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3.15 External Structures of Cells - Advanced

• Distinguish between cilia and flagella.

What propels a bacteria along?

Bacteria, being single-celled organisms, cannot just get up and walk from place to place. So they have to "swim." Todo this, they must have some sort of structure that propels them through their environment. Such a tail-like structureis a flagellum or set of flagella. These protein containing structures spin around a biological motor, allowing thebacteria to move.

External Structures of the Cell

Flagella ( flagellum, singular) are long, thin structures that protrude from the cell membrane. Both eukaryotic andprokaryotic cells can have flagella. Flagella help single-celled organisms move or swim towards food. The flagellaof eukaryotic cells are normally used for movement too, such as in the movement of sperm cells, which have onlya single flagellum. The flagella of either group are very different from each other. Prokaryotic flagella, shown inFigure 3.18, are spiral-shaped and stiff. They spin around in a fixed base much like a screw does, which moves thecell in a tumbling fashion. Eukaryotic flagella are made of microtubules that bend and flex like a whip.

Cilia ( cilium, singular) are made up of microtubule containing extensions of the cell membrane. Although bothcilia and flagella are used for movement, cilia are much shorter than flagella. Cilia cover the surface of some single-celled organisms, such as paramecium. Their cilia beat together to move the little animal-like protists through thewater. In multicellular animals, including humans, cilia are usually found in large numbers on a single surface ofcells. Multicellular animals’ cilia usually move materials inside the body. For example, the mucociliary escalator ofthe respiratory system is made up of mucus-secreting ciliated cells that line the trachea and bronchi. These ciliatedcells, shown in Figure 3.19, move mucus away from the lungs. This mucus catches spores, bacteria, and debris andmoves to the esophagus, where it is swallowed.

A video showing flagella and cilia can be viewed at http://www.youtube.com/watch?v=QGAm6hMysTA (3:12).

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FIGURE 3.18Bacterial flagella spin about in place,which causes the bacterial cell to "tum-ble."

MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/252

FIGURE 3.19Left: Scanning electron micrograph(SEM), of the cilia protruding from humanlung cells. Right: Electron micrographof cross-section of two cilia, showing thepositions of the microtubules inside. Notehow there are nine groups of two mi-crotubules (called dimers) in each cilium.Each dimer is made up of an alpha anda beta tubulin protein that are connectedtogether.

Vocabulary

• cilia (singular, cilium): Short, hairlike projection, similar to flagella, that allows some cells to move.

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• flagella (singular, flagellum): A "tail-like" appendage that protrudes from the cell body of certain prokaryoticand eukaryotic cells; used for locomotion.

• microtubules: Largest component of the cytoskeleton; hollow protein cylinders made of alpha and betatubulin; also found in flagella.

Summary

• Cilia and flagella are extensions of the cell membrane that contain microtubules, and are usually used formovement.

• Cilia cover the surface of some single-celled animals, such as paramecium, but cover only one side of cells insome multicellular organisms.

Explore More

Use this resource to answer the questions that follow.

• Structure and Function of Bacterial Cells at http://textbookofbacteriology.net/structure_2.html .

1. What is the role of the flagellum motor?2. What powers the flagulla motor?3. Describe the process needed to propel the bacterium.4. Describe the structure and function of the basal body and hook of the flagella.

Review

1. Compare and contrast cilia and flagella.

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3.16 The Nucleus - Advanced

• Outline the form and function of the nucleus.

Where does the DNA live?

The answer depends on if the cell is prokaryotic or eukaryotic. The main difference between the two types of cellsis the presence of a nucleus. In eukaryotic cells, DNA lives in the nucleus.

The Nucleus

The nucleus is a membrane-enclosed organelle found in most eukaryotic cells. The nucleus is the largest organellein the cell and contains most of the cell’s genetic information (mitochondria also contain DNA, called mitochondrialDNA, but it makes up just a small percentage of the cell’s overall DNA content). The genetic information, whichcontains the information for the structure and function of the organism, is found encoded in DNA in the form ofgenes. A gene is a short segment of DNA that contains information to encode an RNA molecule or a protein strand.DNA in the nucleus is organized in long linear strands that are attached to different proteins. These proteins helpthe DNA to coil up for better storage in the nucleus. Think how a string gets tightly coiled up if you twist one endwhile holding the other end. These long strands of coiled-up DNA and proteins are called chromosomes. Eachchromosome contains many genes. Humans have about 20,000 to 22,000 genes scattered among 23 chromosomes.

Essentially, the nucleus is the control center of the cell. The function of the nucleus is to maintain the integrity ofthe genes and to control the activities of the cell by regulating gene expression. Gene expression is the process bywhich the information in a gene is "decoded" by various cell molecules to produce a functional gene product, such asa protein molecule or an RNA molecule. Gene expression is a highly regulated process, ensuring RNA and proteinsare only produced when necessary.

The degree of DNA coiling determines whether the chromosome strands are short and thick or long and thin. Be-tween cell divisions, the DNA in chromosomes is more loosely coiled and forms long thin strands called chromatin.DNA is in this uncoiled form during the majority of the cell cycle, making the DNA available to the proteins

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involved in DNA replication and transcription. Before the cell divides, the chromatin coils up more tightly and formchromosomes. Only chromosomes stain clearly enough to be seen under a microscope. The word chromosomecomes from the Greek word chroma (color), and soma (body) due to its ability to be stained strongly by dyes.

Nuclear Envelope

The nuclear envelope is a double membrane of the nucleus that encloses the genetic material. It separates thecontents of the nucleus from the cytoplasm. The nuclear envelope is made of two phospholipid bilayers, an innermembrane and an outer membrane. The outer membrane is continuous with the rough endoplasmic reticulum. Manytiny holes called nuclear pores are found in the nuclear envelope. These nuclear pores help to regulate the exchangeof materials (such as RNA and proteins) between the nucleus and the cytoplasm.

Nucleolus

The nucleus of many cells also contains an organelle called a nucleolus, shown in Figure 3.20. The nucleolusis mainly involved in the assembly of ribosomes. Ribosomes are organelles made of protein and ribosomal RNA(rRNA), and they build cellular proteins in the cytoplasm. The function of the rRNA is to provide a way of decodingthe genetic messages within another type of RNA, called mRNA for messenger RNA, into amino acids. After beingmade in the nucleolus, ribosomes are exported to the cytoplasm where they direct protein synthesis.

FIGURE 3.20The eukaryotic cell nucleus. Visible in thisdiagram are the ribosome-studded doublemembranes of the nuclear envelope, theDNA (as chromatin), and the nucleolus.Within the cell nucleus is a viscous liquidcalled nucleoplasm, similar to the cyto-plasm found outside the nucleus. Thechromatin (which is normally invisible), isvisible in this figure only to show that it isspread out throughout the nucleus.

Vocabulary

• chromatin: Grainy material form of uncoiled DNA; form of DNA during interphase of the cell cycle.

• chromosome: The coiled structure of DNA and histone proteins; allows for the precise separation of replicatedDNA; forms during prophase of mitosis and meiosis.

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• gene: A segment of DNA that contains information to encode an RNA molecule or a single polypeptide.

• gene expression: The process by which the information in a gene is "decoded" to produce a functional geneproduct, such as an RNA molecule or a polypeptide/protein molecule.

