08_The_Cell

Cellular Basis of Life

A Closer Look at Cells

• The cell

– Is the lowest level of organization that can perform all activities required for life

25 µmFigure 1.5

The Cell’s Heritable Information

• Cells contain chromosomes made partly of DNA, the substance of genes

– Which program the cells’ production of proteins and transmit information from parents to offspring

Egg cell

Sperm cell

NucleicontainingDNA

Fertilized eggwith DNA fromboth parents

Embyro’s cells with copies of inherited DNA Offspring with traits

inherited fromboth parentsFigure 1.6

Discovery of the Cell

• Robert Hooke (1665)

– English scientist, first described and named cells.

– Observed in a slice a cork and saw that the compartments / tiny boxes or cells were unique to cork.

• Anton van Leeuwenhoek (1674)

– Dutchman who first saw live cells using grains of sand polished into magnifying glasses.

– Saw a microbial world in droplets of pond water and also observed blood cells and sperm cells of animals.

• Matthias Schleiden and Theodor Schwann (1839)

– German biologists, reached a generalization based on many concurring observations reaching a generalization that all living things consists of cells.

• Rudolph Virchow (1858)

– German doctor, concluded that all cells come from pre-existing cells based on his study on how cells reproduce.

Cell Theory (Classic)

• 1. All living things are composed of cells.

• 2. A cell is the smallest unit with the properties of life.

• 3. Each new cell arises from division of another, preexisting cell.

• 4. Each cell passes hereditary material to its offspring.

• To study cells, biologists use microscopes and the tools of biochemistry

Microscopy

• Scientists use microscopes to visualize cells too small to see with the naked eye

• Light microscopes (LMs)

– Pass visible light through a specimen

– Magnify cellular structures with lenses

• Different types of microscopes

– Can be used to visualize different sized cellular structures

0.1 nm

100 µm

10 µ m

1 µ m

100 nm

Length of somenerve and muscle cells

Chicken egg

Frog egg

Most plant and Animal cells

Smallest bacteria

Viruses

Ribosomes

Proteins

Lipids

Small molecules

NucleusMost bacteriaMitochondrion

Figure 6.2

Human height

Measurements1 centimeter (cm) = 102 meter (m) = 0.4 inch1 millimeter (mm) = 10–3 m1 micrometer (µm) = 10–3 mm = 10–6 m1 nanometer (nm) = 10–3 mm = 10–9 m

– Use different methods for enhancing visualization of cellular structures

TECHNIQUE RESULT

Brightfield (unstained specimen). Passes light directly through specimen. Unless cell is naturally pigmented or artificially stained, image has little contrast. [Parts (a)–(d) show a human cheek epithelial cell.]

Brightfield (stained specimen). Staining with various dyes enhances contrast, but most staining procedures require that cells be fixed (preserved).

Phase-contrast. Enhances contrast in unstained cells by amplifying variations in density within specimen; especially useful for examining living, unpigmented cells.

50 µm

Figure 6.3

Differential-interference-contrast (Nomarski). Like phase-contrast microscopy, it uses optical modifications to exaggerate differences indensity, making the image appear almost 3D.

Fluorescence. Shows the locations of specific molecules in the cell by tagging the molecules with fluorescent dyes or antibodies. These fluorescent substances absorb ultraviolet radiation and emit visible light, as shown here in a cell from an artery.

Confocal. Uses lasers and special optics for “optical sectioning” of fluorescently-stained specimens. Only a single plane of focus is illuminated; out-of-focus fluorescence above and below the plane is subtracted by a computer. A sharp image results, as seen in stained nervous tissue (top), where nerve cells are green, support cells are red, and regions of overlap are yellow. A standard fluorescence micrograph (bottom) of this relatively thick tissue is blurry.

