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A Tour of the Cell (Chapter 6). Figure 6.1. Theodore Schwann’s cell theory (1839):. 1) All organisms are composed of one or more cells 2) Cells are the basic units of organization of all organisms 3) Cells arise only by division of a previously existing cell. How do we study cells?.

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Theodore schwann s cell theory 1839
Theodore Schwann’s cell theory (1839):

  • 1) All organisms are composed of one or more cells

  • 2) Cells are the basic units of organization of all organisms

  • 3) Cells arise only by division of a previously existing cell


How do we study cells
How do we study cells?

Few cells are big enough to be viewed with the naked eye. Therefore, we use:

  • Cytology (microscopy) – study of cell structure

    - magnification – ratio of object’s image size to its real size

    - resolution – measure of clarity of the image, minimum distance two points can be separated and still be distinguished as two points

    - limits the effective study


Microscopy
Microscopy

  • Light microscopes (LMs)

    • First used during the Renaissance (1600s)

    • Pass visible light through a specimen

    • Magnify cellular structures with lenses


10 m

Human height

1 m

Length of some

nerve and

muscle cells

0.1 m

Light microscope

Chicken egg

1 cm

Frog egg

1 mm

100 µm

Most plant and Animal cells

Electron microscope

10 µ m

NucleusMost bacteriaMitochondrion

1 µ m

Electron microscope

Smallest bacteria

100 nm

Viruses

10 nm

Ribosomes

Proteins

Lipids

1 nm

Small molecules

Figure 6.2

Atoms

0.1 nm

Smaller subcellular structures require higher powered microscopes

1 micrometer = 1 micron =

0.000001 or 1/100,000 meter =

1 nanometer =

0.000000001 or 1/100,000,000 meter =


Microscopy1

RESULT

TECHNIQUE

(a)

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.]

50 µm

(b)

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

(c)

Phase-contrast. Enhances contrast

in unstained cells by amplifying

variations in density within specimen;

especially useful for examining living,

unpigmented cells.

Figure 6.3

Microscopy

  • Different methods for enhancing visualization of cellular structures


(d)

(e)

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.

50 µm

(f)

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

Research Methods

Differential-interference-contrast (Nomarski).

Like phase-contrast microscopy, it uses optical

modifications to exaggerate differences in

density, making the image appear almost 3D.


TECHNIQUE

RESULTS

1 µm

Cilia

Scanning electron micro-

scopy (SEM). Micrographs taken

with 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 move

inhaled debris upward toward

the throat.

(a)

  • 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


TEM

The transmission electron microscope (TEM)

  • Provides for detailed study of the internal ultrastructure of cells

Longitudinal

section of

cilium

Cross section

of cilium

1 µm

(b)

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.

Figure 6.4 (b)


Electron micrographs of rabbit trachea cells
Electron micrographs of rabbit trachea cells

Transmission EM Scanning EM

Figure 6.4

- Beam through object - beam on surface


How do we study cells1
How do we study cells?

1) Microscopy

2) Cell fractionation

- allows for determination of the function of subcellular structures

  • Take cells apart and separate the major organelles from each other

  • Done in a centrifuge – spin tubes at various speeds

  • Isolates cell components, based on size and density

  • Ultracentrifuges – most powerful, can spin as fast as 130K/minute = applies forces on particles of more than 1 million times the force of gravity


Homogenization

Tissue

cells

1000 g

(1000 times the

force of gravity)

10 min

Homogenate

Differential centrifugation

Supernatant poured

into next tube

20,000 g

20 min

80,000 g

60 min

Pellet rich in

nuclei and

cellular debris

150,000 g

3 hr

Pellet rich in

mitochondria

(and chloro-

plasts if cells

are from a

plant)

Pellet rich in

“microsomes”

(pieces of

plasma mem-

branes and

cells’ internal

membranes)

Pellet rich in

ribosomes

Cell Fractionation

Cells are homogenized in a blender to

break them up. The resulting mixture (cell homogenate) is then

centrifuged at various speeds and durations to fractionate the cell

components

Increasing speed (force)

Results: 1)Researchers used microscopy to identify the organelles in each pellet

2) Researchers used biochemical methods to determine the metabolic functions of organelles.

