ZOOLOGY Chap 3

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Chapter 3

Cells as

Units of

Life


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3.1. Cell Concept

A. History

1. English scientist Robert Hooke described little boxes of cells in 1663

using a compound microscope.

2. Dutch microscopistAnton van Leeuwenhoek made extensive

observations and reported them in letters to Royal Society of London.

3. Advanced high-quality lenses in the early 19th

century made itpossible to examine cells.

B. Cell Theory

1. The cell theory asserts that all living organisms are composed of cells.

2. In 1838, Matthias Schleiden announced plant tissue was made of cells.3. In 1839, Theodore Schwann concluded animals were made of cells.

4. In 1840, J. Purkinje described cell contents as protoplasm; modern

understanding of cell organelles makes cytoplasm the preferred term.

5. 1858, another German, Rudolf Virchow, recognized that all cells

came from pre-existing cells.


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Figure 03.01


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C. How Cells Are Studied

1. Light microscopes use light rays; they are limited in magnification

and resolution.

2. The transmission electron microscope (TEM) uses electrons

passing through the specimen.

a. The wavelength of the electron is 0.00001 that of light, allowing

greater magnification.

b. Specimens must be prepared in thin section; the electrons pass

through to a photographic plate.3. Scanning electron microscope (SEM) scans electrons across a

metal-coated specimen; it has a lower magnification than TEM.

4. X-ray crystallography and nuclear magnetic resonance (NMR)

spectroscopy may reveal the shape of molecules.

5. Cytology, the study of cells, has its own methods.

a. Cells are disrupted in a blender and separated by a centrifuge;

organelles are then recovered.

b. Use of radioisotopes allow for tracing of metabolic pathways.

c. Proteins are extracted and purified; antibodies prepared against

the protein can be combined with fluorescent substances to detect

the location of the protein.


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Figure 03.02


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3.03

Separation of cell organelles by

centrifugation


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3.2. Cell Organization: Complex Organelles

and Energy Transformations

A. Prokaryotic and Eukaryotic Cells

1. Prokaryotes lack a membrane-bound nucleus found in eukaryotes.

2. However, both have DNA, use the same genetic code,

synthesize proteins and use ATP.

B. Major Components of Eukaryotic Cells and Their Functions

1. The cell membrane is the outermost membrane and regulates

the entrance and exit of molecules.

2.A

double membrane that separates the nucleus from cytoplasmencloses the nucleus.

3. Plant cells usually contain plastids for photosynthesis and have a

cellulose-based cell wall.


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Figure 03.01


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A bacterial cell


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Figure 03.05


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C. Cell Membrane

1. The current model of membrane structure is the fluid-mosaicmodel.

2. The cell membrane is phospholipid bilayer in which protein

molecules are partially or wholly embedded.

3. The phospholipid molecules have their water-soluble hydrophilic

ends toward the outside and their fat-soluble hydrophobicportions toward the inside of the membrane.

4. The layer is liquid, providing flexibility; embedded cholesterols

decrease this fluidity.

5. Glycoproteins are proteins with carbohydrates attached.

6. Some proteins catalyze transport of substances such as ions

across the membrane.7. Others are receptors for specific molecules.


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Figure 03.06


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D. Nucleus (Figure 3.7)

1. Nuclear envelopes contain less cholesterol than cell membranes;

pores allow relatively large molecules to readily move through.

2. Chromatin is a threadlike material that coils into chromosomes

just before cell division occurs; it contains DNA, protein

histones and nonhistone proteins.

3. Nucleoli are dark-staining spherical bodies in nucleus and

synthesize ribosomal RNA.4. After transcription from DNA, ribosomal RNA joins proteins to form

ribosomes.

5. The outer membrane of nucleus is continuous with endoplasmic

reticulum


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E. Endoplasmic reticulum

1. The endoplasmic reticulum (ER) is a system of membrane channels

continuous with outer membrane of the nuclear envelope.

2. The space between membranes of nuclear envelope communicates

with channels (cisternae) in ER.

3. The rough ER is studded with ribosomes on cytoplasm side;products enter cisternae for transport to the Golgi apparatus.

4. Ribosomes on the rough ER synthesize peptides or proteins that

enter the ER cisternae or membrane.

5. Some ribosome products are destined for incorporation into the

cell membrane or for export from the cell.6. The smooth ER functions to synthesize lipids and phospholipids.


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Endoplasmic reticulum Electron micrograph

showing endoplasmic

reticulumRough ER - studded with ribosomesSmooth ER - few ribosomes


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Figure 03.09

Golgi ComplexElectron micrograph of

a Golgi Complex


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Figure 03.10


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F. Lysosomes

1. Lysosomes are membrane-bound vesicles produced by the Golgi

complex.

