All complex life — including plants, animals and fungi — is made up of eukaryotic cells, cells with a nucleus and other complex internal machinery used to perform the functions an organism needs to stay alive and healthy. Humans, for example, are composed of 220 different kinds of eukaryotic cells — which, working in groups, control everything from thinking and locomotion to reproduction and immune defense.
For the most part, scientists agree that eukaryotic cells arose from a symbiotic relationship between bacteria and archaea. Archaea — which are similar to bacteria but have many molecular differences — and bacteria represent two of life’s three great domains. The third is represented by eukaryotes, organisms composed of the more complex eukaryotic cells.
In prokaryotes there are either no or very few organelles bounded by a single membrane in comparison to eukaryotes which have many of them including the Golgi apparatus and the endoplasmic reticulum.
PETERSON/ECHS Describe the similarities and differences between the biochemical pathways of aerobic respiration and photosynthesis in eukaryotic cells.
Eukaryotic cells are characterized by an elaborate inner architecture. This includes, among other things, the cell nucleus, where genetic information in the form of DNA is housed within a double membrane; mitochondria, membrane-bound organelles, which provide the chemical energy a cell needs to function; and the endomembrane system, which is responsible for ferrying proteins and lipids about the cell.
A potentially more fruitful avenue to explore, he suggests, would be to look for intermediate forms of cells with some, but not all, of the features of a full-blown eukaryote. “The implication is that intermediates that did exist went extinct, most likely because of competition with fully-developed eukaryotes.”
One catch for fleshing out the evolutionary history of the eukaryotic cell, however, is that unlike many other areas of biology, the fossil record is of little help. “When it comes to individual cells, the fossil record is rarely very helpful,” explains David Baum. “It is even hard to tell a eukaryotic cell from a prokaryotic cell. I did look for evidence of microfossils with protrusions, but, not surprisingly, there were no good candidates.”
Modern eukaryotic cells, says Buzz Baum, can be interrogated in the context of the new theory to answer many of their unexplained features, including why nuclear events appear to be inherited from archaea while other features seem to be derived from the bacteria.
From time to time, David Baum dusted off his rudimentary idea and shared it with others, including the late Lynn Margulis, the American scientist who developed the theory of the origin of eukaryotic organelles. Over the past year, Buzz and David Baum refined and detailed their idea, which, like any good theory, makes predictions that are testable.
Prevailing theory holds that eukaryotes came to be when a bacterium was swallowed by an archaeon. The engulfed bacterium, the theory holds, gave rise to mitochondria, whereas internalized pieces of the outer cell membrane of the archaeon formed the cell’s other internal compartments, including the nucleus and endomembrane system.
Prokaryotic cells have naked DNA which is found in the cytoplasm in a region named the nucleoid. On the other hand, eukaryotes have chromosomes that are made up of DNA and protein. These chromosomes are found in the nucleus enclosed in a nuclear envelope.
The first step in in all living cells is , which can take place without the presence of molecular oxygen. If oxygen is present in the cell, then the cell can subsequently take advantage of via the to produce much more usable energy in the form of than any anaerobic pathway. Nevertheless, the anaerobic pathways are important and are the sole source of ATP for many anaerobic bacteria. Eukaryotic cells also resort to anaerobic pathways if their oxygen supply is low. For example, when muscle cells are working very hard and exhaust their oxygen supply, they utilize the anaerobic pathway to lactic acid to continue to provide ATP for cell function.
Plant chloroplasts are commonly found in guard located in plant . Guard cells surround tiny pores called , opening and closing them to allow for gas exchange required for photosynthesis. Chloroplasts and other plastids develop from cells called proplastids. Proplastids are immature, undifferentiated cells that develop into different types of plastids. A proplastid that develops into a chloroplast, only does so in the presence of light. Chloroplasts contain several different structures, each having specialized functions. Chloroplast structures include:
occurs in structures called chloroplasts. A chloroplast is a type of organelle known as a plastid. Plastids assist in storing and harvesting needed substances for energy production. A chloroplast contains a green pigment called chlorophyll, which absorbs light energy for photosynthesis. Hence, the name chloroplast indicates that these structures are chlorophyll-containing plastids. Like , chloroplasts have their own , are responsible for energy production, and reproduce independently from the rest of the cell through a division process similar to bacterial . Chloroplasts are also responsible for producing and components needed for chloroplast membrane production. Chloroplasts can also be found in other such as .