Cell structure c&m c 7

•C-7
• Inside the Cell
• A tour of the Cell
© 2011 Pearson Education, Inc.

Key Concepts
The structure and function of a cell’s overall shape and
composition, as well as individual cell components, are closely
related.

Molecular “zip codes” aid material transport within a cell.

The cell’s cytoskeleton provides a structural framework within
the cell, and plays a role in cell division, movement, and
transport.


Key Concepts
Cells are highly dynamic and integrated; within a cell, thousands
of chemical reactions occur every second, molecules are
constantly moving across the plasma membrane, cell products are
transported along protein fibers, and elements of the cell’s
internal skeleton grow and shrink.


Classifying Cells
• According to morphology, there are two broad groupings of life:
1. Prokaryotes, which lack a membrane-bound nucleus
2. Eukaryotes, which have such a nucleus

• According to phylogeny, or evolutionary history, there are three
domains:
1. Bacteria
prokaryotic
2. Archaea
3. Eukarya – eukaryotic


Prokaryotic Cells – Structural Characteristics
• All prokaryotes lack a membrane-bound nucleus.
.

• Bacterial cells vary greatly in size and shape, but most bacteria
contain several structural similarities:
– Plasma membrane
– A single chromosome
– Ribosomes, which synthesize proteins
– Stiff cell wall


Prokaryotic Cells – Genetic Information
• Most prokaryotic species have one supercoiled circular
chromosome found in the nucleoid region of the cell.
– The chromosome contains a long strand of DNA and a few
supportive proteins.

• In addition to the large chromosome, many bacteria contain
plasmids.
– Small, supercoiled, circular DNA molecules
– Plasmids usually contain genes that help the cell adapt to
unusual environmental conditions.


Prokaryotic Cells – Internal Structure
• In addition to the nucleoid chromosome and plasmids, other
structures are contained within the cytoplasm:

– All prokaryotic cells contain ribosomes, consisting of RNA
molecules and protein, for protein synthesis.

– Many prokaryotes have internal photosynthetic membranes.

– Some prokaryotes have membrane-enclosed organelles.

– The inside of many prokaryotic cells is supported by a
cytoskeleton of long, thin protein filaments.


Bacterial Organelles
• Recently, internal compartments in many bacterial species were
discovered.
– These compartments qualify as organelles (“little organs”).
– An organelle is a membrane-bound compartment inside the
cell that contains enzymes or structures specialized for a
particular function.
– Organelles are common in eukaryotic cells.

• Each type of bacterial organelle is found in certain species.

• Bacterial organelles perform an array of tasks.


Prokaryotic Cells – External Structure
• Some prokaryotes have tail-like flagella on the cell surface that
spin around to move the cell. 256 H+ per turn

• Most prokaryotes have a cell wall.
– Bacterial and archaeal cell walls are a tough, fibrous layer that
surrounds the plasma membrane.

• Many species have an additional layer outside the cell wall
composed of glycolipids.


Eukaryotes
• Eukaryotes range from microscopic algae to 100-meter-tall
redwood trees.

• Most eukaryotes are multicellular, some are unicellular.

• Most eukaryotic cells are larger than most prokaryotic cells.


Eukaryotic Cells
• The relatively large size of the eukaryotic cell makes it difficult for
molecules to diffuse across the entire cell.
– This problem is partially solved by breaking up the large cell
volume into several smaller membrane-bound organelles.
– Surface area to volume problems

• The compartmentalization of eukaryotic cells offers two primary
advantages:
1. Separation of incompatible chemical reactions
2. Increasing the efficiency of chemical reactions


Eukaryotes and Prokaryotes Compared
• Four key differences between eukaryotic and prokaryotic cells have
been identified:

1. Eukaryotic chromosomes are found inside a membrane-
bound compartment called a nucleus.

2. Eukaryotic cells are often much larger.

3. Eukaryotic cells contain membrane bound organelles.

4. Eukaryotic cells feature a diverse and dynamic cytoskeleton.


The Nucleus
• The nucleus is large and highly organized.

• STRUCTURE:
– The nucleus is surrounded by a double-membrane nuclear
envelope.
– The nucleus has a distinct region called the nucleolus.

• FUNCTION:
– Information storage and processing
– Contains the cell’s chromosomes
– Ribosomal RNA synthesis (in the nucleolus)


Rough Endoplasmic Reticulum
• STRUCTURE:
– The rough endoplasmic reticulum (rough ER, RER) is a
network of membrane-bound tubes and sacs studded with
ribosomes.
– The interior is called the lumen.
– The rough ER is continuous with the nuclear envelope.

• FUNCTION:
– Ribosomes associated with the rough ER synthesize proteins.
– New proteins are folded and processed in the rough ER
lumen.


Smooth Endoplasmic Reticulum
• STRUCTURE:
– The smooth endoplasmic reticulum (smooth ER, SER) lacks
the ribosomes associated with the rough ER.

• FUNCTION:
– Enzymes within the smooth ER may synthesize fatty acids
and phospholipids, or break down poisonous lipids.
– Reservoir for Ca2+ ions


Golgi Apparatus
• STRUCTURE:
– The Golgi apparatus is formed by a series of stacked flat
membranous sacs called cisternae.

