Cell Growth And Reproduction PDF

Copyright © 2010 Scott A. Bowling REPRODUCTION CELL GROWTH AND.  A. CELL STRUCTURE AND FUNCTIONS A.1 The Cellular Level of Organization The cell marks...

Copyright © 2010 Scott A. Bowling REPRODUCTION CELL GROWTH AND.  A. CELL STRUCTURE AND FUNCTIONS A.1 The Cellular Level of Organization The cell marks the boundary between the Copyright © 2010 Scott A. Bowling nonliving and the living. The molecules that serve as food for a cell and the macromolecules that make up a cell are not alive, and yet the cell is alive. Cell contains tiny specialized structures called organelles that perform specific cellular functions..  A. CELL STRUCTURE AND FUNCTIONS Antoine Van Leeuwenhoek of Holland is famous for making his own microscopes and Copyright © 2010 Scott A. Bowling observing all sorts of tiny things that no one had seen before. Robert Hooke, an Englishman, confirmed Leeuwenhoek’s observations and was the first to use the term cell. The tiny chambers he observed in the honeycomb structure of cork reminded him of the rooms, or cells, in a monastery..  A. CELL STRUCTURE AND FUNCTIONS In the 1830s—the German microscopist Matthias Schleiden said that plants are Copyright © 2010 Scott A. Bowling composed of cells; His counterpart, Theodor Schwann, said that animals are also made up of living units called cells. This was quite a feat, because aside from their own exhausting work, both had to take into consideration the studies of many other microscopists..  A. CELL STRUCTURE AND FUNCTIONS Rudolf Virchow, another German microscopist, later came to the conclusion Copyright © 2010 Scott A. Bowling that cells don’t suddenly appear; rather, they come from pre-existing cells. Today, the cell theory, which states that all organisms are made up of basic living units called cells and that cells come only from pre- existing cells, is a basic theory of biology..  Cell Theory All organisms are composed of one or more cells. Copyright © 2010 Scott A. Bowling Cells are the basic living unit of structure and function in organisms. All cells come only from other cells..  The sizes of living things and their components. Copyright © 2010 Scott A. Bowling.  Cell Size The figure above outlines the visual ranges of the eye, light microscope, and electron Copyright © 2010 Scott A. Bowling microscope. Cells are usually quite small. A frog’s egg, at about one millimeter (mm) in diameter, is large enough to be seen by the human eye..  Cell Size But most cells are far smaller than one millimeter; some are even as small as one Copyright © 2010 Scott A. Bowling micrometer (μm)—one-thousandth of a millimeter. Cell inclusions and macromolecules are even smaller than a micrometer and are measured in terms of nanometers (nm)..  Copyright © 2010 Scott A. Bowling.  Cell Size To understand why cells are so small and why we are multicellular, consider the surface/ Copyright © 2010 Scott A. Bowling volume ratio of cells. Nutrients enter a cell and wastes exit a cell at its surface; therefore, the amount of surface affects the ability to get material in and out of the cell..  Cell Size A large cell requires more nutrients and produces more wastes than a small cell. Copyright © 2010 Scott A. Bowling In other words, the volume represents the needs of the cell. Yet, as cells get larger in volume, the proportionate amount of surface area actually decreases..  Problems with cell size A large cell requires "much more" in terms of the cellular components. Copyright © 2010 Scott A. Bowling Uptake from the environment is also a problem for large cells: there is less surface area compared to the volume. Distribution of nutrients from one portion of a large cell to another is also a problem, simply because of the distance required for the nutrients to travel.. Copyright © 2010 Scott A. Bowling Thus most cells are small: sufficient surface area to accommodate the volume. Larger organisms do not generally have larger cells than smaller organisms, - just simply more cells..  Prokaryotic cells vs. Eukaryotic Cells Prokaryotic cells, without true nucleus. An organism whose DNA is not contained within Copyright © 2010 Scott A. Bowling a nucleus is a prokaryotic organism, e.g. a bacterium. Eukaryotic cells, have a nucleus. A nucleus is a large structure that controls the workings of the cell because it contains the genes. Both animals and plants have eukaryotic cells..  