Cell Theory – Definition, History, Principles, Modern Theory, Examples

What is Cell Theory?

• The realm of biology underwent a paradigm shift in the 19th century with the advent of advanced microscopy. Prior to this technological leap, the existence of microorganisms remained an enigma, leading to the presumption that individual organisms represented the fundamental units of life. However, the microscope unveiled a more intricate reality, revealing the cellular structure that underpins all living entities.
• The cell theory, a foundational tenet in biology, posits that all living organisms are composed of fundamental units known as cells. These cells, irrespective of their size or complexity, form the structural and functional basis of life.
• From the vastness of a frog egg to the minuteness of certain bacterial cells, the cellular composition remains a consistent feature. Notably, viruses stand as an exception, as they consist of reproducing DNA or RNA that may not always be encapsulated within a cellular structure.
• At its core, the cell theory elucidates the principles governing the constitution, structure, and function of organisms. It underscores the concept that every living being originates from a singular cell, a product of the fusion of cells from its progenitors.
• A cell, in its entirety, operates as an autonomous unit, capable of self-sustenance, deriving from a preceding cell, and in turn, producing subsequent cells.
• This scientific doctrine, which delineates the construction and functionality of living organisms, accentuates that every living entity is an assemblage of these fundamental life units – the cells. Furthermore, it emphasizes the continuity of life, highlighting that every cell traces its lineage back to a pre-existing cell, maintaining a consistent chemical composition throughout.
• The architects of this groundbreaking theory, Matthias Schleiden and Theodor Schwann, laid its foundational principles in the mid-19th century.
• Their work was later augmented by Rudolf Virchow, who posited that cells emerge exclusively from pre-existing cells, challenging the then-prevailing notion of spontaneous generation. Collectively, their contributions coalesced into the three cardinal principles of the cell theory, which remain instrumental in shaping our understanding of the biological world.

Definition of Cell Theory

The cell theory is a fundamental principle in biology that states all living organisms are composed of cells, cells are the basic units of life, and all cells arise from pre-existing cells.

3 Parts Of Cell Theory – The three principles of cell theory

The cell theory, a cornerstone in the realm of biological sciences, is underpinned by three cardinal principles. These principles collectively elucidate the significance, origin, and functionality of cells in all living organisms.

1. Constitution of Living Organisms by Cells:
   • Premise: All living entities, whether plant or animal, are composed of cells.
   • Elaboration: Through meticulous examination of plant tissues, Schleiden discerned that every plant is an assemblage of cells. Concurrently, Schwann’s observations on animal tissues revealed a similar cellular composition. Their synergistic findings cemented the idea that cells serve as the foundational building blocks for all forms of life.
2. Cells as the Fundamental Units of Life:
   • Premise: Cells are the quintessential structural and functional units in living organisms.
   • Elaboration: Schleiden and Schwann’s investigations transcended mere structural observations. They discerned that cells are dynamic entities, pivotal for executing specific biological functions. Every cellular activity, from metabolic processes to energy transduction, contributes to the organism’s holistic functionality.
3. Derivation of Cells from Pre-existing Cells:
   • Premise: Cells emerge from the division of their antecedent cells, negating the notion of spontaneous generation.
   • Elaboration: A critical observation made by both scientists was the non-spontaneous origin of cells. They identified that cells proliferate through a process termed cell division, ensuring the continuity of life. This principle underscores the cyclical nature of life, where every cell traces its lineage to a predecessor, facilitating growth, development, and propagation of living organisms.

