Introduction to Molecular and Cell Biology, Biol. 220
Lecture 7: The three roles of rNA.
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Introduction to Molecular and Cell Biology, Biol. 220 Lecture 7: The three roles of rNA. |
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Eukaryotic organelles
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Houses genetic information.
Functions that occur in the nucleus:
DNA replication
Transcription
RNA processing
(translation occurs in the cytoplasm, unlike prokaryotes)
Nuclear envelope
Nuclear membranes act as barriers that prevent the free passage of molecules between the nucleus and the cytoplasm.
Two concentric membranes (phospholipid bilayers) are called the inner and outer nuclear membranes (Fig. 5.42b).
The outer nuclear membrane is continuous with the ER so the space between the inner and outer membrane is directly connected with the lumen of the ER. This space is termed the perinuclear space.
Channels through membrane provided by nuclear pores. Selective traffic of proteins and RNAs through these pores (Fig. 5.50).
Under the inner membrane you have the nuclear lamina. The nuclear lamina is a fibrous meshwork that provides structural support to the nucleus. Network of proteins known as lamins. The lamins associate with each other to form filaments. In addition to providing structural support, the nuclear lamina is thought to serve as a site of chromatin attachment (Fig. 5.42a). Chromatin within the nucleus is organized into large loops of DNA, some of which appear to be bound to the nuclear envelope. The lamins may help to mediate this interaction.
The nucleolus (Fig. 5.42b) is the site at which the rRNA genes are transcribed and ribosomal subunits are assembled.
Extensive maze of branching membranes that extends throughout the cytoplasm. Evidence suggests that the ER is continuous with the plasma membrane and with the outer membrane surrounding the nucleus.
Therefore, the membranes of the ER divide the cytoplasm up into interconnected compartments in which different types of reactions take place.
The ER plays an important role in the synthesis of proteins and some lipids. Expanded regions of the ER may serve as storage areas. The ER also functions as a system for transporting materials from one part of the cell to another.
Smooth ER
The smooth ER is more tubular and its outer membrane surfaces have a smooth appearance. It is the primary site of phospholipid, steroid, and fatty acid metabolism (Fig. 5.47).
The smooth ER also contains enzymes that detoxify harmful chemicals, breaking them down into water-soluble substances that can be excreted.
Rough ER
The Rough ER has a more granular appearance under EM due to the presence of ribosomes (Fig. 5.47).
Ribosomes of course serve as the sites of protein synthesis and can be found associated with the ER or free in the cytoplasm. RER is especially abundant in cells that synthesize proteins destined for exocytosis.
Fig. Movie of Protein Synthesis on the ER and Subsequent Protein Transport.
Factory for packaging proteins
Consists of stacks of flattened membranous sacs that may be distended in certain regions because they are filled with cell products (Fig. 5.49).
The golgi complex processes, sorts, modifies, and packages proteins. Proteins that pass through the golgi are:
secreted from the cell plasma membrane proteins and proteins routed to other intracellular organelles pass through the golgi. During their passage through the golgi, proteins are modified in different ways. Carbohydrates are often added here or previously added carbohydrates are modified.
In some cases, the carbohydrates and other modifications act as "sorting signals" allowing the GOLGI to route the protein to different parts of the cell. The GOLGI of plant cells also produces some of the extracellular polysaccharides used in components of the cell wall.
The Golgi Complex has a distinct polarity. The cis face of the golgi {closest to the nucleus} is the site where membrane-bound vesicles from the ER first fuse. The golgi stack refers to the middle portion of the golgi. The trans face of the golgi refers to the portion of the golgi closest to the plasma membrane and serves as the site where membrane-bound vesicles bud and then exit the golgi complex.
The Golgi also performs important functions in non-secreting cells by packaging intracellular digestive enzymes in the little organelles known as lysosomes.
Small membrane-bound vesicles that contain digestive (hydrolytic enzymes). The vesicles are released from the GOLGI COMPLEX and are dispersed throughout the cytoplasm. Over 50 different enzymes present.
Lysosomes play an important role in the digestion of internalized particles (phagocytosis) and macromolecules (pinocytosis/receptor-mediated endocytosis) (Fig. 5.44a).
Another important function of lysosomes is the digestion and break down of old cellular components, which otherwise tend to accumulate and interfere with proper cell function. The lysosome membrane itself is able to resist the digestive action of its own enzymes (Fig. 5.44b).
In addition, when a cell dies, the lysosome membrane breaks down, releasing digestive enzymes into the cytoplasm, where they break down the cell itself.
Some forms of tissue damage as well as the aging process may be related to "leaky lysosomes".
Whether or not lysosomes are present in plant cells is an open debate.
May carry out many of the functions of the lysosome in
the plant cell and algal cell.
Contains acid hydrolases and the pH of the vacuole is maintained at a low value.
As much as 90% of the volume of a plant cell may be occupied by a large central vacuole containing water, stored food, salts, pigments, and wastes (Fig. 5.43a).
Plants lack systems for disposing of metabolic waste products that are toxic
to the cells. Such waste products often aggregate and form small crystals inside
the vacuole.
The vacuole plays an important function as a storage compartment.
Compounds noxious to predators or various compounds used in the plant defense against pathogens may be stored here.
Vacuoles are also involved in plant cell enlargement and in maintaining cell rigidity by the osmotic flow of water into the vacuole to create turgor pressure.
