DNA, in the form of a circular or linear molecule, is found in the matrix. The mitochondrial DNA encodes many of the components for mitochondrial function, while nuclear DNA encodes the remaining components. Components of the protein synthesizing machinery specific for mitochondria-ribosomes, tRNAs and specific proteins and enzymes-are also found in the matrix.
All eukaryotic cells have within them a functionally interrelated membrane system, the endomembrane system which consists of the nuclear envelope, endoplasmic reticulum ER , Golgi apparatus, vesicles and other organelles derived from them for example, lysosomes, peroxisomes , and the plasma membrane.
Many materials, including some proteins, are sorted by the functionally cellular membranes of the endomembrane system. The various membranes involved, though interrelated, differ in structure and function.
The endomembrane system plays a very important role in moving materials around the cell, notably proteins and membranes the latter is called membrane trafficking. For example, while many proteins are made on ribosomes that are free in the cytoplasm and remain in the cytoplasm, other proteins are made on ribosomes bound to the rough endoplasmic reticulum RER.
The latter proteins are inserted into the lumen of the RER, carbohydrates are added to them to produce glycoproteins, and they are then moved to cis face of the Golgi apparatus in transport vesicles that bud from the ER membrane. Within the Golgi, the protein may be modified further and then be dispatched from the trans face in a new transport vesicle. These vesicles move through the cytoplasm to their final desinations using the cytoskeleton. We can think of the system as analogous to a series of switching yards and train tracks, where materials are sorted with respect to their destinations at the switching yards and sent to those destinations along specific tracks in the cytoskeleton.
Proteins destined for secretion are made on ribosomes bound to the RER. The proteins move through the endomembrane system and are dispatched from the trans face of the Golgi apparatus in transport vesicles that move through the cytoplasm and then fuse with the plasma membrane releasing the protein to the outside of the cell.
Examples of secretory proteins are collagen, insulin, and digestive enzymes of the stomach and intestine. In a similar way, proteins destined for a particular cell organelle move to the organelle in transport vesicles that deposit their contents in the organelle by membrane fusion.
Like secretory proteins and some other proteins, proteins destined for lysosomes are made on ribosomes bound to the RER and move through the endomembrane system. In this case the lysosomal protein-containing vesicle that buds from the trans face of the Golgi apparatus is the lysosome itself. The figure below illustrates at a glance the structures that are common to both animal and plant cells, as well as the structures that are unique to each.
Structures that are common to both plant and animal cells are labeled between the cells; structures that are unique to plants are labeled on the left of the cells and those unique to animals are labeled on the right.
Chloroplasts are plant cell organelles that contain chlorophyll and the enzymes required for photosynthesis, the light-dependent synthesis of carbohydrates from carbon dioxide CO2 and water H2O. Oxygen O2 is a product of the photosynthesis process, and is released into the atmosphere. Chloroplasts are large organelles bounded by a double membrane and containing DNA. Unlike the mitochondrial double membrane, the inner membrane is not folded. Distinctly separate from the double membrane is an internal membrane system consisting of flattened sacs and called thylakoids.
Figure 5. Peroxisomes are membrane-bound organelles that contain an abundance of enzymes for detoxifying harmful substances and lipid metabolism. Reactive oxygen species ROS such as peroxides and free radicals are the highly reactive products of many normal cellular processes, including the mitochondrial reactions that produce ATP and oxygen metabolism. Some ROS are important for certain cellular functions, such as cell signaling processes and immune responses against foreign substances.
Free radicals are reactive because they contain free unpaired electrons; they can easily oxidize other molecules throughout the cell, causing cellular damage and even cell death. Free radicals are thought to play a role in many destructive processes in the body, from cancer to coronary artery disease. Peroxisomes, on the other hand, oversee reactions that neutralize free radicals. Peroxisomes produce large amounts of the toxic H2O2 in the process, but peroxisomes contain enzymes that convert H2O2 into water and oxygen.
These byproducts are safely released into the cytoplasm. Like miniature sewage treatment plants, peroxisomes neutralize harmful toxins so that they do not wreak havoc in the cells. The liver is the organ primarily responsible for detoxifying the blood before it travels throughout the body, and liver cells contain an exceptionally high number of peroxisomes. Defense mechanisms such as detoxification within the peroxisome and certain cellular antioxidants serve to neutralize many of these molecules.
Some vitamins and other substances, found primarily in fruits and vegetables, have antioxidant properties. Antioxidants work by being oxidized themselves, halting the destructive reaction cascades initiated by the free radicals.
Sometimes though, ROS accumulate beyond the capacity of such defenses. Oxidative stress is the term used to describe damage to cellular components caused by ROS.
Due to their characteristic unpaired electrons, ROS can set off chain reactions where they remove electrons from other molecules, which then become oxidized and reactive, and do the same to other molecules, causing a chain reaction. ROS can cause permanent damage to cellular lipids, proteins, carbohydrates, and nucleic acids. Damaged DNA can lead to genetic mutations and even cancer. It is noteworthy that these diseases are largely age-related.
Many scientists believe that oxidative stress is a major contributor to the aging process. Aging and the… Cell: The Free Radical Theory The free radical theory on aging was originally proposed in the s, and still remains under debate. Generally speaking, the free radical theory of aging suggests that accumulated cellular damage from oxidative stress contributes to the physiological and anatomical effects of aging.
