UNIT 3 – CELL BIOLOGY
THEMES COVERED:
1. SCIENCE AS A PROCESS: The discovery and early study of cells
progressed with the invention and improvement of the
microscopes.
2. EVOLUTION: The matching machinery of all eukaryotic cells
evidences a broad evolutionary connection between eukaryotes.
5. RELATIONSHIP OF STRUCTURE TO FUNCTION: Many of the cell
organelles show clear correlation between structure and
function.
8. SCIENCE AND TECHNOLOGY, AND SOCIETY: Advances in cancer
research depend on progress in our basic understanding of how
cells work.
CHAPTER 6 – A TOUR OF THE CELL
OBJECTIVE QUESTIONS:
How We Study Cells
1. Distinguish between magnification and resolving power.
2. Describe the principles, advantages, and limitations of the light
microscope, transmission electron microscope, and scanning electron
microscope.
3. Describe the major steps of cell fractionation and explain why it is a useful
technique.
A Panoramic View of the Cell
4. Distinguish between prokaryotic and eukaryotic cells.
5. Explain why there are both upper and lower limits to cell size.
6. Explain the advantages of compartmentalization in eukaryotic cells
The Nucleus and Ribosomes
7. Describe the structure and function of the nuclear envelope, including the
role of the pore complex.
8. Briefly explain how the nucleus controls protein synthesis in the
cytoplasm.
9. Explain how the nucleolus contributes to protein synthesis.
10. Describe the structure and function of a eukaryotic ribosome.
11. Distinguish between free and bound ribosomes in terms of location and
function.
The Endomembrane System
12. List the components of the endomembrane system, and describe the
structure and functions of each component.
13. Compare the structure and functions of smooth and rough ER.
14. Explain the significance of the cis and trans sides of the Golgi apparatus.
15. Describe three examples of intracellular digestion by lysosomes.
16. Name three different kinds of vacuoles, giving the function of each kind.
Other Membranous Organelles
17. Briefly describe the energy conversions carried out by mitochondria and
chloroplasts.
18. Describe the structure of a mitochondrion and explain the importance of
compartmentalization in mitochondrial function.
19. Distinguish among amyloplasts, chromoplasts, and chloroplasts.
20. Identify the three functional compartments of a chloroplast. Explain the
importance of compartmentalization in chloroplast function.
21. Describe the evidence that mitochondria and chloroplasts are
semiautonomous organelles.
22. Explain the roles of peroxisomes in eukaryotic cells.
The Cytoskeleton
23. Describe the functions of the cytoskeleton.
24. Compare the structure, monomers, and functions of microtubules,
microfilaments, and intermediate filaments.
25. Explain how the ultrastructure of cilia and flagella relates to their
functions.
Cell Surfaces and Junctions
26. Describe the basic structure of a plant cell wall.
27. Describe the structure and list four functions of the extracellular matrix in
animal cells.
28. Explain how the extracellular matrix may act to integrate changes inside
and outside the cell.
29. Name the intercellular junctions found in plant and animal cells and list
the function of each type of junction.
I. OVERVIEW (The Cell Theory):
 All organisms are made up of cells
 Cells are the simplest collection of matter that can live – the basic units of
structure and function in living organisms
 All cells are related by their descent from earlier cells, however, they were
modified in may different ways during the long evolutionary history of life
II. MICROSCOPY AND OTHER TOOLS
A. Light microscope:
 Visible light is passed through the specimen and then through glass
lenses. The lenses refract the light that he image is magnified and
projected into the eye or onto a photographic film or digital sensor.
YOU MUST KNOW THE PARTS OF THE MICROSCOPE