• FUNCTION:
– The Golgi apparatus processes, sorts, and ships proteins
synthesized in the rough ER.
– Membranous vesicles carry materials to and from the
organelle.


Ribosomes
• STRUCTURE:
– Ribosomes are non-membranous (they are not considered
organelles).
– Have large and small subunits, both containing RNA molecules
and protein
– Ribosomes can be attached to the rough ER or free in the
cytosol, the fluid part of the cytoplasm.

• FUNCTION:
– Protein synthesis


Peroxisomes
• STRUCTURE:
– Peroxisomes are globular organelles bound by a single
membrane.

• FUNCTION:
– Center of oxidation reactions

• Specialized peroxisomes in plants called glyoxysomes are packed
with enzymes that oxidize fats to form a compound that can be used
to store energy for the cell.


Lysosomes
• STRUCTURE:
– Lysosomes are single-membrane-bound structures containing
approximately 40 different digestive enzymes.
– Lysosomes are found in animal cells.

• FUNCTION:
– Lysosomes are used for digestion and waste processing.


How Are Materials Delivered to Lysosomes?
• Materials are delivered to the lysosomes by three processes:
– Phagocytosis
– Autophagy
– Receptor-mediated endocytosis

• Endocytosis is a process by which the cell membrane can pinch off
a vesicle to bring outside material into the cell.
– In addition to phagocytosis and receptor-mediated endocytosis,
a third type of endocytosis called pinocytosis brings fluid into
the cell.


Vacuoles
• STRUCTURE:
– Vacuoles are large membrane-bound structures found in plants
and fungi.
– Some contain digestive enzymes.

• FUNCTION:
– Some vacuoles are specialized for digestion.
– Most vacuoles are used for storage of water and/or ions to help
the cell maintain its normal volume.


Mitochondria
• STRUCTURE:
– Mitochondria have two membranes; the inner one is folded
into a series of sac-like cristae. The solution inside the cristae
is called the mitochondrial matrix.
– Mitochondria have their own DNA and manufacture their own
ribosomes.

• FUNCTION:
– ATP production is a mitochondrion’s core function.


Chloroplasts
• STRUCTURE:
– Most plant and algal cells have chloroplasts that, like
mitochondria, have a double membrane and contain their own
DNA.
– Chloroplasts contain membrane-bound, flattened vesicles
called thylakoids, which are stacked into piles called grana.
Outside the thylakoids is the solution called the stroma.

• FUNCTION:
– Chloroplasts convert light energy to chemical energy – in other
words, they perform photosynthesis.


The Cell Wall
• Fungi, algae, and plants have a stiff outer cell wall that protects the
cell.
– In plants and algae, the cell wall’s primary component is
cellulose.
– In fungi, the primary component is chitin.

• Some plants have a secondary cell wall containing lignin.


Cytoskeleton
• The cytoskeleton, composed of protein fibers, gives the cell shape
and structural stability, and aids cell movement and transport of
materials within the cell.

• In essence, the cytoskeleton organizes all of the organelles and
other cellular structures into a cohesive whole.


Structure and Function at the Whole-Cell Level
An organelle’s membrane and its enzymes correlate with its
function, and cell structure (e.g., the type, size, and number of
organelles) correlates with cell function.

• Cells are dynamic living things with interacting parts and
constantly moving molecules.


How Dynamic Are Eukaryotic Cells?
• Your body’s cells use, and synthesize, approximately 10 million
ATP molecules per second.

• Cellular enzymes can catalyze >25,000 reactions per second.

• Each membrane phospholipid can travel the breadth of its organelle
or cell in under a minute.

• The hundreds of trillions of mitochondria inside you are replaced
about every 10 days, for as long as you live. (Aging)

• The fluid plasma membrane’s composition is constantly changing.


The Nuclear Envelope: A Transport Mechanism
• The nuclear envelope has two membranes, each consisting of a
lipid bilayer, and is continuous with the endoplasmic reticulum.

• The inside surface is linked to fibrous proteins that form a lattice-
like sheet called the nuclear lamina.
– Stiffens the membrane’s structure and maintains its shape
– Provides attachment points for each chromosome

• The envelope contains thousands of openings called nuclear pores.
– Function as doors into and out of the nucleus


How Are Molecules Imported into the Nucleus?
• Messenger RNAs and ribosomes are synthesized in the nucleus and
exported to the cytoplasm. Materials such as proteins needed in the
nucleus are imported into the nucleus.

• Movement of proteins and other large molecules into and out of the
nucleus is an energy-demanding process.

Proteins destined for the nucleus have a molecular “zip code”—a
17-amino-acid-long nuclear localization signal (NLS)—which
allows them to enter the nucleus.


The Endomembrane System
• The endomembrane system is composed of the smooth and rough
ER and the Golgi apparatus, and is the primary system for protein
and lipid synthesis.

• Ions, ATP, amino acids, and other small molecules diffuse
randomly throughout the cell, but the movement of proteins and
other large molecules is energy demanding and tightly regulated.


