Cells+and+cell+membranes

=**__Cells and Cell Membranes__**= by:King Hsu

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Vocabulary:
[|all the organelles]

Prokaryotic and eukaryotic cells differ in size and complexity
A major difference between prokaryotic and eukaryotic cells is indicated by their names. The word prokaryote is from Greek pro, "before", and karyon, "kernel", referring here to the nucleus. The prokaryotic cell has no nucleus. Its genetic material (DNA) is concentrated in a region called the nucleotide, but no membrane separates this region from the rest of the cell. In contrast, the eukaryotic cell has a true nucleus enclosed by a membranous under envelope. The entire region between the nucleus and the membrane bounding the cell is called the cytoplasm. It consists of a semi fluid medium called the cyosol, in which are located organelles of specialized from the function, most of them absent in prokaryotic cells. Thus the presence or absence of a true nucleus is just one example of the disparity in structural complexity between the two types of cells.

The endoplasmic reticulum manufactures membranes and performs many other biosynthetic functions
The endoplasmic reticulum (ER) is a membranous mabyrinth so extensive that it accounts for more than half the total membrane in many eukaryotic cells. The ER consists of a network of membranous tubules and sacs called cisternae. The ER membrane separates its internal compartment, the cisternal space, from the cytosol. And because the ER membrane is continuous with the nuclear envelope, the space between the two membranes of the envelope is continuous with the cisternal space of the ER.

The Golgi apparatus finishes, sorts, and ships cell products
After leaving the ER, many transport vesicles travel to the golgi apparatus. We can think of the golgi as a center of manufacturing, warehousing, sorting and shipping. Harem products of the ER are modified and stored, and then sent to other destinations. Not surprisingly, the golgi apparatus is especially extensive in cells specialized for the secretion

lysosomes are digestive compartments
A lysosome is a membrane-bounded sac of hydrolytic enzymes that the cell uses to gihest macromolecules. There are lysosmal enzymes that can hydrolyze proteins, polysaccharides, fats, and nucleic acids---all the major acidic environment, at about pH5. The lysosmal membrane maintains this low internal pH by pumping hydrogen ions from the cytosol into the mumien of the lysosome. If a lysosome breaks open or leaks its contents, the enzymes are not very active in the neutral environment of the cytosol. However excessive leakage from a large number of lysosmes can destroy a cell by auto digestion. From this example we can once again how important compartmental organization is to the functions of the cell: the lysosme provides a space where the cell can digest macromolecules safely, without the general destruction that would occur if hydrolytic enzymes roamed at large.

Peroxisomes consume oxygen in various metabolic function
The peroxisome is a specialized metabolic compartment bounded by a single membrane. Peroxisomes contain enzymes that transfer hydrogen from carious substrates to oxygen, producing hydrogen peroxide as a by-product, from which the organelle derives its name. These reactions may have many different functions. Some peroxisomes use oxygen to break fatty acids down into smaller molecules that can then be transported to mitochondria’s fuel for cellular respiration. Peroxisomes in the liver detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to oxygen. The H2O2 to water. Enclosing in the same space both the enzymes that produce hydrogen peroxide and those that dispose of this toxic compound is another example of how the cell's compartmental structure is crucial to it's function

Mitochondria and chloroplasts are the main energy transformers of cells
One of this book's themes is that organisms are open systems that transform energy they acquire from their surroundings. In eukarytic cells, mitochondria and chloroplasts are the organelles that convert energy to forms that ells can use for work. Mitochondria are the sites of cellular respiration, the catabolic process that generates ATP by extracting energy from sugars, fats, and other fuels with the help of oxygen. Chloroplasts, found only in plants and eukarytoic algae, are the sites of photosynthesis. They convert solar energy to chemical energy by absorbing sunlight and using it to drive the synthesis of organic compounds from carbon dioxide and water.

Providing structural support to the cell, the cytoskeleton also functions in cell motility and regulation
Action of the cytoskeleton is to give mechanical support to the cell and maintain this shape. This is especially important for animal cells, which lack walls. The remarkable strength and resilience of the cytoskeleton as a whole is based on its architecture. Like a geodesic dome, a balance between opposing forces exerted by its elements stabilizes the cytoskeleton. And just as the skeleton of an animal helps fix the positions of their body parts, the cytoskeleton provides anchorage for many organelles even cytosolic enzyme molecules. The cytoskeleton is more dynamic than an animal skeleton, however. It can be quickly dismantled in one part of the cell and reassembled in a new location, charging the shape of the cell.

A membrane is a fluid mosaic of lipids, proteins, and carbohydrates
Membranes are not static sheets of molecules locked righidly in place. A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds. Most of the lipids and some of the proteins can drift about randomly in the place of the membrane. It is rare, however for a molecule to flip-flop transversely across the membrane, switching from one phospholipid layer to the other: to do so, the hydrophilic part of the molecule would have to cross the hydrophobic core of the membrane.