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Cell Structure and Function


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Title: Cell Structure and Function

Cell Structure and Function
  • This is a transmission electron micrograph of a
    neutrophil, a cell found in bone marrow
  • Color has been added to highlight the various
    organelles (magnification 27,500)

Life Is Cellular
  • Look closely at a part of a living thing, and
    what do you see?
  • Hold a blade of grass up against the light, and
    you see tiny lines running the length of the
  • Examine the tip of your finger, and you see the
    ridges and valleys that make up fingerprints
  • Place an insect under a microscope, and you see
    the intricate structures of its wings and the
    spikes and bristles that protect its body
  • As interesting as these close-up views may be,
    however, they're only the beginning of the story
  • Look closer and deeper with a more powerful
    microscope, and you'll see that there is a common
    structure that makes up every living thing the

The Discovery of the Cell
  • Seeing is believing, an old saying goes
  • It would be hard to find a better example of this
    than the discovery of the cell
  • Without the instruments to make them visible,
    cells remained out of sight and, therefore, out
    of mind for most of human history
  • All of this changed with a dramatic advance in
    technology the invention of the microscope

Early Microscopes 
  • It was not until the mid-1600s that scientists
    began to use microscopes to observe living things
  • In 1665, Englishman Robert Hooke used an early
    compound microscope to look at a thin slice of
    cork, a plant material
  • Under the microscope, cork seemed to be made of
    thousands of tiny, empty chambers
  • Hooke called these chambers cells because they
    reminded him of a monastery's tiny rooms, which
    were called cells
  • One of Hooke's illustrations of cells is shown in
    the figure to the right
  • The term cell is used in biology to this day
  • We now know, however, that cells are not empty
    but contain living matter

Cork Cells   
  • Using an early microscope, Hooke made this
    drawing of cork cells
  • In Hooke's drawings, the cells look like empty
    chambers because he was looking at dead plant
  • Today, we know that living cells are made up of
    many structures

Early Microscopes
  • In Holland around the same time, Anton van
    Leeuwenhoek used a single-lens microscope to
    observe pond water and other things
  • To his amazement, the microscope revealed a
    fantastic world of tiny living organisms that
    seemed to be everywhere, even in the very water
    he and his neighbors drank

The Cell Theory 
  • Soon, numerous observations made it clear that
    cells were the basic units of life
  • In 1838, German botanist Matthias Schleiden
    concluded that all plants were made of cells
  • The next year, German biologist Theodor Schwann
    stated that all animals were made of cells
  • In 1855, the German physician Rudolf Virchow
    concluded that new cells could be produced only
    from the division of existing cells
  • These discoveries, confirmed by other biologists,
    are summarized in the cell theory, a fundamental
    concept of biology

The Cell Theory
  • The cell theory states
  • All living things are composed of cells
  • Cells are the basic units of structure and
    function in living things
  • New cells are produced from existing cells

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Exploring the Cell
  • Following in the footsteps of Hooke, Virchow, and
    others, modern biologists still use microscopes
    to explore the cell
  • However, today's researchers use microscopes and
    techniques more powerful than the pioneers of
    biology could have imagined
  • Researchers can use fluorescent labels and light
    microscopy to follow molecules moving through the
  • Confocal light microscopy, which scans cells with
    a laser beam, makes it possible to build
    three-dimensional images of cells and their parts
  • High-resolution video technology makes it easy to
    produce movies of cells as they grow, divide, and

Exploring the Cell
  • These new technologies make it possible for
    researchers to study the structure and movement
    of living cells in great detail
  • Unfortunately, light itself limits the detail, or
    resolution, of images that can be made with the
    light microscope
  • Like all forms of radiation, light waves are
    diffracted, or scattered, as they pass through
    matter, making it impossible to visualize tiny
    structures such as proteins and viruses with
    light microscopy

Exploring the Cell
  • By contrast, as shown in the figure below,
    electron microscopes are capable of revealing
    details as much as 1000 times smaller than those
    visible in light microscopes because the
    wavelengths of electrons are much shorter than
    those of light
  • Transmission electron microscopes (TEMs) make it
    possible to explore cell structures and large
    protein molecules
  • Because beams of electrons can only pass through
    thin samples, cells and tissues must be cut first
    into ultrathin slices before they can be examined
    under a microscope

Exploring the Cell
Exploring the Cell
  • With scanning electron microscopes (SEMs), a
    pencillike beam of electrons is scanned over the
    surface of a specimen
  • For SEM images, specimens do not have to be cut
    into thin slices to be visualized
  • The scanning electron microscope produces
    stunning three-dimensional images of cells
  • Because electrons are easily scattered by
    molecules in the air, samples examined in both
    types of electron microscopes must be placed in a
    vacuum in order to be studied
  • As a result, researchers chemically preserve
    their samples first and then carefully remove all
    of the water before placing them in the
  • This means that electron microscopy can be used
    to visualize only nonliving, preserved cells and

Exploring the Cell
  • In the 1990s, researchers perfected a new class
    of microscopes that produce images by tracing the
    surfaces of samples with a fine probe
  • These scanning probe microscopes have
    revolutionized the study of surfaces and made it
    possible to observe single atoms
  • Unlike electron microscopes, scanning probe
    microscopes can operate in ordinary air and can
    even show samples in solution
  • Researchers have already used scanning probe
    microscopes to image DNA and protein molecules as
    well as a number of important biological

