Chapter 4: Cell Structure & Function (Outline)

1. Chapter 4: Cell Structure & Function (Outline) • Cell Theory • Cell Size • Prokaryotic Cells • Eukaryotic Cells • Organelles • Nucleus • Endomembrane System • Cytoskeleton • Centrioles, Cilia and Flagella

2. Development of Cell Theory • In 1665, English Scientist Robert Hooke discovered cells while looking at a thin slice of cork • In 1673, Anton van Leuwenhoek observed pond scum & discovered single-celled organisms using a handmade microscope • In 1831, English botanist Robert Brown described the nucleus of cells • In 1838, German Botanist, Matthias Schleiden, stated that all plant parts are made of cells • In 1839, German physiologist Theodor Schwann stated that all animal tissues are composed of cells • In 1858, Rudolf Virchow German physician concluded that cells must arise from preexisting cells

3. Cell Theory • A unifying concept in biology • Originated from the work of biologists Schleiden, Schwann & Virchow • States that: • All organisms are composed of cells (Schleiden & Schwann, 1838-39) • The cell is the basic unit of structure & function in organisms (Schleiden & Schwann, 1838-39) • All cells come only from preexisting cells since cells are self-reproducing (Virchow, 1858)

4. Cell Size • Most much smaller than one millimeter (mm) • Some as small as one micrometer (mm) • Size restricted by Surface/Volume (S/V) ratio • Surface is membrane, across which cell acquires nutrients and expels wastes • Volume is living cytoplasm, which demands nutrients and produces wastes • As cell grows, volume increases faster than surface • Cells specialized in absorption modified to greatly increase surface area per unit volume

5. Surface to Volume Ratio • TotalSurfaceArea (Height  Width  Number Of Sides  Number Of Cubes) 96 cm2 192 cm2 384 cm2 • TotalVolume (Height  Width  Length x Number Of Cubes) 64 cm3 64 cm3 64 cm3 • SurfaceAreaPerCube/VolumePerCube (Surface Area/ Volume) 1.5/1 3/1 6/1

6. Sizes of living things and their component

7. Prokaryotic Cells • Prokaryotes – lack a membrane-bounded nucleus and are structurally less complicated than the eukaryotes • Prokaryotes are responsible for either all or significant portions of all of the following • Nutrient recycling – mineralization; nitrogen fixing • Decomposition of dead organisms • Disease (infectious) – tuberculoses; anthrax • Commercial uses – foodstuffs; antibiotics; insulin • Prokaryotes are divided into two domains • Domain Bacteria • Domain Archaea

8. Prokaryotic Cells • Nuclear body is not bounded by a nuclear membrane • Usually contains one circular chromosome composed of deoxyribonucleic acid (DNA) • The nuclear body is called a nucleoid • Extra chromosomal piece of DNAcalled plasmid • Structurally simple • Three basic shapes: • Bacillus (rod) • Coccus (spherical) • Spirilla (spiral)

9. Prokaryotic Cells:The Envelope • Cell Envelopes include • Glycocalyx • Layer of polysaccharides outside cell wall • May be slimy and easily removed, or • Well organized and resistant to removal (capsule) • Cell wall • Consist of peptidoglycan (amino disaccharide & peptide) • Maintains shape of the cell • Plasma membrane • Like in eukaryotes – a phospholipid bilayer with proteins • Form internal pouches (mesosomes), why?

10. Prokaryotic Cells:Cytoplasm • Cytoplasm - semifluid solution bounded by a plasma membrane containing • Nucleoid – location of the single bacterium chromosome (coiled) • Plasmid – extrachromosomal piece of circular DNA • Inclusion bodies – Stored granules of various substances • Ribosomes – tiny particles where protein is synthesized (contain RNA & protein in 2 subunits) • Thylakoids – extensive internal membranes found in cyanobacteria,function?

11. Prokaryotic Cells:Appendages • Appendages are made of protein that include • Flagella – the most common form of bacterial motility (made up of a filament, hook & basal body) • Fimbriae – small, bristle-like fibers that sprout from the cell surface (attach bacteria to a surface) • Conjugation pili – rigid tubular structures used to pass DNA from cell to cell

12. Prokaryotic Cells: Visual Summary

13. Eukaryotic Cells • Domain Eukarya • Protists • Fungi • Plants • Animals • Eukaryotic cells contain: • a true nucleus, bound by a double membrane • a complex collection of organelles • a plasma membrane

14. Eukaryotic Cells :Organelles • Compartmentalization: • Allows eukaryotic cells to be larger than prokaryotic cells • Isolates reactions from others • Two classes: • Endomembrane system: • Organelles that communicate with one another • via membrane channels and small vesicles • Energy related organelles • Mitochondria & chloroplasts • Basically independent & self-sufficient

