Chapter 5
Cell Membrane Structure and Organelles
Part II
Principles of Individual Cell Function
Chapter 5
Cell Membrane Structure and Organelles
Cell structures consist of biological membranes – essentially mobile lipid bilayers to
which many membrane proteins attach. The cell membrane separates the interior
of the cell from the outer environment, acts as a barrier against exterior forces, and
regulates the flow of materials and information across the membrane.
Many bacteria, including E. coli, have one biological membrane – the cell
membrane. Nucleated cells of organisms such as yeasts, animals and plants
have organelles surrounded by biological membranes intertwined with particular
proteins, forming a unique intercellular environment. Many intracellular functions
operate smoothly through the collaboration of organelles. In addition, biological
membranes are not static; they are continuously created and move/fuse together,
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thus creating flow between membranes. Cells can grow, divide and perform a
range of movements thanks to the fluid structure of their membranes.
I. Cell Membrane Structure
Prokaryotic and Eukaryotic Cells
Membranes found in cells are called biological membranes. Their basic structure
consists of a lipid bilayer to which many proteins are bound. The cell membrane
surrounding a cell is also a biological membrane. It is believed that primitive cells
were created as a result of genetic material and its reproduction structures
becoming surrounded by membranes. As shown in Figure 5-1, bacteria (an
example of prokaryotic cells) have a simple structure with just a plasma
membrane. Conversely, animal and plant cells (examples of eukaryotic cells) are
surrounded by a plasma membrane containing – as shown in Table 5-1 –
organelles surrounded by double lipid bilayers consisting of inner and outer
membranes (such as the nucleus, mitochondria and chloroplasts) and organelles
surrounded by a single lipid bilayer (such as the plasma membrane, endoplasmic
reticula, Golgi apparatuses and lysosomes).
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Figure 5-1 Prokaryotic and eukaryotic cells
Table 5-1
Main functions of eukaryotic cell compartments
divided by cell membranes
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Organelles of Eukaryotic Cells
❖ Plasma membrane
The biological structure that separates the interior of a cell from its outer environment
is called the plasma membrane. Bacteria and animal cells are separated from
the outside by a lipid bilayer plasma membrane, whereas plant cells have a
strong cell wall outside the plasma membrane (Fig. 5-1). The plasma membrane
is characteristic in that many of the membrane proteins located on its outer surface
are modified by sugar chains. The plasma membrane has fluidity, and parts of it
continually diffuse into the cell. In this membrane (as discussed later in the chapter),
many channels and transporters carry materials, and receptors pass information
from the outer environment to the interior of the cell.
❖ Nucleus and Nuclear Envelope
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Eukaryotic cells normally have one nucleus, which contains a genome – a
complete set of hereditary information for an organism. In the nucleus, DNA
replication and RNA transcription occur. Linear DNA and binding proteins (e.g.,
histones) form a complex (i.e., chromatin) in the nucleus. There are two types of
nuclear chromatin: euchromatin – a lightly packed form under active transcription
– and heterochromatin, a tightly packed form in which transcription is limited.
During cell division, DNA in the nucleus becomes increasingly condensed until it
forms rod-like structures called chromosomes that are then distributed to the two
daughter cells. The nuclear envelope has many holes known as nuclear pores,
which control the movement of materials across the envelope. As an example,
mRNA generated by transcription passes through the nuclear pores out into the
cytoplasm, where it is translated into proteins (see Chapter 3).
❖ Endoplasmic Reticula and Golgi Apparatuses
Endoplasmic reticulam and Golgi apparatuses are involved in the synthesis and
transport of secretory proteins and the constituents of membranes. Endoplasmic
reticula synthesize and process proteins by sorting, adding sugar chains and
providing other modifications. Such endoplasmic reticula have a ribosome-rich
surface, and are called rough endoplasmic reticula. Those without attached
ribosomes are known as smooth endoplasmic reticula.
