The Cell and Its Functions pdf - (Introduction to Physiology (The Cell and General Physiology

posed of combinations of a few molecules in tubular-globular form. These

are an entirely different type of protein, usually com-

The

ligaments, and so forth.

lagen and elastin fibers of connective tissue and in blood vessel walls, tendons,

compartments. Extracellularly, fibrillar proteins are found especially in the col-

hold the parts of the cytoplasm and nucleoplasm together in their respective

spindles of mitosing cells, and a tangled mass of thin filamentous tubules that

“cytoskeletons” of such cellular organelles as cilia, nerve axons, the mitotic

that themselves are polymers of many individual protein molecules. A promi-

functional proteins.

which normally constitute 10 to 20 per cent of the cell mass. These can be

After water, the most abundant substances in most cells are proteins,

trochemical impulses in nerve and muscle fibers.

instance, ions acting at the cell membrane are required for transmission of elec-

necessary for operation of some of the cellular control mechanisms. For

The ions provide inorganic chemicals for cellular reactions. Also, they are

relations between the intracellular and extracellular fluids.

These are all discussed in more detail in Chapter 4, which considers the inter-

sodium, chloride,

sulfate, bicarbonate,

potassium, magnesium, phosphate,

The most important ions in the cell are

the surfaces of the suspended particles or membranes.

particulates. Chemical reactions take place among the dissolved chemicals or at

chemicals are dissolved in the water. Others are suspended in the water as solid

cells, except for fat cells, in a concentration of 70 to 85 per cent. Many cellular

The principal fluid medium of the cell is water, which is present in most

Water.

trolytes, proteins, lipids, and carbohydrates.

Protoplasm is composed mainly of five basic substances: water, elec-

The different substances that make up the cell are collectively called

plasma membrane.

cell membrane,

and the cytoplasm is separated from the

nuclear membrane,

the cytoplasm by a

The nucleus is separated from

A typical cell, as seen by the light microscope, is shown in Figure 2–1. Its two

Organization of the Cell

parts.

organs and other structures of the body, it is essen-

To understand the function of

priate nutrients.

provided its surrounding fluids contain appro-

years,

The Cell and Its Functions

Each of the 100 trillion cells in a human being is a
living structure that can survive for months or many

tial that we first understand the basic organization
of the cell and the functions of its component

major parts are the nucleus and the cytoplasm.

surrounding fluids by a

also called the

proto-

plasm.

Ions.

and smaller quantities of

and calcium.

Proteins.

divided into two types: structural proteins and

Structural proteins are present in the cell mainly in the form of long filaments

nent use of such intracellular filaments is to form microtubules that provide the

functional proteins

The

hydrophobic.

is soluble only in fats; that is, it is

The other end

hydrophilic.

soluble in water; that is, it is

molecules. One end of each phospholipid molecule is

The basic lipid bilayer is composed of phospholipid

lar protein molecules.

surface. Interspersed in this lipid film are large globu-

lipid bilayer,

Figure 2–3 shows the structure of the cell membrane.

Lipid Barrier of the Cell Membrane Impedes Water Penetration.

other lipids, 4 per cent; and carbohydrates, 3 per cent.

phospholipids, 25 per cent; cholesterol, 13 per cent;

The approximate composition is proteins, 55 per cent;

is composed almost entirely of proteins and lipids.

elastic structure only 7.5 to 10 nanometers thick. It

brane), which envelops the cell, is a thin, pliable,

The cell membrane (also called the plasma mem-

discussed here and in subsequent chapters.

catalyze a multitude of different chemical reactions,

Also, many other membrane proteins are

passage of specific substances through the membrane.

pores,

pathways, often organized into actual

way through the membrane, thus providing specialized

because water is not soluble in lipids. However, protein

impedes the movement of water and water-soluble

The lipids of the membranes provide a barrier that

apparatus.

branes of the mitochondria, lysosomes,

cell membrane, nuclear membrane,

composed primarily of lipids and proteins. These mem-

Membranous Structures of the Cell

in Figure 2–2.

ents would cease immediately. The most important

than 95 per cent of the cell’s energy release from nutri-

mitochondria,

without one of the organelles, the

chemical constituents for cell function. For instance,

nature of each organelle is as important as the cell’s

The physical

intracellular organelles.

structures, called

chemicals; it also contains highly organized physical

The cell is not merely a bag of fluid, enzymes, and

Physical Structure of the Cell

the cells’ energy needs.

available to the cell. Also, a small amount of carbohy-

cent in liver cells. However, carbohydrate in the

as 3 per cent in muscle cells and, occasionally, 6 per

carbohydrates; the amount usually averages about 1

cell. Most human cells do not maintain large stores of

ecules, but they play a major role in nutrition of the

fat stored in these cells represents the body’s main

account for as much as 95 per cent of the cell mass. The

fat cells,

triglycerides,

In addition to phospholipids and cholesterol, some

ferent cell compartments.

therefore, are used to form the cell membrane and

cell mass. The significance of phospholipids and cho-

cholesterol,

of being soluble in fat solvents. Especially important

enzymes.

ical reactions. For instance, the chemical reactions

branous structures inside the cell. The enzymes come

cell fluid. Also, many of them are adherent to mem-

trast to the fibrillar proteins, are often mobile in the

of the cell and, in con-

Introduction to Physiology: The Cell and General Physiology

Unit I

proteins are mainly the enzymes

into direct contact with other substances in the cell
fluid and thereby catalyze specific intracellular chem-

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

Lipids.

Lipids are several types of substances that are

grouped together because of their common property

lipids are phospholipids and

which

together constitute only about 2 per cent of the total

lesterol is that they are mainly insoluble in water and,

intracellular membrane barriers that separate the dif-

cells contain large quantities of

also

called neutral fat. In the

triglycerides often

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 mol-

per cent of their total mass but increases to as much

form of dissolved glucose is always present in the

surrounding extracellular fluid so that it is readily

drate 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

organelles and other structures of the cell are shown

Most organelles of the cell are covered by membranes

branes include the
membrane of the endoplasmic reticulum, and mem-

and Golgi

substances from one cell compartment to another

molecules in the membrane often do penetrate all the

for

enzymes that

Cell Membrane

Its basic structure is a

which is a thin,

double-layered film of lipids—each layer only one
molecule thick—that is continuous over the entire cell

Nucleoplasm

Cytoplasm

Nucleus

Nucleolus

Cell

membrane

Nuclear

membrane

Structure of the cell as seen with the light microscope.

Figure 2–1

enzymes.

transport.” Still others act as

natural direction of diffusion, which is called “active

etrate the lipid bilayer. Sometimes these even trans-

stances over others.

fluids. These protein channels also have selective prop-

and water-soluble substances, especially ions, can

channels

all the way through the membrane, and

Two

glycoproteins.

brane proteins, most of which are

masses floating in the lipid bilayer. These are mem-

Figure 2–3 also shows globular

body fluids. Cholesterol controls much of the fluidity

ability) of the bilayer to water-soluble constituents of

in the bilayer of the membrane. They mainly help

fat soluble. These molecules, in a sense, are dissolved

The cholesterol molecules in the membrane are also

with ease.

alcohol, can penetrate this portion of the membrane

substances, such as oxygen, carbon dioxide, and

such as ions, glucose, and urea. Conversely, fat-soluble

impermeable to the usual water-soluble substances,

The lipid layer in the middle of the membrane is

water on the outside surface.

complete cell membrane, in contact with

brane, as shown in Figure 2–3. The hydrophilic phos-

attracted to one another, they have a natural tendency

the fatty acid portion is hydrophobic.

phosphate end of the phospholipid is hydrophilic, and

Chapter 2

The Cell and Its Functions

Because the hydrophobic portions of the phospho-

lipid molecules are repelled by water but are mutually

to attach to one another in the middle of the mem-

phate portions then constitute the two surfaces of the

intracellular

water on the inside of the membrane and extracellular

lipid in nature because their steroid nucleus is highly

determine the degree of permeability (or imperme-

of the membrane as well.

