Cell Structure and Function
Robert Hooke-First person to see cells, he coined the term "cell" for
the many boxes he empty boxes he saw under the microscope
Endosymbiosis theory: All organelles seem to share
many properties with bacteria. Lynn Margulis
proposed endosymbiont hypothesis: that organelles
derived from ancient colonization of large bacteria
(became the eukaryotic cell) by smaller bacteria
(became the mitochondria, chloroplast, etc.)
Symbiosis = "living together". Eventually, organelles
lost ability to exist as separate organisms, cannot be
separated from cell. Recent evolutionary taxonomy
by comparing ribosomal RNA shows that this idea
has lots of merit. Mitochondrial and plastid ribosomes
are very similar to current bacteria, very different from
eukaryotes.
The Cell Theory
1. Every living organism is made of one or more cells
2. Cells are the functional unit of multicellular organisms
3. Cells arise from pre-existing cells
Cell Size
atoms --> DNA --> virus --> bacteria -->mitochondria--> Eukaryotic cells
Cells must remain small in size due to the ratio of surface area and volume
As the cell increases in size, its surface area becomes too small to support its internal structures. Oxygen and other important
substances cannot diffuse fast enough. Cells that get too large, may divide.
All Cells Have Three Basic Features:
Cell Membrane, Genetic Material, Cytoplasm
Plasma Membrane (aka Cell Membrane)
1. Isolates cytoplasm from external environment
2. regulates flow or material into and out of the cell
3. allows interaction with other cells
Genetic Material
1. provides cellular "blueprint" that controls the functions of the cell
2. In the form of DNA (Deoxyribonucleic acid)
3. DNA is universal for all cells, an all living things - evidence of common ancestry
4. Chromatin is the complex of proteins and DNA, it condenses into chromosomes before cell division
Cytoplasm (aka cytosol)
1. inside plasma membrane
2. contains water, salts, and other chemicals
3. organelles float within this jelly-like substance
Prokaryote vs Eukaryote Cells
Prokaryotes
no membrane bound nucleus, chromosomes
grouped together in an area called the "nucleoid"
no membrane bound organelles
smaller than eukaryotes
consist of bacteria and archaebacteria
Eukaryotes
has a membrane bound nucleus
has membrane bound organelles in cytoplasm
much larger than prokaryotes
animals, plants, fungi, protists
The Nucleus--nuclear
envelope contains pores
for some things to enter
and exit
-- chromatin is DNA and
proteins, when the cell
begins to divide, chromatin
condenses and forms
chromosomes
--DNA remains in the
nucleus, it sends
instructions to the
cytoplasm via messenger
RNA
--RNA directs the
synthesis of proteins on
ribosomes in the
cytoplasm
--Nucleolus assembles
ribosomes within the
nucleus, ribosomes
contain the "tools" to
construct proteins
Endoplasmic
Reticulum -Transport System
(endoplasm means "within
cytoplasm", reticulum
means "little net"
--divides cell into
compartments
--channels molecules
through the cell's interior,
like a little highway
Rough ER
--has ribosomes which
give it its "rough"
appearance
--functions in protein
synthesis
--ER transports newly
assembled proteins to the
Golgi Apparatus
Smooth ER
--mostly contains enzymes
that unction in lipid
synthesis (such as
hormones like estrogen
and testosterone)
Golgi Apparatus -Delivery System
--flattened stacks of
membranes
--functions in collection,
packaging and distribution
of molecules made in the
cell and used elsewhere
-- front end (cis) faces the
ER, and the back end
(trans) faces the cell
membrane
--unprocessed proteins
enter the Golgi apparatus,
are processed and exit
near the cell membrane for
export
--the folded stacks are
called cisternae
Jobs of the Golgi
Apparatus (aka Golgi
Complex)
1. separates proteins
according to their
destinations
2. modifies proteins (adds
sugar and makes
glycoproteins)
3. packages materials into
vesicles which are
exported outside the cell
Lysosomes Intracellular
Digestion Centers-vesicles that are used to
digest
--contain high levels of
degrading enzymes (to
"lyse" means to dissolve)
--recycle old and worn out
cell parts
--"suicide sac"
--digest other particles
taken in by phagocytosis
--this "food" is stored in
food vacuoles, the
lysosomes fuse with the
vacuoles and release
digestive enzymes
--found in animal cells
Ribosomes - Sites
of Protein
Synthesis
--each is composed of two
subunits, one large and
one small
--mRNA is "read" by the
ribosomes and amino
acids are assembled into
proteins
--ribosomes are
manufactured by the
nucleolus inside the
nucleus
Mitochondria The Cell's
Chemical
Furnaces
--contains its own DNA,
support for Endosymbiosis
Theory
--singular is
"mitochondrion"
--2 membranes, one
smooth outer membrane,
and an inner membrane
folded into layers called
cristae
--Cristae has two
compartments: the matrix
and the intermembrane
space
--mitochondria divide
before cell division, they
are not synthesized like
other cell parts
--function to store energy
for cell use. Energy is
stored in the form of ATP adenosine triphosphate
Chloroplasts Where
Photosynthesis
Takes Place
--only found in plant cells
--has its own DNA, like
mitochondrion
--functions to convert light
energy to ATP
--consists of grana, closed
compartments that are
stacked
--thylakoids are the
individual disk shaped
compartments that make
up the grana
--stroma is the fluid
surrounded the thylakoids
Cytoskeleton Support System
(page 97)
Cell is not "just a bag in a
bubble". Lots of internal
fibers = internal "skeleton".
