Cells: the building blocks
of life!!!
What is a cell?
A cell is the basic structural and
functional unit of a living organism
What kind of cells can you think of ?
Cells: An Overview Generalized
There are trillions of cells in the human
body
Of those trillions, there are over 200
different kinds that vary in size, shape
and function
Cells vary in size and shape
1 micrometer (1μm) = 1 x10-6 m)
Red Blood Cell
(RBC) 7.5 μm
Smooth Muscle Cell
Human Egg Cell (Ovum)
20-500 μm long
140 μm
Nerve Cell
White Blood Cell
Can be many cm in
length
10-12 μm
Despite all their differences….ALMOST
all human cells have 3 common
components
Plasma membrane – the outer layer of the cell
Cytoplasm – fluid inside the cells. Contains
organelles
Nucleus – the control center of the cell
*****RBC’s do not have a nucleus!!!!
The Plasma Membrane…What is it?
• The outer layer of the cell. Think of it as the
“traffic controller” for the cell
• It is a semi-permeable membrane that is
selective about what can enter or leave the
cell…the “bouncer”
• Separates the body’s 2 main fluid
compartments
– Intracellular fluid – fluid inside the cell
– Extracellular fluid- fluid outside and in between
the cells
The Plasma Membrane: Structure
Phospholipids 75%
Glycolipids 5%
Cholesterol 20%
Double layer membrane composed primarily of
phospholipids
Phospholipids
• The polar heads are attracted to water so they
lie on the inner and outer surface of the
membrane
***remember intracellular fluid and extracellular
fluid = water
The nonpolar tails avoid the fluid and line up in the
center of the membrane
Glycolipids
• Lipid with an attached sugar group
• Found only in the outer surface of the
plasma membrane
• Combine with other glycolipids to make
glycocalyx (sugar coating)
Glycocalyx
• The fuzzy, sticky carbohydrate-rich area
surrounding the cell
• Every cell has a different pattern of sugars in
its glycocalyx; therefore, the glycocalyx
provides a very specific biological marker for
cell recognition
• Essentially I.D. tags for the cell to cell
recognition
Cholesterol
It’s not the devil!
• Wedges between the phospholipid tails
• Stabilize the membrane
Membrane Proteins
Integral Proteins
Peripheral Proteins
Most are transmembrane (span
the entire width of the
membrane and protrude on
both sides)
Not embedded in the
membrane. Attached loosely to
integral proteins or
phospholipids
Most are involved in transport
as channels or carriers
May act as enzymes, and others
help to bring about changes in
cell shape during cell division
What 3 factors bind cells together?
1) Glycoproteins in the glycocalyx act as an
adhesive
2) Wavy contours of membranes fit together in
a tongue and groove fashion
3) Special membrane junctions are formed
Special Membrane Junctions:
Tight Junction:
“impermeable” junction helps prevent molecules
from passing through the extracellular space
between adjacent cells
Desmosome:
 “anchoring” junctions scattered like rivets along the sides
of adjacent cells that prevent separation
Button like plaque on cytoplasmic side held together by
thin linker proteins (cadherins) on the cellular side
Thicker protein filaments “lock” together with the plaque
on the opposite side to anchor them together
Strong yet flexible junctions
Special Membrane Junctions:
Gap Junctions:
Allows chemical substances to pass
between adjacent cells
Connected to other cells by a hollow
cylinder (connexons)
Ions, sugars, and other small molecules
pass through these channels
Cell
Junction
Junction…
What’s
Your
Function?
Membrane Transport
The plasma membrane is a selectively permeable
membrane that allows nutrients to enter the cell
while keeping unwanted elements out of the cell
as well as ridding itself of toxic waste products.