Cell Membrane Proteins.

types of proteins occur: integral proteins that protrude

peripheral pro-

teins that are attached only to one surface of the mem-
brane and do not penetrate all the way through.

Many of the integral proteins provide structural

(or pores) through which water molecules

diffuse between the extracellular and intracellular

erties that allow preferential diffusion of some sub-

Other integral proteins act as carrier proteins for

transporting substances that otherwise could not pen-

port substances in the direction opposite to their

Lysosome

Nucleolus

Cell
membrane

Secretory

granule

Mitochondrion

Centrioles

Microtubules

Nuclear

membrane

Granular

endoplasmic

reticulum

Smooth

(agranular)

endoplasmic

reticulum

Ribosomes

Glycogen

Golgi
apparatus

Microfilaments

Chromosomes and DNA

cell, showing the internal

Figure 2–2

Reconstruction of a typical

organelles in the cytoplasm
and in the nucleus.

teins, electrolytes, and glucose.

of the cytoplasm in which the particles are dispersed

persed particles and organelles. The clear fluid portion

The cytoplasm is filled with both minute and large dis-

Cytoplasm and Its Organelles

immune reactions, as discussed in Chapter 34.

enzymes. (4) Some carbohydrate moieties enter into

that, in turn, activate a cascade of intracellular

for binding hormones, such as insulin; when bound,

other cells, thus attaching cells to one another. (3)

charge that repels other negative objects. (2) The gly-

(1) Many of them have a negative electrical charge,

The carbohydrate moieties attached to the outer

Thus, the entire outside surface of the cell often has a

cell surface. Many other carbohydrate compounds,

to the outside of the cell, dangling outward from the

membrane lipid molecules are glycolipids. The “glyco”

proteins are glycoproteins, and about one tenth of the

In fact, most of the integral

glycolipids.

“pores.”

the integral proteins. These peripheral proteins func-

environment to the cell interior.

this way, integral proteins spanning the cell membrane

lular part of the receptor to the interior of the cell. In

sengers,

and proteins in the cytoplasm that act as

tional changes in the receptor protein. This, in turn,

mones, that do not easily penetrate the cell membrane.

for water-soluble chemicals, such as peptide hor-

Introduction to Physiology: The Cell and General Physiology

Unit I

Integral membrane proteins can also serve as recep-

tors

Interaction of cell membrane receptors with specific
ligands that bind to the receptor causes conforma-

enzymatically activates the intracellular part of the
protein or induces interactions between the receptor

second mes-

thereby relaying the signal from the extracel-

provide a means of conveying information about the

Peripheral protein molecules are often attached to

tion almost entirely as enzymes or as controllers of
transport of substances through the cell membrane

Membrane Carbohydrates—The Cell “Glycocalyx.”

Mem-

brane carbohydrates occur almost invariably in
combination with proteins or lipids in the form of gly-
coproteins or

portions of these molecules almost invariably protrude

called proteoglycans—which are mainly carbohydrate
substances bound to small protein cores—are loosely
attached to the outer surface of the cell as well.

loose carbohydrate coat called the glycocalyx.

surface of the cell have several important functions:

which gives most cells an overall negative surface

cocalyx of some cells attaches to the glycocalyx of

Many of the carbohydrates act as receptor substances

this combination activates attached internal proteins

is called cytosol; this contains mainly dissolved pro-

Integral protein

Extracellular

fluid

Intracellular

fluid

Cytoplasm

Lipid
bilayer

Carbohydrate

Integral protein

Peripheral

protein

George V. Kevin.)

from Lodish HF, Rothman JE: The

and to additional protein mole-

attached to the protein molecules

Also, carbohydrate moieties are

protruding through the layer.

numbers of protein molecules

Structure of the cell membrane,

Figure 2–3

showing that it is composed
mainly of a lipid bilayer of phos-
pholipid molecules, but with large

on the outside of the membrane

cules on the inside. (Redrawn

assembly of cell membranes. Sci
Am 240:48, 1979. Copyright

in the chapter.

other cytoplasmic components that are discussed later

apparatus to form lysosomes, secretory vesicles, and

endoplasmic reticulum to the Golgi apparatus. The

fuse with the Golgi apparatus. In this way, substances

reticulum vesicles, or

small “transport vesicles” (also called endoplasmic

the endoplasmic reticulum. As shown in Figure 2–5,

The Golgi apparatus functions in association with

in secretory cells, where it is located on the side of

one side of the nucleus. This apparatus is prominent

stacked layers of thin, flat, enclosed vesicles lying near

reticulum. It usually is composed of four or more

related to the endoplasmic reticulum. It has mem-

The Golgi apparatus, shown in Figure 2–5, is closely

cells promoted by intrareticular enzymes.

The agranular reticulum functions for the syn-

smooth, endoplasmic reticu-

agranular,

reticulum has no attached ribosomes. This part is

synthesize new protein molecules in the cell, as dis-

mixture of RNA and proteins, and they function to

The ribosomes are composed of a

present, the reticulum is called the

Where these are

ribosomes.

conducted to other parts of the cell. Also, the vast

membrane.

fluid in the cytosol outside the endoplasmic reticulum.

plasmic reticulum is shown in Figure 2–4. The space

The detailed structure of a small portion of endo-

cells, for instance—can be as much as 30 to 40 times

of proteins, similar to the cell membrane. The total

with one another. Also, their walls are constructed

The tubules and vesicles interconnect

ular structures in the cytoplasm; this is the

Figure 2–2 shows a network of tubular and flat vesic-

peroxisomes.

somes,

Golgi apparatus, mitochondria, lyso-

five especially important organelles: the

glycogen granules, ribosomes, secretory vesicles, and

Dispersed in the cytoplasm are neutral fat globules,

Chapter 2

The Cell and Its Functions

endoplasmic

reticulum, the

and

Endoplasmic Reticulum

endoplas-

mic reticulum.

of lipid bilayer membranes that contain large amounts

surface area of this structure in some cells—the liver

the cell membrane area.

inside the tubules and vesicles is filled with endoplas-
mic matrix, a watery medium that is different from the

Electron micrographs show that the space inside the
endoplasmic reticulum is connected with the space
between the two membrane surfaces of the nuclear

Substances formed in some parts of the cell enter

the space of the endoplasmic reticulum and are then

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

granular endoplas-

mic reticulum.

cussed later in this chapter and in Chapter 3.

Agranular Endoplasmic Reticulum.

Part of the endoplasmic

called the

lum.
thesis of lipid substances and for other processes of the

Golgi Apparatus

branes similar to those of the agranular endoplasmic

the cell from which the secretory substances are
extruded.

