The Plant Cell and the Cell Cycle

1. The Plant Cell and the Cell Cycle Chapter 3

2. Cell • Basic unit of plant structure and function • Robert Hooke • Looked at cork tissue under microscope • “little boxes or cells distinct from one another ….that perfectly enclosed air” • Nehemiah Grew • Recognized leaves as collections of cells filled with fluid and green inclusions

3. Cell Theory

4. Basic Similarities of Cells • Cells possess basic characteristic of life • Movement • Metabolism • Ability to reproduce • Organelles • “little organs” • Carry out specialized functions within cells

5. Light Microscope • View cells 20-200 µm in diameter • Can view living or stained specimens • Resolution (resolving power) • Ability to distinguish separate objects • Limited by lenses and wavelengths of light used • Smallest object that can be resolved is ~ 0.2 µm in diameter

6. Confocal Microscope • Laser illumination • Detecting lens focuses on single point at a time • Scans entire sample to assemble picture • No reduction in contrast due to scattered light • Can generate 3-D images

7. Transmission Electron Microscope • Responsible for discovery of most of smaller organelles in cell • Greater resolution • Uses beams of electrons rather than light • Magnets for lenses • Ultrathin section examined in vacuum • View image on fluorescent plate or photographic film

8. Scanning Electron Microscope • Collected electrons used to form picture in television picture tube • High resolution view of surface structures • Requires vacuum • Recent refinements • Can operate in low vacuum • Can view live plant cells and insects

9. Microscope Comparisons

10. Generalized Plant Cell

11. Boundaries Between Inside and Outside the Cell Plasma Membrane and Cell Wall

12. Plasma Membrane • Surrounds cell • Controls transport into and out of cell • Selectively permeable

13. Plasma Membrane • Composed of approximately half phospholipid and half protein, small amount of sterols • Phospholipid bilayer • Separates aqueous solution inside cell from aqueous layer outside cell • Prevents water-soluble compounds inside cell from leaking out • Prevents water-soluble compounds outside cell from diffusing in

14. Plasma Membrane • Proteins in bilayer • Perform different functions • Ion pumps • Move ions from lower to higher concentration • Require ATP energy • Proton pump – moves H+ ions from inside to outside of cell • Ca+2 pump – moves Ca2+ to outside of cell • Channels – allow substances to diffuse across membrane

15. Plasma Membrane • Plasmodesmata • Connects plasma membranes of adjacent plant cells • Extends through cell wall • Allows materials to move from cytoplasm of one cell to cytoplasm of next cell • Symplast – name for continuous cytoplasm in set of cells

16. Plasma Membrane • Apoplast – • Space outside cell • Next to plasma membrane within fibrils of cell wall • Area of considerable metabolic activity • Important space in plant but questionable as to whether it is part of the plant’s cells

17. Cell Wall • Rigid structure made of cellulose microfibrils • Helps prevent cell rupture • Process of osmosis allows water to enter cell • Inflow of water expands cell • Expansion forces cell membrane against cell wall • Resistance of cell wall to expansion balances pressure of osmosis • Stops flow of water into cell • Keeps cell membrane from further expansion

18. Cell Wall • Osmotic forces balanced by pressure exerted by cell wall • Creates turgor pressure • Causes cells to become stiff and incompressible • Able to support large plant organs • Loss of turgor pressure – plant wilts

19. Cell Wall • Place cell in salt solution • Water leaves cytoplasm • Protoplast (space inside plasma membrane) shrinks • Plasma membrane pulls away from cell wall • Cell lacks turgor pressure - wilts

20. Cell Wall Structure • Primary cell wall • Cell wall that forms while cell is growing • Secondary cell wall • Additional cell wall layer deposited between primary cell wall and plasma membrane • Generally contains cellulose microfibrils and water-impermeable lignin • Provides strength to wood

21. Cell Wall Structure • Specialized types of cell walls • cutin covering cell wall or suberin imbedded in cell wall • Waxy substances impermeable to water • Cutinized cell walls • Found on surfaces of leaves and other organs exposed to air • Retard evaporation from cells • Barrier to potential pathogens

22. Organelles of Protein Synthesis and Transport Nucleus, Ribosomes, Endoplasmic Reticulum, and Golgi Apparatus

23. Nucleus • Ovoid or irregular in shape • Up to 25 µm in diameter • Easily stained for light or electron microscopy

24. Nucleus • Surrounded by double membrane – nuclear envelope • Protein filaments of lamin line inner surface of envelope and stabilize it • Inner and outer membranes connect to form pores • Nucleoplasm • Portion of nucleus inside nuclear envelope

25. Nucleus • Nucleoli (singular, nucleolus) • Densely staining region within nucleus • Accumulation of RNA-protein complexes (ribosomes) • Site where ribosomes are synthesized • Center of nucleoli • DNA templates • Guide synthesis of ribosomal RNA

