Chap 7

Inside the Cell

Ki-Young Kim
kiyoung@khu.ac.kr
국경대 324호

Roadmap 7
In this chapter you will learn that
Life’s properties emerge from the collaboration
of internal structures in a cell

by asking
What are the examining
examining
parts of the cell? Nuclear
Prokaryotic Eukaryotic transport 7.4
Cell structures Cell structures How the parts fit Endomembrane
7.1 7.2 into a whole system 7.5
7.3 Dynamic
looking cytoskeleton 7.6
closer at

The Cell Theory

• All cells have
1. Nucleic acids: store and transmit information
2. Proteins: perform most of the cell’s functions
3. Carbohydrates: chemical energy, carbon, support, identity
4. Plasma membrane: selectively permeable membrane barrier
Grouping Cells
• According to morphology, there are two broad groupings of life:
1. Prokaryotes
• Lack a membrane-bound nucleus
2. Eukaryotes
• Have a nucleus

• The three domains according to phylogeny, or evolutionary history,

are
1. Bacteria—prokaryotic
2. Archaea
3. Eukarya—eukaryotic

Prokaryotic Cells: Structural Overview

• All prokaryotes lack a membrane-bound nucleus
• Recent advances in microscopy reveal more complexity in prokaryotic
structure
• Archaeal cell structure is still relatively poorly understood
• Bacterial cells vary greatly in size and shape
• Most bacteria contain several structural similarities
- Plasma membrane
- A single chromosome
- Ribosomes, which synthesize proteins
- Stiff cell wall

(Spherical)

(Rod-like)

A-Monotrichous
B-Lophotrichous
C-Amphitrichous
D-Peritrichous

(Elongated) (DNA-like)
Figure 7.1 Overview of a Prokaryotic Cell.

Ribosomes

Plasmid

Cytoplasm

Chromosome
Plasma
membrane
Cell wall

1 m

Prokaryotic Cells: Genetic Information

• Most prokaryotic species have
- One supercoiled circular chromosome
- In the nucleoid region of the cell

• The chromosome contains

- A long strand of DNA
- A few supportive proteins
- The DNA double helix coils on itself with the aid of enzymes to form a
compact, “supercoiled” structure

Prokaryotic Cells: Genetic Information

• In addition, many bacteria contain plasmids
- Small, supercoiled, circular DNA molecules
- Usually contain genes that help the cell adapt to unusual environmental
conditions
- Are physically independent of the cellular chromosome
 (a) Compared to the cell, chromosomal DNA is very long.
Figure 7.2 Bacterial
DNA Is Supercoiled.
E. coli chromosome

0.5  m

(b) DNA is packaged by supercoiling.

0.5  m

Prokaryotic Cells: Internal Structure

• Other structures contained within the cytoplasm

• Ribosomes
- Consist of RNA molecules and protein
- Used for protein synthesis

• Many prokaryotes have internal photosynthetic membranes

• Cytoskeleton
- The inside of the cell is supported by a network of long, thin protein filaments

Figure 7.3 Photosynthetic Membranes in Bacteria.

Photosynthetic
membranes

0.5 m
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
• 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
• Bacterial organelles perform an array of tasks
- Storing calcium ions or other key molecules
- Holding crystals of the mineral magnetite
• Function like a compass needle to
• Help cells sense a magnetic field
• Swim in a directed way
- Organizing enzymes
• Responsible for synthesizing complex carbon compounds from carbon dioxide
- Sequestering enzymes
• Generate chemical energy

Plasma Membrane Separates Life from

Nonlife
• The plasma membrane
- Consists of a phospholipid bilayer
- Has proteins that either span the bilayer or attach to one side

• Inside the membrane, all the contents of a cell

- Excluding the nucleus (eukaryotes)
- Are collectively termed the cytoplasm
 Prokaryotic Cells: External Structure
• The cell wall forms a protective “exoskeleton”

• Most prokaryotes have a cell wall

• Bacterial and archaeal cell walls

- Composed of a tough, fibrous layer
- Surrounds the plasma membrane

• Many species have an additional layer:

- Outside the cell wall
- Composed of glycolipids

Prokaryotic Cells: External Structure

• Some prokaryotes have tail-like flagella
- On the cell surface
- Spin around to move the cell
• The base of this structure
- Is embedded in the plasma membrane
- Its rotation spins a long, helical filament
- Propels cells through water
• Fimbriae (singular: fimbria)
- Are needlelike projections
- Extend from the plasma membrane of some bacteria
- Promote attachment to other cells or surfaces

Figure 7.4 Extracellular Appendages Found on Bacteria.

