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The NucleusNucleus = A generally conspicuous membrane-bound cellular organelle
in a eukaryotic cell; contains most of the genes that control the entire cell
Nuclear envelope = A double membrane which encloses the nucleus in a eukaryotic cell.
• Is two lipid bilayer membranes separated by a space of about 20 to 40 nm. Each lipid bilayer has its own specific proteins.
• Attached to proteins on the envelope’s nuclear side is a network of protein filaments, the nuclear lamina, which stabilizes nuclear shape.
The envelope’s inner and outer membranes are fused at the lip of each
pore.
Pore complex regulates molecular traffic into and out of the nucleus.The nucleus contains most of the cell's DNA which is organized with
proteins into a complex called chromatin.
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Chromatin = Complex of DNA and histone proteins, which makes up chromosomes in eukaryotic cells; appears as a mass of stained material in non dividing cells.
Chromosomes = Long threadlike association of genes, composed of chromatin and found in the nucleus of eukaryotic cells.
• Each species has a characteristic chromosome number.
• Human cells have 46 chromosomes, except egg and sperm cells, which have half or 23.
The most visible structure within the nondividing nucleus is the nucleolus.
Nucleolus = Roughly spherical region in the nucleus of nondividing cells, which consists of nucleolar organizers and ribosomes in various stages of production.
Nucleolar organizers = Specialized regions of some chromosomes, with multiple copies of genes for rRNA (ribosomal RNA) synthesis.
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• The nucleus controls protein synthesis in the cytoplasm:
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Ribosome = A cytoplasmic organelle that is the site for protein synthesis
Are complexes of RNA and protein
• Constructed in the nucleolus in eukaryotic cells
• Cells with high rates of protein synthesis have prominent nucleoli and many ribosomes (e.g., human liver cell has a few million).
Ribosomes function either free in the cytosol or bound to endoplasmic reticulum.
Bound and free ribosomes are structurally identical and interchangeable.
Free ribosomes = Ribosomes suspended in the cytosol.
• Most proteins made by free ribosomes will function in the cytosol.
Bound ribosomes = Ribosomes attached to the outside of the endoplasmic reticulum.
• Generally make proteins that are destined for membrane inclusion or export.
• Cells specializing in protein secretion often have many bound ribosomes (e.g.,pancreatic cells).
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The Endomembrane SystemBiologists consider many membranes of the eukaryotic cell to be part of an
endomembrane system.
• Membranes may be interrelated directly through physical contact.
• Membranes may be related indirectly through vesicles.
Vesicles = Membrane-enclosed sacs that are pinched off portions of membranes moving from the site of one membrane to another.
The endomembrane system includes:
• Nuclear envelope
• Endoplasmic reticulum
• Golgi apparatus
• Lysosomes
• Vacuoles
Plasma membrane (not actually an endomembrane, but related to
endomembrane system)
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Endoplasmic reticulum (ER) = (Endoplasmic = within the cytoplasm; reticulum = network); extensive membranous network of tubules and sacs (cisternae) which sequesters its internal lumen (cisternal space) from the cytosol.
• Most extensive portion of endomembrane system.
• Continuous with the outer membrane of the nuclear envelope; therefore, the space between the membranes of the nuclear envelope is continuous with cisternal space.
There are two distinct regions of ER that differ in structure and function: smooth ER and rough ER
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Functions of smooth ER
1. Participates in the synthesis of lipids, phospholipids and steroids
2. Participates in carbohydrate metabolism: Smooth ER in liver contains an embedded enzyme that catalyzes the final step in the conversion of glycogen to glucose
3. Detoxifies drugs and poisons: Smooth ER, especially in the liver, contains enzymes which detoxify drugs and poisons. Enzymes catalyze the addition of hydroxyl groups to drugs and poisons. This makes them soluble in the cytosol, so they may be excreted from the body.
4. Stores calcium ions necessary for muscle contraction: In a muscle cell, the ER membrane pumps Ca++ from the cytosol into the cisternal space.
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Rough ER and protein synthesis
Glycoprotein = Protein covalently bonded to carbohydrate.
Oligosaccharide = Small polymer of sugar units.
Transport vesicle = Membrane vesicle in transit from one part of the cell to
another.
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The Golgi apparatus finishes, sorts, and ships cell products
Many transport vesicles leave the ER and travel to the Golgi apparatus.
Golgi apparatus = Organelle made of stacked, flattened membranous sacs (cisternae), that modifies, stores and routes products of the endoplasmic reticulum Membranes of the cisternae sequester cisternal space from the cytosol.
• Two poles are called the cis face (forming face) and the trans face (maturing face).
• Cis face, which is closely associated with transitional ER, receives products by accepting transport vesicles from the ER. A vesicle fuses its membrane to the cis face of the Golgi and empties its soluble contents into the Golgi's cisternal space.
• Trans face pinches off vesicles from the Golgi and transports molecules to other sites.
During this process, the Golgi:
• Alters some membrane phospholipids.
• Modifies the oligosaccharide portion of glycoproteins.
• Manufactures certain macromolecules itself (e.g., hyaluronic acid).
• Targets products for various parts of the cell.
• Phosphate groups or oligosaccharides may be added to Golgi products as molecular identification tags.
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Lysosomes are digestive compartmentsLysosome = An organelle which is a membrane-enclosed bag of hydrolytic
enzymes that digest all major classes of macromolecules.
• Enzymes include lipases, carbohydrases, proteases, and nucleases.
• Optimal pH for lysosomal enzymes is about pH 5.
• Lysosomal membrane performs two important functions:
• Sequesters potentially destructive hydrolytic enzymes from the cytosol.
• Maintains the optimal acidic environment for enzyme activity by pumping H+s inward from the cytosol to the lumen.
• Hydrolytic enzymes and lysosomal membrane are synthesized in the rough ER and processed further in the Golgi apparatus.
• Lysosomes probably pinch off from the trans face of the Golgi apparatus
Phagocytosis = (Phago = to eat; cyte = cell); cellular process of ingestion, in
which the plasma membrane engulfs particulate substances and pinches off to form a particle-containing vacuole.
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Vacuoles have diverse functions in cell maintenance
Vacuole = Organelle which is a membrane-enclosed sac that is larger than a vesicle (transport vesicle, lysosome, or microbody).
Vacuole types and functions:
Food vacuole = Vacuole formed by phagocytosis which is the site of intracellular digestion in some protists and macrophages.
Contractile vacuole = Vacuole that pumps excess water from the cell; found in some freshwater protozoa.
Central vacuole = Large vacuole found in most mature plant cells
• Is a versatile compartment with many functions:
• Stores organic compounds (e.g., protein storage in seeds)
• Stores inorganic ions (e.g., K+ and Cl-)
• Sequesters dangerous metabolic by-products from the cytoplasm
• Contains soluble pigments in some cells (e.g., red and blue pigments in flowers)
• May protect the plant from predators by containing poisonous or unpalatable compounds
• Plays a role in plant growth by absorbing water and elongating the cell
• Contributes to the large ratio of membrane surface area to cytoplasmic volume. (There is only a thin layer of cytoplasm between the tonoplast and plasma membrane.)
