American University of Beirut Biol 201

Marita Yaghi 0

Chapter 3 : Cell Structure I. The cell theory

II. Prokaryotic cells

III. Eukaryotic cells

IV. The endomembrane system

V. Mitochondria and Chloroplasts

VI. The cytoskeleton

VII. Extracellular structures

VIII. Cell junctions: Cell-Cell interaction

I. The cell theory

Cells were discovered in 1665 by Robert Hooke and Anton Van

Leeuwenhoek with the invention of the first microscopes.

Early studies concerning cells were conducted by :

- Mathias Schleiden in 1838 study of plant cells

- Theodor Schwann in 1839 study of animal cells

Schleiden and Schwann proposed the cell theory

Cell Theory: Unifying Foundation of Cell Biology

The cell theory states the following three principles:

1. All living organisms are composed of cells

2. Cells are the smallest living things basic units of life

3. Cells arise only from pre-existing cells inheritance, mitosis…

All cells today represent a continuous line of descent from the first living

cells:

IG: Eukaryotic cells evolved from Prokaryotic cells

Cell Size:

A cell size is limited

Having a lot but small cells is an advantage for organisms

They have a quicker rate of diffusion (small surface area)

Do not synthesize as much macromolecules as big ones less energy

needed

Removal of metabolic waste + exocytose of molecules for energy and

biosynthesis is faster little distance to the membrane that is of small

area

Surface area-to-volume is another important advantage: if a cell’s size

increases, its volume increases much more that it surface.

(radius x 10 = volume x 1000)

BUT - a cell’s membrane is the only exchange platform with the external

milieu, as substances enter and exit via the membrane

- The membrane plays a key role in controlling the cell’s function.

Small cells, having more surface by unit of volume, are more easily

controlled

Still, some cells remain big

An eukaryotic cell varies from 10 to 100 μm while a Prokaryotic cell varies

from 1 to 10 μm

Microscopes:

Are essential to view cells because of their small size and the eye’s

resolution limitation

They work by magnification using lenses

Light microscopes

- Operate at visible light

- Have two magnifying lenses

- Aim to achieve very high magnification and clarity (the higher the

magnification, the higher the resolution)

- Resolve structures up to 200 nm apart

- We have

i. Bright-field microscope use stains on cells, fixing them, which

can distort or alter their components 1

ii. Dark-field microscope light is only directed at the specimen,

giving a light specimen against a dark background 2

iii. Phase-contrast microscope wavelengths are sent out of phase,

improving the contrast and brightness when they recombine 3

iv. Differential-interference microscope polarized light is split in

two beams with two paths, giving a great contrast around edges 4

v. Fluorescent microscope fluorescent stains absorb a wavelength

and emit another that is absorbed by the filters 5

vi. Confocal microscope laser light is focused on a point and

scanned across fluorescent dies in two directions giving images

of one plane of the material; by superposing different images of

different planes we get a 3-D image 6;

Electron microscopes

- Employs electron beams that have shorter wavelengths (the shorter the

wavelength, the higher the resolution)

- We have two types:

i. Transmission Electron Microscope TER Resolve structure up to

0.2 nm apart by sending light into the material exposing films.

Dark areas absorb the e- and false coloring improves contrast 7

ii. Scanning Electron Microscope SER Electrons can be sent only

on the material surface and reflected, and their image is recorded

by topography, and also using false coloring. 8

Using stains or dyes help us increase the contrast between different cellular

components. In fact, some stains only bind to specific molecules.

IG: antibodies, that bide only certain proteins, can be used. This is

Immunohistochemistry, where purified antibodies are injected in a living

organism with fluorescent or radioactive stains, and they bind to cellular

structures with the target molecules, making them observable by a microscope.

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Common Structures in all Cells:

General plans of cells vary between organisms but a lot of fundamental

structures are common among all cells

1. Nucleus or Nucleoid

Place where genetic material is recorded hereditary molecule DNA

that codes for the cell’s protein synthesis

Is called Nucleoid in Prokaryotic cells near the center of the cell

where a single circular molecule of DNA resides. It is not separated from

the rest of the cell by membranes

Is called Nucleus in Eukaryotic cells it is surrounded by a double

membrane called nuclear envelope and contains the complex DNA

2. Cytoplasm

Semifluid matrix (aqueous medium that is like jello)

It fills the inside of the cell

Contains all the carbs, sugars and lipids that the cell uses for its

everyday activity

Contains specialized macromolecules organelles

Cytosol part of the cytoplasm that contains ions and organic

molecules ≠ from the free organelles suspended in the fluid

3. Ribosomes universal orgnalles

Synthesize proteins and made of two subunits.

