Cells Of The Human Body Essay, Research Paper
Cells are the basic living units of all plants and animals. The cell is the structural and functional unit of all living organisms. There are a wide variety of cell types, such as nerve, muscle, bone, fat, and blood cells. Each cell type has many characteristics, which are important to the normal function of the body as a whole. One of the important reasons for maintaining hemostasis is to keep the trillions of cells that form the body functioning normally. An averaged size cell is one-fifth the size of the smallest dot you can make on a sheet of paper with a sharp pencil.
Although cells may have quite different structures and functions, all cells share some common characteristics. The plasma, or cell membrane, forms the outer boundary of the cell through which the cells interacts with its external environment. The nucleus is usually located centrally and functions to direct cell activities, most of which take place in the cytoplasm, located between the plasma membrane and the nucleus.
PLASMA (CELL) MEMBRANE
The plasma membrane is the outer part of a cell. The plasma membrane is made up of 45% – 50% lipids, 45% – 50% proteins, and 4% – 8% carbohydrates. The main lipids are phospholipids and cholesterol. Phospholipids easily come together to form a lipid bilayer, a double layer of lipid molecules, because they have a polar head and a nonpolar tail. The charged water-loving heads are exposed to water inside and outside the cell, whereas the uncharged water-fearing tails face one another in the interior of the plasma membrane. The other major lipid in the plasma membrane is cholesterol, which is mixed among the phospholipids and makes up about a third of the total lipids in the plasma membrane. Cholesterol is too hydrophobic to extend to the hydrophilic surface of the membrane but lies within the hydrophobic region of the phospholipids. The amount of cholesterol in a given membrane is a major factor in determining the fluid nature of the membrane, which is important to its function.
The fluid-mosaic model suggests that the plasma membrane is highly flexible and can change its shape and composition through time. The lipid bilayer functions as a liquid in which other molecules such as proteins “float”. The fluid nature of the lipid bilayer is very important. It provides an important means of distributing molecules within the plasma membrane. In addition, slight damage to the membrane can be repaired because the phospholipids tend to reassemble around damaged sites and seal them closed. The fluid nature of the lipid bilayer enables membranes to fuse with one another.
Although the basic structure of the plasma membrane is determined mainly by its lipids, the functions of the plasma membrane are determined mainly by its proteins. Integral, or intrinsic proteins, penetrate the lipid bilayer from one surface to the other. Peripheral, or extrinsic proteins, are attached to either the inner or outer surfaces of the lipid bilayer. Integral proteins consist of regions made up of amino acids with hydrophobic R groups and other regions of amino acids with hydrophilic R groups. The hydrophobic regions are located within the hydrophobic part of the membrane, and the hydrophilic regions are located at the inner or outer surface of the membrane or line channels through the membrane. Peripheral proteins are usually bound to integral proteins. Some membrane proteins form channels through the membrane or act as carrier molecules. Other membrane proteins are receptors, markers, enzymes, or structural supports in the membrane. The ability of membrane proteins to function depends on their three-dimensional shape.
Channel proteins are one or more integral proteins arranged so that they form a tiny channel through the plasma membrane. The hydrophobic regions of the proteins face outward toward the hydrophobic part of the cell membrane, and the hydrophilic regions of the proteins line channel. Small molecules or ions of the right shape, size, and charge can pass through the channel. The charges in the hydrophilic part of the channel protein determine which typed of ions can pass through the channel.
The function of a channel protein is determined by its shape. The channel can be open or closed, depending on the shape of the channel proteins. Some channel proteins change shape to open the channel when a ligand binds to a specific receptor site on the protein. This is called a ligand-gated channel. Other channel proteins change shape to open the channel when there is a change in charge across the cell membrane. This is called a voltage-gated channel.
Receptor molecules are proteins in the cell membrane with an exposed binding site on the outer cell surface, which can attach to specific ligand molecules. The receptors and the ligands they bind are part of an intercellular communication system that controls coordination of cell activities. The binding acts as a signal that triggers a response, such as contraction in the muscle cell. The same chemical messenger would have no effect on another cell that lacks the receptor molecule. Some receptor molecules function by means of a G protein complex located on the inner surface of the cell membrane. G proteins may function in one of several ways. For example, when a ligand such as a hormone attaches to the receptor molecule, the G protein complex binds guanosine triphosphate (GTP) and is activated. The activated G protein, in turn, activated adenylate cyclase, which catalyzes the conversion of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP). cAMP functions as a second messenger inside the cell, stimulating a variety of cell functions.
Marker molecules are cell surface molecules that allow cells to identify and attach to each other. They are mostly glycoproteins or glycolipids.
THE NUCLEUS
The nucleus, which contains most of the genetic information of the cell, is a large, membrane-bound structure usually located near the center of the cell. It may be spherical, elongated, or lobed, depending on the cell type. All cells of the body have a nucleus at some point in their life cycle, although some cells, such as red blood cells, lose their nuclei as they develop. Other cells, such as skeletal muscle cells and certain bone cells, called osteoclasts, contain more than one nucleus. The nucleus is surrounded by a nuclear envelope composed of surface of the nuclear envelope, the inner and outer membranes fuse to form porelike structures, the nuclear pores. Molecules move between the nucleus and the cytoplasm through these nuclear pores.