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Endoplasmic reticula are connected to the outer membrane of the nuclear
envelope and form a mesh-like structure. Consisting of a stack of flattened
membrane structures, Golgi apparatuses are located near endoplasmic reticula,
adding sugar chains to membrane proteins and secretory proteins as well as
sorting proteins. Transportation of lipids and membrane proteins between
organelles is performed by small bag-like structures made of biological membranes
called transport vesicles (see Fig. 5-10).
❖ Endosomes and Lysosomes
Endosomes and lysosomes play a role in the incorporation and digestion of
extracellular materials. Part of the membrane invaginates and pinches off to form
an endosome inside the cell. This process is called endocytosis. Endocytosed
proteins and lipids are transported to organelles after being sorted within
endosomes or digested within lysosomes. Plant vacuoles have functions similar to
those of lysosomes, and regulate cell turgor. Lysosomes contain enzymes that
digest nucleic acids, proteins and lipids, and the interior of lysosomes is kept
acidic (pH 5) by proton pumps. Vesicular transport also plays an important role
in the movement of lipids and membrane proteins between these organelles.
❖ Mitochondria, Chloroplasts and Peroxisomes
The inner mitochondrial membrane is compartmentalized into many cristae. The
space it encloses is called a matrix (see Fig. 8-2 in Chapter 8), and contains
DNA unique to mitochondria. Mitochondria are found in almost all eukaryotic
cells, and perform oxidative phosphorylation via the electron transport chain to
synthesize ATP.
Chloroplasts are flattened, disk-shaped organelles found in plants and algae, and
play a role in photosynthesis. The material inside the inner membrane is called the
stroma, which contains stacks of flattened, bag-like structures called thylakoids that
perform photosynthesis. The stroma contains DNA unique to chloroplasts.
Peroxisomes contain many oxidases, and perform lipid oxidization and the
metabolism of various materials. As an example, peroxisomes in plants synthesize
carbohydrates from stored lipids.
Vesicular transport does not occur between mitochondria and chloroplasts or
between peroxisomes and other organelles.
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II. Lipids and Membrane Proteins in Biological Membranes
Characteristics of a Lipid Bilayer
Lipid is a collective term for materials that do not dissolve readily in water but do
so easily in organic solvents. The lipids that constitute biological membranes are
made of hydrocarbon chains with hydrophilic heads and hydrophobic tails. The
most abundant constituents of biological membranes are phospholipids (Fig.
(A)
5-2A), and other constituents include sterols and glycolipids.
When placed in water, the hydrophilic heads of phospholipids face the water,
while the hydrophobic tails line up against one another, thus forming a bylayer
or a ball-shaped structure (i.e., a micelle) (Fig. 5-2B).
Lipid molecules in the bilayer continually move in a horizontal direction, but do
not move between the inner and outer sides of the membrane except under
special conditions. As shown in Figure 5-2C, lipid composition differs between
the inner and outer sides of the membrane, and in animal cells glycolipids are
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known to be found more on the outside.
(B)
(C)
Figure 5-2 Lipids of biological membranes
(A) The phospholipid molecule that constitutes membranes
(B) Model of lipid bilayer and ball-shaped micelles
(C) Membrane lipids readily flow in a horizontal direction, but not between the inside and outside of the membrane.
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Membrane Proteins
In cells, the plasma membrane – which has a lipid bilayer as its basic framework
– forms a ball-shaped structure and separates the cytoplasm from the aqueous
system outside the cell. Electron microscopy shows that the plasma membrane
consists of a lipid bilayer to which numerous membrane proteins are attached.
Membrane proteins take various forms; as shown in Figure 5-3, single- or
multipass transmembrane proteins, tunnel-shaped proteins similar to ion channels,
and proteins that attach to one side of the lipid membrane.