15. Animal and Plant Cells

16. Nucleus • Command center of cell, why? • Separated from cytoplasm by nuclear envelope • Consists of double layer of membrane • Nuclear pores permit exchange of ribosomal subunits & mRNA between nucleoplasm & cytoplasm • Contains chromatin in semifluid nucleoplasm • Chromatin contains DNA of genes • Condenses to form chromosomes • Dark nucleolus composed of ribosomal RNA (rRNA) • Produces subunits of ribosomes

17. Anatomy of the nucleus • Messenger RNA (mRNA) carries information about a protein sequence to the ribosome • Transfer RNA (tRNA) assembles the amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis

18. Ribosomes • Serve in protein synthesis • Composed of rRNA • Consists of a large subunit and a small subunit • Each subunit is composed of protein and rRNA • Subunits made in nucleolus • Number of ribosomes in a cell varies depending on function (e.g. pancreatic cells) • May be located: • On the endoplasmic reticulum (ER), thereby making it “rough”, or • Free in the cytoplasm, either singly or in groups called polyribosomes

19. Ribosome Function • Ribosome binding to the endoplasmic reticulum occurs through a signal peptide on the synthesized protein • Signal peptide combines with a signal recognition particle (SRP) • SRP attaches to SRP receptor, thus allowing protein to enter the lumen of the ER • The signal peptide is removed from the protein (via signal peptidase) in the lumen of the ER • Ribosomal subunits & mRNA break away and protein folds into its final shape

20. Nucleus, Ribosomes, & ER

21. Endomembrane System • Restrict enzymatic reactions to specific compartments within cell • Consists of: • Nuclear envelope • Membranes of endoplasmic reticulum • Golgi apparatus • Vesicles • Several types • Transport materials between organelles of the system

22. Endomembrane System:The Endoplasmic Reticulum • A membrane network within the cytoplasm of cells involved in the synthesis, modification and transport of cellular materials • Rough ER • Studded with ribosomes on cytoplasmic side • Protein anabolism • Synthesizes proteins • Modifies proteins - adds sugar to protein (i.e. glycoproteins) • Forms vesicles - transport of large molecules to other parts of cell (i.e. Plasma membrane or Golgi apparatus) • Smooth ER • Continuous with rough ER; No attached ribosomes • Synthesis of lipids (i.e. phospholipids & steroids)

23. Endoplasmic Reticulum

24. Endomembrane System:The Golgi Apparatus • Golgi Apparatus • Consists of 3-20 flattened, curved membrane-bound saccules called cisternae • Resembles stack of deflated balloons • Modifies proteins (i.e. glycosylation) and lipids • Packages them in vesicles • Receives vesicles from ER on cis face • Prepares for “export” in vesicles from trans face • Within cell • Export from cell (secretion, exocytosis)

25. Golgi Apparatus

26. Endomembrane System:Lysosomes • Membrane-bound vesicles (common in animal cells but rare in plant cells) • Produced by the Golgi apparatus • Low pH • Contain hydrolytic enzymes • Digestion of large molecules • Recycling of cellular resources • Destroying nonfunctional organelles • Lysosomes participate in apoptosis • Normal part of development • Example: tadpole → frog

27. Peroxisomes • Similar to lysosomes • Membrane-bounded vesicles • Enclose oxidative enzymes • However • Enzymes synthesized by free ribosomes in cytoplasm (instead of ER) • Active in lipid metabolism • Catalyze reactions that produce hydrogen peroxide H2O2 • Toxic molecule • Broken down to H2O and O2 by catalase enzyme • Alcohol detoxification in liver • Germinating seeds oxidize fatty acids to sugars → growth

28. Peroxisomes & Vacuoles

29. Energy-Related Organelles:Chloroplast Structure • An organelle found within the cells of green plants & eukaryotic algae • Bounded by a double membrane • Inner membrane infolded • Forms disc-like thylakoids, which are stacked to form grana • Suspended in semi-fluid stroma • Green due to chlorophyll • Chlorophyll absorbs light between the red and blue spectrums and reflects green light, making leaves appear green • Found ONLY in inner membranes of chloroplast

30. Energy-Related Organelles:Chloroplasts • Chloroplasts are a type of plastid & are considered to have originated as endosymbiotic cyanobacteria • Has its own DNA and reproduces independently of the cell • Captures light energy to drive cellular machinery • Photosynthesis • Synthesizes carbohydrates from CO2 and H2O • Makes own food using CO2 as only carbon source