ER vesicles) continually pinch off

from the endoplasmic reticulum and shortly thereafter

entrapped in the ER vesicles are transported from the

transported substances are then processed in the Golgi

Matrix

Agranular
endoplasmic
reticulum

Granular

endoplasmic

reticulum

tis EDP, Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadel-

Structure of the endoplasmic reticulum. (Modified from DeRober-

Figure 2–4

phia: WB Saunders, 1975.)

Golgi
apparatus

Endoplasmic
reticulum

ER vesicles

Golgi vesicles

reticulum (ER) and the nucleus.

A typical Golgi apparatus and its relationship to the endoplasmic

Figure 2–5

and shape. Some are only a few hundred nanometers

its energy metabolism. They are also variable in size

on the amount of energy required by the cell. Further,

less than a hundred up to several thousand, depending

cytoplasm, but the total number per cell varies from

Mitochondria are present in all areas of each cell’s

would cease.

the nutrients, and essentially all cellular functions

called the “powerhouses” of the cell. Without them,

The mitochondria, shown in Figures 2–2 and 2–7, are

duct and thence into the duodenum, where they

not yet activated). The proenzymes are secreted later

tory vesicles inside pancreatic acinar cells; these

Figure 2–6 shows typical secre-

secretory granules.

released from the Golgi apparatus into the cytoplasm

tion of special chemical substances. Almost all such

Secretory Vesicles

liver cells in this manner.

sonous to the cell. For instance, about half the alcohol

enzyme present in large quantities in peroxisomes, to

catalase,

). Hydro-

they contain oxidases rather than hydrolases. Several

lum) rather than from the Golgi apparatus. Second,

they are different in two important ways. First, they are

Peroxisomes are similar physically to lysosomes, but

Peroxisomes

somes are discussed later in the chapter.

glucose. Some of the more specific functions of lyso-

tive enzymes. These enzymes then split the organic

some of the lysosomes, allowing release of the diges-

therefore, prevents their digestive actions. However,

Ordinarily, the membrane surrounding the lysosome

hydrolyzed to form glucose, and lipids are hydrolyzed

is hydrolyzed to form amino acids, glycogen is

other part of the compound. For instance, protein

lase (digestive) enzymes.

granules 5 to 8 nanometers in diameter, which are

in diameter. It is surrounded by a typical lipid bilayer

types of cells, but it is usually 250 to 750 nanometers

bacteria. The lysosome is quite different in different

ingested by the cell, and (3) unwanted matter such as

lular structures, (2) food particles that have been

plasm. The lysosomes provide an

apparatus and then dispersing throughout the cyto-

Lysosomes, shown in Figure 2–2, are vesicular

Lysosomes

Introduction to Physiology: The Cell and General Physiology

Unit I

organelles that form by breaking off from the Golgi

intracellular digestive

system that allows the cell to digest (1) damaged cel-

membrane and is filled with large numbers of small

protein aggregates of as many as 40 different hydro-

A hydrolytic enzyme is

capable of splitting an organic compound into two or
more parts by combining hydrogen from a water mol-
ecule with one part of the compound and combining
the hydroxyl portion of the water molecule with the

to form fatty acids and glycerol.

prevents the enclosed hydrolytic enzymes from
coming in contact with other substances in the cell and,

some conditions of the cell break the membranes of

substances with which they come in contact into small,
highly diffusible substances such as amino acids and

believed to be formed by self-replication (or perhaps
by budding off from the smooth endoplasmic reticu-

of the oxidases are capable of combining oxygen with
hydrogen ions derived from different intracellular
chemicals to form hydrogen peroxide (H

gen peroxide is a highly oxidizing substance and is
used in association with

another oxidase

oxidize many substances that might otherwise be poi-

a person drinks is detoxified by the peroxisomes of the

One of the important functions of many cells is secre-

secretory substances are formed by the endoplasmic
reticulum–Golgi apparatus system and are then

in the form of storage vesicles called secretory vesicles
or

vesicles store protein proenzymes (enzymes that are

through the outer cell membrane into the pancreatic

become activated and perform digestive functions on
the food in the intestinal tract.

Mitochondria

cells would be unable to extract enough energy from

the mitochondria are concentrated in those portions
of the cell that are responsible for the major share of

Secretory

granules

pancreas.

Secretory granules (secretory vesicles) in acinar cells of the

Figure 2–6

Outer membrane

Inner membrane

Oxidative

phosphorylation

enzymes

Outer chamber

Matrix

Crests

Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB

Structure of a mitochondrion. (Modified from DeRobertis EDP,

Figure 2–7

Saunders, 1975.)

reticulum, as shown in Figure 2–9.

with the endoplasmic reticulum of the cell cytoplasm,

inside the other. The outer membrane is continuous

is actually two separate bilayer membranes, one

lope,

nuclear membrane,

The

illustrated in the next chapter.

then be easily identified using the light microscope, as

chromosomes,

During mitosis, the chromatin material organizes in

chromatin material

period between mitoses), revealing darkly staining

activities. Figure 2–9 shows the light microscopic

Unfortunately, the appearance of the nucleus under

detail in the next chapter.

of which receives one of the two sets of DNA genes.

to form two daughter cells, each

identical sets of genes; then the cell splits by a special

the cell itself. The genes first reproduce to give two

The genes also control and promote reproduction of

plasmic and nuclear activities.

well as the intracellular enzymes that control cyto-

the cell’s proteins, including the structural proteins, as

The genes determine the characteristics of

genes.

nucleus contains large quantities of DNA, which are

The nucleus is the control center of the cell. Briefly, the

certain parts of cells.

Thus, a primary function of microtubules is to act as

composed of stiff microtubules.

and is illustrated in Figure 2–17. Also, both the

cilium. This structure is discussed later in the chapter

ates upward from the cell cytoplasm to the tip of the

Figure 2–8 shows typical microtubules that were

microtubules.

struct very strong tubular structures, the

traction, as discussed in detail in Chapter 6.

support for the cell membrane. Also, in muscle cells,

the cytoplasm, called the

As an example, large numbers of

filaments.

cytoplasm. The precursor molecules then polymerize

into filaments or tubules. These originate as precursor

The fibrillar proteins of the cell are usually organized

Filament and Tubular Structures of the Cell

controlling replication of the mitochondrion itself.

The DNA of the mitochondrion plays a similar role,

In Chapter 3 we will see that DNA is the basic chem-

similar to that found in the cell nucleus.

DNA

increased amounts of ATP. Indeed, the mitochondria

and so on, whenever there is a need in the cell for

one mitochondrion can form a second one, a third one,

Mitochondria are self-replicative, which means that

are introduced later in this chapter.

67, but some of the basic functions of ATP in the cell

forming cellular functions.The chemical details of ATP

release its own energy wherever it is needed for per-

mitochondrion, and it diffuses throughout the cell to

ATP is then transported out of the

triphosphate (ATP).

synthesize a “high-energy” substance called

time releasing energy. The liberated energy is used to

the shelves to cause oxidation of the nutrients, thereby

extracting energy from nutrients. These enzymes

are attached. In addition, the inner cavity of the mito-

inner membrane.

bilayer–protein membranes: an

in Figure 2–7, is composed mainly of two lipid

The basic structure of the mitochondrion, shown

filamentous.