Two important parameters of light microscopes:
o Magnification – the ratio of an object’s image size to its real size
(to get it, multiply the magnification of the objective lens and the
eyepiece) – light microscopes are most effective up to X1000 of
magnification
o Resolution – the measure of the clarity of the image (the
minimum distance that still distinguishes two points as separate).
The maximum resolution of light microscopes is about 200 nm.
B. Electron microscope (EM)
Focuses a beam of electrons through a specimen or onto its surface and
is able to have a resolution of about 0.002 nm.
The two basic types of electron microscopes:
o Scanning electron microscopes (SEM) – excellent to study the
surface of the specimen that has to be coated with gold. Provides
a three dimensional images of dead specimens.
o Transmission electron microscope (TM) – Excellent device to
study the internal structure of the specimen that must be stained
with heavy metals. The specimen also must be dead. Gives twodimensional images.
C. Cell Fractionation (Centrifuges)
Cell Fractionation – taking cells apart and separate the major
organelles from one another by their different densities and size.
This procedure is done by centrifuges. Ultracentrifuges are the most
powerful ones of these machines that can apply forces that are 1 million
times the force of gravity.
III. PROKARYOTES AND EUKARYOTES
 All cells contain the same general features such as a plasma membrane,
cytosol where the organelles are found, chromosomes that carry DNA
and all of them have ribosomes to perform protein synthesis.
 Three structural units are found in every cell:
o Plasma membrane
o Nucleus (nucleoid)
o Cytoplasm
 Prokaryotes – single celled organisms in which the DNA is concentrated
around a nucleoid region but they are lacking a membrane that
separates the DNA from the rest of the cell. Many other organelles are
also missing. Size: 1-10 μm. Two domains of prokaryotes are Bacteria
and Archaea.
 Eukaryotes – have true nuclei that are bounded by a nuclear envelope.
The region between the nucleus and the cell membrane is called

cytoplasm (prokaryotes also have it). These cells also have a large
number of organelles. Size: 10 – 100 μm.
The size differences are the result of the various metabolic requirements.
Cells cannot grow larger than the speed of gas exchange and nutrient –
waste exchange between the border and the inside of the cell.
Organelles’ compartmentalization helps this process in eukaryotes, so
they can have a larger cell. The surface to volume ratio is a factor that
will limit cell size.
IV. THE NUCLEUS
 The nucleus contains most of the genes of an eukaryotic cells (some
genes are found in the mitochondria and chloroplasts)
 It is enclosed by the nuclear envelope – a double membrane, each with a
phospholipid bilayer and proteins. The envelope also contains pores that
are lined with a pore complex of proteins. This complex regulates what
enters and leaves the cell.
 The nuclear side of the envelope is lined by a nuclear lamina (network
of protein filaments) that extend inward into the nuclear matrix.
 Chromosomes – tightly packed DNA are found in the nucleus (46 for
humans or 23 pairs, egg and sperm cells have half)

Nucleolus – densely stained granules and fibers in the center of a
nondividing nucleus that assembles rRNA and its protein components to
form the large and small subunit of ribosomes.
V. RIBOSOMES
 Very small particles that are made up of rRNA and proteins. They are
assembled from a small and large subunit.
 Some ribosomes are free floating in the cytoplasm while others are bound
to the nuclear envelope or to the endoplasmic reticulum. Free ribosomes
make proteins that function in the cytoplasm, while bound ribosomes
make proteins that either are attached to membranes or are packaged
into membrane structures.
VI. THE ENDOMEMBRANE SYSTEM
 Endomembrane system – many different membranes in the cytoplasm
of an eukaryotic cell that carry out a wide range of functions. Each
membrane is related to the others by either direct contact or by exchange
of materials through vesicles
 This system includes the endoplasmic reticulum, nuclear envelope, Golgi
apparatus, lysosomes, vacuoles, and even the cell membrane
A. The Endoplasmic Reticulum


An extensive network of tubules and sacs (cisternae) that is continuous
with the nuclear envelope
Rough endoplasmic reticulum (RER) – exists as flattened, fluid-filled,
membrane sacs that are interconnected. Its appearance is due to the
large number of ribosomes on its surface. Almost all of the proteins of
the cell are entered through a pore into the lumen of the RER where they
are folded into their 3D shape and other nonprotein parts are attached to
them. RER also provides catalytic surfaces for some of the chemical
activities of the cell. The proteins are than packaged into transport
vesicles and moved to various parts of the cell or out of the cells
(secretory proteins). The rough ER is also the main membrane factory of
the cell where membrane proteins and phospholipids are made.