Deoxyribonucleic acid (DNA) and associated proteins are mixed throughout the nucleus as thin strands about 4-5 nanometers (nm) in diameter. The proteins include histones and other proteins that play a role in the regulation of DNA function. The DNA and protein strands can be stained with dyes and are called chromatin. Chromatin is distributed throughout the nucleus but is more condensed and more readily stained in some areas than in others. The more highly condensed chromatin apparently is less functional than the more evenly distributed chromatin, which stains lighter. During cell division the chromatin condenses to form the more solid bodies called chromosomes.
DNA ultimately determines the structure of proteins. Many structural components of the cell and all the enzymes, which regulate most chemical reactions in the cell, are proteins. By determining protein structure, DNA therefore ultimately controls the structural and functional characteristics of the cell. DNA does not leave the nucleus, but works by means of an intermediate, ribonucleic acid (RNA), which can leave the nucleus. DNA determines the structure of messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). mRNA moves out of the nucleus through the nuclear pores into the cytoplasm, where it determines the structure of proteins.
Because mRNA synthesis occurs within the nucleus, cells without nuclei accomplish protein synthesis only as long as the mRNA produced before the nucleus degenerates remains functional. The nuclei of developing red blood cells are expelled from the cells before the red blood cells enter the blood, where they survive without a nucleus for about 120 days. In comparison, many cells with nuclei, such as nerve and skeletal muscle cells, survive as long as the individual person survives.
A nucleolus is a somewhat rounded, dense region within the nucleus that lacks a surrounding membrane. There is usually one nucleolus per nucleus, but several smaller, accessory nucleoli may also be seen in some nuclei, especially during the latter phases of cell division. The nucleolus contains portions of 10 chromosomes, called nucleolar organizer regions. These regions contain DNA from which rRNA is produced. Within the nucleolus, the subunits of ribosomes are manufactured.
CYTOPLASM
Cytoplasm, the cellular material outside the nucleus but inside the plasma membrane, is about half cytosol and half organelles.
CYTOSOL
Cytosol consists of a fluid portion, a cytoskeleton, and cytoplasmic inclusions. The fluid portion of cytosol is a solution with dissolved ions and molecules and a colloid with suspended molecules, especially proteins. Many of these proteins are enzymes that catalyze the breakdown of molecules for energy or the synthesis of sugars, fatty acids, nucleotides, amino acids, and other molecules.
CYTOSKELETON
The cytoskeleton supports the cell and holds the nucleus and organelles in place. It is also responsible for cell movements, such as changes in cell shape or movement of cell organelles. The cytoskeleton consists of three groups of proteins: microtubules, actin filaments, and intermediate filaments.
Microtubules are hollow tubules composed primarily of protein units called tubulin. The microtubules are about 25 nm in diameter, with walls that are about 5 nm thick. Microtubules vary in length but are normally several micrometers (um) long. Microtubules play a variety of roles within cells. They help provide support and structure to the cytoplasm of the cell, much like an internal scaffolding. They are involved in the process of cell division and form essential parts of certain cell organelles, such as centrioles, spindle fibers, cilia, and flagella.
Actin filaments, or microfilaments, are small fibrils about 8 nm in diameter that form bundles, sheets, or networks in the cytoplasm of cells. Actin filaments provide structure to the cytoplasm and mechanical support for microvilli. Actin filaments support the plasma membrane and define the shape of the cell. Changes in cell shape involve the breakdown and reconstruction of actin filaments. Actin filaments are involved in cell movement. Cell movement in cells that can move about is accomplished by changes in cell shape controlled by the actin cytoskeleton. Muscle cells contain a large number of highly organized actin filaments responsible for the muscle’s contractile capabilities.
Intermediate filaments are protein fibers about 10 nm in diameter. They provide mechanical strength to cells. For example, intermediate filaments support the extensions of nerve cells, which have a very small diameter but can be a meter in length.
CYTOPLASMIC INCLUSIONS
The cytosol also contains cytoplasmic inclusions, which are collections of chemicals either produced by the cell or taken in by the cell. Dust, minerals, and dyes can also accumulate in the cytoplasm.
ORGANELLES
Organelles are small structures within cells that are specialized for particular functions, such as manufacturing proteins or producing ATP. Most organelles have membranes that are similar to the plasma membrane. The membranes separate the organelles from the rest of the cytoplasm, creating a subcellular compartment with its own enzymes that is able to carry out its own unique chemical reactions. The nucleus is an example of an organelle.