Eukaryotic vs. prokaryotic cells prokaryotic cells do not have internal membranes (thus no nuclear membrane) Copyright © 2010 Scott A. Bowling – main DNA molecule (chromosome) is typically circular; its location is called the nuclear area – other small DNA molecules (plasmids) are often present, found throughout the cell.  Eukaryotic vs. prokaryotic cells plasma membrane of prokaryotic cells is typically enclosed in a cell wall often the cell wall is covered with a sticky Copyright © 2010 Scott A. Bowling layer of proteins and/or sugars called a capsule do not completely lack organelles; have: – plasma membrane – ribosomes – generally just called bacteria – prokaryotic cells are typically 1-10 mm in diameter.  Copyright © 2010 Scott A. Bowling Prokaryotic cells.  DNA Structure of prokayrotes Copyright © 2010 Scott A. Bowling Linear or Circular DNA Molecules.  Eukaryotic cells eukaryotic cells have internal membranes and a distinct, membrane-enclosed nucleus Copyright © 2010 Scott A. Bowling typically 10-100 mm in diameter.  Copyright © 2010 Scott A. Bowling.  Copyright © 2010 Scott A. Bowling.  DNA OF EUKARYOTIC CELL Copyright © 2010 Scott A. Bowling DOUBLE HELIX GENETIC MATERIAL.  Outer Boundaries of Animal and Plant Cells Animal and plant cells are surrounded by a plasma membrane that consists of a Copyright © 2010 Scott A. Bowling phospholipid bilayer in which protein molecules are embedded. Plasma membrane surrounds cells and separates their contents from the external environment. Cells are heterogeneous mixtures, with specialized regions and structures (such as organelles)..  Outer Boundaries of Animal and Plant Cells Plant cells (but not animal cells) have a permeable but protective cell wall, in Copyright © 2010 Scott A. Bowling addition to a plasma membrane. Many plant cells have both a primary and secondary cell wall. A main constituent of a primary cell wall is cellulose molecules..  Outer Boundaries of Animal and Plant Cells Cellulose molecules form fibrils that lie at right angles to one another for added Copyright © 2010 Scott A. Bowling strength. The secondary cell wall, if present, forms inside the primary cell wall. Such secondary cell walls contain lignin, a substance that makes them even stronger than primary cell walls..  Phospholipid bilayer The Plasma membrane is also called the Phospholipid bilayer Copyright © 2010 Scott A. Bowling Phospholipids contain a hydrophilic head and a nonpolar hydrophobic tail Hydrogen bonds form between the phospholipid "heads" and the watery environment inside and outside of the cell.  Copyright © 2010 Scott A. Bowling Phospholipid bilayer.  Phospholipid bilayer Hydrophobic interactions force the "tails" to face inward Copyright © 2010 Scott A. Bowling Phospholipids are not bonded to each other, which makes the double layer fluid Cholesterol embedded in the membrane makes it stronger and less fluid.  Copyright © 2010 Scott A. Bowling Phospholipid bilayer.  Copyright © 2010 Scott A. Bowling Phospholipid bilayer.  Proteins embedded in membrane serve different functions 1. Channel Proteins - form small openings for molecules to diffuse through Copyright © 2010 Scott A. Bowling 2. Carrier Proteins - binding site on protein surface "grabs" certain molecules and pulls them into the cell 3. Receptor Proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel proteins) 4. Cell Recognition Proteins - ID tags, to identify cells to the body's immune system 5. Enzymatic Proteins - carry out metabolic reactions.  Copyright © 2010 Scott A. Bowling Transport Across Cell Membranes.  Transport Across Cell Membranes The membrane is differentially permeable (also called semipermeable ) - which means: Copyright © 2010 Scott A. Bowling Passive Transport Simple Diffusion - water, oxygen and other molecules move from areas of high concentration to areas of low concentration, down a concentration gradient..  Transport Across Cell Membranes Facilitated Diffusion - diffusion that is assisted by proteins (channel or carrier Copyright © 2010 Scott A. Bowling proteins) Osmosis - diffusion of water. Salt Sucks Osmosis affects the turgidity of cells, different solution can affect the cells internal water amounts.  Transport Across Cell Membranes Contractile Vacuoles are found in freshwater microorganisms - they pump out excess water Copyright © 2010 Scott A. Bowling Turgor pressure occurs in plants cells as their central vacuoles fill with water..  