cells, tissues, organs, and organ systems of animals

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This photomicrograph is a longitudinal section through skeletal muscle tissue,
one of the four major tissue types discussed in this chapte1:
Because all organisms are made of cells, the cell is as fundamental to an under­ standing of zoology as the atom is to an understanding of chemistty. In the hier­ archy of biological organization, the cell is the simplest organization of matter that
exhibits all of the properties of life (figure 2.1). Some organisms are single celled; others are multicellular. An animal has a body composed of many kinds of special­ ized cells. A division of labor among cells allows specialization into higher levels of organization (tissues, organs, and organ systems). Yet, everything that an animal does is ultimately happening at the cellular level.
2.1 WHAT ARE C LLS?
LEARNING OUTCOMES
1. Differentiate between a prokaryotic and eukaryotic cell. 2. Describe the three patts of a eukaryotic cell.
Cells are the functional units of life, in which all of the chemical reactions neces­ sary for the maintenance and reproduction of life take place. They are the small­
est independent units of life. There are two basic types of cells: prokaryotic and eukaryotic. The prokaryotes lack nuclei and other membrane-bound organelles. These simpler (prokaryotic or prokaryotes; "before nucleus") cells are classified into two domains: Archaea and Eubacteria. The Archaea have unique characteristics and also share features with Eubacteria and the third domain, Eukarya. Eukaryotic cells are larger and more complex than prokaryotic cells. Since animals and protista are composed of eukaryotic cells, this cell type will be emphasized in this chapter. Table 2.1 compares prokaryotic and eukaryotic cells.
All eukaryotes ("true nucleus") have cells with a membrane-bound nucleus
containing DNA. In addition, eukaryotic cells contain many other structures called organelles ("little organs") that perform specific functions. Eukaryotic cells also have a network of specialized structures called microfilaments and microtubules organized into the cytoskeleton, which gives shape to the cell and allows intracel­ lular movement.
All eukaryotic cells have three basic parts:
1. The plasma membrane is the outer boundary of the cell. It separates the internal metabolic events from the environment and allows them to proceed in organized, controlled ways. The plasma membrane also has specific receptors for external molecules that alter the cell's function.
Chapter Outline 2.1 What Are Cells? 2.2 Why Are Most Cells Small? 2.3 Cell Membranes
Structure of Cell Membranes Functions of Cell Membranes
2.4 Movement across Membranes Simple Diffusion Facilitated Diffusion Osmosis Filtration Active Transport: Energy Required Bulk Transport
2.5 Cytoplasm, Organelles, and Cellular Components
Cytoplasm Ribosomes: Protein Workbenches Endoplasmic Reticulum: Production
and Transport Golgi Apparatus: Packaging, Sorting,
and Export Lysosomes: Digestion
of Organelles Mitochondria: Power Generators Cytoskeleton: Microtubules,
Intermediate Filaments, and Microf/laments
Organizing Centers Vacuoles: Cell Maintenance Vaults: Mysterious Symmetrical Shells Exosomes
2.6 The Nucleus: Information Center Nuclear Envelope: Gateway
to the Nucleus Chromosomes: Genetic
Containers Nucleolus: Preassembly Point for
Ribosomes 2.7 Levels of Organization in Various
Animals 2.8 Tissues
Connective Tissue: Connection and Support
Nervous Tissue: Communication Muscle Tissue: Movement
2.9 Organs 2.1 0 Organ Systems
12 LHAPTt.1{ 1 WU
Cl C (/) (1j Q)
u Q)
Structural Hierarchy in a Multicellular Animal. At each level, function depends on the structural organization of that level and those below it
2. Cytoplasm (Gr. kytos, hollow vessel + plasm, fluid) is the portion of the cell outside the nucleus. The semifluid portion of the cytoplasm is called the cytosol. Suspended within the cytosol are the organelles.
3. The nucleus (pl., nuclei) is the cell control center. It contains the chromosomes and is separated from the cytoplasm by its own nuclear envelope. The nucleoplasm is the semifluid material in the nucleus.
Because cells vary so much in form and function, no "typical" cell exists. However, to help you learn as much as possible about cells, figure 2.2 shows an idealized version of a eukaryotic cell and most of its component parts.
T A 2 L :: - . i COMPARISON OF PROKARYOTIC
AND EUKARYOTIC CELLS
Number of One chromosomes
recombination transfer of DNA
Endoplasmic Absent reticulum
Golgi apparatus Absent
Simpler organelles
Ribosomes 70S
Present
Present
Yes
Microscopic in size; membrane bound; usually 20 microtubules in 9 + 2 pattern
Present
Present
Present
Present
Present
Present
Present
Tissues and organs
Prokatyotes are small cells that lack complex internal organiza­ tion. The two proka1yotic domains are Archaea and F.uhacteria. Euka1yotic cells exhibit compattmentalization and various organ­ elles that cany out specific functions. The three parts of a euka1y­ otic cell are the plasma membrane, cytoplasm, and nucleus.