50 µm

• Electron microscopes (EMs)

– Focus a beam of electrons through a specimen (TEM) or onto its surface (SEM)

• The scanning electron microscope (SEM)

– Provides for detailed study of the surface of a specimen

TECHNIQUE RESULTS

Scanning electron micro-scopy (SEM). Micrographs takenwith a scanning electron micro-scope show a 3D image of the surface of a specimen. This SEM shows the surface of a cell from a rabbit trachea (windpipe) covered with motile organelles called cilia. Beating of the cilia helps moveinhaled debris upward toward the throat.

(a)Cilia

Figure 6.4 (a)

• The transmission electron microscope (TEM)

– Provides for detailed study of the internal ultrastructure of cells

Transmission electron micro-scopy (TEM). A transmission electron microscope profiles a thin section of a specimen. Here we see a section through a tracheal cell, revealing its ultrastructure. In preparing the TEM, some cilia were cut along their lengths, creating longitudinal sections, while other cilia were cut straight across, creating cross sections.

Longitudinalsection ofcilium

Cross sectionof cilium 1 µm

Figure 6.4 (b)

Eukaryotic cells have internal membranes that compartmentalize their functions

• Two types of cells make up every organism

– Prokaryotic

– Eukaryotic

Comparing Prokaryotic and Eukaryotic Cells

• All cells have several basic features in common

– They are bounded by a plasma membrane

– They contain a semifluid substance called the cytosol

– They contain chromosomes

– They all have ribosomes

• Prokaryotic cells

– Do not contain a nucleus

– Have their DNA located in a region called the nucleoid

(b) A thin section through the bacterium Bacillus coagulans (TEM)

Pili: attachment structures onthe surface of some prokaryotes

Nucleoid: region where thecell’s DNA is located (notenclosed by a membrane)

Ribosomes: organelles thatsynthesize proteins

Plasma membrane: membraneenclosing the cytoplasm

Cell wall: rigid structure outsidethe plasma membrane

Capsule: jelly-like outer coatingof many prokaryotes

Flagella: locomotionorganelles ofsome bacteria

(a) A typical rod-shaped bacterium

0.5 µmBacterial

chromosome

Figure 6.6 A, B

• Eukaryotic cells

– Contain a true nucleus, bounded by a membranous nuclear envelope

– Are generally quite a bit bigger than prokaryotic cells

• The logistics of carrying out cellular metabolism sets limits on the size of cells

• A smaller cell

– Has a higher surface to volume ratio, which facilitates the exchange of materials into and out of the cell

Surface area increases whiletotal volume remains constant

Total surface area (height width number of sides number of boxes)

Total volume (height width length number of boxes)

Surface-to-volume ratio (surface area volume)

Figure 6.7

• The plasma membrane

– Functions as a selective barrier

– Allows sufficient passage of nutrients and waste

Carbohydrate side chain

Figure 6.8 A, B

Outside of cell

Inside of cell

Hydrophilicregion

Hydrophobicregion

Hydrophilicregion

(b) Structure of the plasma membrane

Phospholipid Proteins

TEM of a plasmamembrane. Theplasma membrane,here in a red bloodcell, appears as apair of dark bandsseparated by alight band.

0.1 µm

A Panoramic View of the Eukaryotic Cell

• Eukaryotic cells

– Have extensive and elaborately arranged internal membranes, which form organelles

• Plant and animal cells

– Have most of the same organelles

• A animal cell

Rough ER Smooth ER

Centrosome

CYTOSKELETON

Microfilaments

Microtubules

Microvilli

Peroxisome

Lysosome

Golgi apparatus

Ribosomes

In animal cells but not plant cells:LysosomesCentriolesFlagella (in some plant sperm)

Nucleolus

Chromatin

NUCLEUS

Flagelium

Intermediate filaments

ENDOPLASMIC RETICULUM (ER)

Mitochondrion

Nuclear envelope

Plasma membrane

Figure 6.9

• A plant cell

In plant cells but not animal cells:ChloroplastsCentral vacuole and tonoplastCell wallPlasmodesmata

CYTOSKELETON

Ribosomes (small brwon dots)

