The CELL
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Some background information
The Cell Theory
The Cell theory has three basic points
1. All living things (organisms) are
2. Cells are the basic units of
living
things.
3. All cells come from PRE-EXISTING
composed of cells.
Structure and Function in all
CELLS.
Cellular History
The first Microscope invented by Anton van Leeuwenhoek
First major observation of CELLS made by Robert Hooke.
The botanist Mirbel (1809) made the determination “All plant tissues are
composed of Cells.”
Robert Brown (1833) determined “Many Cells seem to have a dark structure
at or near the center of the cell.”
Today we know this structure as
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The Nucleus
Theodor Schwann (1938) made the determination “All animals are made of
Cells.”
Rudolf Virchow (1855) determined “All cells arise from the division of PreExisting Cells.
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It was from these observations and determinations that the current CELL
THEORY arose.
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BASIC CELL TYPES
PROKARYOTES
Greek : “before nucleus”.
No true nucleus
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E.g.. Bacterial
The genetic material is concentrated in a region of the cell called
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The Nucleoid
There is no membrane separating the genetic material from the cytosol of the
c e ll.
EUKARYOTES
GREEK: “ True (real) nucleus”
Having a true nucleus
The nucleus is bounded by membrane called
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The nuclear Envelope
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Between the nucleus and the cell membrane is the cytoplasm (cytosol).
Eukaryotes are larger than Prokaryotes.
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CELL STRUCTURE
Despite the differences in origin, shape and size, All living cells have 3
structures in common.
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They are…
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The cell Membrane (plasma membrane)
The Genetic Material
The Cytoplasm (cytosol).
THE CELL MEMBRANE
“T
The Plasma (cell) membrane is the edge of life, the boundary that separates the
living cell from its non-living surroundings. … It controls the traffic into and
out of the cell it surrounds.”
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The function of the Cell (plasma) membrane is three fold.
1. It provides an external limit to
the cell.
2. Provides shape to the cell.
3. Provides a barrier (selective or
SEMI-PERMABLE)
between the
interior of the cell and the exterior
environment.
The cell membrane is composed of a DOUBLE LIPID LAYER. (a lipid BiLayer).
The HYDROXYL end of the lipid chain (the part of the lipid that is hydrophilic)
serves as the outer and inner surfaces.
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The hydrocarbon tail (the HYDROPHOBIC portion) extends into the space
between the two hydrophilic layers.
MEMBRANE MAKE-UP
Lipid and Proteins are the MAIN ingredients of the Cell membrane.
Carbohydrates also are important compounds in the membrane.
The most abundant lipids found in the cell membrane are the
PHOSPHOLIPIDS.
PHOSPHOLIPIDS
A phospholipid is a lipid with a PHOSPHATE group added to the carbon
chain.
It is an AMPHIPATHIC molecule. (It has a Hydrophobic and hydrophilic
region).
It is this feature that gives the Phospholipid the ability to form membranes.
There are other forms of lipids involved in the cell membrane. These are
also amphipathic.
We will discuss these a little later.
How are the lipids and proteins arranged?
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The Lipids serve as the main fabric of the membrane, with the proteins being
embedded in the lipid.
This arrangement has been called…
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A FLUID MOSAIC.
The proteins have a Hydrophilic and a Hydrophobic portion also.
The proteins also have hydrophilic and hydrophobic sections.
The hydrophilic sections extend from the membrane surface .
The hydrophobic portion is situated within the membrane.
CHARACTERISTICS OF THE MEMBRANE
Cell membranes are NOT static sheets, held in a rigidly fixed arrangement.
The entire structure is held together primarily by HYDROPHOBIC
INTERACTION.
This form of bond is much weaker that covalent bonds.
The hydrophobic interaction allows the Lipids and some Proteins to drift
about the membrane (laterally).
While the proteins move laterally, they do NOT deviate from the
HYDROPHOBIC – HYDROPHOBIC, HYDROPHILLIC – HYDROPHILIC
arrangement.
In other words, the Proteins do not Flip around not do the move from the
inner membrane to the outer membrane.
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The Proteins, however Do move about the membrane, despite being much
larger than the lipids.
The protein movement is slow, almost a drifting type motion.
While it may seem to be a random style of movement Some proteins move in
a Highly directed manner.
This movement is directed by portions of the cell’s micro-skeleton.
