Each of the
100 trillion cells in a human being is a living structure that can survive for
months or many years, provided its surrounding fluids contain appropriate
nutrients. To understand the function of organs and other structures of the
body, it is essential that we first understand the basic organization of the
cell and the functions of its component parts.
Organization of the
Cell
A typical
cell, as seen by the light microscope, is shown in Figure 2–1. Its two major
parts are the nucleus and the cytoplasm. The nucleus is separated from the
cytoplasm by a nuclear membrane, and the cytoplasm is separated from the
surrounding fluids by a cell membrane, also called the plasma membrane. The
different substances that make up the cell are collectively called protoplasm.
Protoplasm is composed mainly of five basic substances: water, electrolytes,
proteins, lipids, and carbohydrates.
Water
The
principal fluid medium of the cell is water, which is present in most cells,
except for fat cells, in a concentration of 70 to 85 per cent. Many cellular
chemicals are dissolved in the water. Others are suspended in the water as
solid particulates. Chemical reactions take place among the dissolved chemicals
or at the surfaces of the suspended particles or membranes.
Ions
The most
important ions in the cell are potassium, magnesium, phosphate, sulfate,
bicarbonate, and smaller quantities of sodium, chloride, and calcium. These are
all discussed in more detail in Chapter 4, which considers the interrelations
between the intracellular and extracellular fluids. The ions provide inorganic
chemicals for cellular reactions. Also, they are necessary for operation of
some of the cellular control mechanisms. For instance, ions acting at the cell
membrane are required for transmission of electrochemical impulses in nerve and
muscle fibers.
Proteins
After water,
the most abundant substances in most cells are proteins, which normally
constitute 10 to 20 per cent of the cell mass. These can be divided into two
types: structural proteins and functional proteins. Structural proteins are
present in the cell mainly in the form of long filaments that themselves are
polymers of many individual protein molecules. A prominent use of such
intracellular filaments is to form microtubules that provide the “cytoskeletons”
of such cellular organelles as cilia, nerve axons, the mitotic spindles of
mitosing cells, and a tangled mass of thin filamentous tubules that hold the
parts of the cytoplasm and nucleoplasm together in their respective
compartments. Extracellularly, fibrillar proteins are found especially in the
collagen and elastin fibers of connective tissue and in blood vessel walls,
tendons, ligaments, and so forth. The functional proteins are an entirely
different type of protein, usually composed of combinations of a few molecules
in tubular-globular form. These proteins are mainly the enzymes of the cell and
in contrast to the fibrillar proteins, are often mobile in the cell fluid. Also,
many of them are adherent to membranous structures inside the cell. The enzymes
come into direct contact with other substances in the cell fluid and thereby
catalyze specific intracellular chemical reactions. For instance, the chemical
reactions that split glucose into its component parts and then combine these
with oxygen to form carbon dioxide and water while simultaneously providing
energy for cellular function are all catalyzed by a series of protein enzymes.
Lipids
Lipids are several types of substances that are grouped together because of
their common property of being soluble in fat solvents. Especially important
lipids are phospholipids and cholesterol, which together constitute only about
2 per cent of the total cell mass. The significance of phospholipids and
cholesterol is that they are mainly insoluble in water and, therefore, are used
to form the cell membrane and intracellular membrane barriers that separate the
different cell compartments. In addition to phospholipids and cholesterol, some
cells contain large quantities of triglycerides, also called neutral fat. In
the fat cells, triglycerides often account for as much as 95 per cent of the
cell mass. The fat stored in these cells represents the body’s main storehouse
of energy-giving nutrients that can later be dissoluted and used to provide
energy wherever in the body it is needed.
Carbohydrates
Carbohydrates
have little structural function in the cell except as parts of glycoprotein
molecules, but they play a major role in nutrition of the cell. Most human
cells do not maintain large stores of carbohydrates; the amount usually
averages about 1 per cent of their total mass but increases to as much as 3 per
cent in muscle cells and, occasionally, 6 per cent in liver cells. However,
carbohydrate in the form of dissolved glucose is always present in the surrounding
extracellular fluid so that it is readily available to the cell. Also, a small
amount of carbohydrate is virtually always stored in the cells in the form of
glycogen, which is an insoluble polymer of glucose that can be depolymerized
and used rapidly to supply the cells’ energy needs.