• nuclear envelope: Double phospholipid membrane of the nucleus; encloses the genetic material.

• nuclear pore: Tiny hole in the nuclear envelope.

• nucleolus: Section of the nucleus; site of ribosome assembly.

• nucleus (plural, nuclei): The membrane-enclosed organelle found in most eukaryotic cells that contains thegenetic material (DNA); control center of the cell.

• ribosome: A non-membrane bound organelle inside all cells; site of protein synthesis (translation).

Summary

• The nucleus is a membrane-enclosed organelle, found in most eukaryotic cells, which stores the geneticmaterial (DNA).

• The nucleus is surrounded by a double lipid bilayer, the nuclear envelope, which is embedded with nuclearpores.

• The nucleolus is inside the nucleus, and is where ribosomes are made.

Review

1. What is the role of the nucleus of a eukaryotic cell?2. Describe the nuclear envelope.3. What are nuclear pores?4. What is the role of the nucleolus?

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3.17 The Mitochondria - Advanced

• Outline the form and function of the mitochondria.

Sperm cells and muscle cells need lots of energy. What do they have in common?

They have lots of mitochondria. Mitochondria are called the power plants of the cell, as these organelles are wheremost of the cell’s energy is produced. Cells that need lots of energy have lots of mitochondria.

The Mitochondria

A mitochondrion ( mitochondria, plural), is a membrane-enclosed organelle that is found in most eukaryotic cells.Mitochondria are called the "power plants" of the cell because they are the site of cellular respiration. In cellularrespiration, the energy from organic compounds such as glucose, is used to make ATP ( adenosine triphosphate).ATP is the cell’s energy source that is used for such things such as movement and cell division. Some ATP is madein the cytosol of the cell, but most of it is made inside mitochondria. The number of mitochondria in a cell dependson the cell’s energy needs. For example, active human muscle cells may have thousands of mitochondria, while lessactive red blood cells do not have any.

5 Compartments

As the Figure 3.21 (a) and (b) shows, a mitochondrion has two phospholipids membranes. The smooth outermembrane separates the mitochondrion from the cytosol. The inner membrane has many folds, called cristae.

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These cristae greatly increase the membrane surface area for integral proteins. Many proteins involved in cellularrespiration are embedded in this inner membrane. The greater surface area allows more proteins to be locatedthere, resulting in more cellular respiration reactions, and more ATP synthesis. ATP is produced by the enzymeATP synthase, which is a membrane protein of the mitochondria inner membrane. The fluid-filled inside of themitochondrian, called matrix, is where most of the cell’s ATP is made.

FIGURE 3.21(a): Electron micrograph of a single mitochondrion within which you can see many cristae. Mitochondria rangefrom 1 to 10 µm in size. (b): This model of a mitochondrian shows the organized arrangement of the outermembrane and folded inner membrane with cristae, the inter membrane space, the mitochondrial matrix, andATP synthase protein complex.

The mitochondria essentially has five compartments, each with its own function:

1. the outer mitochondrial membrane,2. the intermembrane space (the space between the outer and inner membranes),3. the inner mitochondrial membrane,4. the cristae space (formed by infoldings of the inner membrane), and5. the matrix (space within the inner membrane).

The outer membrane contains large numbers of integral proteins called porins. These porins form channels thatallow small molecules to freely diffuse across the membrane to the other. The inner mitochondrial membrane ishighly impermeable to all molecules. Almost all ions and molecules require special membrane transporters to enteror exit the matrix. ATP synthase, which produces ATP in the matrix, is embedded within this membrane. The cristaegreatly expand the surface area of the inner mitochondrial membrane, enhancing the ability of the mitochondriato produce ATP. The matrix contains a highly-concentrated mixture of hundreds of enzymes, the mitochondrialribosomes, tRNAs, and several copies of the mitochondrial genome. Of the enzymes, the Krebs cycle enzymes arelocated here.

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Mitochondria Genome

Although most of a cell’s DNA is contained in the cell nucleus, mitochondria have their own DNA. Mitochondriaalso have the machinery to manufacture their own RNAs and proteins. The human mitochondrial DNA sequencehas 16,569 base pairs encoding 37 total genes: 22 tRNA genes, 2 rRNA genes, and 13 peptide genes. The 13mitochondrial peptides in humans are integrated into the inner mitochondrial membrane, along with proteins encodedby nuclear genes.

Mitochondria are able to reproduce asexually, like bacteria, and scientists think that they are descended from prokary-otic organisms. According to the Theory of Endosymbiosis, mitochondria were once free-living prokaryotes thatinfected other prokaryotic cells. The invading prokaryotes were protected inside the host cell, and in turn theprokaryote supplied extra ATP to its host. Eventually these two cells turned into one eukaryotic cell, as the twoorganisms evolved so that they could no longer live without each other. Over time, the ancient internal prokaryoteturned into an organelle, resulting in a large cell with an internal organelle. By definition, this is an eukaryotic cell.

Unlike nuclear DNA which is inherited from the father and mother, mitochondrial DNA (mtDNA is most ofteninherited from mothers. However paternal mtDNA occasionally slips through with sperm. The technical processis still unclear but a study was down using Caenorhabditis elegans that showed double membrane vesicles, calledautophagosomes, engulf paternal mitochondria and destroy them.

Since mothers provide the mtDNA and fathers will never pass on a mtDNA, a child shares the same or similarmtDNA sequence as does his/her siblings and mother. This direct inheritance has allowed biologists to track theorigin of modern human and to draw maternal lineages.

Unfortunately, maternal mt(DNA)is susceptible to mutations which are a cause of inherited disease, such as breastcancer. Although, it is important to note that most mutations do not lead to defected mtDNA. Heteroplasmy isthe presence of a mixture of more than one type of mtDNA. Most people have homoplasmic cells, meaning thattheir cells contain only normal, undefected mtDNA. However, people with both normal, undefected mtDNA and notnormal, defected mtDNA, may inherit mitochondrial diseases. The ultimate condition leading to disease is when theproportion of mutant mtDNA reaches a threshold, after which the cell can no longer cope, resulting in disease. Thisthreshold varies among different tissues and different mutations.

Vocabulary

• ATP ( adenosine triphosphate): Energy-carrying molecule that cells use to power their metabolic processes;energy-currency of the cell.

• ATP synthase: Ion channel and enzyme complex; chemically bonds a phosphate group to ADP, producingATP as H+ ions flow through the ion channel.

• cellular respiration: Metabolic process which transfers chemical energy from glucose (a deliverable fuelmolecule) to ATP (a usable energy-rich molecule); most efficient in the presence of oxygen (aerobic).

• cristae: Inner membrane folds of the mitochondrion.

• Heteroplasmy: the presence of a mixture of more than one type of mtDNA (normal or defected).

• Krebs cycle: Stage 2 of aerobic cellular respiration; a series of chemical reactions which completes thebreakdown of glucose begun in stage 1, releasing more chemical energy and producing carbon dioxide; alsocalled the Citric Acid Cycle.

• matrix: Fluid-filled inside of the mitochondrion; space inside of the inner membrane.

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• mitochondria (singular, mitochondrion): Membrane-enclosed organelles that are found in most eukaryoticcells; called the "power plants" of the cell because they use energy from organic compounds to make ATP.