Smooth endoplasmic reticulum (SER) – lacks ribosomes and has a
more tubular surface. Participates in synthesis of lipids (oils,
phospholipids, sterols), metabolism of carbohydrates, detoxification of
drugs and poisons by making them more water soluble so they can be
easily flushed through the body. SER is also important in storing calcium
ions that are vital for normal nerve and muscle function.
B. The Golgi Apparatus





Vesicles ship many of the products of the endoplasmic reticulum here for
further processing. This is the manufacturing, storing and shipping center
of the cell. Secretory cells are especially rich in Golgi apparatus.
It is made up of a stack of flattened membranous sacs (cisternae) that are
surrounded by transport vesicles.
The Golgi apparatus has a distinct polarity because of its different
molecular composition. The cis face of the apparatus is the receiving end
that is located near the ER. Vesicles coming from the ER empty their
contents on the cis end.
Molecules are modified during their trip from the cis to the trans end of
the Golgi apparatus. Molecules that are modified here include
carbohydrates, phospholipids and membrane proteins. Also molecular
identification tags are frequently added to molecules here.
The trans end is the shipping end of the Golgi that gives rise to new
vesicles and ship molecules to various other parts of the cell.

Some polysaccharides (pectin) that are released by the cell are also made
here in the Golgi apparatus.
C. Lysosomes



A lysosome is a membraneous sac of hydrolytic enzymes that an animal
cell uses to digest all kinds of macromolecules. These enzymes work best
in an acidic environment. Large amount of these enzymes leaking out
into the cytoplasm can destroy the cell.
Lysosomes carry out intracellular digestion for a variety of reasons:
o Digest food particles taken in by phagocytosis
o Break down old cell organelles and recycle some of their
components – autophagy
Some diseases result when the lysosomes lack hydrolytic digestion
enzymes and the cell overcomes with indigestible substances (Tay-Sachs
disease)
D. Vacuoles




Plant and fungi cells have one or several vacuoles.
Food vacuoles – formed by phagocytosis
Contractile vacuoles – pump excess water out of the cell to maintain
stable water and salt balance (unicellular animals can have it as well)
Central vacuoles – found in mature plant cells and enclosed by a
membrane called tonoplast. The large central vacuole forms from the
fusion of smaller vacuoles. Because of the selective permeability of the
tonoplast, the vacuole has a different solute composition than the
cytoplasm. The central vacuole can act as a storage compartment of
organic or inorganic substances, can be a pigment storage place and
result in various colors of flowers or can assemble poisons to protect the
plant. Their enlargement can grow plant cells.
VII. ENERGY PROCESSING ORGANELLES
 In eukaryotic cells mitochondria and chloroplasts are the organelles
that convert energy to forms that cells can use for work. Both of these
organelles are enclosed by a double membrane system and most of their
proteins are made by free moving ribosomes that are not attached to the

ER or by ribosomes that are inside of these organelles. They both also
have their own DNA that program the synthesis of their proteins.
Peroxisomes – oxidative organelles that are also not part of the
endomembrane system
A. Mitochondria



Found in almost all eukaryotic cells, their numbers correlate to the cell’s
level of metabolic activity. They are actively moving and dividing
organelles that also change shape easily and frequently.
Label and draw the structure by using the figure above
The matrix of the mitochondrion contains many different enzymes that are
mostly related to cellular respiration including ATP synthase, an enzyme
that makes ATP molecules and is imbedded into the inner membrane.
B. Chloroplasts





Part of the family of plastids:
o Amyloplast – stores starch
o Chromoplast – stores colored pigments of fruits and flowers
o Chloroplast – contains the green pigment chlorophyll,
photosynthetic enzymes and other molecules that are necessary for
photosynthesis.
Label and draw the structure by using the picture above
The fluid outside the thylacoids contains enzymes, DNA.
Chloroplasts are the main organs of photosynthesis.
They are also actively moving in the cell, changing shape and divide.
C. Peroxisomes
 A single membrane organelle that contains powerful oxidative enzymes
that transfer hydrogen from various substances to oxygen and produce
hydrogen peroxide.
 They can break fatty acids down, before the breakdown products enter
the mitochondria for cellular respiration, they can detoxify alcohol and
other poisons.
 They contain catalase enzyme that breaks down H2O2 to eventually
produce water.
VIII. THE CYTOSKELETON
 Cytoskeleton is a network of fibers that extend throughout the
cytoplasm. It plays a major role in organizing the structures and activities
of the cell.
 It is composed of three kinds of structures:
o Microtubules

o Microfilaments
o Intermediate filaments
Functions of the cytoskeleton:
o Supports the cell and maintains its shape
o Provides anchorage for many cell organelles and enzyme molecules
o Involved in many kinds of movements of the cell itself or parts of it.
They work together with motor proteins to accomplish this motion
o They also perform the streaming of the cytoplasm
o Regulate biochemical processes in the cell
A. Microtubules