Central vacuole

MicrofilamentsIntermediate filaments

Microtubules

Rough endoplasmic reticulum Smooth

endoplasmic reticulum

ChromatinNUCLEUS

Nuclear envelope

Nucleolus

Chloroplast

PlasmodesmataWall of adjacent cell

Cell wall

Golgi apparatus

Peroxisome

Tonoplast

Centrosome

Plasma membrane

Mitochondrion

Figure 6.9

The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes

The Nucleus: Genetic Library of the Cell

• The nucleus

– Contains most of the genes in the eukaryotic cell

• The nuclear envelope

– Encloses the nucleus, separating its contents from the cytoplasm

Figure 6.10

Nucleus

NucleusNucleolus

Chromatin

Nuclear envelope:Inner membraneOuter membrane

Nuclear pore

Rough ER

Porecomplex

Surface of nuclear envelope.

Pore complexes (TEM). Nuclear lamina (TEM).

Close-up of nuclearenvelope

Ribosome

1 µm0.25 µm

Ribosomes: Protein Factories in the Cell

• Ribosomes

– Are particles made of ribosomal RNA and protein

– Carry out protein synthesis

Endoplasmic reticulum (ER)

Ribosomes Cytosol

Free ribosomes

Bound ribosomes

Largesubunit

Smallsubunit

TEM showing ER and ribosomes Diagram of a ribosome

0.5 µm

Figure 6.11

The endomembrane system regulates protein traffic and performs metabolic functions in the cell

• The endomembrane system

– Includes many different structures

The Endoplasmic Reticulum: Biosynthetic Factory

• The endoplasmic reticulum (ER)

– Accounts for more than half the total membrane in many eukaryotic cells

• The ER membrane

– Is continuous with the nuclear envelope

Smooth ER

Rough ER

ER lumenCisternae

RibosomesTransport vesicle

Smooth ER

Transitional ER

Rough ER 200 µm

Nuclearenvelope

Figure 6.12

• There are two distinct regions of ER

– Smooth ER, which lacks ribosomes

– Rough ER, which contains ribosomes

Functions of Smooth ER

• The smooth ER

– Synthesizes lipids

– Metabolizes carbohydrates

– Stores calcium

– Detoxifies poison

Functions of Rough ER

• The rough ER

– Has bound ribosomes

– Produces proteins and membranes, which are distributed by transport vesicles

• The Golgi apparatus

– Receives many of the transport vesicles produced in the rough ER

– Consists of flattened membranous sacs called cisternae

The Golgi Apparatus: Shipping and Receiving Center

• Functions of the Golgi apparatus include

– Modification of the products of the rough ER

– Manufacture of certain macromolecules

Golgiapparatus

TEM of Golgi apparatus

cis face(“receiving” side ofGolgi apparatus)

Vesicles movefrom ER to Golgi Vesicles also

transport certainproteins back to ER

Vesicles coalesce toform new cis Golgi cisternae

Cisternalmaturation:Golgi cisternaemove in a cis-to-transdirection

Vesicles form andleave Golgi, carryingspecific proteins toother locations or tothe plasma mem-brane for secretion

Vesicles transport specificproteins backward to newerGolgi cisternae

Cisternae

trans face(“shipping” side ofGolgi apparatus)