• porin: Integral membrane proteins that act as a pore through which molecules can diffuse.

• Theory of Endosymbiosis: Theory that proposes that eukaryotic organelles, such as mitochondria, evolvedfrom ancient, free-living prokaryotes that invaded other prokaryotic cells.

Summary

• Mitochondria are where energy from organic compounds is used to make ATP.• Mitochondria have a double-membrane, resulting in five distinct compartments within the mitochondrion.

They are:

– The outer mitochondrial membrane.– The intermembrane space (the space between the outer and inner membranes).– The inner mitochondrial membrane.– The cristae space (formed by infoldings of the inner membrane).– The matrix (space within the inner membrane).

• Mitochondria are thought to have evolved from ancient prokaryotic cells.• Mitochondria are most often maternally inherited.

Review

1. Identify the reason why mitochondria are called "power plants" of the cell.2. What are the five compartments of a mitochondria?3. If muscle cells become more active than they usually are, they will grow more mitochondria. Explain why

this happens.4. What determines whether a child inherits a mitochondrial disease?

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3.18 Endoplasmic Reticulum - Advanced

• Outline the form and function of the endoplasmic reticulum.

Does a cell have its own ER?

Yes, but in this case, the ER is not just for emergencies. True, there might be times when the cell responds toemergency conditions and the functions of the ER may be needed, but usually the cell’s ER is involved in normalfunctions. Proteins are also made on the outside of the ER, and this starts a whole process of protein transport, botharound the inside of the cell and to the cell membrane and out.

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) (plural, reticuli) is a network of phospholipid membranes that form hollow tubes,flattened sheets, and round sacs. These flattened, hollow folds and sacs are called cisternae. The membrane of theER is continuous with the outer layer of the nuclear envelope. The ER has two major functions:

1. Transport: Molecules, such as proteins, can move from place to place inside the ER, much like on anintracellular highway.

2. Synthesis: Ribosomes that are attached to ER, similar to unattached ribosomes, make proteins. Lipids arealso produced in the ER.

There are two types of endoplasmic reticulum, rough endoplasmic reticulum (RER) and smooth endoplasmic retic-ulum (SER).

• Rough endoplasmic reticulum is studded with ribosomes which gives it a "rough" sandpaper-like appear-ance. The ribosomes on the RER make proteins that are then transported from the ER in small phospholipid

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sacs called transport vesicles. The transport vesicles pinch off the ends of the ER. These vesicles can easilyshuttle proteins between the ER and the Golgi apparatus. The RER works with the Golgi apparatus to movenew proteins to their proper destinations in the cell or to the cell membrane. Proteins that are made on theRER are inserted directly into the ER and then are transported to their various cellular destinations, includingthe cell membrane.

• Smooth endoplasmic reticulum does not have any ribosomes attached to it, and so it has a smooth ap-pearance. SER has many different functions some of which are: lipid synthesis, carbohydrate metabolism,calcium ion storage, steroid metabolism and drug detoxification. Smooth endoplasmic reticulum is foundin both animal and plant cells and it serves different functions in each. The SER is made up of tubulesand vesicles that branch out to form a network. In some cells there are dilated areas like the sacs of RER.Smooth endoplasmic reticulum and RER form an interconnected network of membranous cisternae, tubulesand vesicles.

FIGURE 3.22Image of nucleus, endoplasmic reticulumand Golgi apparatus, and how they worktogether. The process of secretion fromendoplasmic reticuli (orange) to Golgi ap-paratus (pink) is shown.

Protein Transport

The ER plays a significant role in protein transport. Proteins are transported through the ER and then throughout thecell are marked with a signal sequence. This sequence is usually a short peptide of a few amino acids attached tothe N-terminal end of the protein. This short sequence acts as an address "tag," directing the protein to its correctdestination in the cell. At this time, the signal sequence is removed. These proteins packed into transport vesiclesand moved along the cytoskeleton toward their destination.

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Vocabulary

• cisternae (singular, cisterna): Flattened membranous regions of the rough endoplasmic reticulum and theGolgi apparatus.

• endoplasmic reticulum (ER): A network of phospholipid membranes that form hollow tubes, cisternae, andvesicles; involved in transport of molecules, such as proteins, and the synthesis of proteins and lipids.

• rough endoplasmic reticulum: Endoplasmic reticulum embedded with ribosomes.

• smooth endoplasmic reticulum: Endoplasmic reticulum without embedded ribosomes.

• transport vesicle: A vesicle that is able to move molecules between locations inside the cell.

Summary

• The endoplasmic reticulum is a network of phospholipid membranes that form hollow tubes, cisternae, andvesicles.

• The ER is involved in transport of molecules, such as proteins, and the synthesis of proteins and lipids.• The ER can be rough, with embedded ribosomes, or smooth, without ribosomes.

Review

1. What are the main structural and functional differences between rough endoplasmic reticulum and smoothendoplasmic reticulum?

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3.19 Ribosomes - Advanced

• Outline the form and function of ribosomes.

Where are proteins made?

Proteins are made on ribosomes. The chemical structure of a ribosome is shown above. A ribosome is an organellemade out of just protein and RNA. Its role in protein synthesis is extremely important. And it is the structure of theribosome that allows it to function as it does.

Ribosomes

Ribosomes are small organelles and are the site of protein synthesis (translation). Ribosomes can be found alone orin groups within the cytoplasm. They can also be attached to the endoplasmic reticulum, and others are attached tothe nuclear envelope. Unlike other organelles, ribosomes are not surrounded by a membrane.

Translation is the process of ordering the amino acids in the assembly of a protein. The word ribosome comes fromribonucleic acid and the Greek soma (meaning body). Two Nobel Prizes have been awarded for work relating tothe ribosome. The 1974 Nobel Prize in Physiology or Medicine was awarded to Albert Claude, Christian de Duveand George Emil Palade for the discovery of the ribosome, and the 2009 Nobel Prize in Chemistry was awardedto Venkatraman Ramakrishnan, Thomas A. Steitz and Ada E. Yonath for discovering the detailed structure andmechanism of the ribosome.

Ribozymes are RNA molecules that catalyze chemical reactions, such as translation. Ribosomes, which are just madeout of rRNA (ribosomal RNA) and protein, have been classified as ribozymes, because the rRNA has enzymatic

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activity. The rRNA is important for the peptidyl transferase activity that bonds amino acids. Briefly, the ribosomesinteract with other RNA molecules to make chains of amino acids called polypeptide chains, due to the peptide bondthat forms between individual amino acids. Inside the ribosome, three sites participate in the translation process, theA, P and E sites. Translation will be discussed in detail the Protein Synthesis: Translation (Advanced) concept.

Ribosome Structure

Ribosomes from bacteria, archaea and eukaryotes, have significantly different structures and RNA sequences. Theribosomes in the mitochondria of eukaryotic cells significantly resemble those in bacteria, reflecting the likelyevolutionary origin of mitochondria.

Ribosomes are produced in the nucleolus, and then transported to the cytoplasm. Ribosomes are made of ribosomalproteins, called ribonucleoproteins, and ribosomal RNA (rRNA). Each ribosome has two parts, a large and a smallsubunit, as shown in Figure 3.23. The subunits are attached to each other. During translation, the smaller subunitbinds to the mRNA, while the larger subunit binds to the tRNA with attached amino acids. When a ribosome finishesreading an mRNA molecule, the two ribosomal subunits disassociate.

