# Cell (Biology): Definition, Structure, Types, Functions and Cell Theory > Cell is the basic unit of life. Explore its structure, organelles, major types, functions, cell theory, division, and differences across organisms. Canonical URL: https://biologynotesonline.com/cell/ Author: Sourav Pan Last updated: October 3, 2026 ![Cell (Biology): Definition, Structure, Types, Functions and Cell Theory](https://biologynotesonline.com/wp-content/uploads/2024/10/Cell-Structure-Characteristics-Types-Size-and-Shape-Functions.jpg) A cell is the basic structural and functional unit of all living organisms. It is the smallest unit of life which is able to carry out all the fundamental processes such as nutrition, growth, metabolism, and reproduction. Life does not exist below this level. A virus is not considered a cell, as it cannot replicate on its own and depends on a host for its multiplication. A single cell can be a complete organism in itself. Bacteria are made up of one cell only, whereas the human body is made up of a very large number of cells. Cells are also different in their size, shape, and internal complexity. A bacterial cell and a plant cell do not look alike, still all cells carry out the same basic activities of life. All cells have a few common components. The cell membrane (plasma membrane) is the outer covering of the cell which separates its interior from the outside environment. Inside it, the cytoplasm is present, in which all the other components of the cell remain suspended. Every cell carries its genetic material in the form of deoxyribonucleic acid (DNA). Ribosomes are present in all cells, which are used for protein synthesis. But all cells do not have a nucleus. In bacteria, the DNA lies freely in the cytoplasm, whereas in plant and animal cells, it remains inside a membrane-bound nucleus. Depending on these differences, cells are kept under two broad categories- prokaryotic cells and eukaryotic cells. ## Characteristics of Cells Cells show some common characters which are found in all living organisms. Some of the important characteristics of cells are as follows- - All living organisms are made up of one or more cells. A cell is the smallest unit of living matter, and life does not exist below this level. - New cells arise from pre-existing cells only. A cell divides to form two daughter cells. During division, the genetic material of the cell is copied and passed on to both of them. - Every cell is surrounded by a cell membrane (plasma membrane), which separates its interior from the outside environment. Inside it, the cytoplasm is present, in which the other components of the cell remain suspended. - All cells contain their genetic material in the form of deoxyribonucleic acid (DNA) and ribosomes, which are used for protein synthesis. - Most cells are very small. They cannot be seen with the naked eye, so a microscope is used to study them. The small size keeps the surface area of the cell large in comparison to its volume, which helps in an efficient exchange of nutrients and wastes with the environment. - Cells are different in their size, shape, structure and function. - Some cells remain single throughout life, whereas others become specialised, such as a muscle cell or a nerve cell. - Each cell carries out its own metabolism. Energy is processed within the cell and is used for different activities like growth, repair and reproduction. - A cell responds to stimuli from its surrounding environment. Plant cells bend towards a source of light, for example. - The internal conditions of a cell are regulated and kept stable. This is referred to as homeostasis. - Cells grow in size and also divide. In some organisms, a single cell is able to give rise to the complete organism by repeated division. - Depending on the presence or absence of a true nucleus, cells are divided into two broad categories- prokaryotic cells and eukaryotic cells. Bacterial cells lack a membrane-bound nucleus, whereas plant and animal cells possess a well-organised nucleus. ## Types of Cells Cell types are broadly divided into two main cellular organizations, prokaryotic cells and eukaryotic cells. The major difference is the nucleus. Prokaryotic cells do not contain a membrane-bound nucleus and membrane-bound organelles, whereas eukaryotic cells have a nucleus and different membrane-bound organelles. Both contain a plasma membrane, cytoplasm, deoxyribonucleic acid (DNA) and ribosomes. ![Side-by-side comparison of prokaryotic and eukaryotic cells showing shared membrane, cytoplasm, DNA and ribosomes, with a nucleus and membrane-bound organelles only in the eukaryotic cell.](https://biologynotesonline.com/wp-content/uploads/2024/10/Prokaryotic-vs-Eukaryotic-Cell-Structure-1024x768.webp)Side-by-side comparison of prokaryotic and eukaryotic cells showing shared membrane, cytoplasm, DNA and ribosomes, with a nucleus and membrane-bound organelles only in the eukaryotic cell. ### 1. Prokaryotic Cells These are cells in which DNA is not enclosed within a nuclear membrane. The DNA is present in a region of the cytoplasm called the "nucleoid". Membrane-bound organelles are absent. Prokaryotic organisms include Bacteria and Archaea, and they are unicellular forms. [Prokaryotic cells](https://biologynotesonline.com/prokaryotic-cells/) are generally smaller and have a simpler internal organization than eukaryotic cells. They still carry out the basic cellular activities with the structures present in the cell. ### 2. Eukaryotic Cells The DNA is enclosed within a membrane-bound nucleus. These cells contain various membrane-bound organelles