What are some similarities between eukaryotic cells
and the prokaryotic cells of Eubacteria and Arcbaea?
FIGURE 2.2
Smooth
endoplasmic
reticulum
A Generalized Animal Cell. Understanding of the structures in this cell is based mainly on electron microscopy. The sizes of some organelles and structures are exaggerated to show detail.
2. n A E M >
s SMALL?
LEARNING OUTCOMES
1. Explain why most cells are small. 2. Determine how surface area changes as a function
of volume.
Most cells are small and can be seen only with the aid of a microscope. (Exceptions include the eggs of most vertebrates [fishes, amphibians, reptiles, and birds] and some long nerve cells.) One reason for the small size of cells is that the ratio of the volume of the cell's nucleus to the volume of its cytoplasm must not be so small that the nucleus, the cell's major control center, cannot control the cytoplasm.
Another aspect of cell volume works to limit cell size. As the radius of a cell lengthens, cell volume increases more rapidly than cell surface area (figure 2.3). The need for nutri­
ents and the rate of waste production are proportional to cell volume. The cell takes up nutrients and eliminates wastes through its surface plasma membrane. If cell volume becomes too large, the surface-area-to-volume ratio is too small for an adequate exchange of nutrients and wastes.
Cells, Tissues, Org,ms, anc.l Org,111 Systems of Animals 13
Radius (r) 1 cm 2 cm 4 cm
S urface area (SA) 12.57 cm2 50.26 cm2 201.06 cm2
V olume (V) 4.19 cm3 33.51 cm3 268.08 cm3
SAIV 3.0 1.50 0.75
Volume of sphere = 4/31rr3
FIGURE 2.3
The Relationship between Surface Area and Volume. As the radius of a sphere increases, its volume increases more rapidly than its surface area. (SA/V = surface-area-to-volume ratio.)
SECTION REVIEW 2.2
A cell needs a surface area large enough to allow efficient movement of nutrients into the cell and waste material out of the cell. Small cells have a lot more surface area per volume than large cells. For example, a 4-cm cube has a surface-area­ to-volume ratio of only 5.5:1, but a 1-cm cube has a ratio of 6:1.
If the cell radius of a cell increases 10 times, the sur­
face area will increase by 100 times. How much will
the volume increase?
2.3 Ci'..L
I.EARNING OUTCOME
1. Relate the structure of the plasma membrane to the function of the membrane.
The plasma membrane surrounds the cell. Other membranes inside the cell enclose some organelles md have properties similar to those of the plasma membrane.
Structure of Cell Membranes
In 1972, S. Jonathan Singer and Garth Nicolson developed the fluid-mosaic model of membrane structure. According to this model, a membrane is a double layer <bilayer) of proteins and phospholipicls and is fluicl rather than solid. The phospholipicl bilayer forms a fluid "sea" in which specific proteins float like icebergs (figure 2.4). Being fluicl, the membrane is in a constant state of flux-shifting and ch:mging, while retaining its uniform structure. The word mosaic refers to the many different kinds of proteins di.persec.l in the phospholipid bilayer.
14 CHAPTER TWO
J Double layer of phospholipid molecules
Fluid-Mosaic Model of Membrane Structure. Intrinsic globular proteins may protrude above or below the lipid bilayer and may move about in the membrane. Peripheral proteins attach to either the inner surface or the outer surface.
The following are important points of the fluid-mosaic model:
1. The phospholipids have one polar end and one nonpolar end. The polar ends are oriented on one side toward the outside of the cell and into the fluid cytoplasm on the other side, and the nonpolar ends face each other in the middle of the bilayer. The "tails" of both layers of phos­ pholipid molecules attract each other and are repelled by water (they are hydrophobic, "water dreading"). As a result, the polar spherical "heads" (the phosphate por­ tion) are located over the cell surfaces (outer and inner) and are "water attracting" (they are hydrophilic).
2. Cholesterol is present in the plasma membrane and organelle membranes of eukaryotic cells. The cholesterol molecules are embedded in the interior of the mem­ brane and help make the membrane less permeable to water-soluble substances. In addition, the relatively rigid structure of the cholesterol molecules helps stabilize the membrane (figure 2.5).
3. The membrane proteins are individual molecules attached to the inner or outer membrane surface (peripheral proteins) or embedded in it (intrinsic pro­ teins) (see figure 2.4). Some intrinsic proteins are links to sugar-protein markers on the cell surface. Other intrinsic proteins help move ions or molecules across the membrane, and still others attach the membrane to the cell's inner scaffolding (the cytoskeleton) or to vari­ ous molecules outside the cell.