This arrangement allows the membrane to remain FLUID as temperatures
drop.
The higher the concentration of Phospholipids the colder the cell can get
before having the membrane solidify.
This feature is due to the “KINK’ in the phospholipid chain that is caused by
the DOUBLE BONDS.
This is similar to what we saw in the Unsaturated Fat molecule in the
previous chapter.
This KINK prevents the unsaturated hydrocarbons from becoming too tightly
packed.
The membrane, as we look a little closer, also has cholesterol interspersed
between the phospholipids (animal cells).
The cholesterol helps prevent the membrane from becoming too fluid.
Instead, the membrane tends to me as fluid as salad oil.
The real interesting feature of the membrane, is that the cells can ALTER the
lipid composition of the membrane.
WHY ARE THERE PROTEINS IN THE CELL MEMBRANE?
To answer this question, we must first understand HOW the proteins actually are
situated in this structure.
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As we described earlier, some proteins are embedded within the membrane.
These proteins care called
INTEGRAL PROTEINS.
The penetrate the hydrophobic core of the lipid bi-layer.
Many of these proteins are called transmembrane proteins, because the
completely span the membrane. (They have a Hydrophilic end that protrudes
on the cytoplasmic side as well as one that protrudes on the extra cellular
side.)
We also see that there are proteins that do not penetrate the lipid bi-layer,
but rather are loosely attached to the surface of the membrane.
Often they are in contact with the exposed portions of the integral proteins.
Some of these MEMBRANE PROTEINS are held in place by parts of the cell’s
cytoskeleton.
THE ROLE OF THE PROTEINS
The INTEGRAL proteins play a KEY role in the regulation of transport in of
materials through the membrane and into the cell.
The integral proteins help to transport HYDROPHILIC (polar) compounds
through the lipid bi-layer.
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As we have discussed several times already, hydrophilic compounds tend to
“Avoid” hydrophobic substances.
And this is true with materials (water, etc.) as they try to enter the cell.
The hydrophilic materials will avoid the lipid portion of the membrane, and
are attracted to the hydrophilic proteins.
Once they contact the protein, they pass through the membrane via these
TRANSPORT Proteins.
Some of these TRANSPORT PROTEINS have a Hydrophilic channel through
the “Hydrophobic core of the membrane. This allows the substance to pass
through the membrane without contact the core.
They can also play a role in allowing cells to communicate with each other
(foreign body recognition).
Other proteins attach to some to the materials and physically carry these
materials through the membrane and into the cell.
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As we have already learned, proteins are substrate specific, and this is
essential to the Selective Permeability of the membrane.
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PROTEINS ALSO…
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Proteins in the membrane also play an ENZYMATIC role. They help the cell
(membrane) carry out vital steps in the metabolic pathway.
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They can act as a messenger for cellular communication. The specificity of
proteins make it an ideal material for the recognition of specific messengers
(hormones) that cause certain changes within the cell.
They can also play a role in allowing cells to communicate with each other
(foreign body recognition).
They also serve as sites for attachment of the cell’s micro-skeletal system.
This helps give the cell shape and fixes the cell in a specific location.
THE ENDOMEMBRANOUS SYSTEM
The Endomembranous System
Includes:
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PLASMA (CELL) MEMBRANE
ENDOPLASMIC RETICULUM
• SMOOTH
• ROUGH
THE GOLGI APARATUS
VACUOLES
THE NUCLEAR ENVELOPE
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ENDOPLASMIC RETICULUM
Latin for “Little Tubes”.
A series of membranous labyrinths
Accounts for over ½ of the cells’ total membrane
Consists of a network of tubules and sacs (cisternae).
The endoplasmic reticular membrane separates the internal surface
(cisternae) from the cytosol.
Types of endoplasmic reticulum
SMOOTH ENDOPLASMIC RETICULUM (SER).
ROUGH ENDOPLASMIC RETICULUM (RER).
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S.E.R.
Functions is a diverse array of metabolic processes.
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Synthesis of Lipids
Metabolism of carbohydrates
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Detoxification of drugs and poisons
Contains enzymes that are important to the synthesis of lipids.
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OILS
PHOSPHOLIPIDS
STEROIDS
SEX HORMONES
STEROID HORMONES
Liver Cells have a large amount of SER.
The liver functions in:
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Carbohydrate metabolism (hydrolysis of glycogen to release Glucose).