Copyright © 2010 Scott A. Bowling Transport Across Cell Membranes.  Transport Across Cell Membranes Active Transport - involves moving molecules "uphill" against the concentration gradient, Copyright © 2010 Scott A. Bowling which requires energy Endocytosis (Phagocytosis)- taking substances into the cell (pinocytosis for water, phagocytosis for solids) Exocytosis - pushing substances out of the cell, such as the removal of waste.  Transport Across Cell Membranes Sodium-Potassium Pump - pumps out 3 sodium for ever 2 potassium's taken in against Copyright © 2010 Scott A. Bowling gradient.  Organelles of Animal and Plant Cells Originally the term organelle referred to only membranous structures, but we will use it to Copyright © 2010 Scott A. Bowling include any well-defined subcellular structure. Just as all the assembly lines of a factory are in operation at the same time, so all the organelles of a cell function simultaneously. Raw materials enter a factory and then are turned into various products by different departments..  Organelles of Animal and Plant Cells In the same way, chemicals are taken up by the cell and then processed by the organelles. Copyright © 2010 Scott A. Bowling The cell is a beehive of activity the entire 24 hours of every day. Both animal cells and plant cells contain mitochondria, while only plant cells have chloroplasts. Only animal cells have centrioles..  Compartments in eukaryotic cells Two general regions inside the cell: cytoplasm and nucleoplasm Copyright © 2010 Scott A. Bowling – Cytoplasm – everything outside the nucleus and within the plasma membrane contains fluid cytosol and organelles – Nucleoplasm – everything within the nuclear membrane.  The Nucleus The control center of the cell Typically large ( about 5 μm) and singular Copyright © 2010 Scott A. Bowling Nuclear envelope – double membrane surrounding the nucleus Nuclear pores – protein complexes that cross both membranes and regulate passage.  Copyright © 2010 Scott A. Bowling.  The Nucleus chromatin – DNA-protein complex have granular appearance; easily stained for Copyright © 2010 Scott A. Bowling microscopy (“chrom-” = color) “unpacked” DNA kept ready for message transcription and DNA replication proteins protect DNA and help maintain structure and function chromosomes – condensed or “packed” DNA ready for cell division (“-some” = body).  Copyright © 2010 Scott A. Bowling.  The Nucleus Nucleoli – regions of ribosome subunit assembly Copyright © 2010 Scott A. Bowling – appears different due to high RNA and protein concentration (no membrane) – ribosomal RNA (rRNA) transcribed from DNA there proteins (imported from cytoplasm) join with rRNA at a nucleolus to form ribosome subunits – ribosome subunits are exported to the cytoplasm through nuclear pores..  The Nucleus The structural features of the nucleus include the following. Copyright © 2010 Scott A. Bowling Chromatin: DNA and proteins Nucleolus: Chromatin and ribosomal subunits Nuclear envelope: Double membrane with pores.  Copyright © 2010 Scott A. Bowling.  Ribosomes Ribosomes are composed of two subunits, one large and one small. Copyright © 2010 Scott A. Bowling Each subunit has its own mix of proteins and rRNA. Site of protein synthesis. Ribosomes can be found within the cytoplasm, either singly or in groups called polyribosomes..  Copyright © 2010 Scott A. Bowling.  Ribosomes Ribosomes can also be found attached to the endoplasmic reticulum, a membranous Copyright © 2010 Scott A. Bowling system of saccules and channels. Proteins synthesized at ribosomes attached to the endoplasmic reticulum have a different fate. They are eventually secreted from the cell or become a part of its external surface..  Copyright © 2010 Scott A. Bowling.  Endomembrane system Endomembrane system – a set of membranous organelles that interact with Copyright © 2010 Scott A. Bowling each other via vesicles – includes ER, Golgi apparatus, vacuoles, lysosomes, microbodies, and in some definitions the nuclear membrane and the plasma membrane.  Copyright © 2010 Scott A. Bowling.  Endoplasmic reticulum Endoplasmic reticulum (ER) – membrane network that winds through the cytoplasm Copyright © 2010 Scott A. Bowling – winding nature of the ER provides a lot of surface area – many important cell reactions or sorting functions require ER membrane surface.  