4. Plasma membrane

Encloses the cell separating it from its surrounding

Phospholipid bilayer

5-10 nm of thickness

Responsible of the cell-environment interaction

i. Transport proteins help molecules and ions move across the

plasma membrane IG: aquaporin

ii. Receptor proteins/markers induce changes in the cell after

interacting with enzymes, hormones or molecules present on

another cell’s surface. These molecules can act as markers that

identify the cell’s type. (these interactions are important as they

lead to forming tissues)

II. Prokaryotic Cells

This terminology refers the absence of a membrane-bounded nucleus as

Prokaryotic cells do not have an internal membrane system or membrane-

bounded organelles.

Prokaryote’s simple organization:

Simplest organisms Cytoplasm surrounded by a

plasma membrane and into a rigid cell wall made of cellulose or chitin (those have usually a pathogen effect)

They have no distinct compartment no endomembrane system

Free-DNA in the nucleoid Contain only ribosomes but lack

membrane-bounded organelles They lack an elaborate

cytoskeleton BUT contain molecules related to actin (MreB) and tubulin(FtsZ) their strength and shape is determined by the cell wall BUT is influenced by the cytoskeleton-like structures

Their plasma membrane carries functions usually carried by organelles The membrane folds bacterial pigments connected with photosynthesis Since the DNA, enzymes and other constituents lie free in the

cytoplasm, they all have access to all parts of the cell Prokaryotic cells operate as a single unit

We have two main domains in prokaryotes: 1. Bacteria 2. Archaea intermediate between Bacteria and Eukarya

Prokaryotic cell walls:

In general, these walls protect the cell and give it its function, as Prokaryotic

cells lack cytoskeletons. They also prevent excessive water intake or loss.

1. Bacteria cell walls

Most bacteria have a strong cell wall

It is usually composed of peptidoglycan polymer of sugars and amino

acids that is specific to Bacteria (does not exist in other domains like

animals or Protista their cell walls have a different composition)

Bacteria can be Gram Positive or Gram negative:

- Gram positive thick peptidoglycan membrane, superficial to the cell

membrane, that retains the Gram violet stain

- Gram negative thin peptidoglycan membrane, located between the

two cell membranes, that does not retain the Gram violet stain BUT

these bacteria are more resistant to antibiotics because they have an

additional lipid membranes

Δ Antibiotics usually works only on one type of bacteria. But bacteria

have a very high mutation level and they can transmit genetic info

between them. So if a bacterium becomes resistant due to a certain

mutation, it might transmit this mutation to the non-resistant bacteria,

making the antibiotic useless.

Bacteria cell walls are made of both saturated and unsaturated fats

their structure can vary as they have the ability to convert the fats to the

way they want. NOTE: unsaturated fats are usually messy while saturated

ones are linear and organized

Some bacteria have a protective capsule of polysaccharide around

them, making them able to adhere to any surface and cause diseases

practically anywhere that supports their growth. It prevents their

destruction.

2. Arachaea cell walls

Research is still happening.

Cell walls lack peptidoglycan BUT contain many chemical pounds like

polysaccharides and proteins.

The lipid-membrane of Archaea is different from the Bacteria’s

contains saturated hydrocarbons attached to hydroxyl making them more

thermal-stable BUT less adaptive to changing temperatures in the

environment.

Their DNA replication is closer to the Eukaryote’s BUT there cellular

architecture is similar to Prokaryotes.

Flagella and Prokaryotes:

Some Prokaryotes have rotating flagella that helps them move.

It is a long threadlike structure, made of protein fibers, that extends from

the cell and is used for locomotion.

Prokaryotes may have one, multiple or no flagella.

The rotation uses energy stored in a gradient that transfers protons across

the cell membranes same principle in Eukaryotic mitochondria and

chloroplasts by an enzyme that synthesis ATP.

III. Eukaryotic cells

Eukaryotic cells are more complex than Prokaryotic ones. In fact,

Eukaryotes are very compartmentalized as they have

- An endomembrane system

- A lot of organelles that are membrane-bounded system that form

compartments in which multiple biochemical processes happen, in a

simultaneous and independent way.

Plants have a large sac called central vacuole it is used to store proteins,

pigments and waste material.

Both plants and animals have vesicles small sacs that transport and

store a lot of material.

The DNA is wounded tightly inside the nucleus it is packaged into

compact subunits called Chromosomes.

All Eukaryotes have a cytoskeleton it is an internal protein scaffold or a

skeleton made of filaments that is in the cytoplasm and plays a role in

intracellular transport and cellular division.

Animals and some Protista lack cell walls BUT Fungi, Plantae and other

Protista have strong cell walls made of cellulose or chitin fibers, fixed in a

matrix of polysaccharides and proteins.