Interstitial Fluid (the cellular super highway)
Fluid between the cells that contains nutrients
such as vitamins, sugars and amino acids,
hormones and neurotransmitters, and waste
products
Passive Transport
(See Table 3.2 on Page 80)
• Simple Diffusion
• Facilitated Diffusion
• Osmosis
• Filtration
Active Transport
(See Table 3.2 on Page 80)
• Solute Pumping
• Vesicular Transport
–Exocytosis
–Phagocytosis (Endocytosis)
–Bulk-phase Endocytosis
–Receptor-mediated Endocytosis
Diffusion
• The tendency of molecules or ions to scatter
evenly throughout the environment
• Molecules move from areas of higher
concentration to lower concentration
Simple Diffusion
• Substances that are nonpolar and lipid soluble
(oxygen, carbon dioxide, fat-soluble vitamins,
and alcohol) diffuse directly through the lipid
bilayer
• However, polar and charged particles can
selectively pass through channel protein pores
if they are small enough
Diffusion Model
Facilitated Diffusion
Certain molecules (glucose and other simple sugars) are too polar to
dissolve in the lipid bilayer and too large to pass through membrane
channels so they must be helped across
Transport proteins in the plasma membrane allow entrance to the
cell bypassing the non polar portion of the cell by engulfing then
releasing the molecule into the cell
Facilitated Diffusion Model
Osmosis
• The diffusion of a solvent, such as water
through a selectively permeable membrane
• Occurs whenever the water concentration
differs on the two sides of the membrane
***Even though water is highly polar, it passes
easily through the lipid bilayer****
Osmosis Model
Tonicity
• Osmotic imbalances cause cells to shrink or
swell until the solute concentration on both
sides of the plasma membrane is the same, or
the membrane is stretched to it’s breaking
point
• Tonicity is the ability of a solution to change
the tone or shape of cells by altering their
internal water volume
Tonicity
• Isotonic
– solutions with the same concentration of solutes as cells
– Cells retain normal shape and have no loss or gain of water
• Hypertonic
– Solutions with higher concentration of solutes than the cell
– Cells in a hypertonic solutions lose water and shrink
(crenate)
• Hypotonic
– Solutions with a lower concentration of solutes than the
cell
– Cells in a hypotonic solution gain water and swell and
sometimes burst (lyse)
Active Transport
• Similar to facilitated diffusion in that it needs
carrier proteins that combine with the
transported substances.
• Solute pumps move solutes “uphill” against
their concentration gradients
Vesicular Transport
• The transport of large particles and macromolecules
across the plasma membrane
• The substance or cell product to be released is 1st
enclosed in a membranous sac called a vesicle
• 2 types of vesicular transport
– Exocytosis - movement of substances from the cell interior
to the extracellular space
– Endocytosis – movement of substances from the
extracellular space into the cell
Exocytosis
•
•
•
•
•
Moves materials out of the cell
Material is carried in a membranous vesicle
Vesicle migrates to plasma membrane
Vesicle combines with plasma membrane
Material is emptied to the outside
Exocytosis
Endocytosis
• Extracellular substances are engulfed by being
enclosed in a membranous vesicle
• Types of endocytosis
• Phagocytosis—cell eating‖
• Pinocytosis—cell drinking‖
• Receptor mediated
Exocytosis/Endocytosis Model
The Cytoplasm
• The “stuff” between the plasma membrane and the
nucleus…includes
– Cytosol:
• Viscous, semitransparent fluid
• Mostly water
• Contains dissolved proteins, salts, sugars, and other solutes
– Organelles
• Suspended in the cytoplasm
• Metabolic machinery of the cell
– Inclusions
• Extra things that may be stored in some cells
• Ex. melanin, glycogen, lipids, etc…
The Organelles
•The “machinery” of the cell
•Each organelle “little organ” has a specific
job in the cell to maintain the life of the cell
Mitochondria
• The “power plants” of a cell providing most of its ATP supply
• Break down carbohydrate, lipid, and protein molecules to form
ATP
• Complex organelles that contain their own DNA, RNA, and
ribosomes and are able to reproduce themselves
• When ATP needs increase, mitochondria can divide to increase
their number then grow to their full size so they can produce
more ATP!