Detoxification of drugs and poisons. Involves adding a hydroxyl group
to the agent, making it more soluble in water thus easier to remove
from the body.
In muscle cells, the SER contains membrane pumps. These pump Ca++ into
the cisternal space. Upon stimulation of the muscle by the nervous system,
the Ca++ is released into the cytosol causing Muscular contraction.
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R.E.R.
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Play a role in the synthesis of Proteins
Found in many tissues
Have small round “BUMPS” attached.
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RIBOSOMES
In the Pancreas they play a role in INSULIN PRODUCTION.
As a polypeptide chain grows from the attached RIBOSOME, it passes into
the cisternal space of the R. E. R. via a pore in the Endoplasmic membrane.
As the Growing chain enters the cisternal space, the “NEW” protein folds
into its natural configuration.
Most secretory proteins are GLYCOPROTEINS
Proteins that are covalently bonded to the carbohydrate glucose.
Once the secretory proteins are formed the E.R. membrane keeps them
separated from those proteins that are produced by free Ribosomes (in the
cytosol).
Secretory proteins from the E.R. are packed into membrane bond vesicles.
These tiny sacs then bud off at a specialized region of the RER called…
The transitional Endoplasmic Reticulum
These Vesicles are called
Transport vesicles.
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The R. E. R. and the plasma membrane produce PHOSPHOLIPIDS.
As the lipid chains grow from the ribosome, they are inserted into the
membrane of the E.R..
They are then anchored in place by their hydrophobic region
Enzymes are built into the R. E. R. Membrane that assemble the
phospholipids from the precursors in the cytosol.
The Golgi Apparatus
GOLGI
After the transport vesicles leave the R.E.R. they travel to the GOLGI
APPARATUS.
Here the contents of the vesicle are modified, stored and when needed
shipped to OTHER destinations for use.
They are essential in cells that are specialized for secretion.
The G.A. consists of a series of flattened membranous sacs (cisternae).
There may be several hundred of these “STACKS” per cell.
The membrane of each cisterna in each stack separates the internal space
from the cytosol.
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Vesicles that are concentrated near the Golgi are involved in the transport of
materials between the Golgi and other structures.
The Golgi has a distinct polarity between its membranes.
The face located near the E.R. is called the CIS face.
The Face opposite the cis face (where the vesicles Bud off) is called the
TRANS face.
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THE CIS FACE
The cis face is nearest to the Endoplasmic reticulum.
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Transport vesicles from the E.R. arrive here.
They fuse with the surface of the Golgi.
Once fused the contents of the vesicle are added to the Golgi’s lumen.
The trans face
Once the contents of the transport vesicle have been modified and
packaged, they migrate to the inner surface of the lumen nearest the trans
face.
Here they push outwards forming a vesicle, which will eventually bud off and
travel to remotes sites.
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IN THE LUMEN
The Product produced in the E.R. , after being added to the Golgi’s lumen,
undergoes a modification. Prior to being released.
The Golgi’s enzymes change the Original Oligosaccharides on the protein.
The Golgi first removes some sugar monomers and substitutes other Monomers.
The result is the production of a variety or Oligosaccharides.
The Golgi can make its own macromolecules.
Many Polysaccharides are made here
Pectins, and other non-cellulose plant polysaccharides
The Golgi refines products in stages with different cisternae between the –
cis and –trans ends.
Each cisternae has different enzymes.
Products in various stages of modification can be seen between the two
areas.
Before releasing the products by budding off of the trans face, the products
must be sorted and targeted for various parts of the cell.
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Each molecule has an Identification tag (phosphate group) that is added
during the modification process.
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LYSOSOMES
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LYSOSOMES
Membrane bound sacs that contain HYDROLYTIC ENZYMES.
They function in the digestion of macromolecules
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Proteins
Polysaccharides
Fats
Nucleic Acids
The Environment of the CYTOSOL is normally a pH of 5.
This is maintained by the pumping of H+ out of the cytosol and into the
lumen of the lysosome.
If the Lysosomal contents leak into the cytosol, they would not be much
activity.
The cytosol is considered by comparison as Neutral.
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If there is excessive lysosomal leakage the cell could be destroyed
To prevent this the endomembranous system causes a compartmentalization
of the cell.