For transmembrane proteins, two major polypeptide structures are known. One of
these is the α-helix structure (Fig. 5-3A). Its highly hydrophobic surface, which has
many amino acids attached that have hydrophobic side chains (e.g., leucine and
Figure 5-3 Types of attachment for cell-membrane proteins
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isoleucine), faces the lipids. Multiple α-helices can form a path that penetrates the
plasma membrane. In this case, as shown in Figure 5-3B, many hydrophobic
amino acids are located on the outside facing the lipids, whereas many hydrophilic
amino acids are found on the inside facing the aqueous solution.
Membrane proteins can also form a hole that penetrates the plasma membrane
using a β-sheet structure (Fig. 5-3C). In this case, hydrophobic amino acids are
concentrated on the outside facing the lipid bilayer, and hydrophilic amino acids
are located on the inside of the tunnel. This β–sheet structure is called a β-barrel
because of its barrel-like shape.
These membrane proteins are involved in the transport of ions and chemical
compounds through the membrane. They also stabilize the membrane by
undercoating it, act as receptors that pass extracellular information to the cell (see
Chapter 9), and bind to the cytoskeleton by connecting to specific lipids in the
plasma membrane (see Chapter 6).
Some proteins, as shown in Figure 5-3E, do not have a structure that penetrates the
membrane; rather, they bind to it via fatty acids (see the Column on p.96). Many
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proteins that form complexes with membrane proteins also accumulate on the
membrane (Fig. 5-3F).
Proteins located on the outside of the plasma membrane of eukaryotic cells often
have oligosaccharide chains attached. Since extracellular fluid, such as blood,
contains many proteases, the attachment of sugar chains makes it difficult for
proteases to digest the proteins, thus stabilizing the proteins outside the cell.
Sugar chains often found in the proteins located on the cell surface are also used
as cell markings, and play an important role when cells recognize and attach
themselves to each other.
III. Functions of Biological Membranes
Barrier Functions and Selective Transport of Materials
A cell is a compartment separated from its outer environment by a plasma
membrane. The interior of a eukaryotic cell is also compartmentalized into many
organelles, and different reactions occur in each compartment.
Since lipids are the basic constituents of biological membranes (as shown in Fig.
5-4A), small, electrically non-charged solutes such as ethanol, oxygen and
carbon dioxide can pass through a lipid bilayer by simple diffusion following the
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concentration gradient. Water-soluble ions (Na+, K+ and Cl – ), sugars (e.g.,
glucose) and amino acids, however, cannot pass through the membrane. Proteins
– high molecules – are also unable to penetrate. By rigorously regulating the
transport of these materials, the plasma membrane keeps the intracellular
environment relatively stable even when the outside environment changes.
As shown in Figure 5-4B, the plasma membrane has membrane proteins such as
transporters that transport specific molecules, and channels that let specific
molecules pass. Most solutes can pass through the membrane only when
transported by membrane proteins. In this case, passive transport (i.e., transport
following the concentration gradient) occurs without relying on energy, whereas
Figure 5-4
The plasma membrane – a lipid
bilayer that serves as a barrier
against solutes and regulates the
passing of material by transporters
and channels
The plasma membrane lets small, electrically
non-charged solutes with a low molecular weight
pass, but forms a barrier against ions and large
solutes that have a high molecular weight (A) and
regulates transport by transporters and channels.
Passive transport occurs when the concentration
gradient is followed, but active transport, which
requires energy, takes place using ATP-derived or
other energies when transport occurs against the
concentration gradient (B).
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(A)
(B)
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active transport (i.e., transport against the concentration gradient) requires
energy. Active transport is performed by transporters.
The mechanism of active transport, which uses energy, is shown in Figure 5-5.
Animal cells have a higher K+ concentration and a lower Na+ concentration than
blood. To maintain these conditions, an Na+/K+ pump – a type of transporter
– transports Na+ ions to the outside and K+ ions to the inside of the cell against
the concentration gradient using the energy generated when ATP is hydrolyzed
into ADP. A pump protein containing Na+ attached to (1) is phosphorylated
using the energy generated through the hydrolysis of ATP and changes its structure
(2), releases Na+ to the outside of the cell (3), and catches Ka+ instead (4). The
pump protein is then dephosphorylated (5) and changes its structure, releasing
K+ into the cell (6). These reactions occur continually, using 30% of the energy
generated within the cell in some animal cells.