Big small

Objects in the pellet

Figure 6.5


Cells
Cells

  • Two types of cells

    • Prokaryotic

    • Eukaryotic

  • All cells have several basic features in common

    • Bounded by a plasma membrane

    • Contain a semifluid substance - cytosol

    • Contain chromosomes

    • Have ribosomes


Two types of cells
Two types of cells

  • Circular DNA molecule (chromosome), not contained within a nucleus

    • Have their DNA located in a region called the nucleoid

  • Small in size  1-10 micrometers

  • Prokaryotic cells – basic, No interior membranes

2) Eukaryotic cells - Interior membranes

  • Linear DNA molecules (chromosomes) Contained within a true nucleus, bounded by a membranous nuclear envelope

  • 10-100 micrometers

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


Prokaryotic cells
Prokaryotic cells

Pili: attachment structures on

the surface of some prokaryotes

Nucleoid: region where the

cell’s DNA is located (not

enclosed by a membrane)

Ribosomes: organelles that

synthesize proteins

Plasma membrane: membrane

enclosing the cytoplasm

Cell wall: rigid structure outside

the plasma membrane

Capsule: jelly-like outer coating

of many prokaryotes

Bacterialchromosome

0.5 µm

Flagella: locomotion

organelles of

some bacteria

(a) A typical

rod-shaped bacterium

(b) A thin section through the

bacterium Bacillus coagulans

(TEM)

Figure 6.6


Animal cell eukaryotic
Animal cell (eukaryotic)

Nuclear envelope

ENDOPLASMIC RETICULUM (ER)

NUCLEUS

Nucleolus

Smooth ER

Rough ER

Chromatin

Flagelium

Plasma membrane

Centrosome

CYTOSKELETON

Microfilaments

Intermediate filaments

Ribosomes

Microtubules

Microvilli

Golgi apparatus

Peroxisome

In animal cells but not plant cells:

Lysosomes

Centrioles

Flagella (in some plant sperm)

Lysosome

Mitochondrion

Figure 6.9


Plant cell eukaryotic
Plant Cell (eukaryotic)

Nuclear envelope

Rough

endoplasmic

reticulum

Nucleolus

NUCLEUS

Chromatin

Smooth

endoplasmic

reticulum

Centrosome

Ribosomes (small brown dots)

Central vacuole

Tonoplast

Golgi apparatus

Microfilaments

Intermediate

filaments

CYTOSKELETON

Microtubules

In plant cells

(but not in

Animal cells):

Chloroplasts

Central vacuole

and tonoplast

Cell wall

Plasmodesmata

Mitochondrion

Peroxisome

Plasma membrane

Chloroplast

Cell wall

Plasmodesmata

Figure 6.9

Wall of adjacent cell


Prokaryotes vs eukaryotes
Prokaryotes vs. Eukaryotes

Prokaryotes

Eukaryotes

Size 1-10 mm 10-100 mm

Genetic circular linear

Material chromosome chromosomes

Plasma yes yes

Membrane

Cell Wall yes some-plants, etc.

Ribosomes yes yes

Membrane-bound NOYES

organelles


Why are cells so small
Why are cells so small?

  • 1) Diffusion

    • main way of intracellular communication

    • diffusion is very slow!

