Cells as Units of Life
Chapter 3
Cell Theory
 Cells represent the basic structural and
functional unit of life.
 Important unifying concept in biology.
 All organisms are composed of one or more cells.
 All tissues & organs are composed of cells.
 There is no life without cells!
Cell Theory
 Cell theory states that all living organisms are
composed of cells.
 Cells come from preexisting cells.
Prokaryotic vs. Eukaryotic
Cells
 All cells:




Have DNA
Use the same genetic code
Synthesize proteins
Use ATP in similar ways
 This implies common ancestry.
Prokaryotic vs. Eukaryotic
Cells
 Prokaryotic cells – no nucleus or other
membrane-bound organelles.
 Kingdom Archaebacteria
 Kingdom Eubacteria
 Eukaryotic cells – do have nucleus and
membrane-bound organelles.




Kingdom Protista
Kingdom Fungi
Kingdom Plantae
Kingdom Animalia
Components of Eukaryotic
Cells
 The plasma
membrane
surrounds the cell.
 The nucleus is the
largest organelle.
 Double layered
nuclear envelope.
Cell Model
Components of Eukaryotic
Cells
 Cytoplasm refers to the cellular material
between the cell membrane and nuclear
envelope.
 Organelles such as the mitochondria, Golgi
complex, centrioles, and endoplasmic
reticulum are found in the cytoplasm.
Plasma Membrane
 Plasma membrane
structure is
described using the
fluid mosaic
model.
http://youtu.be/Qqsf_UJcfBc
Plasma Membrane
 Two layers of
phospholipid
molecules oriented
with hydrophilic
heads toward the
outside and
hydrophobic tails
inside.
 Fluid-like – flexible
Plasma Membrane
 The nonpolar nature of the hydrophobic ends in the
interior of the membrane prohibit polar substances from
crossing the membrane.
 Glycoproteins embedded in the membrane function in
the transport of molecules across the membrane.
Nucleus
 The nuclear envelope
contains pores to allow
molecules to move
between nucleus &
cytoplasm.
 Chromosomes are
contained in the
nucleus.
 Chromatin refers to
loosely condensed
DNA & proteins.
Nucleus
 Nucleoli are specialized parts of certain
chromosomes that carry multiple copies of the
DNA used to synthesize ribosomal RNA.
 This rRNA combines with protein to from the two
subunits of ribosomes.
 Ribosomes leave the nucleus through pores in the
nuclear envelope.
Endoplasmic Reticulum
 The nuclear envelope joins with a cytoplasmic
membranous system – the endoplasmic
reticulum (ER).
 Rough endoplasmic reticulum (RER) is
covered with ribosomes. Smooth (SER) is not.
Endoplasmic Reticulum
 Ribosomes on the RER synthesize proteins
that enter the ER that will either be
incorporated into the plasma membrane,
exported from the cell, or they may be bound
for lysosomes.
 Lipids and phospholipids are synthesized in the
SER.
Golgi Complex
 The Golgi complex is a stack of membranous
vesicles where storage, modification, and
packaging of protein products occurs.
Assembling & Secreting
Proteins
Lysosomes
 Lysosomes contain
enzymes (proteins)
that can breakdown
foreign material like
bacteria or worn out
cellular components.
 Contents of lysosome
would kill cell if
membrane ruptured.
 May pour enzymes
into food vacuoles.
Mitochondria
 Mitochondria are the powerhouses of cells –
they contain enzymes that carry out the
energy-yielding steps of aerobic metabolism.
ATP is produced here.
 Composed of a double membrane – the inner
membrane is folded into cristae.
 Mitochondria are self-replicating, containing their
own circular DNA molecule.
Cytoskeleton
 Eukaryotic cells have a
cytoskeleton that provides
support and often locomotion
and movement of organelles.
 Composed of microfilaments,
microtubules, and
intermediate filaments.
Cytoskeleton
 Microfilaments are made of the proteins actin
and myosin and function in a cell’s ability to
contract as seen in muscle cells.
 Actin microfilaments move molecules and organelles
through the cytoplasm.