In this case, the energy of ATP is used twice for the structural changes of pump
proteins caused by phosphorylation and dephosphorylation, which allows the
transport of Na+ and K+.
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Figure 5-5
Continual transport of ions by the
Na+ /K+ pump using ATP
Each time the Na +/K + pump hydrolyzes one
molecule of ATP, three Na + ions leave the cell and
two K + ions enter it.
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Column
Cholesterol in the Plasma membrane
(A)
The nature of the plasma membrane varies greatly depending on its
lipid composition. Main lipids of the plasma membrane include
phospholipids, sterols and glycolipids. A typical phospholipid is
phosphatidylcholine (Column Fig. 5-1A). The head, consisting of
choline and phosphate, is connected by glycerol with hydrocarbon
tails that look like two legs. The fluidity of the membrane changes
significantly depending on how many double bonds these
hydrocarbon tails have. A double bond formed between carbon
and carbon bends the hydrocarbon chain from that point.
The most abundant constituent in the membrane of animal cells is
cholesterol (Column Fig. 5-1B). This short, hard molecule is mainly
located on the inside of the plasma membrane, and fills the gaps
created by the double-bond bending of phospholipids.
The plasma membrane has sites called rafts where cholesterol and
glycolipids are concentrated. In rafts, membrane lipids become like
liquid crystal and hence have low fluidity. It is known that membrane
proteins involved in signaling tend to be concentrated in rafts.
Lipid-modified proteins move from the cytoplasm to the inside of
(B)
rafts, whose outside has many glycolipids and attracts glycolipidmodified proteins.
Membrane regions rich in cholesterol are associated with
neurological and immunological functions, Alzheimer's disease and
viral infection, and are therefore quite well known. Lipid-rich regions
of the plasma membrane, such as rafts, are known as the microdomains of the membrane.
(C)
Column Figure 5-1
Cholesterol in
biological membranes
(C)Cholesterol located inside the curve of
a phospholipid tail created by a
hydrocarbon double bond makes the
membrane rigid.
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Membrane Potential
In a resting (non-excited) cell, electrical potential difference exists whereby the
interior of the cell separated by the plasma membrane has negative potential
(i.e., resting membrane potential). This is due to the difference in ion concentration
between the inside and the outside of the cell (Table 5-2) and the selective
permeability of the plasma membrane for ions (Fig. 5-6). With the plasma
membrane in a resting state, certain K+ channels are kept open; the membrane
can therefore be thought of as a semipermeable membrane that allows K+ ions to
pass. When a difference in the concentration of K+ exists between either side of
Table 5-2
Ion concentration inside and
outside the cells of spinal
motor neurons in mammals
such a semipermeable membrane, the resting membrane potential is calculated
as approximately –90 mV by the Nernst equation (see the Column on p.98). This
potential is similar to the value for an actual cell, but differs slightly because small
amounts of Na+ and Cl– also pass through actual plasma membranes.
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Figure 5-6
Generation of membrane potential by a K+ channel
+
+
When a K channel opens, releasing only K ions inside the
cell to the outside, membrane potential is generated unless
ions with a negative charge are also released to balance
+
out the positive ions. The movement of K stops at the point
+
where the driving force of K following the gradient of
+
membrane potential and the driving force of K following the
concentration gradient balance each other out. Refer to the
Column on p.98 for the calculation of this point.
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Additionally, in excitable cells such as neurons, further special changes in
membrane potential occur in response to changes in the resting membrane
potential caused by stimuli (see the Column on p.100).
