Berne & Levy Physiology, 6th Updated Edition
The human body is composed of billions of cells, each with a distinct function. Despite this diversity in cell function, all cells share certain common elements and functions. This chapter provides an overview of these common elements and focuses on the important function of transport of molecules and water into and out of the cell across its plasma membrane.
OVERVIEW OF EUKARYOTIC CELLS
Eukaryotic cells are distinguished by the presence of a membrane-delimited nucleus. With the exception of mature human red blood cells, all cells within the body contain a nucleus. The cell is therefore effectively divided into two compartments: the nucleus and the cytoplasm. The cytoplasm is an aqueous solution containing numerous organic molecules, ions, cytoskeletal elements, and a number of organelles. A brief description of the components of a typical eukaryotic cell follows (Fig. 1-1). Readers who desire a more indepth presentation of this material are encouraged to consult one of the many cellular and molecular biology textbooks currently available.
The nucleus contains the genome of the cell, which in somatic cells is present on 46 chromosomes, 22 pairs of autosomes, and one pair of sex chromosomes. Both sperm and eggs contain 23 chromosomes, a copy of each autosome and either a male (X) or a female (Y) sex chromosome. The chromosome is a highly ordered structure containing genes (DNA) and associated proteins (i.e., histones). The nucleus also contains the enzymatic machinery for repair of damaged DNA and for its replication, as well as the enzymes needed to transcribe DNA and yield messenger RNA (mRNA).
The plasma membrane surrounds the cell and separates the contents of the cell from the surrounding extracellular fl uid. It serves a number of important functions and is described in greater detail later in the chapter.
It is currently thought that mitochondria evolved from an aerobic prokaryote that lived within primitive eukaryotic cells. Mitochondria synthesize ATP and thus provide the energy needed to power many vital cell functions. They contain their own DNA, which codes for a number of the enzymes needed for oxidative phosphorylation (other mitochondrial enzymes are synthesized in the cytoplasm and imported into the mitochondria), as well as the RNA needed for the transcription and translation of mitochondrial DNA. Mitochondria are composed of two membranes separated by an intermembrane space. The outer mitochondrial membrane lets molecules up to 5 kDa in size cross. Thus, the composition of the intermembrane space is similar to that of cytoplasm with respect to small molecules and ions. The inner membrane is folded into numerous cristae and is the site where ATP is generated through the process of oxidative phosphorylation. The interior of mitochondria (i.e., matrix) contains the enzymes involved in the citric acid cycle and those involved in oxidation of fatty acids. In addition to producing ATP, mitochondria can serve as a site for sequestration of Ca++.
Rough Endoplasmic Reticulum
The rough endoplasmic reticulum (rER) is an extensive membrane network throughout the cytoplasm and is especially well developed in cells that produce and secrete proteins (e.g., pancreatic acinar cell, plasma cell). Attached to the membrane are ribosomes, which when viewed with an electron microscope, impart the “rough” appearance characteristic of this organelle. The rER is the site of translation of mRNA and posttranslational modifi cation of proteins that are destined to be secreted from the cell or are targeted to the plasma membrane or other membranous organelles (e.g., Golgi apparatus, lysosomes).
Proteins synthesized in the rER are transferred to the Golgi apparatus via coated vesicles. On electron micrographs the Golgi apparatus appears as a stack of fl attened membrane sacs. Vesicles from the rER fuse with sacs that are in close proximity to the rER (i.e., the cis-Golgi network). The proteins then traverse through the Golgi membrane sacs, also via coated vesicles, and in this process they may undergo additional posttranslational modifi cation (e.g., glycosylation). The Golgi apparatus also sorts the proteins and packages them for delivery to other parts of the cell (e.g., plasma membrane, lysosome, secretory granule). The sorting and packaging of proteins occur in the trans-Golgi network.
Smooth Endoplasmic Reticulum
The smooth endoplasmic reticulum (sER) is devoid of ribosomes and therefore appears “smooth” on electron micrographs. It is a site where many substances are modifi ed and detoxifi ed (e.g., pesticides). Hydrophobic molecules can be converted to water-soluble molecules in the sER, thus facilitating their excretion from the body by the liver and kidneys. The sER is also the site for the synthesis of fats and lipids. For example, the cells of the adrenal gland that secrete the steroid hormone cortisol have an extensive sER. Similarly, the cells within the ovaries and testes that secrete estrogens and testosterone have a well-developed sER. In skeletal and cardiac muscle, the sER, which is called the sarcoplasmic reticulum in these cells, serves to sequester Ca++. Thus, it plays an important role in controlling contraction.
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