Prokaryotes and Eukaryotes
  • Cells come in a great variety of shapes and an
    amazing range of sizes
  • Although typical cells range from 5 to 50
    micrometers in diameter, the tiniest mycoplasma
    bacteria are only 0.2 micrometers across, so
    small that they are difficult to see under even
    the best light microscopes
  • In contrast, the giant amoeba Chaos chaos may be
    1000 micrometers in diameter, large enough to be
    seen with the unaided eye as a tiny speck in pond
  • Despite their differences, all cells have two
    characteristics in common
  • They are surrounded by a barrier called a cell
    membrane and, at some point in their lives, they
    contain the molecule that carries biological

Prokaryotes and Eukaryotes
  • Cells fall into two broad categories, depending
    on whether they contain a nucleus
  • The nucleus (plural nuclei) is a large
    membrane-enclosed structure that contains the
    cell's genetic material in the form of DNA
  • A membrane is a thin layer of material that
    serves as a covering or lining
  • The nucleus controls many of the cell's
  • Eukaryotes are cells that contain nuclei
  • Prokaryotes are cells that do not contain nuclei
  • Both words derive from the Greek words karyon,
    meaning kernel, or nucleus, and eu, meaning
    true, or pro, meaning before
  • These words reflect the idea that prokaryotic
    cells evolved before nuclei developed

  • Prokaryotic cells are generally smaller and
    simpler than eukaryotic cells, although there are
    many exceptions to this rule
  • Prokaryotic cells have genetic material that is
    not contained in a nucleus
  • Some prokaryotes contain internal membranes, but
    prokaryotes are generally less complicated than
  • Despite their simplicity, prokaryotes carry out
    every activity associated with living things
  • They grow, reproduce, respond to the environment,
    and some can even move by gliding along surfaces
    or swimming through liquids
  • The organisms we call bacteria are prokaryotes

  • Eukaryotic cells are generally larger and more
    complex than prokaryotic cells
  • Eukaryotic cells generally contain dozens of
    structures and internal membranes, and many are
    highly specialized
  • Eukaryotic cells contain a nucleus in which their
    genetic material is separated from the rest of
    the cell
  • Eukaryotes display great variety
  • Some eukaryotes live solitary lives as
    single-celled organisms
  • Others form large, multicellular organisms.
    Plants, animals, fungi, and protists are

Cytoplasmic Organelles
  • Little organs
  • Specialized cellular compartments, each
    performing its own job to maintain the life of
    the cell
  • Membranous organelles
  • Bounded by a membrane similar in composition to
    the plasma membranre (minus the glycocalyx)
  • This membrane enables them to maintain an
    internal environment different from that of the
    surrounding cytosol
  • Examples
  • Mitochondria
  • Peroxisomes
  • Lysosomes
  • Endoplasmic reticulum
  • Golgi apparatus
  • Nonmembranous organelles
  • Examples
  • Cytoskeleton
  • Centrioles
  • Ribosomes

Eukaryotic Cell Structure
  • In some respects, the eukaryotic cell is like a
  • The first time you look at a microscope image of
    a cell, the cell seems impossibly complex
  • Look closely at a eukaryotic cell, however, and
    patterns begin to emerge
  • To see those patterns more clearly, we'll look at
    some structures that are common to eukaryotic
    cells, shown in the figure at right
  • Because many of these structures act as if they
    are specialized organs, these structures are
    known as organelles, literally little organs

Eukaryotic Cell Structure
Characteristics of Cells
  • All cells have the same basic parts and some
    common functions
  • A generalized human cell contains the plasma
    membrane, the cytoplasm, and the nucleus

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Eukaryotic Cell Structure
  • Cell biologists divide the eukaryotic cell into
    two major parts
  • Nucleus
  • Cytoplasm
  • The cytoplasm is the portion of the cell outside
    the nucleus
  • As you will see, the nucleus and cytoplasm work
    together in the business of life

  • In the same way that the main office controls a
    large factory, the nucleus is the control center
    of the cell
  • The nucleus contains nearly all the cell's DNA
    and with it the coded instructions for making
    proteins and other important molecules
  • The structure of the nucleus is shown in the
    figure at right

  • The nucleus is the control center of the cell and
    contains the cellular DNA
  • Most cells have only one nucleus, but very large
    cells may be multinucleate
  • Presence of more than one nucleus usually
    signifies that a larger-than-usual cytoplasmic
    mass must be regulated
  • All body cells except mature red blood cells
    (anucleate) have nuclei
  • The nucleus is larger than the cytoplasmic
  • It has three regions
  • Nuclear envelope (membrane)
  • Nucleoli
  • Chromatin

Nuclear Envelope
  • The nucleus is surrounded by a nuclear envelope
    composed of two membranes
  • The nuclear envelope is dotted with thousands of
    nuclear pores, which allow material to move into
    and out of the nucleus
  • Like messages, instructions, and blueprints
    moving in and out of a main office, a steady
    stream of proteins, RNA, and other molecules move
    through the nuclear pores to and from the rest of
    the cell

Nuclear Envelope
  • Is a double-membrane barrier (separated by a
    fluid-filled space) surrounding the nucleus
  • Outer membrane is continuous with the rough ER of
    the cytoplasm and is studded with ribosomes on
    its external face
  • Inner membrane is lined by a network of protein
    filaments ( the nuclear lamina) that maintains
    the shape of the nucleus
  • At various points, nuclear pores penetrate areas
    where the membranes of the nuclear envelope fuse
  • A complex of proteins, called a pore complex,
    lines each nuclear pore and regulates passage of
    large particles into and out of the nucleus
  • Like other cell membranes, the nuclear envelope
    is selectively permeable, but here passage of
    substances is much freer than elsewhere
  • Protein molecules imported from the cytoplasm and
    RNA molecules exported from the nucleus pass
    easily through the relatively large pores
  • The nuclear envelope encloses the fluid and
    solutes of the nucleus, the nucleoplasm