The number and type of cytoplasmic organelles within each cell are related to the specific structure and function of the cell. Cells secreting large amounts of protein contain well-developed organelles that synthesize and secrete protein. Cells actively transporting substances such as sodium ions across their plasma membrane contain highly developed organelles that produce ATP. The following sections describe the structure and main functions of the major cytoplasmic organelles found in cells.
RIBOSOMES
Ribosomes are the sites of protein synthesis. Each ribosome is composed of a large subunit and a smaller one. The ribosomal subunits, which consist of ribosomal RNA (rRNA) and proteins, are assembled separately in the nucleolus of the nucleus. The ribosomal subunits then move through the nuclear pores into the cytoplasm, where they come together to form the functional ribosome during protein synthesis. Ribosomes can be found free in the cytoplasm or associated with a membrane called the endoplasmic reticulum. Free ribosomes primarily synthesize proteins used inside the cell, whereas endoplasmic reticulum ribosomes can produce proteins that are secreted from the cell.
ENDOPLASMIC RETICULUM
The outer membrane of the nuclear envelope is continuous with a series of membranes distributed throughout the cytoplasm of the cell, referred to as the endoplasmic reticulum. The endoplasmic reticulum consists of broad, flattened, interconnecting sacs and tubules. The interior spaces of those sacs and tubules are called cisternae and are isolated from the rest of the cytoplasm.
Rough endoplasmic reticulum is endoplasmic reticulum with attached ribosomes. The ribosomes of the rough endoplasmic reticulum produce proteins for secretion for internal use. The amount and make up of the endoplasmic reticulum within the cytoplasm depend on the cell type and function. Cells with abundant rough endoplasmic reticulum synthesize large amounts of protein that are secreted for use outside the cell.
Smooth endoplasmic reticulum, which is endoplasmic reticulum without attached ribosomes, produces lipids, such as phospholipids, cholesterol, steroid hormones, and carbohydrates such as glycogen. Cells that synthesize large amounts of lipid contain dense accumulations of smooth endoplasmic reticulum. Enzymes required for lipid synthesis are associated with the membranes of the smooth endoplasmic reticulum. Smooth endoplasmic reticulum also participates in the detoxification processes by which enzymes act on chemicals and drugs to change their structure and reduce their toxicity. The smooth endoplasmic reticulum of skeletal muscle stores calcium ions that function in muscle contraction.
GOLGI APPARATUS
The Golgi apparatus is composed of flattened membranous sacs, containing cisternae, that are stacked on each other like dinner plates. The Golgi apparatus modifies, packages, and distributes proteins and lipids manufactured by the rough and smooth endoplasmic reticula. Proteins produced at the ribosomes of the rough endoplasmic reticulum are surrounded by a vesicle, or little sac, that forms from the membrane of the endoplasmic reticulum. The vesicle moves to the Golgi apparatus, fuses with the membrane of the Golgi apparatus, and releases the protein into the cisterna of the Golgi apparatus. The Golgi apparatus concentrates and, in some cases, chemically modifies the proteins by synthesizing and attaching carbohydrate molecules to the proteins to form glycoproteins or attaching lipids to proteins to form lipoproteins. The proteins are then packaged into vesicles that pinch off from the margins of the Golgi apparatus and are distributed to various locations. Some vesicles carry proteins to the plasma membrane where the proteins are secreted from the cell by exocytosis; other vesicles contain proteins that become part of the plasma membrane; and still other vesicles contain enzymes that are used within the cell.
The Golgi apparatuses are most numerous and most highly developed in cells that secrete large amounts of protein or glycoproteins, such as cells in the salivary glands and the pancreas.
SECRETORY VESICLES
The membrane-bound secretory vesicles that pinch off from the Golgi apparatus move to the surface of the cell, their membranes fuse with the plasma membrane, and the contents of the vesicle are released to the exterior by exocytosis. The membranes of the vesicles are then incorporated into the plasma membrane.
Secretory vesicles accumulate in many cells, but their contents frequently are not released to the exterior until a signal is received by the cell. For example, secretory vesicles that contain the hormone insulin do not release it until the concentration of glucose in the blood increases and acts as a signal for the secretion of insulin from the cells.
LYSOSOMES
Lysosomes are membrane-bound vesicles that pinch off from the Golgi apparatus. They contain a variety of hydrolytic enzymes that work as intracellular digestive systems. Vesicles taken into the cell fuse with the lysosomes to form one vesicle and to expose the phagocytized materials to hydrolytic enzymes. Various enzymes within lysosomes digest nucleic acids, proteins, polysaccharides, and lipids. Certain white blood cells have large numbers of lysosomes that contain enzymes to digest phagocytized bacteria. Lysosomes also digest organelles of the cell that are no longer functional in a process called autophagia. Also, when tissues are damaged cells release their enzymes, which digest both damaged and healthy cells. In other cells the lysosomes move to the plasma membrane, and the enzymes are secreted by exocytosis. For example, the normal process of bone remodeling involves the breakdown of bone tissue by specialized bone cells. Enzymes responsible for that degradation are released into the extracellular fluid from lysosomes produced by those cells.