The Nucleus: Center of Information

The largest organelle in a Eukaryotic cell is the nucleus.

Nuclei have a roughly spherical shape and, in animals, it is in the center

of the cell.

The nucleus is the storehouse of the genetic info that enables the

synthesis of proteins. Most Eukaryotic cells have one nucleus but some,

like for fungi, may have more.

Red blood cells lose their nuclei when they mature.

Many nuclei show a dark-staining zone it is a zone where intensive

ribosomal-RNA synthesis is taking place.

The nuclear envelope:

The nuclear envelope is made of two phospholipid membrane bilayers.

The outer membrane of the nuclear envelope is continuous with the

cytoplasm’s interior membrane endoplasmic reticulum.

Scattered over the nuclear membranes, we found nuclear pores these

holes form where two membranes layers of the envelope come together,

typically at 50-80 nm apart.

The pores allow ions and small molecules to diffuse freely between the

nucleoplasm and cytoplasm

BUT they control the transport of proteins and RNA it consists mainly of

the import of proteins that function inside the nucleus and the export of

RNA and RNA-protein complexes formed inside the nucleus.

Nuclear lamins are intermediate filaments that cover the inner surface of

the nuclear envelope. These filaments can only be found in the nucleus;

they play a role in giving the nucleus its shape and play a role in the

destruction of the cell membrane before a division and its reconstitution

after. (they disintegrate and reform)

Chromatin: DNA packaging

The DNA is divided into multiple linear chromosomes that are organized

with proteins (especially histones) to form a complex structure called

chromatin. (chromatin in their combination)

Chromatin has important functions:

1. Package DNA into small units to fit the nucleus

2. Strengthen the DNA to allow mitosis

3. Prevent DNA damage

4. Control gene expression and DNA replication

The structure of the chromatin affects the DNA function changes in the

gene expression that are not caused by changes in the DNA sequence

involve changes in the chromatin structure epigenetic changes.

When cells divide, the chromatins must be further compacted into a highly

condensed state that forms the chromosomes X-shape, visible in the light

microscope.

DNA + histones = nucleosomes (we have 4 types of histones, every DNA

molecule is added to 8 histones– 2 of each type)

Ribosomal subunits: manufactured by the Nucleus

Before going through protein synthesis, cells should synthesize a large

number of ribosomes to carry out this protein synthesis.

The genes that encode the ribosomal RNA group together on the

chromosome, to facilitate the construction of these ribosomes.

The cells then transcribe very quickly a large number of the needed

molecules needed to construct ribosomes.

The cluster (group) of RNA genes, the RNA produced and the ribosomal

proteins all come together in the nucleus during the ribosome production.

Theses ribosome-assembling areas are visible in the nucleus as they form

dark-staining regions, called nucleoli, which can be seen with a light

microscope.

Ribosomes: proteins’ synthesis machinery

Although the DNA that encodes the proteins is in the nucleus, the protein

synthesis happens in the cytoplasm.

Protein synthesis in associated with large RNA-protein complexes found in

the cytoplasm.

Ribosomes are very complex molecules found in the cells. Each ribosome is

made of one ribosomal RNA (rRNA) and proteins. These two subunits join

to form a functional ribosome only to actively synthesis proteins.

Ribosomes associate with two other forms of RNA for the proteins

synthesis: messenger RNA (mRNA), which carries encoding info from DNA

into the cytoplasm that is used by ribosomes, and transfer RNA (tRNA),

which carries the amino acids.

Ribosomes are considered universal organelles found in all three domains

of life.

Ribosomes are found either free in the cytoplasm or bound to internal

membranes; each synthesizes a specific type of proteins:

Free ribosomes Membrane-bound ribosomes - Proteins in the cytoplasm - Nuclear proteins - Mitochondrial proteins - Some organelles proteins

(not related to the endomembrane system)

- Membrane proteins - Proteins found in the

endomembrane system - Proteins that export from the

cell

The individual subunits form in the nucleus and move through the pores

to the cytoplasm where they assemble to form the ribosomes that will translate mRNA and synthesis proteins with the tRNA.

Ribosomes sites or protein synthesis

We can see that the plasma membrane contains the cell, which contains the cytoskeleton and a lot of organelles and other interior structures suspended in the cytoplasm, semi-fluid matrix. Some animal cells show the finger-like projections called microvilli, while other eukaryotic cells have flagella, that aid movement, or cilia, that have other functions.

Most plant cells have central vacuoles, which occupy a very large portion of its internal volumes. Vacuoles segregate toxic items, store material and deal with tonicity Also, most plant cells have chloroplasts organelles in which photosynthesis takes place. The cells of plants, fungi and some protists have cell walls, although the composition is not the same in the different domains. Plant cells have cytoplasmic connections within one other called plasmodemata. Flagella occurs in the sperm of some cell plants but is usually absent from them as well as from fungi cells. Centrioles are also usually absent.