Flagellum

Fimbriae

0.5 m
Figure 7.5 Close-up View
of a Prokaryotic Cell.

Chromosome

Ribosome

Cytoskeleton
Plasma
Flagellum membrane

Cell wall
Fimbria
50 nm Glycolipids

An Introduction to Eukaryotes
• Eukaryotes range in size from very small to very large:
- From microscopic algae to 100-meter-tall redwood trees

• Many eukaryotes are multicellular

- Others 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
• 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
• Termed the nucleus
2. Eukaryotic cells are often much larger
3. Eukaryotic cells contain extensive amounts of internal membrane
4. Eukaryotic cells feature a diverse and dynamic cytoskeleton

Summary Table 7.1 How Do the Structures of Prokaryotic and Eukaryotic Cells Differ?

Eukaryotic Parts List

• Cell component structure correlates with its function
 (a) Generalized animal cell Nuclear envelope

Figure 7.6 Overview of Nucleolus

Chromosomes
Nucleus

Eukaryotic Cells. Rough endoplasmic

reticulum
G olgi apparatus
Ribosomes

Peroxisome
Centrioles
Smooth endoplasmic
reticulum
Structures that
occur in animal cells
but not plant cells
Lysosome

Mitochondrion

Cytoskeletal element

Plasma membrane

(b) Generalized plant cell Nuclear envelope

Nucleolus Nucleus

Chromosomes

Structures that Rough endoplasmic

occur in plant cells reticulum
but not animal cells Ribosomes
Smooth endoplasmic
reticulum
G olgi apparatus
Cell wall
Vacuole
Chloroplast
Peroxisome
Mitochondrion

Plasma membrane

Cytoskeletal element

On average, prokaryotes are about 10

times smaller than eukaryotic cells in
diameter and about 1000 times smaller
than eukaryotic cells in volume.

The Nucleus
• The nucleus is large and highly organized
• Structure
- Surrounded by a double-membrane nuclear envelope
- The nuclear envelope is studded with pore-like openings
- The inside surface is linked to fibrous proteins
• They form a lattice-like sheet called the nuclear lamina
- The nucleus has a distinct region called the nucleolus

The Nucleus
• Function
- Information storage and processing
• Contains the cell’s chromosomes
- Ribosomal RNA synthesis (in the nucleolus)
Figure 7.7 The Nucleus Stores and Transmits Information.
Nucleus

Loosely
packed sections
of chromosomes
Nucleolus
Densely
packed sections
of chromosomes

Nuclear envelope

2 m

Ribosomes
• Structure:
- Ribosomes are non-membranous
• Are not considered organelles
- Have large and small subunits
• Both contain RNA molecules and protein
• Can be attached to the rough ER
• Can be free in the cytosol, the fluid part of the cytoplasm

• Function:
- Protein synthesis

Figure 7.8 Ribosomes Are the Site of Protein Synthesis.

Ribosomes

Prokaryotic ribosome
Ribosome Subunit rRNAs r-proteins
23S (2904 nt)
50S 31
70S 5S (120 nt)
30S 16S (1542 nt) 21

Eukaryotic cytosolic ribosomes

Ribosome Subunit rRNAs r-proteins
28S (4718 nt)
60S 5.8S (160 nt) 49
80S
5S (120 nt)
40S 18S (1874 nt) 33

100 nm
 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
- Is continuous with the nuclear envelope

• Function:
- Synthesis of specific proteins that will be
• Inserted into the plasma membrane
• Secreted to the cell exterior
• Shipped to an organelle

Smooth Endoplasmic Reticulum

• Structure:
- The smooth endoplasmic reticulum (smooth ER, SER) is a network of
membrane-bound tubes and sacs lacking ribosomes

• Function:
- Contains enzymes that catalyze reactions involving lipids
- These enzymes may
• Synthesize lipids needed by the organism
• Break down lipids and other molecules that are poisonous
- Is a reservoir for Ca2+ ions

Figure 7.9 The Endoplasmic Reticulum Is a Site of Synthesis, Processing, and Storage.

