Title: Cell Structure and Function
1Cell 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)
2Life 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
blade - 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
cell
3The 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
4Early 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
5Cork 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
matter - Today, we know that living cells are made up of
many structures
6Early 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
7The 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
8The 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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10Exploring 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
cell - 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
develop
11Exploring 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
12Exploring 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
13Exploring the Cell
14Exploring 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
microscope - This means that electron microscopy can be used
to visualize only nonliving, preserved cells and
tissues
15Exploring 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
structures
16Prokaryotes 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
water - 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
informationDNA
17Prokaryotes 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
activities - 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
18Prokaryotes
- 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
eukaryotes - NO MEMBRANE BOUND ORGANELLES
- 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
19Eukaryotes
- 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 - MEMBRANE BOUND ORGANELLES
- 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
eukaryotes
20Cytoplasmic 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
21Eukaryotic Cell Structure
- In some respects, the eukaryotic cell is like a
factory - 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
22Eukaryotic Cell Structure
23Characteristics 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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25Eukaryotic 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
26Nucleus
- 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
27THE NUCLEUS
- 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
organelles - It has three regions
- Nuclear envelope (membrane)
- Nucleoli
- Chromatin
28Nucleus
29Nuclear 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
30Nuclear 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
31Chromatin
- 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
32Chromatin
- (a) Appears as a fine, unevenly stained network,
but special techniques reveal it as a system of
bumpy threads weaving their way through the
nucleoplasm - 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
33Chromatin
- Histones provide physical means for packing the
very long DNA molecules in a compact, orderly
way, they also play an important role in gene
regulation - 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
34Nucleolus
- Most nuclei also contain a small, dense region
known as the nucleolus - The nucleolus is where the assembly of ribosomes
begins
35Nucleoli
- 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
36Nucleus
37NUCLEUS
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
held.
38Endoplasmic 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
39Endoplasmic Reticulum
40Ribosomes
- 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
41Ribosomes
- (a)Small staining granules consisting of protein
and ribosomal RNA - Each ribosome has two globular subunits that fit
together - Site of protein synthesis
42Ribosome
43Ribosomes
- Some float freely in the cytoplasm
- Make soluble proteins that function in the
cytosol - 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
44Endoplasmic 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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46Endoplasmic 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
47ER PROTEIN SYNTHESIS
48Endoplasmic 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
49Endoplasmic 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
50ENDOPLASMIC RETICULUM
51Golgi 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
52Golgi Apparatus
53Golgi 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
54GOLGI ROLE
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56Lysosomes
- Even the neatest, cleanest factory needs a
cleanup crew, and that's what lysosomes are - Lysosomes are small organelles filled with
enzymes - 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
57Lysosomes
- 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
58Lysosomes
- 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
excreted - Hence, lysosomes provide sites where digestion
can proceed safely within a cell
59LYSOSOMES
60LYSOSOMES
- 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
61Lysosomes
- Function as a cells demolition crew
- Digesting particles taken in by endocytosis,
particularly ingested bacteria, viruses, and
toxins - 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
62Vacuoles
- 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
63Vacuoles
- 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 - Homeostasis is the maintenance of a controlled
internal environment
64VACUOLES
- 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
65VACUOLES
Vacuoles are a second common characteristic of
plant cells. These are fluid-filled organelles
that store enzymes and wastes.
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67Mitochondria 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
68Mitochondria
- 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
organelle
69Mitochondria
- One of the most interesting aspects of
mitochondria is the way in which they are
inherited - 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!