The Secretory Pathway Hypothesis
• The secretory pathway hypothesis proposes that proteins intended
for secretion from the cell are synthesized and processed in a
highly prescribed set of steps.

• Proteins are packaged into vesicles when they move from the RER
to the Golgi apparatus and from the Golgi apparatus to the cell
surface.
– The RER and Golgi apparatus function as an integrated
endomembrane system.


The Signal Hypothesis
The signal hypothesis predicts that proteins bound for the
endomembrane system have a “zip code” that directs the growing
polypeptide to the ER.
– This “zip code” is a 20-amino-acid-long ER signal sequence.

• The ER signal sequence binds to a signal recognition particle
(SRP) that then binds to a receptor in the ER membrane.

• In the RER lumen, proteins are folded and glycosylated.
– Carbohydrates are attached to the protein.


From ER to Golgi
• Proteins are transported from the ER to the Golgi apparatus in
vesicles that bud off the ER, then fuse with the Golgi apparatus
membrane and deposit their contents inside.


Inside the Golgi Apparatus
• The Golgi apparatus’s composition is dynamic.
– New cisternae form at the cis face.
– Old cisternae break off from the trans face.

• Protein products enter the Golgi apparatus at the cis face and pass
through cisternae containing enzymes for attaching specific
carbohydrate chains, before exiting on the far side (trans face) of
the Golgi.


How Are Products Shipped from the Golgi?
• Each protein that comes out of the Golgi apparatus has a molecular
tag that places it in a particular type of transport vesicle.
– Each type of transport vesicle also has a tag that allows it to be
transported to the correct destination.

Proteins produced in a cell have distinctive molecular address
labels, which allow proteins to be shipped to the compartments
where they function.


Exocytosis
• Some proteins are sent to the cell surface in vesicles that fuse with
the plasma membrane, releasing their contents to the exterior of the
cell in a process called exocytosis.


The Dynamic Cytoskeleton
The cytoskeleton is a complex network of fibers that helps
maintain cell shape by providing structural support. The
cytoskeleton is dynamic; it changes to alter the cell’s shape, to
transport materials in the cell, or to move the cell itself.

• There are three types of cytoskeletal elements:
– Actin filaments (microfilaments)
– Intermediate filaments
– Microtubules


Actin Filaments
• Actin filaments are the smallest cytoskeletal elements.

• Actin filaments form by polymerization of individual actin
molecules.

• Actin filaments are grouped together into long bundles or dense
networks that are usually found just inside the plasma membrane
and help define the cell’s shape.
• Rubber bands


Actin-Myosin Interactions
• Actin filaments can also be involved in movement by interacting
with the motor protein myosin.

• Actin-myosin interactions can cause cell movements such as cell
crawling, cytokinesis, and cytoplasmic streaming.


Intermediate Filaments
• Intermediate filaments are defined by size rather than
composition. Many types of intermediate filaments exist, each
consisting of a different protein.

• Intermediate filaments provide structural support for the cell. They
are not involved in movement.

• Intermediate filaments form a flexible skeleton that helps shape the
cell surface and hold the nucleus in place.
• Thick rope


Microtubule Structure
• Microtubules are large, hollow tubes made of tubulin dimers (two-
part compounds).

• Microtubules have polarity, are dynamic, and usually grow at their
plus ends.

• Microtubules originate from the microtubule organizing center
and grow outward, radiating throughout the cell.

• Animal cells have just one microtubule organizing center called the
centrosome. Centrosomes contain two bundles of microtubules
called centrioles.
• Steel beams


Microtubule Function
• Microtubules provide stability and are involved in movement; they
may also provide a structural framework for organelles.
– Microtubules can act as “railroad tracks”; transport vesicles
move through the cell along these microtubule tracks in an
energy-dependent process.

• Microtubules require ATP and kinesin for vesicle transport to
occur. Kinesin is a motor protein that converts chemical energy in
ATP into mechanical work.


Cilia and Flagella: Moving the Entire Cell
• Flagella are long, hairlike projections from the cell surface that
move cells.
– Bacterial flagella are made of flagellin and rotate like a
propeller.
– Eukaryotic flagella are made of microtubules and wave back
and forth.

• Closely related to eukaryotic flagella are cilia, which are short,
filament-like projections.

• Cells generally have just one or two flagella but may have many
cilia.


Cilia and Flagella Structure
• The axoneme of cilia and flagella is a complex “9 + 2”
arrangement of microtubules connected by links and spokes.

• The axoneme attaches to the cell at a structure called the basal
body.


A Motor Protein in the Axoneme
• The motor protein dynein forms the arms between doublets and
changes shape when ATP is hydrolyzed to “walk” up the
microtubule.

• When the dynein arms on just one side of the axoneme move, cilia
and flagella bend instead of elongating because the links and
bridges constrain movement of the microtubule doublets.


Chapter Summary
Taken together, the data reviewed in this chapter can be summed
up in six words: Cells are dynamic, highly integrated
structures.


Cell structure c&m c 7

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