General Definition of a Cell, Cell Organelles, Cycles and Types
What distinguishes living creatures from the nonliving? There’s no specific definition for life. However, most biologists agree that living organisms must have some specific fundamental characteristics that make them alive. One of these fundamental characteristics is that all living organisms are made up of one or more cell, according to cell theory. This makes the cell the smallest unit of life. A cell is an essential component of living organisms, and this is where all living functions in an organism take place.
There are two types of cells, prokaryotic and eukaryotic. Eukaryotic cells are found in organisms such as animals, plants, fungi, algae and protozoa, while prokaryotic cells are found in bacteria and archaea. The two types have some similarity, as well as differences (Freeman 102). They both have plasma membrane made of a double layer of lipids. Most of their mechanisms function in a similar way, for example, protein synthesis, metabolism, transcription and translation of genetic material. They both have the following cell organelles in common; cytoplasm, cytoskeleton, plasma membrane and ribosomes. Both cells are filled with a fluid which contains dissolved nutrients referred to as cytoplasm, a fluid responsible for material movement within the cell. The cytoskeleton is found in the cytoplasm, and prevents the cell from wear and tear. It also helps to keep the cell position and enable cell movement. The cell membrane is the boundary between the inside and outside of the cell. The membrane is permeable and is extended by support of cytoskeleton elements. Synthesis of proteins is common in both prokaryotic and eukaryotic cells. This means that both contain ribosomes, the site of protein synthesis. The ribosomes in both cells are numerous, but in eukaryotic cells, they are larger and more complex (Freeman 107).
A number of organelles exist inside the plasma membrane of eukaryotic cells but not present in the prokaryotes membrane, for example, a nucleus, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosymes and vacuoles.
All the activities of the cell are controlled by the nucleus. The mitochondria, enclosed by two membranes with cristae, provide the site for metabolism, a process where food energy is broken down to chemical energy. In the chloroplast, it provides a surface where photosynthesis takes place, a process that depends on light energy. There are two types of endoplasmic reticulum, rough and smooth. When the lipid molecules are produced by smooth ER in, Golgi apparatus transport them to their destination. Lysosomes are responsible for the breakdown of protein, membranes and materials absorbed by the cell. The vacuoles provide storage facility for materials and water in plants. The prokaryotic cell contains a specialized cell organelle; flagella, which control movement in the cell. The only Eukaryotic cell that has flagella is a sperm.
Cells divide for repair and reproduction purposes. Somatic cells divide by a process called mitosis in which one cell divides into two identical daughter cells. In interphase, which is the first step of mitosis, DNA is replicated. In prophase, chromosomes condense, spindle microtubules form, nuclease disappears and the nuclear envelope breaks down. In Metaphase, chromosomes are lined up at the cell’s equator. In Telophase, each set of chromosomes (sister chromatids) goes to the opposite pole. Also, nuclear envelope forms and spindle microtubules disappear. Cytokinesis is when the cell completely divides into two daughter cells, each one has a nucleus with identical genetic material. A gamete is a sex cell that contains only half the genetic material. Gametes are produced by a slightly different cell cycle called meiosis. In prophase I, crossing over occurs between chromatids of the homologous chromosomes. In metaphase I, homologous chromosome line up at the cell’s equator. In anaphase I, homologues separate to opposite poles (sister chromatids are still attached). In the end of Telophase I, the two daughter cells are haploid because each cell contains one member of each homologue. The second part of meiosis is similar to mitosis, except that four unidentical haploid cells are produced in meiosis (Pierce 18).
What makes us have different types of somatic cells and gametes is a process called gene expression. The end result of gene expression is proteins that are translated from RNA which is transcribed from the active genes (Lodish 279). This means that differentiated cells are specialized because only certain genes which produce certain proteins are active. Undifferentiated cells are sometimes called stem cells because they can turn into specialized cells when certain genes are turned on. Stem cells can reproduce to give more stem cells and can also generate daughter cells which became specialized. An example of how stem cells work is tissue renewal (Slack 3). For example, our melanocytes, the skin cells that make our pigments, are produced and get renewed by stem cells. The mechanism of stem cells gives us hope to treat some disabilities and conditions such as Alzheimer’s disease. This is the main reason why many countries are investing in stem cell research.
Freeman, Scott. Biological Science. San Fransisco: Benjamin Cummings, 2011. Print.
Lodish, Harvey, Arnold Berk, Chris Kaiser, Monty Krieger, Anthony Bretscher, Hidde Ploegh, Angelika Amon, Matthew Scott. Molecular Cell Biology. New York: W.H Freeman and Company, 2013. Print.
Pierce, Benjamin. Genetics: A Conceptual Approach. New York: W.H Freeman and Company, 2011. Print.
Slack, Jonathan. Stem Cells: A Very Short Introduction. New York: Oxford University Press, 2012. Print.