0.1 0 µm16

• Functions of the Golgi apparatus

Figure 6.13

Lysosomes: Digestive Compartments

• A lysosome

– Is a membranous sac of hydrolytic enzymes

– Can digest all kinds of macromolecules

• Lysosomes carry out intracellular digestion by

– Phagocytosis

Figure 6.14 A(a) Phagocytosis: lysosome digesting food

Lysosome containsactive hydrolyticenzymes

Food vacuole fuses with lysosome

Hydrolyticenzymes digestfood particles

Digestion

Food vacuole

Plasma membraneLysosome

Digestiveenzymes

Lysosome

Nucleus

• Autophagy

Figure 6.14 B(b) Autophagy: lysosome breaking down damaged organelle

Lysosome containingtwo damaged organelles 1 µ m

Mitochondrionfragment

Peroxisomefragment

Lysosome fuses withvesicle containingdamaged organelle

Hydrolytic enzymesdigest organellecomponents

Vesicle containingdamaged mitochondrion

Digestion

Lysosome

Vacuoles: Diverse Maintenance Compartments

• A plant or fungal cell

– May have one or several vacuoles

• Food vacuoles

– Are formed by phagocytosis

• Contractile vacuoles

– Pump excess water out of protist cells

• Central vacuoles

– Are found in plant cells

– Hold reserves of important organic compounds and water

Central vacuole

Cytosol

Tonoplast

Centralvacuole

Nucleus

Cell wall

Chloroplast

5 µmFigure 6.15

The Endomembrane System: A Review

• The endomembrane system

– Is a complex and dynamic player in the cell’s compartmental organization

Plasma membrane expandsby fusion of vesicles; proteinsare secreted from cell

Transport vesicle carriesproteins to plasma membrane for secretion

Lysosome availablefor fusion with anothervesicle for digestion

Nuclear envelope isconnected to rough ER, which is also continuous

with smooth ER

Nucleus

Rough ER

Smooth ERcis Golgi

trans Golgi

Membranes and proteinsproduced by the ER flow in

the form of transport vesiclesto the Golgi Nuclear envelop

Golgi pinches off transport Vesicles and other vesicles

that give rise to lysosomes and Vacuoles

Plasmamembrane

• Relationships among organelles of the endomembrane system

Figure 6.16

• Mitochondria and chloroplasts change energy from one form to another

• Mitochondria

– Are the sites of cellular respiration

• Chloroplasts

– Found only in plants, are the sites of photosynthesis

Mitochondria: Chemical Energy Conversion

• Mitochondria

– Are found in nearly all eukaryotic cells

• Mitochondria are enclosed by two membranes

– A smooth outer membrane

– An inner membrane folded into cristae

Mitochondrion

Intermembrane spaceOuter

membrane

Freeribosomesin the mitochondrialmatrix

MitochondrialDNA

Innermembrane

Cristae

Matrix

100 µmFigure 6.17

Chloroplasts: Capture of Light Energy

• The chloroplast

– Is a specialized member of a family of closely related plant organelles called plastids

– Contains chlorophyll

• Chloroplasts

– Are found in leaves and other green organs of plants and in algae

Chloroplast

ChloroplastDNA

RibosomesStromaInner and outermembranes

Thylakoid

Granum

Figure 6.18

• Chloroplast structure includes

– Thylakoids, membranous sacs

– Stroma, the internal fluid

Peroxisomes: Oxidation

• Peroxisomes

– Produce hydrogen peroxide and convert it to water

ChloroplastPeroxisome

Mitochondrion

1 µmFigure 6.19

The cytoskeleton is a network of fibers that organizes structures and activities in the cell

• The cytoskeleton

– Is a network of fibers extending throughout the cytoplasm

Figure 6.20

Microtubule

0.25 µm MicrofilamentsFigure 6.20

Roles of the Cytoskeleton: Support, Motility, and Regulation

• The cytoskeleton

– Gives mechanical support to the cell

– Is involved in cell motility, which utilizes motor proteins

VesicleATPReceptor formotor protein

Motor protein(ATP powered)

Microtubuleof cytoskeleton

(a) Motor proteins that attach to receptors on organelles can “walk”the organelles along microtubules or, in some cases, microfilaments.

