ATP synthesis takes place on the inner membrane. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration. Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids.
They also detoxify many poisons that may enter the body. Many of these oxidation reactions release hydrogen peroxide, H 2 O 2 , which would be damaging to cells; however, when these reactions are confined to peroxisomes, enzymes safely break down the H 2 O 2 into oxygen and water.
For example, alcohol is detoxified by peroxisomes in liver cells. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars. Figure 6. Membrane and secretory proteins are synthesized in the rough endoplasmic reticulum RER. It includes the nuclear envelope, lysosomes, vesicles, and the endoplasmic reticulum and Golgi apparatus, which we will cover shortly.
Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles. The endomembrane system does not include the membranes of either mitochondria or chloroplasts. Figure 6 illustrates the connections of the endomembrane system as a green integral membrane protein in the ER is modified by attachment of a purple carbohydrate.
Vesicles with the integral protein bud from the ER and fuse with the cis face of the Golgi apparatus. The endoplasmic reticulum ER Figure 6 is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids.
The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope. The rough endoplasmic reticulum RER is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope Figure 7. Figure 7. This transmission electron micrograph shows the rough endoplasmic reticulum and other organelles in a pancreatic cell.
Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles—or secreted from the cell such as protein hormones or enzymes.
The RER also makes phospholipids for cellular membranes. Since the RER is engaged in modifying proteins such as enzymes, for example that will be secreted from the cell, you would be correct in assuming that the RER is abundant in cells that secrete proteins. This is the case with cells of the liver, for example.
The smooth endoplasmic reticulum SER is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. Functions of the SER include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storage of calcium ions.
In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that are needed to trigger the coordinated contractions of the muscle cells. Heart disease is the leading cause of death in the United States. This is primarily due to our sedentary lifestyle and our high trans-fat diets. Heart failure is just one of many disabling heart conditions. Heart failure does not mean that the heart has stopped working.
Left untreated, heart failure can lead to kidney failure and failure of other organs. The wall of the heart is composed of cardiac muscle tissue.
Heart failure occurs when the endoplasmic reticula of cardiac muscle cells do not function properly. As a result, an insufficient number of calcium ions are available to trigger a sufficient contractile force. Cardiologists can make a diagnosis of heart failure via physical examination, results from an electrocardiogram ECG, a test that measures the electrical activity of the heart , a chest X-ray to see whether the heart is enlarged, and other tests.
If heart failure is diagnosed, the cardiologist will typically prescribe appropriate medications and recommend a reduction in table salt intake and a supervised exercise program. Figure 8. The Golgi apparatus in this white blood cell is visible as a stack of semicircular, flattened rings in the lower portion of the image.
Several vesicles can be seen near the Golgi apparatus. We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need to be sorted, packaged, and tagged so that they wind up in the right place.
Sorting, tagging, packaging, and distribution of lipids and proteins takes place in the Golgi apparatus also called the Golgi body , a series of flattened membranes Figure 8.
The receiving side of the Golgi apparatus is called the cis face. The opposite side is called the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted.
The most frequent modification is the addition of short chains of sugar molecules. These newly modified proteins and lipids are then tagged with phosphate groups or other small molecules so that they can be routed to their proper destinations.
Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the trans face of the Golgi. While some of these vesicles deposit their contents into other parts of the cell where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell. In another example of form following function, cells that engage in a great deal of secretory activity such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies have an abundance of Golgi.
In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which are used in other parts of the cell. Many diseases arise from genetic mutations that prevent the synthesis of critical proteins. One such disease is Lowe disease also called oculocerebrorenal syndrome, because it affects the eyes, brain, and kidneys.
In Lowe disease, there is a deficiency in an enzyme localized to the Golgi apparatus. Children with Lowe disease are born with cataracts, typically develop kidney disease after the first year of life, and may have impaired mental abilities. Lowe disease is a genetic disease caused by a mutation on the X chromosome. Females possess two X chromosomes while males possess one X and one Y chromosome.
In females, the genes on only one of the two X chromosomes are expressed. However, males only have one X chromosome and the genes on this chromosome are always expressed. Therefore, males will always have Lowe disease if their X chromosome carries the Lowe disease gene. The location of the mutated gene, as well as the locations of many other mutations that cause genetic diseases, has now been identified.