Cell Theory Examples

1. Single-Celled Organisms

The cell theory, a cornerstone of biological sciences, posits that all living entities are composed of cells, which serve as the quintessential units of life, both structurally and functionally. This theory is not exclusive to multicellular entities but is equally pertinent to unicellular organisms. Let’s explore this through the lens of various single-celled organisms:

1. Bacteria:
   • Representing the vast realm of prokaryotes, bacteria are unicellular entities that encapsulate the essence of the cell theory. Each bacterial cell is a self-contained unit, equipped with essential structures such as a plasma membrane, DNA, ribosomes, and a cell wall. These cells undertake vital processes, from metabolism to reproduction. Notably, bacteria reproduce via binary fission, where a single cell divides to yield two genetically identical offspring. This process exemplifies the principle that cells arise from pre-existing cells. Thus, bacteria stand as living testimonies to the cell theory, emphasizing its foundational role in biology.
2. Archaea:
   • Archaea, akin to bacteria, are unicellular prokaryotes. However, they distinguish themselves with unique molecular and genetic features. Each archaeal cell is a holistic entity, encompassing all vital structures for life. These cells engage in metabolic activities, reproduce via binary fission, and adapt to their surroundings. By embodying the principles of the cell theory, archaea accentuate the theory’s universal applicability across diverse life forms.
3. Protists:
   • Protists, a diverse assemblage of eukaryotic unicellular organisms, further underscore the cell theory’s significance. Organisms like Paramecium and Amoeba showcase intricate cellular architectures, replete with specialized organelles such as contractile vacuoles and locomotory structures like cilia or pseudopods. Each protist cell is a self-sufficient unit, underscoring the theory’s assertion that the cell is the fundamental unit of life.
4. Yeast:
   • Yeast, a subset of unicellular fungi, offers a compelling illustration of the cell theory. Each yeast cell is a self-reliant entity, housing critical organelles like the nucleus, mitochondria, and endoplasmic reticulum. These cells partake in metabolic activities, ensuring energy flow and vital cellular functions. Furthermore, yeast reproduction, particularly budding, epitomizes the principle that new cells arise from pre-existing ones. Thus, yeast serves as a tangible demonstration of the cell theory in action, emphasizing its pivotal role in elucidating the nature of life.

In summation, single-celled organisms, spanning bacteria to yeast, not only validate the cell theory but also illuminate its profound implications in understanding the intricate tapestry of life.

2. In Plants – Cell Theory in Botany

The realm of botany and the foundational cell theory share a rich, intertwined history, tracing back to the seminal observations of Robert Hooke in the 17th century. Hooke’s pioneering examination of cork slices under the microscope led him to discern tiny, box-like compartments, which he aptly termed “cells” due to their resemblance to small chambers. This revelation not only marked the dawn of cellular biology but also set the stage for a deeper understanding of plant physiology.

Plants and the Cell Theory: An Inextricable Bond Hooke’s initial observations of cork cells paved the way for the realization that the entirety of the plant kingdom is cellular in nature. The cell theory, with its assertion that all living organisms are composed of cells, finds robust validation in plant biology. This theory elucidates the intricate architecture, growth patterns, developmental processes, and reproductive mechanisms of plants.

1. Structural and Functional Units:
   • Every facet of a plant, from its towering stems to its delicate leaves, is a testament to the cell theory. Delving into plant tissues, one encounters a myriad of cell types, each tailored for a specific function. For instance, the verdant leaves house photosynthetic cells adept at capturing sunlight, while the sprawling roots are replete with specialized cells adept at absorbing water and minerals.
2. Growth and Development:
   • The cell theory offers profound insights into the growth dynamics of plants. As plants burgeon in stature, this growth is attributed to the meticulous division and differentiation of their constituent cells. Processes like mitosis facilitate the genesis of new cells, which are instrumental in plant growth and tissue regeneration.
3. Reproductive Mechanisms:
   • The cell theory’s tenets extend seamlessly to the reproductive strategies of plants. In the realm of sexual reproduction, the fusion of male and female gametes (reproductive cells) culminates in the formation of a zygote. This zygote, a fertilized cell, adheres to the principles of the cell theory, eventually blossoming into a nascent plant. This reproductive cycle embodies the three cardinal principles of the cell theory: cellular composition, functionality, and origin.