This plays a role in protecting the various areas and organelles of the cell
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The endomembranous system provides space for the lysosomes to digest
the macromolecules safely without
Causing a general destruction of the cell.
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WHERE DO HYDROLYTIC ENZYMES COME FROM ?
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The Hydrolytic Enzymes and the Lysosomal membrane are made by the
Rough Endoplasmic Reticulum and are Modified by the Golgi Apparatus.
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WHAT DO LYSOSOMES DO ?
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Lysosomes carry out a process called …
PHAGOCYTOSIS.
Some Lysosomal enzymes carry out AUTOPHAGOCYTOSIS
They digest the cell or parts of the cells.
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This facilitates the recycling of materials.
When phagocytosis occurs a new cellular structure is formed.
This is called…
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A VACUOLE
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VACUOLES
TYPES OF VACUOLES
Large vacuoles are called…
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VACUOLES
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Small vacuoles are called…
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VESICLES.
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FUNCTIONS
Those that function in food storage are called …
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FOOD VACUOLES.
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Those that function to remove excess water from the cell are called…
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CONTRACTILE VACUOLES
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MITOCHONDRIA
The Mitochondria is the site where CELLULAR RESPIRATION TAKES
PLACE.
Cellular respiration is the Catabolic process that generates ATP by
extracting energy from sugars, fats and other fuels – with the help of
OXYGEN.
Mitochondria are found in nearly all eukaryotic cells.
The more active the tissue the more numerous the mitochondria.
The mitochondria move around the interior of the cell and are constantly
changing shape as well as dividing.
STRUCTURE
The Mitochondria are enclosed by 2 membranes (phospholipid bi-layer) with
a unique collection of embedded proteins.
The outer membrane is SMOOTH.
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The inner membrane is convoluted.
THE INNER MEMBRANE
The Convolutions are a series of infoldings called CRISTAE.
They serve to increase the surface area of the inner membrane and thereby
increases production.
The inner membrane divides the mitochondria into 2 distinct compartments.
The intermembranous space a narrow gap between two membranes.
The mitochondrial matrix.
Contains many different enzymes and ribosomes.
The mitochondrial matrix is where some of the reactions of cellular
respiration occur.
A structure that is closely related to the mitochondria that is found strictly in
plants and some algae is called…
The CHLOROPLAST
CHLOROPLAST
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Specified member of the plastids.
It is a type of chromoplast (color container).
It contains the pigment…
CHLOROPHYL.
Chlorophyll is a green pigment compound that functions in
PHOTOSYNTHESIS.
Enzymes and other structures that play a role in photosynthesis are located
in the CHOLOROPLAST.
STRUCTURAL ARRANGEMENT
The Chloroplast’s contents are partitioned from the cytosol by a 2 layer
membrane.
Inside are stacks of flattened sacs that are formed from another
membranous structure.
These stacks are called…
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THYLAKOIDS
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The thylakoids are arranged in stacks called GRANUM.
The fluid outside the Thylakoids is called the STROMA
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The stroma contain the chloroplast DNA, ribosomes and enzymes.
The Thylakoid membrane divides the chloroplast into two compartments:
The THYLAKOID SPACE
And the STROMA.
We will discuss these when we discuss PHOTOSYNTHESIS.
PEROXISOMES
Specialized metabolic compartments that are bonded by a single membrane.
They contain the enzyme that transfers H+ from various substrates to O2
producing H2O.
They carry out different functions
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Break down fatty acids
Detoxify alcohol
They all have an enzyme that changes H2O2into water.
This enzyme is called…
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PEROXYDASE
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SPECIALIZED PEROXISOMES ARE CALLED
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GLYOXYSOMES.
These play a role in fat storage in plants.
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RIBOSOMES
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Made from particles of RIBOSOMAL RNA and PROTEIN.
Carry out PROTEIN SYNTHESIS
Each ribosome is composed of 2 subunits
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The Large subunit
The Small subunit
Cells with a high rate of protein synthesis have a high number of Ribosomes.
Ribosomes build proteins in 2 cytoplasmic locals:
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Free Ribosomes (in the cytoplasm).
And attached to R.E.R (or the nuclear envelope).
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MOST proteins made by FREE RIBOSOMES
will function in the cytosol.
Proteins made on the RER will be utilized else where.
Ribosomes take messenger RNA (from the nucleus), read the code that is
contained and attach the APPROPRIATE amino acid via a process known
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DEHYDRATION SYNTHESIS.