Vacuoles are also present in several different types of animal cells and in many protozoa.
Protozoa often have food or digestion vacuoles which fuse with lysosomes so that the food can be digested.
Many also have contractile vacuoles which function to remove excess water from the cell.
Single membrane-bound organelles that contain enzymes that regulate many different metabolic reactions. One type of microbody, the peroxisome, regulates the converstion of fats to carbohydrates. During the breakdown of fats, hydrogen peroxide is produced. Peroxisomes contain enzymes (including Catalase) that split hydrogen peroxide into water and oxygen, making it harmless. Proteins found in the peroxisome are synthesized in the cytoplasm of the cell and then transported to the peroxisome.
Peroxisomes often contain a dense crystalline core consisting of one of the oxidative enzymes (Fig. 5.44b, Fig. 5.47). Peroxisomes in the liver and kidney cells may be important in detoxifying certain compounds such as ethanol in alchoholic beverages. Peroxisomes occur in both plant and animal cells.
Glyoxysome
Abundant in the seeds of certain plants (Fig.
5.43b).
Its enzymes convert stored fats to sugars.
These sugars are used as an energy source and as a component for making needed
compounds during the germination of seeds.
Contains catalase and the enzymes for fatty acid oxidation as do peroxisomes.
Typical mitochondrion is sausage-shaped and similar in size to a bacterium (Fig. 5.45). Sometimes they can assume a more spherical shape and sometimes even a long thread-like and even branching shape. In other words, the structure of the mitochondrion is dynamic, changing.
The mitochondria play a central role in making chemical energy available to the cell. [ATP SYNTHESIS] Cells which require and expend a lot of energy typically have a lot of mitochondria (Muscle cells).
The internal structure of the mitochondrion can be diagrammed schematically. See diagrams in text.
Each mitochondrion contains an outer membrane and a complex inner membrane system. The outer membrane completely encloses the mitochondrion, serving as its outer boundary. The inner membrane lies beneath the outer membrane but has deep folds or invaginations called cristae. In some cells the cristae are wide sheets that cut across the entire diameter of the mitochondrion. In most plant cells, the cristae have more of a tubular shape. These infoldings greatly increase the amount of surface available to house machinery needed for aerobic respiration.
Inner aqueous compartment is called the matrix.
Between the outer and inner membrane is the inter membrane space.
AT least 60 different types of polypeptides are found in the inner membrane of the mitochondrion. Mass protein/lipid ratio is 3:1 by weight which translates into 1 protein for every 15 phospholipids. The inner membrane is devoid of cholesterol and rich in an unusualy phospholipid- cardiolipin. What I have just described to you is VERY similar to the structure of the bacterial membrane.
The outer membrane contains PORINs which are integral membrane proteins that form large, non-selective membrane channels (also found on outer membrane of certain bacterial cells as part of the cell wall). Outer membrane of mitochondrion is especially permeable and allows molecules up to 10,000 Daltons to pass freely into the intermembrane space. The inner membrane is highly impermeable; virtually all molecules and ions require special transporters. In addition to containing a variety of transport systems, the inner mitochondrial membrane contains most of the enzymes required for the synthesis of ATP.
Matrix
The matrix contains a variety of enzymes, ribosomes, and molecules of double-stranded
DNA (usually circular). This nonchromosomal DNA is important because it encodes
a small number of mitochondrial polypeptides that are integrated into
the inner mitochondrial membrane together with polypeptides encoded by genes
residing within the nucleus and imported from their site of synthesis in the
cytosol.
Observations of living cells show that mitochondria are dynamic organelles that change their shape, move from place to place within the cytoplasm, and undergo branching. Mitochondria arise by fission from pre-existing mitochondria.
Mitochondria are often described as miniature power plants. They extract energy from organic materials and store it in the form of electrical energy.
Following glycolysis, [which occurs in the cytoplasm of the cell], the pyruvate is transported across the inner mitochondrial membrane and into the matrix where it is further broken down in the tricarboxylic acid cycle (Krebs cycle). All but one of the enzymes used in this cycle are in the matrix- one is positioned in inner membrane.
Final phase of aerobic cellular respiration is OXIDATIVE PHOSPHORYLATION which results in the largest net gain of ATP. The ELECTRON TRANSPORT chain is located in the inner mitochondrial membrane.
Chloroplasts are the organelles responsible for photosynthesis and function to generate metabolic energy.
Structural characteristics (Fig. 5.46):
Unlike the mitochondria, the electron transport and energy production of chloroplast is located in the thylakoids instead of the inner membrane. In chloroplasts hydrogen ions are pumped into the thylakoid during photosynthesis and ATP is generated as these ions pass out into the stoma.
Like mitochondria, chloroplasts contain their own genetic material, perhaps reflecting their evolutionary origins as photosynthetic bacteria. The chloroplast genome is much larger, 120 - 160 kbp, and codes for about 120 genes.
About 90% of choroplast proteins are encoded by nuclear genes and are transported from the cytoplasm of the cell into the chloroplast via signal peptides.
Other Plastids
Chromoplasts lack chlorophyll but contain carotenoids and other pigments and are responsible for the color of flowers and fruits.
Leucoplasts are nonpigmented plasteds which sore energy sources in nonphotosynthetic
tissue.
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