Microtubule Vesicles 0.25 µm

(b) Vesicles containing neurotransmitters migrate to the tips of nerve cell axons via the mechanism in (a). In this SEM of a squid giant axon, two vesicles can be seen moving along a microtubule. (A separate part of the

experiment provided the evidence that they were in fact moving.)Figure 6.21 A, B

Components of the Cytoskeleton

• There are three main types of fibers that make up the cytoskeleton

Table 6.1

Microtubules

• Microtubules

– Shape the cell

– Guide movement of organelles

– Help separate the chromosome copies in dividing cells

Centrosomes and Centrioles

• The centrosome

– Is considered to be a “microtubule-organizing center”

– Contains a pair of centrioles

Centrosome

Microtubule

Centrioles0.25 µm

Longitudinal sectionof one centriole

Microtubules Cross sectionof the other centrioleFigure 6.22

Cilia and Flagella

• Cilia and flagella

– Contain specialized arrangements of microtubules

– Are locomotor appendages of some cells

• Flagella beating pattern

(a) Motion of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the flagellum. Propulsion of a human sperm cell is an example of flagellatelocomotion (LM).

Direction of swimming

Figure 6.23 A

• Ciliary motion

(b) Motion of cilia. Cilia have a back- and-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, a freshwater protozoan (SEM).

Figure 6.23 B

15 µm

• Cilia and flagella share a common ultrastructure

Outer microtubuledoubletDynein arms

CentralmicrotubuleOuter doublets cross-linkingproteins inside

Radialspoke

Plasmamembrane

Microtubules

Plasmamembrane

Basal body

0.5 µm

0.1 µm

Cross section of basal body

Triplet

Figure 6.24 A-C

Extracellular components and connections between cells help coordinate cellular activities

Cell Walls of Plants

• The cell wall

– Is an extracellular structure of plant cells that distinguishes them from animal cells

• Plant cell walls

– Are made of cellulose fibers embedded in other polysaccharides and protein

– May have multiple layersCentral vacuoleof cell

PlasmamembraneSecondarycell wallPrimarycell wall

Middlelamella

Centralvacuoleof cell

Central vacuole CytosolPlasma membrane

Plant cell walls

PlasmodesmataFigure 6.28

The Extracellular Matrix (ECM) of Animal Cells

• Animal cells

– Lack cell walls

– Are covered by an elaborate matrix, the ECM

• The ECM

– Is made up of glycoproteins and other macromolecules

Collagen

Fibronectin

Plasmamembrane

EXTRACELLULAR FLUID

Micro-filaments

CYTOPLASM

Integrins

Polysaccharidemolecule

Carbo-hydrates

Proteoglycanmolecule

Coreprotein

Integrin

Figure 6.29

A proteoglycan complex

• Functions of the ECM include

– Support

– Adhesion

– Movement

– Regulation

Intercellular Junctions

Plants: Plasmodesmata

• Plasmodesmata

– Are channels that perforate plant cell walls

Interiorof cell

0.5 µm Plasmodesmata Plasma membranes

Cell walls

Figure 6.30

Animals: Tight Junctions, Desmosomes, and Gap Junctions

• In animals, there are three types of intercellular junctions

– Tight junctions

– Desmosomes

– Gap junctions

• Types of intercellular junctions in animals

Tight junctions prevent fluid from moving across a layer of cells

Tight junction

0.5 µm

Spacebetweencells

Plasma membranesof adjacent cells

Extracellularmatrix

Gap junction

Tight junctions

0.1 µm

Intermediatefilaments

Desmosome

Gapjunctions

At tight junctions, the membranes ofneighboring cells are very tightly pressedagainst each other, bound together byspecific proteins (purple). Forming continu-ous seals around the cells, tight junctionsprevent leakage of extracellular fluid acrossA layer of epithelial cells.

Desmosomes (also called anchoringjunctions) function like rivets, fastening cellsTogether into strong sheets. IntermediateFilaments made of sturdy keratin proteinsAnchor desmosomes in the cytoplasm.

Gap junctions (also called communicatingjunctions) provide cytoplasmic channels fromone cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars,amino acids, and other small molecules maypass. Gap junctions are necessary for commu-nication between cells in many types of tissues,including heart muscle and animal embryos.

TIGHT JUNCTIONS

DESMOSOMES

GAP JUNCTIONS

Figure 6.31

The Cell: A Living Unit Greater Than the Sum of Its Parts

• Cells rely on the integration of structures and organelles in order to function

Figure 6.32

08_The_Cell

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