2. Lysosomes contain digestive enzymes.

3. These enzymes help digest foreign material or engulfed bacteria:

lysosome vesicles pour enzymes into a food vacuole or

phagosome.

4. They destroy injured or diseased cells; a healthy cell must

maintain the membrane.

G. Contractile vacuoles contain fluid and regulate ions and water.

H. Mitochondria are present in nearly most eukaryotic cells.

1. Mitochondria are bound by a double membrane; inner membrane

folds (cristae) project into the inner space (matrix).

2. Enzymes on the cristae break down carbohydrate-derived

products; ATP production occurs here.

3. Mitochondria are self-replicating; their own DNA specifies some

proteins; nuclear DNA codes other proteins.


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Figure 03.11

Structure of a typical mitochondrion

Electron micrograph of amitochondrion


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I. Cytoskeleton

1. The cytoskeleton is a network of filaments and tubules that maintain

support and form.2. In many cells, they provide locomotion and translocation of organelles.

3. The cytoskeleton is composed of microfilaments, microtubules

and intermediate filaments.

4. Microfilaments are thin, linear structures first recognized in muscle cells.

5. Actin filaments are long, thin protein fibers that act with several

dozen other proteins.

6. One of these is myosin; the interaction causes contraction in muscle

and other cells.

7. Microtubules are composed of the protein tubulin; they move

chromosomes during cell division.

8. Microtubules radiate out from a microtubule organizing center:a centrosome.

9. Centrosomes are not membrane bound. (Figure 3.14)


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10. Centrioles are short cylinders with 9 triplets of microtubules;centrosomes contain two centrioles lying at right angles

to each other.

11. Intermediate filaments are larger than microfilaments and smaller

than microtubules in size.

12. Study of the type of intermediate filaments in cancerous cells can

help in the identification of the original cell type and in the

determination of treatment options.


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Figure 03.12

Electron micrograph

of a cytoskeleton


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Figure 03.13

A microtubule

composed oftubulin molecules


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Figure 03.14


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Figure 03.14a


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J. Surfaces of Cells and Their Specializations

1. Free surfaces of epithelial cells of tubes and cavities sometimes bear

cilia or flagella.a. Single celled organisms may use cilia or flagella to propel forward.

b. Flagella provide locomotion for male reproductive cells (sperm).

c. Locomotory cilia and flagella both have a cylinder of nine pairs of

microtubules encircling two single microtubules (9 + 2 pattern of

microtubules).

d. At the base of each cilium or flagellum is a basal body (kinetosome).

2. Ameboid movement uses pseudopodia.

a. Ameboid movement is seen in embryonic cells, white blood cells and

protozoa.

b. Cytoplasmic streaming utilizes actin microfilaments to extendpseudopodia.

c. Some specialized pseudopodia have cores of microtubules that

assemble and dissemble.


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Figure 03.15


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Figure 03.16


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Fluid Mosaic Model

Plasma membrane is composed of bothlipids and globular proteins.

Membrane proteins are not very soluble in

water.


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Components of the Cell Membrane

1. Lipid bilayer

2. Transmembrane proteins - proteins that float on or in the lipid

bilayer; provide passageway that allow substances and

information to cross the membrane

3. Network of supporting proteins - intracellular proteins that

reinforce the membranes shape

4. Exterior proteins and glycolipids - membrane proteins from

the ER to the golgi complex are transported, and then to

the plasma membrane. ER adds sugar molecules tomembrane proteins creating a sugar coating

or glycocalyx.


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3.3. Membrane Function

A. Plasma Membrane; Dynamic and Selective1. Also called the plasmalemma, it maintains cellular integrity.

2. It separates the interior environment from the exterior and regulates

molecule traffic flow.

3. It provides many unique functional properties of specialized cells.

4. Internal membranes divide a cell into compartments; they are sites

for most enzymatic reactions.

B. Cell Membrane Function

1. The membrane is the gatekeeper to substances that enter and

exit a cell.

2. Because the interior and exterior are different, the membrane is acritical controller.

3. Three principal methods are used for crossing a cell membrane:

a. Diffusion along a concentration gradient,

b. Substances bind to a site in a mediated transport system, and

c. Endocytosis encloses a particle in a vesicle that is engulfed.


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4. Diffusion and Osmosis (Figure 3.17)

a. Diffusion is movement of particles from higher to lower concentration,

or along a concentration gradient.b. If a membrane is permeable to a solute, diffusion will continue until

concentrations are equal.

c. Most membranes are selectively permeable, only allowing some

molecules to pass.

d. Most membranes allow free passage of water, gases, urea, and

lipid-soluble solutes.

e. Water-soluble molecules (e.g., sugar), electrolytes and some

macromolecules move across by carrier-mediated processes.


