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Biology Chapter 7.docx

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Lovaye Kajiura

Biology Chapter 7: Inside the Cell What is Inside the Cell − According to phylogeny, organisms fall into three groups: Bacteria, Archaea, Eukarya − Members of Bacteria and Archaea are prokaryotic and members of Eukarya are eukaryotic − Prokaryotic cells o Most bacterial and archaeal species contain a plasma membrane that encloses a single compartment, meaning cell has few or no substructures separated from the rest of the cell by internal membranes o Inside membrane, contents of cell are collectively called the cytoplasm (cell-formed) o Because cytoplasm contains a high concentration of solutes, in most habitats it is hypertonic relative to the surrounding environment o Water enters the cell via osmosis and makes the cell’s volume expand o In virtually all bacteria and archaea, pressure is resisted by a stiff cell wall o Bacterial and archaeal cell walls are a tough, fibrous layer that surrounds the plasma membrane o Cell wall protects organism and gives it shape and rigidity o In addition, many bacteria have another protein layer outside the cell wall that consists of lipids with polysaccharides attached (glycolipids) o In the cytoplasm, the most prominent structure is the chromosome o Most prokaryotic species have a single, circular chromosome that consists of large DNA molecule associated with small number of proteins o DNA molecule contains informant, while the proteins provide structural support for the DNA o Prokaryotic chromosomes are found in localized area of the cell called the nucleoid, usually found in the centre of the cell and typically represents about 20% of the cell’s total volume o To fit into the cell, the DNA double helix coils on itself with the aid of enzymes to form the highly compact, “supercoiled” structure o Depending on species and population under consideration, prokaryotic cells may also contain one to about a hundred small, usually circular, supercoiled DNA molecules called plasmids o Plasmids contain genes, but are physically independent of the main, cellular chromosome o Usually, genes carried by plasmids are not required under normal conditions, instead they help cells adapt to unusual circumstances, such as the presence of a poison in the environment o Ribosomes manufacture proteins and are found throughout the cytoplasm  Bacterial ribosomes consist of three distinct RNA molecules and over 50 different proteins  These molecular components are divided into the large subunit and the small subunit  Lack a membrane o Flagella rotate to power swimming in aquatic species  Located on surface of cell o Lack a nucleus, however many species contain membrane-bound storage containers and extensive internal membranes occur in bacteria and archaea that perform photosynthesis o Photosynthetic membranes observed in prokaryotes contain the enzymes and pigment molecules required for these reactions to occur and develop as infoldings of plasma membrane o In addition, recent research indicates that several bacterial species have internal components that qualify as organelles o An organelle is a membrane-bound compartment in the cytoplasm that contains enzymes or structures specialized for a particular function o Recent research has also shown that bacteria and archaea contain long, thin fibres that serve a structural role inside the cell  These filaments are essential for cell division to take place  Protein filaments such as these form the basis of the cytoskeleton − Eukaryotic cells o Much larger than prokaryotic cells o Size has a downside, ions and other molecules like ATP or amino acids cannot diffuse a large volume quickly o Eukaryotic cells solve this problem with their numerous organelles o Huge volume inside eukaryotic cell is compartmentalized into a large number of bacterium-sized parts o Compartmentalization offers two key advantages  Incompatible chemical reactions can be separated • New fatty acids can be synthesized in one organelle, while excess or damaged fatty acids are degraded and recycled in a different organelle  Efficiency of chemical reactions is increased • Substrates required for particular reactions can be localized and maintained at high concentration within organelles • If substrates are used up in a particular part of an organelle, they can be replaced by substrates that have only a short distance to diffuse • Groups of enzymes that work together can be clustered on internal membranes instead of floating free in the cytoplasm o Nucleus  Among the largest organelles and is highly organized  Enclosed by a complex double membrane called the nuclear envelope  Nuclear envelope is studded with pore-like openings and its inside