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, 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, the cellular material outside the nucleus but inside the plasma membrane, is about half cytosol and half organelles.
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.
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.
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 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 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.
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.
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.
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 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.
Peroxisomes are membrane-bound vesicles that are smaller than lysosomes. Peroxisomes contain enzymes that break down fatty acids and amino acids. Hydrogen peroxide which breaks down hydrogen peroxide to water and oxygen. Cells that are active in detoxification, such as liver and kidney cells, have many peroxisomes.
Mitochondria usually are small, rod-shaped structures. In living cells, time lapse photomicrography shows that mitochondria constantly change shape from spherical to rod-shaped or even to long, threadlike structures. Mitochondria are the major sites of ATP production, which is the major energy source for most endergonic chemical reactions within the cell. Each mitochondrion has an inner and outer membrane separated by an intermembranous space. The outer membrane has a smooth contour, but the inner membrane has numerous infoldings called cristae that project like shelves into the interior of the mitochondria.
A complex series of mitochondrial enzymes forms two major enzyme systems that are responsible for oxidative metabolism and most ATP synthesis. The enzymes of the citric acid (or Krebs) cycle are found in the matrix, which is the substance located in the space formed by the inner membrane. The enzymes of the electron transport chain are embedded within the inner membrane. Cells with a greater energy requirement have more mitochondria with more cristae than cells with lower energy requirements. Within the cytoplasm of a given cell, the mitochondria are more numerous in areas in which ATP is used.
Increases in the number of mitochondria result from the division of preexisting mitochondria. When muscles enlarge as a result of exercise, the number of mitochondria within the muscle cells increases to provide the additional ATP required for muscle contraction.
The information for making some mitochondrial proteins is stored in DNA contained within the mitochondria themselves, and those proteins are synthesized on ribosomes within the mitochondria. The structure of many other mitochondrial proteins is determined by nuclear DNA, however, and these proteins are synthesized on ribosomes within the cytoplasm and then transported into the mitochondria. Both the mitochondrial DNA and mitochondrial ribosomes are very different from those within the nucleus and cytoplasm of the cell. In addition, unlike nuclear DNA, mitochondrial DNA does not have associated proteins.
CENTRIOLES AND SPINDLE FIBERS
The centrosome is a special area of the cytoplasm close to the nucleus that contains two centrioles. Each centriole is a small, cylindrical organelle about 0.3 – 0.5 um in length and 0.15 um in diameter, and the two centrioles are usually found perpendicular to each other within the centrosome. The wall of the centriole is made up of nine evenly spaced, side by side units, or triplets. Each unit consists of three microtubules located side by side and joined together.
The centrosome is the center of microtubule formation. Microtubules appear to have some control over the distribution of actin and intermediate filaments. Through its control of microtubule formation, the centrosome is closely involved in determining cell shape and movement. The microtubules extending from the centrosomes are constantly growing and shrinking.
Before there is cell division, the two centrioles double in number, the centrosome divides into two, and one centrosome, containing two centrioles, moves to each end of the cell. Spindle fibers extend out in all directions from the centrosome. These microtubules grow and shrink even more rapidly than those of nondividing cells. If a spindle fiber comes in contact with a kinetochore, the fiber attaches itself to the kinetochore and stops growing or shrinking. Eventually spindle fibers from each centromere attach to the kinetochores of all the chromosomes. Then the chromosomes are pulled apart and moved by the microtubules toward the two centrosomes during cell division.
CILIA AND FLAGELLA
Cilia are appendages that come from the surface of cells and are capable of movement. They are usually found on only one surface of a given cell and vary in number from one to thousands per cell. Cilia are cylindrical in shape, about 10 um in length and 0.2 um in diameter, and the shaft of each cilium is covered by the plasma membrane. Two centrally located microtubules and nine peripheral pairs of fused microtubules extend from the base to the tip of each cilium. Movement of the microtubules past each other, a process that requires energy from ATP, is responsible for movement of the cilia. A basal body is located in the cytoplasm at the base of the cilium. There are many cilia on surface cells that line the respiratory tract and the female reproductive tract. In these regions cilia move with a power stroke in one direction and a recovery stroke in the other direction. Their motion moves materials over the surface of the cells.
Flagella have a similar structure like cilia but are longer, and there is usually only one per cell. Whereas, cilia moves small particles across the cell surface, flagella moves the cell.
Microvilli are cylindrically shaped extensions of the plasma membrane about 0.5-1 um in length and 90 nm in diameter. Many microvilli are on each cell increasing the cell surface area. Microvilli are only one tenth to one twentieth the size of cilia. Microvilli does not move, and they are supported with actin filaments, not microtubules. They are found in the intestine, kidney, and other areas in which absorption is an important function. In some locations of the body, microvilli are highly modified to work as sensory receptors.