Found in the cytoplasm of all eukaryotic cells
Hollow rods, 25 nm in diameter and about 200nm – 25 μm in length
Their wall is constructed from a globular protein called tubulin that is
composed of two different polypeptide chains (dimmers)
These tubulin dimmers can be taken apart and rearranged again in a new
location in the cell
Microtubules shape and support the cell and form tracks to move motor
proteins
Centrosomes and centrioles – the centrosome is a microtubule
organizing center. In the centrosome of animal cells are a pair of
centrioles that are each composed of nine triplets of microtubules – these
duplicate before the cell divides


Cilia (sing. Cilium) and Flagella (sing. Flagellum) – Located outside
of the cells, these organelles move the cell or can move substances on the
surface of the cell. Found in many unicellular organisms such as Euglena
(flagellum) and Paramecium (cilia) or in many cells of multicellular
organisms (sperm cells, cells of the oviduct, windpipe etc). Cilia are
usually short and there are many of them on the cell’s surface while
flagella are fewer but longer. Cilia has a back-and-force motion while
flagella has an udulating motion.
The ultrastructure of cilia and flagella are the same. They have a core of
9 pairs + 2 single central microtubules that are covered by an extension
of the plasma membrane. This arrangement of microtubules is uniform in
eukaryotes but different in prokaryotes. Flexible proteins connect the
pairs of microtubules to each other like wagon-wheels. The cilium and
flagellum are anchored in the cell by a basal body which is identical
structurally to the centriole (it enters the egg from the sperm and
becomes the centriole of the developing embryo). The protein that
extends from one pair of microtubules to the next is called dynein, which
is a complex protein with several polypeptide chains. Dynein proteins
bend the cilia and flagella microtubules by using ATP and cause the
movement of these organelles.
Watch:
http://programs.northlandcollege.edu/biology/Biology1111/animations/flagell
um.html
B. Microfilaments
 Solid rods, about 7 nm in diameter
 They are built of the globular protein called actin
 The actin molecules form two long chains that twist together. Some
other proteins can form cross bindings between the actin molecules so
this way microfilaments can form networks as well.
 The 3D network of actin filaments help to support the shape of the
cell. They also make up the core of microvilli that help to enlarge the
cell’s surface for making transport of materials more efficient.
 Microfilaments also form the contractile structure of muscles where
they are connected to a thicker protein called myosin.
 The contraction of the actin-myosin complex is also important in
amoeboid movement (pseudopods) and in cytoplasmic streaming
Figure 6.27
Cytoplasmic streaming: http://www.youtube.com/watch?v=6hJ_i_-K--k
Amoeboid movement: http://www.youtube.com/watch?v=7pR7TNzJ_pA
C. Intermediate Filaments:
 Their diameter is 8 – 12 nm
 They are specialized for bearing tension, they reinforce the structure of
cells and keep organelles in position, they also make up the nuclear
lamina
 They are formed from a wider range of proteins and are more permanent
fixtures in the cell.
IX. EXTRACELLULAR COMPONENTS
A. Cell Walls of Plants
 This extracellular structure is not found in animals. In plants the cell wall
protects the cell, maintains its shape, prevents excessive water uptake.
The cell wall also holds the entire plant against the force of gravity
 Fungi, protists, prokaryotes also have cell walls but they are different in
composition
 Although the composition of the plant cell wall can vary from species to
species, the general structure is basically the same among all plants.
Microfibrils of cellulose are imbedded into a matrix that is made up of
other polysaccharides and proteins compose the main structure of the cell
wall.
 Primary cell wall – created in young cells that are still growing. Thin
wall that is fairly flexible.
 Adjacent cells are held together by a middle lamella that is a thin layer
of polysaccharides called pectins. This middle lamella glues cells together.
 Secondary cell wall – produced when the cell stops growing and the
cell deposits several layers of durable matrix (forms wood).
Figure 6.28
B. Extracellular Matrix of Animal Cells
 A network of glycoproteins that are excreted by the cells. Types of
glycoproteins:
o Collagen – forms strong fibers outside the cells.
