11 Biology Module 1: Cells as the Basis of Life Module 1.1. Cell Structure INQUIRY QUESTION: WHAT DISTINGUISHES ONE CELL FROM ANOTHER? Cell Theory Up until 400 years ago, microscopic organisms were not able to be examined. There were magnifying glasses, however they were only able to study macroscopic life. Two Dutch lens makers, Hans and Zacharias Janssen, are credited with making the first compound microscope in 1590, which used two adjustable lenses to magnify an object. However it wasn’t until 1665 that Robert Hooke first recorded observations of living tissue using a compound microscope. Hooke’s publication was titled Micrographia: physiological studies of minute bodies made by a magnifying class. It was the first time the word cell was used to describe the units making up the tissue of cork. In 1676, Dutch scientist Anton van Leeuwenhoek developed a simple microscope with a single powerful x300 lens. He is credited with discovering bacteria through observations in a drop of water. Fun Fact In 1677, Leeuwenhoek linked a “stinking mouth” to the “animals living in the scum on the teeth (plaque). He also compared the semen of a man with gonorrhoea with that of his own, to which he was careful to point out that the material was “residue after conjugal coitus” and not the product of “sinfully defiling myself”. Sure it was … In 1831, the Scottish botanist Robert Brown made several observations on plant cells from a variety of backgrounds. He noticed that they all seemed to contain a dark region near the centre, which he identified as the nucleus. In the decades following Brown’s observations, the collaborated work of biologists Schwann and Schleiden, with later contributed observations by Virchow and Flemming led to the development of cell theory. Cell theory had three main points: 1. The cell is the unit of structure of all living things 2. The cell exists as a distinct entity and as a building block for the construction of organisms 3. All cells come from pre-existing cells A scientific theory is a broad explanation accepted by scientists and based on repeatable observations, tests and investigations. Theories are also changeable as new information is collected. The observations made with microscopes by Hooke and up to the mid 19th century were repeatable. It took Schwann and Schleiden to explain the structure with their initial theory of living things using these observations and Virchow and Flemming to improve the theory when further observations were made on the reproduction of cells. It was the first time a common basic structure had been suggested for all living things. Greater detail into cell function and structure was achieved from the late1920s with the development of the scanning electron microscope, with showed a whole new level of detail. The Evolution of the Cell The first proto-life forms would have been replicating molecules of genetic information, such as DNA or more likely the related RNA (details of these molecules in Module 5: Heredity). Some of this material was able to settle itself in an oily lipid bubble to protect itself from the external environment. This was the first cell membrane, and with it, prokaryotic cells were formed. (Details on the cell membrane later in this Module) Prokaryotes were successful as their free floating genetic material allowed for easy asexual binary fission as a reproductive mechanism. Many also developed a microscopic whip-like appendage enabling them to swim, a flagellum. From here prokaryotic cells diversified into two main domains; bacteria and archaea, each with a huge diversity of species. Bacteria and archaea were classified as the same group until the late 1970s, when the differences were discovered. The differences are mainly chemical make up, such as the proteins embedded within their membrane and cell wall structure, and the behaviour of their RNA polymerase and enzymes (more on this later in the course). It was also discovered that archaea are more closely related to eukaryotes (including us) than they are to bacteria. Fun Fact ALH 84001 is a 1.93 kg meteorite found in Antarctica in 1984 which is thought to be from Mars. It is famous because in 1996 a group of scientists claim to have found evidence of Martian prokaryotic fossils on it. It is inconclusive one way or the other, but still rather interesting. The next major step in cell evolution would have been the formation of a nuclear membrane around the genetic material of the prokaryote. It’s not completely known how this happened, but most likely from infolding of the cellular membrane which helped protect the DNA/RNA. This was the first eukaryotic cell, however further evolution became even more interesting. The nucleus is not the only organelle to contain genetic material. The evolution of the mitochondria and chloroplasts in the timeline of eukaryotes did not come from the organelle developing inside the cell, but rather a hijacking of bacteria and an eventual mutual relationship. The bacteria benefit from the protection of the larger cell and the eukaryote benefits from the products of the bacteria (energy/glucose production). The eukaryotic domain diversified into four main groups: Those that harboured chloroplast became the first plants The immobile fungus, with cells that include a cell wall but no chloroplasts The multicellular heterotrophic animals Those unicellular eukaryotic cells that fit in none of the above, protists. (note that “protist” is not a true taxonomic term, more of “other misc”. Also there are unicellular fungi and plants that are not considered protists) This step gave rise to the major kingdoms of eukaryotes and is an important step in the evolution of all life on Earth. It is also important to our relationship with the other domains; we might be closer related to archaea, but we host bacteria within our very cells. Prokaryotes vs Eukaryotes The membrane bound organelles are mainly what distinguishes eukaryotic cells from prokaryotic cells. Eukaryotes are also on average larger with a more complex interior, often part of a multicellular organism (except protists) and reproduces via mitosis rather than binary fission. And now for the list of organelles… Nucleus: Contains chromosomal DNA Has its own double membrane Considered the control centre of a cell Nucleolus: Located within the nucleus Produced ribosomal RNA Important in protein synthesis Vacuole: Stores water and soluble nutrients in cells Primarily found in plant cells Chloroplasts: Has its own membrane and DNA Contains the chemical chlorophyll Responsible for photosynthesis Only found in plant cells Mitochondria: Has its own membrane and DNA Responsible for respiration Is considered the “powerhouse” of the cell Cell Wall: Found in plant cells, fungi cells and unicellular organisms Gives structure to the cell Membrane: Surrounds the cell and some other organelles Selectively permeable Allows substances into and out of the cell Cen be seen with electron microscope only Golgi Apparatus: Have large surface area to volume ratio Receive, sort, store and secrete proteins Cen be seen with electron microscope only Ribosome: Smallest major organelle Found everywhere in the cell Location of protein synthesis Cen be seen with electron microscope only Endoplasmic Reticulum: A system of membranes within the cell Transports nutrients and wastes into and out of every part of the cell Cen be seen with electron microscope only Lysosomes: Small membrane-bound sacks that contain enzymes. These enzymes break down fats, proteins and other unwanted materials inside the cell Cen be seen with electron microscope only Cytoplasm: Aqueous body of the cell. Contains all organelles Key Terms electron microscope prokaryotic cells eukaryotic cell Microscopic Nucleus Robert Hooke Nucleolus compound microscope Vacuole Cell Chloroplasts Leeuwenhoek Mitochondria Robert Brown Cell Wall Schwann and Schleiden Golgi Apparatus cell theory Membrane scientific theory Endoplasmic Reticulum Scanning Ribosome DNA Cytoplasm RNA Lysosomes The End