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The process will continue until the Ribosome reaches a “STOP CODE”.
The Ribosome will then separate into the large and small subunits and
release the protein.
THE NUCLEAR ENVELOPE
THE NUCLEAR ENVELOPE
Comprised of a double lipid membrane with associated proteins.
Very similar to the plasma membrane.
With one very big difference
And the difference IS.
The Nuclear Envelope has pores that perforate the membrane.
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The inner and outer membrane of the Envelope fuse at the lip of each pore.
Each pore is lined by a very intricate protein structure called a
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PORE COMPLEX.
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It regulates the entering and exiting of large macromolecules and particles.
The nuclear side of the Envelope is lined with a net-like protein called the
NUCLEAR LAMINA (is not found covering the pores).
The Nuclear Lamina helps the nuclear envelope maintain its shape.
THE NUCLEUS
Contains most of the genes in the EUKARYOTIC CELL.
It is the Most conspicuous Organelle in EUKARYOTIC CELLS.
Beside Genes, What else is in the NUCLEUS ?
WITHIN THE NUCLEUS…
DNA is organized with proteins into fibrous molecules called
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CHROMATIN.
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These are very thin strands
As the cell prepares to divide the chromatin fibers coil up (condense),
becoming thick enough to be discerned as separate structures. These are
called CHROMOSOMES.
Each Eukaryotic cells as a specific number of chromosomes.
HUMANS HAVE 23 PAIRS OF CHROMOSOMES.
WHAT IS THE PURPOSE OF THE CHORMOSOME?
The Nucleus is often considered as the “Control Center of the Cell.”
We know that DNA is responsible for cellular replication as well as proteins
synthesis.
So the Chromosomes are responsible for cell replication and Protein
synthesis
The Nucleus direct protein synthesis.
The Nucleus Determines when the DNA will be split and its code read.
It dictates the mRNA to form and leave the Nucleus for the cytosol.
Once in the cytosol the mRNA attaches to a RIBOSOME, where the protein
code is read, translated and the protein is made.
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The process by which new proteins are made is called…
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DEHYDRATION
SYNTHESIS
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THEREFORE, THE NUCLEUS DIRECTS ALL CELLULAR ACTIVITIES.
THE NUCLEOLUS
The most prominent structure within the nucleus is called…
The NUCLEOLUS.
The does not divide.
It appears as a mass of densely stained granules and fibers.
BUT, WHAT DOES IT DO ?
The Nucleolus is the site of synthesis of RIBOSOMAL RNA.
The subunit components are made in the nucleolus.
They then pass out of the Nucleus via the nuclear pores and enter the
cytosol where they are assembled into the Ribosome.
THE CYTOSKELETON
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WHAT IS IT ?
It is a network of fibers extending throughout the cytoplasm,
It plays a role in organizing the structure and activity of the cell.
SUPPORT & MOTILITY
The most obvious function of the cytoskeleton is to provide support and
shape for the cell.
This is very important in animals cells because they lack cell WALLS.
It provides strength and resilience based on structure.
It stabilizes the cell by balancing the opposing forces that are acting on the
c e ll.
It also provides an anchorage for many organelles and cytosolic enzyme
molecules.
The cytoskeleton can be quickly dismantled moved to a new location and the
reassembled.
MOTILITY? CELLS ?
Yes!
This includes changing cells location and more limited movement of parts of
the cell.
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This generally requires the interaction of the cytoskeleton and proteins
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Motor Molecules
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The motor molecules bring about movements of cilia and flagella.
They allow components to slide past one another, as in muscular
contraction.
Vesicles travel through the cell to their destination on “RAILS” from the
cytoskeleton
WHAT IS THIS “SKELETON” MADE OF ?
COMPONENTS
The cytoskeleton is composed of several parts.
They are:
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MICROTUBULES
MICROFILIMENTS
And INTERMEDIATE FILIMANTS
MICROTUBULES
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Are found in the cytoplasm of all eukaryotic cells.
They are hollow rods
They are made up of a globular protein called tubulin.
Tubulin is a dimer of slightly different polypeptides (α tubulin and β tubulin)
They grow in length by adding dimers.
Can be disassembled and reassembled.
Provide shape and support for the cell.
Serve as tracks for the organelles equipped with motor molecules.