  • The granular material you can see in the nucleus
    is called chromatin
  • Chromatin consists of DNA bound to protein
  • Most of the time, chromatin is spread throughout
    the nucleus
  • When a cell divides, however, chromatin condenses
    to form chromosomes
  • These distinct, threadlike structures contain the
    genetic information that is passed from one
    generation of cells to the next

  • (a) Appears as a fine, unevenly stained network,
    but special techniques reveal it as a system of
    bumpy threads weaving their way through the
  • Is roughly half DNA, the genetic material of the
    cell, and half globular histone proteins
  • Nucleosomes are the fundamental unit of
    chromatin, consisting of discus-shaped cores or
    clusters of eight histone proteins connected like
    beads on a string by a DNA molecule
  • DNA winds around each nucleosome and continues on
    to the next cluster via linker DNA segments

  • Histones provide physical means for packing the
    very long DNA molecules in a compact, orderly
    way, they also play an important role in gene
  • In a nondividing cell, addition of phosphate or
    methyl groups to histone exposes different DNA
    segments, or genes, so that they can dictate the
    specifications for protein synthesis
  • When a cell is preparing to divide, chromatin
    condenses into dense, rodlike chromosomes
  • Chromosome compactness avoids entanglement and
    breakage of the delicate chromatin strands during
    the movements that occur during cell division

  • Most nuclei also contain a small, dense region
    known as the nucleolus
  • The nucleolus is where the assembly of ribosomes

  • Dark-staining spherical bodies within the nucleus
  • NOT membrane bound
  • There are typically one or two nucleoli per
    nucleus, but there may be more
  • Site of the assembly of ribosomal subunits
  • Therefore, large in actively growing cells that
    are making large amounts of tissue proteins

The nucleus is surrounded by a double membrane
called the nuclear envelope. Inside the envelope
is chromatin (combo of DNA and protein) which
will become chromosomes. The nucleolus is the
site where ribosomes are synthesized and
partially assembled. The nucleus is porous and
is the site where our genetic information is
Endoplasmic Reticulum
  • Eukaryotic cells also contain an internal
    membrane system known as the endoplasmic
    reticulum, or ER, as shown in the figure at right
  • The endoplasmic reticulum is the site where lipid
    components of the cell membrane are assembled,
    along with proteins and other materials that are
    exported from the cell

Endoplasmic Reticulum
  • One of the most important jobs carried out in the
    cellular factory is making proteins
  • Proteins are assembled on ribosomes
  • Ribosomes are small particles of RNA and protein
    found throughout the cytoplasm
  • They produce proteins by following coded
    instructions that come from the nucleus
  • Each ribosome, in its own way, is like a small
    machine in a factory, turning out proteins on
    orders that come from its bossthe cell nucleus
  • Cells that are active in protein synthesis are
    often packed with ribosomes

  • (a)Small staining granules consisting of protein
    and ribosomal RNA
  • Each ribosome has two globular subunits that fit
  • Site of protein synthesis

  • Some float freely in the cytoplasm
  • Make soluble proteins that function in the
  • Some are attached to membranes, forming a complex
    called the rough endoplasmic reticulum
  • Synthesize proteins destined either for
    incorporation into cell membranes or for export
    from the cell
  • Ribosomes can switch back-and-forth between the
    two types

Endoplasmic Reticulum
  • The portion of the ER involved in the synthesis
    of proteins is called rough endoplasmic
    reticulum, or rough ER
  • It is given this name because of the ribosomes
    found on its surface
  • Newly made proteins leave these ribosomes and are
    inserted into the rough ER, where they may be
    chemically modified

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Endoplasmic Reticulum
  • Proteins that are released, or exported, from the
    cell are synthesized on the rough ER, as are many
    membrane proteins
  • Rough ER is abundant in cells that produce large
    amounts of protein for export
  • Other cellular proteins are made on free
    ribosomes, which are not attached to membranes

Endoplasmic Reticulum
  • The other portion of the ER is known as smooth
    endoplasmic reticulum (smooth ER) because
    ribosomes are not found on its surface
  • In many cells, the smooth ER contains collections
    of enzymes that perform specialized tasks,
    including the synthesis of membrane lipids and
    the detoxification of drugs
  • Liver cells, which play a key role in detoxifying
    drugs, often contain large amounts of smooth ER

Endoplasmic reticulum
  • Is an extensive system of interconnected tubes
    and parallel membranes enclosing fluid-filled
    cavities, called cisternae, that coils and twist
    throughout the cytosol
  • Continuous with the nuclear membrane
  • Two varieties
  • Rough ER
  • Smooth ER

Golgi Apparatus
  • Proteins produced in the rough ER move next into
    an organelle called the Golgi apparatus,
    discovered by the Italian scientist Camillo Golgi
  • As you can see in the figure at right, Golgi
    appears as a stack of closely apposed membranes
  • The function of the Golgi apparatus is to modify,
    sort, and package proteins and other materials
    from the endoplasmic reticulum for storage in the
    cell or secretion outside the cell
  • The Golgi apparatus is somewhat like a
    customization shop, where the finishing touches
    are put on proteins before they are ready to
    leave the factory
  • From the Golgi apparatus, proteins are then
    shipped to their final destinations throughout
    the cell or outside of the cell

Golgi Apparatus
Golgi Apparatus
  • Is a series of stacked, flattened, membranous
    sacs, shaped like hollow dinner plates,
    associated with swarms of tiny groups of
    membranous vesicles
  • The main function of the Golgi apparatus is to
    modify, concentrate, and package the proteins and
    lipids made at the rough ER
  • The transport vesicles that bud off from the
    rough ER move to and fuse with the membranes at
    its convex cis face (receiving side), of the
    Golgi apparatus