Physical Structure of the Cell
The cell is
not merely a bag of fluid, enzymes, and chemicals; it also contains highly
organized physical structures, called intracellular organelles. The physical
nature of each organelle is as important as the cell’s chemical constituents
for cell function. For instance, without one of the organelles, the
mitochondria, more than 95 per cent of the cell’s energy release from nutrients
would cease immediately.
Membranous Structures of the Cell
Most organelles
of the cell are covered by membranes composed primarily of lipids and proteins.
These membranes include the cell membrane, nuclear membrane, membrane of the
endoplasmic reticulum, and membranes of the mitochondria, lysosomes, and Golgi
apparatus. The lipids of the membranes provide a barrier that impedes the
movement of water and water-soluble substances from one cell compartment to
another because water is not soluble in lipids. However, protein molecules in
the membrane often do penetrate all the way through the membrane, thus
providing specialized pathways, often organized into actual pores, for passage
of specific substances through the membrane. Also, many other membrane proteins
are enzymes that catalyze a multitude of different chemical reactions,
discussed here and in subsequent articles.
Cell
Membrane The cell membrane (also called the plasma membrane), which envelops
the cell, is a thin, pliable, elastic structure only 7.5 to 10 nanometers
thick. It is composed almost entirely of proteins and lipids. The approximate
composition is proteins, 55 per cent; phospholipids, 25 per cent; cholesterol,
13 per cent; other lipids, 4 per cent; and carbohydrates, 3 per cent.
Lipid
Barrier of the Cell Membrane Impedes Water Penetration.
Its basic structure is
a lipid bilayer, which is a thin, double-layered film of lipids—each layer only
one molecule thick—that is continuous over the entire cell surface.
Interspersed in this lipid film are large globular protein molecules. The basic
lipid bilayer is composed of phospholipid molecules. One end of each
phospholipid molecule is soluble in water; that is, it is hydrophilic. The
other end is soluble only in fats; that is, it is hydrophobic. The phosphate
end of the phospholipid is hydrophilic, and the fatty acid portion is
hydrophobic. Because the hydrophobic portions of the phospholipid molecules are
repelled by water but are mutually attracted to one another, they have a
natural tendency to attach to one another in the middle of the membrane. The
hydrophilic phosphate portions then constitute the two surfaces of the complete
cell membrane, in contact with intracellular water on the inside of the
membrane and extracellular water on the outside surface. The lipid layer in the
middle of the membrane is impermeable to the usual water-soluble substances,
such as ions, glucose and urea. Conversely, fat-soluble substances, such as
oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane
with ease. The cholesterol molecules in the membrane are also lipid in nature
because their steroid nucleus is highly fat soluble. These molecules, in a
sense, are dissolved in the bilayer of the membrane. They mainly help determine
the degree of permeability (or impermeability) of the bilayer to water-soluble
constituents of body fluids. Cholesterol controls much of the fluidity of the
membrane as well.
Cell Membrane Proteins
The diagram also
shows globular masses floating in the lipid bilayer. These are membrane
proteins, most of which are glycoproteins.
Two types of
proteins occur: integral proteins that protrude all the way through the
membrane and peripheral proteins that are attached only to one surface of the
membrane and do not penetrate all the way through. Many of the integral
proteins provide structural channels (or pores) through which water molecules
and water-soluble substances, especially ions, can diffuse between the
extracellular and intracellular fluids. These protein channels also have
selective properties that allow preferential diffusion of some substances over
others. Other integral proteins act as carrier proteins for transporting
substances that otherwise could not penetrate the lipid bilayer. Sometimes
these even transport substances in the direction opposite to their natural
direction of diffusion, which is called “active transport.” Still others act as
enzymes.
Integral
membrane proteins can also serve as receptors for water-soluble chemicals, such
as peptide hormones, that do not easily penetrate the cell membrane.
Interaction of cell membrane receptors with specific ligands that bind to the
receptor causes conformational changes in the receptor protein. This, in turn,
enzymatically activates the intracellular part of the protein or induces
interactions between the receptor and proteins in the cytoplasm that act as
second messengers, thereby relaying the signal from the extracellular part of
the receptor to the interior of the cell. In this way, integral proteins
spanning the cell membrane provide a means of conveying information about the
environment to the cell interior. Peripheral protein molecules are often
attached to the integral proteins. These peripheral proteins function almost
entirely as enzymes or as controllers of transport of substances through the
cell membrane “pores.”
Membrane
Carbohydrates—the Cell “Glycocalyx.”