o Fibronectin – they attach to integrins (receptor proteins in the
plasmamembrane) and this complex will transmit changes between
the inside and outside of the cell. With these interactions integrins
and fibronectins regulate the cell’s behavior.
Figure 6.29
X. INTERCELLULAR JUNCTIONS:
 Neighboring cells often interact and communicate with each other through
special patches of direct physical contact.
 Plants have plasmodesmata – thin channels of cytoplasm between the
cell walls. This continuous channel of cytoplasm unifies the plant into one
living organism. Water and small molecules are able to pass through
plasmodesmata freely, but even some specific proteins and RNA can also
move through by moving along fibers of the cytoskeleton.
 Animal cells have tight junctions, desmosomes and gap junctions:
o Tight junctions – specific proteins tightly press the cell
membranes of neighboring cells together
o Desmosomes – intermediate keratin filaments anchor these rivers
of cytoplasm together
o Gap junctions – pores form by special proteins that allow various
ions, sugars, amino acids and other small molecules pass through
the cell membrane. These pores are important in cell
communication.
Figure 6.31
CHAPTER 7 – MEMBRANE STRUCTURE AND FUNCTION
OBJECTIVE QUESTIONS
Membrane Structure
1. Explain why phospholipids are amphipathic molecules.
2. Describe the fluidity of the components of a cell membrane and explain
how membrane fluidity is influenced by temperature and membrane
composition.
3. Explain how cholesterol resists changes in membrane fluidity with
temperature change.
Traffic Across Membranes:
4. Distinguish between peripheral and integral membrane proteins.
5. List six major functions of membrane proteins.
6. Explain the role of membrane carbohydrates in cell-cell recognition.
7. Explain how hydrophobic molecules cross cell membranes.
8. Distinguish between channel proteins and carrier proteins.
9. Define diffusion. Explain why diffusion is a spontaneous process.
10. Explain why a concentration gradient of a substance across a membrane
represents potential energy.
11. Distinguish among hypertonic, hypotonic and isotonic solutions.
12. Define osmosis and predict the direction of water movement based on
differences in solute concentrations.
13. Describe how living cells with and without cell walls regulate water
balance.
14. Explain how transport proteins facilitate diffusion.
15. Distinguish among osmosis, facilitated diffusion, and active transport.
16. Describe the two forces that combine to produce an electrochemical
gradient.
17. Explain how an electrogenic pump creates voltage across a membrane
18. Describe the process of cotransport.
19. Explain how large molecules are transported across a cell membrane.
20. Distinguish between pinocytosis and receptor-mediated endocytosis.
I.
OVERVIEW
 The plasma membrane is the outer boundary of the cell that separates the
cell from its nonliving environment. It controls traffic into and out of the
cell.
 It is semipermeable so it allows some substances to cross easily while
does not allow others. With this ability, the cell membrane creates a
different environment inside the cell than outside.
II.
THE STRUCTURE OF THE CELL MEMBRANE:
 The cell membrane is made up of phospholipids, proteins, carbohydrates
and sterols.
 Because phospholipids and many proteins are amphipathic molecules
(have both hydrophobic and hydrophilic areas) they form the main body
of the cell membrane. The fluid mosaic model describes the
arrangement of phospholipids and proteins in the cell membrane.
 The main body of the membrane is formed by the hydrophobic attractions
between the nonpolar parts of the phospholipids molecules that form a
double layer. Within this semifluid double lipid layer the protein molecules
are imbedded by their similar hydrophobic attractions and are able to
move sideways in it. It is however, very rare that the proteins move from
one layer to the next.
A. Phospholipids
 The phospholipids molecules can also move sideways rapidly and can also
flip-flop from one layer to the next but less frequently. The fluidity of the
membrane depends on the temperature (the higher the temperature the
more fluid the membrane), the number of unsaturated fatty acids in the
chain (the more unsaturated fatty acids the phospholipids have the more
fluid the membrane is) and the concentration of cholesterol molecules (on
moderate temperatures, cholesterol makes the membrane less fluid, but
on lower temperatures it slows down the solidification of the membrane).