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  • Even the neatest, cleanest factory needs a
    cleanup crew, and that's what lysosomes are
  • Lysosomes are small organelles filled with
  • One function of lysosomes is the digestion, or
    breakdown, of lipids, carbohydrates, and proteins
    into small molecules that can be used by the rest
    of the cell

  • Lysosomes are also involved in breaking down
    organelles that have outlived their usefulness
  • Lysosomes perform the vital function of removing
    junk that might otherwise accumulate and
    clutter up the cell
  • A number of serious human diseases, including
    Tay-Sachs disease, can be traced to lysosomes
    that fail to function properly

  • Spherical membranous organelles that contain
    digestive enzymes
  • Abundant in phagocytes, the cells that dispose of
    invading bacteria and cell debris
  • Digest almost all kinds of biological molecules
    functioning best in acidic environments
  • Thus called acid hydrolases
  • The lysosomal membrane is adapted to serve
    lysosomal functions in two important ways
  • 1.Contains H (proton) pumps, ATPases that gather
    hydrogen ions from the surrounding cytosol to
    maintain the organelles acidic pH
  • 2.It retains the dangerous acid hydrolases while
    permitting the final products of digestion to
    escape so that they can be used by the cell or
  • Hence, lysosomes provide sites where digestion
    can proceed safely within a cell

  • Contains 40 different kinds of digestive enzymes
  • Digest food particles, bacteria, and worn-out or
    broken cell parts
  • Small, spherical organelles surrounded by a
    single membrane
  • Exist primarily in animal and fungal cells
  • Role in early (embryonic) development
  • Enzymes selectively destroy tissue

  • Function as a cells demolition crew
  • Digesting particles taken in by endocytosis,
    particularly ingested bacteria, viruses, and
  • Degrading worn-out or nonfunctional organelles
  • Performing metabolic functions, such as glycogen
    breakdown and release
  • Breaking down nonuseful tissues, such as the webs
    between fingers and toes of a developing fetus
    and the uterine lining during menstruation
  • Breaking down bone to release calcium ions into
    the blood

  • Every factory needs a place to store things, and
    cells contain places for storage as well
  • Some kinds of cells contain saclike structures
    called vacuoles that store materials such as
    water, salts, proteins, and carbohydrates
  • In many plant cells there is a single, large
    central vacuole filled with liquid
  • The pressure of the central vacuole in these
    cells makes it possible for plants to support
    heavy structures such as leaves and flowers

  • Vacuoles are also found in some single-celled
    organisms and in some animals
  • The paramecium contains a vacuole called a
    contractile vacuole
  • By contracting rhythmically, this specialized
    vacuole pumps excess water out of the cell
  • The control of water content within the cell is
    just one example of an important process known as
  • Homeostasis is the maintenance of a controlled
    internal environment

  • Fluid-filled cavities or sacs
  • Organelles often found in plants
  • Store enzymes and waste products (like lysosomes
    in animal cells)
  • Some of the waste products are toxic and need to
    be kept away from the rest of the cell
  • In a mature cell, makes up about 90 of the volume

Vacuoles are a second common characteristic of
plant cells. These are fluid-filled organelles
that store enzymes and wastes.
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Mitochondria and Chloroplasts
  • All living things require a source of energy
  • Factories are hooked up to the local power
    company, but what about cells?
  • Most cells get energy in one of two waysfrom
    food molecules or from the sun

  • Nearly all eukaryotic cells, including plants,
    contain mitochondria (singular mitochondrion)
  • Mitochondria are organelles that convert the
    chemical energy stored in food into compounds
    that are more convenient for the cell to use
  • Mitochondria are enclosed by two membranesan
    outer membrane and an inner membrane
  • The inner membrane is folded up inside the

  • One of the most interesting aspects of
    mitochondria is the way in which they are
  • In humans, all or nearly all of our mitochondria
    come from the cytoplasm of the ovum, or egg cell
  • This means that when your relatives are
    discussing which side of the family should take
    credit for your best characteristics, you can
    tell them that you got your mitchondria from Mom!

  • Sausage-shaped membranous organelle
  • In living cells they squirm, elongate, and change
    shape almost continuously
  • Power plants of the cell, providing most of its
    ATP supply
  • Enclosed by two membranes, each with the general
    structure of the plasma membrane
  • Outer membrane is smooth and featureless
  • Inner membrane folds inward, forming shelflike
    cristae that protrude into the matrix, the
    gel-like substance within the mitochondrion
  • Intermediate products of food fuels are broken
    down to water and carbon dioxide by teams of
    enzymes, some dissolved in the mitochondrial
    matrix and others forming part of the crista

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  • Site of aerobic respiration (requires oxygen)
  • Contain their own DNA and RNA and are able to
    reproduce themselves
  • Capable of fission
  • Contain approximately 37 genes that direct the
    synthesis of some proteins required for
    mitochondrial functions
  • Believed that mitochondria arose from bacteria
    that invaded the ancestors of plant and animal

  • Large organelles in plants
  • Store food or pigments
  • Organelle where solar energy is converted into
    chemical energy and stored
  • Types
  • Chloroplast
  • Contains a green pigment (chlorophyll) that
    absorbs sunlight as the first step in
  • Chromoplasts
  • Synthesis and store pigments such as orange
    carotenes, yellow xanthophylls, and various red
    pigments, some of which function in trapping
    sunlight for energy
  • Give certain plants their distinctive colors
  • Leucoplasts
  • Store food such as starches, proteins, and lipids

  • Plants and some other organisms contain
  • Chloroplasts are organelles that capture the
    energy from sunlight and convert it into chemical
    energy in a process called photosynthesis
  • Chloroplasts are the biological equivalents of
    solar power plants
  • Like mitochondria, chloroplasts are surrounded by
    two membranes
  • Inside the organelle are large stacks of other
    membranes, which contain the green pigment