SUMMARY OF CELL PARTS
CELL PARTS STRUCTURE FUNCTION
Plasma Membrane Lipid bilayer composed of phospholipids and Outer boundary of cells that controls entry
cholesterol with proteins that extend across and exit of substances; receptor
or are buried in either surface of the lipid molecules function in intercellular
bilayer communication; marker molecules enable cells to recognize one another
Nuclear envelope Double membrane around the nucleus; the Separates nucleus from cytoplasm and
outer membrane is continuous with the controls movement of materials into and
endoplasmic reticulum; nuclear pores go out of the nucleus
through the nuclear envelope
Chromatin Thin strands of DNA, histones, and DNA controls protein synthesis and the
other proteins; condenses to form chemical reactions of the cell; DNA is
chromosomes during cell division the genetic or hereditary material
Nucleolus One to four dense bodies making up of Large and small ribosomal subunits are made
ribosomal RNA and proteins here
Fluid Part Water with dissolved ions and molecules; Contains enzymes that start
colloid with suspended proteins decomposition and synthesis reactions; ATP is produced in glycolysis reactions
Microtubules Hollow tubes composed of the protein Support the cytoplasm and form centrioles,
tubulin; 25 nm in diameter spindle fibers, cilia, and flagella; responsible for cell movements
Actin filaments Small fibrils of the protein actin; 8 nm in Support the cytoplasm and form centrioles,
diameter microvilli, responsible for cell movement
SUMMARY OF CELL PARTS
CELL PARTS STRUCTURE FUNCTION
Intermediate filaments Protein fibers; 10 nm in diameter Support the cytoplasm
Cytoplasmic inclusions Groups of molecules made or taken in Function depends on the molecules; energy
by the cell; may be surrounded storage, oxygen transport, skin color,
by a membrane and others
Ribosome Ribosomal RNA and proteins form large and Site of protein synthesis
small subunits; attached to endoplasmic
reticulum or free
Rough endoplasmic reticulum Membranous tubules and flattened sacs with Protein synthesis and transport to Golgi
attached ribosomes apparatus
Smooth endoplasmic reticulum Membranous tubules and flattened sacs with Makes lipids and carbohydrates;
attached ribosomes makes harmful chemical; stores calcium
Golgi apparatus Flattened membrane sacs stacked on each other Modification, packaging, and distribution of proteins and lipids for secretion or internal use
Secretory vesicle Membrane-bound sac pinched off Golgi Carries proteins and lipids to cell surface
apparatus for secretion
Lysosome Membrane-bound vesicle pinched off Golgi Contains digestive enzymes
Peroxisome Membrane-bound vesicle One site of lipid and amino acid breakdown and breaks down hydrogen peroxide
Mitochondria Round, rod-shaped, or threadlike Major site of ATP production when oxygen
structures; surrounded by double membrane; is available
inner membrane forms cristae
SUMMARY OF CELL PARTS
CELL PARTS STRUCTURE FUNCTION
Centrioles Pair of cylindrical organelles in the centrosome Centers for microtubule formation;
consisting of triplets of parallel microtubules determine cell polarity during cell division; form the basal bodies of cilia and flagella
Spindle fibers Microtubules extending from the centrosome to Assist in the separation of chromosomes
chromosomes and other parts of the cell during cell division
Cilia Extensions of the plasma membrane containing Move materials over the surface of cells
doublets of parallel microtubules
Flagellum Extensions of the plasma membrane containing In humans, responsible for movement of
doublets of parallel microtubules spermatozoa
Microvilli Extension of the plasma membrane containing Increase surface area of the plasma
microfilaments membrane for absorption and secretion; modified to form sensory receptors
Cell metabolism is all the decomposition and synthesis reactions in the cell. The breakdown of food molecules such as carbohydrates, lipids, and proteins releases energy that is used to synthesize ATP. Each ATP molecule has a portion of the energy stored from the chemical bonds of the food molecules. The ATP molecules are small energy packets that are used to drive other chemical reactions or processes such as active transport.
ATP production takes place in cytosol and in mitochondria through lots of chemical reactions. Food molecules transfer energy to ATP. If a cell was to receive all the energy from food molecules, it would literally burn up.
To show ATP production from food molecules: the breakdown of sugar glucose. For example: sugar from a candy bar. Once glucose is put into a cell, lots of reactions takes place inside the cytosol. These chemical reactions, glycolysis, change the glucose to pyruvic acid. Pyruvic acid can go into different biochemical pathways, if oxygen is available.
Aerobic respiration happens when oxygen is available. Pyruvic acid molecules enter mitochondria through chemical reactions called citric acid cycle and the electron transport chain, which are then changed to carbon dioxide and water. Energy stored in each glucose molecule can produce 36-38 ATP molecules through aerobic respiration.
Anaerobic respiration happens without oxygen and includes the change of pyruvic acid to lactic acid. There is a production of two ATP molecules for each glucose molecule used. Anaerobic respiration doesn’t produce as much ATP as aerobic respiration. But it does allow cells to work for a short time when oxygen is too
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