Through prenatal testing, a woman can find out if the fetus she is carrying may be afflicted with one of several genetic diseases. Geneticists analyze the results of prenatal genetic tests and may counsel pregnant women on available options. They may also conduct genetic research that leads to new drugs or foods, or perform DNA analyses that are used in forensic investigations. Vesicles , like vacuoles which we will address soon , are membrane-bound sacs that function in storage and transport.
Also located in the nucleus is the nucleolus or nucleoli, organelles in which ribosomes are assembled. The nucleus is bounded by a nuclear envelope, a double membrane perforated with pores and connected to the rough endoplasmic reticulum membrane system. The cytoskeleton consists of microtubules, intermediate fibers, and microfilaments, which together maintain cell shape, anchor organelles, and cause cell movement.
The microtubules and microfilaments are frequently assembled and disassembled according to cellular needs for movement and maintaining cell shape. Intermediate filaments are more permanent than microtubules and microfilaments. The cell diagrams shown here represent intestinal epithelial cells with fingerlike projections, the microvilli. The location and appearance of cytoskeletal fibers in different cell types will vary.
A ribosome is the site of protein synthesis in the cell. Each ribosome consists of a large subunit and a small subunit, each of which contains rRNA ribosomal RNA and ribosomal proteins. The amino acids are joined to produce the protein. You may access more information on From Gene to Protein: Translation. Ribosomes exist free in the cytoplasm and bound to the endoplasmic reticulum ER.
Free ribosomes synthesize the proteins that function in the cytosol, while bound ribosomes make proteins that are distributed by the membrane systems, including those which are secreted from the cell. The plasma membrane also called the cell membrane is a phospholipid bilayer with embedded proteins that encloses every living cell.
This membrane blocks uncontrolled movements of water-soluble materials into or out of the cell. The various proteins embedded in the phospholipid bilayer penetrate into and through the bilayer three-dimensionally. It is the proteins of the membrane that are responsible for the specific functions of the plasma membrane. These functions include controlling the flow of nutrients and ions into and out of the cells, mediating the response of a cell to external stimuli a process called signal transduction , and interacting with bordering cells.
All membranous eukaryotic cell organelles have the common feature of a phospholipid bilayer, although the proteins differ in each case. All eukaryotic cells contain mitochondria, often many hundreds per cell. Each mitochondrion is about um long. Mitochondria contain the enzymes and other components needed for the enzyme complexes that catalyze respiration.
The primary function of mitochondria is to synthesize ATP adenosine triphosphate from ADP adenosine diphosphate and Pi inorganic phosphate. Mitochondria are large organelles containing DNA and surrounded by a double membrane. The inner membrane is highly convoluted, with deep folds called cristae.
Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities. It is important to know what organism the cell comes from. There are two general categories of cells: prokaryotes and eukaryotes. Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies.
Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of eukaryotic cells. Most of the functions of organelles, such as mitochondria and the Golgi apparatus, are taken over by the prokaryotic plasma membrane.
Eukaryotes are about 10 times the size of a prokaryote and can be as much as times greater in volume. The major and extremely significant difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bounded compartments in which specific metabolic activities take place, and have small specialized structures called organelles that are dedicated to performing certain specific functions.
The outer lining of a eukaryotic cell is called the plasma membrane. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of proteins and lipids, fat-like molecules. Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell. A form of plasma membrane is also found in prokaryotes, but in this organism it is usually referred to as the cell membrane.
The cytoskeleton is an important, complex, and dynamic cell component. It acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis the uptake of external materials by a cell ; and moves parts of the cell in processes of growth and motility.
Inside the cell there is a large fluid-filled space called the cytoplasm , sometimes called the cytosol. In prokaryotes, this space is relatively free of compartments. In eukaryotes, the cytosol is the "soup" within which all of the cell's organelles reside. It is also the home of the cytoskeleton.
The cytosol contains dissolved nutrients, helps break down waste products, and moves material around the cell. The nucleus often flows with the cytoplasm changing its shape as it moves. The cytoplasm also contains many salts and is an excellent conductor of electricity, creating the perfect environment for the mechanics of the cell. The function of the cytoplasm, and the organelles which reside in it, are critical for a cell's survival.
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