Column
Proteins that Bind to the Membrane without a Transmembrane Structure
Among the proteins that are attracted to the membrane, many do not have
a membrane-spanning structure; rather, they covalently bond to lipids. The
(A)
hydrophobic part of the hydrocarbon of lipids bonded to these proteins is
incorporated into the lipid bilayer of biological membranes and accumulates
on them. As shown in Column Figure 5-2, whether a protein binds to the
inside or the outside of the membrane depends on the lipid type attached
to it. As an example, proteins modified by palmitic acid or myristic acid
bond to the inside of the plasma membrane, while those modified by
glycolipids bond to the outside.
Other types of protein attracted to the membrane include those that recognize
the special structure of membrane lipids. As an example, phosphatidylinositol
– a membrane lipid – is phosphorylated at the proteins 3’, 4’ and 5’ in
Column Figure 5-2C. In such cases, specific proteins that bind to phosphorylated
(B)
lipids exist, and different proteins are attracted to the membrane each time the
membrane lipids are phosphorylated or dephosphorylated.
(C)
Column Figure 5-2
On the plasma membrane, proteins modified by lipids and
proteins that attach themselves to particular lipids also accumulate
(A) A ccumulation of lipid-modified proteins on the biological membrane
(B) S tructure of phosphatidylinositol
(C) Proteins that recognize phosphorylated phosphatidylinositol
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Signaling by Receptors
Information on the outside of the cell is transmitted to the inside via receptors in
the plasma membrane. This mechanism is discussed in detail in Chapter 9-I.
Connection to the Cytoskeleton and
the Extracellular Matrix via the Plasma membrane
The plasma membrane bonds to both the cytoskeleton – the frame of the cell – and
the extracellular matrix. This mechanism is discussed in detail in Chapters 6 and 11.
IV. Formation of Organelles and Transport
Selective Transport of Proteins to Organelles
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Each organelle has proteins with specific functions. Synthesis of most proteins
begins in ribosomes in the cytoplasm. The destination of each protein is determined
by the selective signal sequence included in the amino acid sequence of the
protein. This signal sequence binds to other proteins that are involved in
transporting the target protein to a particular organelle. Proteins without this signal
sequence remain in the cytoplasm. As shown in Figure 5-7, selective transport
has three mechanisms. The first is the transport of proteins from the cytoplasm to
Figure 5-7
Transport of Proteins to Organelles
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the nucleus through holes in the nuclear envelope (nuclear pores). The second is
the transport of proteins from the cytoplasm to endoplasmic reticula, mitochondria,
chloroplasts and peroxisomes, which is performed by protein translocators
located in the membrane of organelles. In this case, the higher-order structure of
proteins is unwound before passing through the membrane, and is folded back
into its functional structure after passage. The third mechanism is transport
between membrane structures, which is performed by other small membrane
structures known as transport vesicles.
Transport to and from the Nucleus
The nuclear envelope that houses the DNA-containing nucleus consists of two lipid
bilayers (the inner and outer membranes), and gateways known as nuclear pores
cross the envelope (Fig. 5-7). Although DNA replication and DNA transcription
into RNA occur in the nucleus, protein translation by ribosomes occurs in the
cytoplasm. Transcribed RNA is therefore carried through the nuclear pores to the
outside of the nucleus. Conversely, proteins involved in replication and transcription
– such as enzymes and transcription factors – are transported into the nucleus
through the nuclear pores after being synthesized in the cytoplasm.
Column
Nernst Equation for Calculating Plasma Membrane Potential
How can the membrane potential be calculated from ion concentration? If
only the K+ channel works (allowing ions to pass through the membrane) but
negatively charged ions cannot pass, as shown in Figure 5-6, the movement
of K+ ions stops when the concentration gradient of K+ and the membrane
potential gradient balance each other out. In such cases, the theoretical
resting membrane potential can be calculated by the Nernst equation based
on the ratio of ion concentration inside and outside the cell.
Assuming that only the membrane potential created by K + (V k) exists, the
Nernst equation is expressed as follows, using Kout and Kin as the potassium
ion concentration outside and inside the cell, respectively:
V k = (RT/F) ln (Kout/Kin)
Where R is the gas constant, T is the absolute temperature and F is the
Faraday constant.