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Organelle DNA 
  • Unlike other organelles that contain no DNA,
    chloroplasts and mitochondria contain their own
    genetic information in the form of small DNA
  • Lynn Margulis, an American biologist, has
    suggested that mitochondria and chloroplasts are
    actually the descendants of ancient prokaryotes
  • Margulis suggests that the prokaryotic ancestors
    of these organelles evolved a symbiotic
    relationship with early eukaryotes, taking up
    residence within the eukaryotic cell
  • One group of prokaryotes had the ability to use
    oxygen to generate ATP
  • These prokaryotes evolved into mitochondria
  • Other prokaryotes that carried out photosynthesis
    evolved into
  • Chloroplasts
  • This idea is called the endosymbiotic theory

  • A supporting structure and a transportation
    system complete our picture of the cell as a
  • As you know, a factory building is supported by
    steel or cement beams and by columns that support
    its walls and roof
  • Eukaryotic cells have a structurethe
    cytoskeletonthat helps support the cell
  • The cytoskeleton is a network of protein
    filaments that helps the cell to maintain its
  • The cytoskeleton is also involved in movement
  • Microfilaments and microtubules are two of the
    principal protein filaments that make up the

  • Series of rods running through the cytosol,
    supporting cellular structures and aiding in cell
  • There are three types of rods in the
    cytoskeletonnot covered by membranes
  • Microtubules
  • Microfilaments
  • Intermediate filaments

  • The cytoskeleton is a network of protein
    filaments that helps the cell to maintain its
    shape and is involved in many forms of cell

  • Microfilaments are threadlike structures made of
    a protein called actin
  • They form extensive networks in some cells and
    produce a tough, flexible framework that supports
    the cell
  • Microfilaments also help cells move
  • Microfilament assembly and disassembly is
    responsible for the cytoplasmic movements that
    allow cells, such as amoebas, to crawl along

  • Thinnest elements of the cytoskeleton
  • Strands of the protein actin (ray)
  • Each cell has its own unique arrangements (NO TWO
  • Nearly all cells have a fairly dense cross-linked
    network of microfilaments attached to the
    cytoplasmic side of their plasma membrane that
    strengthens the cell surface

  • Microtubules, as shown in the figure to the
    right, are hollow structures made up of proteins
    known as tubulins
  • In many cells, they play critical roles in
    maintaining cell shape
  • Microtubules are also important in cell division,
    where they form a structure known as the mitotic
    spindle, which helps to separate chromosomes
  • In animal cells, tubulin is also used to form a
    pair of structures known as centrioles
  • Centrioles are located near the nucleus and help
    to organize cell division
  • Centrioles are not found in plant cells

  • Microtubules also help to build projections from
    the cell surface, which are known as cilia
    (singular cilium) and flagella (singular
    flagellum), that enable cells to swim rapidly
    through liquids
  • Cilia and flagella can produce considerable
    force and in some cells they move almost like
    the oars of a boat, pulling or pushing cells
    through the water

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  • Largest diameter
  • Hollow tubes made of spherical protein subunits
    called tubulins
  • Most radiate from a small region of cytoplasm
    near the nucleus called the centrosome
  • Constantly growing from the centrosome,
    disassembling, and then reassembling

Cell Boundaries
  • When you first study a country, you may begin by
    examining a map of the country's borders
  • Before you can learn anything about a nation,
    it's important to understand where it begins and
    where it ends
  • The same principle applies to cells
  • Among the most important parts of a cell are its
    borders, which separate the cell from its
  • All cells are surrounded by a thin, flexible
    barrier known as the cell membrane
  • Many cells also produce a strong supporting layer
    around the membrane known as a cell wall

Cell Membrane
  • The cell membrane regulates what enters and
    leaves the cell and also provides protection and
  • The composition of nearly all cell membranes is a
    double-layered sheet called a lipid bilayer
  • As you can see in the figure at right, there are
    two layers of lipids, hence the name bilayer
  • The lipid bilayer gives cell membranes a flexible
    structure that forms a strong barrier between the
    cell and its surroundings

Cell Membrane
  • In addition to lipids, most cell membranes
    contain protein molecules that are embedded in
    the lipid bilayer
  • Carbohydrate molecules are attached to many of
    these proteins
  • There are so many kinds of molecules in cell
    membranes that scientists describe the membrane
    as a mosaic of different molecules
  • A mosaic is a work of art made of individual
    tiles or other pieces assembled to form a picture
    or design
  • Some of the proteins form channels and pumps that
    help to move material across the cell membrane
  • Many of the carbohydrates act like chemical
    identification cards, allowing individual cells
    to identify one another

The Fluid Mosaic Model
  • The inward-facing and outward-facing surfaces of
    the plasma membrane differ in the kinds and
    amounts of lipids they contain
  • The majority of membrane phospholipids are
    unsaturated (like phosphatidyl choline), a
    condition which kinks their tails (increasing the
    space between them) and increases fluidity
  • Glycolipids, phospholipids with attached sugar
    groups, are found only in the outer membrane (5
    of membrane)
  • Sugar group makes that end of the glycolipid
    molecule polar, whereas the fatty acid tails are
  • Cholesterol (20 of membrane) stabilizes the
    lipid membrane by wedging its platelike
    hydrocarbon rings between the phospholipid tails
    and restraining movement of the phospholipids
  • Lipid rafts (20), dynamic assemblies of
    saturated phospholipids (which pack together
    tightly) associated with unique lipids called
    sphinolipids and lots of cholesterol are also
    found only in the outer membrane
  • More stable and orderly and less fluid than the
    rest of the membrane
  • Include or exclude specific proteins to various
  • Assumed to function in cell signaling

