In this video we are going to take a look at cell theory. Cell theory is the theory that all living organisms are composed of cells. There are three principles of cell theory: The first principle is that the cells are the smallest units of life. This means that the cell is the smallest unit of life, if we went smaller and looked at just an organelle within a cell, such as the nucleus or the mitochondria or any other organelle, they are not capable of the processes required for life alone, they cannot exist on their own, and must work together to be able to do so within the cell, making the cell the smallest unit of life. The second principle of cell theory is that all living things are composed of cells. If you look at any living thing, you will find it is composed of cells. If you look at a bacteria, they are living and are made up of a cell. If you look at a plant or an animal, they are living and made up of many cells. If you look at a virus, they are not alive, and are not made up of cells. The third principle of cell theory is that cells come from pre-existing cells. Cell’s are formed by cell division, a cell will divide to form 2 smaller cells, whether through binary fission in prokaryotes, or through fission, mitosis or meiosis in eukaryotes. What evidence is there that supports cell theory? The main evidence that supports cell theory, is that these principles have never been observed to not be true. The subcellular components, the organelles, have not been observed to be able to perform the functions of life on their own, whereas the cell has. Tissues examined from plants, animals, fungi, bacteria, protists all show the presence of one or more cell. The process of cell division, and cells forming from the division of other cells has been observed, but spontaneous generation has not been observed. The evidence that developed this theory occurred over time, starting with the invention of the compound microscope by Zacharias Janssen in 1590. In 1665, Robert Hooke used a light microscope to look at slivers of cork tissue from a plant, discovering tiny chambers that he called “cells”. In 1674 Anton van Leeuwenhoek observed living microscopic organisms living in pond water, terming then ‘animalcules’. In 1838 Matthias Schleiden, after spending much time observing plants under the microscope, realized that all plants are made up of cells. He is co-credited with developing the first two principles of cell theory. He is co-credited with Theodor Schwann who in 1839 realized that all animals are made up of cells. In 1855, Rudolf Virchow, concluded that all cells come from cells, and is credited for the third principle of cell theory. What evidence is there against cell theory? Cell theory is based on the idea that living things are made up of cells, of distinct small units defined by the cell membrane or cell wall. Most organisms conform to cell theory, but there are some exceptions as some cells types of not fit within the constraints of what is considered a cell. Striated muscle fibres are an example of an atypical cell. Cells are generally considered to have a single nucleus and be small. Striated muscle fibres challenge the idea that cells are autonomous units, as they are a formed by the fusion of multiple cells. This fusion forms long fibres that consist of a single plasma membrane, but have multiple nuclei. Aseptate fungal hyphae are another example of an atypical cell. Fungal hyphae are very large filamentous structures. Cells are generally considered to be small, have a single nucleus, and to be separated into distinct cells. Aseptate fungal hyphae challenge the idea that the cell is a composed of discrete cells. This is because they are long and have incomplete separation of the cells, creating a continuous cytoplasm with multiple nuclei along the hyphae. Unicellular giant algae such as Acetabularia are another example of an atypical cell. These algae are complex in form, they have a root like rhizoid bottom, a long stalk and a series of branches that appear umbrella like called the cap. Cell theory is based on the idea that larger organisms are made up of many much smaller cells. These algae challenge this as they are a larger organism (up to 10cm in size) whilst still being a single cell. Thank you for watching, if you found this video helpful be sure to subscribe and hit that like button.