Cell
Structure
and
Membrane
Function
Correlates with Chapters 6 & 7 in
Campbell & Reece Biology
1
Points to ponder:
• What is a cell?
• Why are most cells small?
• What do prokaryotic and eukaryotic cell
have in common?
• How are cells organized?
• How do things move across the plasma
membrane?
• What is cellular respiration?
2
Cellular level organization
• Cell = fundamental
unit of life
• Some organisms =
1 cell only
• Large organisms =
trillions of cells
Cellular level organization
• All cells contain
mostly the same
components
• Specialized
organelles
perform
processes
needed for life
3.1 What is a cell?
What does the Cell Theory tell us?
• A cell is the basic unit of life
• All living things are made up of cells
• New cells arise from preexisting cells
1660’s
The cell was first discovered by Robert Hooke.
Living cells - Anton van Leewenhoek; earliest microscope
1830’s
Matthias Schleiden
Theodor Schwann
Rudolf Virchow
5
Compound Light Microscope
• May have one or two
eyepieces to look
through
• 10x ocular lens in the
eyepiece
• Rotating nosepiece
has low (scanning),
medium, high power
objective lenses (e.g.
4x, 10x, 40x)
• Total magnification=
ocular x objective
6
Stereomicroscope (Binocular
Dissecting Microscope)
• Used for relatively
low power
examination of (1)
large or (2) whole
specimens (opaque,
light does not have
to pass through)
• Three dimensional
images
7
Microscopy: Two things to take into
account:
Magnification
• The process of enlarging
the size of something, as
an optical image
• Needed to see the detail
in very small things
Resolving power,
or “resolution”
• ‘Resolving power' describes
the shortest distance that is
found between two
specimens that can be
distinguished as separate.
• The ability to clearly
determine two closely
placed objects.
• Affects clarity, ability to
focus
• Poor resolution: the objects
or points will blur together. 8
3.1 What is a cell?
What are some common
microscopes used to view cells?
• Compound light microscope
– Lower magnification
– Uses light beams to view images
– Can view live specimens in 2-D
• Transmission electron microscope
– 2-D image
– Uses electrons to view internal structure
– High magnification, no live specimens
• Scanning electron microscope
– 3-D image
– Uses electrons to view surface structures
– High magnification, no live specimens
9
How small are cells?
• Most cells are measured in micrometers
– µm
– One millionth of a meter;1×10−6 m,or 1⁄1,000,000 m
– Viruses are measured in nanometers (nm), a billionth
of a meter!
10
3.1 What is a cell?
Why are most cells small?
• Consider the cell surface-area-to-volume
ratio:
– Small cells have a larger amount of surface
area compared to the volume
– An increase in surface area allows for more
nutrients to pass into the cell and wastes to
exit the cell more efficiently
– There is a limit to how large a cell can be and
be an efficient and metabolically active cell;
lots of Volume means lots to feed, etc.
11
3.1 What is a cell?
Thinking about surface area to volume
in a cell
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1 mm
Surface area
(square mm)
Volume
(cubic mm)
Surface area
Volume
2 mm
6 ´ 1 mm2 = 6 mm2
6 ´ 4 mm2 = 24 mm2
(1 mm)3 = 1 mm3
(2 mm)3 = 8 mm3
6
1
24 =
8
3
1
12
3.2 How cells are organized
What are the two major types of
cells in all living organisms?
• Prokaryotic cells
– Thought to be the first
cells to evolve
– Lack a nucleus
– Represented by bacteria
and archaea
• Eukaryotic cells
– Have a nucleus that
houses DNA
– Many membrane-bound
organelles
13
3.2 How cells are organized
What do prokaryotic and eukaryotic
cells have in common?
• A plasma membrane that surrounds and
delineates the cell
• Cytoplasm that is the semi-fluid portion inside
the cell that contains organelles
• DNA
• Ribosomes
14
3.2 How cells are organized
Where did eukaryotic cells come from?
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Original
prokaryotic cell
The
Endosymbiosis
Hypothesis
DNA
1. Cell gains a nucleus by the
plasma membrane invaginating
and surrounding the DNA
with a double membrane.
2. Cell gains an endomembrane
system by proliferation
of membrane.
3. Cell gains protomitochondria.
protomitochondrion
4. Cell gains
protochloroplasts.
mitochondrion
protochloroplast
chloroplast
15
Animal cell
Plant cell
3.2 How cells are organized
What do eukaryotic cells look like?
