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Cell is the basic unit, which forms the structure and performs the essential function of any living organism. There is a description of what a cell consists of and two basic types of cells discovered. These are Eukaryotic and prokaryotic cells.
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Key Terms: Cell theory, chloroplast, prokaryotic cell, eukaryotic cell, thylakoids, membrane
What is a Cell?
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A cell is defined as the independent, smallest, basic, structural, and functional unit of a living organism. All living organisms, whether unicellular, multicellular, plant and animal all have cells that perform all the essential functions of the living organism. The cluster of cells forms the tissue, which together forms an organ and completes the organ system of multicellular organisms. Some important functions that a cell performs are:
- Transportation of substances.
- Helps in the metabolic process.
- Absorb nutrition for survival.
- Respond to the stimuli.
- Helps in reproduction, respiration and excretion.
Robert Hooke was the first one to discover cells in 1665, and Anton Von Leeuwenhoek was the one to discover the live cell in bacteria and protozoa. Later on, Robert Brown introduced the term nucleus.
Read More: Difference between Plasma Membrane and Cell Membrane
What is Cell Theory?
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Two scientists, Matthias Schleiden and Theodor Schwann proposed a cell theory for all living organisms. There were 3 main points:
- The cell is the structural and functional unit of a body.
- All plants and animals are made up of cells and their components.
- A new cell generates from the pre-existing cell (Explained by Rudolf Virchow in 1855).
Classification of Cell
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In most organisms, the cell comprises a cell membrane, cytoplasm, nucleus, and other cell organelles. However, there is a slight difference between animal and plant cells with respect to the components present and their function and anatomy.
Organisms have additional cell organelles, which take on important processes. Some maor classifications of cells and its building blocks are:
- Endoplasmic Reticulum
- Lysosomes
- Golgi Apparatus
- Mitochondria
- Microbodies
- Vacuoles
- Ribosomes
- Plastids
- Centrosome
What is Prokaryotic Cell?
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Prokaryotic cell is found in blue-green algae, PPLO, bacteria and mycoplasma. These types of cells reproduce faster and are smaller in size compared to eukaryotic cells. They are found in unicellular organisms in different shapes and sizes. There are no membrane-bound organelles present in this type of cell.
Even the nucleus is not present inside the cell; hence, the genetic component is not enclosed inside the nucleus. However, their structure may have a cell envelope, capsule, slime layer, mesosomes, vesicles, lamellae, tubules, filament, hook, cell wall, plasma membrane, basal bodies, pili, fimbriae, inclusion bodies, polysome, etc., for functions.
Eukaryotic Cells
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Organisms like plants, fungi, animals and protists, all exhibit eukaryotic cells. They are more complex and larger in size than prokaryotic cells. Moreover, the organelles and components of cells vary from organism to organism.
For example, cell walls and plastids are absent in animal cells but present in plant cells. The other example is centrioles, which are found in animal cells but lack in plant cells. The cell contains membrane-bound organelles, including the nucleus.
Previous Year Questions
- A cell organelle containing hydrolytic enzymes is … [NEET 2016]
- Water soluble pigments found in plant cell vacuoles are … [NEET 2016]
- Which of the following are not membrane-bound? … [NEET 2015]
- All types of plastids possess essentially the same structure because they … [NEET 1992]
- Flagella of prokaryotic and eukaryotic cells differ in … [NEET 2004]
- Besides giving out secretory vesicles … [NEET 1994]
Things to Remember
- Cell is the basic unit of an organism, which is responsible for the structural formation and function of an organism.
- The new cell arises from the pre-existing cells via asexual and sexual reproduction.
- Cells are of two types on the basis of structure and functions: Prokaryotic and Eukaryotic Cells.
- The plant cell and animal cell differentiate from each other.
- Microbodies are present in animal and plant cells.
- Nucleoplasm is the nucleus matrix containing chromatin and nucleolus.
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Sample Questions
Q1. Mention 4 essential functions of the cell. (3 marks)
Ans. The given below are the 4 essential functions that a cell performs, resulting in the development and growth of an organism:
- Helps in reproduction.