which form separate compartments inside the cytoplasm. Eukaryotic cells are generally larger and structurally more complex. Animal cells and plant cells are [eukaryotic cells](https://biologynotesonline.com/eukaryotic-cell/). The cells of fungi and protists are also eukaryotic, although their structure, shape and specialized cellular components can differ considerably. Lists mentioning exactly 10 or 11 types of cells usually represent selected examples of specialized cells, rather than the two broad cellular organizations. Nerve cells, muscle cells, epithelial cells and different blood cells are examples of specialized animal cells, while guard cells and root hair cells are examples found in plants. ## Structure of a Cell The structure of a cell is built up of many small components, which are as follows- ![Comparison of animal and plant cell structures showing shared organelles such as the nucleus, ER, Golgi and mitochondria, plus the plant cell wall, chloroplasts and large central vacuole.](https://biologynotesonline.com/wp-content/uploads/2024/10/Animal-and-Plant-Cell-Structure-and-Organelles-1024x768.webp)Comparison of animal and plant cell structures showing shared organelles such as the nucleus, ER, Golgi and mitochondria, plus the plant cell wall, chloroplasts and large central vacuole. - Every cell is enclosed by a [cell membrane](https://biologynotesonline.com/cell-membrane-plasma-membrane-structures-and-functions/) (plasma membrane). It is made up of a lipid bilayer and separates the interior of the cell from the outside environment. The exchange of materials between the cell and its surroundings also takes place through this membrane. - The cytoplasm is the entire region present between the plasma membrane and the nuclear envelope. It consists of a jelly-like fluid, which is referred to as the cytosol, in which the organelles remain suspended. - The genetic material of the cell is present in the form of deoxyribonucleic acid (DNA). In eukaryotic cells, it is enclosed within a membrane-bound nucleus, whereas in prokaryotic cells, it lies freely in the cytoplasm. The nucleus is generally the most prominent organelle of the cell. A dense area inside it, the nucleolus, is the site where ribosomes are assembled. - [Ribosomes](https://biologynotesonline.com/ribosomes-structure-and-functions/) are small particles made up of proteins and ribonucleic acid (RNA). They are found freely in the cytoplasm or attached to the surface of the endoplasmic reticulum, and protein synthesis takes place on them. - The [endoplasmic reticulum (ER)](https://biologynotesonline.com/endoplasmic-reticulum/) is a network of membranes which is continuous with the nuclear envelope. Two types are present in the cell. Rough ER carries ribosomes on its surface, whereas smooth ER does not have ribosomes and is mainly used for lipid synthesis and detoxification. - The Golgi apparatus receives, modifies and packages lipids and proteins for distribution inside or outside the cell. - [Mitochondria](https://biologynotesonline.com/mitochondria/) are double membrane-bound organelles. Cellular respiration takes place in them, and they are responsible for the production of most of the adenosine triphosphate (ATP) of the cell. Due to this, mitochondria are also called the "powerhouse of the cell". - In plant cells, chloroplasts are present, in which photosynthesis takes place. - Lysosomes are the digestive organelles of the cell. They contain hydrolytic enzymes which break down different macromolecules. Most plant cells do not have lysosomes. - Peroxisomes hydrolyse fatty acids, amino acids and some toxins. - Vacuoles are the storage and transport compartments of the cell. Plant cells contain a large central vacuole, which stores water and also helps in enlarging the cell. - The [cytoskeleton](https://biologynotesonline.com/cytoskeleton/) is a network of protein fibres spread in the cytoplasm. It maintains the shape of the cell, holds the organelles in position and also helps in the movement of the cell. - Outside the plasma membrane, a rigid cell wall is present in plant cells, fungal cells and bacterial cells. Animal cells do not have a cell wall. It gives protection to the cell and also maintains its shape. - Animal cells also contain a centrosome with centrioles, which is absent in most plant cells. ## Cell Theory The [cell theory](https://biologynotesonline.com/cell-theory/) is a fundamental theory of biology, which explains the role of the cell in all living organisms. The theory was formulated in the years 1838-39. All modern biology is based on this theory, and it is accepted even today. ![Scientific figure showing the three principles of classical cell theory and a timeline from Hooke](https://biologynotesonline.com/wp-content/uploads/2024/10/Cell-Theory-Principles-and-Historical-Development-819x1024.png)Scientific figure showing the three principles of classical cell theory and a timeline from Hooke's 1665 cork observations through Leeuwenhoek, Schleiden, Schwann, Remak and Virchow. ### The Three Principles of Classical Cell Theory The classical cell theory consists of three principles, which are as follows- - All living organisms are made up of one or more cells. No life exists without a cell. - The cell is the basic unit of structure and function in all living organisms. The body of an organism is built up from cells only. - Every new cell is produced from a pre-existing cell only. ### Discovery of Cells and Development of Cell Theory The cell theory did not come in one day. Almost 200 years passed between the first observation of cells and