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In humans, assuming a body temperature of 37˚C, an extracellular ion
concentration of 5.5 mM and an intracellular concentration of 150 mM, the
membrane potential V k (mV) is calculated as follows:
Vk = 62 log10 ( Kout/Kin)
= 62 log10 (5.5/150)
≒
-90 mV.
Transport of Proteins to Mitochondria and Chloroplasts
Each protein synthesized in ribosomes within the cytoplasm and then transported
to mitochondria or chloroplasts has a selective signal sequence. When the signal
sequence of Protein A is replaced with that of Protein B in experimental genetic
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engineering conditions, Protein A is transported to a different organelle to which
Protein B is supposed to be transported, indicating that the signal sequence
determines the destination of proteins in the cell.
When proteins are transported to mitochondria or chloroplasts, their structure is
unwound before passing through the outer membrane of the organelle, and is
folded back into its original functional structure once inside (Fig. 5-8).
Figure 5-8
Transport of proteins
to mitochondria
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Column
Nerve Excitation and Signaling
Neurons rapidly amplify and propagate changes in the potential of the
plasma membrane as a result of ion flux. When stimuli applied to the plasma
membrane temporarily open the Na+ channel, Na+ ions flow into the cell
following the concentration and electric potential gradients across the
membrane, thus raising its potential. This opens the Na+ channel that responds
to changes in the membrane potential, which in turn causes further inflow of
large amounts of Na+ ions, thus greatly raising the potential of the membrane.
This phenomenon is known as action potential. The Na+ channel immediately
closes, and the K+ channel that responds to changes in the membrane
potential opens instead. As a result, K+ flows out of the cell following the
concentration and electric potential gradients, which rapidly lowers the
membrane potential. This is the mechanism of nerve impulse generation.
Such local changes in membrane potential trigger the opening and closing of
nearby Na+ channels, which spreads (in one direction) the potential changes
to the surrounding areas. This is the propagation of nerve excitation.
Column Figure 5-3
Action potential and
the subsequent refractory
period in neurons
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Column
Cell Membrane Structure and Organelles
G-Protein Involved in Nuclear Pore Transport
Nuclear pores are giant protein complexes with a molecular weight of over
100 million and a diameter of over 120 nm. While molecules with a weight
of less than 10,000 can pass through the pores by diffusion, those with a
larger molecular weight are selectively transported using the energy derived
from ATP. Proteins transported to the nucleus have an amino acid sequence
called the nuclear localization signal, and pass through the nuclear pores
with the help of a GDP G-protein known as Ran and by bonding with
importin (a transport protein). In the nucleus, Ran becomes a GTP protein,
helping importin and transported proteins to dissociate from each other. The
reverse reaction occurs when mRNA and proteins are transported to the
outside of the nucleus.
Proteins with activity that transforms Ran to GDP proteins are abundant in the
cytoplasm, and those with activity that transforms Ran to GTP proteins are
abundant in the nucleus. The direction of transport into and out of the nucleus
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is determined in line with the difference in location of these proteins.
Column Figure 5-4
System of transporting
proteins to the cell nucleus
Proteins with a nuclear localization
signal attach to importin – a transport
protein – before passing through
nuclear pores into the nucleus. This
process is regulated by a lowmolecular G protein called Ran.
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Transport of Proteins to Endoplasmic Reticula
Proteins incorporated into the plasma membrane, enzymes in lysosomes
and proteins secreted to the outside of the cell are synthesized in ribosomes
attached to the endoplasmic reticulum membrane. Endoplasmic reticula
with ribosomes attached are called rough endoplasmic reticula. The
synthesis of a protein starts in the cytoplasm; as soon as the signal sequence
of the protein is synthesized, the protein complex (SRP) recognizes and
attaches itself to the sequence, and the SRP-bonded protein attaches itself to
the receptor on the endoplasmic reticulum membrane, where the synthesis of
the protein continues (Fig. 5-9). The protein about to be fully synthesized is
transported into an endoplasmic reticulum through the protein transport
channel. In the endoplasmic reticulum, the protein, with the help of proteins
called chaperones, is folded into a functional form. A pair of cysteine side
chains is oxidized to form a disulfide bond in the endoplasmic reticulum. In
addition, many membrane proteins and proteins secreted to the outside of
the cell are covalently bonded with short oligosaccharide chains.
