chapter2 The cell and its functions Flashcards
1
Q
A typical cell has
Its two major parts are the :
A
- the nucleus and the
- cytoplasm.
2
Q
What separates the nucleus from the cytoplasm?
A
The nucleus is separated from the cytoplasm
by a nuclear membrane.
3
Q
The cytoplasm is separated
from the surrounding fluids by a _____________
A
cell membrane, also called the plasma membrane.
4
Q
Whta is a protoplasm?
A
The different substances that make up the cell are
collectively called protoplasm.
5
Q
Protoplasm is composed
mainly of five basic substances:
A
- water,
- electrolytes,
- proteins,
- lipids,
- and carbohydrates
6
Q
The principal fluid medium of the cell is ________
which is present in most cells, except for fat cells, in a concentration of 70 to 85 percent.
A
water
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.
7
Q
Ions. Important ions in the cell include ____________
These are all discussed
in more detail in Chapter 4, which considers the interrelations
between the intracellular and extracellular fluids.
A
- potassium,
- magnesium,
- phosphate,
- sulfate,
- bicarbonate,
- and smaller quantities of sodium, chloride, and calcium.
8
Q
The ions provide inorganic chemicals for :
A
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.
9
Q
After water, the most abundant substances are
____________
A
After water, the most abundant substances
in most cells are proteins, which normally constitute 10 to
20 percent of the cell mass.
10
Q
Proteins can be divided into two
types:
A
- structural proteins
- and functional proteins
11
Q
What are structural proteins?
A
Structural proteins are present in the cell mainly in the
- *form of long filaments that 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 atangled 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.**
12
Q
What are functional proteins?
A
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.
13
Q
What are lipids?
A
Lipids are several types of substances that are
grouped together because of their common property of
being soluble in fat solvents.
14
Q
Especially important lipids
are____________ which together constitute
only about 2 percent of the total cell mass.
A
phospholipids and cholesterol,
15
Q
The significance
of phospholipids and cholesterol is that they are
________________________________
A
mainly insoluble in water and, therefore, are used to form
the cell membrane and intracellular membrane barriers
that separate the different cell compartments.
16
Q
In addition to phospholipids and cholesterol, some cells
contain large quantities of_________________
A
- *triglycerides, also called neutral**
- *fat.**
In the fat cells, triglycerides often account for as much
as 95 percent 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.
17
Q
Carbohydrates have little structural function in the cell **except** as parts of \_\_\_\_\_\_\_\_\_\_\_ but they **play a major role in nutrition of the cell.**
A
glycoprotein molecules,
Most human cells do not maintain large stores of carbohydrates; the amount usually averages about 1 percent of their total mass but increases to as much as 3 percent in muscle cells and, occasionally, 6 percent 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 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
18
Q
Physical Structure of the Cell
A
- Membranous Structures of the Cell
- Cell Membrane
- Lipid Barrier of the Cell Membrane Impedes Water
Penetration. - Integral and Peripheral Cell Membrane Proteins.
- Membrane Carbohydrates—The Cell “Glycocalyx
- Cytoplasm and Its Organelles
- Endoplasmic Reticulum
- the Golgi apparatus,
- mitochondria,
- lysosomes,
- and peroxisomes.
- Secretory Vesicles
*
19
Q
What are intracellular organelles?
A
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 mito chondria, more than 95 percent of the
cell’s energy release from nutrients would cease immediately.
20
Q
Most organelles of the cell are covered by membranes
composed primarily of ____________.
A
lipids and proteins
21
Q
Most organelles of the cell are covered by membranes
composed primarily of lipids and proteinsThese membranes
include the :
A
- cell membrane,
- nuclear membrane,
- membrane of the endoplasmic reticulum, and membranes of the
- mitochondria,
- lysosomes,
- and Golgi apparatus.
22
Q
What is the function of lipid in the cell membrane?
A
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.
23
Q
Whar are the function of proteins with regards to the cell membranes?
A
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 chapters.
24
Q
Discuss the cell membrane.
A
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.
25
What are the composition of cell membrane?
It is composed almost
**entirely of proteins and lipids.**
The approximate composition
is:
* **proteins, 55 percent;**
* **phospholipids, 25 percent;**
* cholesterol, 13 percent;
* other lipids, 4 percent;
* and carbohydrates, 3 percent
26
What is the basic structure of lipid membrane?
