma Membrane
- Cytoplasm
- Ribosomes
Cells in different organisms: Animal Cell and Plant cell
The nature and components of the cells vary depending on the type of organism put under consideration. Plant cells and animal cells differ in a few components of the cells.
Animal Cell and Plant cell
The differences are mentioned below in the table:
| Animal Cell | Plant Cell |
|---|---|
| Cell wall is absent | A hard cell wall is present |
| The shape of an animal cell is round and irregular. | The shape of the plant cell is mostly rectangular and fixed. |
| Animal Cells do not have any chloroplast and plastids. | Plant cells have chloroplast which imparts them a green colour by the synthesis of chlorophyll and food. They also have plastids |
| Centrioles, flagella, and cilia are present in a few cells | Centrioles, flagella, and cilia are present in a few cells |
| Vacuoles are not present or are present in small numbers and sizes. | Large vacuoles are present in the animal cells. |
Different cells in the human body
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There are several different types of cells present in the human body. A few types of cells are mentioned below:
Stem Cells: They are totipotent and can be transformed into any type of cell.
Bone Cells: They are of two types, osteoblast and osteoclast. Osteoblastic cells form new bone cells and osteoclastic cells result in decomposition of the cells.
Different cells in the human body
Blood Cells: Red and white blood cells in the bloodstream are the blood cells, red blood cell carries the oxygen, and the white blood cells play a crucial role in the immunity of a person.
Muscle Cells: They are divided into three types: skeletal muscles, cardiac muscles, smooth muscle cells.
Fat Cells: They are also called adipose cells. They are present below the skin and around the organs. They store the fat.
Nerve Cells: Nerve cells or neurons are the most basic unit of the nervous system. Nerves send signals between the brain, spinal cord, and other body organs via nerve impulses. Structurally, a neuron consists of a cell body and nerve processes
Different cells in the human body
Endothelial Cells: They form the inner lining of different types of organ systems.
Sex cells: They are also called gametes. These cells are produced by the reproductive organs and have the ability to produce new organisms. They are produced by miosis.
Pancreatic cells: They are present in the pancreas. Pancreatic cells are important for regulating blood glucose concentration levels as well as for the digestion of proteins, carbohydrates, and fats.
Different cells in the human body
Cancer cells: Cancer results from the development of abnormal cell properties that cause cells to divide uncontrollably and spread to other locations. They behave so due to mutations.
Skin Cells: The skin is made of the topmost epidermal cell layer, followed by a dermal layer.
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Things to Remember
- Cells are the living building blocks that differentiate living organisms from non-living organisms.
- Cells combine together to form a tissue, tissues combine together to form an organ, organs together form an organ system, organ systems and rest of the components then form an organism.
- Plasma Membrane prevents the entry of harmful foreign substances into the cell.
- Endoplasmic reticulum is of two types, rough endoplasmic reticulum, and smooth endoplasmic reticulum.
- Ribosomes play a crucial role in the final step of protein synthesis.
- Golgi apparatus stores all the secretions modifies the secretions and packages them.
- Mitochondria is defined as the powerhouse of the cell.
- Lysosomes are called the scavengers of the cell.
- Centrioles help in the division of the cells.
- Microtubules help in the movement of the cells and provide physical support.
- The nucleus is the central part of the cell.
- Cells are divided into prokaryotes and Eukaryotes based on evolution.
- Plant cells have a cell wall and animal cells do not have it.
- There are different types of cells in the human body.
Previous Year Questions
- Which of the following cell organelles are called suicide bags ?...[BHU UET 2012]
- Lysosomes have acidic environment inside their vesicles due to….[BHU UET 2012]
- Which of the following process requires ATP ?...[BHU UET 2007]
- Histones are...[BHU UET 2007]
- Function of endoplasmic reticulum is..[BCECE 2006]
- In chloroplast which is most abundant...[BCECE 2006]
- Cell wall of a cell is removed, the remaining is called..[BCECE 2004]
- Which cell organelle is present in both prokaryotic and eukaryotic cell?..[BCECE 2004]
- Continuity of cytoplasm from cell to cell is maintained through...[COMEDK UGET 2012]
- Which of the following sequence is correct for the origin of lysosome?...[COMEDK UGET 2015]
- Protein synthesis take place in...[CUCET 2012]
- Fluid mosaic model of plasma membrane was proposed by...[COMEDK UGET 2005]
- Larger subunit of the ribosome helps in producing...[COMEDK UGET 2008]
- Wliich is characteristic feature of plant cell...[COMEDK UGET 2013]
- One of the major reasons for apoptosis is...[COMEDK UGET 2007]
Sample Questions
Ques. What is the difference between prokaryotes and eukaryotes? (2 marks)
Ans. They differ on several criteria eukaryotic cells have a membrane-bound nucleus and prokaryotic cells do not have it, prokaryotes are smaller in size, Prokaryotes have no membrane-bound organelles, Eukaryotic DNA consists of several molecules of double-stranded linear DNA, while that of prokaryotes is double-stranded and circular.
Ques. What is a nucleus? (2 marks)
Ans. The nucleus was termed by Robert Brown and it comprises of two parts, nuclear membrane, and nucleolus. The nuclear membrane is a thin lipid membrane characterised by the presence of pores that act as channels to transport substances. The nucleus is the organelle that stores the genetic material in the form of chromatic fibres.
Ques. Who first identified and observed the cell? (2 marks)
Ans. Anton Von Leeuwenhoek was the first scientist who noticed and termed the unit as a cell. The discovery of microscopes and other instruments then further helped to determine the structural components of the cell.
Ques. What are the different types of muscular cells? (2 marks)
Ans. Muscular cells are of three types cardiac cells, smooth muscle cells, skeletal muscle cells.