Endoplasmic reticulum – ER lumen – internal aqueous compartment in ER Copyright © 2010 Scott A. Bowling – separated from the rest of the cytosol – typically continuous throughout ER and with the lumen between the nuclear membranes – enzymes within lumen and imbedded in lumen side of ER differ from those on the other side, thus dividing the functional regions.  Endoplasmic reticulum smooth ER – primary site of lipid synthesis, many detoxification reactions, and Copyright © 2010 Scott A. Bowling sometimes other activities rough ER – ribosomes that attach there insert proteins into the ER lumen as they are synthesized.  Copyright © 2010 Scott A. Bowling.  Endoplasmic reticulum rough ER – ribosomes that attach there insert proteins into the ER lumen as they are Copyright © 2010 Scott A. Bowling synthesized ribosome attachment directed by a signal peptide at the amino end of the polypeptide.  Endoplasmic reticulum a protein/RNA signal recognition particle (SRP) binds to the signal peptide and pauses Copyright © 2010 Scott A. Bowling translation at the ER the assembly binds to an SRP receptor protein SRP leaves, protein synthesis resumes (now into the ER lumen), and the signal peptide is cut off.  Copyright © 2010 Scott A. Bowling Endoplasmic reticulum.  Endoplasmic reticulum proteins inserted into the ER lumen may be membrane bound or free Copyright © 2010 Scott A. Bowling proteins are often modified in the lumen (example, carbohydrates or lipids added) proteins are transported from the ER in transport vesicles.  Endoplasmic reticulum vesicles – small, membrane-bound sacs – buds off of an organelle (ER or other) Copyright © 2010 Scott A. Bowling – contents within the vesicles (often proteins) transported to another membrane surface – vesicles fuses with membranes, delivering contents to that organelle or outside of the cell.  Fig. 5.16d (TEArt) Protei n Copyright © 2010 Scott A. Bowling Vesicle Migrating Fusion budding transport of vesicle from rough vesicle with Golgi endoplasmi apparatus c reticulum Ribosom e.  Golgi apparatus Golgi apparatus (aka Golgi complex) – a stack of flattened membrane sacs (cisternae) Copyright © 2010 Scott A. Bowling where proteins further processed, modified, and sorted [the “post office” of the cell] not contiguous with ER, and lumen of each sac is usually separate from the rest has three areas: cis, medial, and trans.  Copyright © 2010 Scott A. Bowling.  Golgi apparatus cis face: near ER and receives vesicles from it; current model (cisternal maturation model) Copyright © 2010 Scott A. Bowling holds that vesicles actually coalesce to continually form new cis cisternae medial region: as a new cis cisterna is produced, the older cisternae mature and move away from the ER.  Golgi apparatus medial region: in this region proteins are further modified (making Copyright © 2010 Scott A. Bowling glycoproteins and/or lipoproteins where appropriate, and ) maturing cisternae may make other products; for example, many polysaccharides are made in the Golgi some materials are needed back a the new cis face and are transported there in vesicles.  Golgi apparatus trans face: nearest to the plasma membrane; a fully matured cisterna breaks into many Copyright © 2010 Scott A. Bowling vesicles that are set up to go to the proper destination (such as the plasma membrane or another organelle) taking their contents with them.  Copyright © 2010 Scott A. Bowling.  Lysosomes lysosomes – small membrane-bound sacs of digestive enzymes Copyright © 2010 Scott A. Bowling serves to confine the digestive enzymes and their actions allows maintenance of a better pH for digestion (often about pH 5) formed by budding from the Golgi apparatus; special sugar attachments to hydrolytic enzymes made in the ER target them to the lysosome.  Lysosomes used to degrade ingested material, or in some cases dead or damaged organelles Copyright © 2010 Scott A. Bowling – ingested material is found in vesicles that bud in from the plasma membrane; the complex molecules in those vesicles is then digested – can also fuse with dead or damaged organelles and digest them.  Lysosomes digested material can then be sent to other parts of the cell for use Copyright © 2010 Scott A. Bowling found in animals, protozoa; debatable in other eukaryotes, but all must have something like a lysosome.  