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction
or display.
plasma
membrane
nuclear
envelope
nucleolus
chromatin
endoplasmic
reticulum
Plasma membrane:
outer surface that
regulates entrance and
exit of molecules
protein
50 nm
phospholipid
NUCLEUS:
Nuclear envelope: double
membrane with nuclear pores
that encloses nucleus
Chromatin: diffuse threads
containing DNA and protein
Nucleolus: region that produces
subunits of ribosomes
CYTOSKELETON: maintains
cell shape and assists movement
of cell parts:
Microtubules: cylinders of
protein molecules present
in cytoplasm, centrioles,
cilia, and flagella
Intermediate filaments:
protein fibers that
provide support
and strength
Actin filaments: protein
fibers that play a role in
movement of cell
and organelles
ENDOPLASMIC RETICULUM:
Rough ER: studded with
ribosomes, processes proteins
Smooth ER: lacks
ribosomes, synthesizes
lipid molecules
Ribosomes:
particles that carry
out protein synthesis
Centrioles: short
cylinders of microtubules
of unknown function
Centrosome: microtubule
organizing center that
contains a pair of centrioles
Mitochondrion: organelle
that carries out cellular respiration,
producing ATP molecules
Lysosome: vesicle that
digests macromolecules
and even cell parts
Polyribosome: string of
ribosomes simultaneously
synthesizing same protein
Vesicle: membrane-bounded
sac that stores and transports
substances
Cytoplasm: semifluid
matrix outside nucleus
that contains organelles
Golgi apparatus: processes, packages,
and secretes modified cell products
© Alfred Pasieka/Photo Researchers, Inc.
16
What are some characteristics of
the plasma membrane?
• It is a phospholipid
bilayer
• It is embedded with
proteins that move in
space
• It contains cholesterol
for support
• It contains
carbohydrates on
proteins and lipids
• Selectively permeable
17
3.3 The plasma membrane and how substances cross it
What does selectively permeable mean?
• The membrane allows some things in while keeping
other substances out
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
charged molecules
and ions
-+
H2O
aquaporin
noncharged
molecules
+
-
macromolecule
phospholipid
molecule
protein
18
3.3 The plasma membrane and how substances cross it
How do things move across the
plasma membrane?
1. Diffusion
2. Osmosis
3. Facilitated
transport
4. Active transport
5. Endocytosis and
exocytosis
19
Passive Transport: Diffusion and
Osmosis
Diffusion is the random
movement of molecules
from a higher
concentration to a lower
concentration .
How Diffusion Works - narrated animation
– This can be through the air (ex: the smell of coffee wafting
through your house), or…
– Through a liquid (ex: tea bag steeping in a mug of water), or…
– Molecule passing through the cell membrane pores, until they
are evenly spread out, or until a state of equilibrium is reached.
20
Facilitated Diffusion
• Facilitated transport is the transport of
molecules across the plasma membrane from
higher concentration to lower concentration via a
protein carrier embedded in the membrane
– Useful when molecules are large, or hydrophilic
How Facilitated Diffusion works – narrated animation
21
OSMOSIS
• Osmosis is the diffusion of water
molecules across a semi-permeable
membrane…
– from an region of high water concentration to an
region of lower concentration
How Osmosis
Works –
narrated
animation
22
How does tonicity change a cell?
Rarely, in nature do
we fine PURE water.
It usually has solutes
dissolved in it. We
refer to the amount
of solute in water by
its “tonicity”.
Turgor pressure: the pressure exerted on a plant cell wall by water
passing into the cell by osmosis Also called hydrostatic pressure.
Water potential: the tendency of water to leave one place in favor of
another. Water always moves from an area of higher water potential to an
area of lower water potential.
23
3.3 The plasma membrane and how substances cross it
How does tonicity change a cell?
• Hemolysis and
Crenation: narrated
animation
• Hypertonic solutions have more solute than the inside
of the cell = water will osmose OUT of cell and lead to shrinking (crenation,
or plasmolysis)
• Hypotonic solutions have less solute than the inside of the cell = water will
osmose INTO cell and lead to swelling, and possibly cytolysis (bursting)
• Isotonic solutions have equal amounts of solute inside and outside the cell
and thus does not affect the cell
24
Tonicity
be patient and wait and watch…
25
Self-quiz: what is being demonstrated
in this picture?
26
Same principle can be used for
water purification
27
3.3 The plasma membrane and how substances cross it
What is Active Transport?
• Active transport is the
movement of molecules
from a lower to higher
concentration
• Because molecules are
being moved against a
concentration gradient…
• …it requires ENERGY
expenditure on the part
of the cell; using ATP as
energy
• Requires a protein
carrier to move the
molecules
Outside
K+
K+
K+
P
ATP
ADP
K+
Inside
K+
28
The Sodium-Potassium Pump
• An example of active transport
• In order to maintain the cell membrane potential and osmolarity cells
must keep a low concentration of sodium ions and high levels of
potassium ions within the cell (intracellular).
