The endomembrane system regulates protein traffic and performs metabolic functions in the cell (6.4) : ⭐
The Endoplasmic Reticulum:
Biosynthetic Factory
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The ( ER) is such an extensive network of membranes that it accounts for more than half the total membrane in many eukaryotic cells.
It consists of a network of membranous tubules and sacs
called cisternae
There are two distinct, , regions of the ER
that differ in structure and function:
Smooth ER is so named because its outer surface lacks ribosomes.
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Rough ER has ribosomes on the outer surface of the membrane and thus appears rough through the electron microscope
Functions of Smooth ER
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functions in diverse metabolic processes, which vary with cell type. (These processes include synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons)
Functions of Rough ER
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Many types of cells secrete proteins produced by ribosomes attached to rough ER. For example, certain pancreatic cells synthesize the protein insulin on the ER and secrete this hormone into the bloodstream.
The Golgi Apparatus: Shipping
and Receiving Center
After leaving the ER, many transport vesicles travel to the
Goigi apparatus.. The membrane of each cisterna in a stack separates its internal space from the cytosol.
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A Golgi stack has a distinct structural polarity, with the membranes of cisternae on opposite sides of the stack differing in thickness and molecular composition. The two poles of a Golgi stack are referred to as the cis face and the trans face;
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Avesicle that buds from the ER can add its membrane and the contents of its lumen to the cis face by fusing with a Golgi membrane. The trans face gives rise to vesicles, which pinch off and travel to other sites.
Lysosomes: Digestive Compartments
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A lysosome is a membranous sac of hydrolytic enzymes (that an animal cell uses to digest macromolecules). Lysosomal enzymes work best in the acidic environment found in lysosomes.
Lysosomes carry out intracellular digestion in a variety of
circumstances (digestion products, including sugars, amino acids, and other monomer, to become nutrients for the cell)
Hydrolytic enzymes and lysosomal membrane are made by rough ER and then transferred to the Golgi apparatus for further processing. At least some lysosomes probably arise by budding from the trans face ofthe Golgi apparatus
Lysosomes also use their hydrolytic enzymes to recycle the
cell's own organic material, a process called autophagy.
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Vacuoles: Diverse Maintenance
Compartments
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Vacuoles are membrane-bounded vesicles whose functions vary in different kinds of cells. Food vacuoles, formed by phagocytosis .Many freshwater protists have contractile vacuoles that pump excess water out of the cell,
Mature plant
cells generally contain a large central vacuole
The plant cell's central vacuole is a versatile compartment It can hold reserves of important organic compounds, such as the proteins stockpiled in the vacuoles of storage cells in seeds.