The membrane has to be liquid to function properly because its
permeability changes and enzymes in the membrane become inactive.
There are many animal adaptations that make them resistant to low
temperatures (higher unsaturated fat and cholesterol concentrations).
B. Membrane Proteins
 Membrane proteins are embedded in the fluid matrix. These proteins
determine the membrane’s specific functions. There are two major types
of proteins in the cell membrane:
o Integral proteins – penetrate the hydrophobic part of the
membrane completely and reach across the entire membrane.
These proteins contain long stretches of nonpolar amino acids to be
able to fit into the nonpolar phospholipid layer. The outer edges of
these proteins are hydrophilic so they can fit into the aqueous
environment of the cell.
o Peripheral proteins – they are appendages that are only loosely
bound to the surface of the cell membrane. On the cytoplasmic
side they may be held in place by the cytoskeleton, while on the
outside of the cell membrane they are attached to fibers of the
extracellular matrix.

Membrane proteins have 6 major functions:
o Transport proteins – provide hydrophilic channels for ions or
molecules to pass through or actively change shape to shuttle a
substance.
o Enzymatic activity – in some cases enzymes are organized into a
team that will catalyze an entire metabolic pathway.
o Signal transduction – The protein has a binding site that fits
specific messenger molecules and causes changes inside the cell.
o Cell-cell recognition – glycoproteins that act as identifying tags
for the cell
o Intercellular joining – May hook various cells together at
intercellular junctions
o Attachments of the cytoskeleton and extracellular matrix –
anchoring proteins help to maintain the cell’s shape or stabilize the
location of the cell
C. Carbohydrates:
 These are usually short, branched chains of carbohydrates (15 or fewer
monosaccharides)
 Some of the carbohydrates are covalently bonded to lipids, forming
glycolipids, most are covalently bonded to proteins forming
glycoproteins
 Carbohydrates are responsible for tagging the cell. These tags are
important for cell recognition, the proper specialization of the cell and for.
If the proper carbohydrate identification tags are not available on the
surface of the cell, the immune system destroys the cell.
 These markers vary from organism to organism and from species to
species.
D. Cholesterol
 Cholesterol provides rigidity to animal cell membranes. Plant cell
membranes usually do not contain cholesterol, because they have their
cell wall
 http://www.wiley.com/college/pratt/0471393878/student/animations/me
mbrane_transport/index.html
E. Synthesis and Sidedness of Membranes:
 The cell membrane has distinct inside and outside surface due to a
different lipid composition and the directional orientation of the membrane
proteins.
 Vesicles fuse into the plasma membrane and their contents are released
into the outside.
 The process of synthesizing, modifying and releasing plasma proteins:
1. ribosomes make polypeptides of the membrane proteins
on the surface of the rough ER
2. Phospholipids are synthesized and membrane proteins are
modified in the rough ER. Membrane proteins frequently
get a carbohydrate chain and become glycoproteins.
3. Vesicles transport these proteins and phospholipids into
the Golgi apparatus
4. Inside the Golgi apparatus glycoproteins are further
modified and the lipids can gain carbohydrates and
become glycolipids.
5. All proteins and lipids are transported to the plasma
membrane in vesicles
6. Vesicles fuse with the membrane to release secretory
proteins and to attach membrane glycoproteins and
glycolipids to the cell membrane
III.
THE SEMIPERMEABLE MEMBRANE
 A steady traffic of small molecules and ions moves across the plasma
membrane in both directions. However, these molecules and ions move
at a different rate.
 Sugars, amino acids, nutrients and oxygen enter the cell, waste products
leave the cell. Various ion concentrations are also regulated by the cell
membrane.
 The cell membrane is semipermeable – because it acts as a barrier
between the environment and the inside of the cell.
 Hydrophobic molecules such as hydrocarbons, oxygen, and carbon dioxide
can easily cross the lipid bilayer without the aid of membrane proteins.
 Polar molecules cannot pass freely through the membrane. Sugars, water
and some other small polar molecules move through the membrane very
slowly.
 The cell membrane is permeable to a variety of polar molecules with the
help of transport proteins (form a hydrophilic channel). For example
water molecules move through transport proteins called aquaporins.
Other proteins are carrier proteins that change shape and shuttles
molecules across the membrane (ex. Glucose carriers).
IV.
PASSIVE TRANSPORT
 Passive transport – the movement of substances across a biological
membrane without a need from the cell to expel energy. The energy for
the transport is fueled by potential energy from the concentration gradient
across the cell membrane. During passive transport, particles move from
higher to lower concentration area.
 Diffusion – The spreading out of molecules due to thermal motion.
Although the movement of molecules may be random, the sum of the
movement is directional from the higher to the lower cc. area or down the
concentration gradient.
 Important substance that moves by diffusion through the cell membrane
is oxygen.