• Outside cells (extracellular), there are high concentrations of sodium
and low concentrations of potassium, so diffusion occurs through ion
channels in the plasma membrane.
• In order to keep the appropriate concentrations, the sodiumpotassium pump pumps sodium out and potassium in through active
transport.
• This requires the hydrolysis of ATP (energy)
•
Narrated animation “How the Sodium Potassium Pump Works”
29
30
Nerve cells
31
Resting potential vs
Action Potential in a
Neuron
32
3.3 The plasma membrane and how substances cross it
•
What are endocytosis and
exocytosis?
Endocytosis
transports molecules or
cells into the cell via
invagination of the
plasma membrane to
form a vesicle (storage
compartment)
• Exocytosis transports
molecules outside the cell
via fusion of a vesicle
with the plasma
membrane
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Outside
Inside
substances
taken in
vesicle
a. Endocytosis
Outside
substances
released
vesicle
Narrated animation: Endocytosis
and Exocytosis
Inside
b. Exocytosis
33
Phagocytes
• Phagocytes are white blood cells that engulf and
break down (enzymatically) invasive organisms
in your body.
Narrated
animation:
Phagocytosis
34
Aside from the Plasma Membrane
and its importance, what other
structures would you find in
eukaryotic cells?
35
The Extracellular Matrix (ECM) of Animal
Cells
• Animal cells
– Lack cell walls
– Are covered by an elaborate matrix, the ECM made up
of glycoproteins and other macromolecules
• Functions of the
ECM include
– Support
– Adhesion
– Movement
– Regulation
Intercellular Junctions
• A cell junction is a structure within a
tissue of a multicellular organism.
• Cell junctions are especially abundant in
epithelial tissues.
• They consist of protein complexes and
provide contact between neighboring cells,
between a cell and the extracellular matrix,
or control the paracellular transport.
Animals: Tight Junctions, Desmosomes, and Gap
Junctions
• In animals, there are three types of
intercellular junctions
– Tight junctions
– Desmosomes
– Gap junctions
•Invertebrates have several other types of specific junctions, for
example Septate junctions or C. elegans apical junction.
• Types of intercellular junctions in animals
Desmosomes act like spot
welds to hold together
tissues that undergo
considerable stress (such
as skin or heart muscle).
Tight junctions are tightly
stitched seams between
cells. The junction
completely encircles each
cell, preventing the
movement of material
between the cell. Tight
junctions are characteristic
of cells lining the digestive
tract, where materials are
required to pass through
cells (rather than
intercellular spaces) to
penetrate the bloodstream.
Gap junctions are narrow tunnels
between cells that consist of proteins
called connexons. The proteins allow
only the passage of ions and small
molecules. In this manner, gap junctions
allow communication between cells
through the exchange of materials or the
transmission of electrical impulses.
Cell Wall
• Animal cells do not
have cell walls, nor do
most Protozoans
• All plant cells have
primary cell wall
composed of cellulose
• Some plant cells have
secondary cell wall
composed of lignin for
even greater strength
Plants: Plasmodesmata
• Plasmodesmata
– Are channels that perforate plant cell walls
– Allow for movement of ions, small molecules like sugars
and amino acids, and even macromolecules like RNA
and proteins, between cells
Cell walls
Interior
of cell
Interior
of cell
Figure 6.30
0.5 µm
Plasmodesmata
Plasma membranes
Nucleus
• Surrounded by a
double membrane
• Holds and protects the
DNA
• DNA inside stores
genetic information in
the form of genes
• Normally can’t see
DNA because its
“unspooled” into
thread-like chromatin
Nucleus, continued
– DNA is usually pictured
as forming distinctly Xshaped chromosomes
– Chromatin coils into
chromosomes right
when cells are about to
divide
– Most of time DNA is
uncoiled into long, thin
chromatin molecules
– Looks grainy under
microscope
Nucleus, continued
– Every cell in an
organism has identical
DNA
– DNA codes for many
different traits
– Which genes (recipes)
are activated depends
on cell
– When a gene is
activated it sends mRNA
(m is for messenger)
into cytoplasm resulting
in construction of
proteins
Nucleus, continued
• Nucleus Continued
– Most cell nuclei have
region of darker material the nucleolus
– DNA there produces rRNA
– rRNA joins with proteins in
cytoplasm to make
ribosomes
– Nuclear membrane thus
needs passageways
(nuclear pores) through it
to allow stuff to enter and
leave the nucleus
What is the structure and function of
ribosomes?