Membrane
carbohydrates occur almost invariably in combination with proteins or lipids in
the form of glycoproteins or glycolipids. In fact, most of the integral
proteins are glycoproteins and about one tenth of the membrane lipid molecules
are glycolipids. The “glyco” portions of these molecules almost invariably
protrude to the outside of the cell, dangling outward from the
cell
surface. Many other carbohydrate compounds, called proteoglycans—which are
mainly carbohydrate substances bound to small protein cores—are loosely
attached to the outer surface of the cell as well. Thus, the entire outside
surface of the cell often has a loose carbohydrate coat called the glycocalyx.
The carbohydrate moieties attached to the outer surface of the cell have
several important functions:
(1) Many of
them have a negative electrical charge, which gives most cells an overall
negative surface charge that repels other negative objects.
(2) The
glycocalyx of some cells attaches to the glycocalyx of other cells, thus
attaching cells to one another.
(3) Many of
the carbohydrates act as receptor substances for binding hormones, such as
insulin; when bound, this combination activates attached internal proteins
that, in turn, activate a cascade of intracellular enzymes.
(4) Some
carbohydrate moieties enter into immune reactions.
Cytoplasm and Its Organelles
The
cytoplasm is filled with both minute and large dispersed particles and
organelles. The clear fluid portion of the cytoplasm in which the particles are
dispersed is called cytosol; this contains mainly dissolved proteins,
electrolytes, and glucose. Dispersed in the cytoplasm are neutral fat globules,
glycogen granules, ribosomes, secretory vesicles, and five especially important
organelles: the endoplasmic reticulum, the Golgi apparatus, mitochondria,
lysosomes, and peroxisomes.
Endoplasmic Reticulum
Diagram
shows a network of tubular and flat vesicular structures in the cytoplasm; this
is the endoplasmic reticulum. The tubules and vesicles interconnect with one
another. Also, their walls are constructed of lipid bilayer membranes that
contain large amounts of proteins, similar to the cell membrane. The total
surface area of this structure in some cells the liver cells, for instance can
be as much as 30 to 40 times the cell membrane area. The detailed structure of
a small portion of endoplasmic reticulum is shown in diagram. The space inside
the tubules and vesicles is filled with endoplasmic matrix, a watery medium that
is different from the fluid in the cytosol outside the endoplasmic reticulum.
Electron micrographs show that the space inside the endoplasmic reticulum is
connected with the space between the two membrane surfaces of the nuclear
membrane. Substances formed in some parts of the cell enter the space of the
endoplasmic reticulum and are then conducted to other parts of the cell. Also,
the vast surface area of this reticulum and the multiple enzyme systems
attached to its membranes provide machinery for a major share of the metabolic
functions of the cell.
Ribosomes and the Granular Endoplasmic Reticulum
Attached to
the outer surfaces of many parts of the endoplasmic reticulum are large numbers
of minute granular particles called ribosomes. Where these are present, the
reticulum is called the granular endoplasmic reticulum. The ribosomes are
composed of a mixture of RNA and proteins, and they function to synthesize new
protein molecules in the cell.
Agranular Endoplasmic Reticulum
Part of the
endoplasmic reticulum has no attached ribosomes. This part is called the
agranular, or smooth, endoplasmic reticulum. The agranular reticulum functions
for the synthesis of lipid substances and for other processes of the cells
promoted by intrareticular enzymes.
Golgi
apparatus
The Golgi
apparatus, is closely related to the endoplasmic reticulum. It has membranes
similar to those of the agranular endoplasmic reticulum. It usually is composed
of four or more stacked layers of thin, flat, enclosed vesicles lying near one
side of the nucleus. This apparatus is prominent in secretory cells, where it
is located on the side of the cell from which the secretory substances are
extruded. The Golgi apparatus functions in association with the endoplasmic
reticulum. Shown in the diagram, small “transport vesicles” (also called
endoplasmic reticulum vesicles, or ER vesicles) continually pinch off from the
endoplasmic reticulum and shortly thereafter fuse with the Golgi apparatus. In
this way, substances entrapped in the ER vesicles are transported from the
endoplasmic reticulum to the Golgi apparatus. The transported substances are
then processed in the Golgi apparatus to form lysosomes, secretory vesicles, and
other cytoplasmic components.
Lysosomes
Lysosomes,
shown in diagram, are vesicular organelles that form by breaking off from the
Golgi apparatus and then dispersing throughout the cytoplasm.