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.
27
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**
28
Discuss the hyrdrophobic portion of the lipid bilayer?
Because the hydrophobic portions of the phospholipid
molecules are **repelled by water but are mutually attracted**
**to one anothe**r, they have a **natural tendency to attach to one another in the middle of the membrane,**
29
Discuss the hydrophilic phosphate portion of the lipid bilayer?
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.**
30
The lipid layer in the middle of the membrane is
impermeable to the usual water-soluble substances, such
as i**ons, glucose, and urea.**
Conversely, fat-soluble substances,
such as oxygen, carbon dioxide, and alcohol, can
penetrate this portion of the membrane with ease.
31
What is the role of the cholesterol in the lipid bilayer?
The cholesterol molecules in the membrane are also
* *lipid in nature** because **their steroid nucleus is highly fat
soluble. **
These molecules, in a sense, a**re dissolved in the
bilayer of the membrane**.
They **mainly help determine thedegree 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.**
32
What are glycoproteins?
**globular masses floating in the lipid**
bilayer.
These are **membrane proteins, most of which
are glycoproteins.**
33
There are two types of cell membrane
proteins:
1. integral proteins that protrude all the way through the membrane
2. and peripheral proteins that are attached only to one surface of the membrane and do not penetrate all the way through.
34
What are your integral proteins' functions?
* channels (or pores)
* carrier proteins
* enzymes
* receptors
* ligands
* second messengers
35
What are your channel pores?
Many of the integral proteins provide structural channels
(or pores) through which **water molecules and watersoluble 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.**
36
What are your carrier proteins?
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 electrochemical gradients** for diffusion, which is called “active transport.” Still others act as enzymes.
37
What are receptors?
Integral membrane proteins can also serve as **receptors for water-soluble chemicals, such as peptide hormones,
that do not easily penetrate the cell membrane.**
38
WHat are ligands?
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.
39
What is the function of your peripheral protein?
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.”**
40
What are glycoproteins or glycolipids?
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.
41
What are proteogylcans?
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.
42
What are glycocalyx
Thus, the entire outside surface
of the cell often has a **loose carbohydrate coat** called the
glycocalyx
43
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
44
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, as shown in
Figure 2-3.
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
45
What is a cytosol?
The cytoplasm is filled with both minute and large dispersed particles and organelles. Th**e clear fluid portion
of the cytoplasm in which the particles are dispersed is
called cytosol**; this contains mainly dissolved proteins,
electrolytes, and glucose.
46
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.
47
What is an ER?
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.
48
What is the endoplasmic matrix?
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.
49
What is the function of the vast surface of the ER?
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.
50
What are ribosomes?
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 f**unction to synthesize new protein molecules** in the cell, as discussed later in this chapter and
in Chapter
51
What are agranular ER?
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.
52
What are Golgi appratus?
The Golgi apparatus, shown in Figure 2-5, is **closely
related to the endoplasmic reticulum**.
It has membranes similar to those of the agranular endoplasmic reticulum. It is usually composed of four or more stacked layers of thin, flat, enclosed vesicles lying near one side of the nucleus.
T**his apparatus is prominent in secretory cells**, where it is located on the side of the cell from which the secretory
substances are extruded.
53
What are ER vesicles?
The Golgi apparatus functions in association with the
endoplasmic reticulum. As shown in Figure 2-5, 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
that are discussed later in the chapter.
54
What are lysosomes?
are vesicular organelles
that **form by breaking off from the Golgi apparatus**
and then dispersing throughout the cytoplasm.
55
What are the function of lysosomeS?
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 lysosomeis quite different in different cell types, 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.
56
What are 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 poisonousto the cell. For instance, about half the alcohol a person
drinks is detoxified by the peroxisomes of the liver cells
in this manner
57
What are 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.**
Figure 2-6 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.
58
What are mitochondria?
The mitochondria, shown in Figures 2-2 and 2-7, 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.**
59
Mitochondria are present in all areas of each cell’s
cytoplasm, but the total number per cell varies from lessthan a hundred up to several thousand, depending on theamount of energy required by the cell.