Osmosis – the movement of water molecules across a selectively
permeable membrane when the dissolved substances are not allowed
across the membrane. Osmosis moves water down the concentration
gradient.
Diffusion:
Watch the animation on
http://bcs.whfreeman.com/thelifewire/content/chp05/0502001.html
Osmosis:
Watch the animation on osmosis:
http://www.stolaf.edu/people/giannini/flashanimat/transport/osmosis.swf


When considering the behavior of a cell in a solution, both the solute
concentration and the membrane permeability must be considered.
Tonicity, the ability of a solution to cause a cell to gain or lose water,
depends on both of the factors above.
According to tonicity, solutions can be three kinds:
o Isotonic – a solution to a cell if the net water movement across
the plasma membrane is zero
o Hypertonic -- a solution to the cell if the cell loses water when
inserted into this solution. In this solution the cell loses water,
than shrivels and dies.
o Hypotonic – a solution if the cell gains water from the solution,
swells and bursts.




Animal cells without rigid cells walls cannot tolerate various
environmental concentrations, unless they have various adaptations to
regulate their water concentration (osmoregulation). Ex. Contractile
vacuole of Paramecium.
Plant cells have cell walls to prevent the bursting of the cell in
hypotonic solutions. However, they are not protected against water
loss and the plant cell can also shrivel (plamsolysis).
Facilitated diffusion – The transport of ions or polar molecules
across the cell membrane with the help of transport proteins. These
transport proteins are very specific and transport only certain
molecules. Facilitated diffusion can be done with both carrier proteins
and channel proteins.
Examples of channel proteins are ion channels or gated channels –
only opened by a certain stimulus. Ex. Neurotransmitters open gated
sodium channels of the connecting nerve cells to forward a nerve
stimulus.
Completing transport processes
Facilitated diffusion: Look at the animations on
http://bcs.whfreeman.com/thelifewire/content/chp05/0502001.html
Active transport: Watch the animation on
http://bcs.whfreeman.com/thelifewire/content/chp05/0502002.html
Endocytosis: Watch the animation on
http://bcs.whfreeman.com/thelifewire8e/content/cat_010/0504003.html
V.
ACTIVE TRANSPORT
 Active transport – pumps molecules across the cell membrane against
the concentration gradient by using cellular energy in the form of ATP.
 Active transport mostly uses carrier proteins and not ion channels.
 Active transport enables cells to maintain internal concentrations of small
molecules that differ from the concentrations of the environment. (Ex.
Higher K-ion concentration inside the cell than outside, but higher Na-ion
concentration outside the cell than inside).
 ATP fuels active transport by attaching Pi directly to the carrier proteins.
The attached phosphate causes conformation change in the protein and
moves attached ions or molecules across the membrane. Ex. Sodiumpotassium pump:



All cells have voltages (potential energy caused by separation of opposite
charges) across the cell membrane. This voltage across the membrane is
called membrane potential (it is negative inside the cytoplasm
compared to the environment). This membrane potential favors the
transport of positive ions into the cytoplasm by diffusion and the transport
of anions out of the cell. The combination of the two forces (chemical
from ion concentration and electrical from the difference in charges) is
called electrochemical gradient will determine the movement of ions.
A transport protein that generates voltage across the cell membrane is
called an electrogenic pump (ex. Proton pumps).
Cotransport – the coupling of the “downhill” diffusion of one substance
with the “uphill” diffusion of an other substance against its own
concentration gradient. (ex. Plants’ couple proton pump with the transport
of glucose or other substances):
VI.
BULK TRANSPORT:
 Bulk transport – transport of larger particles across the cell membrane
by using vesicles.
 Exocytosis – fusion of vesicles with the plasma membrane and releasing
various substances from the cell into the environment (hormones,
neurotransmitters, proteins, carbohydrates).
 Endocytosis – the cell takes in macromolecules by forming vesicles off
the plasma membrane (moving cholesterol into the cell)
 Three main types of endocytosis are:
o Phagocytosis
o Pinocytosis
o Receptor-mediated endocytosis