Cell Organelles

• General Attributes of the Cell:

  • Phospholipid bilayer separates the cell from its environment
  • Allows passage of oxygen, nutrients, and wastes
• Plasma Membrane:

  • Semifluid substance where organelles are suspended
  • Cytoplasm is the region between the plasma membrane and nucleus
• Chromosomes contain the genetic material in the form of DNA
• Ribosomes:

  • Tiny organelles that make proteins using the instructions contained in genes
• Prokaryotic Cells:

  • Include Bacteria and Archaea
  • Have a nucleoid and no membrane-bound nucleus
  • Contain a circular strand of DNA
  • Have few organelles
• Eukaryotic Cells:

  • Include Protists, Fungi, Plants, and Animals
  • Have a nucleus bound by a membrane
  • DNA is in several linear chromosomes
  • Contain many specialized membrane-bound organelles
• Animal Cell Nucleus:

  • Contains the eukaryotic cell's genetic library
  • Separated from cytoplasm by a double membrane
  • Nuclear pore allows large macromolecules and particles to pass through
  • Some genes are contained in mitochondria and chloroplasts
• Ribosomes:

  • Build a cell's proteins through translation
  • Site of protein synthesis
  • Free ribosomes are suspended in the cytosol and synthesize proteins that function in the cytosol
  • Bound ribosomes are attached to the rough endoplasmic reticulum (RER) or nuclear envelope and synthesize proteins included in the membrane or for export from the cell
• Endomembrane System:

  • Many internal membranes in a eukaryotic cell are part of this system
  • Plays a key role in the synthesis and hydrolysis of macromolecules in the cell
  • Various components modify macromolecules for their various functions
  • Components include the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles/vesicles, and plasma membrane
• Endoplasmic Reticulum (ER):

  • Manufactures membranes and performs many other biosynthetic functions
  • ER membrane is continuous with the nuclear envelope
  • Cisternal lumen is continuous with the space between 2 membranes of nuclear envelope
• Golgi Apparatus:

  • Finishes, sorts, and ships cell products
  • Transport vesicles from ER travel to Golgi apparatus for modification
  • Abundant in cells specialized for secretion of glycoproteins
  • Flattened membranous sacs CISTERNAE
  • CIS SIDE –receives by fusing with vesicles
  • TRANS SIDE – buds off vesicles
  • Manufactures polysaccharides such as pectin
• Lysosomes:

  • Membranous sac of hydrolytic enzymes
  • Digests macromolecules and works best at pH 5
  • Can destroy an entire cell by autodigestion if there is massive leakage
  • Can fuse with food vacuoles, formed when a food item is brought into cell by phagocytosis or other organelles or other parts of the cytosol for autophagy, which is the recycling process that renews the cell
• Vacuoles:

  • Membrane-bound sacs with varied functions
  • Functions include stockpiling proteins or inorganic ions (K & Cl), depositing metabolic byproducts, storing pigments, and defensive compounds against herbivores
  • Major role in the growth of plant cells: Cells enlarge as their vacuoles absorb water, with minimal investment in new cytoplasm
  • Food Vacuole: From phagocytosis, fuse with lysosomes
  • Contractile Vacuole: In freshwater protists, pump excess water out of the cell
  • Central Vacuoles: Found in many mature plant cells
• Mitochondria and Chloroplasts:

  • Mitochondria are the site of cellular respiration and generate ATP from catabolism of sugars, fats and other fuels in the presence of oxygen
  • Chloroplasts are the site of photosynthesis and convert solar energy to chemical energy and synthesize new organic compounds from CO2 and H2O
  • Both are the main energy transformers of cells and convert energy to forms that cells can use for work
• Mitochondria:

  • Have a smooth outer membrane and a highly folded inner membrane called cristae
  • The intermembrane space is fluid-filled and located between the outer and inner membrane
  • Cristae provide ample surface area for enzymes that synthesize ATP
  • The inner membrane encloses the mitochondrial matrix, a fluid-filled space with DNA, ribosomes, and enzymes
  • Mitochondria grow and reproduce as autonomous organelles
  • Mitochondria are mobile and move around the cell along tracks in the cytoskeleton
• Chloroplast:

  • Belongs to a generalized class of plant structures called plastids
  • Contains amyloplasts that store starch and chromoplasts that store pigments such as carotene
  • Produces sugar via photosynthesis
  • Found in leaves and other green structures
  • Reproduces by pinching into 2
  • Chloroplast processes are separated from the cytosol by 2 membranes
  • Contains a fluid-filled space called stroma and membranous sacs called thylakoids
  • Stroma contain DNA, ribosomes, enzymes
  • Thylakoids are flattened sacs stacked into grana and are critical for converting light to chemical energy
• Peroxisomes:

  Oxidation generate & degrade hydrogen peroxide in performing various metabolic functions

  • Bound by a single membrane
  • Generated and degrade hydrogen peroxide in various metabolic functions
  • Split into 2 upon reaching a certain size
  • Contain enzymes that transfer hydrogen from various substrates to oxygen
  • Intermediate product is H2O2 but converted into H2O by catalase
  • Some peroxisomes break fatty acids into smaller molecules for transport to mitochondria
  • Some detoxify alcohol and other harmful compounds
• Cytoskeleton:

  • Network of fibers that organizes structures and activities in the cell
  • Composed of microtubules, microfilaments, and intermediate filaments
  • Provides mechanical support and maintains cell shape
  • Anchors organelles and cytosolic enzymes
  • Dynamic, can dismantle in one part and reassemble in another to change cell shape
• Microtubules:

  • Composed of alpha and beta tubulin dimers
  • Move chromosomes during cell division
  • Grow out from a centrosome near the nucleus
  • In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring
  • Centrioles replicate during cell division
  • Central structural support in cilia and flagella
  • Have a core of microtubules sheathed by the plasma membrane
  • 9 doublets of microtubules arranged around 1 pair at the center(9+2 pattern)
  • Flagella have just one or a few in a cell, while cilia occur in large numbers
• Microfilaments:

  • Thinnest among the cytoskeleton fibers
  • Composed of solid rods of the globular protein actin
  • Resist tension and support cell shape
  • Involved in motility, cell division, cytoplasmic streaming, and distribution of materials
• Intermediate Filaments:

  • Composed of keratin
  • Found only in some animal cells
  • Bear tension and reinforce cell shape
  • More permanent fixture of the cytoskeleton
  • Fix organelle location
• Cell Wall:

  • Found in prokaryotes, fungi, some protists, and plants
  • Provides protection, maintains shape, and prevents excessive uptake of water
  • Basic design includes microfibrils of cellulose embedded in a matrix of proteins and other polysaccharides
  • Mature cell wall consists of primary cell wall (cellulose), middle lamella with sticky polysaccharides (pectin), and layers of secondary wall
• Extracellular Matrix (ECM):

  • Functions in support, adhesion, movement, and regulation in animal cells
  • Contains glycoproteins like collagen fibers embedded in a network of proteoglycans
  • Fibronectins in ECM connect to integrins, intrinsic membrane proteins
  • Integrins connect ECM to the cytoskeleton
  • Allows interaction of changes inside and outside the cell
• Intracellular Junctions:

  • Help integrate cells into higher levels of structure and function
  • Neighboring cells in tissues adhere, interact, and communicate through direct physical contact
  • Plant cells have plasmodesmata, channels allowing cytosol to pass between cells
• Animal Intercellular Links:

  • Tight junctions fuse adjacent cells to prevent leakage of extracellular fluid
  • Desmosomes (anchoring junctions) fasten cells together into strong sheets, reinforced by intermediate filaments
  • Gap junctions (or communicating junctions) provide cytoplasmic channels between adjacent cells, facilitating chemical communication during development in embryos
• Plant Crystals:

  • Waste or excretory products of protoplasts
  • Calcium carbonate forms cystolith & worm-like cystolith
  • Lithocyst - cells containing cystoliths
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  • Calcium oxalate forms raphide, prismatic, druse, and styloid
• Nuclear Lamina:

  • Network of intermediate filaments
  • Maintain shape of the nucleus
  • Mechanically support of nuclear membrane
• Nucleolus:

  • Site of ribosomal RNA (rRNA) synthesis
  • rRNA combines with proteins in the cytoplasm to form ribosomal subunits
• Rough Endoplasmic Reticulum:

  • With ribosomes
  • Abundant in cells that secrete proteins
  • Secretory proteins are packaged into TRANSPORT VESICLES that carry them into the next stage
  • Synthesis of membrane bound proteins
  • Enzymes in RER synthesize phospholipids from precursors in the cytosol
• Smooth Endoplasmic Reticulum:

  • No ribosomes
  • Synthesis of lipids, oil, phospholipids & steroids
  • Metabolism of carbohydrates
  • Catalyzes key step in mobilization of glucose from stored glycogen in the liver
  • Stores calcium ions espin muscle cells
  • Other enzymes in SER of the liver help detoxify drugs, poisons, alcohol
• Tonoplast:

  • membrane surrounding the central vacuole
  • selective in its transport of solutes into the central vacuole
  • contains glycolipids (instead of phospholipids)
• Cytoskeleton: Role in cell motility

  • Cilia & flagella, motor proteins pull components of cytoskeleton past each other
  • Interactions of motor proteins and the cytoskeleton circulates materials within a cell via streaming.
  • Also in muscle cells