Ques. How are plant cells different from animal cells? (2 marks)
Ans. Plant cells have a definite shape, cell walls, plastids, chloroplasts, and large vacuoles which differentiate them from animal cells.
Ques. What are unicellular organisms and multicellular organisms? (2 marks)
Ans. Unicellular organisms are organisms comprising of one cell, e.g., bacteria, and multicellular organisms consist of more than one cell e.g., humans, plants, fungi.
Ques. What is a mesosome in a prokaryotic cell? Mention the functions that it performs. (2 marks)
Ans. Mesosome is a membranous structure in a prokaryotic cell, which is formed by the extensions of the plasma membrane into the cell in form of vesicles, tubules and lamellae. Mesosomes are equal to mitochondria in eukaryotes, as they perform aerobic cellular respiration in prokaryotes. It helps in DNA replication and the distribution of genetic material to daughter cells. Mesosomes also help in respiration, increase the surface area of the plasma membrane and enzymatic content and cell wall formation.
Ques. What is Mitochondria? What are its functions? (3 marks)
Ans. Mitochondria and chloroplast are double membrane-bound organelles. Mitochondria: Mitochondria are cylindrical or sausage-shaped cell organelles and contains two membranes, outer and inner. The inner compartment is called the matrix containing DNA, RNA, ribosomes, enzymes of Krebs cycle etc and the outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a number of infoldings called the cristae which increase the surface area. Oxysomes are present on the inner mitochondrial membrane. Mitochondria are semiautonomous organelles, i.e., have their own DNA and ribosomes.
Mitochondria
Functions of mitochondria:
- Mitochondria are essential for aerobic respiration.
- Mitochondria provide intermediates for the synthesis of important biomolecules such as chlorophyll, cytochrome, steroids etc.
- Mitochondria regulate the calcium ion concentration in the cell.
- The mitochondrial matrix contains enzymes for the synthesis of fatty acids.
- Synthesis of many amino acids takes place here
Ques. What is Chloroplast? What are its functions? (3 marks)
Ans. They are green coloured plastids that are disc-shaped. The space limited by the inner membrane of the chloroplast is called the stroma. Stroma has organised flattened membranous sacs called the thylakoids. Thylakoids are arranged in stacks called grana. The matrix of a chloroplast contains DNA, RNA, ribosomes and enzymes. Chloroplast is also a semiautonomous organelle.
Chloroplast
Functions of chloroplast:
- Photosynthesis is performed by chloroplast.
- Chloroplast stores starch grains.
- Maintains balance of CO2 concentration in the air.
- Keeps oxygen balance constant in the atmosphere by liberating O2 into the atmosphere, used during respiration and combustion.
Ques. What are the characteristics of prokaryotic cells? (3 marks)
Ans. Characteristics of prokaryotic cells are as follows:
- The prokaryotic cell is essentially a single–envelope system.
- Prokaryotes lack membrane-bound cell organelles.
- Prokaryotes have 70S ribosomes.
- DNA is naked and lies coiled in the cytoplasm. It is not covered by a nuclear membrane and is termed a nucleoid.
- Nuclear components, like, nuclear envelope, nucleolus, nucleoplasm are absent.
- The cell wall is present in bacteria and cyanobacteria, but absent in mycoplasma.
- Multiplies by asexual reproduction.
- Transcription and translation take place in the cytoplasm.
Ques. The cell is the basic unit of life. Discuss in brief. (3 marks)
Ans. The cell is the fundamental, structural and functional unit of life, as no living organism can have life without being cellular. All life begins as a single cell. An organism is either made of a single cell (unicellular) or many cells (multicellular). In a unicellular organism, a single cell is capable of independent existence and perform all essential functions of life, while in a multicellular organism, each group of cells is specialised for a specific function. Life passes from one generation to the next in form of cells, and new cells always arise from the division of pre-existing cells. Cells are totipotent, i.e., a single cell has the ability to form a whole organism. The activities of an organism are sum total of activities of its cells, therefore, a cell is the basic unit of life.
Ques. Both lysosomes and vacuoles are endomembrane ‘structures, yet they differ in terms of their functions. Comment. (2 marks)
Ans. Organelles of the endomembrane systems such as lysosomes and vacuoles function in close coordination with one another but are specialised to perform different functions. Lysosomes break down the ageing and dead cells, they help in the digestion of food as they contain hydrolytic digestive enzymes. They are involved in cell division also. Vacuoles on other hand, help in excretion and osmoregulation in Amoeba (contractile vacuole) or provides buoyancy, mechanical strength in prokaryotes (air vacuoles).
Ques. Describe the structure of the nucleus with the help of labelled diagrams. (3 marks)
Ans. Nucleus: Nucleus is a double membrane-bound principle cell organelle that contains all genetic information for controlling cellular metabolism and transmission of genetic information.
The nucleus is differentiated into the following four parts:
(a) Nuclear envelope: It is a double membrane-bound envelope that surrounds the nucleus and separates the latter from the cytoplasm.
Nuclear envelope
(b) Nucleoplasm: It is a clear, non-staining, fluid material present in the nucleus, which contains raw materials (nucleotides), enzymes (DNA/RNA polymerases) and metal ions for the synthesis of RNAs and DNA. The nuclear matrix or the nucleoplasm is composed of nucleolus and chromatin.
(c) Nucleolus: It is a naked, round and slightly irregular structure, which is attached to the chromatin at a specific region. It is a site for active ribosomal RNA synthesis.
(d) Chromatin: It has the ability to get stained with certain basic dyes. It is known to be the hereditary DNA protein fibrillar complex. The chromatin fibres are distributed throughout the nucleoplasm.
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