the final theory, the contributions being as follows- - Robert Hooke first observed the cells in the year 1665. He saw a thin section of cork under a microscope of his own design and found small box-like chambers, which he named "cells" in his book Micrographia. These were dead cells. - Antonie van Leeuwenhoek, in 1675, observed living organisms in a drop of rain water with the help of a simple microscope made by himself. These single-celled organisms were described as "animalcules" by him. He was the first person to observe a living cell. - Matthias Schleiden, a German botanist, studied a large number of plant tissues under the microscope and stated in 1838 that all plants are made up of cells. - Theodor Schwann, a zoologist, made similar observations on the animal tissues and stated that all animals are also made up of cells. In 1839, Schleiden and Schwann together formulated the cell theory. - Rudolf Virchow added the third principle in the year 1855. According to him, a new cell is always formed from a pre-existing cell. This is expressed as "omnis cellula e cellula". The idea was first published by Robert Remak in 1852, but it was popularised by Virchow only. ### Modern Extensions of Cell Theory With new discoveries, some more statements were added to the cell theory. These are not part of the classical three principles, but they extend the theory further- - Hereditary information is present inside the cell in the form of deoxyribonucleic acid (DNA), and this information is passed from a parent cell to its daughter cells during [cell division](https://biologynotesonline.com/cell-division/). - Energy flow takes place inside all cells. Metabolic activities like cellular respiration and photosynthesis are carried out within the cell only. - The cells of all organisms of a same species are mostly similar, both in their structure and in their chemical composition. All cells also share a common chemical basis. ## Functions of Cells - Cells carry out different metabolic reactions for their survival. Nutrients are broken down or used for the formation of new cellular materials, and energy is produced during these processes. - The plasma membrane controls the movement of substances into and outside the cell. This maintains the required internal cellular environment. - Protein synthesis takes place in cells. The proteins formed are used for different structural, metabolic and cellular activities. - Cells contain genetic information in the form of deoxyribonucleic acid (DNA). The genetic material directs cellular activities and is copied before cell division. - Cells receive and respond to different signals from their surroundings. Cell-to-cell communication also occurs, especially in multicellular organisms. - Cells grow by producing new cellular components. Many cells then divide to form daughter cells. - In multicellular organisms, different cells perform specialized functions according to the tissue or organ in which they occur. ## Cells at a Glance FeatureQuick summaryDefinitionCell is the smallest structural and functional unit of life.Basic structureAll cells contain plasma membrane, cytoplasm, deoxyribonucleic acid (DNA) and ribosomes.Main typesCells are broadly of two types, prokaryotic and eukaryotic cells.Prokaryotic cellIt has no membrane-bound nucleus or membrane-bound organelles. DNA is present in the nucleoid. Bacteria and Archaea are prokaryotes.Eukaryotic cellA membrane-bound nucleus is present. These cells also contain different membrane-bound organelles. Animals, plants, fungi and protists are eukaryotes.Plasma membraneIt forms the cell boundary and regulates movement of substances across the cell.CytoplasmThe internal cellular region where different cell components are present and many metabolic reactions take place.RibosomesPresent in all cells. Protein synthesis takes place on ribosomes.Genetic materialDNA carries the genetic information of the cell. In eukaryotic cells, most DNA is enclosed within the nucleus.Major functionsCells carry out metabolism, protein synthesis, energy utilization, transport, growth, response and cell division.Cell theoryLiving organisms consist of one or more cells. Cell is the basic unit of life, and new cells arise from pre-existing cells. ## References - Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell (4th ed.). Garland Science. [https://www.ncbi.nlm.nih.gov/books/NBK21054/](https://www.ncbi.nlm.nih.gov/books/NBK21054/) - Brown, T. A. (2002). Genomes (2nd ed.). Wiley-Liss. [https://www.ncbi.nlm.nih.gov/books/NBK21120/](https://www.ncbi.nlm.nih.gov/books/NBK21120/) - Clark, M. A., Douglas, M., & Choi, J. (2018). Biology 2e. OpenStax. [https://openstax.org/books/biology-2e/pages/1-introduction](https://openstax.org/books/biology-2e/pages/1-introduction) - Cooper, G. M. (2000). The cell: A molecular approach (2nd ed.). Sinauer Associates. [https://www.ncbi.nlm.nih.gov/books/NBK9839/](https://www.ncbi.nlm.nih.gov/books/NBK9839/) - Fowler, S., Roush, R., & Wise, J. (2013). Concepts of biology. OpenStax. [https://openstax.org/books/concepts-biology/pages/1-introduction](https://openstax.org/books/concepts-biology/pages/1-introduction) - Parker, N., Schneegurt, M., Tu, A.-H. T., Lister, P., & Forster, B. M. (2016). Microbiology. OpenStax. [https://openstax.org/books/microbiology/pages/1-introduction](https://openstax.org/books/microbiology/pages/1-introduction) - Rye, C., Wise, R., Jurukovski, V., DeSaix, J., Choi, J., & Avissar, Y. (2016). Biology. OpenStax. [https://openstax.org/books/biology/pages/1-introduction](https://openstax.org/books/biology/pages/1-introduction)