26. Nucleus • Chromosomes • Found in nucleoplasm • Contain DNA and protein • Each chromosome composed of long molecule of DNA wound around histone proteins forming a chain of nucleosome • Additional proteins form scaffolds to hold nucleosomes in place

27. Nucleus • DNA in chromosomes • Stores genetic information in nucleotide sequences • Information used to direct protein synthesis • Steps in protein synthesis • Transcription – DNA directs synthesis of RNA • Most RNA stays in nucleus or is quickly broken down • Small amount of RNA (mRNA) carries information from nucleus to cytoplasm

28. Nuclear Components

29. Ribosomes • Small dense bodies formed from ribosomal RNA (rRNA) and proteins • Function in protein synthesis • Active ribosomes in clusters called polyribosomes • Attached to same mRNA • All ribosomes in one polyribosome make same type of protein

30. Ribosomes • In living cell, ribosomes are not fixed • Move rapidly along mRNA • Read base sequence • Add amino acids to growing protein chain • At end of mRNA, ribosome falls off, releasing completed protein into cytoplasm

31. Endoplasmic Reticulum • ER • Branched, tubular structure • Often found near edge of cell • Function • Site where proteins are synthesized and packaged for transport to other locations in the cell • Proteins injected through membrane into lumen

32. Endoplasmic Reticulum • Packaging of proteins by ER • Considered to be packaged when separated from cytoplasm by membrane • Sphere (vesicle) of membrane-containing proteins may bud off from ER • Vesicle carries proteins to other locations in cell

33. Endoplasmic Reticulum • Types of ER • rough ER – ribosomes attached to surface • smooth ER – does not have attached ribosomes • Carbohydrate transport • Often attached to proteins in ER • Helps protect carbohydrate from breakdown by destructive enzymes

34. Golgi Apparatus • Also called a dictyosome • Consists of stack of membranous, flattened bladders called cisternae

35. Golgi Apparatus • Directs movements of proteins and other substances from ER to other parts of cell • Cell wall components (proteins, hemicellulose, pectin) pass through cisternae • Move to plasma membrane inside membranous sphere • Sphere joins with plasma membrane • Membrane of sphere becomes part of plasma membrane • Protein, hemicellulose, and pectin contents released to outside the cell

36. Endomembrane System • Complex network that transports materials between Golgi apparatus, the ER, and other organelles of the cell • Movement • Rapid • Continuous

37. Organelles of Energy Metabolism Plastids and Mitochondria

38. Plastids • Found in every living plant cell • 20-50/cell • 2-10 µm in diameter • Surrounded by double membrane • Contain DNA and ribosomes • Protein-synthesizing system similar to but not identical to one in nucleus and cytoplasm

39. Plastids • Proplastids • Small plastids always found in dividing plant cells • Have short internal membranes and crystalline associations of membranous materials called prolamellar bodies • As cell matures, plastids develop • Prolamellar bodies reorganized • Combined with new lipids and proteins to form more extensive internal membranes

40. Plastids • Types of plastids • Chloroplasts • Leukoplasts • Amyloplasts • Chromoplasts

41. Plastids • Chloroplasts • Thylakoids • Inner membranes • Have proteins that bind to chlorophyll • Chlorophyll • Green compound that gives green plant tissue its color • Stroma • Thick solution of enzymes surrounding thylakoids

42. Plastids • Chloroplasts • Function • Convert light energy into chemical energy (photosynthesis) • Accomplished by proteins in thylakoids and stromal enzymes • Can store products of photosynthesis (carbohydrates) in form of starch grains

43. Chloroplast

44. Leukoplasts • leuko – “white” • Found in roots and some nongreen tissues in stems • No thylakoids • Store carbohydrates in form of starch • Microscopically appear as white, refractile, shiny particles

45. Amyloplasts • amylo – “starch” • Leukoplast that contains large starch granules

46. Chromoplasts • chromo – “color” • Found in some colored plant tissues • tomato fruits, carrot roots • High concentrations of specialized lipids – carotenes and xanthophylls • Give plant tissues orange-to-red color

47. Plastids

48. Mitochondria • Double-membrane structure • Contain DNA and ribosomes • Inner membrane infolded • Folds called cristae • Increase surface area available for chemical reactions

49. Mitochondria • Matrix • Viscous solution of enzymes within cristae • Function • source of most ATP in any cell that is not actively photosynthesizing • Site of oxidative respiration • Release of ATP from organic molecules • ATP used to power chemical reactions in cell

50. Other Cellular Structures Vacuoles, Vesicles, Peroxisomes, Glyoxysomes, Lysosomes, and Cytoskeleton