- Cells form the structure of an organism and simultaneously provide it optimum support.
- To regulate chemical processes like respiration or photosynthesis, cells provide energy for the process.
- The transportation of substances/nutrients like oxygen and carbon dioxide takes place through cells.
Q2. Who modified the cell theory and what was the modification? (2 marks)
Ans. Rudolf Virchow, 1855 introduced the advanced feature in the cell theory by explaining cell division. He gave the hypothesis of ‘Omnis cellula-e cellula,’ which means new cell formulates from the pre-existing cell.
Q3. Which cell organelle is known as the powerhouse of the cell? (1 mark)
Ans. Mitochondria are also known as the ‘Powerhouse of the Cell’ due to the generation of ATP molecules (Adenosine Triphosphate).
Q4. Name the different kinds of cells on the basis of structure and function. Mention 1 functional and structural difference between the two types. (2 marks)
Ans. There are two types of cells, namely, prokaryotic and eukaryotic cells. These are divided on the basis of structure and functions. Prokaryotic cell lacks a nucleus; however, a nucleoid is present instead of a nucleus. Also, eukaryotic cells reproduce via., asexual as well as sexual reproduction. But prokaryotic cells reproduce through asexual reproduction.
Q5. Define plastids. (2 marks)
Ans. Plastid is the double-membraned cell organelles, which mainly work as a storage house and manufacturer of food. This is usually present in the cells of euglenoids and plants. There are 3 types of plastids based on pigmentation, ie., chromoplasts, chloroplasts and leucoplasts.
Q6. Define prokaryotes. (2 marks)
Ans. Prokaryotes are unicellular organisms, having membrane-less organelles and a nucleoid instead of a nucleus. Bacteria, mycoplasma, PPLO (Pleuro Pneumonia Like Organisms) and blue-green algae. One can find bacteria in water bodies, soil, ocean water, etc. The size of prokaryotes is smaller than that of eukaryotes, ranging between 0.1 micrometers to 5 micrometers (μm).
Q7. What are the components of an endomembrane system? (3 marks)
Ans. The Endomembrane system is present in the eukaryotic cells. These are basically membrane-bound cell organelles present in an organism. Their functions are coordinated with each other. However, the peroxisomes, mitochondria and chloroplast are not part of this system. The given below are the various components present in the endomembrane system:
- Endoplasmic Reticulum (ER)
- Golgi Apparatus
- Vacuoles
- Lysosomes
Q8. With the help of the diagrams describe the anatomy of a chloroplast. (3 marks)
Ans. Chloroplasts are present in the mesophyll cells of the plant cell. The size of a chloroplast may range between width (2-4µm) and length (5-10µm) in the discoid, oval, lens, spherical or ribbon-like shapes. It is covered with two-layer membranes; namely, outer and inner membranes. The space covered by the inner membrane is filled with colorless fluid called the stroma. There are numerous coin-like flattened membranous sacs present known as thylakoids. A pile of thylakoids is called granum and, collectively, named intergranal thylakoid or grana. Chlorophyll is present in the thylakoids. Stroma lamellae is the tubular flat membrane that joins the granum.
Structure of Chloroplast
Q9. When was the cell discovered? (2 marks)
Ans. Robert Hooke, a British scientist, was the first one to recognize the cell on a cork slice. In 1655, the cell word was invented describing it as the basic, functional and structural unit present in any living organism. Later on, Robert Brown discovered the nucleus, followed by the formulation of cell theory by Matthias Schleiden & Theodore Schwann.
Q10. Which of the following is correct:
(1) Thylakoids are disc-shaped sacs in the Golgi apparatus.
(2) Cells of all living organisms have a nucleus.
(3) Virchow explained that cells are formed from pre-existing cells. (3 marks)
Ans. Option- 3
Explanation: Option 1- Thylakoids are a lens, oval, spherical, discoid, or ribbon-shaped sac present in the mesophyll cells in the leaves of plants.
Cisternae are the disc-shaped sacs in the Golgi apparatus.
Option 2- Prokaryotic cells don’t have a nucleus. Hence, not all living organisms’ cells have a nucleus.
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