The cell was once viewed as a bag of fluid, enzymes, and chemicals. Now, we understand that the cell is an extremely complex living the entity. With the advent of electron microscopy in the early 1940's, several distinct cellular structures called organelles were clearly recognized. A typical animal cell contains several types of these organelles (Figure 2-1). Each organelle has an important role in the functioning of the cell. It is important for you to become familiar with these organelles.
a. Cell Membrane. (Animal cells do not have cell walls; they have cell membranes only. Plant cells have both cell walls and cell membranes.)
(1) Practically all the structures within the cell, as well as the cell itself, are lined with a porous, elastic membrane. The cell membrane is composed primarily of lipids (fats) and proteins that are arranged in layers at right angles to each other (Figure 2-1).
Protein Lipids Protein
Protein Lipids Protein
(2) The lipids of the cell wall are composed of two portions: a long hydrocarbon chain (that is insoluble in water) and a glycerol-phosphate head (that is soluble in water). The long chains are in the center of the protein and the glycerol-phosphate group is attached to the end of the protein.
(3) The cell membrane contains many pores. It is through these pores that lipid-insoluble particles, such as water and urea, pass between the interior and the exterior of the cell. Diffusion experiments have shown that particles up to approximately 8-Angstrom units in diameter pass through the pores freely.
(4) The main function of the cell membrane is to regulate the flow of substances into and out of the cell. This regulation of flow is accomplished by the membrane's selective permeability. That is, only certain substances may pass through the pores. This is important, since the cell must obtain the nutrients for its growth from the extracellular fluid (fluid outside the cell) and discard waste products back into the extracellular fluid.
b. Cytoplasm (Figure 2-2). Cytoplasm is the fluid or semifluid contained inside the cell membrane, but outside the nucleus. The cytoplasm functions as a medium to contain many substances, such as fats, glucose, proteins, water, and electrolytes. The clear portion of the cytoplasm is called hyaloplasm. Located within the cytoplasm are the organelles that perform highly specialized functions in the cell.
c. Nucleus (Figure 2-2). The nucleus is the control center for the cell. It controls the reproduction of the cell as well as the chemical reactions that occur within the cell. The nucleus contains large amounts of deoxyribonucleic acid (DNA). The DNA is responsible for controlling the characteristics of the protein enzymes of the cytoplasm, and thus, it controls cytoplasmic activities. The DNA is also responsible for controlling the hereditary characteristics of individuals.
d. Mitochondria (Figure 2-2). The mitochondria may be called the power house" of the cell. The mitochondria are the site of cell respiratory activity. The mitochondria are found in the cytoplasm. They are usually located near energy requiring structures (that is, nodes of nerves, contracting ligaments of muscles, active transport mechanisms in membranes and ribosomes). Their numbers depend on the amount of energy required by the cell to perform its function. Several infoldings of the inner unit membrane form shelves on which practically all of the oxidative enzymes of the cell are said to be absorbed. When nutrients and oxygen meet these enzymes, they combine to form carbon dioxide, water, and energy. The liberated energy is used to synthesize ATP (adenosine triphosphate). This ATP then diffuses throughout the cell and releases its energy whenever it is needed for cellular functions.
e. Lysosomes (Figure 2-2). Lysosomes may be called the digestive organs of the cell. Lysosomes are surrounded by a membrane and contain digestive (hydrolytic) enzymes. When this membrane ruptures, it releases the digestive enzymes that will break down particles or molecules located near the ruptured area. For example, they surround pinocyticle vesicles containing food particles and digest them. If a sufficient number of lysosomes rupture, the entire cell may be digested. When the lysosomes function properly, products of digestion can be used by the cell.
f. Nucleoli (Figure 2-2). In the nucleus of many cells, there may be one or more structures called nucleoli. The nucleoli do not have a limiting membrane, as do most organelles. These structures are primarily aggregate of loosely bound granules composed mainly of ribonucleic acid (RNA). Hereditary units called genes are thought to synthesize and store in the nucleolus. This stored RNA diffuses into the cytoplasm where it controls cytoplasmic function. Therefore, the main functions of the nucleolus are the synthesis of RNA and the storage of RNA.
g. Endoplasmic Reticulum (Figure 2-2). The endoplasmic reticulum is a network of tubules and vesicles (saclike structures) in the cytoplasm. The inside of the tubules and vesicles is filled with endoplasmic matrix, a fluid medium, which is different from the fluid outside the endoplasmic reticulum. In the matrix, there are enzyme systems. The first function of the endoplasmic reticulum is to use these enzymes to synthesize various substances (that is, lipids). The endoplasmic reticulum is connected to the nuclear membrane and, in some cases, it is connected directly through small openings to the exterior of the cell. A second function of the endoplasmic reticulum is to transport various substances, through the vast network of tubules, from one part of the cell to another area of the cell. A third function of the endoplasmic reticulum is to store various substances within the cell.
h. Ribosomes (Figure 2-2). Ribosomes are small particles that are usually attached to the endoplasmic reticulum. Ribosomes are the site of protein synthesis and are referred to as "protein factories" of the cell. Ribosome is composed mainly of ribonucleic acid (RNA).
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