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The Fluid Mosaic Model
  • Two distinct populations of membrane proteins
  • Integral
  • Peripheral

Functions of Membrane Proteins
  • Proteins make up about 50 of the plasma membrane
    by mass and are responsible for most of the
    specialized membrane functions
  • Transport
  • Enzymatic activity
  • Receptors for signal transduction
  • Intercellular joining
  • Cell-cell recognition
  • Attachment to the cytoskeleton and extracellular
    matrix (ECM)

Plasma Membrane Structure
  • Plasma membrane (cell membrane) defines the
    extent of the cell, separating two of the bodys
    major fluid compartments
  • Intracellular fluid within cells
  • Extracellular fluid outside cells

Cell Membrane
The Fluid Mosaic Model
  • Plasma membrane is composed of a double layer of
    phospholipids embedded with small amounts of
    cholesterol and proteins dispersed in it
  • The phospolipid bilayer is composed of two layers
    of phospholipids lying tail to tail
  • Polar head is charged and hydrophilic
    (hydrowater, philicloving)
  • Exposed to water inside (intracellular) and
    outside (extracellular) the cell
  • Attracted to water
  • Nonpolar tail is made of two fatty acid chains
    and is hydrophobic (phobiahating)
  • Avoid water
  • Line up in the center of the membrane

Cell Walls
  • Cell walls are present in many organisms,
    including plants, algae, fungi, and many
  • Cell walls lie outside the cell membrane
  • Most cell walls are porous enough to allow water,
    oxygen, carbon dioxide, and certain other
    substances to pass through easily
  • The main function of the cell wall is to provide
    support and protection for the cell

Cell Walls
  • Most cell walls are made from fibers of
    carbohydrate and protein
  • These substances are produced within the cell and
    then released at the surface of the cell membrane
    where they are assembled to form the wall
  • Plant cell walls are composed mostly of
    cellulose, a tough carbohydrate fiber
  • Cellulose is the principal component of both wood
    and paper, so every time you pick up a sheet of
    paper, you are holding the stuff of cell walls in
    your hand

  • Plant cells
  • Surround the cell membrane and helps support and
    protect the cell
  • Rigid covering
  • Made primarily of long chains of cellulose
    embedded in hardening compounds such as pectin
    and lignin
  • Pores allow ions and molecules to pass to and
    from the cell membrane

  • Two types both give strength to the cell
  • Primary
  • Formed during cell growth
  • Secondary
  • Formed after growth has ceased

  • Middle lamella forms between the new cells
  • Intercellular glue of pectin (substance that
    makes jelly gel)
  • Primary wall is formed between the middle lamella
    and the cell membrane
  • Cellulose fibers are laid down in layers (all of
    the fibers in one direction)
  • Growing cell is thus able to expand sequentially
    at a right angle to the most recent fiber layer
  • Functions both to protect and allow the cell to
  • Secondary wall forms after growth of the primary
    wall ceases
  • Made of cellulose and lignin (makes wall woody)
    fibers interwoven so that no further expansion of
    the cell is possible
  • Functions to strengthen the mature cell
  • When you hold a piece of wood, you are holding
    secondary cell walls

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Diffusion Through Cell Boundaries
  • Every living cell exists in a liquid environment
    that it needs to survive
  • It may not always seem that way yet even in the
    dust and heat of a desert, the cells of cactus
    plants, scorpions, and vultures are bathed in
  • One of the most important functions of the cell
    membrane is to regulate the movement of dissolved
    molecules from the liquid on one side of the
    membrane to the liquid on the other side

Measuring Concentration 
  • The cytoplasm of a cell contains a solution of
    many different substances in water
  • Recall that a solution is a mixture of two or
    more substances
  • The substances dissolved in the solution are
    called solutes
  • The concentration of a solution is the mass of
    solute in a given volume of solution, or
  • Example
  • If you dissolved 12 grams of salt in 3 liters of
    water, the concentration of the solution would be
    12 g/3 L, or 4 g/L (grams per liter)
  • If you had 12 grams of salt in 6 liters of water,
    the concentration would be 12 g/6 L, or 2 g/L
  • The first solution is twice as concentrated as
    the second solution

  • In a solution, particles move constantly
  • They collide with one another and tend to spread
    out randomly
  • As a result, the particles tend to move from an
    area where they are more concentrated to an area
    where they are less concentrated, a process known
    as diffusion
  • When the concentration of the solute is the same
    throughout a system, the system has reached

Passive ProcessDiffusion
  • Diffusion is a process in which substances
    scatter evenly throughout the environment from an
    area of higher concentration to an area of lower
  • Molecules move randomly, collide and ricochet off
    one another, changing direction with each
  • Overall effect of this erratic movement is that
    molecules or ions move away from areas where they
    are in higher concentration to areas where their
    concentration is lower, so we say that molecule
    diffuse along, or down, their concentration
    gradient until equilibrium
  • The greater the difference in concentration
    between the two areas, the faster the net
    diffusion of the particles

Passive ProcessDiffusion
  • Driving force is the kinetic energy of the
    molecules themselves
  • The speed is influenced by
  • Molecular size
  • Smaller the faster
  • Temperature
  • Warmer, the faster
  • Example
  • Peeling an onion, releases volatile substances
    that diffuse through the air, dissolving in the
    fluid film covering your eyes forming irritating
    sulfuric acid