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They are responsible for the separation of the Chromosomes during cellular
division.
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Centrioles and Centrosomes
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In many cells microtubules grow out from a centrosome.
This is a region, often located near the nucleus.
It functions as a compression resisting girder of he skeleton.
Within the Centrosome of an animal are a pair of centrioles.
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Each are comprised of 9 sets of a triplet of microtubules arranged in a ring.
Before a cell divides the centrioles replicate.
They may help organize the microtubule assembly.
CILIA AND FLAGELLA
Specialized arrangements of microtubules that are responsible for
locomotion. Both have a common arrangement.
9 doublets arranged in a ring with two single microtubules in the center (9 +
2 arrangement).
This arrangement allows for locomotion appendages to beat or “walk”.
CILIA.
Small hair like projections on the cell.
They are numerous
Beating pattern similar to oars.
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Alternating power and recovery strokes.
Flagella
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Long whip like structures
Usually single or a pair per cell
Beating pattern is an undulating motion.
Generates motion force in the same direction as the flagella’s axis.
HOW DOE THE MOTION OCCUR ?
Flexible “wheels” of protein are evenly distributed along the length of the
appendage.
These connect the outer doublets to each other and to the 2 central
microtubules.
Each doublet has a pair of arms spaced along its length (toward the
neighboring doublet).
Microtubules anchor the assembly to the basal bodies.
Motor molecules extending from each microtubule doublet are made of a
large protein called DYNEIN.
These are responsible for the bending movement of the cilia and flagella.
They form complex movements that change the protein configuration with
ATP thus supplying energy.
The dynein arms reach out and attach to an adjacent doublet and pull.
This creates a sliding motion.
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They then release. Recoil. Reattach and pull again.
Microfilaments
Actin filaments
Solid rod of a globular protein that is twisted into a double chain.
Can handle tension
It gives the membrane shape.
INTERMEDIATE FILAMENTS
Specialize for bearing tension.
Diverse class of cytoskeletal components
Each is constructed from different molecular subunits
Called KERATIN
More permanent fixture of cells.
CELL SURFACES
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PLANTS
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Cell Walls
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Protect cell
Maintain shape
Protects from excess water loss or uptake
Supports plant against gravity.
Thick cell (plasma) membrane.
Microfibrils made of cellulose are embedded in a matrix of other
polysaccharides and proteins.
Primary cell wall – found in young plants.
Very thin and flexible
Between the primary cell wall is a middle lamella. This is a thin layer
Made of a sticky polysaccharide – pectin. It holds the wall together.
In mature plants
Strengthens the cell wall by secreting hard substances into the primary cell
wall.
Other add a secondary cell wall between the plasma membrane and the
primary wall.
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This is often layered (laminated for more support.)
EXTRACELLULAR MATERIAL
Found in animals only.
Animals lack cell walls
They do have a very elaborate extracellular matrix.
Made of glycoproteins (collagen)
These are very strong fibers on the outside of the cell.
Collagen is embedded in a network of woven proteoglycans. These are
especially rich in carbohydrates.
Some cells are attached to the ECM by fibronectins.
These bind to receptor proteins on the cell membrane.
The ECM plays a role in cell to cell communication as well as the regulation
of cell behavior.
INTERCELLULAR JUNCTIONS
The adhering of neighboring cells.
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Produces Intercellular junctions.
They are means of connecting and communication between cells.
TYPES
Plasmodesmosomes (plants)
Perforated channels in the cell walls
Connection of living components of the cells
Unifies most of the plant into one living continuum.
Allows for communication.
Exchange of materials
In animals there are three types:
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TIGHT JUNCTIONS
DESOMOSOMES
GAP JUNCTIONS
All three are common to epithelial tissues.
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TIGHT JUNCTIONS
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Membrane of neighboring cells are actually fused together
There is a continuous band around each cell.
Prevents leakage
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E.g.. Intestinal epithelium
DESOMOSOMES
Act like rivets between cells
They hold the cells together in large strong sheets
Intermediate filaments (keratin) reinforces these junctions.
GAP JUNCTIONS
Cytoplasmic channels between adjacent cells.
Special membrane protects and surround each pore
The pores are wide enough for small molecules of sugars, salt, ions and
other small molecules to pass through.
In the heart muscle tissue, GAPS allow the flow of ions through the cells
This allows for the coordination of muscular contractions of the heart