7 micrometers long; still others are branching and

in diameter and globular in shape, whereas others are

Chapter 2

The Cell and Its Functions

elongated—as large as 1 micrometer in diameter and

outer membrane and

Many infoldings of the inner

membrane form shelves onto which oxidative enzymes

chondrion is filled with a matrix that contains large
quantities of dissolved enzymes that are necessary for

operate in association with the oxidative enzymes on

forming carbon dioxide and water and at the same

adenosine

formation by the mitochondrion are given in Chapter

contain

ical of the nucleus that controls replication of the cell.

protein molecules synthesized by ribosomes in the

to form
actin filaments frequently occur in the outer zone of

ectoplasm, to form an elastic

actin and myosin filaments are organized into a special
contractile machine that is the basis for muscle con-

A special type of stiff filament composed of poly-

merized tubulin molecules is used in all cells to con-

teased from the flagellum of a sperm.

Another example of microtubules is the tubular

skeletal structure in the center of each cilium that radi-

centri-

oles and the mitotic spindle of the mitosing cell are

a cytoskeleton, providing rigid physical structures for

Nucleus

the

process called mitosis

All these activities of the nucleus are considered in

the microscope does not provide many clues to the
mechanisms by which the nucleus performs its control

appearance of the interphase nucleus (during the

throughout the nucleoplasm.

the form of highly structured

which can

Nuclear Membrane

also called the nuclear enve-

and the space between the two nuclear membranes is
also continuous with the space inside the endoplasmic

formerly the Ciba Foundation.)

1967. Figure 4, page 314. Copyright the Novartis Foundation

Wolstenholme GEW, O’Connor M, and The publisher, JA Churchill,

Microtubules teased from the flagellum of a sperm. (From

Figure 2–8

exactly the same structure as its progenitor.

tion after generation, each new cell having almost

vides a control center for all cellular activities, and it

of cell from all lower forms of life; the nucleus pro-

itself. The nucleus distinguishes this type

organelles developed, the most important of which is

Finally, in the nucleated cell, still more complex

the organism, representing physical structures of

ettsial and bacterial stages,

In still later stages of life, particularly in the rick-

mining the organism’s activities.

catalyzing chemical reactions and, therefore, deter-

matrix appeared. Specialized chemicals then devel-

around the virus, and inside the membrane, a fluid

in different parts of the virus. A membrane formed

organism, and specialized functions began to develop

As life evolved, other chemicals besides nucleic acid

human being are living structures.

appropriate conditions. Thus, the virus propagates its

cells, and it is capable of reproducing itself under

acid constituents (DNA or RNA) found in mammalian

This nucleic acid is composed of the same basic nucleic

The essential life-giving constituent of the small

cell are also far more complex than those of the virus.

ingly, the functions and anatomical organization of the

billion times that of the smallest virus. Correspond-

of the smallest virus and, therefore, a volume about 1

rickettsia,

smallest known virus, (2) a large virus, (3) a

earth. Figure 2–10 shows the relative sizes of (1) the

virus,

develop after the earliest form of life, an organism

However, the cell is a very complicated organism that

Many of us think of the cell as the lowest level of life.

with Precellular Forms of Life

forming cytoplasmic proteins, as discussed more fully

“mature” ribosomes that play an essential role in

through the nuclear pores into cytoplasm. Here, it is

in the nucleoli, but most of it is transported outward

cause RNA to be synthesized. Some of this is stored

nucleus. First, specific DNA genes in the chromosomes

the cytoplasm outside the nucleus) begins in the

Formation of the nucleoli (and of the ribosomes in

ing proteins.

types found in ribosomes. The nucleolus becomes con-

mulation of large amounts of RNA and proteins of the

have a limiting membrane. Instead, it is simply an accu-

unlike most other organelles discussed here, does not

The nucleolus,

The nuclei of most cells contain one or more highly

Nucleoli and Formation of Ribosomes

pass through with reasonable ease.

nanometers in diameter. Even this size is large enough

nuclear pores.

The nuclear membrane is penetrated by several

Introduction to Physiology: The Cell and General Physiology

Unit I

thousand

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

to allow molecules up to 44,000 molecular weight to

staining structures called nucleoli.

siderably enlarged when the cell is actively synthesiz-

used in conjunction with specific proteins to assemble

in Chapter 3.

Comparison of the Animal Cell

required many hundreds of millions of years to

similar to the present-day

first appeared on

(4) a bacterium, and (5) a nucleated cell, demonstrat-
ing that the cell has a diameter about 1000 times that

virus is a nucleic acid embedded in a coat of protein.

lineage from generation to generation and is therefore
a living structure in the same way that the cell and the

and simple proteins became integral parts of the

oped inside the fluid to perform special functions;
many protein enzymes appeared that were capable of

organelles developed inside

chemical aggregates that perform functions in a more
efficient manner than can be achieved by dispersed
chemicals throughout the fluid matrix.

the nucleus

provides for exact reproduction of new cells genera-

Nuclear envelope –

Endoplasmic
reticulum

Nucleoplasm

Cytoplasm

outer and inner
membranes

Pores

Nucleolus

Chromatin material (DNA)

Structure of the nucleus.

Figure 2–9

15 nm — Small virus

150 nm — Large virus

350 nm — Rickettsia

m Bacterium

5 – 10

Cell

average cell in the human body.

Comparison of sizes of precellular organisms with that of the

Figure 2–10

a second to surround the entire particle; then,

2. The edges of the membrane around the points of

ligands of the particle.

1. The cell membrane receptors attach to the surface

Phagocytosis occurs in the following steps:

terium along with it. This intermediation of antibodies

attaches to the phagocyte receptors, dragging the bac-

to a specific antibody, and it is the antibody that

of bacteria, each bacterium usually is already attached

receptors on the surface of the phagocyte. In the case

bacterium, a dead cell, or tissue debris binds with

Phagocytosis is initiated when a particle such as a

tissue macrophages and some of the white blood cells.

have the capability of phagocytosis, most notably the

particles rather than molecules. Only certain cells

way as pinocytosis, except that it involves large

Phagocytosis occurs in much the same

membrane.

cellular fluid, which probably react with contractile

energy substance discussed later in the chapter. Also,

from within the cell; this is supplied by ATP, a high-

remains mainly a mystery. This process requires energy

necessary contortions to form pinocytotic vesicles

What causes the cell membrane to go through the

inside the cytoplasm of the cell.

from the surface of the cell, forming a

amount of extracellular fluid. Immediately thereafter,

invaginates inward, and the fibrillar proteins sur-

with the receptors, the surface properties of the local

teins, perhaps including contractile filaments of

coated pits.

the cell membrane, called

protein that is to be absorbed. The receptors generally

attaching to the membrane. These molecules usually

pinocytosis, showing three molecules of protein

Figure 2–11 demonstrates the successive steps of

molecules attach to the cell membrane.

enter cells. In fact, the rate at which pinocytotic vesi-

macromolecules, such as most protein molecules, can

Pinocytosis is the only means by which most large

tron microscope.

each minute. Even so, the pinocytotic vesicles are so

in some cells. For instance, it occurs so rapidly in

membranes of most cells, but it is especially rapid

Pinocytosis occurs continually in the cell

tissue.

as bacteria, whole cells, or portions of degenerating

Phagocytosis means ingestion of large particles, such

particulate constituents inside the cell cytoplasm.