  • What do diffusion and equilibrium have to do with
    cell membranes?
  • Suppose a substance is present in unequal
    concentrations on either side of a cell membrane,
    as shown in the figure at right
  • If the substance can cross the cell membrane, its
    particles will tend to move toward the area where
    it is less concentrated until equilibrium is
  • At that point, the concentration of the substance
    on both sides of the cell membrane will be the

  • Because diffusion depends upon random particle
    movements, substances diffuse across membranes
    without requiring the cell to use energy
  • Even when equilibrium is reached, particles of a
    solution will continue to move across the
    membrane in both directions
  • However, because almost equal numbers of
    particles move in each direction, there is no
    further change in concentration

Passive ProcessSimple Diffusion
  • Nonpolar and lipid-soluble substances diffuse
    directly through the lipid bilayer
  • Examples oxygen, carbon dioxide, fat-soluble
    vitamins, and alcohol

  • Although many substances can diffuse across
    biological membranes, some are too large or too
    strongly charged to cross the lipid bilayer
  • If a substance is able to diffuse across a
    membrane, the membrane is said to be permeable to
  • A membrane is impermeable to substances that
    cannot pass across it
  • Most biological membranes are selectively
    permeable, meaning that some substances can pass
    across them and others cannot

  • Water passes quite easily across most membranes,
    even though many solute molecules cannot
  • An important process known as osmosis is the
  • Osmosis is the diffusion of water through a
    selectively permeable membrane

How Osmosis Works 
  • Look at the beaker on the left in the figure at
  • There are more sugar molecules on the left side
    of the selectively permeable membrane than on the
    right side
  • That means that the concentration of water is
    lower on the left than it is on the right
  • The membrane is permeable to water but not to
  • This means that water can cross the membrane in
    both directions, but sugar cannot
  • As a result, there is a net movement of water
    from the area of high concentration to the area
    of low concentration

How Osmosis Works
  • When equal volumes of aqueous solutions of
    different osmolarity are separated by a membrane
    that is permeable to all molecules in the system,
    net diffusion of both solute and water occurs,
    each moving down its own concentration gradient
  • Eventually, equilibrium is reached when the water
    concentration on the left equals that on the
    right, and the solute concentration on both sides
    is the same

How Osmosis Works
  • Water will tend to move across the membrane to
    the left until equilibrium is reached
  • At that point, the concentrations of water and
    sugar will be the same on both sides of the
  • When this happens, the two solutions will be
    isotonic, which means same strength
  • When the experiment began, the more concentrated
    sugar solution was hypertonic, which means above
    strength, as compared to the dilute sugar
  • The dilute sugar solution was hypotonic, or
    below strength

  • If we consider the same system, but make the
    membrane impermeable to solute molecules, we see
    quite a different result
  • Water quickly diffuses from the left to the right
    compartment and continues to do so until its
    concentration is the same on the two sides of the
  • Notice that in this case equilibrium results from
    the movement of WATER ALONE (the solutes are
    prevented from moving)
  • Also the movement of water leads to dramatic
    changes in the volumes of the two compartments

Osmotic Pressure 
  • For organisms to survive, they must have a way to
    balance the intake and loss of water
  • Osmosis exerts a pressure known as osmotic
    pressure on the hypertonic side of a selectively
    permeable membrane
  • Osmotic pressure can cause serious problems for a
  • Because the cell is filled with salts, sugars,
    proteins, and other molecules, it will almost
    always be hypertonic to fresh water
  • This means that osmotic pressure should produce a
    net movement of water into a typical cell that is
    surrounded by fresh water
  • If that happens, the volume of a cell will
    increase until the cell becomes swollen
  • Eventually, the cell may burst like an
    overinflated balloon
  • The effects of osmosis are shown in the figure at

Osmotic Pressure
Osmotic Pressure
  • Fortunately, cells in large organisms are not in
    danger of bursting
  • Most cells in such organisms do not come in
    contact with fresh water
  • Instead, the cells are bathed in fluids, such as
    blood, that are isotonic
  • These isotonic fluids have concentrations of
    dissolved materials roughly equal to those in the
    cells themselves

Osmotic Pressure
  • Other cells, such as plant cells and bacteria,
    which do come into contact with fresh water, are
    surrounded by tough cell walls
  • The cell walls prevent the cells from expanding,
    even under tremendous osmotic pressure
  • However, the increased osmotic pressure makes the
    cells extremely vulnerable to injuries to their
    cell walls

Facilitated Diffusion
  • A few molecules, such as the sugar glucose, seem
    to pass through the cell membrane much more
    quickly than they should
  • One might think that these molecules are too
    large or too strongly charged to cross the
    membrane, and yet they diffuse across quite

Passive ProcessFacilitated Diffusion
  • In facilitated diffusion substances are moved
    through, even though they are unable to pass
    through the lipid bilayer of the plasma membrane,
    by either
  • Binding to protein carriers in the membrane
  • Moving through channels
  • Examples glucose and other sugars, amino acids,
    and ions

Facilitated Diffusion
  • How does this happen?
  • The answer is that cell membranes have protein
    channels that make it easy for certain molecules
    to cross the membrane
  • Red blood cells, for example, have a cell
    membrane protein with an internal channel that
    allows glucose to pass through it
  • Only glucose can pass through this channel, and
    it can move through in either direction
  • This cell membrane protein is said to facilitate,
    or help, the diffusion of glucose across the
  • The process, shown to the right, is known as
    facilitated diffusion
  • Hundreds of different protein channels have been
    found that allow particular substances to cross
    different membranes

Facilitated Diffusion
  • During facilitated diffusion, molecules, such as
    glucose, that cannot diffuse across the cell
    membrane's lipid bilayer on their own move
    through protein channels instead