In conclusion, the cell theory, with its universal principles, finds profound resonance in plant biology. From the rudimentary observations of cork cells by Hooke to the intricate cellular processes that govern modern botany, the cell theory remains an indomitable pillar, guiding our understanding of the verdant tapestry of plant life.

3. In Animals – Cell Theory in Zoology

The principles of cell theory, while universally applicable across all living organisms, find profound significance in the realm of zoology. This theory, foundational to biology, offers invaluable insights into the intricate architecture, physiological processes, and developmental dynamics of animals. The vast spectrum of animal species, from the tiniest insects to the majestic blue whales, are all bound by the unifying tenets of the cell theory.

Animals and the Cell Theory: A Symbiotic Relationship

1. Structural and Functional Paradigm:
   • Animals, as multicellular entities, are composed of a myriad of specialized cells, each tailored for a distinct function. The intricate neural networks, for instance, owe their functionality to nerve cells, while the rhythmic contractions of the heart are orchestrated by muscle cells. Each cell type, with its unique role, collectively ensures the seamless operation of the animal organism.
2. Growth and Development:
   • The cell theory elucidates the developmental trajectory of animals. Beginning as a solitary fertilized egg, the embryonic journey involves a series of meticulous cell divisions. This cascade of cellular proliferation and differentiation culminates in the genesis of diverse cell types, tissues, and organs, sculpting a fully developed animal. The theory also underscores the pivotal role of mitosis in animal growth, tissue regeneration, and overall maintenance. In the face of injuries, cellular divisions are accelerated to replenish damaged tissues, underscoring the animal’s innate resilience and adaptability.
3. Physiological Implications:
   • The cell theory extends its relevance to the physiological processes that govern animal life. At the cellular level, myriad functions such as cellular respiration, digestion, and metabolism are executed. These processes are instrumental in harnessing energy from ingested nutrients, thereby fueling the myriad life processes that sustain animals.

In summation, the cell theory stands as a beacon in zoology, illuminating our understanding of the cellular underpinnings of animal life. From the cellular intricacies that dictate physiological processes to the developmental dynamics that shape animal growth, the cell theory remains an indomitable pillar in the scientific exploration of the animal kingdom.

4. Other Organisms

The vast tapestry of life on Earth is woven with cells, the quintessential units of life. From the towering sequoias to the minuscule amoeba, the principles of cell theory remain consistent. However, as we delve deeper into the realm of biological diversity, certain organisms present intriguing deviations from the conventional understanding of cell theory.

1. Fungi: The Cellular Bridge:
   • Fungi, an intriguing kingdom of organisms, serve as a cellular nexus between plants and animals. While they are devoid of chlorophyll-bearing chloroplasts, a hallmark of plants, they do possess rigid cell walls. This unique cellular architecture positions fungi as a transitional entity, bridging the cellular characteristics of both plants and animals.
2. Viruses: The Cellular Enigma:
   • Venturing beyond the conventional cellular landscape, we encounter viruses, the enigmatic entities that challenge the traditional boundaries of cell theory. Encased in a protein sheath, viruses harbor genetic material, either DNA or RNA. Their ambiguous status in the realm of life has sparked debates among scientists. While some argue that their lack of cellular machinery excludes them from the living world, others contend that their ability to reproduce, albeit within a host cell, qualifies them as living. Regardless of their classification, viruses undeniably represent an exception to the standard cell theory. Their replication, contingent upon hijacking the cellular apparatus of a host, underscores their unique biological strategy.
3. Universality of Cell Theory:
   • Despite the intriguing exceptions and nuances, the cell theory stands robust across the vast spectrum of organisms. From the simplest prokaryotes to the most complex eukaryotes, and spanning unicellular to multicellular entities, the foundational principles of cell theory remain unchallenged. Each organism, in its cellular intricacy, reaffirms the theory’s universal applicability.