Pinocytosis means ingestion of minute

phagocytosis.

principal forms of endocytosis are

The

endocytosis.

Very large particles enter the cell by a specialized

membrane. These active transport mechanisms are so

membrane.

soluble substances, through the lipid matrix of the

through cell membrane pores or, in the case of lipid-

ecules of the substance; substances move either

rounding fluids. Most substances pass through the cell

obtain nutrients and other substances from the sur-

If a cell is to live and grow and reproduce, it must

In the remainder of this chapter, we discuss several

Chapter 2

The Cell and Its Functions

Functional Systems of the Cell

representative functional systems of the cell that make
it a living organism.

Ingestion by the Cell—Endocytosis

membrane by diffusion and active transport.

Diffusion involves simple movement through the

membrane caused by the random motion of the mol-

Active transport involves the actual carrying of a

substance through the membrane by a physical pro-
tein structure that penetrates all the way through the

important to cell function that they are presented in
detail in Chapter 4.

function of the cell membrane called

pinocytosis and

particles that form vesicles of extracellular fluid and

Pinocytosis.

macrophages that about 3 per cent of the total macro-
phage membrane is engulfed in the form of vesicles

small—usually only 100 to 200 nanometers in diame-
ter—that most of them can be seen only with the elec-

cles form is usually enhanced when such macro-

attach to specialized protein receptors on the surface
of the membrane that are specific for the type of

are concentrated in small pits on the outer surface of

On the inside of

the cell membrane beneath these pits is a latticework
of fibrillar protein called clathrin, as well as other pro-

actin

and myosin. Once the protein molecules have bound

membrane change in such a way that the entire pit

rounding the invaginating pit cause its borders to close
over the attached proteins as well as over a small

the invaginated portion of the membrane breaks away

pinocytotic

vesicle

it requires the presence of calcium ions in the extra-

protein filaments beneath the coated pits to provide
the force for pinching the vesicles away from the cell

Phagocytosis.

is called opsonization, which is discussed in Chapters
33 and 34.

attachment evaginate outward within a fraction of

progressively more and more membrane receptors

Receptors

Actin and myosin

Dissolving clathrin

Proteins

Coated pit

Clathrin

Figure 2–11

Mechanism of pinocytosis.

vesicles and tubules, that is, into the

protein molecules directly into the cytosol, but they

somes. The ribosomes extrude some of the synthesized

membrane. As we discuss in Chapter 3, protein mole-

The granular portion of the endoplasmic reticulum is

Golgi apparatus.

before being released into the cytoplasm. But first, let

Golgi apparatus, where they are further processed

The products formed there are then passed on to the

lipid bilayer membranes similar to the cell membrane,

emphasized. These structures are formed primarily of

The extensiveness of the endoplasmic reticulum and

and Golgi Apparatus

Synthesis and Formation of Cellular

inactivates bacterial metabolic systems.

pH of about 5.0, which activates the hydrolases and

they can promote bacterial growth; and (3) acid at a

which dissolves the bacterial cell membrane; (2)

lysozyme,

cellular damage. These agents include (1)

can kill phagocytized bacteria before they can cause

The lysosomes also contain bactericidal agents that

cell to take the place of the old one.

pletely removed, and a new cell of the same type ordi-

In this way, the cell is com-

autolysis.

the damage is severe, the entire cell is digested, a

the cell is removed, followed by repair of the cell. If

substances. If the damage is slight, only a portion of

somes to rupture. The released hydrolases immedi-

trauma, chemicals, or any other factor—induces lyso-

tissues. Damage to the cell—caused by heat, cold,

much of this regression. The mechanism by which lack

the end of lactation. Lysosomes are responsible for

long periods of inactivity, and in mammary glands at

occurs in the uterus after pregnancy, in muscles during

body often regress to a smaller size. For instance, this

Tissues of the

Regression of Tissues and Autolysis of Cells.

the cells.

Thus, the pinocytotic and phagocytic vesicles con-

opposite of endocytosis.

exocytosis,

represents indigestible substances. In most instances,

residual body,

left of the digestive vesicle, called the

membrane of the vesicle into the cytoplasm. What is

phosphates, and so forth that can diffuse through the

digestion are small molecules of amino acids, glucose,

and other substances in the vesicle. The products of

begin hydrolyzing the proteins, carbohydrates, lipids,

the cell cytoplasm in which the vesicular hydrolases

Figure 2–12. Thus, a

to the inside of the vesicle, as shown in

vesicle appears inside a cell, one or more

Almost immediately after a pinocytotic or phagocytic

the Cell—Function of the Lysosomes

Digestion of Pinocytotic and

pinocytotic vesicles are formed.

separates from the cell membrane, leaving the

4. The contractile proteins then pinch the stem of

interior.

around its outer edge, pushing the vesicle to the

surround the phagocytic vesicle and contract

3. Actin and other contractile fibrils in the cytoplasm

phagocytic vesicle.

suddenly in a zipper-like manner to form a closed

attach to the particle ligands. All this occurs

Introduction to Physiology: The Cell and General Physiology

Unit I

the vesicle so completely that the vesicle

vesicle in the cell interior in the same way that

Phagocytic Foreign Substances Inside

lysosomes

become attached to the vesicle and empty their acid
hydrolases

digestive vesicle is formed inside

this is finally excreted through the cell membrane by
a process called

which is essentially the

taining lysosomes can be called the digestive organs of

of activity in a tissue causes the lysosomes to increase
their activity is unknown.

Another special role of the lysosomes is removal of

damaged cells or damaged portions of cells from

ately begin to digest the surrounding organic

process called

narily is formed by mitotic reproduction of an adjacent

lyso-

ferrin, which binds iron and other substances before

Structures by Endoplasmic Reticulum

Specific Functions of the
Endoplasmic Reticulum

the Golgi apparatus in secretory cells has already been

and their walls are loaded with protein enzymes that
catalyze the synthesis of many substances required by
the cell.

Most synthesis begins in the endoplasmic reticulum.

us note the specific products that are synthesized in
specific portions of the endoplasmic reticulum and the

Proteins Are Formed by the Granular Endoplasmic Reticulum.

characterized by large numbers of ribosomes attached
to the outer surfaces of the endoplasmic reticulum

cules are synthesized within the structures of the ribo-

also extrude many more through the wall of the endo-
plasmic reticulum to the interior of the endoplasmic

endoplasmic

matrix.

Lysosomes

Pinocytotic or
phagocytic
vesicle

Digestive vesicle

Residual body

Excretion

enzymes derived from lysosomes.

Figure 2–12

Digestion of substances in pinocytotic or phagocytic vesicles by

lar use.

Some vesicles, however, are destined for intracellu-

membrane’s outer surface and extrusion of its contents

brane, followed by exocytosis—that is, opening of the

entry of calcium ions into the cell; calcium ions inter-

Exocytosis, in most cases, is stimulated by the

cytosis.

membrane, then fuse with it and empty their sub-

brane. These secretory vesicles first diffuse to the cell

In a highly secretory cell, the

Vesicles and Lysosomes.