Not rigid like bone;
capable of being
assembled, broken down
in minutes. Allows cell
movement, cell division,
internal motion of
compartments.
Composed of
Microtubules &
Microfilaments
Centrioles Microtubule Assembly
Centers
--usually occur in pairs
arranged at right angles
--assemble microtubules
which influence the cell
shape and movement part of the cytoskeleton
--also function in cell
division, mitosis
--only found in animal cells
Cilia & Flagella
--function in movement
-- 9+ 2 Arrangement of
microtubules
Vacuoles Storage Areas
--in plants the vacuoles
are large and centralized,
storage of water makes
the cell turgid
--in animals, they store
food, water and other
substances
Cell Membrane
The Plasma Membrane
--the fluid mosaic model (S.J Singer)
-- semi-permeable
--fluid portion is a double layer of phospholipids, called the phospholipid bilayer
Jobs of the cell membrane




Isolate the cytoplasm from the external environment
Regulate the exchange of substances
Communicate with other cells
Identification
Phospholipid bilayer
Phospholipids contain a hydrophilic head and a nonpolar hydrophobic tail
Hydrogen bonds form between the phospholipid "heads" and the watery environment inside and outside of the cell
Hydrophobic interactions force the "tails" to face inward
Phospholipids are not bonded to each other, which makes the double layer fluid
Cholesterol embedded in the membrane makes it stronger and less fluid
Proteins embedded in membrane serve different functions
Transport Proteins - regulate movement of substance
Channel Proteins - form small openings for molecules to diffuse through
Carrier Proteins- binding site on protein surface "grabs" certain molecules and pulls them into the cell
Gated Channels - similar to carrier proteins, not always "open"
Receptor Proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel proteins)
Recognition Proteins - ID tags, to identify cells to the body's immune system
Transport Across Membrane
Passive Transport
Simple Diffusion - water, oxygen and other molecules move from areas of high concentration to areas of low concentration, down a
concentration gradient
Facilitation Diffusion - diffusion that is assisted by proteins (channel or carrier proteins)
Osmosis - diffusion of water. Salt Sucks
Osmosis affects the turgidity of cells, different solution can affect the cells internal water amounts
Contractiles Vacuoles are found in freshwater microorganisms - they pump out excess water
Turgor pressure occurs in plants cells as their central vacuoles fill with water.
Active Transport - involves moving molecules "uphill" against the concentration gradient, which requires energy
Endocytosis - taking substances into the cell (pinocytosis for water, phagocytosis for solids)
Exocytosis - pushing substances out of the cell, such as the removal of waste
Sodium-Potassium Pump - pumps out 3 sodiums for ever 2 potassium's taken in against gradient
Demo - Starch in the baggie, iodine in the beaker. What happens and why?
Observation of elodea cells in salt water. What happens and why?
Cell Connections and Communication
Tight junctions are composed of protein fibers that seal Desmosomes anchor adjacent cells together by making
adjacent cells to prevent leakage, something which can be
useful in organs such as the bladder and the lining of the
digestive tract. Tight junctions literally fuse the cells together
forming a sheet of cells restricting molecules to one side of
the sheet or the other.
Tight junctions can also partition the cells in which they are
found. Certain membrane proteins can be restricted to one
side of the junction, as well, since the tight junction prevents
protein migration within the membrane.
*Plants have plasmodesmata - channels between the
cell wall that cytosol can pass through
connections that work like staples or rivets that attach to
components of the cytoskeleton. Many epithelial cells must
adhere to adjacent membranes to prevent free passage or free
movement, and to not break apart under stress. Desmosome
filaments are composed of specialized glycoproteins proteins.
Intermediate filaments of keratin in the desmosomes help
strengthen the junction. Actin microfilaments can also attach to
desmosomes, but have less strength
Gap junctions are protein channels, called connexons,
between adjacent cells that permit the transfer of small molecules,
such as nutrient monomers, between the cells. They are common
in brain cells, forming the synapse, in many glands, and in cells in
the heart muscle that coordinate contraction for heartbeat. Gap
junctions can be gated.