In essence, while the cell theory provides a cohesive framework for understanding the cellular underpinnings of life, the diverse array of organisms, with their unique cellular configurations, enriches our understanding of life’s complexity and adaptability.

Contributions To Cell Theory

The cell theory, a cornerstone of modern biology, has been shaped and refined by numerous scientific luminaries over the centuries. Their collective contributions have provided a comprehensive understanding of the cellular basis of life, paving the way for groundbreaking discoveries in various biological disciplines. Here, we chronicle the significant milestones and the scientists behind them that have sculpted the cell theory.

1. Robert Hooke (1665):
   • A pioneer in microscopy, Hooke was the first to observe and describe the cellular structure in cork. His seminal work led to the term “cells”, laying the groundwork for future cellular studies.
2. Antonie van Leeuwenhoek (1674):
   • With his enhanced microscopes, Leeuwenhoek unveiled the world of living cells, including bacteria and protists. His observations marked the discovery of bacteria, expanding the horizons of cellular biology.
3. Matthias Schleiden (1838) & Theodor Schwann (1839):
   • Schleiden posited that all plants are cellular in nature. Schwann extended this concept to the animal kingdom, asserting that animal tissues are cellular. Together, their insights formed the foundational pillars of the cell theory.
4. Rudolf Virchow (1855):
   • Virchow’s assertion that “all cells arise from pre-existing cells” crystallized the principles of cell theory, emphasizing the continuity of life at the cellular level.
5. Carl Zeiss and Ernst Abbe (Late 1800s):
   • Their advancements in microscopy, particularly the modern compound microscope, revolutionized cellular studies, offering unprecedented clarity and detail.
6. Camillo Golgi (1873) & Santiago Ramón y Cajal (Late 1800s – Early 1900s):
   • While Golgi unveiled the eponymous Golgi apparatus, Cajal’s work illuminated the cellular nature of the nervous system, emphasizing the discrete nature of neurons.
7. Robert Brown (1831):
   • Brown’s identification of the cell nucleus underscored its significance as the repository of genetic material in eukaryotic cells.
8. James Watson and Francis Crick (1953):
   • Their elucidation of the double-helix structure of DNA unraveled the molecular code underpinning cellular function and inheritance.
9. Lynn Margulis (1967):
   • Margulis introduced the endosymbiotic theory, suggesting a symbiotic origin for organelles like mitochondria, reshaping our understanding of eukaryotic cell evolution.
10. Rita Levi-Montalcini (1986) & Shinya Yamanaka and John Gurdon (2012):
    • While Levi-Montalcini unveiled the role of nerve growth factor in cellular differentiation, Yamanaka and Gurdon’s work on cellular reprogramming heralded new frontiers in regenerative medicine.

In conclusion, the cell theory, while seemingly straightforward, is the culmination of centuries of rigorous scientific inquiry. The contributions of these trailblazing scientists have not only elucidated the cellular nature of life but have also paved the way for myriad discoveries that continue to shape the landscape of modern biology.

Viruses and the Cell Theory

Viruses, with their unique biological characteristics, present a conundrum in the realm of cell theory. These entities, while not fitting neatly into the conventional definitions of life, play pivotal roles in the biological tapestry, influencing everything from evolutionary trajectories to medical research. Here, we explore the nuances of viruses and their relationship with the foundational principles of cell theory.