There are two significantly different versions of this theory: one states that the aging process itself is a result of oxidative damage, and the other states that oxidative damage causes age-related disease and disorders. The latter version of the theory is more widely accepted than the former. However, many lines of evidence suggest that oxidative damage does contribute to the aging process. Research has shown that reducing oxidative damage can result in a longer lifespan in certain organisms such as yeast, worms, and fruit flies.
Conversely, increasing oxidative damage can shorten the lifespan of mice and worms. Interestingly, a manipulation called calorie-restriction moderately restricting the caloric intake has been shown to increase life span in some laboratory animals. It is believed that this increase is at least in part due to a reduction of oxidative stress.
However, a long-term study of primates with calorie-restriction showed no increase in their lifespan. A great deal of additional research will be required to better understand the link between reactive oxygen species and aging. The Cytoskeleton Much like the bony skeleton structurally supports the human body, the cytoskeleton helps the cells to maintain their structural integrity.
The cytoskeleton is a group of fibrous proteins that provide structural support for cells, but this is only one of the functions of the cytoskeleton. Cytoskeletal components are also critical for cell motility, cell reproduction, and transportation of substances within the cell. The cytoskeleton forms a complex thread-like network throughout the cell consisting of three different kinds of protein-based filaments: microfilaments, intermediate filaments, and microtubules Figure 6.
The thickest of the three is the microtubule, a structural filament composed of subunits of a protein called tubulin. Microtubules maintain cell shape and structure, help resist compression of the cell, and play a role in positioning the organelles within the cell. Other ribosomes are found on the endoplasmic reticulum. Endoplasmic reticulum with attached ribosomes is called rough ER.
It looks bumpy under a microscope. The attached ribosomes make proteins that will be used inside the cell and proteins made for export out of the cell. There are also ribosomes attached to the nuclear envelope.
Those ribosomes synthesize proteins that are released into the perinuclear space. Two Pieces Make the Whole There are two pieces or subunits to every ribosome. In eukaryotes, scientists have identified the S large and S small subunits. Even though ribosomes have slightly different structures in different species, their functional areas are all very similar. For example, prokaryotes have ribosomes that are slightly smaller than eukaryotes. It's a small difference, but one of many you will find in the two different types of cells.Peroxisomes produce large amounts of the toxic H2O2 in the process, but peroxisomes contain enzymes that convert H2O2 into water and oxygen. The interior of the thylakoid is the lumen. When it is time to make the protein, the two subunits come together and combine with the mRNA. Many scientists believe that oxidative stress is a major contributor to the aging process. Since there are no membrane-bound organelles in prokaryotes, the ribosomes float free in the cytosol.
One of the organ systems in the body that uses huge amounts of ATP is the muscular system because ATP is required to sustain muscle contraction. The proteins move through the endomembrane system and are dispatched from the trans face of the Golgi apparatus in transport vesicles that move through the cytoplasm and then fuse with the plasma membrane releasing the protein to the outside of the cell. For example, while many proteins are made on ribosomes that are free in the cytoplasm and remain in the cytoplasm, other proteins are made on ribosomes bound to the rough endoplasmic reticulum RER.
Mitochondria are large organelles containing DNA and surrounded by a double membrane. The amino acids are joined to produce the protein. The cytoskeleton is a group of fibrous proteins that provide structural support for cells, but this is only one of the functions of the cytoskeleton. You may access more information on From Gene to Protein: Translation.
Then, the SRP is released, and the protein-ribosome complex is at the correct location for movement of the protein through a translocation channel. The resulting proteins carry out cell functions.
The Cellular Level of Organization 17 3. This organelle contains the enzymes involved in lipid synthesis, and as lipids are manufactured in the ER, they are inserted into the organelle's own membranes. Soluble proteins are carried in the lumens of vesicles. Notably, prokaryotic cells lack a nucleus and membranous organelles. It is composed of two ribosomal RNA subunits that wrap around mRNA to start the process of translation, followed by protein synthesis. Many scientists believe that oxidative stress is a major contributor to the aging process.
When it is time to make the protein, the two subunits come together and combine with the mRNA. Therefore, an individual neuron will be loaded with over a thousand mitochondria.
The mitochondrial DNA encodes many of the components for mitochondrial function, while nuclear DNA encodes the remaining components.
What is the primary role of the endomembrane system? Most of the components of photosynthesis are located in the thylakoids.
Cytosol, the jelly-like substance within the cell, provides the fluid medium necessary for biochemical reactions. These reactions convert energy stored in nutrient molecules such as glucose into adenosine triphosphate ATP , which provides usable cellular energy to the cell. Structures that are common to both plant and animal cells are labeled between the cells; structures that are unique to plants are labeled on the left of the cells and those unique to animals are labeled on the right. The ER can be thought of as a series of winding thoroughfares similar to the waterway canals in Venice. The cytoskeleton consists of microtubules, intermediate fibers, and microfilaments, which together maintain cell shape, anchor organelles, and cause cell movement.
The smooth and rough endoplasmic reticula are very different in appearance and function source: mouse tissue. Like the ER, these discs are membranous.
In the case of damaged or unhealthy cells, lysosomes can be triggered to open up and release their digestive enzymes into the cytoplasm of the cell, killing the cell. In eukaryotes, scientists have identified the S large and S small subunits.