• Many mitochondria are found in very active cells and fewer in
less active…ex. Liver, kidneys = LOTS of mitochondria
Ribosomes
• Made of protein and RNA, these are the site of protein
synthesis (production)
• Some are free floating in the cytoplasm and others are
attached to membranes forming Rough ER
• Can attach and detach from the ER depending on they
type of protein they are making
• Attached ribosomes make proteins that are designated
for exporting from the cell or for the plasma membrane
itself
Endoplasmic
Reticulum (ER)
• Extensive system of inter-connected tubes and layered
membranes enclosed in fluid filled cavities (cisternae)
• Coil and twist through the cytosol
• Continuous with the nuclear membrane
• Accounts for about half of the cell’s membranes!
• 2 types
 Rough ER – has ribosomes
 Smooth ER – no ribosomes
Rough ER
• Endoplasmic reticulum that is studded with ribosomes,
which make all the proteins secreted from the cells
• Manufactures the integral proteins and phospholipids
that form the plasma membrane –considered a
“membrane factory”
• Once made, proteins are enclosed in vesicles for
transport to the Golgi Aparatus where they are further
processed
Smooth ER
• Play no role in protein synthesis – WHY?
• Catalyses (fuels the fire) reactions involved in:
 Lipid metabolism, cholesterol production (why is this
important?)
 Synthesis (creation) of steroid-based hormones (such
as estrogen and testosterone)
 Absorption, synthesis, and transport of fats (cells of the
GI tract)
 Detoxification of drugs, pesticides, and carcinogens (in
liver and kidneys)
 Breakdown of stored glycogen to form glucose (esp. in
liver)
Golgi Aparatus
• Made of stacked, flattened membranous sacs with many
tiny vesicles that pinch of for “shipping proteins.
• Main function is to modify, concentrate and package the
proteins and lipids made by the rough ER. --See figure
3.20 on page 86
• “Packages” are shipped 1 of 3 ways
1) Vesicle is destined for exocytosis
2) Vesicle is to become part of the plasma
membrane
3) Vesicles becomes a lysosomes
Lysosomes
• Vesicles produced by the Golgi Aparatus that contain
digestive enzymes
• Function as a cell’s “demolition crew”by
1. Digesting particles taken in by phagocytosis (esp. bacteria,
viruses, and toxins)
2. Geting rid of worn-out or non-functioning organelles
3. Performing metabolic functions such as glycogen
breakdown and release
4. Breaking down non-useful tissues such as the webbing
between the fingers and toes of a developing fetus
5. Breaking down bone to release Calcium ions into the blood
Peroxisomes
•Membranous sacs containing powerful enzymes (oxidases and
catalases) which detoxify harmful substances and neutralize free
radicals
•Especially numerous in the liver and kidney cells which are very
active in detoxification
•Free Radicals- normal byproducts of cellular metabolism that
can have harmful effects on cells if allowed to accumulate
The Cytoskeleton
• The “cell skeleton” – it is a network of rods running through
the cystosol
• Supports cell structure and aids in cell movement
• 3 types of rods from smallest to larges
1. Microfilaments
2. Intermediate filaments
3. microtubules
Check Your Understanding
What organelle is the major site of ATP synthesis?
What are 3 organelles involved in protein synthesis and
how do they interact?
How does the function of lysosomes compare to that of
peroxisomes?
Microtubules
•Cylindrical structures made of tubulin proteins
•Support the cell and give it shape
•Involved in intracellular and cellular movement
•Form the centrioles
Centrioles
•Paired cylindrical bodies each composed of 9 triplets of
microtubules
•Organize a microtubule network during mitosis to form
the spindle and asters
•Form the bases of cilia and flagella
Cilia
•Whip-like cellular extensions on the surface of certain cells
Example: cells that line the respiratory tract have cilia that propel
mucus laden with bacteria and dust particles upward away from the
lungs
Flagella
•Long tail-like projection formed by centrioles
Example: sperm which have one flagellum used for movement
NOTE: Cilia propel other substances across the cell’s surface whereas
the flagella propels the cell itself
Microvilli
“Little Shaggy Hairs”
•Tiny finger like extensions of the plasma membrane
•Increase the plasma membrane surface are tremendously
The Nucleus
• The control center of the cell
• Has 3 regions or structures
1. The nuclear envelope
2. Nucleoli
3. chromatin
• Most cells only have 1 nucleus but some are multinucleate –
having more than 1 nucleus (ex. Skeletal muscles)
• All human cells except red blood cells have at least 1 nucleus.