1. Acellular Nature:
   • Unlike traditional living organisms, viruses lack the quintessential cellular structures that define life. Their absence of organelles, cellular membranes, and an independent metabolic system raises intriguing questions about their classification within the biological hierarchy.
2. Masters of Genetic Manipulation:
   • Despite their acellular status, viruses possess an uncanny ability to commandeer the machinery of host cells. Armed with their genetic material, either DNA or RNA, they serve as templates for their own replication, effectively turning host cells into viral factories.
3. Dependence on Hosts:
   • Viruses epitomize the concept of “obligate intracellular parasites.” Devoid of the means to execute vital life processes autonomously, they are inextricably dependent on host cells for their survival, propagation, and evolution.
4. Mechanisms of Host Invasion:
   • Evolution has equipped viruses with specialized mechanisms to breach host defenses. By binding to specific receptors on host cell surfaces, they gain entry, subsequently harnessing the cell’s resources to churn out new viral progeny.
5. The Living vs. Non-Living Debate:
   • The very nature of viruses straddles the boundary between life and non-life. This has spurred ongoing debates among scientists. While some argue for their classification as “biological entities” that defy traditional definitions, others contend that their reliance on host cells for basic functions precludes them from being deemed truly alive.

In summation, viruses, in their defiance of the traditional tenets of cell theory, underscore the complexity and diversity of life. Their unique attributes and interactions with host cells offer invaluable insights, prompting a reevaluation of established biological paradigms and enriching our understanding of the vast molecular landscape.

Drawbacks of classical cell theory – Limitations of Classical Cell Theory

Classical cell theory, while groundbreaking in its time, has certain limitations that have been highlighted with the advancement of scientific knowledge. The theory, as proposed by Schleiden and Schwann, provided a foundational understanding of cellular biology. However, several aspects of cellular life were not adequately addressed or explained by this theory. Here are the primary drawbacks:

1. Viruses and the Cell Theory:
   • Classical cell theory posits that the function of an organism is a coordinated activity of its cells. However, this principle is not applicable to viruses. Unlike typical cells, viruses remain inert outside a host and require the cellular machinery of a host organism (be it bacteria, plants, or animals) to complete their life cycle. Thus, viruses challenge the traditional understanding of cell theory.
2. Prokaryotic Cells:
   • Prokaryotic cells, characterized by the absence of a well-defined nucleus, present another challenge to the classical cell theory. The theory emphasizes the presence of a distinct nucleus in cells, but prokaryotes defy this characterization.
3. Multinucleate Condition:
   • The classical cell theory asserts that each cell contains a single nucleus. However, certain cells, such as tapetal cells, exhibit a multinucleate or coenocytic condition, contradicting this aspect of the theory.
4. Cells Lacking a Nucleus:
   • The theory’s emphasis on the presence of a nucleus in every cell is further challenged by specific cell types that lack a nucleus altogether. Notable examples include human red blood cells (RBCs) and the sieve tubes of angiosperms.

In conclusion, while the classical cell theory laid the groundwork for understanding cellular biology, it is not without its limitations. These shortcomings underscore the importance of continuous scientific inquiry and the evolution of theories in light of new discoveries.

Modern cell theory

The understanding of cellular biology has undergone significant evolution, culminating in the formulation of the modern cell theory. This theory, a product of collaborative scientific endeavors, serves as the cornerstone for comprehending the fundamental principles governing cellular life.

1. Foundational Principles:
   • Modern cell theory posits that all living entities are composed of cells, which are the quintessential structural and functional units. Furthermore, it emphasizes the principle of biogenesis, asserting that all cells originate from pre-existing cells.
2. Key Postulates:
   • Cellular Composition: All living organisms, whether unicellular or multicellular, are constituted by cells and their by-products.
   • Fundamental Unit: The cell stands as the primary structural and functional unit, underpinning the myriad activities of living organisms.
   • Origins of Cells: Rudolf Virchow’s seminal declaration, “Omnis cellula-e-cellula,” underscores the principle that every cell arises from another pre-existing cell.
   • Cellular Activity: The overall activity of an organism is intrinsically tied to the independent actions of its constituent cells.
   • Metabolic Processes: Cells are the epicenters of energy flow, with intricate metabolic and biochemical reactions unfolding within their confines.
   • Genetic Continuity: Cells harbor genetic information within DNA molecules and chromosomes. This genetic blueprint is faithfully transmitted from one cell generation to the next.
3. Accepted Tenets of Modern Cell Theory:
   • Universality of Cells: All recognized living entities, irrespective of their complexity, are cellular in nature.
   • Cellular Division: Every living cell is a progeny of a pre-existing cell, emerging through the process of cellular division.
   • Cell as a Functional Entity: Beyond being a structural unit, the cell is pivotal in orchestrating the diverse functions that sustain life.
   • Independence of Cellular Activity: The collective actions of individual cells dictate the overall behavior and activity of an organism.
   • Cellular Biochemistry: The cell is the nexus of metabolic activities, where energy transformations and biochemical pathways are executed.
   • Genetic Repository: Cells enshrine genetic information, primarily in the form of DNA and RNA, ensuring the continuity of genetic traits.
   • Chemical Homogeneity: Within organisms of a similar species, cells exhibit a remarkable consistency in their chemical composition.