Types of Vesicles Formed by the Golgi Apparatus—Secretory

and within 1 to 2 hours, radioactive proteins are

proteins are already present in the Golgi apparatus,

within 3 to 5 minutes. Within 20 minutes, newly formed

acids, newly formed radioactive protein molecules can

When a glandular cell is bathed in radioactive amino

To give an idea of the timing of these processes:

turn, the vesicles diffuse throughout the cell.

with them the compacted secretory substances, and in

ually break away from the Golgi apparatus, carrying

proceed. Finally, both small and large vesicles contin-

Golgi apparatus, the compaction and processing

secretions into highly concentrated packets. As the

secretions. Also, an important function of the Golgi

the vesicular spaces of the Golgi apparatus. Here,

The transport vesicles instantly fuse with the Golgi

of the Golgi apparatus. Inside these

deepest layer

the Golgi apparatus. At this point, small

endoplasmic reticulum, especially the proteins, they

Golgi apparatus. As substances are formed in the

Figure 2–13 summarizes the

Formation of Vesicles.

cartilage and bone.

between collagen fibers and cells; and (3) they are

the cells in the interstitial spaces, acting as filler

in mucus and other glandular secretions; (2) they are

chondroitin sulfate in the body are as follows: (1) they

chondroitin sulfate.

bound with small amounts of protein; the most impor-

formed in the endoplasmic reticulum. This is especially

in the endoplasmic reticulum, it also has the capability

in other ways.

hydrolysis, conjugation with glycuronic acid, and

detoxification by coagulation, oxidation,

that might damage the cell. It achieves

capable of detoxifying substances, such as drugs,

2. It provides a vast number of enzymes that are

energy.

1. It provides the enzymes that control glycogen

cially the smooth reticulum, include the following:

nificant functions of the endoplasmic reticulum, espe-

migrate rapidly to the Golgi apparatus.

from the smooth reticulum; most of these vesicles then

beyond the needs of the cell, small vesicles called

To keep the endoplasmic reticulum from growing

reticulum to grow more extensive. This occurs mainly

plasmic reticulum itself, thus causing the endoplasmic

especially phospholipids and cholesterol. These are

The endoplasmic reticulum also synthesizes lipids,

Chapter 2

The Cell and Its Functions

Synthesis of Lipids by the Smooth Endoplasmic Reticulum.

rapidly incorporated into the lipid bilayer of the endo-

in the smooth portion of the endoplasmic reticulum.

vesicles or transport vesicles continually break away

Other Functions of the Endoplasmic Reticulum.

Other sig-

breakdown when glycogen is to be used for

Specific Functions of the Golgi Apparatus
Synthetic Functions of the Golgi Apparatus.

Although the

major function of the Golgi apparatus is to provide
additional processing of substances already formed

of synthesizing certain carbohydrates that cannot be

true for the formation of large saccharide polymers

tant of these are hyaluronic acid and

A few of the many functions of hyaluronic acid and

are the major components of proteoglycans secreted

the major components of the ground substance outside

principal components of the organic matrix in both

Processing of Endoplasmic Secretions by the Golgi Apparatus—

major functions of the endoplasmic reticulum and

are transported through the tubules toward portions
of the smooth endoplasmic reticulum that lie nearest

transport vesi-

cles composed of small envelopes of smooth endo-
plasmic reticulum continually break away and diffuse
to the
vesicles are the synthesized proteins and other prod-
ucts from the endoplasmic reticulum.

apparatus and empty their contained substances into

additional carbohydrate moieties are added to the

apparatus is to compact the endoplasmic reticular

secretions pass toward the outermost layers of the

be detected in the granular endoplasmic reticulum

secreted from the surface of the cell.

vesicles formed by the Golgi apparatus are mainly
secretory vesicles containing protein substances that
are to be secreted through the surface of the cell mem-

stances to the exterior by the mechanism called exo-

act with the vesicular membrane in some way that is
not understood and cause its fusion with the cell mem-

outside the cell.

Transport

Ribosomes

Lysosomes

Secretory

vesicles

Protein

formation

Glycosylation

vesicles

Smooth

endoplasmic

reticulum

Golgi

apparatus

Granular

endoplasmic

reticulum

Lipid

formation

plasmic reticulum and Golgi apparatus.

Formation of proteins, lipids, and cellular vesicles by the endo-

Figure 2–13

released during this conversion, but this amount

amount of ADP is changed into ATP by the energy

). A small

pyruvic acid

On entry into the cells, glucose is sub-

Chemical Processes in the Formation of ATP—Role of the

ually, having a turnover time of only a few minutes.

these reasons, ATP has been called the

and the entire process repeats over and over again. For

and phosphoric acid to recombine to form new ATP,

energy derived from the cellular nutrients causes ADP

To reconstitute the cellular ATP as it is used up,

tually all of the cell’s other functions, such as synthe-

is formed. This released energy is used to energize vir-

radical is split away, and

When ATP releases its energy, a phosphoric acid

other intracellular reactions.

bond is very labile, so that it can be split instantly on

Further, the high-energy phosphate

average chemical bond, thus giving rise to the term

12,000 calories of energy per mole of ATP, which is

body,

Under the physical and chemical conditions of the

high-energy phosphate bonds,

The last two phosphate rad-

phosphate radicals.

ribose,

adenine,

ATP is a nucleotide composed of (1) the nitrogenous

energy released in the proper direction. The details of

chemically with oxygen, under the influence of

ing the cell. Inside the cell, the foodstuffs react

stuffs—glucose, fatty acids, and amino acids—all enter-

Figure 2–14 shows oxygen and the food-

fatty acids.

before they reach the other cells of the body. Similarly,

human body, essentially all carbohydrates are con-

oxygen—carbohydrates, fats, and proteins. In the

The principal substances from which cells extract

Extraction of Energy from Nutrients—

In summary, the membranous system of the endo-

apparatus continually replenish the cell membrane.

totic vesicle, and the vesicular membranes of the Golgi

substance every time it forms a phagocytic or pinocy-

up. For instance, the cell membrane loses much of its

This increases the expanse of these membranes and

Use of Intracellular Vesicles to Replenish Cellular Membranes.

Introduction to Physiology: The Cell and General Physiology

Unit I

Some of the intracellular vesicles formed by the Golgi
apparatus fuse with the cell membrane or with the
membranes of intracellular structures such as the
mitochondria and even the endoplasmic reticulum.

thereby replenishes the membranes as they are used

plasmic reticulum and Golgi apparatus represents a
highly metabolic organ capable of forming new intra-
cellular structures as well as secretory substances to be
extruded from the cell.

Function of the Mitochondria

energy are foodstuffs that react chemically with

verted into glucose by the digestive tract and liver

proteins are converted into amino acids and fats into

enzymes that control the reactions and channel the

base

(2) the pentose sugar

and (3)

three
icals are connected with the remainder of the molecule
by so-called

which are

represented in the formula above by the symbol ~.

each of these high-energy bonds contains about

many times greater than the energy stored in the

high-energy bond.

demand whenever energy is required to promote

adenosine diphosphate (ADP)

sis of substances and muscular contraction.

energy currency

of the cell because it can be spent and remade contin-

Mitochondria.

jected to enzymes in the cytoplasm that convert it
into

(a process called glycolysis

36 ATP

2ADP

2ATP

ATP

Pyruvic acid

Amino acids

Cell

membrane

Fatty acids

Glucose

Acetoacetic
acid

Acetyl-CoA

Mitochondrion

Nucleus

Acetyl-CoA

ADP

36 ADP

that most of the ATP is formed in the mitochondria. ADP, adeno-

Formation of adenosine triphosphate (ATP) in the cell, showing

Figure 2–14

sine diphosphate.