T or F
T
all cells need an energy
60
Descirbe the mitochondria.
Further, the mitochondriaare 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.
61
The basic structure of the mitochondrion, shown
in Figure 2-7, is composed mainly of two lipid bilayer protein membranes:
* an outer membrane
* and an inner membrane.
62
Describe the matrix of the mitochondria.
Many infoldings 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.**
63
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.
T
T
64
Indeed, the mitochondria contain DNA
similar to that found in the cell nucleus. In Chapter 3 we
will see that DNA is the basic chemical o**f the nucleus that controls replication of the cell.**
T or F
T
The DNA of the mitochondrion
plays a similar role, controlling replication of
the mitochondrion.
65
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
66
What are filaments?
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, as discussed in
detail in Chapter 6.
67
What are microtubules?
A special type of stiff filament composed of polymerized
**tubulin molecules** is used in all cells to construct
strong tubular structures, the **microtubules**. Figure 2-8
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
Figure 2-17.
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.
68
What is 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.
69
What is the 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,
70
Describe the nuclear pore.
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.
71
Describe the nucleo
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.
72
How are nucleoli formed?
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, as
discussed more fully in Chapter 3.
73
Functional Systems of the Cell
* Ingestion by the Cell—Endocytosis
* Digestion of Pinocytotic and Phagocytic Foreign
Substances Inside the Cell—Function of the
Lysosomes
* Synthesis and Formation of Cellular Structures by
Endoplasmic Reticulum and Golgi Apparatus
* Extraction of Energy from Nutrients—Function
of the Mitochondria
74
If a cell is to live and grow and reproduce, it must obtain
nutrients and other substances from the surrounding fluids.
Most substances pass through the cell membrane by
\_\_\_\_\_\_\_\_\_\_\_
diffusion and active transport.
75
What is diffusion?
Diffusion involves simple movement through the
membrane caused by the **random motion of the molecules** of the substance; substances move either through **cell membrane pores or, in the case of lipid-soluble substances,**
through the lipid matrix of the membrane.
76
What is an active transport?
Active transport involves the actual carrying of a substance through the membrane by a physical protein structure
that penetrates all the way through the membrane.
77
What is an endocytosis?
Very large particles enter the cell by a specialized function of the cell membrane called endocytosis.
78
The principal
forms of endocytosis are \_\_\_\_\_\_\_\_\_\_\_
pinocytosis and phagocytosis.
79
Describe pinocytosis.
Pinocytosis means ingestion of minute particles that form
vesicles of extracellular fluid and particulate constituents
inside the cell cytoplasm.
80
Describe phagocytosis.
Phagocytosis means ingestion
of large particles, such as bacteria, whole cells, or portions
of degenerating tissue.
81
How does pinocytosis works?
Pinocytosis occurs continually in the cell
membranes of most cells, but it is especially rapid in some cells.
For instance, **it occurs so rapidly in macrophages**
that about 3 percent of the total macrophage membrane is engulfed in the form of vesicles each minute.
Even so,
the pinocytotic vesicles are so small—usually only 100 to 200 nanometers in diameter—that most of them can be seen only with the electron microscope.
Pinocytosis is the only means by which most large macromolecules,
such as most protein molecules, can enter
cells.
In fact, the rate at which pinocytotic vesicles form
is usually enhanced when such macromolecules attach to the cell membrane.
These molecules usually attach tospecialized
protein receptors on the surface of the membrane
that are specific for the type of protein that is to
be absorbed. The receptors generally are concentrated in small pits on the outer surface of the cell membrane, called **coated pits**.
On the inside of the cell membrane
beneath these pits is a latticework of fibrillar protein
called **clathrin,** as well as other proteins, perhaps including contractile filaments of actin and myosin.
Once the
protein molecules have bound with the receptors, the
surface properties of the local membrane change in such a way that the entire pit invaginates inward and the fibrillar proteins surrounding the invaginating pit cause its borders to close over the attached proteins, as well as over a small amount of extracellular fluid.
Immediately
thereafter, the invaginated portion of the membrane
breaks away from the surface of the cell, forming a pinocytotic vesicle inside the cytoplasm of the cell.
82
What causes the cell membrane to go through the
necessary contortions to form pinocytotic vesicles is still
unclear.