IV. The endomembrane system:

The interior of a Eukaryotic cell is packed with very thin membranes that

- Fill the cell

- Divide the cell into compartments

- Channel the passage of molecules through the interior of the cell

- Provide surface for the synthesis of some lipids and proteins

The ensemble of these membranes forms Endomembrane System that marks the

distinction between Eukaryotic and Prokaryotic cells.

The largest of the internal membranes is the Endoplasmic Reticulum that means

“a little net within the cytoplasm”. It is made of a phospholipid bilayer fixed to

proteins.

The ER contains the two largest compartments present in the Eukaryotic cells:

1- The cisternal space or lumen inner region

2- The cytosol outer region fluid part of the

cytoplasm that contains dissolved organic molecules (proteins and ions)

The Rough ER:

The RER is primarily composed of flattened sacks whose surfaces are

bumpy because of ribosomes.

The RER is not easily seen with a light microscope, it requires an electron

microscope.

The RER is a site for proteins synthesis, which happens on its surface. The

synthesized proteins are:

- Exported from the cell

- Sent to lysosomes or vacuoles

- Integrated in the plasma membrane.

Entering the cisternal space is the first step in the pathway of sorting the

proteins out. This pathway also involves vesicles and the Golgi apparel. The

sequence of AAs in every protein determines if it stays in the ER or remains a

cytoplasmic ribosome.

In the ER, short-chain carbs are added to some of the new proteins forming

glycoproteins. These proteins are designated for secretion; they are isolated

into vesicles and moved to the Golgi for modification and transport to other

cells.

The Smooth ER:

It is connected to the RER.

It has less bounded-ribosomes and its structure can vary from:

- Network of tubules

- Flattened sacks

- Tubular arrays

The SER membranes contain a lot of enzymes that are involved in the

synthesis of carbs and lipids. Lipid membrane are in fact made in the SER and

sent to other parts of the cell.

Steroids are also synthesized in the SER.

Membrane proteins and plasma membrane are inserted by the ribosomes

on the RER.

The SER also stores intracellular Ca2+, that allows the cytoplasmic level to

be low and is used as a signaling molecule.

The SER can also detoxify foreign substances.

Some organs have and extensive smooth ER: ovaries, testes, liver, etc…

The ratio of SER and RER varies in each type of cell. In fact, cells that have

extensive lipid membranes have a bigger SER while cells that synthesize secreted

proteins (IG: antibodies) have a bigger RER.

The Golgi body/apparatus:

Flattened sacks of membrane form the Golgi apparatus complex.

The individual sacks are called cisternae and vary in number in the Golgi

body of different organisms: 1 or a few in Protista, around 20 for Animalia

and hundreds for Plantae.

Individual Golgi can group and form a ribbon; especially in glandular cells

that manufacture and secret substances.

The Golgi body collects, packages and distributes the molecules

synthesized in a specific location in the cell but are used in another

location within or outside the cell.

The Golgi body has a front and a back made of different membrane

compositions.

The front end – called cis face – is usually near the ER. It receives the

material sent to the Golgi via vesicles sent from the ER.

The back end – called the Trans face – is the exit. It discharges the material

that entered from the cis face after it had been modified or sorted out.

The transition of the material is made

primarily by cisternal maturation, although

transportation by vesicles or direct tubular

connections might also occur.

Proteins and lipids made on the rough and

smooth RER are transported to Golgi and

modified. Usually, short sugars chains are

added or modified, forming glycoproteins and

glycolipids. The modified proteins are then

packaged into small membrane-bounded

vesicles that exit from the Trans face of the

Golgi and diffuse the newly-synthesized

molecules to their appropriate destination.

The Golgi apparatus also synthesizes cell-

wall components non-cellulose

polysaccharides are made in the Golgi and then

sent to the plasma membrane where the

cellulose is added, but assembled in the

exterior of the cell. Other plants

polysaccharides are made in the Golgi.

In brief Proteins synthesized by ribosomes on the RER are transported into the internal compartments of the ER. These proteins may be used further inside the cell or secreted outside of it. Transported by vesicles that got out of the RER, these proteins travel to the cis face of the Golgi apparatus. They are modified and packed in other vesicles that get out of the trans face of the Golgi. These vesicles transport the proteins to other locations in the cell or fuse with the plasma membrane releasing their content out of the cell.

Lysosomes:

Lysosomes are membrane-bounded

digestive vesicles that are part of the

endomembrane system.

Lysosomes arise from the Golgi apparatus.