  • 2) Surface area-to-volume ratio

    • A smaller cell - has a higher surface to volume ratio, which facilitates the exchange of materials into and out of the cell

    • volume increases more rapidly than surface area

    • Cell must receive nutrients and expel wastes across the surface area of the cell

      • Large volume = greater nutrient need and more waste production

    • consider a cube


Surface area and volume
Surface area and volume

Surface area increases while

total volume remains constant

5

1

1

Total surface area

(height  width 

number of sides 

number of boxes)

6

150

750

Total volume

(height  width  length

 number of boxes)

125

125

1

Surface-to-volume

ratio

(surface area  volume)

6

12

6

Area is proportional to the surface area squared

Volume is proportional to the surface area cubed


TEM of a plasma

membrane. The

plasma membrane,

here in a red blood

cell, appears as a

pair of dark bands

separated by a

light band.

(a)

Plasma Membrane

  • functions as a selective barrier

    • Allows sufficient passage of nutrients and waste

Outside of cell

Carbohydrate side chain

Hydrophilic

region

Inside of cell

0.1 µm

Hydrophobic

region

Hydrophilic

region

Phospholipid

Proteins

(b) Structure of the plasma membrane

Figure 6.8 A, B


Nucleus
Nucleus

  • Contains the eukaryotic cell’s genetic instructions

    • Chromosomes, structures that carry genetic information

      • Made of chromatin (a complex of protein and DNA)

    • Each typical human cells has 46 chromosomes, 23 pairs

    • Fruit flies – 8 chromosomes

    • Dog – 78

    • Cat - 38

  • Enclosed by the nuclear envelope (double membrane), separating its contents from the cytoplasm


Nucleus genetic information center
Nucleus-genetic information center

Nucleus

Nucleus

1 µm

Nucleolus

Chromatin

Nuclear envelope:

Inner membrane

Outer membrane

Nuclear pore

Pore

complex

Rough ER

Surface of nuclear envelope.

1 µm

Ribosome

0.25 µm

Close-up of nuclear

envelope

Nuclear lamina (TEM).

Pore complexes (TEM).


Nuclear envelope
Nuclear envelope

  • Nucleus is surrounded by a double membrane

  • Transport to-and-from cytoplasm via nuclear pores

Inner membrane – lined with nuclear lamina

Nuclear Pore

Complex

nucleus

cytoplasm

ER

Outer membrane


Nucleolus
Nucleolus

  • Densely stained portion of the nucleus

  • Site of ribosomal RNA (rRNA) synthesis

  • Site of beginning of ribosomal subunit assembly


Ribosomes protein producing machinery
Ribosomes - protein-producing machinery

Ribosomes

Cytosol

Endoplasmic reticulum (ER)

Free ribosomes

Bound ribosomes

Large

subunit

Bound Ribosomes - make proteins for insertion into membranes, packaging within certain organelles, or export from the cell

Free Ribosomes – produce proteins which function in cytosol

Small

subunit

0.5 µm

TEM showing ER and ribosomes

Diagram of a ribosome


Ribosomes site of protein synthesis
Ribosomessite of protein synthesis

Large

subunit

-composed of protein

and ribosomal RNA (rRNA)

Small

subunit

- assembled in the nucleolus

- active in the cytoplasm

- prokaryotes have a smaller version (70S) than

eukaryotes (80S)

- actively growing or secreting eukaryotic cell may have millions of ribosomes


Endomembrane system
Endomembrane system

  • Extensive network of membranes

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

  • Regulates protein synthesis and traffic

  • Performs metabolic functions in the cell

  • Metabolism and movement of lipids

  • The endomembrane system

    • Includes many different structures – nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles


The endoplasmic reticulum (ER) membrane

  • Is continuous with the nuclear envelope

Smooth ER

Nuclear

envelope

Rough ER

ER lumen

Cisternae

Ribosomes

Transitional ER

Transport vesicle

200 µm

Smooth ER

Rough ER


Endoplasmic reticulum er accounts for more than half the total membrane in many eukaryotic cells
Endoplasmic reticulum (ER)Accounts for more than half the total membrane in many eukaryotic cells

1) Smooth ER

- no/few ribosomes

- lipid synthesis (example -testes and ovaries)

- carbohydrate metabolism

- drug detoxification (liver cells)