O-

Phosphate

Adenosine triphosphate

Adenine

Ribose

Functional Characteristics of ATP

quent intracellular metabolic reactions.

Then, ATP, not the original foodstuffs, is used

ATP.

inside the mitochondria, and the energy that is

Briefly, almost all these oxidative reactions occur

all these digestive and metabolic functions are given
in Chapters 62 through 72.

released is used to form the high-energy compound

throughout the cell to energize almost all the subse-

cell it is needed. To replace the ATP used by the cell,

In summary, ATP is always available to release its

energy for all these types of mechanical work is ATP.

are described later in this chapter. The source of

ways, especially by

energy. Other cells perform mechanical work in other

requires expenditure of tremendous quantities of ATP

special cells to perform mechanical work. We see in

The final major use of ATP is to supply energy for

the growth phase of cells.

cially protein molecules; this is particularly true during

simply to synthesize new chemical compounds, espe-

much as 75 per cent of all the ATP formed in the cell

protein molecule is formed. Indeed, some cells use as

ATP molecules must release their energy as each

down of four high-energy bonds; thus, many thousand

another by peptide linkages; the formation of each of

chemical compound requires energy. For instance, a

host of other substances. Synthesis of almost any

phospholipids, cholesterol, purines, pyrimidines, and a

In addition to synthesizing proteins, cells synthesize

for this purpose alone.

use as much as 80 per cent of the ATP that they form

that some cells—the renal tubular cells, for instance—

and many other ions and various organic substances.

phate ions, chloride ions, urate ions, hydrogen ions,

of potassium ions, calcium ions, magnesium ions, phos-

energy from ATP is required for membrane transport

protein synthesis by the ribosomes, and (3) to supply

of sodium through the cell membrane, (2) to promote

in Figure 2–15: (1) to supply energy for the transport

These uses of ATP are illustrated by examples

mechanical

throughout the cell, and (3)

synthesis of chemical

membranes in the cell, (2)

functions: (1)

Energy from ATP is

Uses of ATP for Cellular Function.

metabolic functions of ATP in the body are presented

presented in Chapter 67, and many of the detailed

of ATP formation. The

chemiosmotic mechanism

This overall process for formation of ATP is called

energize multiple cell functions.

plasm and nucleoplasm, where its energy is used to

out of the mitochondria into all parts of the cell cyto-

ADP to ATP. The newly formed ATP is transported

Finally, the enzyme ATP synthetase uses the energy

from the membranes of the mitochondrial shelves.

ATP synthetase,

teins, called

ion. The terminal event is combination of hydrogen

the hydrogen atom, thus converting it to a hydrogen

matrix. The initial event is removal of an electron from

ATP. The processes of these reactions are complex,

chondria to convert very large amounts of ADP to

dous amount of energy, which is used by the mito-

diffused into the mitochondria. This releases a tremen-

The hydrogen atoms, conversely, are highly reactive,

from the body through the lungs.

and eventually out of the cell; finally, it is excreted

The carbon dioxide diffuses out of the mitochondria

carbon dioxide.

component parts,

In this citric acid cycle, acetyl-CoA is split into its

These chemical reactions are

Krebs cycle.

cycle,

mitochondrion matrix, undergoing dissolution in a

turn, is further dissoluted (for the purpose of extract-

in the matrix of the mitochondrion. This substance, in

acids from lipids, and amino acids from proteins are

The pyruvic acid derived from carbohydrates, fatty

cell, about 95 per cent, is formed in the mitochondria.

By far, the major portion of the ATP formed in the

Chapter 2

The Cell and Its Functions

accounts for less than 5 per cent of the overall energy
metabolism of the cell.

eventually converted into the compound acetyl-CoA

ing its energy) by another series of enzymes in the

sequence of chemical reactions called the citric acid

so important that they are explained in detail in
Chapter 67.

hydrogen atoms and

and they combine instantly with oxygen that has also

requiring the participation of large numbers of protein
enzymes that are integral parts of mitochondrial mem-
branous shelves that protrude into the mitochondrial

ions with oxygen to form water plus the release of
tremendous amounts of energy to large globular pro-

that protrude like knobs

from the hydrogen ions to cause the conversion of

the
chemical and physical details of this mechanism are

in Chapters 67 through 71.

used to promote three major categories of cellular

transport of substances through multiple

compounds
work.

the energy needed during muscle contraction.

In addition to membrane transport of sodium,

Membrane transport is so important to cell function

single protein molecule might be composed of as many
as several thousand amino acids attached to one

these linkages requires energy derived from the break-

Chapter 6 that each contraction of a muscle fiber

ciliary and ameboid motion, which

energy rapidly and almost explosively wherever in the

ATP

ADP

ATP

ADP

Mitochondrion

ADP

Muscle contraction

ADP

Protein synthesis

Ribosomes

Membrane

transport

Endoplasmic
reticulum

brane transport, protein synthesis, and muscle contraction. ADP,

drion) to provide energy for three major cellular functions: mem-

Use of adenosine triphosphate (ATP) (formed in the mitochon-

Figure 2–15

adenosine diphosphate.

whiplike movement of cilia on the surfaces of cells. This

ciliary movement,

certain, it is known that the side of the cell most exposed

ameboid locomotion? Although the answer is not

tive chemotaxis.

cells move away from the source, which is called

positive chemotaxis.

source of a chemotactic substance—that is, from an area

chemotactic substance.

ance of certain chemical substances in the tissues.

This results from the appear-

chemotaxis.

The most

development of special structures.

instance, embryonic cells often must migrate long dis-

embryo and fetus after fertilization of an ovum. For

toward a cut area to repair the rent. Finally, cell loco-

skin, though ordinarily completely sessile cells, move

repair the damage, and even the germinal cells of the

instance, fibroblasts move into a damaged area to help

ameboid locomotion under certain circumstances. For

macrophages.

The most

Types of Cells That Exhibit Ameboid Locomotion.

membrane.

the ectoplasm of the cell body, where a preexisting

enlarging pseudopodium. Contraction also occurs in

the pseudopodium of a moving cell, where such

high-energy compound ATP. This is what happens in

The whole process is energized by the

merize to form a filamentous network, and the network

do not provide any motive power; however, these poly-

the following as an explanation: In the cytoplasm of all

direction of the pseudopodium. Experiments suggest

The second essential effect for locomotion is to

the pseudopodial end of the cell, where they are used

cles. Then, inside the cell, these vesicles stream toward

away from their ligands and form new endocytotic vesi-

At the opposite end of the cell, the receptors pull

and attach to ligands in the surrounding tissues.

outside, and the receptors now protrude to the outside

membrane, they open so that their insides evert to the

cles. When the vesicles become part of the pseudopodial

that line the insides of exocytotic vesi-

point of attachment. This attachment is effected by

that it becomes fixed in its leading position, while the

forward movement of the cell. The first effect is attach-

of the cell. Also, two other effects are essential for

general principle of ameboid motion. Basically, it results

Figure 2–16 shows the

following along as the cell moves.