Rough endoplasmic Smooth endoplasmic

reticulum reticulum

Lumen of
rough ER

Ribosomes
on outside
of rough ER

Lumen of
smooth ER

Free
ribosomes
in cytoplasm 200 nm 200 nm
 Golgi Apparatus
• Structure:
- The Golgi apparatus
• Is formed by a series of stacked flat membranous sacs called cisternae
- Has a distinct polarity, or sidedness
• The cis (“this side”) surface is closest to the nucleus
• The trans (“across”) surface is oriented toward the plasma membrane

Golgi Apparatus
• Function:
- Processes, sorts, and ships proteins synthesized in the rough ER
- cis side of a Golgi apparatus receives products from the rough ER
- trans side ships them out to other organelles or the cell surface
- Membranous vesicles carry materials to and from the organelle

Figure 7.10 The Golgi Apparatus Is a Site of Protein Processing,

cis
Sorting, and Shipping.
Golgi apparatus

trans

Vesicle

Lumen of Golgi
apparatus

Cisternae

Vesicles
100 nm
 Lysosomes
• Structure:
- Lysosomes are single-membrane-bound structures
• Contain approximately 40 different digestive enzymes
- Found only in animal cells

• Function:
- Lysosomes are used for
• Digestion
• Waste processing

Figure 7.11 Lysosomes Are Recycling Centers.

Lysosome

Material being
digested within
lysosomes

250 nm

How Are Materials Delivered to Lysosomes?

• Materials are delivered to the lysosomes by three processes:
1. Phagocytosis
2. Autophagy
3. Receptor-mediated endocytosis

• Endocytosis is a process
- The cell membrane can pinch off a vesicle
• To bring outside material into the cell
- A third type of endocytosis, pinocytosis
• Brings fluid into the cell
 Vacuoles
• Structure:
- Vacuoles are large, membrane-bound structures found in plants and fungi
• Function:
- Some vacuoles are specialized for digestion
- Some contain digestive enzymes
- Most are used for storage of water and/or ions
• To help the cell maintain its normal volume
- Inside seeds, they are filled with proteins
- In flower petals or fruits, they are filled with colorful pigments
- They may be packed with noxious compounds
• To protect leaves and stems from being eaten by predators

Figure 7.12 Vacuoles Are Generally Storage Centers in Plant and Fungal Cells.
Vacuole

Vacuole

1 m

Peroxisomes
• Structure:
- Peroxisomes are globular organelles bound by a single membrane
- They originate as buds from the ER

• Function:
- Peroxisomes are the center of oxidation reactions
- Liver cell peroxisomes contain enzymes that remove electrons from, or
oxidize, the ethanol in alcoholic beverages
- Specialized peroxisomes in plants, called glyoxysomes
• Are packed with enzymes
• Oxidize fats to form a compound for energy storage
Figure 7.13 Peroxisomes Are the Site of Oxidation Reactions.
Peroxisome

Peroxisome
membrane
Enzyme
core
Peroxisome
lumen

100 nm

Mitochondria
• Structure:
- Mitochondria have two membranes:
• The inner one is folded into a series of sac-like cristae
• The solution inside the cristae is the mitochondrial matrix
- Have their own DNA
- Manufacture their own ribosomes

• Function:
- ATP production is a mitochondrion’s core function

Figure 7.14 Mitochondria Are Power-Generating Stations.