FIGURE 3.23The two subunits that make up a ribo-some, small organelles that are intercel-lular protein factories.

The two ribosomal subunits are named base on their sedmentation rate in a centrifuge. The unit of measurement isthe Svedberg unit, a measure of the rate of sedimentation, not the size. This accounts for why fragment names donot add up (70S is made of 50S and 30S).

• Prokaryotes have 70S ribosomes, each consisting of a small (30S) and a large (50S) subunit. Their smallsubunit has a 16S RNA subunit (consisting of 1540 nucleotides) bound to 21 proteins. The large subunit iscomposed of a 5S RNA subunit (120 nucleotides), a 23S RNA subunit (2900 nucleotides) and 31 proteins.

• Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large (60S) subunit. Their 40S subunithas an 18S RNA (1900 nucleotides) and 33 proteins. The large subunit is composed of a 5S RNA (120nucleotides), 28S RNA (4700 nucleotides), a 5.8S RNA (160 nucleotides) subunits and about 49 proteins.

• The ribosomes found in chloroplasts and mitochondria of eukaryotes also consist of large and small subunitsbound together with proteins into one 70S particle. These organelles are believed to be descendants of bacteriaand as such their ribosomes are similar to those of bacteria.

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Vocabulary

• ribonucleoprotein: A nucleoprotein that contains RNA; includes the ribosome, vault ribonucleoproteins, andsmall nuclear RNPs (snRNPs).

• ribosome: A non-membrane bound organelle inside all cells; site of protein synthesis (translation).

• ribozyme: An RNA molecule with a tertiary structure that enables it to catalyze a chemical reaction.

• Svedberg unit: A non-SI unit for sedimentation rate; technically a measure of time that offers a measure ofparticle size; 10−13 seconds (100 fs).

• translation: The process of synthesizing a polypeptide/protein from the information in a mRNA sequence;occurs on ribosomes.

Summary

• Ribosomes are small organelles and are the site of protein synthesis. They are found in all cells.• Ribosomes are composed of a large and small subunit. Prtokaryotic ane eukaryotic ribosomal subunits differ

in size.

Review

1. What is the role of the ribosome?2. What is a significant difference between the structure of a ribosome and other organelles?3. Describe the structural differences between prokaryotic and eukayrotic ribosomes.

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3.20 The Golgi Apparatus - Advanced

• Outline the form and function of the Golgi apparatus.

Why balloons?

The Golgi apparatus is said to look like a stack of deflated balloons. In essence, that is what the Golgi apparatus is.Not balloons, but plasma membrane. The Golgi apparatus is a series of stacks of membrane, with some extremelyimportant functions.

The Golgi Apparatus

The Golgi apparatus, which is also known as the Golgi complex or Golgi body, is a large organelle found in mosteukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi.

The Golgi apparatus is usually made up of five to eight cup-shaped, membrane-covered stacks of discs calledcisternae (singular, cisterna), as shown in Figure 3.26. Both plant and animal cells have a Golgi apparatus. Atypical mammalian cell will have 40 to 80 of these stacks. While plant cells can have up to several hundred Golgistacks scattered throughout the cytoplasm. In plants, the Golgi apparatus contains enzymes that synthesize some ofthe cell wall polysaccharides.

The Golgi apparatus modifies, sorts, and packages different substances for secretion out of the cell, or for use withinthe cell. The Golgi apparatus is found close to the nucleus of the cell where it modifies proteins that have beendelivered in transport vesicles from the RER. It is also involved in the transport of lipids around the cell. Pieces ofthe Golgi membrane pinch off to form vesicles that transport molecules around the cell. The Golgi apparatus canbe thought of as similar to a post office; it packages and labels "items" and then sends them to different parts of thecell. The Golgi apparatus tends to be larger and more numerous in cells that synthesize and secrete large quantitiesof materials; for example, the plasma B cells and the antibody-secreting cells of the immune system have prominentGolgi complexes.

The stack of cisternae has four functional regions: the cis-Golgi network, medial-Golgi, endo-Golgi, and trans-Golginetwork. Vesicles from the ER fuse with the network and subsequently progress through the stack from the cis- tothe trans-Golgi network, where they are packaged and sent to their destination. Each cisterna includes special Golgi

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enzymes which modify or help to modify proteins that travel through it. Proteins may be modified by the additionof a carbohydrate group (glycosylation) or phosphate group (phosphorylation). These modifications may form asignal sequence on the protein, which determines the final destination of the protein. For example, the addition of amannose-6-phosphate signals the protein for lysosomes.

FIGURE 3.24This animal cell depicts the Golgi appara-tus as a stack of flattened discs. The nu-cleus with the adjacent endoplasmic retic-ulum, and numerous mitochondria arealso easily identifiable.

The Endomembrane System

Together with the ER and transport vesicles, the Golgi apparatus is part of the cell’s endomembrane system, whichtransports molecules around the cell. This system transports molecules, such as proteins, in vesicles. The vesiclesthat leave the RER are transported to the cis face of the Golgi apparatus, where they fuse with the Golgi membraneand empty their contents into the lumen. Once inside the lumen, the molecules are modified, then sorted for transportto their next destinations. In addition to the ER, Golgi apparatus, the endomembrane system includes the nuclearenvelope, lysosomes, vacuoles, vesicles, peroxisomes and the cell membrane.

Those proteins destined for areas of the cell other than the ER or Golgi apparatus are moved towards the trans face ofthe Golgi complex, to a complex network of membranes and associated vesicles known as the trans-Golgi network(TGN). This area of the Golgi is the point at which proteins are sorted and shipped to their intended destinations bytheir placement into one of at least three different types of vesicles, depending upon the molecular signal they carry.

Vocabulary

• cisternae (singular, cisterna): Flattened membranous regions of the rough endoplasmic reticulum and theGolgi apparatus.

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FIGURE 3.25The secretory process (vesicular trans-port) from endoplasmic reticulum to Golgiapparatus. Note how the ER is attachedto the nuclear envelope and the flow ofvesicles from the cis to the trans face ofthe Golgi apparatus.

• endomembrane system: Divide the cell into functional and structural compartments (organelles); composedof the different membranes that are suspended in the cytoplasm within a eukaryotic cell; includes the nuclearenvelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vacuoles, vesicles, peroxisomes and thecell membrane.

• Golgi apparatus: A large organelle that is usually made up of five to eight cup-shaped, membrane-covereddiscs called cisternae; modifies, sorts, and packages different substances for secretion out of the cell, or foruse within the cell.

• trans-Golgi network (TGN): A major sorting pathway that directs newly synthesized proteins to differentsubcellular destinations.

Summary

• The Golgi apparatus is a large organelle that is usually made up of five to eight cup-shaped, membrane-covereddiscs called cisternae.

• The Golgi apparatus modifies, sorts, and packages different substances for secretion out of the cell, or for usewithin the cell.

Review

1. Describe the structure and role of the Golgi apparatus.2. Describe the endomembrane system.

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3.21 Vesicles and Vacuoles - Advanced

• Outline the form and function of organelles.

What’s a little ball of plasma membrane?

A vesicle. Because vesicles are made of phospholipids, they can break off of and fuse with other membraneousmaterial. This allows them to serve as small transport containers, moving substances around the cell and to the cellmembrane.