This process **requires energy from within the cell;**
this is supplied by ATP, a high-energy substance discussed
later in the chapter.
Also, it **requires the presence of calcium**
ions in the extracellular fluid, which probably react
with contractile protein filaments beneath the coated pits to provide the force for pinching the vesicles away from the cell membrane.
83
Phagocytosis occurs in much the same
way as pinocytosis, **except tha**t it involves \_\_\_\_\_\_\_\_\_\_\_
large particles rather than molecules.
Only certain cells have the capability
of phagocytosis, **most notably the tissue macrophages**
**and some of the white blood cells.**
84
What initiates phagocytosis?
Phagocytosis is initiated when a particle such as a **bacterium, a dead cell, or tissue debris binds with receptors on the surface of the phagocyte**. In the case of bacteria, each bacterium is usually already attached to a specific antibody, and it is the antibody that attaches to the phagocyte receptors, dragging the bacterium along with it.
This
intermediation of antibodies is called **opsonization**, which
85
Phagocytosis occurs in the following steps:
1. The cell membrane receptors attach to the surface ligands of the particle.
2. The edges of the membrane around the points of
attachment evaginate outward within a fraction of a
second to surround the entire particle; then, progressively more and more membrane receptors attach to the particle ligands.
All this occurs suddenly in a zipper-
like manner to form a closed phagocytic vesicle.
3. Actin and other contractile fibrils in the cytoplasm
surround the phagocytic vesicle and contract around
its outer edge, pushing the vesicle to the interior.
4. The contractile proteins then pinch the stem of the
vesicle so completely that the vesicle separates from
the cell membrane, leaving the vesicle in the cell interior in the same way that pinocytotic vesicles are
formed.
86
Describe Digestion of Pinocytotic and Phagocytic Foreign
Substances Inside the Cell—**Function of the
Lysosomes**
Almost immediately after a pinocytotic or phagocytic
vesicle appears inside a cell, **one or more lysosomes
become attached to the vesicle and empty their
acid hydrolases to the inside of the vesicle**, as shown in Figure 2-12.
Thus, a **digestive vesicle is formed inside
the cell cytoplasm** in which the vesicular hydrolases
begin hydrolyzing the proteins, carbohydrates, lipids,
and other substances in the vesicle.
The products of
digestion are small molecules of amino acids, glucose,
phosphates, and so forth that can diffuse through the
membrane of the vesicle into the cytoplasm. What is
left of the digestive vesicle, called the residual body, represents
indigestible substances.
In most instances, this
is finally excreted through the cell membrane by a process called **exocytosis,** which is essentially the opposite of endocytosis.
87
What are your digestive organs of the cell?
Thus, the pinocytotic and phagocytic vesicles containing
lysosomes can be called the digestive organs of the
cells.
88
Whos is responsible for the Regression of Tissues and Autolysis of Cells
Lysosomes
89
What is an autolysis?
Another special role of the lysosomes is removal of
damaged cells or damaged portions of cells from tissues.
Damage to the cell—caused by heat, cold, trauma,
chemicals, or any other factor—induces lysosomes to
rupture.
The released hydrolases immediately begin to
digest the surrounding organic substances. If the damage is slight, only a portion of the cell is removed and the cell is then repaired.
If the damage is severe, the entire cell is digested, a process called **autolysis.** In this way, the cell is
completely removed and a new cell of the same type ordinarily
is formed by mitotic reproduction of an adjacent
cell to take the place of the old one.
90
The lysosomes also contain bactericidal agents that
can kill phagocytized bacteria before they can cause cellular
damage. These agents include
(1) lysozyme, which dissolves the bacterial cell membrane;
(2) lysoferrin, which binds iron and other substances before they can promote
bacterial growth; and
(3) acid at a pH of about 5.0, which activates the hydrolases and inactivates bacterial metabolic
systems.
91
Specific Functions of the Endoplasmic Reticulum
1. Proteins Are Formed by the Granular Endoplasmic
Reticulum.
2. Synthesis of Lipids by the Smooth Endoplasmic
Reticulum.
3. Other Functions of the Endoplasmic Reticulum
* It provides the enzymes that control glycogen breakdown
when glycogen is to be used for energy.