- calcium ion storage

2) Rough ER

- ribosomes attached

- continuous with nuclear membrane

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

(pancreatic cells)

- protein glycosylation (sugars added)


Golgi apparatus
Golgi apparatus

  • Consists of flattened membranous sacs called cisternae

    • Polarity – opposite sides of the stack differ in thickness and molecular composition, cis face (receiving) and trans face (shipping)

  • Functions

    • Modification of the products of the rough ER

    • Manufacture of certain macromolecules

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

    • Products of ER are modified (glycosylation), stored, sent out

  • Vesicles pinch off and carry cargo to appropriate locations in the cell


Golgi apparatus functions

5

6

3

4

1

2

Golgi Apparatus functions

cis face

(“receiving” side of

Golgi apparatus)

Vesicles coalesce to

form new cis Golgi cisternae

Vesicles move

from ER to Golgi

0.1 0 µm

Vesicles also

transport certain

proteins back to ER

Cisternae

Cisternal

maturation:

Golgi cisternae

move in a cis-

to-trans

direction

Vesicles form and

leave Golgi, carrying

specific proteins to

other locations or to

the plasma mem-

brane for secretion

Vesicles transport specific

proteins backward to newer

Golgi cisternae

trans face

(“shipping” side of

Golgi apparatus)

Figure 6.13


Lysosome acidic digestive compartments
Lysosome – acidic digestive compartments

1 µm

Nucleus

  • Is a membranous sac of hydrolytic enzymes (acidic environments) from ER to Golgi to lysosome

  • Lysosomes carry out intracellular digestion by

    • Phagocytosis

Lysosome

Hydrolytic

enzymes digest

food particles

Food vacuole fuses with lysosome

Lysosome contains

active hydrolytic

enzymes

Digestive

enzymes

Lysosome

Plasma membrane

Digestion

Food vacuole

(a) Phagocytosis: lysosome digesting food


Lysosome-acidic digestive compartment

Lysosome containing

two damaged organelles

1 µ m

Mitochondrion

fragment

Peroxisome

fragment

Lysosome fuses with

vesicle containing

damaged organelle

Hydrolytic enzymes

digest organelle

components

Lysosome

Digestion

Vesicle containing

damaged mitochondrion

(b) Autophagy: lysosome breaking down damaged organelle

  • Autophagy –

    • Process of lysosomes which use their hydrolytic enzymes to recycle the cell’s own organic material

    • Damaged organelle or small amount of cytosol

    • Cell renews itself

    • Tay-Sachs disease

      • Lipid accumulation

Figure 6.14 B


Central vacuole

Cytosol

Tonoplast

Nucleus

Central

vacuole

Cell wall

Chloroplast

5 µm

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

    • Surrounded by tonoplast


The endomembrane system
The endomembrane system

  • Complex and dynamic

  • Relationships among organelles of the endomembrane system

1

Nuclear envelope is

connected to rough ER, which is also continuous

with smooth ER

Nucleus

Rough ER

2

Membranes and proteins

produced by the ER flow in

the form of transport vesicles

to the Golgi

Smooth ER

cis Golgi

Nuclear envelop

3

Golgi pinches off transport

Vesicles and other vesicles that give rise to lysosomes and

Vacuoles

Plasma

membrane

trans Golgi

4

5

6

Lysosome available

for fusion with another

vesicle for digestion

Transport vesicle carries

proteins to plasma membrane for secretion

Plasma membrane expands

by fusion of vesicles; proteins

are secreted from cell


Cell power
Cell Power!