Cytoskeleton
 Microtubules are
larger tubular
structures composed
of the protein
tubulin.
 Move
chromosomes
during cell division.
 Part of the structure
of cilia & flagella.
Cytoskeleton
 Microtubules radiate out
from the centrosome –
the microtubule organizing
center.
 Located near nucleus.
 Not membrane bound.
 Centrioles are found in
the centrosome.
 Centrioles composed
of 9 triplets of
microtubules.
 Replicate before cell
division.
Cytoskeleton
 Intermediate fibers fall in between
microfilaments and microtubules in size.
 There are five biochemically distinct types of
intermediate fibers.
Cilia & Flagella
 Cilia & flagella are motile extensions of the cell
surface.
 In many single celled organisms they are a source of
locomotion.
 In multicellular animals they usually sweep material
past the fixed cell.
 Nine pairs of microtubules enclose a central pair.
 At the base is a basal body - identical to a centriole.
Pseudopodia
 Some single-celled organisms, migrating cells
in embryos, and white blood cells show
ameboid movement.
 Cytoplasmic streaming through the action of actin
microfilaments extends a pseudopodium outward.
 Some have specialized pseudopodia with
microtubules that are assembled & disassembled to
allow movement.
http://www.youtube.com/watch?v=zTFsn9xt7b0&feature=player_embedded
Junctions
 Tight junctions form
when cell membranes
ofadjacent cells fuse.
 Function as seals.
 Adhesion junctions
occur under tight
junctions.
Transmembrane
proteins link across a
small space and
connect to
microfilaments.
Junctions
 Desmosomes act as spot welds and increase
the strength of the tissue.
 Hemidesmosomes are found at the base of
cells and anchor them to connective tissue.
 Gap junctions are canals between cells that
provide intercellular communication.
Microvilli
 Microvilli are small
fingerlike projections
that have bundles of
actin microfilaments.
 They serve to
increase the surface
area of the tissue as
in the intestine.
Membrane Function
 Membranes surround the outside of the cell
and the organelles inside it.
 The plasma membrane acts as a selective
gatekeeper.
 A substance may cross the membrane:
 By diffusion
 By a mediated transport system
 By endocytosis
Diffusion & Osmosis
 Diffusion is the movement of molecules from
an area of high concentration to an area of low
concentration. This tends to equalize the
concentration.
 Down the concentration gradient.
 Solutes are molecules (e.g. salt) that are found in a
solution.
Diffusion & Osmosis
 Cell membranes are
selectively
permeable – water
can pass through,
but not most solutes.
 Gases (oxygen &
carbon dioxide),
urea, lipid soluble
solutes can cross
the membrane.
http://www.youtube.com/watch?v=sdiJtDRJQEc&feature=player_embedded#!
Diffusion & Osmosis
 Osmosis - if there is a membrane between two
solutions with unequal concentration of solutes
that can not cross the membrane, water will
flow toward the side with less water / more
solute until the two sides have equal
concentrations.
Diffusion & Osmosis
Diffusion & Osmosis
 Animals utilize osmosis to control internal fluid
and solute levels.
 The blood of marine fishes has 1/3 the salt content of
the water. They are hypoosmotic to seawater.
 Freshwater fishes have blood that is saltier than the
water. They are hyperosmotic to the water.
 If the solute concentrations were the same, the two
solutions would be isoosmotic.
Diffusion Through Channels
 Charged substances,
like water and dissolved
ions, can’t simply diffuse
across the cell
membrane.
 They pass through
channels created by
transmembrane proteins.
 Some channels always
open.
 Some are gated
channels.
http://www.youtube.com/watch?v=PYP4JYL-KQk&list=FL9N_Px072WuVorSwDfqf-9w&index=87&feature=plpp_video
Diffusion Through Channels
 Gated channels require a signal to open or
close them.
 Chemically-gated channels open or close when
a signaling molecule binds to a binding site on the
transmembrane protein.
 Voltage-gated channels open or close when the
ionic charge across the membrane changes.
Carrier Mediated Transport
 Sugars & amino acids must be able to enter
cells and waste products must be able to leave.