Mitochondrion

Outer
and inner
membranes

Matrix

Cristae

0.1 m
 Chloroplasts
• Structure:
- Most plant and algal cells have chloroplasts
• Have a double membrane
• Contain their own DNA
- Chloroplasts contain membrane-bound, flattened vesicles called thylakoids
• Are stacked into piles called grana
- Outside the thylakoids is the solution called the stroma

• Function:
- Chloroplasts convert light energy to chemical energy
- They perform photosynthesis

Figure 7.15 Chloroplasts Are Sugar-Manufacturing Centers in Plants and Algae.

Chloroplast

Stroma

Thylakoids

Granum

Outer and inner

membranes

1 m

Cytoskeleton
• Structure:
- The cytoskeleton
• Is composed of protein fibers
• Gives the cell shape and structural stability

• Function:
- The cytoskeleton organizes
• All of the organelles
• Other cellular structures into a cohesive whole
- Aids cell movement
- Transport of materials within the cell
 The Cell Wall
• Fungi, algae, and plants have a stiff outer cell wall
- Protects the cell

• The cell wall in plants and algae

- Primary component is cellulose

• The cell wall in fungi

- Primary component is chitin

• Some plants have a secondary cell wall containing lignin

Structure and Function at the Whole-Cell

Level
• An organelle’s membrane and its enzymes correlate with its function
• Cell structure correlates with cell function
- Type
- Size
- Number of organelles

• Cells are dynamic living things

- Have interacting parts
- Contain constantly moving molecules

Figure 7.16 Cell Structure Correlates with Function.

(a) Animal pancreatic cell: Exports (c) Plant leaf cell: Manufactures ATP
digestive enzymes. and sugar.

0.5 m 1 m

(b) Animal testis cell: Exports (d) Brown fat cells: Burn fat to generate
lipid-soluble signals. heat in lieu of ATP.

0.5 m 1 m
Tissue

Tissue

How Dynamic Are Eukaryotic Cells?

• Your body’s cells use and synthesize approximately 10 million ATP
molecules per second
• Cellular enzymes can catalyze more than 25,000 reactions per second
• Each membrane phospholipid can travel the breadth of its organelle or
cell in under a minute
How Dynamic Are Eukaryotic Cells?
• The hundreds of trillions of mitochondria inside you
- Are replaced about every 10 days
- This process continues for as long as you live

• The plasma membrane composition is constantly changing

The Nuclear Envelope: A Transport

Mechanism
• The nuclear envelope has two membranes:
1. Each consists of a lipid bilayer
2. It is continuous with the endoplasmic reticulum
• The inside surface is linked to fibrous proteins
- Forms 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
• 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
 How Are Molecules Imported into the
Nucleus?
• Nuclear proteins
• Are synthesized by ribosomes in the cytosol
• Contain a zip code
- A molecular address tag
- Marks them for transport through the nuclear pore complex

• Proteins destined for the nucleus

• Have a molecular zip code
- A 17-amino-acid-long nuclear localization signal (NLS)
- Allows them to enter the nucleus

Figure 7.17 Structure of the Nuclear Envelope and Nuclear Pore Complex.

Nuclear pore complex

Cross-sectional view of nuclear envelope

Nuclear matrix

Ribosomes, DNA in nucleus

mRNA

Nuclear lamina

Inner
membrane Nuclear
Outer envelope
membrane

0.1 m Proteins and

building blocks
Cytosol of DNA and RNA

Figure 7.18 Does the Nucleoplasmin Does the nucleoplasmin protein contain a
Protein Contain a “Send to “Send to nucleus” signal?

Nucleus” Signal? Nucleoplasmin contains a discrete “S end to nucleus” signal that

resides in either the tail or core region.
Nucleoplasmin does not require a signal to enter the
nucleus, or the entire protein serves as the signal.

