Eukaryotic cells contain many sacs and tubes bounded by a single membrane. Although these are often rather similar in appearance, they can be subdivided into different types specialized to carry out distinct functions.
Figure 3.6. Mitochondrion and chloroplast.
Figure 3.6. Mitochondrion and chloroplast.
Mitochondria and chloroplasts are frequently found close to another membrane-bound organelle, the peroxisome. In human cells peroxisomes have a diameter of about 500 nm, and their dense matrix contains a heterogeneous collection of proteins concerned with a variety of metabolic functions, some of which are only now beginning to be understood. Peroxisomes are so named because they are frequently responsible for the conversion of the highly reactive molecule hydrogen peroxide (H2O2), which is formed as a by-product of the reactions in the mitochondrion, into water:
This reaction is carried out by a protein called catalase, which sometimes forms an obvious crystal within the peroxisome. Catalase is an enzyme—a protein catalyst that increases the rate of a chemical reaction (page 241). In fact, it was one of the first enzymes to be discovered. In humans, peroxisomes are primarily associated with lipid metabolism. Understanding peroxisome function is important for a number of inherited human diseases such as X-linked adrenoleukodystrophy where peroxisome malfunction and the consequent inability to metabolise lipid properly typically leads to death in childhood or early adulthood unless dietary lipid is extremely restricted.
In addition to Camillo Golgi's Nobel prize in 1906, modern studies of cell organelles have also been recognized by the Nobel committee. George Palade, Albert Claude, and Christian de Duve shared the 1974 Nobel prize in physiology or medicine for their work on the identification and isolation of cell organelles. Gunter Blobel, a former student of Palade's, was awarded the same prize in 1999 for the discovery that proteins carry so-called targeting sequences that determine where in the cell they should reside (page 215).
The endoplasmic reticulum is a network of membrane enclosed channels that run throughout the cell, forming a continuous network whose lumen (inside) is at all points separated from the cytosol by a single membrane. The membrane of the endoplasmic reticulum is continuous with the outer nuclear membrane (Fig. 3.5). Two regions can be recognized in most cells, known as smooth endoplasmic reticulum and rough endoplasmic reticulum (Fig. 1.2 on page 3). The basic difference is that the rough endoplasmic reticulum is covered in ribosomes, which give it its rough appearance in the electron microscope.
The function of the smooth endoplasmic reticulum varies from tissue to tissue. In the ovaries, testes, and the adrenal gland it is where steroid hormones are made; in the liver it is the site of detoxication of foreign chemicals including drugs. Probably the most universal role of the smooth endoplasmic reticulum is the storage and sudden release of calcium ions. Calcium ions are pumped from the cytosol into the lumen of the smooth endoplasmic reticulum to more than 100 times the concentration found in the cytosol. Many stimuli can cause this calcium to be released back into the cytosol, where it activates many cell processes (Chapter 16).
The rough endoplasmic reticulum is where the cells make the proteins that will end up as integral membrane proteins in the plasma membrane, and proteins that the cell will export to the extracellular medium (such as the proteins of the extracellular matrix, page 13).
The Golgi apparatus, named after its discoverer, 1906 Nobel prize winner Camillo Golgi, is a distinctive stack of flattened sacks called cisternae. The Golgi apparatus is the distribution point of the cell where proteins made within the rough endoplasmic reticulum are further processed and then directed to their final destination.
Appropriately, given this central role, the Golgi apparatus is situated at the so-called cell center, a point immediately adjacent to the nucleus that is also occupied by a structure called the centrosome (Fig. 1.2 on page 3). The centrosome helps to organize the cytoskeleton (Chapter 18).
Lysosomes are sometimes called "cell stomachs" because they contain enzymes that digest cellular components. They are particularly plentiful in cells that digest and destroy other cells, such as macrophages. Lysosomes are roughly spherical and usually 250-500 nm in diameter.
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