4. When carbohydrates unite with proteins, they form glycoproteins, and when they unite with lipids,
Hydrophilic (polar)
More fluid region
The Arrangement of Cholesterol between Lipid Molecules of a Lipid Bilayer. Cholesterol stiffens the outer lipid bilayer and causes the inner region of the bilayer to become slightly more fluid. Only half the lipid bilayer is shown; the other half is a mirror image.
they form glycolipids on the surface of the plasma membrane. Surface carbohydrates and portions of the proteins and lipids make up the glycocalyx ("cell coat") (figure 2.6). This arrangement of distinctively shaped groups of sugar molecules of the glycocalyx acts as a molecular "fingerprint" for each cell type. The glyco­ calyx is necessary for cell-to-cell recognition and the behavior of ·ei·Lain c lls, and it is a key comp n nl in coordl­ nating cell b havior in animals.
MP3
How Do Zoologists Investigate the Inner
Workings of the Tiny Structures within a Cell?
T he small size of cells is the greatest obstacle to dis­
covering their nature and the anatomy of the tiny structures
within cells. The evolution of sci­ ence often parallels the invention of instruments that extend human
senses to new limits. Cells were
discovered after microscopes were invented, and high-magnification
microscopes are needed to see the smallest structures within a cell.
Extracellular fluid (outside of cell)
Cytoplasm
Most commonly used are the light microscope, the transmission electron microscope (TEM), the
scanning electron microscope, the fluorescence microscope, the
scanning tunneling microscope, and the atomic force microscope.
modern cell biology developed
biochemistry, the study of molecules
and the chemical processes of metabolism. Throughout this book, many photographs are presented
using various microscopes to show different types of cells and the various
tiny structures within. From these
photographs, it will become apparent that similarities among cells reveal
the evolutionary unity of life.
Microscopes are the most impor­ tant tools of cytology, the study of cell
structure. But simply describing the diverse structures within a cell reveals little about their function. Today's
Glyco­ calyx
Lipid bilayer
specific cell identification markers that differentiate one cell type from another.
The ability of the plasma membrane to let some sub­ stances in and keep others out is called selective permeability
(L. permeare or per, through + meare, pass) and is essen­ tial for maintaining a "steady state" within the cell. How­ ever, before you can fully understand how substances pass into and out of cells and organelles, you must know how the molecules of those substances move from one place to another.
SECTION REVIEW 2.3
The major components of the plasma membrane are as follows: a phospholipid bilayer, cholesterol, membrane proteins, and the glycocalyx. This strncture creates the outer boundary of the cell, it separates the internal metabolic events from the environ­ ment, and it allows the events to proceed in an organized, con­ trolled way. The plasma membrane also has specific structures for movement of materials into and out of the cell and recep­ tors for external molecules that alter the cell's function.
The Glycocalyx, Showing the Glycoproteins and Glycolipids. Note that all of the attached carbohydrates are on the out5ide of the plasma membrane.
If the plasma membrane of a cell were just a single layer
of phospholipids, how would this affect its function?
Functions of Cell Membranes
Cell membranes (1) regulate material moving into and out of the cell, and from one part of the cell to another; (2) separate the inside of the cell from the outside; (3) separate various organelles within the cell; ( 4) provide a large surface area on which specific chemical reactions can occur; (5) separate cells from one another; and (6) are a site for receptors containing
.l.
M1
( u .
1. Differentiate the different processes by which material can move into and out of the cell through the plasma membrane.
2. Explain the movement of water by osmosis.
16 CHAPTER TWO
Molecules can cross membranes in a number of ways, both by using their own energy and by relying on an outside
energy source. Table 2.2 summarizes the various kinds of transmembrane movement, and the sections that follow dis­ cuss them in more detail.
of the short-distance transport of substances moving into and out of cells. Figure 2.7 shows the diffusion of sugar
particles away from a sugar cube placed in water.
5..... Animation
Simple Diffusion Facilitated Diffusion
Molecules move randomly at all temperatures above absolute zero (-273° C) (due to spontaneous molecular motion) from areas where they are highly concentrated to areas where they are less concentrated, until they are evenly distributed in a state of dynamic equilibrium. This process is simple diffusion
(L. dif.fundere, to spread). Simple diffusion accounts for most
Polar molecules (not soluble in lipids) may diffuse through protein channels (pores) in the lipid bilayer (figure 2.8). The protein channels offer a continuous pathway for specific mol­ ecules to move across the plasma membrane so that they never come into contact with the hydrophobic layer or the membrane's polar surface.
Simple diffusion
Facilitated diffusion
Exocytosis
No cell energy is needed. Molecules move "down" a concentration gradient. Molecules spread out randomly from areas of higher concentration to areas of lower concentration until they are distributed evenly in a state of dynamic equilibrium.