Nucleoplasmin 1. Cleave tails

protein Core from cores.
“Tails”

Labeled tails Labeled cores 2. Label tails

and cores

3. Inject tails
and cores
into cells.

4. Locate
fragments.

Labeled tail fragments Labeled core fragments still

located in nucleus located in cytoplasm
The Endomembrane System
• Most of the proteins found in peroxisomes, mitochondria, and
chloroplasts
- Are actively imported from the cytosol
- Contain special signal sequences
• Target them to the appropriate organelles

• The endomembrane system is

- Composed of the smooth and rough ER and the Golgi apparatus
- The primary system for protein and lipid synthesis

The Endomembrane System

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

The Secretory Pathway Hypothesis

• The secretory pathway hypothesis proposes
- Proteins intended for secretion from the cell are synthesized
- They are 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
 RNA Ribosome Rough ER
Figure 7.19 The Secretory 1. Ribosome
deposits protein
Pathway Hypothesis. in ER.

cis face Protein

of Golgi 2. Protein exits
apparatus Golgi apparatus ER.

3. Protein enters
Golgi for
processing.

trans face 4. Protein exits

of Golgi Golgi.
apparatus

Plasma membrane
5. Protein exits
cell.

(a) Setup for a pulse-chase experiment

Figure 7.20 Tracking Protein Before

experiment Pulse Chase

Movement in a Pulse-Chase
Experiment.

Rough
ER

Golgi
apparatus
S ecretory Labeled
vesicles proteins

(b) Tracking pulse-labeled proteins during the chase

Location of labeled proteins (%)

Rough ER

Golgi
apparatus

S ecretory
vesicles

Incubation time after pulse (min)

The Signal Hypothesis

• The signal hypothesis predicts
- Proteins bound for the endomembrane system have a zip code
- It 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
 The Signal Hypothesis
• In the RER lumen, proteins are folded and glycosylated
- Carbohydrates are attached to the protein

Figure 7.21 The Signal Hypothesis Explains How Proteins

Destined for Secretion Enter the Endomembrane System.

RNA

Ribosome

SRP
Signal
Cytosol sequence

Lumen of
rough ER SRP receptor

Protein

1. Signal sequence 2. Signal binds 3. SRP binds to 4. Growing 5. Signal

is synthesized. to SRP. receptor. protein sequence is
enters ER. removed.

Moving from ER to Golgi

• Proteins are transported in vesicles that
- Bud off from the ER
- Move away
- Fuse with the membrane on the cis face of the Golgi apparatus
- Dump their cargo 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
- They are replaced by the cisternae behind them
- This process is called cisternal maturation
• Protein products
- Enter the Golgi apparatus at the cis face
- Pass through cisternae containing enzymes for attaching specific carbohydrate
chains
- Exit 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
- This tag 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
- The labels allow proteins to be shipped to the compartments where they
function

How Are Products Shipped from the Golgi?

• Each cargo protein
- Has a molecular tag
- The tag directs the protein to particular vesicle budding sites
- By interacting with receptors in the trans cisterna

• These receptors, along with other cytosolic proteins

- Direct the transport vesicles to the correct destinations
 Exocytosis
• Some proteins
- Are sent to the cell surface in vesicles
- Fuse with the plasma membrane
- Release their contents to the exterior of the cell

• This process is called exocytosis

Figure 7.22 In the Golgi Apparatus, Proteins Are Sorted into

Vesicles That Are Targeted to a Destination.

1. Proteins are
Lumen of tagged.
Golgi apparatus

“Tags” 2. Proteins are

sorted.

Receptors 3. Vesicles bud.

Cytosol

To other organelle
Transport
vesicles 4. Proteins interact
with receptors.

To pre-lysosomal To plasma
compartment membrane 5. Delivery.
for secretion

Recycling Material in the Lysosome

• How is cargo brought into the cell?
• Endocytosis (“inside-cell-act”) refers to
- Any pinching off of the plasma membrane
- Resulting in the uptake of material from outside the cell

• The sequence of events begins when

- Macromolecules outside the cell
- Bind to receptors on the plasma membrane
Figure 7.23 Receptor-Mediated Endocytosis
1. Macromolecules
Is a Pathway to the Lysosome. bind to receptors.

Recycling
of membrane
proteins
Endocytic 2. Endocytic vesicle
vesicle forms.

H+
3. Endocytic vesicle
Early fuses with early
endosome endosome; protons
lower pH.
H+ H+

4. Early endosome
Late matures; digestive
Vesicle endosome
from Golgi enzymes received.
apparatus

Lysosome 5. Mature lysosome;

macromolecules
digested.