Copyright © 2010 Scott A. Bowling.  Vacuoles vacuoles – large membrane-bound sacs that perform diverse roles; have no internal structure Copyright © 2010 Scott A. Bowling distinguished from vesicles by size in plants, algae, and fungi, performs many of the roles that lysosomes perform for animals central vacuole – typically a single, large sac in plant cells that can be 90% of the cell volume – usually formed from fusion of many small vacuoles in immature plant cells.  Vacuoles – storage sites for water, food, salts, pigments, and metabolic wastes Copyright © 2010 Scott A. Bowling – important in maintaining turgor pressure – tonoplast – membrane of the plant vacuole food vacuoles – present in most protozoa and some animal cells; usually bud from plasma membrane and fuse with lysosomes for digestion contractile vacuoles – used by many protozoa for removing excess water.  Copyright © 2010 Scott A. Bowling Vacuoles.  Microbodies microbodies – small membrane-bound organelles that carry out specific cellular functions; examples: Copyright © 2010 Scott A. Bowling – lysosomes could be consider a type of microbody – peroxisomes – sites of many metabolic reactions that produce hydrogen peroxide (H2O2), which is toxic to the rest of the cell peroxisomes have enzymes to break down H2O2, protecting the cell peroxisomes are abundant in liver cells in animals and leaf cells in plants.  Microbodies peroxisomes are normally found in all eukaryotes example: detoxification of ethanol in Copyright © 2010 Scott A. Bowling liver cells occurs in peroxisomes – glyoxysomes – in plant seeds, contains enzymes that convert stored fats into sugar.  Copyright © 2010 Scott A. Bowling Microbodies.  Energy-Related Organelles Life is possible only because of a constant input of energy used for maintenance and growth. Copyright © 2010 Scott A. Bowling Chloroplasts and mitochondria are the two eukaryotic membranous organelles that specialize in converting energy to a form that can be used by the cell. Chloroplasts use solar energy to synthesize carbohydrates, and carbohydrate-derived products are broken down in mitochondria (sing., mitochondrion) to produce ATP molecules..  Copyright © 2010 Scott A. Bowling.  Energy-Related Organelles energy obtained from the environment is typically chemical energy (in food) or light Copyright © 2010 Scott A. Bowling energy mitochondria are the organelles where chemical energy is placed in a more useful molecule chloroplasts are plastids where light energy is captured during photosynthesis.  Mitochondria mitochondria – the site of aerobic respiration recall aerobic respiration: Copyright © 2010 Scott A. Bowling sugar + oxygen carbon dioxide + water + energy the “energy” is actually stored in ATP.  Copyright © 2010 Scott A. Bowling Mitochondria.  Copyright © 2010 Scott A. Bowling Mitochondria.  Mitochondria mitochondria have a double membrane – space between membranes = intermembrane Copyright © 2010 Scott A. Bowling space – inner membrane is highly folded, forming cristae; provides a large surface area – inner membrane is also a highly selective barrier – the enzymes that conduct aerobic respiration are found in the inner membrane.  Mitochondria inside of inner membrane is the matrix, analogous to the cytoplasm of a cell Copyright © 2010 Scott A. Bowling mitochondria have their own DNA, and are inherited from the mother only in humans mitochondria have their own division process, similar to cell division; each cell typically has many mitochondria, which can only arise from mitochondrial division – some cells require more mitochondria than others.  Mitochondria mitochondria can leak electrons into the cell, Copyright © 2010 Scott A. Bowling allowing toxic free radicals to form mitochondria play a role in initiating apoptosis (programmed cell death).  Plastids plastids – organelles of plants and algae that produce and store food Copyright © 2010 Scott A. Bowling – include amyloplasts (for starch storage), chromoplasts (for color, often found in petals and fruits), and chloroplasts (for photosynthesis) like mitochondria, have their own DNA (typically a bit larger and more disk-shaped than mitochondria, however).  Plastids – derive from undifferentiated proplastids, although role of mature plastids can Copyright © 2010 Scott A. Bowling sometimes change – numbers and types of plastids vary depending on the organism and the role of the cell.  