* It provides a vast number of enzymes that are capable of detoxifying substances, such as drugs, that might damage the cell. It achieves detoxification by coagulation,
oxidation, hydrolysis, conjugation with glycuronic acid, and in other ways.
92
Describe the granular portion of the ER.
The granular portion of the endoplasmic
reticulum is characterized by large numbers of ribosomes attached to the outer surfaces of the endoplasmic reticulum membrane.
As discussed in Chapter 3,
**protein molecules are synthesized within the structure**s of the ribosomes.
The ribosomes extrude some
of the synthesized protein molecules directly into
the cytosol, but they also extrude many more through the wall of the endoplasmic reticulum to the interior of
the endoplasmic vesicles and tubules, into the endoplasmic
matrix.
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What is being synthesized in the smoothe endoplasmic reticulum?
The endoplasmic reticulum also synthesizes
lipids, especially phospholipids and cholesterol.
These are
rapidly incorporated into the lipid bilayer of the endoplasmic reticulum itself, thus causing the endoplasmic reticulum
to grow more extensive.
This occurs mainly in the
smooth portion of the endoplasmic reticulum.
To keep the endoplasmic reticulum from growing
beyond the needs of the cell, small vesicles called **ER vesicles**
or transport vesicles continually break away from the
smooth reticulum; most of these vesicles then migrate
rapidly to the Golgi apparatus
94
Whta is the synthetic function of 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 in the endoplasmic reticulum, it also has the capability of synthesizing certain carbohydrates that cannot be formed in the endoplasmic reticulum.
This is especially true for
the formation of large saccharide polymers bound with
small amounts of protein; important examples include
hyaluronic acid and chondroitin sulfate.
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A few of the many functions of hyaluronic acid and
chondroitin sulfate in the body are as follows:
(1) they are the major components of proteoglycans secreted in mucus and other glandular secretions;
(2) they are the major components of the ground substance outside the cells in the interstitial spaces, acting as fillers between collagen
fibers and cells;
(3) they are principal components of the organic matrix in both cartilage and bone; and
(4) they are important in many cell activities including migration and proliferation.
96
Describe the formation of proteins, lipids, and cellular vesicles by
the endoplasmic reticulum and Golgi apparatus.
As substances are formed in
the endoplasmic reticulum, especially the proteins, they are transported through the tubules toward portions of the smooth endoplasmic reticulum that lie nearest the Golgi apparatus.
At this point, small transport vesicles
composed of small envelopes of smooth endoplasmic
reticulum continually break away and diffuse to the deepest layer of the Golgi apparatus.
Inside these vesicles are
the synthesized proteins and other products from the
endoplasmic reticulum.
The transport vesicles instantly fuse with the Golgi
apparatus and empty their contained substances into thevesicular spaces of the Golgi apparatus.
Here, additional
carbohydrate moieties are added to the secretions. Also, an important function of the Golgi apparatus is to compact the endoplasmic reticular secretions into highly concentrated packets.
As the secretions pass toward
the outermost layers of the Golgi apparatus, the compaction and processing proceed.
Finally, both small and
large vesicles continually break away from the Golgi
apparatus, carrying with them the compacted secretory substances, and in turn, the vesicles diffuse throughout the cell.
97
Types of Vesicles Formed by the Golgi Apparatus—
Secretory Vesicles and Lysosomes
In a **highly secretory**
**cell,** the **vesicles formed by the Golgi apparatus** are **mainly secretory vesicles** containing protein substances that are to be secreted through the surface of the cell membrane.
These secretory vesicles first diffuse to the cell membrane, then fuse with it and empty their substances to the exterior by the mechanism called exocytosis.
98
What stimulates exocytosis?
Exocytosis, in most
cases, is stimulated by the entry of calcium ions into the cell; calcium ions interact with the vesicular membrane in some way that is not understood and cause its fusion with the cell membrane, followed by exocytosis—that is, opening of the membrane’s outer surface and extrusion of its contents outside the cell.
Some vesicles, however, are destined for intracellular
use.
99
What is the use of intracellular vesicles
Use of Intracellular Vesicles to Replenish Cellular
Membranes.
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.
This
increases the expanse of these membranes and thereby replenishes the membranes as they are used up.