31. Chloroplast Structure

32. Other Plastids • Different types of plastids are classified according to the kinds of pigments they contain • Chromoplasts lack chlorophyll but contain carotenoids • responsible for the yellow, orange, & red colors of some flowers and fruits • Leucoplasts are colorless plastids, which synthesize and store a variety of energy sources in non-photosynthetic tissues • Amyloplasts (starch) • Elaioplasts (lipids)

33. Energy-Related Organelles:Mitochondria • Mitochondria are rod-shaped organelles that can be considered the power generators of the cell • Bounded by double membrane • Cristae – Infoldings of inner membrane that encloses matrix, why? • Matrix – Inner semifluid containing respiratory enzymes • Involved in cellular respiration – process by which chemical energy of sugar is converted to ATP • Produce most of ATP utilized by the cell • Has its own DNA and reproduces independently of the cell

34. Mitochondrial Structure

35. The Cytoskeleton • Maintains cell shape • Assists in movement of cell and organelles • Three types of macromolecular fibers • Actin Filaments • Intermediate Filaments • Microtubules • Dynamic, assemble and disassemble as needed • Protein phosphorylation (e.g. protein kinases) • Phosphorylation → disassembly • Dephosphorylation → assembly

36. Cytoskeleton Protein Fibers

37. The Cytoskeleton:Actin Filaments • Extremely thin filaments like a twisted pearl necklace • Dense web just under plasma membrane maintains cell shape • Support for microvilli in intestinal cells • Intracellular traffic control • For moving stuff around within cell • Cytoplasmic streaming in plant cells • Function in pseudopods of amoeboid cells • Pinches off dividing animal cells apart during mitosis • Important component in muscle contraction (other is myosin)

38. Actin Filaments

39. Actin Filament Operation • Actin filaments interact with motor molecules (proteins that can attach, detach and reattach to the actin filament) • Myosin pulls actin filaments in the presence of ATP • In muscle cells, cytoplasmic myosin tails are bound to membranes, while heads interact with actin

40. The Cytoskeleton:Intermediate Filaments • Intermediate in size between actin filaments and microtubules • Rope-like assembly of fibrous polypeptides • Vary in nature (i.e. from tissue to tissue and from time to time) • Functions: • Mechanical stability of the plasma- and the nucleus-membranes • Cell-cell interaction, like those holding skin cells tightly together (keratin)

41. The Cytoskeleton:Microtubules • Hollow cylinders made of two globular proteins called a and b tubulin giving rise tostructures called dimers • Dimers then arrange themselves into tubular spirals of 13 dimers around • Assembly: • Under control of Microtubule Organizing Center (MTOC) • Most important MTOC is centrosome • Interacts with proteins kinesin and dynein to cause movement of organelles

42. Microtubule Operation

43. Microtubules • Microtubules disassemble and then reassemble into a spindle during cellular division • Colchicine - a plant toxic (defense mechanism) that inhibits polymerization by binding to tubulin and preventing microtubule assembly

44. The Cytoskeleton (Summary) • Microfilaments regulate: • Cell shape • Cell movement • Intermediate filaments effect: • The mechanical stability of the plasma- & the nucleus-membranes • Cell-cell interaction • Microtubules effect: • Localization and transport of organelles • Cell division

45. Microtubular Arrays:Centrioles • Short, hollow cylinders • Composed of 27 microtubules • Microtubules are arranged in 9 sets of 3 each (9 + 0) pattern • One pair per animal cell • Located on centrosome of animal cells • Oriented at right angles to each other • Separate during mitosis (cell division) • May give rise to basal bodies of cilia & flagella • Plant cells do not have centrioles

46. Centrioles

47. Microtubular arrays:Cilia and Flagella • Hair-like projections from cell surface that aid in cell movement • Very different from prokaryote flagella • Outer covering of plasma membrane • Inside is a cylinder of 18 microtubules arranged in 9 pairs • Two single microtubules run down the centre of the shaft (9 + 2)pattern found in cilia and flagella • In eukaryotes, cilia are much shorter and numerous than flagella • Cilia move in coordinated waves like oars • Flagella move like a propeller or cork screw

48. Cilia and Flagella • The pairs of microtubules are connected by short arms of protein dymein • Movement of the cilia or flagella is the result of sliding movements between microtubule pairs • Beneath each cilium of flagellum in the cytoplasm of the cell is a basal body • The two central microtubules of the cilia/flagellum do not extend into the basal boy. • The nine pairs of microtubule do and they are joined by a third microtubule. • Centrioles are needed to create basal bodies in order to produce cilia and/or flagella