Lysosomes contain a lot of degrading

enzymes, which catalyze the breakdown of

proteins, carbs and lipids.

Lysosomal enzymes break down old

organelles and recycle their components

making rooms for new ones. IG: mitochondria

are recycled every 10 days

Lysosomes eliminate all engulfed cells via

phagocytosis by their main cell. IG: pathogens

phagocytized by white blood cells fuse within

the lysosomes that release their enzymes to

degrade the pathogen.

The digestive enzymes of lysosomes are

optimally active at acid pH.

Fusing with a “food vesicle” (made by

phagocytosis) or with old organelles activate

the lysosomes, as it leads to lowering their

internal pH and thus, making the enzymes

work, degrading the food vesicle or old

organelle.

Lysosome aiding in the breakdown of an old organelle

Lysosome aiding in the digestion of phagocytized particles

In Brief: Lisosomes are formed from vesicles that bud off the Golgi. They contain digestive enzymes that digest phogocyted cells or break down old organelles.

Microbodies: a diverse category of organelles

Microbodies are NOT part of the

endomembrane system.

Microbodies are membrane bounded

vesicles that contain a lot of enzymes.

They are found in cells of plants,

animals, fungi and protists.

They are formed by the addition then

division of proteins and lipids.

The distribution of the enzymes

inside these Microbodies is very

important for the cell’s metabolic

organization.

An important type of microbody is peroxisome. These spherical organelles

may contain a crystal full of proteins. They also contain digestive and

detoxifying enzymes that produce hydrogen peroxide as a by-product of

their oxidizing activity. Hydrogen peroxide is dangerous as it reacts

violently; but peroxisomes also contain catalase, an enzyme that breaks

down hydrogen peroxide to oxygen and water.

They are:

- Formed by the fusion of ER-derived vesicles, added to peroxisomal

proteins mature peroxisomes

- Also formed by the division of large peroxisomes

- Contain enzymes that oxidize fatty acids. (If these enzymes were free in

the cytoplasm, they would short circuit the metabolism of the cytoplasm

by adding hydrogen to oxygen.)

Vacuoles:

Vacuoles are membrane bounded structures that exist mainly in plants,

but also in fungi and Protista.

The vacuole is surrounded by a membrane called tonoplast it contains

channels for water to help maintain the cell’s tonicity or osmotic balance.

Different vacuole types with different structures are found in different

cells, depending on their functions:

- The central vacuole found in plants

i. Maintains the tonicity of the cell due to the water channels of the

tonoplast

ii. Involved in cell growth as it occupies most of the cell’s volume

iii. Can store molecules, ions and waste products

- Contractile vacuole found in protists; it pumps water and maintains its

balance in the cell

- Other vacuoles for storage and for isolation of toxic material from the

rest of the cytoplasm

V. Mitochondria and Chloroplasts: Cellular Generators

Mitochondria and Chloroplasts have a lot of structural and functional similarities:

- Structural : they are both surrounded by a double membrane

both contain their own DNA

both have their own protein synthesis machinery

- Functional: they are both involved in energy metabolism

Mitochondria:

Mitochondria are tube-like organelles found in all Eukaryotic cells.

They are bounded by two

membranes:

1. An outer membrane smooth

2. An inner membrane folded and

made of contiguous layers called

“cristae” that increases the surface

of the inner membrane and

partition the mitochondrion into

two compartments:

i. A matrix inside the inner

membrane

ii. An intermediate membrane

between the inner and the outer

membrane

The surface of the inner membrane is embedded with proteins that

execute oxidative metabolism, a process that requires oxygen and gives the

necessary energy for ATP production.

Mitochondria have their own DNA, whose genes encode proteins

necessary for the mitochondria’s role in oxidative metabolism. the

mitochondrion acts like a cell inside the cell; but is not fully independent of

the main cell, as most of the genes encoding the enzymes used during the

oxidative metabolism are in the nucleus.

During mitosis, when the cell divides, the mitochondria within it divide

also, doubling in number then partitioned between the two cells. The

required components for the mitochondria divisions are encoded by genes

in the nucleus and translated into proteins by the ribosomes in the

cytoplasm. Therefore, mitochondrial replication is impossible without the

nucleus.

Mitochondria cannot be grown in a cell-free structrure

Mitochondria can also replicate by fusion

Chloroplasts:

Chloroplasts are contained in all cells that carry out photosynthesis,

mainly plants.

Chloroplasts can make their own food as they contain a green pigment,

called chlorophyll.

The inner membrane surrounds a membrane system of stacks called thylakoids, that contain chlorophyll vesicles. Photosynthesis occurs in the thylakoids. These are stacked to form columnns called grana.