 These molecules cross the membrane with the
help of transporter proteins.
 Transporter proteins are specific.
 Facilitated diffusion
 Active transport
Facilitated Diffusion
 In facilitated diffusion,
the transporter protein
binds to the substrate
molecule on one side of
the plasma membrane
then changes shape to
release it on the other
side.
 Takes place in the
direction of the
concentration gradient.
Active Transport
 Active transport requires energy (ATP) to
transport molecules in the direction opposite
the concentration gradient.
http://www.youtube.com/watch?v=STzOiRqzzL4
Endocytosis
 Endocytosis is the ingestion of material by cells.
 Phagocytosis – cell eating – method of feeding by singlecelled organisms.
 Pinocytosis – small molecules or ions are enclosed in vesicles
called caveolae.
 Receptor-mediated endocytosis – method of bringing large
molecules into a cell with the help of the protein clathrin.
Endocytosis and Exocytosis
http://www.youtube.com/watch?v=W6rnhiMxtKU&feature=player_embedded#!
http://www.youtube.com/watch?v=U9pvm_4-bHg&feature=player_embedded
Exocytosis
 Exocytosis - membranes of a vesicle inside
the cell can fuse with the plasma membrane to
discharge the contents of the vesicle outside
the cell.
 Transcytosis – a substance may be picked up
on one side of the cell, transported completely
across the cell and discharged on the other
side.
Mitosis and Cell Division
 Mitosis is the process of nuclear cell division in
nonreproductive, or somatic, cells.
 A fertilized egg, or zygote, divides by mitosis to
produce a multicellular organism.
 Damaged cells are replaced by mitosis.
Chromosomes
 In cells that are not dividing, the DNA is loosely
organized so that individual chromosomes
can’t be distinguished – it is now referred to as
chromatin.
 Before division, chromatin becomes more
compact and chromosomes can be
recognized.
Chromosomes
 All nonreproductive cells in a species have the
same number of chromosomes.
 46 in humans
 Half of these chromosomes come from each
parent.
 Result is two sets of chromosomes.
 Diploid
 Chromosome 1 from Mom and chromosome 1 from
Dad are called homologous chromosomes.
The Cell Cycle
 Cells come from
preexisting cells
through the process
of cell division.
 Cell division –
mitosis and
cytokinesis – occupy
a very small portion
of the cell cycle.
The Cell Cycle
 Interphase includes:
 G1 – growth phase where RNA and functional
proteins are synthesized.
 S – DNA replication.
 G2 – growth phase where structural proteins are
made.
 Mitosis
 Cytokinesis
Chromosome Structure
 During S phase, each
of the 2 homologues
replicates, resulting in
identical copies called
sister chromatids.
 Chromatids remain
connected at a linkage
site called the
centromere.
Cell Division
 There are two phases of cell division:
 Mitosis – nuclear cell division
 Prophase
 Metaphase
 Anaphase
 Telophase
 Cytokinesis – division of the cytoplasm
 Multiple nuclear divisions not accompanied by
cytokinesis result in a multinucleate cell.
Cell Division
 Prophase –
Chromosomes
condense enough to
be seen with a light
microscope.
 Spindle forms
between the 2
centrioles.
 Spindle fibers
attach to
kinetochores.
Cell Division
 Metaphase –
Alignment of the
chromosomes along
center of cell
(metaphase plate).
 Fibers attached to
kinetochores on
both sides of each
chromosome.
Cell Division
 Anaphase –
Separation of the
sister chromatids.
 Centromere splits
apart – sister
chromatids move
toward opposite
poles.
 Disassembly of the
tubulin subunits
shortens the
microtubules.
Cell Division
 Telophase – reformation of the
nuclei once the
chromosomes are at
opposite poles.
 Chromosomes
unwind.
Cell Division
 Cytokinesis –
division of the
cytoplasm.
 Two complete,
diploid cells that are
identical to the
original cell.
Cytokinesis
 During cytokinesis
in animal cells, the
cell pinches in two.
 A cleavage furrow
produced by
microfilaments
deepens until the
cell splits.
http://www.youtube.com/watch?v=VGV3fv-uZYI