Vesicles

A vesicle is a small, spherical compartment that is separated from the cytosol by at least one lipid bilayer. Manyvesicles are made in the Golgi apparatus and the endoplasmic reticulum, or are made from parts of the cell membraneby endocytosis. Vesicles can also fuse with the cell membrane and release their contents to the outside. This processis called exocytosis. In addition to the Golgi apparatus and ER, vesicles can also fuse with other organelles withinthe cell.

Vesicles from the Golgi apparatus can be seen in Figure 3.26. Because a vesicle is essentially a small organelle,the space inside the vesicle can be chemically different from the cytosol. It is within the vesicles that the cell canperform various metabolic activities, as well as transport and store molecules.

Types of Vesicles

Vesicles can be classified by their contents and function.

• Transport vesicles are part of the endomembrane system. They are able to move molecules such as proteinsbetween locations inside the cell. For example, transport vesicles move proteins from the rough endoplasmicreticulum to the Golgi apparatus.

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FIGURE 3.26Vesicles from the Golgi apparatus can beseen in this figure.

• Lysosomes are vesicles that are formed by the Golgi apparatus. They contain powerful enzymes that couldbreak down (digest) the cell. Lysosomes break down harmful cell products, waste materials, and cellulardebris and then force them out of the cell. They also digest invading organisms such as bacteria. Lysosomesalso break down cells that are ready to die, a process called autolysis.

• Peroxisomes are vesicles that use oxygen to break down toxic substances in the cell. Unlike lysosomes,which are formed by the Golgi apparatus, peroxisomes self-replicate by growing bigger and then dividing.They are common in liver and kidney cells that break down harmful substances. Peroxisomes are namedfor the hydrogen peroxide (H2O2) that is produced when they break down organic compounds. Hydrogenperoxide is toxic, and in turn is broken down into water (H2O) and oxygen (O2) molecules.

• Secretory Vesicles contain materials that are to be excreted from the cell, such as wastes or hormones.Secretory vesicles include synaptic vesicles and vesicles in endocrine tissues. Synaptic vesicles store neu-rotransmitters. They are located at presynaptic terminals in neurons. When a signal reaches the end of anaxon, the synaptic vesicles fuse with the cell membrane and release the neurotransmitter. The neurotransmittercrosses the synaptic junction, and binds to a receptor on the next cell. Some cells also produce molecules, suchas hormones produced by endocrine tissues, needed by other cells. These molecules are stored in secretoryvesicles and released when needed. Secretory vesicles also hold enzymes needed to make extracellularstructures, such as the extracellular matrix of animal cells.

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Vesicles and Transport

Most vesicles are involved in transporting some sort of molecule, such as a hormone or neurotransmitter. Thesevesicles must first form around the substance being transported. This requires numerous vesicle coats to surroundand bind to the proteins being transported. They also trap various transmembrane receptor proteins, called cargoreceptors, which in turn trap the cargo molecules.

The Vesicle Coat

The vesicle coat selects specific proteins as cargo. It selects cargo proteins by binding to sorting signals. Thesecomplexes cluster in the membrane, forming a vesicle buds, or coated pit. There are three types of vesicle coats:clathrin, COPI and COPII. Clathrin coats are found on vesicles trafficking between the Golgi and plasma membrane,the Golgi and endosomes, and the plasma membrane and endosomes. COPI ( coat protein complex) coated vesiclesare responsible for transport from the cis-Golgi to the ER (retrograde transport), while COPII coated vesicles areresponsible for transport from the ER to the Golgi (anterograde transport). Low-density lipoprotein (LDL) receptorsaggregate in clathrin coated pits prior to internalization.

SNAREs

The vesicle fuses to the membrane phospholipids to release its materials. This process is mediated by a classof proteins known as SNAREs, for Soluble NSF Attachment Protein Receptors. SNAREs are divided into twocategories, depending on their location. Vesicle or v-SNAREs are incorporated into the membranes of transportvesicles, and target or t-SNAREs are located in the membranes of target compartments. The v-SNAREs identifythe vesicle’s cargo, while the t-SNAREs on the target membrane cause the fusion of the vesicle with the targetmembrane.

Vesicle Fusion

For a vesicle to release its contents to a cell organelle or to the outside of the cell, the vesicle and target membranemust fuse. This process is called vesicle fusion. Fusion between the vesicle and a target membrane occurs in one oftwo ways: full fusion or "kiss-and-run" fusion. In a full fusion process, the vesicle phospholipids fully incorporateinto the plasma membrane. The vesicle can only be reformed and by a clathrin-coat-dependent process. With kiss-and-run fusion, the vesicle reforms after the release of its material. This allows the rapid release of materials froma synaptic vesicle. In this type of fusion, the vesicle forms a fusion pore or porosome in the presynaptic membraneand releases its neurotransmitters across the synapse, after which the vesicle reforms, allowing it to be reused.

Vacuoles

Vacuoles are membrane-bound organelles that can have secretory, excretory, and storage functions. Vacuoles areusually much larger than vesicles. Many organisms will use vacuoles as storage areas and some plant cells havevery large vacuoles. The large central vacuole of the plant cell is used for osmotic control (storage of water) andnutrient storage. Contractile vacuoles are found in certain protists. These vacuoles take water from the cytoplasmand excrete it from the cell to avoid bursting due to osmotic pressure.

Vocabulary

• clathrin: A protein that plays a major role in the formation of coated vesicles.

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• contractile vacuole: An organelle found in freshwater protists involved in osmoregulation; pumps excesswater out of a cell.

• endocytosis: The cellular process of capturing a material/substance from outside the cell by vesicle formation.

• exocytosis: The cellular process of secreting materials by vesicle fusion.

• hormone: A chemical messenger molecule.

• lysosome: A vesicle that contains powerful digestive enzymes.

• neurotransmitter: Chemical messages which are released at the synapse; relay the message/signal onto thenext neuron or other type of cell.

• peroxisome: Vesicles that use oxygen to break down toxic substances in the cell.

• porosome: A cup-shaped structure in the cell membranes of eukaryotic cells where vesicles dock in theprocess of vesicle fusion and secretion.

• secretory vesicle: Vesicle with materials that are to be excreted/secreted from the cell.

• SNARE: Soluble NSF Attachment Protein Receptor; mediate vesicle fusion through full fusion exocytosis orkiss-and-run fusion exocytosis.

• synaptic vesicle: Vesicle located at presynaptic terminals in neurons; store neurotransmitters.

• transport vesicle: A vesicle that is able to move molecules between locations inside the cell.

• vacuole: Membrane-bound organelle that can have secretory, excretory, and storage functions; plant cells havea large central vacuole.

• vesicle: A small, spherical compartment that is separated from the cytosol by at least one lipid bilayer; usedfor transport and storage.

• vesicle coat: Clusters selected membrane cargo proteins into regions of the plasma membrane for internaliza-tion; develops vesicle buds.

Summary

• Vesicles store and transport materials with the cell. Some of these materials are transported to other organelles,other materials are secreted from the cell.

• Examples of vesicles include secretory vesicles, transport vesicles, synaptic vesicles and lysosomes.• Vacuoles are membrane-bound organelles that can have secretory, excretory, and storage functions.They are

usually larger than vesicles.