Recycling Material in the Lysosome

• Recycling material via autophagy and phagocytosis
• Autophagy (literally, “same-eating”)
• Damaged organelles
- Are enclosed within an internal membrane
- Delivered to a lysosome
- Components are digested and recycled

Recycling Material in the Lysosome

• Phagocytosis (“eat-cell-act”)
• The plasma membrane of a cell
Specific material
- Surrounds a smaller cell or food particle
- Engulfs it
- Forms a structure called a phagosome
- Is delivered to a lysosome, where it is taken in and digested
Figure 7.24 Two More Ways to Deliver Materials to Lysosomes.

1. Damaged
organelle 1. Detection.
surrounded by
Damaged membrane.
Lysosome organelle
Phagosome 2. Phagosome
2. Delivery to
lysosome. formation.

Lysosome

3. Small molecules 3. Delivery to

recycled. lysosome and
digestion.

4. Small molecules
recycled.

The Dynamic Cytoskeleton

• The cytoskeleton
- Is a dense and complex network of fibers
- Helps maintain cell shape by providing structural support
- Is not a static structure like scaffolding used at construction sites

• Its fibrous proteins move and change to

- Alter the cell’s shape
- Shift its contents
- Even move the cell itself

The Dynamic Cytoskeleton

• Three types of cytoskeletal elements:
- Actin filaments (microfilaments)
- Intermediate filaments
- Microtubules
Actin Filaments
• Actin filaments are
- The smallest cytoskeletal elements
- Formed by polymerization of individual actin molecules
- Grouped together into long bundles or dense networks
- Usually found just inside the plasma membrane
- Structures that help define the cell’s shape

Actin Filaments
• The two distinct ends of an actin filament are referred to as plus and
minus ends
• The structural difference results in
- Different rates of assembling new actin subunits
- The plus end growing faster than the minus end

Actin–Myosin Interactions
• Actin filaments are
- Involved in movement
- Dependent on the protein myosin
• Myosin is a motor protein
- Converts the potential energy in ATP
- Into the kinetic energy of mechanical work
• Actin–myosin interactions can cause cell movements such as
- Cytokinesis
- Cytoplasmic streaming
 Actin–Myosin Interactions
• Cytokinesis (“cell-moving”) is the process of cell division
• In animals,
- Cytokinesis occurs by the use of actin filaments
- Connected to the plasma membrane
- Arranged in a ring around the circumference of the cell
• Myosin causes the filaments to
- Slide past one another
- Draw in the membrane
- Pinch the cell in two

Actin–Myosin Interactions
• Cytoplasmic streaming is the directed flow of cytosol and organelles
• In plant cells
- The movement occurs along actin filaments
- The process is powered by myosin

• Cytoplasmic streaming is especially common in large cells

- Where the circulation of cytoplasm facilitates material transport

(a) Actin and myosin interact to cause movement.

Myosin
Figure 7.25 Many Cellular
When myosin “head”
Movements Are Based on ATP attaches to actin
“Head” and moves, the actin
Actin–Myosin Interactions. ADP + P i region filament slides

+ end Actin − end

(b) Examples of movement caused by actin–myosin interactions

Cytokinesis in animals
Actin–myosin
interactions pinch
membrane in two

Cytoplasmic streaming
in plants
Actin–myosin interactions
move cytoplasm around
cell

Cell wall
Intermediate Filaments
• Intermediate filaments
- Are defined by size rather than composition
- Many types exist, each consisting of a different protein
- Provide structural support for the cell
- Are not involved in movement

• About 20 types of keratin

- Includes fingernails, toenails, and hair

Intermediate Filaments
• Nuclear lamins
- Form a dense mesh under the nuclear envelope
- Give the nucleus its shape
- Anchor the chromosomes
- Break up and reassemble the nuclear envelope when cells divide

Intermediate Filaments
• Project from the nucleus
- Through the cytoplasm
- To the plasma membrane
- Linked to other intermediate filaments that run parallel to the cell surface

• Form a flexible skeleton that helps

- Shape the cell surface
- Hold the nucleus in place
 Microtubule Structure
• Microtubules
- Are large, hollow tubes made of tubulin dimers
- Have two polypeptides, called α-tubulin and β-tubulin
- Have polarity
- Are dynamic
- Usually grow at their plus ends

• Originate from the microtubule organizing center

- Plus ends grow outward
- Radiating throughout the cell

Microtubule Structure
• In animal cells, this center is called the centrosome
- It contains two bundles of microtubules called centrioles

(a) In animals, microtubules originate from centrosomes.