  • Mitochondria and chloroplasts change energy from one form to another

  • Enclosed by membranes

    • not part of the endomembrane system

  • Mitochondria

    • cellular respiration

  • Chloroplasts

    • photosynthesis


Mitochondria chemical energy conversion

Mitochondrion

Intermembrane space

Outer

membrane

Free

ribosomes

in the

mitochondrial

matrix

Inner

membrane

Cristae

Matrix

Mitochondrial

DNA

100 µm

Mitochondria: Chemical Energy Conversion

  • Mitochondria are enclosed by two membranes

    • Smooth outer membrane

    • Inner membrane folded into cristae

  • Matrix – contains enzymes,

  • mitochondrial

  • DNA, and

  • ribosomes

  • divide and

  • partition

  • into daughter

  • cells

Figure 6.17


Chloroplasts capture of light energy
Chloroplasts: Capture of Light Energy

  • Chloroplast - Contains chlorophyll

    • Specialized member of a family of plant organelles called plastids

    • Found in leaves/other green organs of plants and algae

Chloroplast

Stroma

the internal fluid

Ribosomes

Chloroplast

DNA

Inner and outer

membranes

Granum

1 µm

Thylakoid - membranous sacs


Peroxisomes
Peroxisomes

  • Removal of electrons and hydrogen - oxidation

  • Produce hydrogen peroxide as a byproduct, by transferring hydrogen from substrates to oxygen

  • Hydrogen peroxide is broken down to water

  • Break down fatty acids, detoxify alcohol/toxins

    • highly reactive and very destructive to cell

  • Contain catalase

    2 H2O2 2 H2O + O2

catalase


Peroxisomes oxidation

Chloroplast

Peroxisome

Mitochondrion

1 µm

Peroxisomes: Oxidation

Often have a granular or crystal core - thought to be a dense collection of enzymes molecules


Cytoskeleton support motility and regulation

Microtubule

Microfilaments

0.25 µm

Figure 6.20

Cytoskeleton: Support, Motility, and Regulation

  • Network of fibers - organizes structures and activities in the cell

  • Gives mechanical support to the cell


Cytoskeleton support motility and regulation1

Vesicle

ATP

Receptor for

motor protein

Motor protein

(ATP powered)

Microtubule

of cytoskeleton

(a) Motor proteins that attach to receptors on organelles can “walk”

the organelles along microtubules or, in some cases, microfilaments.

Vesicles

Microtubule

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

Cytoskeleton: Support, Motility, and Regulation

  • Involved in cell motility, which utilizes motor proteins

  • Railroad tracks

    for the cell,

    Trafficking of

    vesicles and

    organelles


Cytoskeleton
Cytoskeleton

Three main types of fibers make up the cytoskeleton


Cytoskeleton1
Cytoskeleton

Table 6.1


Microtubules
Microtubules

  • Microtubules

    • Shape the cell

    • Guide movement of organelles

    • Help separate the chromosome copies in dividing cells

  • The centrosome

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


Centrosomes and centrioles

Centrosome

Microtubule

Centrioles

0.25 µm

Longitudinal section

of one centriole

Cross section

of the other centriole

Microtubules

Figure 6.22

Centrosomes and Centrioles


Cilia and flagella

(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).

(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

1 µm

Cilia and Flagella

  • Specialized arrangements of microtubules

  • Are locomotor appendages of some cells

Direction of organism’s movement

Direction of

Active stroke

Direction of

Recovery stroke


Cilia and flagella share a common ultrastructure
Cilia and flagella share a common ultrastructure

Cilia – shorter than flagella

- more abundant/cell than flagella

  • Cilia – shorter than flagella

  • more abundant/

  • cell than flagella

Outer microtubule

doublet

Plasma

membrane

0.1 µm

Dynein arms

Central

microtubule

Outer doublets

cross-linking

proteins inside

Microtubules

Radial

spoke

Plasma

membrane

(b) Cross-section through the

Cilium shows “9+2” arrangement

Of microtubules

Basal body

0.5 µm

0.1 µm

(a) A longitudinal section of cilium shows microtubules running the length of the structure

Triplet

Figure 6.24 A-C

(c) Basel Body – (anchor) nine outer doublets of a cilium or flagellum exend into the basel body where each doublet joins another microtubule to form a ring of nine triplets

Cross section of basal body


Microfilaments actin filaments

Muscle cell

Actin filament

Myosin filament

Myosin arm

Myosin motors in muscle cell contraction.