Review

1. Compare vesicles to vacuoles.2. Describe three types of vesicles.3. How does a vesicle export materials from the cell?

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3.22 Other Structures of Cells - Advanced

• Outline the form and function of cellular structures.

How are chromosomes separated during cell division?

They are pulled apart by spindle fibers. The fibers are made of microtubules and are organized by the centrioles.Two pairs of centrioles are seen on opposite sides of the cell during prophase, the first phase of mitosis.

Centrioles

Centrioles are rod-like structures made of short microtubules. Though they are found in most eukaryotic cells,centrioles are absent in some plants and most fungi.

Nine groups of three microtubules (nine triplets) make up each centriole. The nine triplets are arranged in acartwheel-like orientation. Two perpendicularly placed centrioles make up the centrosome. Centrioles are veryimportant in cellular division, where they arrange the mitotic spindles that pull the chromosome apart duringmitosis. The position of the centriole determines the position of the nucleus, thus playing a crucial role in thespatial arrangement of the cell.

Centrioles are a very important part of centrosomes, which are involved in organizing microtubules in the cytoplasm.Centrosomes are associated with the nuclear membrane during prophase of the mitosis. In mitosis, the nuclearmembrane breaks down and the microtubule organizing center (MTOC) of the centrosome arranges microtubulessuch that they interact with the chromosomes to build the mitotic spindle.

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FIGURE 3.27Here the centrosome is shown as a pairof orange cylindrical centrioles. They aremade of nine triplets of microtubules.

Junctions

Junctions are areas between cells that either allow or prevent the movement of materials. Junctions are usuallycomposed of numerous proteins, forming a large molecular complex. Gap junctions, desmosomes and tight junctionsare three examples of junctions.

Gap Junctions

A gap junction or nexus is a specialized intercellular connection between a variety of animal cell-types. Thisjunction is a type of "opening," or channel, directly connecting the cytoplasm of two cells, which allows variousmolecules and ions to pass freely between these cells. One gap junction channel is composed of two connexonswhich connect across the intercellular space. Six connexins proteins create one connexon (hemichannel) channel.Each connexin protein has four transmembrane domains. The complete gap junction is a macromolecular complexcomposed of several to hundreds of individual junctions. Gap junctions are especially important in cardiac musclecells. The action potential signaling contraction is passed efficiently and effortlessly through gap junctions, allowingthe heart muscle cells to contract in tandem. Electrical synapses in the brain also pass through gap junctions. Thisallows action potentials at the synaptic terminals to be transmitted across to the postsynaptic cell without the needof a neurotransmitter.

Gap junctions are analogous to the plasmodesmata that join plant cells.

Desmosomes

A desmosome is a cell junction specialized for cell-to-cell adhesion. They are found in simple and stratified squa-mous epithelium, and in muscle tissue where they bind muscle cells to one another. These junctions are composedof complexes of cell surface adhesion proteins and linking proteins. These proteins have both an intracellular andextracellular region. Inside the cell, they attach to intracellular filaments of the cytoskeleton. Outside the cell, theyattach to other adhesion proteins.

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The cell adhesion proteins of the desmosome, desmoglein and desmocollin, are members of the cadherin familyof adhesion proteins. These proteins are transmembrane proteins that bridge the space between adjacent epithelialcells. The extracellular domains of these cells bind to other cadherin proteins on an adjacent cell. The extracellulardomain of the desmosome is called the Extracellular Core Domain (ECD). This is where the two adhesion proteinsinteract.

Tight Junction

Tight junctions are the closely associated areas of two cells. It is a type of junctional complex present only invertebrates. The corresponding junctions that occur in invertebrates are septate junctions. An example of a tightjunction is between epithelial cells in the distal convoluted tubule and the collecting duct part of the nephron in thekidney.

Tight junctions are common at epithelia, which are sheets of cells that form a boundary between a mass of cellsand a cavity or space (a lumen). The membranes of these cells join together, forming a virtually impermeablebarrier to fluid. Tight junctions essentially seal adjacent epithelial cells in a narrow layer just beneath their apicalsurface, which is the portion of the cell exposed to the lumen. The rest of the cell surface is known as the basolateralsurface. Tight junctions prevent integral membrane proteins from moving between the apical and basolateral surface,maintaining the properties of those distinct surfaces. For example, receptor-mediated endocytosis occurs at the apicalsurface and exocytosis at the basolateral surface.

Tight junctions are composed of strands of transmembrane proteins embedded in the plasma membranes of twoadjacent cells. The extracellular domains of these proteins directly join to one another. These joining proteinsassociate with peripheral membrane proteins located on the intracellular side of plasma membrane. These peripheralproteins anchor the strands to the actin component of the cytoskeleton, effectively forming a molecular complexthat joins together the cytoskeletons of adjacent cells. The major types anchoring proteins of tight junctions are theclaudins and the occludins.

In addition to holding cells together, tight junctions play a role in the transport of materials. Tight junctions preventthe passage of molecules and ions through the space between cells. So these molecules and ions must actuallyenter cells (either by diffusion or active transport) in order to proceed through a tissue. This allows tight junctionsto indirectly play a role over what substances are allowed into a specific cell. Tight junctions play this role inmaintaining the blood-brain barrier.

Vocabulary

• centriole: A cylindrical shaped cell structure composed of nine triplets of microtubules; structure from whichspindle fibers originate.

• centrosome: An organelle that serves as the main microtubule organizing center (MTOC) of the animal cell.

• connexon: An assembly of six connexin proteins; part of a gap junction channel between the cytoplasm oftwo adjacent cells.

• desmosome: A junctional complex cell structure specialized for cell-to-cell adhesion.

• gap junction: A specialized intercellular connection between a various animal cell-types; directly connectsthe cytoplasm of two cells; the narrow gap between the pre- and post-synaptic cells in electrical synapses.

• microtubule organizing center (MTOC): A structure found in eukaryotic cells from which microtubulesemerge.

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• plasmodesmata: Microscopic channels which traverse the cell walls of plant cells; enables transport andcommunication between them.

• tight junction: The closely associated areas of two cells whose membranes join together; forms a virtuallyimpermeable barrier to fluid.

Summary

• Centrioles are made of short microtubules and are very important in cell division.• Cellular junctions allow cell association communication, and adhesion.

Explore More

Use this resource to answer the questions that follow. Inter Cellular Junctions - The Tissue Level of Organizationat https://www.youtube.com/watch?v=ARaj3Kz1cCQ .

1. List the 5 types of intercellular junctions.2. Briefly describe each type of junction.3. What is a connexon?

Review

1. What is a cell junction?2. Describe the structure of a gap junction. How does the structure relate to its function?3. Distinguish between tight junctions and desmosomes.

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3.23 Plant Cells - Advanced

• Identify and describe three structures that are present solely in plant cells.

What do plants have to do that animals don’t?

When an animal needs energy, it eats food. That’s why animals use mitochondria to convert food into energy.Plants, on the other hand, don’t seem to eat anything. Instead, they receive energy from water and sunlight. Theyuse chloroplasts to convert light into energy through photosynthesis. The focus of this concept is to delineate thedistinct differences between plant and animal cells.