surface is associated with fibrous proteins that form a lattice-like sheet called the nuclear lamina (stiffens structure and maintains shape)  Each chromosome occupies a distinct area and is attached to the nuclear lamina in at least one location  Nucleus also includes a distinctive region called the nucleolus, where the RNA molecules found in ribosomes are manufactured and large and small ribosomal subunits are assembled o Ribosomes  Fluid portion of cytoplasm is called cytosol  Many of the cell’s millions of ribosomes are scattered throughout the cytosol  Ribosomes are composed of one large and one small subunit, made of proteins and RNA  Large subunit contains three RNA while the small subunit contains one  Neither subunit is enclosed by a membrane  When the two subunits come together, they form a complex molecular machine that synthesizes proteins o Rough Endoplasmic Reticulum  Hundreds of thousands of ribosomes are attached to a network of membrane-bound sacs and tubules called rough endoplasmic reticulum  Continuous with the outer membrane of the nuclear envelope  Ribosomes associated with rough ER are responsible for synthesizing proteins that will be inserted into the plasma membrane, secreted to the cell exterior, or shipped to the lysosome  As they are being manufactured by the ribosomes, the proteins move to the interior of the sac-like component of the rough ER, called the lumen  In the lumen, newly manufactured proteins undergo folding and other types of processing  Proteins produced in the rough ER have a variety of functions • Some carry messages to other cells • Some act as membrane transport proteins or pumps • Others are enzymes o Golgi Apparatus  In many cases products of rough ER pass through Golgi apparatus before they reach their final destination  Consists of a flattened, membranous sacs called cisternae, which are stacked on top of one another  Also has a distinct polarity  Cis surface is closest to the rough ER and nucleus  Trans surface is oriented toward the plasma membrane  Cis side of a Golgi apparatus receives products from the rough ER and trans side ships them out towards the cell surface  In between, within the cisternae, the rough ER’s products are processed and packaged for delivery o Smooth Endoplasmic Reticulum  Parts of the ER that don’t have ribosomes attached  Contains enzymes that catalyze reactions involving lipids  Manufacturing site for phospholipids used in cell membranes  Functions as a reservoir for calcium ions that act as a signal triggering a wide array of activities inside the cell  Functions primarily as a lipid-processing centre  Together with the Golgi apparatus and lysosomes, the endoplasmic reticulum forms the endomembrane system o Peroxisomes  Globular organelles found in eukaryotic cells  Have a single membrane and grow and divide independently of other organelles  Centres for oxidation reactions  In many cases the products of these reactions include hydrogen peroxide, which is highly corrosive and if it escapes from the peroxisome, it would quickly damage organelle membranes and plasma membrane  Inside peroxisome, enzyme catalase quickly detoxifies hydrogen peroxide by converting it to water and oxygen  Different types of peroxisomes contain different oxidative enzymes  As a result, each is specialized for oxidizing particular compounds  In plant leaves, specialized peroxisomes called glyoxisomes are packed with enzymes that convert one of the products of photosynthesis into a sugar that can be used to store energy for the cell o Lysosomes  Major structures involved in solid-waste processing and materials storage in the cell  Referred to as vacuoles in plants and fungi  Function as digestive centres  Lumen of organelle is acidic because proton pumps in the lysosome membrane import enough hydrogen ions to maintain a pH of 5.0  Also contains many enzymes to break polymers into monomers, which are then excreted or recycled into new polymers  Two ways materials are delivered to lysosomes in animal cells • Phagocytosis o Plasma membrane detects a smaller cell or food particle and begins to engulf it o Resulting membrane-bound particle is a phagosome or food particle o Phagosome is delivered to lysosome, which takes it in and begins digesting it o Small molecules from digested food particles are released into the cytosol • Autogaphy o Damaged organelle is surrounded by a membrane o Membrane-bound organelle is delivered to a lysosome, which takes it in and begins digesting it o Small molecules from the digested organelle are recycled into cytosol  Materials can also be processed by lysosomes as a result of receptor-mediated endocytosis • Macromolecules outside cell bind to membrane proteins that act as receptors • Plasma membrane folds in and pinches