For instance, the cell membrane loses much of its substance every time it forms a phagocytic or pinocytotic vesicle, and the vesicular membranes of the Golgi apparatus continually replenish the cell membrane.
In summary, the membranous system of the endoplasmic
reticulum and Golgi apparatus represents a highly
metabolic organ capable of forming new intracellular
structures, as well as secretory substances to be extruded
from the cell.
100
What are the function of Mitochondria
1. Mitochondria Extraction of Energy from Nutrients
2. Chemical Processes in the Formation of ATP—Role
of the Mitochondria
101
Briefly, almost all these oxidative reactions occur
inside the \_\_\_\_\_\_\_\_\_and the energy that is released is
used to form the high-energy compound ATP.
Then, ATP,
not the original foodstuffs, is used throughout the cell to
energize almost all the subsequent intracellular metabolic
reactions.
mitochondria
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ATP is a nucleotide composed of
(1) the nitrogenous base
adenine,
(2) the pentose sugar ribose,
and
(3) three phosphate
radicals.
The last two phosphate radicals are connected
with the remainder of the molecule by so-called
high-energy phosphate bonds, which are represented in the formula shown by the symbol ~. Under the physical and chemical conditions of the body, each of these highenergy bonds contains about 12,000 calories of energy per mole of ATP, which is many times greater than the energy stored in the average chemical bond, thus giving rise to the term high-energy bond.
Further, the high-energy phosphatebond is very labile so that it can be split instantly on demand whenever energy is required to promote other intracellular reactions.
103
When is ADP formed?
When ATP releases its energy, a phosphoric acid radical
is split away and adenosine diphosphate (ADP) is
formed.
This released energy is used to energize virtually
many of the cell’s other functions, such as synthesis of
substances and muscular contraction
104
How does the cell reconstitute ATP?
To reconstitute the cellular ATP as it is used up, energy
derived from the cellular nutrients causes ADP and phosphoric acid to recombine to form new ATP, and the
entire process repeats over and over again. For these reasons,
**ATP has been called the energy currency of the cell**
because i**t can be spent and remade continually**, having a turnover time of only a few minutes.
105
On entry into the cells, glucose is
subjected to enzymes in the cytoplasm that convert it into pyruvic acid (a process called glycolysis).
A small amount
of ADP is changed into ATP by the energy released during this conversion, but this amount accounts for less than 5 percent of the overall energy metabolism of the cell.
About 95 percent of the cell’s ATP formation occurs
in the \_\_\_\_\_\_\_\_\_\_.
mitochondria
The pyruvic acid derived from carbohydrates,
fatty acids from lipids, and amino acids from
proteins is eventually converted into the compound
acetyl- CoA in the matrix of the mitochondrion.
This substance, in turn, is further dissoluted (for the purpose of extracting its energy) by another series of enzymes in the mitochondrion matrix, undergoing dissolution in a sequence of chemical reactions called the **citric acid cycle, or Krebs cycle.**
106
In this citric acid cycle, acetyl-CoA is split into its
component parts, \_\_\_\_\_\_\_\_\_\_-
hydrogen atoms and carbon dioxide.
The carbon dioxide diffuses out of the mitochondria and eventually out of the cell; finally, it is excreted from the body through the lungs.
The hydrogen atoms, conversely, are highly reactive,
and they combine instantly with oxygen that has
also diffused into the mitochondria.
This releases a tremendous
amount of energy, which is used by the mitochondria
to convert large amounts of ADP to ATP. The
processes of these reactions are complex, requiring the participation of many protein enzymes that are integral
parts of mitochondrial membranous shelves that protrude
into the mitochondrial matrix. The initial event
is removal of an electron from the hydrogen atom, thus
converting it to a hydrogen ion. The terminal event
is combination of hydrogen ions with oxygen to form
water plus the release of tremendous amounts of energy
to large globular proteins, called ATP synthetase, that
protrude like knobs from the membranes of the mitochondrial
shelves. Finally, the enzyme ATP synthetase
uses the energy from the hydrogen ions to cause the
conversion of ADP to ATP. The newly formed ATP is
transported out of the mitochondria into all parts of
the cell cytoplasm and nucleoplasm, where its energy is
used to energize multiple cell functions.
This overall process for formation of ATP is called the
chemiosmotic mechanism of ATP formation.