In essence, the modern cell theory offers a comprehensive framework for understanding the cellular underpinnings of life. It encapsulates the principles that govern cell structure, function, and inheritance, providing invaluable insights into the intricate tapestry of biological existence.

Cell Theory Key Points

Cell theory is a cornerstone of biological sciences, providing a foundational understanding of the cellular basis of life. Here are the key points encapsulating the essence of cell theory:

1. Cellular Composition:
   • All living organisms are composed of cells and their by-products. This cellular composition underscores the universality of cells across diverse life forms.
2. Structural and Functional Units:
   • Cells serve as both the structural and functional units in living organisms. Each cell is characterized by a protoplasmic mass, which houses vital organelles, a nucleus, and is enclosed by a protective membrane.
3. Basis of Life’s Activities:
   • The collective activities of an organism are essentially the aggregated functions of its individual cells. In essence, life’s processes are cellular processes.
4. Life’s Existence in Cells:
   • All vital activities, from metabolism to reproduction, occur within cells, making them the very crux of life.
5. Growth Mechanism:
   • An organism’s growth is intrinsically linked to the growth and multiplication of its constituent cells.
6. Genetic Information:
   • Cells are the repositories of genetic information. This information, encoded in DNA, governs the characteristics and functions of cells and, by extension, the organism.
7. Continuity of Life:
   • The perpetuation of life from one generation to the next is facilitated through cells. The living cell serves as the conduit for transmitting life.
8. Cellular Reproduction:
   • New cells originate from pre-existing cells via cellular division. This process ensures that the genetic information in the offspring cells mirrors that of the parent cell, maintaining genetic consistency.

In summary, cell theory offers profound insights into the cellular nature of life, emphasizing the centrality of cells in structure, function, and the perpetuation of life.

Importance of Cell Theory

The cell theory is a foundational concept in biology, and its importance cannot be overstated. Here are some of the key reasons why cell theory is crucial:

1. Fundamental Understanding of Life: Cell theory provides the basic understanding that the cell is the fundamental unit of life. All living organisms, from the simplest bacteria to the most complex multicellular organisms, are composed of cells.
2. Basis for Biological Research: Understanding that all life is cellular in nature has paved the way for advancements in various biological disciplines, from genetics to physiology.
3. Medical Advancements: Knowledge of cellular processes and structures has led to significant advancements in medicine, including the development of treatments for diseases, understanding cellular pathologies, and advancements in regenerative medicine.
4. Evolutionary Insights: The cellular basis of life provides insights into evolutionary processes. By studying cells, scientists can trace the evolutionary history of organisms and understand the shared ancestry of life.
5. Biotechnological Applications: The principles of cell theory are applied in biotechnology, where cells are used to produce a variety of products, from insulin to biofuels.
6. Understanding Growth and Development: Cell theory helps explain how organisms grow and develop. Growth results from the proliferation of cells, and development involves specific sequences of cellular differentiation.
7. Explains Genetic Continuity: The principle that new cells arise from pre-existing cells ensures the continuity of genetic information from one generation to the next.
8. Framework for Further Discoveries: The acceptance of the cell theory paved the way for the discovery of the cell membrane, cytoplasm, organelles, and the nucleus, leading to a deeper understanding of cellular functions.
9. Microbial Understanding: Cell theory has also been fundamental in understanding the microbial world, leading to the development of microbiology as a discipline.
10. Therapeutic Applications: Stem cell research, which holds promise for treating various diseases and injuries, is based on understanding cellular differentiation and the principles of cell theory.