Carrier (transport) proteins in a plasma membrane temporarily bind with molecules and help them pass across the membrane. Other proteins form channels through which molecules move across the membrane.
Water molecules diffuse across selectively permeable membranes from areas of higher concentration to areas of lower concentration.
Essentially protein-free plasma muves acruss capillary walls due to a pressure gradient across the wall.
Specific carrier proteins in the plasma membrane bind with molecules or ions to help them cross the membrane against a concentration gradient. Cellular energy is required.
The bulk movement of material into a cell by the formation of a vesicle.
The plasma membrane encloses small amounts of fluid droplets (in a vesicle) and takes them into-the-cell.···
The plasma membrane forms a vesicle around a solid particle or other cell and draws it into the phagocytic cell.
Extracellular molecules bind with specific receptor proteins on a plasma membrane, causing the membrane to invaginate and draw molecules into the cell.
The bulk movement of material out of a cell. A vesicle (with particles) fuses with the plasma membrane and expels particles or fluids from the cell across the plasma membrane. The reverse of endocytosis.
A frog inhales air containing oxygen, which inoves into the lungs and then diffuses into the bloodstream.
Glucose in the gut of a frog combines with carrier proteins to pass through the gut cells into the bloodstream.
Water molecules move into a frog's red blood cell when the concentration of water molecules outside the blood cell is greater than it is inside.
A frog's blood pressure forces water and dissolved wastes into the kidney tubules during urine formation.
Sodium ions move from inside the neurons of the sciatic nerve of a frog (the sodium­ ptitassium pump) to the outside of the neurons.
The kidney cells of a frog take in fluid to - maintain'fluid·batance. --
The white blood cells of a frog engulf and digest harmful bacteria.
The intestinal cells of a frog take up large molecules from the inside of the gut.
The sciatic nerve of a frog releases a chemical (neurotransmitter).
(a) (b)
FIGURE 2.7
(c) (d)
Simple Diffusion. When a sugar cube is placed in water (a), it slowly dissolves (b) and disappears. As this happens, the sugar molecules diffuse from a region where they are more concentrated to a region (c) where they are less concentrated. Even distribution of the sugar molecules throughout the water is diffusion equilibrium (d).
Inside the cell (cytoplasm)
Higher concentration
Transport Proteins. Molecules can move into and out of cells through integrated protein channels (pores) in the plasma membrane without using energy.
Large molecules and some (e.g., glucose and amino acids) of those not soluble in lipids require assistance in passing across the plasma membrane. These molecules use facilitated
diffusion, which, like simple diffusion, requires no energy input. To pass across the membrane, a molecule temporarily binds with a carrier (transpo1t) protein in the plasma membrane and is trans­ potted from an area of higher concentration to ""' Animation
an area of lower concentration (figure 2.9). :;J • Facilitated
Diffusion
Osmosis
The diffusion of water across a selectively permeable mem­ brane from an area of higher concentration to an area of lower concentration is osmosis (Gr. osmos, pushing). Osmosis is just a special type of diffusion, not a different method (figure 2.10).
Inside the cell (cytoplasm)

Higher concentration
Facilitated Diffusion and Carrier (Transport) Proteins. Some molecules move across the plasma membrane with the assistance of carrier proteins that transport the molecules down their concentration gradient, from a region of higher concentration to a region of lower concentration. A carrier protein alternates between two configurations, moving a molecule across a membrane as the shape of the protein changes. The rate of facilitated diffusion depends on how many carrier proteins are available in the membrane and how fast they can move their specific molecules.
Recent studies show that water, despite its polarity, can cross cell membranes, but this flow is limited. Water flow in living cells is facilitated by specialized water channels called aquaporins. Aquaporins fall into two general classes: those that are specific only for water, and others that allow small hydro­ philic molecules (e.g., urea, glycerol) to cross the membrane.
The term tonicity (Gr. to11w, tension) refers to the relative concentration of solutes in the water inside and outside the cell. For example, in an isotonic ( Gr. isos equal + tomi,:, tension)
18 CHAPTER TWO
Selectively permeable membrane
(a)
Time --
(b)
Osmosis. (a) A selectively permeable membrane separates the beaker into two compartments. Initially, compartment 1 contains sugar and water molecules, and compartment 2 contains only water molecules. Due to molecular motion, water moves down the concentration gradient (from compaitment 2 to compartment 1) by osmosis. The sugar molecules remain in compa1tment 1 because they are too large to pass across the membrane. (b) At osmotic equilibrium, the number of sugar molecules in compaLtment 1 does not increase, but the number of water molecules does.
solution, the solute concentration is the same inside and out5ide a red blood cell (figure 2.lla). The concentration of water mol­ ecules is also the same…