Chapter 5 Cell Membrane Structure and Organelles

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1 Part II Principles of Individual Cell Function Chapter 5 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, 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). CSLS / THE UNIVERSITY OF TOKYO 85

2 Figure 5-1 Prokaryotic and eukaryotic cells Table 5-1 Main functions of eukaryotic cell compartments divided by cell membranes CSLS / THE UNIVERSITY OF TOKYO 86

3 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 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. CSLS / THE UNIVERSITY OF TOKYO 87

4 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. CSLS / THE UNIVERSITY OF TOKYO 88

5 Chapter 5 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 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. C S L S / T H E U N IV E R S IT Y OF T OK YO 89

6 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 CSLS / THE UNIVERSITY OF TOKYO 90

7 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 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 CSLS / THE UNIVERSITY OF TOKYO 91

8 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). (A) (B) CSLS / THE UNIVERSITY OF TOKYO 92

9 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 +. 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. CSLS / THE UNIVERSITY OF TOKYO 93

10 Column Cholesterol in the Plasma membrane (B) (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 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. CSLS / THE UNIVERSITY OF TOKYO 94

11 Chapter 5 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. 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. C S L S / T H E U N IV E R S IT Y OF T OK YO 95

12 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 (A) (B) 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 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 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) Accumulation of lipid-modified proteins on the biological membrane (B) Structure of phosphatidylinositol (C) Proteins that recognize phosphorylated phosphatidylinositol CSLS / THE UNIVERSITY OF TOKYO 96

13 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 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 CSLS / THE UNIVERSITY OF TOKYO 97

14 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. CSLS / THE UNIVERSITY OF TOKYO 98

15 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 log 10 (Kout/Kin) = 62 log 10 (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 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 CSLS / THE UNIVERSITY OF TOKYO 99

16 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 CSLS / THE UNIVERSITY OF TOKYO 100

17 Column 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 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. CSLS / THE UNIVERSITY OF TOKYO 101

18 Chapter 5 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 C SLS / THE UNIVERSITY OF TOKYO 10 2

19 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 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 CSLS / THE UNIVERSITY OF TOKYO 103

20 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). CSLS / THE UNIVERSITY OF TOKYO 104

21 Incorporation Pathways of Extracellular Materials Extracellular materials are incorporated through the three pathways shown in Figure 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 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 CSLS / THE UNIVERSITY OF TOKYO 1

22 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. Mitochondria 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. CSLS / THE UNIVERSITY OF TOKYO 106

23 Problems [1] Explain the transport mechanism by which certain molecules and ions cross a biological membrane from the inside to the outside against the concentration gradient (assuming that the concentration is higher outside the membrane). [2] 1) Provide an outline of the transport pathways through which proteins synthesized in the cytoplasm are transported to different organelles. 2) Are proteins synthesized anywhere other than in the cytoplasm? If so, please specify. [3] It has been suggested that mitochondria and chloroplasts have evolved from symbiotic bacteria that parasitized primitive eukaryotic cells. Explain the basis of this theory. [4] In a biological membrane, it is easy for lipid molecules to move on the surface of the bilayer, but molecules rarely move between the inside and outside lipid surfaces. Outline the reasons for this. [5] Nerve excitation transmission occurs only in one direction. Outline the reasons for this. [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 other through vesicular transport, they are actually quite different. Explain the mechanism of this phenomenon. (Answers on p.253) CSLS / THE UNIVERSITY OF TOKYO 107

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