The
lysosomes provide an intracellular digestive system that allows the cell to
digest
(1) Damaged
cellular structures,
(2) Food
particles that have been ingested by the cell and
(3) Unwanted
matter such as bacteria.
The lysosome
is quite different in different types of cells, but it is usually 250 to 750
nanometers in diameter. It is surrounded by a typical lipid bilayer membrane
and is filled with large numbers of small granules 5 to 8 nanometers in
diameter, which are protein aggregates of as many as 40 different hydrolase
(digestive) enzymes. A hydrolytic enzyme is capable of splitting an organic
compound into two or more parts by combining hydrogen from a water molecule
with one part of the compound and combining the hydroxyl portion of the water
molecule with the other part of the compound. For instance, protein is
hydrolyzed to form amino acids, glycogen is hydrolyzed to form glucose and
lipids are hydrolyzed to form fatty acids and glycerol. Ordinarily, the
membrane surrounding the lysosome prevents the enclosed hydrolytic enzymes from
coming in contact with other substances in the cell and, therefore, prevents
their digestive actions. However, some conditions of the cell break the
membranes of some of the lysosomes, allowing release of the digestive enzymes.
These enzymes then split the organic substances with which they come in contact
into small, highly diffusible substances such as amino acids and glucose. Some
of the more specific functions of lysosomes are discussed later in the chapter.
Peroxisomes
Peroxisomes
are similar physically to lysosomes, but they are different in two important
ways. First, they are believed to be formed by self-replication (or perhaps by
budding off from the smooth endoplasmic reticulum) rather than from the Golgi
apparatus. Second, they contain oxidases rather than hydrolases. Several of the
oxidases are capable of combining oxygen with hydrogen ions derived from
different intracellular chemicals to form hydrogen peroxide (H2O2). Hydrogen
peroxide is a highly oxidizing substance and is used in association with
catalase, another oxidase enzyme present in large quantities in peroxisomes, to
oxidize many substances that might otherwise be poisonous to the cell. For
instance, about half the alcohol a person drinks is detoxified by the
peroxisomes of the liver cells in this manner.
Secretory
Vesicles One of the important functions of many cells is secretion of special
chemical substances. Almost all such secretory substances are formed by the
endoplasmic reticulum–Golgi apparatus system and are then released from the
Golgi apparatus into the cytoplasm in the form of storage vesicles called
secretory vesicles or secretory granules. Diagram shows typical secretory
vesicles inside pancreatic acinar cells; these vesicles
store protein proenzymes (enzymes that are not yet activated).The proenzymes
are secreted later through the outer cell membrane into the pancreatic duct and
thence into the duodenum, where they become activated and perform digestive
functions on the food in the intestinal tract.
Mitochondria
The
mitochondria, shown in diagram, are called the “powerhouses” of the cell.
Without them, cells would be unable to extract enough energy from the
nutrients, and essentially all cellular functions would cease. Mitochondria are
present in all areas of each cell’s cytoplasm, but the total number per cell
varies from less than a hundred up to several thousand, depending on the amount
of energy required by the cell. Further, the mitochondria are concentrated in
those portions of the cell that are responsible for the major share of its
energy metabolism. They are also variable in size and shape. Some are only a
few hundred nanometers in diameter and globular in shape, whereas others are
elongated—as large as 1 micrometer in diameter and 7 micrometers long; still
others are branching and filamentous. The basic structure of the mitochondrion,
shown in diagram, is composed mainly of two lipid bilayer–protein membranes: an
outer membrane and an inner membrane. Many in folding of the inner membrane
form shelves onto which oxidative enzymes are attached. In addition, the inner
cavity of the mitochondrion is filled with a matrix that contains large
quantities of dissolved enzymes that are necessary for extracting energy from
nutrients. These enzymes operate in association with the oxidative enzymes on
the shelves to cause oxidation of the nutrients, thereby forming carbon dioxide
and water and at the same time releasing energy. The liberated energy is used
to synthesize a “high-energy” substance called adenosine triphosphate (ATP). ATP
is then transported out of the mitochondrion, and it diffuses throughout the
cell to release its own energy wherever it is needed for performing cellular
functions. (The chemical details of ATP formation by the mitochondrion are
given in other article), but some of the basic functions of ATP in the cell are
introduced later in this chapter. Mitochondria are self-replicative, which
means that one mitochondrion can form a second one, a third one, and so on,
whenever there is a need in the cell for increased amounts of ATP. Indeed, the
mitochondria contain DNA similar to that found in the cell nucleus. DNA is the
basic chemical of the nucleus that controls replication of the cell. The DNA of
the mitochondrion plays a similar role, controlling replication of the
mitochondrion itself.