(a)

Figure 6.27 A

Microfilaments (Actin Filaments)

  • Found in microvilli and muscle cells

  • For cellular motility - Contain the protein myosin in addition to actin

Microvillus

Plasma

membrane

Microfilaments (actin

filaments)

Intermediate

filaments

0.25 µm

Figure 6.26


Microfilaments

Nonmoving

cytoplasm (gel)

Chloroplast

Streaming

cytoplasm

(sol)

Parallel actin

filaments

Cell wall

(b) Cytoplasmic streaming in plant cells

Microfilaments

  • Cytoplasmic streaming

    • Is another form of locomotion created by microfilaments


Special plant structures
Special plant structures

  • Cell Wall

    • Protects the cell

    • Provides rigidity to the cell

    • Made of cellulose (polysaccharide)

    • Different from bacterial cell wall

  • Vacuole

    • Storage area for the cell

    • Water, sugars, ions


Plasma

membrane

Central

vacuole

of cell

Secondary

cell wall

Primary

cell wall

Central

vacuole

of cell

Middle

lamella

1 µm

Central vacuole

Cytosol

Plasma membrane

Plant cell walls

Plasmodesmata

Figure 6.28

Plant cell walls

Cellulose fibers embedded in other polysaccharides and protein

a) Rigid structure- support

b) Prevents excessive H2O

uptake

c) Acts against gravity

d) Protects the cell


The extracellular matrix ecm of animal cells

Polysaccharide

molecule

EXTRACELLULAR FLUID

Collagen

A proteoglycan

complex

Carbo-

hydrates

Core

protein

Fibronectin

Proteoglycan

molecule

Plasma

membrane

Integrins

CYTOPLASM

Micro-

filaments

Integrin

Figure 6.29

The Extracellular Matrix (ECM) of Animal Cells

  • Animal cells - Covered by a matrix, the ECM

    • Is made up of glycoproteins and other macromolecules

      Functions of the ECM – support, adhesion, movement, regulation


Intercellular junctions plants

Cell walls

Interior

of cell

Interior

of cell

0.5 µm

Plasmodesmata

Plasma membranes

Figure 6.30

Intercellular Junctions - Plants

  • Plasmodesmata

    • Are channels that perforate plant cell walls


Intercellular junctions animals

TIGHT JUNCTIONS

At tight junctions, the membranes of

neighboring cells are very tightly pressed

against each other, bound together by

specific proteins (purple). Forming continu-

ous seals around the cells, tight junctions

prevent leakage of extracellular fluid across

A layer of epithelial cells.

Tight junction

Tight junctions prevent

fluid from moving

across a layer of cells

0.5 µm

DESMOSOMES

Desmosomes (also called anchoring

junctions) function like rivets, fastening cells

Together into strong sheets. Intermediate

Filaments made of sturdy keratin proteins

Anchor desmosomes in the cytoplasm.

Tight junctions

Intermediate

filaments

Desmosome

Gap

junctions

1 µm

GAP JUNCTIONS

Gap junctions (also called communicating

junctions) provide cytoplasmic channels from

one 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 may

pass. Gap junctions are necessary for commu-

nication between cells in many types of tissues,

including heart muscle and animal embryos.

Extracellular

matrix

Space

between

cells

Gap junction

Plasma membranes

of adjacent cells

Figure 6.31

0.1 µm

Intercellular junctions (animals)


5 µm

Figure 6.32

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


Prokaryotes vs eukaryotes1
Prokaryotes vs. Eukaryotes

Prokaryotes

Eukaryotes

Size 1-10 mm 10-100 mm

Genetic circular linear

Material chromosome chromosomes

Plasma yes yes

Membrane

Cell Wall yes some-plants, etc.

Ribosomes yes yes

Membrane-bound NO YES

organelles


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