off to form a vesicle called an early endosome • Early endosome undergoes a series of processing steps including activation of proton pumps that lowers its pH • Early endosome matures into a late endosome that receives digestive enzymes from Golgi apparatus • Late endosome eventually matures into a function lysosome  Not all materials surrounded by membrane and taken into the cell end up in lysosomes  Endocytosis refers to any pinching off of plasma membrane that results in uptake of material from outside the cell  Can occur in three ways • Phagocytosis • Receptor-mediated endocytosis • Pinocytosis o Brings fluid into the cytoplasm via tiny vesicles that form from infoldings of plasma membrane o Fluid inside these vesicles is not transported to lysosomes, but used elsewhere in the cell  Compared with lysosomes, vacuoles in plant and fungal cells are large • Although some vacuoles contain enzymes that are specialized for digestion, most vacuoles act as storage depots • In many cases, stored material is water, which maintains the cell’s normal volume or ions such as potassium and chloride o Mitochondria  Chemical energy required to build all of these organelles and do other types of work comes from ATP, most of which is produced in the cell’s mitochondria  Has two membranes  Outer membrane defines organelle’s surface, while inner membrane is connected to a series of sac-like cristae  Solution inside inner membrane is called mitochondrial matrix  Each mitochondrion possesses a small chromosome that contains genes, independent of main chromosomes in nucleus  Mitochondrial DNA is a component of circular and supercoiled chromosome that is similar in structure to bacterial chromosomes  Can facture their own ribosomes o Chloroplasts  Most algal and plant cells possess the chloroplast, in which sunlight is converted to chemical energy during photosynthesis  Has a double membrane around its exterior  Interior of chloroplast is dominated by hundreds of membrane- bound, flattened vesicles called thylakoids, which are independent of inner membrane  Thylakoids are stacked like pancakes into piles called grana  Many pigments, enzymes and molecular machines responsible for converting light energy into carbohydrates are embedded in the thylakoid membrane  Certain enzymes and substrates are found outside the thylakoids in a region called the stroma  Each contains a circular chromosome o Cytoskeleton  Contains several distinct types of proteins and fibres and has an array of functions, including giving the cell its shape and structural stability  Involved in moving cell itself and moving materials within the cell o Cell wall  In fungi, algae and plants, cells possess an outer cell wall in addition to plasma membrane  Rods or fibres composed of a carbohydrate run through a stiff matrix made of other polysaccharides and proteins  Some plant cells produce a secondary cell wall that features a tough molecule called lignin, which forms a branching, cagelike network that is almost impossible for enzymes to attack − How cell structure correlates with function o Pancreatic cell  Specialized for the manufacture and export of digestive enzymes  Packed with rough ER and Golgi o Testis cell  Synthesizes steroid hormone called testosterone  Dominated by smooth ER, where processing of steroids and other lipids takes place o Leaf cell  Specialized for absorbing light and manufacturing sugar  Contains hundreds of chloroplasts o Potato tuber cell  Functions as starch storage container  Has a prominent storage vacuole filled with carbohydrates − Examination of cells o A technique called differential centrifugation made it possible to isolate particular cell components and analyze their chemical compositions  Differential centrifugation is based on breaking cells apart to create a complex mixture and then separating components in a centrifuge  Individual parts of the cell can then be purified and studied in detail o Transmission electron microscopy is based on a fixed “snapshot” of the cell that is to be observed o Light microscopy and electron microscopy  Gives info about relative shape and size of cell, but also locations of specific organelles o Confocal microscopy  Can take images of live processes  Follows and tracks them to reduce bluriness Nuclear Envelope: Transport into and out of the Nucleus − Nucleus is separated from rest of cell by nuclear envelope − Structure is supported by fibrous nuclear lamina and is bounded by two membranes, each consisting of a lipid bilayer − The membranes are separated by a space much like the lumen of the endoplasmic reticulum − Envelope contains thousands of openings called nuclear pores, which connect the nucleus with the cytoplasm − Each pore cons
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