In summary, the cell theory has been instrumental in shaping our understanding of life and has had vast implications in various fields of biology and medicine. It provides a unifying theme that underscores the continuity and diversity of life.

Quiz

Who is credited with coining the term “cells” after observing cork under a microscope?
a) Antonie van Leeuwenhoek
b) Matthias Schleiden
c) Theodor Schwann
d) Robert Hooke

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Which scientist proposed that all plants are composed of cells?
a) Robert Hooke
b) Matthias Schleiden
c) Theodor Schwann
d) Rudolf Virchow

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The principle that “all cells arise from pre-existing cells” was formulated by:
a) Robert Hooke
b) Theodor Schwann
c) Antonie van Leeuwenhoek
d) Rudolf Virchow

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Which of the following is NOT a tenet of the classical cell theory?
a) All organisms are composed of cells.
b) The cell is the basic unit of life.
c) Cells can spontaneously generate.
d) Cells arise from pre-existing cells.

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Which organism challenges the classical cell theory due to its acellular nature?
a) Bacteria
b) Fungi
c) Viruses
d) Algae

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Which cell component is responsible for housing genetic material in eukaryotic cells?
a) Cytoplasm
b) Cell membrane
c) Mitochondria
d) Nucleus

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Who extended the cell theory to animals?
a) Robert Hooke
b) Matthias Schleiden
c) Theodor Schwann
d) Rudolf Virchow

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Which process is responsible for the formation of new cells?
a) Photosynthesis
b) Respiration
c) Mitosis
d) Digestion

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Which of the following cells lack a well-defined nucleus?
a) Eukaryotic cells
b) Animal cells
c) Plant cells
d) Prokaryotic cells

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The statement “Omnis cellula-e-cellula” means:
a) Cells are the basic unit of life.
b) All cells have a nucleus.
c) All existing cells arise from pre-existing cells.
d) Cells can spontaneously generate.

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FAQ

References

1. Biology Online. (n.d.). Cell theory. Retrieved from https://www.biologyonline.com/dictionary/cell-theory#Contributions_To_Cell_Theory
2. Biology Dictionary. (n.d.). Cell theory. Retrieved from https://biologydictionary.net/cell-theory/
3. Unacademy. (n.d.). Modern cell theory. Retrieved from https://unacademy.com/content/neet-ug/study-material/biology/modern-cell-theory/
4. GeeksforGeeks. (n.d.). What is the cell theory? Retrieved from https://www.geeksforgeeks.org/what-is-the-cell-theory-definition-history-characteristics-importance/
5. Vedantu. (n.d.). Cell theory. Retrieved from https://www.vedantu.com/biology/cell-theory
6. BYJU’S. (n.d.). Cell theory: An overview. Retrieved from https://byjus.com/biology/cell-theory-an-overview/
7. Wikipedia. (n.d.). Cell theory. Retrieved from https://en.wikipedia.org/wiki/Cell_theory
8. ThoughtCo. (n.d.). Cell theory. Retrieved from https://www.thoughtco.com/cell-theory-373300
9. Aakash Institute. (n.d.). Cell theory and its modification. Retrieved from https://www.aakash.ac.in/important-concepts/biology/cell-theory-and-its-modification
10. OpenStax. (n.d.). Foundations of modern cell theory. Retrieved from https://openstax.org/books/microbiology/pages/3-2-foundations-of-modern-cell-theory

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