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Movement Across Membrane

1. Simple Diffusion - movement of molecules randomly from areas

of greater concentration to areas of lesser concentration untila state of equilibrium is achieved

e.g. diffusion of a sugar molecule away from a sugar cube

placed in water

2. Facilitated Diffusion - movement of molecules from area of


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high concentration to low concentration and requires a

carrier protein

e.g. Polar molecules (not soluble in lipids) may diffuse

through protein channels (pores) in the lipid bilayer.

The protein channels offer a continuous pathway forspecific molecules so that they never come in contact

with the hydrophobic layer of the membranes polar

surface.


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i. Marine fish have one-third the solute concentration as seawater; they

are hyposmotic to seawater.

j. A marine fish swimming up a river delta would pass through a region

where external and internal solutes were equal orisosmotic.

k. In freshwater, its blood solutes would be hyperosmotic to thefreshwater.

5. Diffusion through Channels (Figure 3.18)

a. Water and dissolved ions cannot pass through the phospholipidcomponent of the plasma membrane.

b. Water and ions pass through the membrane by diffusion through pores

created by transmembrane proteins.

c. Some channels are gated and require a signal to open or close them.

d. Gated ion channels may open or close in response to a signaling

molecule (chemically-gated ion channels) or to the change of anionic charge across the plasma membrane (voltage-gated ion

channels).

e. Water channels are called aquaporins.


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f. Osmosis is movement of water molecules down a concentration

gradient across a membrane.

g. A salt solution in a cell will cause diffusion of water inward until the

increase in weight (hydrostatic or osmotic pressure) of the solutioncauses it to be in equilibrium.

h. Osmotic potential is a term used to avoid confusion of term

osmotic pressure in the absence of membrane and pure water

reference.


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i. Marine fish have one-third the solute concentration as seawater; they

are hyposmotic to seawater.

j. A marine fish swimming up a river delta would pass through a region

where external and internal solutes were equal orisosmotic.k. In freshwater, its blood solutes would be hyperosmotic to the

freshwater.


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5. Diffusion through Channels

a. Water and dissolved ions cannot pass through the phospholipid

component of the plasma membrane.b. Water and ions pass through the membrane by diffusion through

pores created by transmembrane proteins.

c. Some channels are gated and require a signal to open or close

them.

d. Gated ion channels may open or close in response to a signalingmolecule (chemically-gated ion channels) or to the change of an

ionic charge across the plasma membrane (voltage-gated ion

channels).

e. Water channels are called aquaporins.


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Figure 03.18


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6. Carrier-Mediated Transport (Figure 3.19)

a. Special proteins (transporters orpermeases) move nutrients and

wastes across the membrane.b. Permeases form a small passageway for very specific solute molecules.

c. When all transporters become saturated with solutes, the rate of influx

does not increase with more solute; this measures the amount of

transporter molecules.

d. With simple diffusion, the greater the difference in solute concentrations,

the higher the flux.e. Two mediated transport mechanisms are recognized.

1) Facilitated diffusion permeases assist a molecule (e.g., sugar)

to diffuse that otherwise cannot. (Figure 3.20)

2) Active transport uses energy to transport molecules against the

concentration gradient.f. Most animal cells require internal potassium levels 2050 times

outside levels; outside sodium levels may be ten times inside levels.

g. In many cells, sodium and potassium pumping are linked using

the same transporter molecule.


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Figure 03.19b


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7. Endocytosis (Figure 3.21)

a. All processes (phagocytosis, pinocytosis and receptor-mediated

endocytosis) require energy.

b. Phagocytosis is common among protozoa and lower metazoa.1) An area of cell membrane coated internally with actin-and-myosin

forms a pocket to engulf material.

2) The membrane-enclosed vesicle detaches from the cell surface for

internal digestion.

c. Pinocytosis1) Small areas of surface membrane invaginate into tiny vesicles

called caveolae.

2) Specific binding receptors for the molecule or ion are on this cell

surface.

3) It is involved in taking in some vitamins, hormones and growth

factors.


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d. Receptor-mediated endocytosis

1) Plasma membrane proteins bind specific particles called

ligands.

2) This occurs in clathrin-coated pits coated with receptors.3) Brought within the cell, the pit is uncoated and the ligand

disassociated to be recycled.

4) Some proteins and peptide hormones are brought into cells by

this method.

8. Exocytosis

a. This is the reverse of the invagination and formation of a vesicle.

b. It removes indigestible residues, secretes hormones and

transport substances


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Figure 03.21


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3.4. Mitosis and Cell Division

A. Cell Types

1. All cells in nearly all multicellular organisms originated from divisionof a single cell, the zygote.

2. Azygote is formed from union of egg and sperm, the gametes.

3. Formation of body (somatic) cells by nuclear division is mitosis.

4. Mitosis delivers chromosomes and their DNA to the cell lineage.

5. Although the cell lineage differentiates, the genes not expressed

are still present.6. Mitosis ensures the equality of genetic material.

