Chapter 4-Cell Structure and Function


4.1 Cellular Level of Organization

• 1830s Mathias Schleiden stated that all plants are composed of cells and Theodor Schwann stated that all animals are composed of cells
• cell-smallest unit of living matter
• a cell is not only the structural unit but also the functional unit of organs, and, therefore, organisms
• once a cell gets to a certain size, it divides (multicellular organisms grow, unicellular organisms reproduce)
• Rudolf Virchow concluded "every cell comes from a preexisting cell"
• cell theory-all living things are composed of cells, and cells come from other cells
• most cells are smaller than one millimeter
• cells need surface areas large enough to allow adequate nutrients to enter and to rid itself of wastes
• small cells have greater surface area per volume than large cells
• cells that specialize in absorption have modifications that greatly increase the surface area per volume of the cell
• columnar cells along the surface of the intestinal wall have surface foldings called microvilli that increase their surface area

4.2 Bacterial Cells

• bacteria-prokaryotic cells in the domain Bacteria
• prokaryotic cells-lacking a membrane bound nucleus and organelles
• main features of bacterial anatomy: cell wall, capsule or slime layer, flagellum, plasma membrane, cytoplasm, cytosol, ribosomes, nucleoid, plasmids, thylakoids
• cell wall-contains peptidoglycan
• capsule-surrounds cell wall
• slime layer-gelatinous sheath in lieu of a capsule
• flagellum-long, very thin appendages possessed by motile bacterium
• some also have fimbriae, short appendages that help attach them to appropriate surfaces
• plasma membrane-membrane that regulates the movement of molecules into and out of the cytoplasm
• cytoplasm-the interior of the cell, consisting of cytosol and ribosomes
• cytosol-semifluid medium
• ribosomes-small bodies that coordinate the synthesis of proteins
• nucleoid-where innumerable enzymes and chromosome (loop of DNA)/genes are located
• plasmids-small accessory rings of DNA
• thylakoids-(cyanobacteria) membranes of flattened discs containing light-sensitive pigments
• cytoplasm is the site of thousands of chemical
• bacteria are adapted to living in almost any kind of environment and are diversified to the extent that almost any type of organic matter can be used as a nutrient for some particular bacterium