Figure 7.26 Centrosomes Are a
Type of Microtubule-Organizing New
microtubules
Center.
Centrioles
Centrosome

Matrix

(b) Centrioles consist of microtubules.

Centrosome Centrioles (oriented
at 90° to each other)
Centrioles

200 m Microtubule
triplets
 Microtubule Function
• Microtubules
- Provide stability
- Are involved in movement
- Also provide a structural framework for organelles

• Microtubules can act as “railroad tracks”

- Transport vesicles move through the cell along these tracks
- In an energy-dependent process

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

Figure 7.27 Transport Vesicles Move along Microtubule Track.

(a) Electron micrograph

Vesicle

0.1 m

(b) Video image

Vesicle

0.1 m
 Microtubule Function
• Kinesin
- The head region binds to the microtubule
- The tail region binds to the transport vesicle

• Kinesin uses these domains

- To “walk” along the microtubule
- Through a series of conformational changes
- As it hydrolyzes ATP

Figure 7.28 Motor Proteins Move Vesicles along Microtubules.

(a) Structure of kinesin (b) Kinesin “walks” along a microtubule track.

Transport
vesicle
Tail

Kinesin

Stalk Every step

requires energy
ATP

ADP + Pi

Microtubule

Head
5 nm
− end + end

Cilia and Flagella: Moving the Entire Cell

• Flagella are long, hairlike projections from the cell surface that move
cells
• Bacterial flagella
- Are helical rods made of a protein called flagellin
- Move the cell by rotating the rod like a ship’s propeller
- Are not surrounded by the plasma membrane
 Cilia and Flagella: Moving the Entire Cell
• Eukaryotic flagella
- Consist of several microtubules constructed from tubulin dimers
- Move the cell by undulating—they whip back and forth
- Are surrounded by the plasma membrane
- Are considered organelles

Cilia and Flagella: Moving the Entire Cell

• Closely related to eukaryotic flagella are cilia
- Short, filament-like projections

• Cells
- Generally have just one or two flagella
- May have many cilia

Figure 7.29 Cilia and Flagella Differ in Length and Number.

Cilia

Flagellum

50 m 10 m
 Cilia and Flagella Structure
• Flagella and cilia were examined with an electron microscope
- Found that their underlying organization is identical

• The axoneme of cilia and flagella is

- A complex “9 + 2” arrangement of microtubules
- Connected by links and spokes

• Basal body
- Where axoneme attaches to the cell

A Motor Protein in the Axoneme

• The motor protein dynein
- Forms the arms between doublets
- Changes shape when ATP is hydrolyzed to “walk” up the microtubule
- Moves dynein along microtubules toward the minus end

• When dynein arms on only one side of the axoneme move

- Cilia and flagella bend instead of elongating
- The links and bridges constrain movement of the microtubule doublets
- A swimming motion results

Figure 7.30 The Structure and Function of Cilia and Flagella.

(a) Transmission electron micrograph of axoneme

Central
microtubules

Microtubule
doublet

(c) Mechanism of axoneme bending

75 nm
Microtubule doublet

(b) Structure of axoneme

Central
1 microtubules
9
Spoke
8 2
Microtubule
Plasma
doublet
membrane 3
7 Dynein arms
Link walk along
6 4 Link
microtubule
Dynein 5
doublets on
arms
one side of
Dynein an axoneme
arms

− end + ATP: Causes dynein to walk toward minus

end and pull toward plus end
Chapter Summary
• Cells are dynamic, highly integrated structures

End of week 7
• Good luck for your health
• See you next week