7. In animals that reproduce asexually, mitosis transfers

genetic information to progeny.

8. In animals that reproduce sexually, parents produce sex cells

(known as gametes or germ cells) with half the number of chromosomes.a. This prevents the union of gametes from doubling the number

of parental chromosomes.

b. This requires reduction division ormeiosis.


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C. Phases in Mitosis

1. Cell division involves division of nuclear chromosomes (mitosis) and

cytoplasm (cytokinesis).a. When a nucleus divides without cytokinesis, a multinucleate cell

results.

b. When several cells fuse, it can also form a multinucleate syncytium.

2. Mitosis is a four-step process with each step merging into the next.

3. When the cell is not actively dividing, it is in interphase during which

DNA replicates and genes are transcribed.

4. Prophase (Figures 3.213.23)

a. In early prophase, centrosomes replicate and the two centrosomes

migrate to opposite sides of the nucleus.

b. Microtubules form a football-shaped spindle between the

centrosomes.c. Other microtubules radiate outward to form asters.

d. Nuclear chromatin condenses into chromosomes; the sister

chromatids were actually formed during interphase.

e. Spindle fibers reach the centrosome and bind to the kinetochore.


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5. Metaphase (Figure 3.24)

a. Kinetochore fibers pull condensed sister chromatids to the central

metaphasic plate.

b. Centromeres line up precisely on the equatorial plate; arms of

chromatids dangle.

6. Anaphase

a. A single centromere that holds the chromatids together now splits.

b. Chromosomes move toward their respective poles, pulled by

kinetochore fibers.

c. At the end of anaphase A, microtubules shorten by disassembly oftubulin units at the kinetochore end.

d. In anaphase B, chromosomes approach respective centrosomes as

the spindle lengthens.

e. Tubulin in microtubules intermingles and pushes the ends of the

spindle apart.

7. Telophase

a. Phase begins as daughter chromosomes reach each pole.

b. Spindle fibers disappear.

c. Chromosomes lose identity and diffuse into chromatin network in

nucleus.

d. Nuclear membranes re-form in daughter cells.


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D. Cytokinesis: Cytoplasmic Division

1. During the final stage of nuclear division, a cleavage

furrow appears on cell surface.

2. Microfilaments of actin just beneath the surface draw

the furrow inward.

3. Infolding edges of cytoskeleton meet and fuse,

completing cell division.


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E. Cell Cycle (Figure 3.25)

1. Cells undergo cycles of growth and replication.

2. A cell cycle is a mitosis-to-mitosis cycle, the interval between one

cell generation and the next.

3. Interphase

a. Nuclear division occupies 510% of the cell cycle; the rest is

interphase.

b. Early concepts of interphase as an inactive stage of rest are

incorrect.c. DNA replication occurs during interphase.

d. S (for synthesis) stage lasts about 6 of the 1824 hours of a cell

cycle in a human.

e. Both strands of DNA replicate a complimentary strand.

f. The G1 period precedes the S stage; transfer RNA, ribosomes,

messenger RNA and enzymes are synthesized.

g. The G2 period follows the S stage; spindle and aster proteins

form in preparation for chromosome separation.


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4. Embryonic Cells

a. Embryonic cells divide rapidly with no cell growth between

divisions, just subdivision.b. DNA synthesis may be hundreds of times faster in embryonic cells

than in adult cells.

5. As organisms develop, the cell cycle of most cells lengthens.

a. A nonproliferative phase or G0 ends the cycle.

b. Neurons do not divide further after birth and are in a permanentG0.


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Figure 03.25


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6. Cell Cycle Control (Figure 3.26)

a. Regulation of the cell cycle is mediated by cyclin-dependentkinases (cdks).

b. Cyclins are activating subunits of cdks.

c. Kinase enzymes add phosphate groups to other proteins to activate

or inactivate them.

d. The passage from one cell cycle to the next is likely regulated by

phosphorylation and dephosphorylation of specific cdks and their

interaction with phase specific cyclins.


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Figure 03.26

7 Flux of Cells


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7. Flux of Cells

a. Cell division is very rapid during early development of an organism.

b. The human infant has 2 trillion cells that originated from one

fertilized egg; this represents 42 cell divisions.c. Five more cell divisions produce an adult with 60 trillion cells.

d. Various cells divide in days, months or years: muscle and nerve

cells stop dividing in childhood or before.

e. A human sheds about 12% of the total number of cells daily from

skin, digestive tract, sperm and short-lived red blood cells.

8. Apoptosis

a. Apoptosis is programmed cell death.

b. Apoptosis is necessary for normal development and suicide of

unhealthy cells.

c. Cells shrink, fragment, and then the remains are taken up bysurrounding cells.

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