• Organelles made of
RNA and protein
• May be (1) bound to the
endoplasmic reticulum
or (2) free floating in the
cell
• Site of protein synthesis
3.4 The nucleus and the production of proteins
What is the endomembrane
system?
• A series of membranes in which molecules
are transported in the cell
• It consists of the nuclear envelope,
endoplasmic reticulum, Golgi apparatus,
lysosomes and vesicles
47
Endoplasmic reticulum (ER)
– Complex system of
transport canals, made of
folded membranes
– Continuous with outer
nuclear membrane
– Rough ER has many
ribosomes attached to it
• Proteins made in ribosomes
enter ER channels and are
modified for transport out of
cell
• Sent to Golgi bodies for
packaging into vesicles for
shipping to elsewhere
Smooth ER
• Lacks ribosomes but
aids in making
carbohydrates and
lipids
– Manufactures the
phospholipid molecules
used to build cell
membranes
– Builds certain hormones
(like testosterone)
– Helps with detoxification
of poisons in the liver
Endomembrane System, continued
• Golgi Apparatus
– Stack of 3-20 slightly
curved folded sacs
– process, package and
deliver proteins and
lipids from the ER
– Either sends them to
cell membrane for
secretion, or it
repackages molecules
into vesicles
Endomembrane System, continued
• Lysosomes
– Membranous sacs made by the Golgis
– Filled with powerful digestive enzymes
– Used to digest large molecules brought into cell
– Ex: white blood cell ingests bacteria, wrapping them
up in vesicles, then their lysosomes fuse with
vesicles, digesting contents
– Enzymes can also be released into cell itself,
resulting in autolysis (death) = “suicide sacs”
– May play a role in APOPTOSIS (programmed cell
death (see
article:http://www.jbc.org/content/284/33/21783#sec10)
LYSOSOMES: narrated animation
• Vacuoles and vesicles
– Vacuoles and vesicles may be
used to store and /or transport
substances (water, waste,
food, enzymes)
– Vacuoles: large membranous
sacs
• Plant cell vacuoles are much
larger and more prominent
• Large central vacuole takes up
most of space in plant cell,
TURGOR
– Vesicles: smaller sacs
• Animal cells
More eukaryotic organelles
• Peroxisomes
– Also membranous sac filled
with enzymes
– Contain enzymes that that
oxidize very long chain fatty
acids (VLCFA) into hydrogen
peroxide
– Hydrogen peroxide immediately
broken down into oxygen and
water by other enzymes
(catalase)
Energy-Related Organelles
• Chloroplasts
– Photosynthesis happens here
– Surrounded by two membranes
– Inside are membranous sacs,
thylakoids, bathed in fluid
stroma
– Chlorophyll molecules in
thylakoid membranes capture
light energy
– Use absorbed solar energy to
convert CO2 and water into
carbohydrates, giving off oxygen
as waste gas
3.6 Mitochondria and cellular metabolism
Energy-Related Organelles
Mitochondria
• A highly folded organelle in
eukaryotic cells
• Produces energy in the
form of ATP
• Glucose (carb) is
converted into ATP (usable
energy molecules) to
power all of an organism’s
metabolic needs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
outer
membrane
intermembrane
space
inner
membrane
200 nm
matrix
cristae
– Cellular respiration
Carbohydrate + O2  CO2 + H20 +ATP
55
© Dr. Don W. Fawcett/Visuals Unlimited
3.5 The cytoskeleton and cell movement
What is the cytoskeleton?
• A series of protein filaments that maintain
cell shape as well as anchors and/or
moves organelles in the cell
• Made of 3 types of fibers:
• large microtubules (tubulin polymers, average length
of 25 µm)
• thin actin filaments
• medium-sized intermediate filaments
56
• Cytoskeleton
– Interconnected
network of filaments
and tubes running
between nucleus and
plasma membrane
– Maintains cell’s shape
– Allows organelles to
move within cell
– Sometimes makes cell
move
3.5 The cytoskeleton and cell movement
What are cilia and flagella?
• Both are made of
microtubules
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Flagellum
microtubules
• Both are used in
movement
cilia
sperm
plasma
membrane
flagellum
secretory cell
a.
b.
• Cilia are about 20x
shorter than flagella
flagellum
c.
58
Both cilia and flagella use the 9+2
arrangement of microtubules to generate
movement.
http://www.northland.cc.mn.us/biology/biology1111/animations/flagellum.html
59
CENTRIOLES: also made
of microtubules; 9+3
arrangement. Used to
separate chromosomes
during cell division
(spindle).
A centriole is a barrelshaped cell structure found
in most animal eukaryotic
cells, though absent in
higher plants and most
fungi.
The walls of each centriole are usually
composed of nine triplets of microtubules
(protein of the cytoskeleton).
60