Chloroplasts is surrounded by two membranes, just like the mitochondria

1. Outer membrane

2. Inner membrane

BUT chloroplasts are more complex: they contain hundreds of

membranous sacks called thylakoids that form grana; closed

compartments in the inner membrane.

The thylakoids are surrounded by a fluid matrix called stroma, which

contains enzymes used to synthesize glucose during photosynthesis.

Chloroplasts also contain DNA but the genes that specify their components

are in the nucleus BUT some of the proteins used for photosynthesis are

entirely made in the chloroplast.

Leucoplasts:

Other DNA containing organelles in plants

Lack pigments and internal structure

They may serve as starch storage sites and will then be called amyloplast

Mitochondria, chloroplasts, amyloplast and Leucoplasts are called plastids; they

are produced by the division of pre-existing plastids.

Endosymbiosis:

Theory of endosymbiosis:

“Some of today’s eukaryotic cells evolved

from the symbiosis of two cells: a

prokaryotic cell engulfed by a second cell,

which would be the ancestor of

eukaryotes”.

Mitochondria would have originated from

aerobic Bacteria (uses dioxygen for

oxidative reaction) while Chloroplasts

would have originated from photosynthetic

Bacteria.

There is much evidence:

- M and C have the size of a Prokaryotic cell

- M and C divide by fission like Bacteria

- They have two membranes cristae

structure

- Their DNA and ribosomes are similar in

size and structure to the ones found in

prokaryotes

- They have genome similarities with α-

protobacteria and cyanobacteria.

VI. The cytoskeleton

The cytoplasm of Eukaryotic cells is crisscrossed by a network of protein

fibers. These fibers support the structure of the cell and keep organelles in

specific locations. Also, they can help move material within the cell. This network

is called the cytoskeleton.

The cytoskeleton is a dynamic system that keeps assembling and

disassembling. Individual fibers consist of polymers of identical proteins that

attract one another and spontaneously form long chains. These fibers also

disassemble in the same way: as a protein after another break away from the end

of the chain.

Inside the cytoskeleton, actin filaments and microtubules organize their

activity to affect the cellular processes.

IG: newly replicated Xmes move to the opposite sides of a dividing cell using

shortening microtubules

IG: a belt of actin pinches the cell by contracting, dividing it into two cells

IG: muscle cells use actin filaments to contract the filaments slide along the

filaments of the motor protein, myosin.

The cytoskeleton also acts as a scaffold that holds certain enzymes and

macromolecules in specific place in the cytoplasm

IG: enzymes responsible for the cell metabolism bind to actin proteins

IG: ribosomes bind to actin proteins

By moving and anchoring particular enzymes near one other, the

cytoskeleton helps organize the cellular activities, just like the endoplasmic

reticulum.

The Three Types of Fibers:

In the cytoskeleton of Eukaryotic cells, we can find three types of fibers. Each is

formed by a different subunit or protein

Actin filaments Microtubules Intermediate filaments

Diameter 7 nm 25 nm 8-10 nm Subunits - Globular protein

-Actin proteins - Globular proteins - Dimers of α and β-tubulin subunits

- fibrous proteins - group of cytoskeletal fibers

Shape - 2 protein chains - loosely twined

- 13 protein photofilaments - tube shape - form from the nucleus center towards the periphery

- several tetramers - twined together and overlapping - very tough

Polarity

+ and – ends that show the direction of growth of the filaments

+ ends meaning “away from the nucleus” and – ends “towards the nucleus”

No polarity

Role Cellular movements like contraction, crawling, tightening during division and formation of cellular extensions

Movement of material within the cell Organize the cell’s structure

Provide structural stability

Stability Polymerization is regulated by “switch proteins” in the cell at appropriate times

Constant state of flux: constantly polymerizing and depolymerizing

Stable and do not usually break down

Centrosomes:

Centrioles are barrel shaped organelles found in animals and most

protists. Plants and fungi lack centrioles.

Centrioles occur in pair, forming a right angle. The region surrounding

them is called centrosome.

Surrounding the centrioles in the centrosome, we can find the

pericentriolar material, which is composed of ring shaped structures called

tubulin. They help organize the assembly of microtubules in animal cells.

These structures are called microtubule-organizing centers.

The centrosome also reorganizes microtubules that occur during cell

divisions.

Although plants and fungi lack centrioles, they have microtubule-

organizing centers.