Facilitated Diffusion
  • Although facilitated diffusion is fast and
    specific, it is still diffusion
  • Therefore, a net movement of molecules across a
    cell membrane will occur only if there is a
    higher concentration of the particular molecules
    on one side than on the other side
  • This movement does not require the use of the
    cell's energy

Facilitated DiffusionCarriers
  • Is a transmembrane integral protein (sometimes
    called a permease) that shows specificity for
    molecules of a certain polar substance or class
    of substances that are too large to pass through
    membrane channels
  • Examples sugars and amino acids
  • Mechanism Carrier-Mediated Facilitated Diffusion
  • Changes in shape of the carrier allow it to first
    envelop and then release the transported
    substance, shielding it en route from the
    nonpolar regions of the membrane
  • Essentially, the binding site is moved from one
    face of the membrane to the other by changes in
    the conformation of the carrier protein

Facilitated DiffusionChannels
  • Transmembrane proteins that serve to transport
    substances, usually ions or water, through
    aqueous channels from one side of the membrane to
    the other
  • Types of Channels are
  • Open Channels
  • Are always open (leakage channels) and simply
    allow ion or water fluxes according to
    concentration gradients
  • Gated and Controlled Channels
  • Gated Binding or association sites exist within
    the channel and the channel is selective due to
    pore size and the charges of the amino acids
    lining the channel
  • Controlled open or close by various chemical or
    electrical signals

Facilitated Diffusion
Integral Membrane Proteins
  • Firmly inserted into the plasma membrane
  • Some protrude from one membrane face only, BUT
    most are transmembrane proteins that span the
    entire width of the membrane and protrude on BOTH
  • All have BOTH hydrophobic and hydrophilic
  • This allows them to interact BOTH with the
    nonpolar lipid tails buried in the membrane and
    with water inside and outside the cell

Integral Membrane Proteins
  • Mainly involved in transport
  • Some cluster together to form channels, or pores,
    through which small, water-soluble molecules or
    ions can move, thus bypassing the lipid part of
    the membrane
  • Some act as carriers that bind to a substance and
    then move it through the membrane
  • Some are receptors for hormones or other chemical
    messengers and relay messages to the cell
    interior (process called signal transduction)

Active Transport
  • As powerful as diffusion is, cells sometimes must
    move materials in the opposite directionagainst
    a concentration difference
  • This is accomplished by a process known as active
  • As its name implies, active transport requires
  • The active transport of small molecules or ions
    across a cell membrane is generally carried out
    by transport proteins or pumps that are found
    in the membrane itself
  • Larger molecules and clumps of material can also
    be actively transported across the cell membrane
    by processes known as endocytosis and exocytosis
  • The transport of these larger materials sometimes
    involves changes in the shape of the cell

Active Transport
  • Similar to facilitated diffusion in that both
    require carrier proteins that combine
    specifically and reversibly with the transported
  • HOWEVER, facilitated diffusion always honors
    concentration gradients because its driving force
    is kinetic energy
  • IN CONTRAST, the active transporters or solute
    pumps move solutes, most importantly ions (such
    as Na, K, and Ca2), uphill against a
    concentration gradient
  • To do this work, cells must expend the energy of
  • Very selective involving chemicals that cannot
    pass by diffusion
  • Classified according to their energy source

Primary Active Transport
  • Because Na and K leak slowly but continuously
    through channels in the plasma membrane along
    their concentration gradient (and cross rapidly
    in stimulated muscles and nerve cells), the
    Na-K pump operates more or less continuously as
    an antiport to simultaneously drive Na out of
    the cell against a steep concentration gradient
    and pump K back in

Molecular Transport Active Transport
  • Small molecules and ions are carried across
    membranes by proteins in the membrane that act
    like energy-requiring pumps
  • Many cells use such proteins to move calcium,
    potassium, and sodium ions across cell membranes
  • Changes in protein shape, as shown in the figure
    at right, seem to play an important role in the
    pumping process
  • A considerable portion of the energy used by
    cells in their daily activities is devoted to
    providing the energy to keep this form of active
    transport working.
  • The use of energy in these systems enables cells
    to concentrate substances in a particular
    location, even when the forces of diffusion might
    tend to move these substances in the opposite

Molecular Transport Active Transport
  • Larger molecules and even solid clumps of
    material may be transported by movements of the
    cell membrane
  • One of these movements is called endocytosis
  • Endocytosis is the process of taking material
    into the cell by means of infoldings, or pockets,
    of the cell membrane
  • The pocket that results breaks loose from the
    outer portion of the cell membrane and forms a
    vacuole within the cytoplasm
  • Large molecules, clumps of food, and even whole
    cells can be taken up in this way
  • Two examples of endocytosis are phagocytosis and

Endocytosis Phagocytosis  
  • Phagocytosis means cell eating
  • In phagocytosis, extensions of cytoplasm surround
    a particle and package it within a food vacuole
  • The cell then engulfs it
  • Amoebas use this method of taking in food
  • Engulfing material in this way requires a
    considerable amount of energy and, therefore, is
    correctly considered a form of active transport

  • Type of endocytosis in which some relatively
    large or solid material, such as a clump of
    bacteria or cell debris, is engulfed by the cell
  • Cytoplasmic extensions called pseudopods form and
    flow around the particle and engulf it
  • Vesicle formed is called a phagosome
  • In most cases, the phagosome then fuses with a
    lysosome and its contents are digested
  • In humans, certain WBC and macrophages

Endocytosis Pinocytosis  
  • In a process similar to endocytosis, many cells
    take up liquid from the surrounding environment
  • Tiny pockets form along the cell membrane, fill
    with liquid, and pinch off to form vacuoles
    within the cell
  • This process is known as pinocytosis

  • Also called fluid-phase endocytosis (cell
  • Bit of in
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