end of the cell is continually moving forward, and the

is a protruding pseudopodium. The membrane of this

showing an elongated cell, the right-hand end of which

pseudopodium. Figure 2–16 demonstrates this process,

Then the remainder of the cell is pulled toward the

body, and partially secures itself in a new tissue area.

pseudopodium projects far out, away from the cell

from one end of the cell. The

Typically, ameboid locomotion begins with protrusion

blood cells through tissues. It receives its name from the

relation to its surroundings, such as movement of white

—occur in other cells.

ciliary movement

these cells are discussed in Chapters 6 through 9. Two

of the entire body mass. The specialized functions of

and smooth muscle, which constitute almost 50 per cent

in the body is that of the muscle cells in skeletal, cardiac,

erhouses” of the cell.

accounts for the mitochondria being called the “pow-

of this ATP is formed in the mitochondria, which

from these to form new ATP. More than 95 per cent

drates, fats, and proteins and use the energy derived

Introduction to Physiology: The Cell and General Physiology

Unit I

much slower chemical reactions break down carbohy-

Locomotion of Cells

By far the most important type of movement that occurs

other types of movement—ameboid locomotion and

Ameboid Movement

Ameboid movement is movement of an entire cell in

fact that amebae move in this manner and have pro-
vided an excellent tool for studying the phenomenon.

of a pseudopodium

membrane at the left-hand end of the cell is continually

Mechanism of Ameboid Locomotion.

from continual formation of new cell membrane at the
leading edge of the pseudopodium and continual
absorption of the membrane in mid and rear portions

ment of the pseudopodium to surrounding tissues so

remainder of the cell body is pulled forward toward the

receptor proteins

to form still new membrane for the pseudopodium.

provide the energy required to pull the cell body in the

cells is a moderate to large amount of the protein actin.
Much of the actin is in the form of single molecules that

contracts when it binds with an actin-binding protein
such as myosin.

a network of actin filaments forms anew inside the

actin network is already present beneath the cell

common cells to exhibit ameboid locomotion in the
human body are the white blood cells when they move
out of the blood into the tissues in the form of tissue

Other types of cells can also move by

motion is especially important in development of the

tances from their sites of origin to new areas during

Control of Ameboid Locomotion—Chemotaxis.

important initiator of ameboid locomotion is the
process called

Any chemical substance that causes chemotaxis to
occur is called a

Most cells

that exhibit ameboid locomotion move toward the

of lower concentration toward an area of higher con-
centration—which is called

Some

nega-

But how does chemotaxis control the direction of

to the chemotactic substance develops membrane
changes that cause pseudopodial protrusion.

Cilia and Ciliary Movements

A second type of cellular motion,

is a

Surrounding tissue

Receptor binding

Endocytosis

Pseudopodium

Exocytosis

Movement of cell

Figure 2–16

Ameboid motion by a cell.

Cell 116:205, 2004.

Danial NN, Korsmeyer SJ: Cell death: critical control points.

76:1, 1996.

docking, and fusion: a molecular description. Physiol Rev

Calakos N, Scheller RH: Synaptic vesicle biogenesis,

and fusion. Cell 116:153, 2004.

Bonifacino JS, Glick BS: The mechanisms of vesicle budding

the Cell. New York: Garland Science, 2002.

Alberts B, Johnson A, Lewis J, et al: Molecular Biology of

arms.

signal, perhaps an electrochemical signal, is transmitted

cilia fail to beat. Therefore, it is presumed that some

do not have the two central single tubules, and these

understood. The cilia of some genetically abnormal cells

The way in which cilia contraction is controlled is not

tubules remain stationary, this will cause bending.

tubule. If the front tubules crawl outward while the back

“crawl” rapidly along the surface of the adjacent double

ATPase dynein arms causes the heads of these arms to

that the release of energy from ATP in contact with the

Given this basic information, it has been determined

tubule.

which has ATPase enzymatic activity, project

Fifth, multiple protein arms composed of the protein

cilium, while those on the back edge remain in place.

motion of the cilium, the double tubules on the front

of magnesium and calcium. Fourth, during forward

ionic conditions, especially appropriate concentrations

cilium: (1) the availability of ATP and (2) appropriate

still beat under appropriate conditions. Third, there are

ments of the cilium besides the axoneme, the cilium can

Second, even after

axoneme.

cross-linkages; this total complex of tubules and cross-

First, the nine double tubules and the two single tubules

ciliary movement are clear, we do know the following:

Mechanism of Ciliary Movement.

from one part of the surface to another.

direction, this is an effective means for moving fluids

in the direction of the fast-forward stroke. Because most

movement. As a result, the fluid is continually propelled

tion that the cilium moves; the slow, dragging movement

The rapid forward-thrusting, whiplike movement

Then it moves backward slowly to its initial position.

whiplike stroke 10 to 20 times per second, bending

shown. The cilium moves forward with a sudden, rapid

In the inset of Figure 2–17, movement of the cilium is

instead of whiplike movements.

of contractile mechanism. The flagellum, however, is

is similar to a cilium; in fact,

The

cell membrane, called the

section shown in Figure 2–17. Each cilium is an out-

tubules down the center, as demonstrated in the cross

located around the periphery of the cilium, and 2 single

covered by an outcropping of the cell membrane, and it

inside the respiratory passageways. The cilium is

cilia often project from a single cell—for instance, as

2 to 4 micrometers from the surface of the cell. Many

As shown in Figure 2–17, a cilium has the appearance

uterus.

the uterine tube toward the uterus cavity; this move-

become trapped in the mucus. In the uterine tubes, the

toward the pharynx, in this way continually clearing

layer of mucus to move at a rate of about 1 cm/min

ratory airways, the whiplike motion of cilia causes a

reproductive tract. In the nasal cavity and lower respi-

occurs in only two places in the human body: on the

Chapter 2

The Cell and Its Functions

sufaces of the respiratory airways and on the inside
surfaces of the uterine tubes (fallopian tubes) of the

these passageways of mucus and particles that have

cilia cause slow movement of fluid from the ostium of

ment of fluid transports the ovum from the ovary to the

of a sharp-pointed straight or curved hair that projects

many as 200 cilia on the surface of each epithelial cell

is supported by 11 microtubules—9 double tubules

growth of a structure that lies immediately beneath the

basal body of the cilium.

flagellum of a sperm

it has much the same type of structure and same type

much longer and moves in quasi-sinusoidal waves

sharply where it projects from the surface of the cell.

pushes the fluid lying adjacent to the cell in the direc-

in the backward direction has almost no effect on fluid

ciliated cells have large numbers of cilia on their sur-
faces and because all the cilia are oriented in the same

Although not all aspects of

are all linked to one another by a complex of protein

linkages is called the
removal of the membrane and destruction of other ele-

two necessary conditions for continued beating of the
axoneme after removal of the other structures of the

edge of the cilium slide outward toward the tip of the

dynein,
from each double tubule toward an adjacent double

along these two central tubules to activate the dynein

References

Tip

Cross section

Forward stroke

Backward stroke

Membrane

Filament

Cell

membrane

Basal body

Rootlet

Basal plate

Ciliary stalk

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Figure 2–17

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Introduction to Physiology: The Cell and General Physiology

Unit I

2003.

of glucose uptake and organization of energy metabolism.

2004.

2003.