Centrioles:

Molecular molecules:

All Eukaryotic cells have to move material inside their cytoplasm. One way

is by using the channels of the ER. Another way is by using vesicles loaded

with material that will move along the cytoskeleton like on a railroad. IG: in a nerve cell with a long axon extended away from the cell body, vesicle can

move material along the microtubules inside the axon away from the cell body

Four components are required to move material along microtubules:

1. Microtubules

2. Vesicles or organelles that should be transported they will ride on 1

3. Motor protein provide the energy-driven motion

4. Connector molecule connects the vesicle to the motor protein

The direction of the movement of the vesicles depends on two factors:

i. The type of motor protein used

ii. The organization of the microtubules, with their plus end towards the

periphery of the cell, from the nucleus

IG: Kinetin complex Kinetin connector protein binds vesicles on the Kinesin

motor protein. This protein uses ATP to move vesicles towards the + end of the

microtubules, from the center of the cell towards its periphery

IG: Dynactin complex Dynactin connector binds the vesicles on the Dynein

motor protein which moves the vesicles towards the – end of the microtubules,

from the periphery towards the nucleus.

The destination of a transport vesicle depends on the nature of the linking protein embedded with the vesicle’s membrane.

VII. Extracellular Structures and Cell Movement

In general, all cell movement is related to the movement of actin filaments and

microtubules. Intermediate filaments act as intracellular tendons, as they

prevent the cell from stretching too much. Actin filaments have a major role in

determining the shape of the cell, as they form and dissolve quickly, enabling

quick changes in the cell’s shape.

Crawling:

Some cells exhibit the ability to crawl. At their edges, actin filaments

polymerize quickly to form an extension that will force the edge of the cell

forward. Then, microtubules stabilize this extension, and the motor

protein myosin slides along the stabilized actin filaments and contracts,

pushing the rest of the cell forward. IG: white blood cells, formed in the bone marrow, are released into the circulatory

system they crawl out of small veins into tissues to destroy pathogens

Crawling occurs when this process is continuously repeated.

Receptors on the cell membrane can detect molecules outside the cell and

extend their filaments into specific directions in order to reach that

molecule.

Crawling is essential for diverse processes:

- Inflammation

- Clotting

- Healing of wounds

- Spread of cancer

Flagella:

In Prokaryotic cells, flagella are protein fibers that extend out of the cell.

They rotate to move the cells.

In Eukaryotic cells, flagella are made of 9 microtubule pairs that surround

2 central microtubules: 9+2 structure. The pairs of microtubules slide

along each other using the motor protein Dynein, so the flagellum

undulates to move the cells or moves and up down.

In Eukaryotic cells, the flagellum in an extension of the cell’s interior: it

contains cytoplasm and is attached to the plasma membrane. The

microtubules derive from a basal body situated below the point they

extend from.

Cilia:

Because of evolution, Eukaryotic cells todays possess no flagella and are

non-motile.

Cilia are structures similar in organization (9+2 and internal) to flagella

that can be found within cells. They’re short cellular projections, usually

numerous. They are arranged in rows on the surface of eukaryotes.

They have several functions:

i. Propel the cells forward through water original function

ii. Move water over the tissue surface

iii. Sensory cilia in ears are bended by sound waves sensory process

The 9+2 structure in flagella and cilia is a fundamental component in Eukaryotes.

Algae with numerous flagella Paramecia with many cilia

Cell walls:

The cells of plants, fungi, and many types of protists have cell walls, which

provide these cells protection and support. They are different from the cell

walls in Prokaryotes.

They are made of:

i. Cellulose fibers in plants and protists

ii. Chitin in fungi

In plants:

a. Primary walls are laid down while the plant is growing

b. Middle lamella glues the different cell walls

c. Secondary walls inside primary walls inside some cells

Extracellular matrix:

Animal cells lack cell walls. Instead they have an extracellular matrix that

surrounds their cells, made by a mixture of fibrous proteins and

glycoproteins embedded within each other:

i. Collagen

ii. Elastin

iii. Proteoglycans

The ECM of the cells is attached to the cytoplasm by a 3rd kind of

glycoproteins Fibronectin that binds to proteins called Integrin. Integrin is

a part of the plasma membrane that extends into the cytoplasm to attach to

microfilaments and intermediate filaments.

Integrin also alters gene expression and cell migration.

The ECM helps coordinate the behavior of all cells in a tissue.

VIII. Cell-to-cell interactions

A basic feature of multicellular animals in the formation of tissues where cells are organized in specific ways IG: skin, blood, muscle…

Cells must be able to communicate and identified. This is possible by the presence of markers on their surface. Membrane proteins and proteins secreted by the cells are responsible for these functions – cell communication, markers of cell identity and cell connection.

Cells acquire their identity by controlling gene expressions, and turning on specific genes that encodes their specific functions.