Filament and Tubular Structures of the Cell
The fibrillar
proteins of the cell are usually organized into filaments or tubules. These
originate as precursor protein molecules synthesized by ribosomes in the
cytoplasm. The precursor molecules then polymerize to form filaments. As an
example, large numbers of actin filaments frequently occur in the outer zone of
the cytoplasm, called the ectoplasm, to form an elastic support for the cell
membrane. Also, in muscle cells, actin and myosin filaments are organized into a
special contractile machine that is the basis for muscle contraction. A special type of stiff filament composed of
polymerized tubulin molecules is used in all cells to construct very strong
tubular structures, the microtubules. Diagram shows typical microtubules that
were teased from the flagellum of a sperm. Another example of microtubules is
the tubular skeletal structure in the center of each cilium that radiates
upward from the cell cytoplasm to the tip of the cilium. This structure is
discussed later in the chapter and is illustrated in diagram. Also, both the centrioles
and the mitotic spindle of the mitosing cell are composed of stiff
microtubules. Thus, a primary function of microtubules is to act as a
cytoskeleton, providing rigid physical structures for certain parts of cells.
Nucleus
The nucleus
is the control center of the cell. Briefly, the nucleus contains large
quantities of DNA, which are the genes. The genes determine the characteristics
of the cell’s proteins, including the structural proteins, as well as the
intracellular enzymes that control cytoplasmic and nuclear activities. The
genes also control and promote reproduction of the cell itself. The genes first
reproduce to give two identical sets of genes; then the cell splits by a
special process called mitosis to form two daughter cells, each of which receives
one of the two sets of DNA genes. All these activities of the nucleus are
considered in detail in the next chapter. Unfortunately, the appearance of the
nucleus under the microscope does not provide many clues to the mechanisms by
which the nucleus performs its control activities. Diagram shows the light
microscopic appearance of the interphase nucleus (during the period between
mitoses), revealing darkly staining chromatin material throughout the
nucleoplasm. During mitosis, the chromatin material organizes in the form of
highly structured chromosomes, which can then be easily identified using the
light microscope, as illustrated in the next chapter.
Nuclear
Membrane
The nuclear
membrane, also called the nuclear envelope, is actually two separate bilayer
membranes, one inside the other. The outer membrane is continuous with the
endoplasmic reticulum of the cell cytoplasm, and the space between the two
nuclear membranes is also continuous with the space inside the endoplasmic
reticulum, as shown in diagram.
The nuclear
membrane is penetrated by several thousand nuclear pores. Large complexes of
protein molecules are attached at the edges of the pores so that the central area
of each pore is only about 9 nanometers in diameter. Even this size is large
enough to allow molecules up to 44,000 molecular weight to pass through with
reasonable ease.
Nucleoli and
Formation of Ribosomes
The nuclei
of most cells contain one or more highly staining structures called nucleoli.
The nucleolus, unlike most other organelles discussed here, does not have a
limiting membrane. Instead, it is simply an accumulation of large amounts of
RNA and proteins of the types found in ribosomes. The nucleolus becomes
considerably enlarged when the cell is actively synthesizing proteins.
Formation of the nucleoli (and of the ribosomes in the cytoplasm outside the
nucleus) begins in the nucleus. First, specific DNA genes in the chromosomes
cause RNA to be synthesized. Some of this is stored in the nucleoli, but most
of it is transported outward through the nuclear pores into cytoplasm. Here, it
is used in conjunction with specific proteins to assemble “mature” ribosomes
that play an essential role in forming cytoplasmic proteins.
 |
BASIC SCHEME OF STRUCTURE OF CELL |
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STRUCTURAL ARRANGEMENT OF CELL ORGANELLES |
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PHOSPHOLIPID BILAYER (CELL MEMBRANE) |
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ROUGH ENDOPLASMIC RETICULUM |
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ARRANGEMENT OF RER & SER |
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FORMATION OF SECRETORY VESICLES |
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STRUCTURE OF MITOCHONDRIUM |
 |
MICROFILAMENTS |
 |
NUCLEUS & NUCLEAR MEMBRANE |
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