107
Energy from ATP
is used to promote three major categories of cellular functions:
* (1) transport of substances through multiple membranes in the cell,
* (2) synthesis of chemical compounds throughout the cell, and (
* 3) mechanical work.
These uses
* of ATP are illustrated by examples in Figure 2-15: (1) to supply energy for the transport of sodium through the cell membrane,
* (2) to promote protein synthesis by the ribosomes,
* and (3) to supply the energy needed during muscle contraction.
108
Explain the function of ATP as membrane transport.
In addition to membrane transport of sodium, energy
from ATP is required for membrane transport of potassium ions, calcium ions, magnesium ions, phosphate ions chloride ions, urate ions, hydrogen ions, and many other ions and various organic substances.
Membrane transport
is so important to cell function that some cells—the renal tubular cells, for instance—use as much as 80 percent of the ATP that they form for this purpose alone
109
Explain the function of the ATP in synthesizing chemical compounds.
In addition to synthesizing proteins, cells make phospholipids, cholesterol, purines, pyrimidines, and a host of other substances.
Synthesis of almost any chemical compound
requires energy.
For instance, a single protein molecule
might be composed of as many as several thousand
amino acids attached to one another by peptide linkages; the formation of each of these linkages requires energy derived from the breakdown of four high-energy bonds; thus, many thousand ATP molecules must release their energy as each protein molecule is formed. Indeed, some cells use as much as 75 percent of all the ATP formed in
the cell simply to synthesize new chemical compounds, especially protein molecules; this is particularly true during
the growth phase of cells.
110
Explain the funciton of ATP in mechanical work.
The final major use of ATP is to supply energy for special cells to perform mechanical work.
We see in Chapter 6 that
each contraction of a muscle fiber requires expenditure of tremendous quantities of ATP energy.
Other cells perform mechanical work in other ways, especially by ciliary and ameboid motion, described later in this chapter. The source of energy for all these types of mechanical work is ATP.
111
Locomotion of Cells
* Ameboid Movement
* Cilia and Ciliary Movements
112
What is an ameboid movement?
Ameboid movement is movement of an entire cell in
relation to its surroundings, such as movement of white
blood cells through tissues. It receives its name from the
fact that amebae move in this manner and have provided
an excellent tool for studying the phenomenon
113
How does ameboid locomotion starts?
Typically, ameboid locomotion begins with protrusion
of a pseudopodium from one end of the cell.
The pseudopodium projects far out, away from the cell body, and partially secures itself in a new tissue area.
Then the remainder of the cell is pulled toward the pseudopodium. elongated
cell, the right-hand end of which is a protruding
pseudopodium. The membrane of this end of the cell is continually moving forward, and the membrane at the left-hand end of the cell is continually following along as the cell moves.
Figure 2-16 demonstrates this process, showing an elongated
114
Mechanism of Ameboid Locomotion
Basically, it results 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 of the cell.
Also, two other effects are essential for forward
movement of the cell.
The **first effect is attachment of the
pseudopodium** to surrounding tissues so that it becomes fixed in its leading position, while the remainder of the cell body is pulled forward toward the point of attachment.
This attachment is effected by receptor proteins that
line the insides of exocytotic vesicles. When the vesicles become part of the pseudopodial membrane, they open so that their insides evert to the outside, and the receptors now protrude to the outside and attach to ligands in the surrounding tissues.
At the opposite end of the cell, the receptors pull away
from their ligands and form new endocytotic vesicles.
Then, inside the cell, these vesicles stream toward the
pseudopodial end of the cell, where they are used to form still new membrane for the pseudopodium.
The **second essential effect for locomotion** is to providethe energy required to pull the cell body in the direction of the pseudopodium.
Experiments suggest the following as
an explanation: In the cytoplasm of all cells is a moderate to large amount of the protein actin.
Much of the actin is in
the form of single molecules that do not provide any motivepower; however, these polymerize to form a filamentousnetwork, and the network contracts when it binds with an actin-binding protein such as myosin.
The whole process is energized by the high-energy compound ATP. This is what happens in the pseudopodium of a moving cell, where such
a network of actin filaments forms anew inside the enlarging pseudopodium.