Copyright © 2010 Scott A. Bowling.  Plastids Etioplast - a microscopic membranous sac with no pigment that occurs inside the cells of Copyright © 2010 Scott A. Bowling plants that are deprived of light. Etioplasts change into chloroplasts if they are exposed to light. Leucoplast - a common minute colorless body plastid found inside plant cells and used for storing food..  Plastids Elaioplasts - are a type of leucoplast that is specialized for the storage of lipids in plants. Copyright © 2010 Scott A. Bowling Elaioplasts house oil body deposits as rounded plastoglobuli, which are essentially fat droplets. Proteinoplast – are specialized organelles found only in plant cells. They contain crystalline bodies of protein and can be the sites of enzyme activity involving those proteins..  Chloroplasts chloroplasts get their green color from chlorophyll, the main light harvesting pigments involved in Copyright © 2010 Scott A. Bowling photosynthesis: carbon dioxide + water + light energy food (glucose) + oxygen.  Chloroplasts chloroplasts have a double membrane – the region within the inner membrane is Copyright © 2010 Scott A. Bowling the stroma; it is analogous to the mitochondrial matrix – inner membrane is contiguous with an interconnected series of flat sacks called thylakoids that are grouped in stacks called grana – the thylakoids enclose aqueous regions called the thylakoid lumen.  Chloroplasts chlorophyll is found in the thylakoid membrane, and the reactions of Copyright © 2010 Scott A. Bowling photosynthesis take place there and in the stroma carotenoids in the chloroplast serve as accessory pigments for photosynthesis.  Copyright © 2010 Scott A. Bowling Chloroplasts.  Copyright © 2010 Scott A. Bowling.  The Cytoskeleton The cytoskeleton is a network of interconnected filaments and tubules that extends from the Copyright © 2010 Scott A. Bowling nucleus to the plasma membrane in eukaryotic cells. Prior to the 1970s, it was believed that the cytoplasm was an unorganized mixture of organic molecules. Then, high-voltage electron microscopes, which can penetrate thicker specimens, showed that the cytoplasm is instead highly organized..  The Cytoskeleton It contains actin filaments, microtubules, and intermediate filaments. Copyright © 2010 Scott A. Bowling The technique of immunofluorescence microscopy identified the makeup of these protein fibers within the cytoskeletal network. The name cytoskeleton is convenient in that it compares the cytoskeleton to the bones and muscles of an animal..  The Cytoskeleton Bones and muscles give an animal structure and produce movement. Copyright © 2010 Scott A. Bowling Similarly, the fibers of the cytoskeleton maintain cell shape and cause the cell and its organelles to move. The cytoskeleton is dynamic; assembly occurs when monomers join a fiber and disassembly occurs when monomers leave a fiber..  The Cytoskeleton Assembly and disassembly occur at rates that are measured in seconds and minutes. Copyright © 2010 Scott A. Bowling The entire cytoskeletal network can even disappear and reappear at various times in the life of a cell..  The Cytoskeleton Actin Filaments Actin filaments (formerly called Copyright © 2010 Scott A. Bowling microfilaments) are long, extremely thin fibers (about 7 nm in diameter) that occur in bundles or meshlike networks. The actin filament contains two chains of globular actin monomers twisted about one another in a helical manner..  Copyright © 2010 Scott A. Bowling.  The Cytoskeleton Actin Filaments Actin filaments play a structural role by forming a Copyright © 2010 Scott A. Bowling dense complex web just under the plasma membrane, to which they are anchored by special proteins. Also, the assembly and disassembly of a network of actin filaments lying the plasma membrane accounts for the formation of pseudopods, extensions that allow certain cells to move in an amoeboid fashion..  The Cytoskeleton Intermediate Filaments Intermediate filaments (8–11 nm in diameter) Copyright © 2010 Scott A. Bowling are intermediate in size between actin filaments and microtubules. They are ropelike assemblies of fibrous polypeptides that support the nuclear envelope and the plasma membrane. In the skin, intermediate filaments made of the protein keratin give great mechanical strength to skin cells..  