70Mitochondria
- 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
membrane
71MITOCHONDRION
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74Mitochondria
- 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
cells
75PLASTIDS
- 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
photosynthesis - 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
76Chloroplasts
- Plants and some other organisms contain
chloroplasts - 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
chlorophyll
77CHLOROPLAST
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80Organelle DNA
- Unlike other organelles that contain no DNA,
chloroplasts and mitochondria contain their own
genetic information in the form of small DNA
molecules - 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
81Cytoskeleton
- A supporting structure and a transportation
system complete our picture of the cell as a
factory - 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
shape - The cytoskeleton is also involved in movement
- Microfilaments and microtubules are two of the
principal protein filaments that make up the
cytoskeleton
82Cytoskeleton
- Series of rods running through the cytosol,
supporting cellular structures and aiding in cell
movement - There are three types of rods in the
cytoskeletonnot covered by membranes - Microtubules
- Microfilaments
- Intermediate filaments
83Cytoskeleton
- The cytoskeleton is a network of protein
filaments that helps the cell to maintain its
shape and is involved in many forms of cell
movement
84Microfilaments
- 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
surfaces
85Microfilaments
- Thinnest elements of the cytoskeleton
- Strands of the protein actin (ray)
- Each cell has its own unique arrangements (NO TWO
CELLS ARE ALIKE) - 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
86Microtubules
- 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
87Microtubules
- 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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89Microtubules
- 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
90Cytoskeleton
91Cell 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
surroundings - 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
92Cell Membrane
- The cell membrane regulates what enters and
leaves the cell and also provides protection and
support - 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
93Cell 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
94The 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
nonpolar - 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
extents - Assumed to function in cell signaling
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96The Fluid Mosaic Model
- Two distinct populations of membrane proteins
- Integral
- Peripheral
97Functions 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)
98Plasma 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
99Cell Membrane
100The 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
101Cell Walls
- Cell walls are present in many organisms,
including plants, algae, fungi, and many
prokaryotes - 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
102Cell 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
103CELL WALL
- 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
104CELL WALL
- Two types both give strength to the cell
- Primary
- Formed during cell growth
- Secondary
- Formed after growth has ceased
105PLANT CELL DIVISIONCELL WALL FORMATION
- 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
grow - 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
106(No Transcript)
107Diffusion 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
liquid - 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
108Measuring 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
mass/volume - 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
109Diffusion
- 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
equilibrium
110Passive ProcessDiffusion
- Diffusion is a process in which substances
scatter evenly throughout the environment from an
area of higher concentration to an area of lower
concentration - Molecules move randomly, collide and ricochet off
one another, changing direction with each
collision - 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
111Passive 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
112Diffusion
- 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
reached - At that point, the concentration of the substance
on both sides of the cell membrane will be the
same
113Diffusion
- 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
114Diffusion
115Passive ProcessSimple Diffusion
- Nonpolar and lipid-soluble substances diffuse
directly through the lipid bilayer - Examples oxygen, carbon dioxide, fat-soluble
vitamins, and alcohol
116Osmosis
- 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
it - 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
117Osmosis
- Water passes quite easily across most membranes,
even though many solute molecules cannot - An important process known as osmosis is the
result - Osmosis is the diffusion of water through a
selectively permeable membrane
118How Osmosis Works
- Look at the beaker on the left in the figure at
right - 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
sugar - 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
119How Osmosis Works
120Osmolarity
- 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
121How 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
membrane - 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
solution - The dilute sugar solution was hypotonic, or
below strength
122Osmolarity
- 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
membrane - 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
123Osmotic 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
cell - 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
right
124Osmotic Pressure
125Osmotic 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
126Osmotic 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
127Facilitated 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
easily
128Passive 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
129Facilitated 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
membrane - 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
130Facilitated 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
131Facilitated 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
132Facilitated 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
133Facilitated 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
134Facilitated Diffusion
135Integral 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
sides - All have BOTH hydrophobic and hydrophilic
regions - This allows them to interact BOTH with the
nonpolar lipid tails buried in the membrane and
with water inside and outside the cell
136Integral 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)
137Active 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
transport - As its name implies, active transport requires
energy - 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
membrane
138Active Transport
- Similar to facilitated diffusion in that both
require carrier proteins that combine
specifically and reversibly with the transported
substances - 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
ATP - Very selective involving chemicals that cannot
pass by diffusion - Classified according to their energy source
139Primary 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
140Molecular 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
direction
141Molecular Transport Active Transport
142Endocytosis
- 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
pinocytosis
143ENDOCYTOSIS
144Endocytosis 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
145Phagocytosis
- 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
146Endocytosis 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
147Pinocytosis
- Also called fluid-phase endocytosis (cell
drinking) - Bit of in