Figure 5-9
Protein synthesis and transport
in endoplasmic reticula
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Vesicular Transport
Transport vesicles are used for the transport of membrane components and
secretory proteins and for the incorporation of materials. This is known as
vesicular transport (Fig. 5-10). A transport vesicle is first formed as a pit
undercoated with coat proteins (Fig. 5-10) in a phenomenon known as budding.
The proteins to be transported are enveloped inside the budding vesicle, which
is then cut off from the endoplasmic reticulum to form a transport vesicle filled with
baggage (proteins). The transport vesicle, following the detachment of the coat
proteins, is carried to its destination. The membrane of the transport vesicle has a
protein called SNARE, which bonds with a particular SNARE protein on the
target membrane. The destination of a transport vesicle is therefore determined by
the t ype of SNARE protein it has. Transport vesicles supply the lipids and proteins
of the plasma membrane from an endoplasmic reticulum to a Golgi apparatus
(Fig. 5-10A) and from a Golgi apparatus to the plasma membrane (Fig. 5-10B).
When the transport vesicle is fused with the plasma membrane, proteins on the
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membrane stay on the cell surface, while those inside the transport vesicle are
released to the outside of the cell.
Figure 5-10 Main endoplasmic reticulum transport system
(A) From an endoplasmic reticulum to a Golgi apparatus, (B) From a Golgi apparatus to the
plasma membrane, (C) From the plasma membrane to an endosome, (D) From an endosome
to a lysosome
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Column
Speculation on the Origin of Organelles
Organisms are divided into the categories of eukaryotes, cells with a nucleus
(a membrane structure containing DNA) and prokaryotes (cells with no
nucleus). Eukaryotic cells are generally larger than prokaryotic cells, feature
an intracellular environment compartmentalized by membrane structures,
and have a more advanced system of intracellular transport. There has been
speculation regarding how organelles developed during the evolution
process from prokaryotic cells to eukaryotic cells. As shown in Column
Figure 5-5, it is suggested that in ancient times, part of the plasma membrane
of a prokaryotic cell with DNA and ribosomes attached was invaginated
into the cell to form a nucleus enveloping DNA with two membranes. On
the other hand, it has also been suggested that mitochondria and chloroplasts
have evolved from different origins. Since both have a small genome that is
unique to each one, these organelles are believed to have derived from
different prokaryotic cells that lived symbiotically with primitive eukaryotic
cells (in line with endosymbiotic theory; see Chapter 1). This speculation
explains why they have two lipid bilayers (inner and outer membranes), as
well as why vesicular transport, which occurs between other organelles, is
not found in mitochondria and chloroplasts.
Column Figure 5-5
Theory: mitochondria and
chloroplasts originally existed
as foreign cells that were
engulfed by other cells
It is suggested that in ancient prokaryotic
cells (1), the plasma membrane with DNA
and ribosomes attached was invaginated to
form a nucleus enveloped by two membranes
(2), and that foreign cells were engulfed by
the cell, establishing themselves as
mitochondria and chloroplasts (3, 4).
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Incorporation Pathways of Extracellular Materials
Extracellular materials are incorporated through the three pathways shown in
Figure 5-12. The first is, as already discussed, the direct transport of water-soluble
ions and other materials to the cytoplasm by transporters or through channels in
the plasma membrane.