Special Structures in Plant Cells

Most of the organelles that have been discussed in other concepts, such as ribosomes, the mitochondria, endoplasmicreticulum, and Golgi complex, are common to both animal and plant cells. However, plant cells also have featuresthat animal cells do not have; they have a cell wall, a large central vacuole, and plastids such as chloroplasts. Theyalso have junctions called plasmodesmata.

Plants have very different lifestyles from animals, and these differences are apparent when you examine the structureof the plant cell. Plants have to make their own food, and they do so in a process called photosynthesis. They take

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in carbon dioxide (CO2) and water (H2O) and convert them into sugars. The features unique to plant cells can beseen in Figure 3.28.

FIGURE 3.28In addition to containing most of the or-ganelles found in animal cells, plant cellsalso have a cell wall, a large central vac-uole, and plastids. These three featuresare not found in animal cells.

Cell Wall

A cell wall is a rigid layer that is found outside the cell membrane and surrounds the cell. The cell wall containsnot only cellulose and protein, but other polysaccharides as well. In fact, two other classes of polysaccharides,hemicelluloses and pectic polysaccharides, can comprise 30% of the dry mass of the cell wall. The cell wall providesstructural support and protection. Pores in the cell wall allow water and nutrients to move into and out of the cell.The cell wall also prevents the plant cell from bursting when water enters the cell.

Microtubules guide the formation of the plant cell wall. Cellulose is laid down by enzymes to form the primary cellwall. Some plants also have a secondary cell wall. The secondary wall contains a lignin, a secondary cell componentin plant cells that have completed cell growth/expansion.

Central Vacuole

Most mature plant cells have a central vacuole that occupies more than 30% of the cell’s volume, but can alsooccupy as much as 90% of the volume of certain cells. The central vacuole is surrounded by a membrane called thetonoplast. The central vacuole has many functions. Aside from storage, the main role of the vacuole is to maintainturgor pressure against the cell wall. Proteins found in the tonoplast control the flow of water into and out of thevacuole. The central vacuole also stores the pigments that color flowers.

The central vacuole contains large amounts of a liquid called cell sap, which differs in composition to the cell cytosol.Cell sap is a mixture of water, enzymes, ions, salts, and other substances. Cell sap may also contain toxic byproductsthat have been removed from the cytosol. Toxins in the vacuole may help to protect some plants from being eaten.

Plastids

Plant plastids are a group of closely related membrane-bound organelles that carry out many functions. Theyare responsible for photosynthesis, for storage of products such as starch, and for the synthesis of many types of

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molecules that are needed as cellular building blocks. Plastids have the ability to change their function betweenthese and other forms. Plastids contain their own DNA and some ribosomes, and scientists think that plastids aredescended from photosynthetic bacteria that allowed the first eukaryotes to make oxygen. The main types of plastidsand their functions are:

• Chloroplasts are the organelle of photosynthesis. They capture light energy from the sun and use it with waterand carbon dioxide to make food (sugar) for the plant. The arrangement of chloroplasts in a plant’s cells canbe seen in Figure 3.29.

• Chromoplasts make and store pigments that give petals and fruit their orange and yellow colors.• Leucoplasts do not contain pigments and are located in roots and non-photosynthetic tissues of plants. They

may become specialized for bulk storage of starch, lipid, or protein. However, in many cells, leucoplasts donot have a major storage function; instead they make molecules such as fatty acids and many amino acids.

FIGURE 3.29Plant cells with visible chloroplasts.

The Chloroplast

Chloroplasts capture light energy from the sun and use it with water and carbon dioxide to produce sugars for food.Chloroplasts look like flat discs that are usually 2 to 10 micrometers in diameter and 1 micrometer thick. A model ofa chloroplast is shown in Figure 3.30. The chloroplast is enclosed by an inner and an outer phospholipid membrane.Between these two layers is the intermembrane space. The fluid within the chloroplast is called the stroma, and itcontains one or more molecules of small circular DNA. The stroma also has ribosomes. Within the stroma are stacksof thylakoids, the sub-organelles which are the site of photosynthesis. The thylakoids are arranged in stacks calledgrana (singular: granum). A thylakoid has a flattened disk shape. Inside it is an empty area called the thylakoidspace or lumen. Photosynthesis takes place on the thylakoid membrane.

Within the thylakoid membrane is the complex of proteins and light-absorbing pigments, such as chlorophylland carotenoids. This complex allows capture of light energy from many wavelengths because chlorophyll andcarotenoids both absorb different wavelengths of light. More about how chloroplasts convert light energy intochemical energy will be presented in the Photosynthesis (Advanced) concepts.

Plasmodesmata

Plasmodesmata (singular, plasmodesma) are microscopic channels which traverse the cell walls of plant cellsand some algal cells. These junctions enable two cells to transport materials and communication between them.

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FIGURE 3.30The internal structure of a chloroplast,with a granal stack of thylakoids circled.

Plasmodesmata are similar to gap junctions of animal cells. Like gap junctions, plasmodesmata enable directintercellular transport of substances between cells. However, unlike other junctions, plasmodesmata do not seemto be protein based. Rather, they are made from membrane and cell wall material. Plasmodesmata move varioustypes of molecules, including transport proteins (including transcription factors), short interfering RNA, messengerRNA and viral genomes from cell to cell. A typical plant cell may have between 1,000 and 100,000 plasmodesmataconnecting it with adjacent cells.

There are two forms of plasmodesmata: primary plasmodesmata, which are formed during cell division, andsecondary plasmodesmata, which can form between mature cells. As plant cells are surrounded by a polysaccharidecell wall, movement of materials and communication between cells is more complicated than in animal cells.Neighboring plant cells are separated by a pair of cell walls and the space between them, forming an extracellulardomain known as the apoplast.

Primary plasmodesmata form during cell division. These junctions form as portions of the endoplasmic reticulumare trapped in the apoplast as new cell wall is formed between two newly divided plant cells. This eventuallybecome the cytoplasmic connections between cells (primary plasmodesmata). Secondary plasmodesmata form asplasmodesmata are inserted into existing cell walls between non-dividing cells. This process forms a cytoplasmicsleeve, a fluid-filled space enclosed by the cell membrane. The cytoplasmic sleeve is a continuous extension ofthe cytosol of the two adjacent cells. Molecules and ions pass through plasmodesmata using this passage. Thesemolecules move by diffusion without the need for additional chemical energy.

Vocabulary

• apoplast: The space outside the plasma membrane of plant cells; formed by the continuum of cell walls ofadjacent cells.

• cell wall: Rigid layer that surrounds the plasma membrane of prokaryotic cells and plant cells; helps supportand protect the cell.

• central vacuole: Large saclike organelle in plant cells; stores substances such as water; helps keep planttissues rigid.

• chloroplast: The organelle of photosynthesis; site of photosynthesis.

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• chromoplast: Plastid responsible for pigment synthesis and storage in specific photosynthetic eukaryotes.

• cytoplasmic sleeve: A fluid-filled space enclosed by the plasmalemma and a continuous extension of thecytosol.

• grana (singular: granum): Structure within the chloroplast; consists of stacks of sac-like thylakoid mem-branes.

• leucoplast: Non-pigmented plastid specialized for bulk storage of starch, lipid or protein; located in roots andnon-photosynthetic tissues of plants.

• photosynthesis: The process by which carbon dioxide and water are converted to glucose and oxygen, usingsunlight for energy.