Contraction also occurs in the ectoplasm
of the cell body, where a preexisting actin network is
already present beneath the cell membrane.
115
Types of Cells That Exhibit Ameboid Locomotion
* white blood cells
* tissue macrophages.
* fibroblasts
* embryonic cells
116
The most common cells to exhibit ameboid locomotion in
the human body are the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_when they move
out of the blood into the tissues to form tissue macrophages
w
117
What is the most important initiator of ameboid locomotion?
The most important initiator of ameboid locomotion
is the process called chemotaxis.
This results from the
appearance of certain chemical substances in the tissues.
Any chemical substance that causes chemotaxis to
occur is called a chemotactic substance.
Most cells that
exhibit ameboid locomotion move toward the source
of a chemotactic substance—that is, from an area of
lower concentration toward an area of higher concentration—which is called **positive chemotaxis.**
Some cells
move away from the source, which is called negative
chemotaxis. But how does chemotaxis control the direction of ameboid locomotion? Although the answer is not certain, it is known that the side of the cell most exposed to the chemotactic substance develops membrane changes that
cause pseudopodial protrusion.
118
What are ciliary movements?
A second type of cellular motion, ciliary movement, is a
**whiplike movement of cilia on the surfaces of cells.**
119
Cilia occurs in only two places in the human body: on ____________ tract.
* the surfaces of the respiratory airways
* and on the inside surfaces of the uterine tubes (fallopian tubes) of the reproductive
In the nasal cavity and lower respiratory airways,
the whiplike motion of cilia causes a layer of mucus to
move at a rate of about 1 cm/min toward the pharynx, in this way continually clearing these passageways of mucus and particles that have become trapped in the mucus.
In the uterine tubes, the cilia cause slow movement of fluid from the ostium of the uterine tube toward the uterus cavity; this movement of fluid transports the ovum from the ovary to the uterus.
120
Describe the appearance of the cilum
a cilium has the appearance
of a sharp-pointed straight or curved hair that projects 2 to 4 micrometers from the surface of the cell. Many cilia often project from a single cell—for instance, as many as 200 cilia on the surface of each epithelial cell inside the respiratory passageways.
The cilium is covered by an
outcropping of the cell membrane, and it is supported
by 11 microtubules—9 double tubules located around
the periphery of the cilium and 2 single tubules down
the center, Each cilium is an outgrowth of a structure
that lies immediately beneath the cell membrane, called the basal body of the cilium.
121
The flagellum of a sperm is similar to a cilium; in fact,
it has much the same type of structure and same type of
contractile mechanism. The flagellum, however, is much
longer and moves in quasi-sinusoidal waves instead of
whiplike movements
122
Describe the movement of the cilium.
The cilium moves forward with a sudden,
rapid whiplike stroke 10 to 20 times per second, bending sharply where it projects from the surface of the cell.
Then it moves backward slowly to its initial position. The rapid forward-thrusting, whiplike movement pushes the fluid lying adjacent to the cell in the direction that the cilium moves; the slow, dragging movement in the backward direction has almost no effect on fluid movement.
As a result, the fluid is continually propelled in the direction of the fast-forward stroke.
Because most ciliated cells
have large numbers of cilia on their surfaces and because
all the cilia are oriented in the same direction, this is an
effective means for moving fluids from one part of the
surface to another.
123
What is the mechanism of cilium?
* First, the nine double tubules and the two single
tubules are all linked to one another by a complex of
protein cross-linkages; this total complex of tubules and cross-linkages is called the axoneme.
* Second, even after removal of the membrane and destruction of other elementsof the cilium besides the axoneme, the cilium can still beat under appropriate conditions.
* Third, there are two necessary conditions for continued beating of the axoneme after removal of the other structures of the cilium:
* Fourth, during forward motion of the cilium, the double tubules on the front edge of the cilium
slide outward toward the tip of the cilium, while those on the back edge remain in place.
* Fifth, multiple protein arms composed of the protein dynein, which has ATPase enzymatic
activity, project from each double tubule toward an adjacent double tubule.
124
there are
two necessary conditions for continued beating of the
axoneme after removal of the other structures of the cilium:
* (1) the availability of ATP
* and (2) appropriate ionic conditions, especially appropriate concentrations of magnesium and calcium.
125