4.3 Eukaryotic Cells

• eukaryotic cells-cells that have a true nucleus, a membrane-bounded structure where DNA is housed with threadlike structures called chromatin
• a membrane is a phospholipid bilayer with embedded proteins
• organelles-small bodies each with a specific structure and function, many are membrane-bounded
• the cytosol, which is a semifluid medium outside the nucleus, is divided up and compartmentalized by the organelles
• compartmentalization keeps the cell organized and keeps its various functions separate from one another
• the cytosol has an organized lattice of protein filaments called the cytoskeleton
• some eukaryotic cells, notably plant cells, have cell walls
• cell wall-supports and protects the cell but does not interfere with the movement of molecules across the plasma membrane; plant cell walls contain cellulose fibris and therefore has a different composition than the cell wall of bacteria
• endosymbiotic relationship (suggested by Lynn Margulis and others)
1. mitochondria and chloroplasts are similar to bacteria in size and in structure
2. both organelles are bounded by a double membrane--the outer membrane may be derived from the engulfing vesicle, and the inner one may be derived from the plasma membrane of the original prokaryote.
3. mitochondria and chloroplasts contain a limited amount of genetic material and divide by splitting. Their DNA is a circular loop like that of bacteria
4. although most of the proteins within mitochondria and chloroplasts are now produced by the eukaryotic host, they do have their own ribosomes and they do produce some proteins. Their ribosomes resemble those of bacteria
5. the RNA (ribonucleic acid) composition of their ribosomes suggests a eubacterial origin for chloroplasts and mitochondria
• nucleus-has a diameter of about 5 µm, prominent structure in eukayotic cell, containg chromatin in nucleoplasm
• chromatin-looks grainy, a network of strands that undergoes coiling into rodlike structures
• chromosomes-rodlike structures containing DNA and much protein, and some RNA
• nucleolus-dark regions of chromatin where ribosomal RNA is produced and where rRNA joins with proteins to form the subunits of ribosomes
• nuclear envelope-double membrane with pores separating the nucleus from the cytoplasm
• nuclear pores-100 nm, permits the passage of proteins into the nucleus and ribosomal subunits outside of the nucleus
• ribosomes are found in both prokaryotes and eukaryotes
• ribosomes are 20 by 30 nm in eukaryotes and slightly smaller in prokaryotes
• ribosomes are composed of two subunits, one large and one small, and each has its own mix of proteins and RNA
• polyribosomes-groups of ribosomes
• in eukaryotic cells some occur free within the cytosol either single or in groups, or attached to the endoplasmic reticulum
• ribosomes are sites of protein synthesis; they receive mRNA from the nucleus and synthesize proteins
• vesicle-tiny membranous sacs where ribosomes are secreted out of the cell
• ribosomes bind to endoplasmic reticulum through receptor proteins which act as docking sites for a particular molecule
• the endomembrane system consists of the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, and several vesicles
• this system compartamentalizes the cell so that particular enzymatic reactions are restricted to specific regions
• membranes that make up the endomembrane system are connected by direct physical contact and/or by the transfer of vesicles from one part to the other
• endoplasmic reticulum-system of membranous channels and saccules (flattened vesicles), physically continuous with the outer membrane of the nuclear envelope
• rough ER-studded with ribosomes on the side of the membrane that faces the cytoplasm, synthesizes and moders proteins
• smooth ER-does not have attached ribosomes, continuous with rough ER, synthesizes phospholipids, steroids, fatty acids; in the testes it produces testosterone, in the liver it detoxifies drugs
• Camillo Golgi discovered the Golgi apparatus in 1898
• Golgi apparatus-stack of three to twenty slightly curved saccules whose appearance can be compared to a stack of pancakes
• in animal cells one side of the stack is directed toward the ER and the other toward the plasma membrane
• Golgi apparatus contains enzymes that alter the carbohydrate chains first attached to proteins in the rough ER
• the Golgi apparatus packages its products in vesicles that depart at the outer face
• some of these vesicles are lysosomes
• lysosomes-membrane bounded vesicles produced by the Golgi apparatus that have a very low pH and powerful hydrolytic digestive enzymes
• lysosomes sometimes break down macromolecules, fusing with these vesicles and being engulfed by white blood cells
• autodigestion is when parts of a cell are digested by its own lysosomes
• apoptosis-programmed cell death
• peroxisomes-membrane-bounded desicles that contain specific enzymes imported from the cytosol
• have enzymes for oxiding small organic molecules with the formation of hydrogen peroxide
• RH2 + O2 → R + H2O2
• in plants, peroxisomes oxidize fatty acids into molecules and carry out a reaction in leaves that releases carbon dioxide that can be used for photosynthesis
• vacuole-large membranous sac
• plant cells have a large central vacuole so filled with a watery fluid that it gives added support to the cell
• vacuoles store substances
• plant vacuoles contain water, sugar, salt, pigments, toxic molecules
• chloroplasts and mitochondria are 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
• mitochondria-where broken down carbohydrate-derived products to produce ATP molecules
• photosynthesis is the process by which solar energy is converted to chemical energy within carbohydrates
• solar energy + carbon dioxide + water → carbohydrate + oxygen
• carbohydrate + oxygen → carbon dioxide + water + energy
• when a cell needs energy, ATP supplies it
• chloroplasts, which are about 4-6 μm in diameter and 1-5 μm in length, are plastids
• stroma-fluid enclosed by two membranes bounding the chloroplast containing DNA, enzymes, ribosomes
• thylakoids-interconnected flattened sacs in grana
• grana-stacked up structures containing thylakoids
• mitochondria are 0.5-1.0 μm in diameter and 2-5 μm in length
• mitochondria are also bounded by two membranes
• cristae-invagination of inner membrane, providing a greater surface area to accommodate the protein complexes and other participants in cellular respiration
• matrix-where the cristae project, inner space filled with semifluid membrane that contains enzymes that break down carbohydrate products, releasing energy that is used for ATP production on the cristae, also contains DNA and ribosomes
• cytoskeleton-network of interconnected filaments and tubules that extends from the nucleus to the plasma membrane in eukaryotic cells
• cytoskeleton change into spindles, contain three types of elements: actin filaments, intermediate filaments, microtubules
• actin filaments are long, thin fibers (7 nm in diameter) containing two chains of globular actin monomers twisted about one another in a helical manner
• they form a dense complex web just under the plasma membrane to which they are anchored by special proteins, they are seen in microvilli that project from intestinal cells, their presence most likely accounts for the ability of microvilli to alternately shorten and extend into the intestine; in plant cells they form the tracks along which chloroplasts circulate or stream in a particular direction; the presence of a network accounts for the formation of pseudopods, extensions that allow certain cells to move in an amoeboid fashion
• motor molecules-proteins that can attach, detach, reattach along an actin filament
• in the presence of ATP, myosin pulls actin filaments along
• intermediate filaments (8-11 nm in diameter) are a ropelike assembly of fibrous polypeptides, but the specific type varies according to the tissue
• some support the plasma membrane and take part in the formation of cell-to-cell junctions
• in the skin filaments made of keratin give mechnanical strength
• need to have phosphate added by soluble enzymes
• microtubules-small hollow cylinders 25 nm in diameter and 0.2-25 μm in length made of a globular protein called tubulin
• when assembly occurs, and tubulin molecules come together as dimers and arrange themselves in rows
• regulation of microtubule assembly is under control of a microtubule organizing center/MTOC
• centrosome-the main MTOC, near the nucleus, radiates microtubules that help maintain the shape of the cell and act as the track along which organelles can move
• centrioles-short cylinders with a 9 + 0 pattern of microtubule triplets
• in animal cells and most protists, a centrosome contains 2 centrioles lying at right angles to each other
• before an animal cell divides, the centrioles replicate and the members of each pair are at right angles to one another. Then, each pair becomes part of a separate centrosome that moves apart and organizes the mitotic spindle
• plant and fungal cells have centrosome equivalents but no centrioles, which means they may not be necessary to the assembly of cytoplasmic microtubules
• in cells with cilia and flagella, centrioles give rise to basal bodies that direct microtubule organization
• a basal body may function for a cilium/flagellum like a centrosome does for a cell
• cilia/flagella-hairlike projections that can move either in an undulating fashion (whip) or stiffly (oar)
• cellular paramecia move by cilia, sperm cells have flagella, our respiratory tract has cilia
• in eukaryotic cells, cilia and flagella have 9 microtubule doublets arranged in a circle around 2 central microtubules called the 9 + 2 pattern, moving when the microtubules slide past one another
• each cilium and flagellum has a basal body lying in the cytoplasm at its base and have the same arrangement of microtubule triplets