• plasmodesmata (singular, plasmodesma): Microscopic channels which traverse the cell walls of plant cells;enables transport and communication between them.

• plastid: Organelle found in the cells of plants and algae; the site of manufacture and storage of importantchemical compounds used by the cell; often contain pigments.

• stroma: Space outside the thylakoid membranes of a chloroplast; site of the Calvin cycle of photosynthesis.

• thylakoid: Sub-organelle within the chloroplast; site of the light reactions of photosynthesis.

• tonoplast: Membrane that surrounds the central vacuole.

Summary

• Plant cells have a cell wall, a large central vacuole, and plastids such as chloroplasts.• The cell wall is a rigid layer that is found outside the cell membrane and surrounds the cell, providing structural

support and protection.• The central vacuole maintains turgor pressure against the cell wall.• Chloroplasts capture light energy from the sun and use it with water and carbon dioxide to produce sugars for

food.• Plasmodesmata are gaps between plant cells, connecting the cytoplasms of plant cells.

Explore More

• Eucaryotic Cell Interactive Animation: Plant Cell at http://www.cellsalive.com/cells/cell_model.htm .

Review

1. List three structures that are found in plant cells but not in animal cells.2. Identify two functions of plastids in plant cells.3. What is the role of the cell wall?4. Describe plasmodesmata.

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3.24 Organization of Cells - Advanced

• Distinguish between a colonial organism and a multicellular organism.• Outline the relationship between cells, tissues, organs, and organ systems.

Why be organized?

It can be said organization leads to efficiency. And in you, cells are organized into tissues, which are organized intoorgans, which are organized into organ systems, which form you. And it can be said that the human body is a veryorganized and efficient system.

Organization of Cells

Biological organization exists at all levels in organisms. It can be seen at the smallest level, in the molecules thatmake up such compounds as DNA and proteins, to the largest level, in an organism such as a blue whale, the largestmammal on Earth. Similarly, single celled prokaryotes and eukaryotes show order in the way their cells are arranged.Single-celled organisms such as an amoeba are free-floating and independent-living. Their single-celled "bodies"are able to carry out all the processes of life such as metabolism and respiration without help from other cells.

Some single-celled organisms such as bacteria can group together and form a colony. A colony refers to a group ofindividual organisms of the same species that live closely together. This is usually done to benefit the group, suchas by providing a stronger defense or the ability to attack bigger prey. A colony can also form from organisms otherthan bacteria. A bacterial colony often defends from a single organism, producing a colony of genetically identicalindividuals.

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A specific type of colony of microorganisms is a biofilm. A biofilm is a large grouping of many microorganismsthat sticks to a surface and makes a protective coating over itself. Biofilms can show similarities to multicellularorganisms, in the sense that a biofilm will have properties and capabilities greater than the capabilities of theindividual organisms.

Division of labor is the process in which one group of cells does one job (such as making the "glue" that sticks thebiofilm to the surface), while another group of cells does another job (such as taking in nutrients). Multicellularorganisms carry out their life processes through division of labor and they have specialized cells that have specificfunctions. However, biofilms are not considered a multicellular organism, but this and other colonial organisms wereprobably the first step toward the evolution of multicellular organisms.

FIGURE 3.31Colonial algae of the genus Volvox.

Colonial Organisms

A colony of single-cell organisms is known as colonial organisms. The difference between a multicellular organismand a colonial organism is that the individual organisms that form a colony or biofilm can, if separated, survive ontheir own, while cells from a multicellular organism (e.g., liver cells) cannot.

Colonial organisms were probably one of the first evolutionary steps towards multicellular organisms. Algae of thegenus Volvox are an example of the bridge between colonial organisms and multicellular organisms. Each Volvox,shown in Figure 3.31, is a colonial organism. It is made of up to 50,000 photosynthetic flagellate algae that aregrouped together into a hollow sphere. Volvox live in a variety of freshwater habitats, and were first reported byAntonie van Leeuwenhoek in 1700.

The Volvox sphere has a distinct front and back end. The colony of cells can swim in a coordinated fashion. Thecells have eyespots, which are more developed in the cells near the front. This enables the colony to swim towardslight.

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Origin of Multicellularity

The oldest known multicellular organism is a red algae Bangiomorpha pubescens, fossils of which were found in1.2 billion year old rock. However, the first organisms were single celled. How multicellular organisms developedis the subject of much debate.

Scientists think that multicellularity arose from cooperation between many organisms of the same species. TheColonial Theory proposes that this cooperation led to the development of a multicellular organism. Many examplesof cooperation between organisms in nature have been observed. For example, a certain species of amoeba (asingle-celled protist) groups together during times of food shortage and forms a colony that moves as one to a newlocation. Some of these amoebas then become slightly differentiated from each other. Volvox, shown in Figure 3.31,is another example of a colonial organism. Most scientists accept that the Colonial theory explains how multicellularorganisms evolved.

Multicellular organisms are organisms that are made up of more than one type of cell and have specialized cellsthat are grouped together to carry out specialized functions. Most life that you can see without a microscope ismulticellular. As discussed earlier, the cells of a multicellular organism would not survive as independent cells. Thebody of a multicellular organism, such as a tree or a cat, exhibits organization at several levels: tissues, organs, andorgan systems. Similar cells are grouped into tissues, groups of tissues make up organs, and organs with a similarfunction are grouped into an organ system.

Levels of Organization in Multicellular Organisms

The simplest living multicellular organisms, sponges, are made of many specialized types of cells that work togetherfor a common goal. Such cell types include digestive cells, tubular pore cells; and epidermal cells. Though thedifferent cell types create a large organized, multicellular structure—the visible sponge—they are not organized intotrue interconnected tissues. If a sponge is broken up by passing it through a sieve, the sponge will reform on theother side. However, if the sponge’s cells are separated from each other, the individual cell types cannot survivealone. Simpler colonial organisms, such as members of the genus Volvox, as shown in Figure 3.31, differ in thattheir individual cells are free-living and can survive on their own if separated from the colony.

A tissue is a group of connected cells that have a similar function within an organism. More complex organismssuch as jellyfish, coral, and sea anemones have a tissue level of organization. For example, jellyfish have tissues thathave separate protective, digestive, and sensory functions. Though most animals have many different types of cells,they only have four basic types of tissue: connective, muscle, nervous, and epithelial.

Even more complex organisms, such as the roundworm shown in Figure 3.32, while also having differentiated cellsand tissues, have an organ level of development. An organ is a group of tissues that has a specific function or groupof functions. Organs can be as primitive as the brain of a flatworm (a group of nerve cells), as large as the stem of asequoia (up to 90 meters, or 300 feet, in height), or as complex as a human liver.

The most complex organisms (such as mammals, trees, and flowers) have organ systems. An organ system is agroup of organs that act together to carry out complex related functions, with each organ focusing on a part of thetask. An example is the human digestive system in which the mouth ingests food, the stomach crushes and liquifiesit, the pancreas and gall bladder make and release digestive enzymes, and the intestines absorb nutrients into theblood.

Vocabulary

• biofilm: A colony of prokaryotes that is stuck to a surface, such as a rock or a host’s tissue.

• colonial organism: Organism formed from a grouping of individuals of the same species living symbioticallytogether; one of the first evolutionary steps towards multicellular organisms.

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