The second pathway is called endocytosis, which involves part of the plasma
membrane being incorporated into the cell in the form of transport vesicles
enveloped by a coat protein called clathrin. When this occurs, proteins and
lipids attached to receptors on the plasma membrane are also incorporated into
the vesicles. This is also the pathway mediated by transport vesicles (Fig. 5-10C).
The endosomes incorporated into the cell gradually lower their pH through the
action of proton pumps, are fused with lysosomes and break down the molecules
incorporated (Fig. 5-10D). Some endosome proteins are then recycled to the
plasma membrane.
The third pathway is called phagocytosis, in which large particles such as
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bacteria are engulfed, and the plasma membrane extends toward and surrounds
the target articles by the action of the cytoskeleton elements such as actin. The
vacuoles formed in the cell are fused with lysosomes, leading to the degradation
of ingested articles.
Figure 5-11
The transport of membrane components through
the budding and fusion of transport vesicles
Figure 5-12
The three pathways for the incorporation of
extracellular materials
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Summary
Chapter 5
• A biological membrane consists of a lipid bilayer with membrane proteins attached.
• Organisms are divided into the categories of eukaryotes, which have membrane-enclosed
organelles in their cells (e.g., the nucleus), and prokaryotes, which have no organelles in
their cells.
• The biological membranes of eukaryotic cells include the plasma membrane, endoplasmic
reticula, Golgi apparatuses and lysosomes (which have a single lipid bilayer) and the
nuclear membrane, mitochondria and chloroplasts (which have two lipid bilayers, i.e., inner
and outer membranes).
• The nucleus contains genetic information, and is host to the occurrence of DNA replication
and RNA transcription.
• Endoplasmic reticula and Golgi apparatuses are involved in the synthesis, processing and
selective transport of lipids, membrane proteins and secretory proteins.
• Endosomes and lysosomes are involved in the uptake and degradation of extracellular
materials, respectively.
•M
itochondria are involved in ATP synthesis, and chloroplasts are involved in plant
photosynthesis. These organelles have DNA that is unique only to them, giving rise to the
possibility that they were originally primitive organisms that become incorporated into cells.
• Biological membranes function as a barrier with limited permeability, and selectively allow
the passage of metabolites and informational molecules.
• Biological membranes have channel and transporter proteins that regulate the transport of
ions and molecules.
• The plasma membrane has receptor proteins that transfer extracellular information to the inside
of the cell by detecting the information and changing the structure of intracellular elements.
• Cells contain a transport system in which membrane structures known as transport vesicles
perform the movement of materials. Transport vesicles bud before being cut off from an
organelle with a membrane structure, and transport proteins and other materials by fusing
with the membrane structure of target organelles.
• Proteins to be transported to a particular organelle have a unique signal sequence, and
membrane proteins on the surface of the organelle incorporate proteins with this sequence.
C SLS / THE UNIVERSITY OF TOKYO
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Chapter 5
Cell Membrane Structure and Organelles
Problems
[1]
[4]
Explain the transport mechanism by which certain molecules
In a biological membrane, it is easy for lipid molecules to move
and ions cross a biological membrane from the inside to the
on the surface of the bilayer, but molecules rarely move between
outside against the concentration gradient (assuming that the
the inside and outside lipid surfaces. Outline the reasons for this.
concentration is higher outside the membrane).
[5]
[2]
Nerve excitation transmission occurs only in one direction.
1) Provide an outline of the transport pathways through which
Outline the reasons for this.
proteins synthesized in the cytoplasm are transported to
different organelles.
2) Are proteins synthesized anywhere other than in the
cytoplasm? If so, please specify.
[6]
Although it may be suggested that the material composition of
endoplasmic reticula, Golgi apparatuses, the plasma membrane
and other organelles should gradually become similar to each
[3]
other through vesicular transport, they are actually quite different.
It has been suggested that mitochondria and chloroplasts have
Explain the mechanism of this phenomenon.
evolved from symbiotic bacteria that parasitized primitive
eukaryotic cells. Explain the basis of this theory.
(Answers on p.253)
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