The Cytoskeleton Intermediate Filaments Recent work has shown intermediate Copyright © 2010 Scott A. Bowling filaments to be highly dynamic. They also are able to assemble and disassemble in the same manner as actin filaments and microtubules..  Copyright © 2010 Scott A. Bowling.  Centrioles a rod-shaped structure in cell: in an animal cell, a two-part rod-shaped structure with the Copyright © 2010 Scott A. Bowling parts lying at right angles to each other, located in pairs near the nucleus. During cell division, centrioles move to opposite ends of the cell and form the poles of the spindle fibers that pull the chromosomes apart..  Centrioles Centrioles are short cylinders with a 9 0 pattern of microtubule triplets—that is, a ring having Copyright © 2010 Scott A. Bowling nine sets of triplets with none in the middle. In animal cells, a centrosome contains two centrioles lying at right angles to each other. The centrosome is the major microtubule organizing center for the cell, and centrioles may be involved in the process of microtubule assembly and disassembly..  Centrioles Before an animal cell divides, the centrioles replicate, and the members of each pair are Copyright © 2010 Scott A. Bowling again at right angles to one another. Then, each pair becomes part of a separate centrosome. During cell division, the centrosomes move apart and may function to organize the mitotic spindle..  Centrioles Plant cells have the equivalent of centrosome, but it does not contain centrioles, suggesting Copyright © 2010 Scott A. Bowling that centrioles are not necessary to the assembly of cytoplasmic microtubules. Centrioles are believed to give rise to basal bodies that direct the organization of microtubules within cilia and flagella. In other words, a basal body does for a cilium (or flagellum) what the centrosome does for the cell..  Copyright © 2010 Scott A. Bowling.  Prokaryotic Cells Prokaryotic cells, the other major type of cell, does not have a nucleus as eukaryotic cells do. Copyright © 2010 Scott A. Bowling Archaea and bacteria are both prokaryotes, cells so small they are just visible with the light microscope..  Copyright © 2010 Scott A. Bowling.  Prokaryotic Cells The previous figure illustrates the main features of bacterial anatomy. Copyright © 2010 Scott A. Bowling The cell wall contains peptidoglycan, a complex molecule with chains of a unique amino disaccharide joined by peptide chains. In some bacteria, the cell wall is further surrounded by a capsule and/or gelatinous sheath called a slime layer..  Prokaryotic Cells Motile bacteria usually have long, very thin appendages called flagella (sing., flagellum) Copyright © 2010 Scott A. Bowling that are composed of subunits of the protein called flagellin. The flagella, which rotate like propellers, rapidly move the bacterium in a fluid medium. Some bacteria also have fimbriae, which are short appendages that help them attach to an appropriate surface..  Prokaryotic Cells The cytoplasm of prokaryotic cells like that of eukaryotic cells is bounded by a plasma Copyright © 2010 Scott A. Bowling membrane. Prokaryotes have a single chromosome (loop of DNA) located within a region called the nucleoid but it is not bounded by membrane. Many prokaryotes also have small accessory rings of DNA called plasmids..  Prokaryotic Cells The cytoplasm has thousands of ribosomes for the synthesis of proteins. In addition, the Copyright © 2010 Scott A. Bowling photosynthetic cyanobacteria have light- sensitive pigments, usually within the membranes of flattened disks called thylakoids. Although prokaryotes are structurally simple, they are actually metabolically complex and contain many different kinds of enzymes..  Prokaryotic Cells Prokaryotes are adapted to living in almost any kind of environment and are diversified to Copyright © 2010 Scott A. Bowling the extent that almost any kind of organic matter can be used as a nutrient for some particular type. The cytoplasm is the site of thousands of chemical reactions, and prokaryotes are more metabolically competent than are human beings..  Prokaryotic Cells Given adequate nutrients, most prokaryotes are able to synthesize any kind of molecule Copyright © 2010 Scott A. Bowling they may need. Indeed, the metabolic capability of bacteria is exploited by humans, who use them to produce a wide variety of chemicals and products for human use..  Copyright © 2010 Scott A. Bowling.