Type of connection

Structure Function Example

Surface markers Variable, integral proteins or glycolipids in plasma membrane

Identify the cell MHC Complex Blood groups Antibodies

Septate junctions Tight junctions

Tightly bound, leak-proof, fibrous claudin protein seal that surround the cell

Holds cell together in a way that material pass through but not between cells

Junctions between epithelial cells in the gut

Adhesive junction or Desmosome

Variant cadherin, desmocollins bind to intermediate filaments of the cytoskeleton

Creates strong & flexible connections in cells ; Found in vertebrates

Epithelium

Adhesive junction or Adherens junction

Classical cadherin binds to microfilaments in the cytoskeleton

Connects cells together; oldest form found in all cells

Tissues with high mechanical stress like the skin

Adhesive junction Hemidesmosome & focal adhesion

Integrins bind the cell to the extracellular matrix

Provide attachment to a substract

Involved in cell movement and development

Communicating junction: Gap junction

6 transmembrane connexon/pannexin proteins create intercellular pores

Allow passage of small molecules from cell to cell in a tissue

Excitable tissue like heart muscle

Communicating junction: Plasmodesmata

Cytoplasmic connections between gaps in adjoining plant cell walls

Communicating junction between plant cells

Plants tissues

Surface proteins:

One important set of genes codes for proteins that will mark the surface of

the cells as being from a particular type.

Cells will identify each other by cell-surface markers such as surface

proteins and act accordingly.

Cells from the same tissue recognize each other and coordinate their

functions by creating connections.

Glycolipids lipids with carbohydrates heads, most common form of

tissue-specific cell-markers IG: glycolipids on red blood cells are responsible for A, B and O blood types

Glycoproteins MHC proteins part of the immune system, they

recognize self and non-self cells (Major HistoCompatibility)

Cell connections mediate cell-to-cell junctions:

The evolution of organisms into multicellularity required the acquisition of

molecules that will connect cells. The type of connections between cells have

conserved despite evolutions.

The nature of the cellular connections in a tissue determines what the tissue is

like. Cell junctions can be characterized by their visible structure or the protein

involved.

1. Adhesive junctions:

These junctions are the oldest and found in all animal species. They

attach the cytoskeleton of a cell to its Extra Cellular Matrix or to another’s cell

cytoskeleton. They’re found in tissues subject to mechanical stress.

i. Adherent junctions

Based on the protein Cadherin (classic types I and II) Ca2+-

dependent adhesion molecule with a phylogenetic distribution

Two extra-cellular domain of 2 Cadherins in 2 cells to join them

Cadherin interacts with actin filaments through other proteins to

form flexible connections between cells

ii. Desmosomes

Cadherin-based junctions only in vertebrates

Desmocollin and desmoglein cadherin to the intermediate filaments

They link cells together

They support tissues against mechanical stress

iii. Hemidesmosomes/ Focal Adhesion

They connect the cell to the ECM or to the basal Lamina

Uses Integrins to bind to protein components in the ECM 20

different types

Focal adhesion connects the cytoskeletons of two cells by linking

actin filaments

Hemidesmosomes connect the cytoskeletons of two cells by linking

their intermediate filaments

Cadherin-Mediated Junction

2. Septate or tight junctions

i. Septate junctions

Found in vertebrates and invertebrates

From a barrier that can seal off a sheet of cells

ii. Tight junctions

Unique to vertebrates

Contain Claudine Proteins permit or block substances from passing

between cells

Act like walls within a tissue between cells, keeping molecules on a

side or another

They partition the plasma membranes into separate compartments

regulating the passage of proteins from one part of the cell to another,

preventing from drifting inside the membrane

iii. Create sheets of cell

Sheets are only one cell thick one face facing the inside of the cell

and the other facing the extracellular space

Each cell is encircled by tight junctions, with no possible leakage

the substances have to pass from inside the cells as they cannot pass

between the cells.

3. Communicating junctions

The evolution of organisms into multicellular required a new form of

cellular connection: communication junctions. These junctions permit the

passage of small molecule and ions to pass from one cell to another by diffusion

through small openings.

i. Gap junctions in animals:

Found in vertebrates by Pannexin proteins and invertebrates by

Connexon proteins

Formed of 6 trans-membrane proteins aligned in circle to form a

channel through the plasma membranbe they protrude several nm

from the cell surface

When two sets of these Connexon/Pannexin are aligned in two cells,

an open channel is created

Only small molecules can pass

Dynamic structures that can open or close gated channels;

regulated of Ca2+ and H+ ions

Gating is important if the cell is damaged and its membrane

became leaky, the Ca2+ flows in and closes the gap junctions isolating

the cell and thus the damage

ii. Plasmodemata

In plants only

Cytoplasmic connections between touching

plasma membranes at gaps in the cell wall

Concern the majority of living cells in a

plant

They are lined with the plasma membrane

and contain a tubule that connects the ER of

two cells.