RBC’s are the only anucleate cells therefore cannot reproduce
and only live for 3-4 months in the blood stream
The Nuclear Envelope
•Surrounds the nucleus in a double layer membrane barrier
separated by a fluid filled space
•The outer membrane is connected with the rough ER of the
cytoplasm and studded with ribosomes & pores
Nucleoli
•Spherical bodies found within the nucleus
•Produce ribosomal RNA molecules for the creation of ribosomes
Chromatin
•Uncoiled chromosomes consisting of DNA and histone
protein molecules
•Histone is responsible for packing long DNA molecules
in a compact, orderly way
Cell Division
•Cells must reproduce if an organism is to grow and repair
damaged tissues
•During cell reproduction, a cell divides its genes equally and
then splits into 2 identical cells
Cell Division
Cell division involves 2 major events
1. Mitosis
2. Cytokinesis
Mitosis- when the chromatin in the nucleus combine into
chromasomes and are equally divided between the 2
forming cells
Cytokinesis – separation of the cytoplasm to produce 2
daughter cells. Each daughter cell has the same number of
chromosomes as the parent cell
What is Interphase?
• Interphase is the period from cell formation to cell
division
• Previously thought to be a “resting” resting phase,
interphase is actually a busy time for the cell. In addition
to its daily routine activities, the cell is also growing
rapidly, replicating DNA and in general, preparing for cell
division
• Calling it a “resting” phase would be like calling
pregnancy a “resting” phase. Just because it’s not active
labor, doesn’t mean there’s nothing being done!
Interphase is divided into 3
subphases
G1- The cell is making protein rapidly, growing
vigorously, and most importantly, begins replicating
organelles including the centriole pair
S phase- DNA is replicated and new histones are
made and assembled into chromatin
G2- Enzymes and other proteins needed for division
are made and moved to the proper places. At this
point the centriole replication is complete
The M (Mitotic) Phase
• The 2nd phase of the cell’s life cycle
• Involves 2 distinctive events
1. Mitosis – division of the nucleus
2. Cytokinesis – division of the cytoplasm
Mitosis is the series of events taking place from prophase
through telophase
Cytokinesis – begins during late anaphase and is
completed after mitosis ends. It occurs as a shrinking of
actin microfilaments forms the cleavage furrow and pinches
the cell apart.
At the end of cytokinesis there are 2 daughter cells
Prophase
Early Prophase
• Chromatin condenses forming chromosomes
• Centrisomes begin moving apart and act as focal
points for the mitotic spindle.
Late Prophase
•
The nuclear envelope breaks down
Metaphase
• The 2 centrioles are now at opposite poles of the cell
• The chromosomes cluster at the middle of the cell with
the centromeres precisely lined up at the equator of the
spindle.
• This imaginary plane midway between poles is called the
metaphase plate.
Anaphase
• The third phase of mitosis
• This is when the chromosomes split
• Spindle fibers pull apart the chromosomes and drag
them to opposite poles of the cell
*Cytokinesis
begins near the end of anaphase when a
cleavage furrow develops along the metaphase plate and
the cell membrane pinches in and continues to telophase.
Telophase
• Begins as soon as chromosomal
movement stops
• Chromosomes uncoil back into
chromatin
• A new nuclear envelope forms around
each chromatin mass, the nucleoli
reappears and the spindle breaks down
and disappears
• As mitosis ends, cytokinesis completes
the division of the cell by pinching the
cell completely apart and leaving 2
identical daughter cells.