chapter 3

Electron microscopy

  • Scanning: study detailed architecture of cell surfaces

  • Transmission: study details of internal cell structure

Nucleus and Ribosomes

  • Nucleus = control center of the cell

    • Contains most of cell’s DNA

    • Directs protein synthesis by making mRNA

    • Nuclear envelope: double membrane and has pores that allow material to flow in and out of the nucleus

      • Attached to a network of cellular membranes called the endoplasmic reticulum

  • Nucleolus = site of ribosomal RNA (rRNA) synthesis

    • Structure in the nucleus

  • Ribosomes = involved in protein synthesis

    • Synthesized from rRNA produced in the nucleolus

    • Not bound by membrane

    • Free ribosomes: suspended in the cytoplasm; typically involved in making proteins that function within the cytoplasm

    • Bound ribosomes: attached to the Endoplasmic Reticulum (ER) associated with the nuclear envelope; associated with proteins packed in certain organelles or exported from the cell

The Endomembrane System 

  • Some membranes within this system are physically connected while others are connected by the transfer of membrane segments by tiny vesicles (sacs made of membrane)

  • Many of these organelles work together in: synthesis, storage, and export of molecules

  • Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and the plasma membrane

  • Two Kind of Endoplasmic Reticulum

    • Rough ER

      • Ribosomes line the outer surface of membranes

      • Makes: additional membranes for itself, proteins designed for secretions

    • Smooth ER

      • Lacks attached ribosomes

      • Produces enzymes important in the synthesis of lipids, oils, phospholipids, and steroids

      • Other enzymes help process drugs, alcohol, and other potentially harmful substances

  • The Golgi Apparatus

    • (molecular factory) Finishes, sorts, and ships cell products

    • Finishing factory for products created by ER

      • Products travel in transport vesicles from ER to Golgi

    • One side = receiving dock and other = shipping dock

  • Lysosome: membranous sac containing digestive enzymes

    • The enzymes and membrane are produced by the ER and processed by the Golgi

    • Membranes function to safely isolate the potent enzymes from the rest of the cell

    • Primary lysosomes originate from the golgi apparatus

      • Contain digestive enzymes and are the site where macromolecules are hydrolyzed into monomers

    • Phagocytes = cells that take materials into cells and break them down

    • Phagocytosis = macromolecules may enter the cell when part of the plasma membrane encloses the material and a phagosome is formed => phagosomes then fuse with primary lysosomes to form secondary lysosomes

      • Enzymes in secondary lysosomes hydrolyze food molecules

    • Lysosomes also help remove or recycle damaged parts of cells

      • The damaged organelle is first enclosed in a membrane vesicle

      • Then a lysosome fuses with the vesicle, dismantles its contents, and breaks down the damaged organelle

    • Lysosomal storage diseases occur when lysosomes fail to digest the components

    • Autophagy = programmed destruction of cell components

      • Occurs in lysosomes

  • Peroxisome

    • Sequester and break down reactive oxygen species

    • Involved in lipid metabolism

    • Detoxify alcohol and other toxins in the liver

  • Vacuole

    • Sac-like structures - occur in eukaryotes but mainly in plants and fungi

    • Functions:

      • Stores materials such as salts, water, proteins, and carbs

      • Structure for plant cell - water enters the vacuole by osmosis, creating turgor pressure

      • Storage of waste products and toxic compounds

    • Tonoplast: membrane surrounding a large central vacuole, separating it from the cytoplasm

    • Contractile Vacuoles: some protists have contractile vacuoles that help to eliminate water from the protists

Energy Processing Organelles

  • Mitochondria: carry out cellular respiration in nearly all eukaryotic cells

    • Two internal compartments:

      • Intermembrane space = narrow region between the inner and outer membranes

      • Mitochondrial matrix => mitochondrial DNA, ribosomes, and many enzymes that catalyze some of the reactions of cellular respiration

  • Chloroplasts: photosynthesizing organelles of all photosynthesizing eukaryotes

    • Intermembrane space

    • Inside inner membrane:

      • Thick fluid called stroma: contains chloroplast DNA, ribosomes, many enzymes, and

        • Thylakoids: network of interconnected sacs

        • Each stack of thylakoids (like poker chips) = granum; where green chlorophyll molecules trap solar energy

  • Mitochondria and Chloroplasts evolved by endosymbiosis

    • Both have DNA and ribosomes

    • Endosymbiont theory: mitochondria and chloroplasts were formerly small prokaryotes and they began living within larger cells

Cytoskeleton and Cell Motility

  • Cytoskeleton: network of protein fibers in cells which helps organize the cell’s structure and activities

    • Interacts with motor proteins to provide motility and cellular regulation

    • Composed of three types of fibers

      • Microfilaments (Actin filaments): support the cell’s shape and are involved in motility

      • Intermediate filaments: reinforce cell shape and anchor organelles

        • More permanent fixtures in the cell

      • Microtubules (made of tubulin): give the cell rigidity and act as tracks for organelle movement

        • Elongate and disassemble

  • Kinesin: motor protein that binds to vesicles in the cell and “walks” them along the microtubule

  • Cilia and Flagella

    • Cilia (movable cell appendage) works like oars of a crew boat

      • Provide movement for unicellular eukaryotes or move fluid and materials for a stationary cell

    • Flagellum (longer than cilia): propels cells by an undulating, whiplike motion

    • Both have common structure and mechanism of movement (move when microtubules bend)

      • Move by bending motor proteins called dynein feet

        • Dynein: motor protein that drives the sliding of doublets by changing its shape

    • Some cells of multicellular organisms have them for different reasons:

      • Cells that sweep mucus out of our lungs have cilia

      • Animal sperm are flagellated

    • Arrangement of microtubules

      • Both cilia and flagella are made of microtubules wrapped in an extension of the plasma membrane

  • Extracellular Matrix

    • (animal cells) functions in support and regulation

    • Helps hold cells tg in tissues and protects and supports the plasma membrane

    • May attach to a cell through glycoproteins that then bind to the membrane proteins called integrins

      • Integrins span the plasma membrane and connect to microfilaments of the cytoskeleton

  • Cell Junctions

    • Tight junctions prevent leakage of extracellular fluid across a layer of epithelial cells

    • Anchoring junctions fasten cells together into sheets

    • Gap junctions are channels that allow molecules to flow between cells

  • Cell Walls

    • Enclose and support plant cells

    • Protect and provide skeletal support that helps keep the plant upright against gravity (primarily composed of cellulose)

    • Plant cells have cell junctions called plasmodesmata that serve in communication between cells