Cells as Units of Life

1. Cells as Units of Life Chapter 3

2. Cell Theory • Cells represent the basic structural and functional unit of life. • Important unifying concept in biology. • All organisms are composed of one or more cells. • All tissues & organs are composed of cells. • There is no life without cells!

3. Cell Theory • Cell theory states that all living organisms are composed of cells. • Cells come from preexisting cells.

4. Prokaryotic vs. Eukaryotic Cells • All cells: • Have DNA • Use the same genetic code • Synthesize proteins • Use ATP in similar ways • This implies common ancestry.

5. Prokaryotic vs. Eukaryotic Cells • Prokaryotic cells – no nucleus or other membrane-bound organelles. • Kingdom Archaebacteria • Kingdom Eubacteria • Eukaryotic cells – do have nucleus and membrane-bound organelles. • Kingdom Protista • Kingdom Fungi • Kingdom Plantae • Kingdom Animalia

6. Components of Eukaryotic Cells • The plasma membrane surrounds the cell. • The nucleus is the largest organelle. • Double layered nuclear envelope. Cell Model

7. Components of Eukaryotic Cells • Cytoplasm refers to the cellular material between the cell membrane and nuclear envelope. • Organelles such as the mitochondria, Golgi complex, centrioles, and endoplasmic reticulum are found in the cytoplasm.

8. Plasma Membrane • Plasma membrane structure is described using the fluid mosaic model. http://www.youtube.com/watch?v=ULR79TiUj80

9. Plasma Membrane • Two layers of phospholipid molecules oriented with hydrophilic heads toward the outside and hydrophobic tails inside. • Fluid-like – flexible

10. Plasma Membrane • The nonpolar nature of the hydrophobic ends in the interior of the membrane prohibit polar substances from crossing the membrane. • Glycoproteins embedded in the membrane function in the transport of molecules across the membrane.

11. Nucleus • The nuclear envelope contains pores to allow molecules to move between nucleus & cytoplasm. • Chromosomes are contained in the nucleus. • Chromatin refers to loosely condensed DNA & proteins.

12. Nucleus • Nucleoli are specialized parts of certain chromosomes that carry multiple copies of the DNA used to synthesize ribosomal RNA. • This rRNA combines with protein to from the two subunits of ribosomes. • Ribosomes leave the nucleus through pores in the nuclear envelope.

13. Endoplasmic Reticulum • The nuclear envelope joins with a cytoplasmic membranous system – the endoplasmic reticulum (ER). • Rough endoplasmic reticulum (RER) is covered with ribosomes. Smooth (SER) is not.

14. Endoplasmic Reticulum • Ribosomes on the RER synthesize proteins that enter the ER that will either be incorporated into the plasma membrane, exported from the cell, or they may be bound for lysosomes. • Lipids and phospholipids are synthesized in the SER.

15. Golgi Complex • The Golgi complex is a stack of membranous vesicles where storage, modification, and packaging of protein products occurs.

16. Assembling & Secreting Proteins

17. Lysosomes • Lysosomes contain enzymes (proteins) that can breakdown foreign material like bacteria or worn out cellular components. • Contents of lysosome would kill cell if membrane ruptured. • May pour enzymes into food vacuoles.

18. Mitochondria • Mitochondria are the powerhouses of cells – they contain enzymes that carry out the energy-yielding steps of aerobic metabolism. ATP is produced here. • Composed of a double membrane – the inner membrane is folded into cristae. • Mitochondria are self-replicating, containing their own circular DNA molecule.

19. Cytoskeleton • Eukaryotic cells have a cytoskeleton that provides support and often locomotion and movement of organelles. • Composed of microfilaments, microtubules, and intermediate filaments.

20. Cytoskeleton • Microfilaments are made of the proteins actin and myosin and function in a cell’s ability to contract as seen in muscle cells. • Actin microfilaments move molecules and organelles through the cytoplasm.

21. Cytoskeleton • Microtubules are larger tubular structures composed of the protein tubulin. • Move chromosomes during cell division. • Part of the structure of cilia & flagella.

22. Cytoskeleton • Microtubules radiate out from the centrosome – the microtubule organizing center. • Located near nucleus. • Not membrane bound. • Centrioles are found in the centrosome. • Centrioles composed of 9 triplets of microtubules. • Replicate before cell division.

23. Cytoskeleton • Intermediate fibers fall in between microfilaments and microtubules in size. • There are five biochemically distinct types of intermediate fibers.

24. Cilia & Flagella • Cilia & flagella are motile extensions of the cell surface. • In many single celled organisms they are a source of locomotion. • In multicellular animals they usually sweep material past the fixed cell. • Nine pairs of microtubules enclose a central pair. • At the base is a basal body - identical to a centriole.

25. Pseudopodia • Some single-celled organisms, migrating cells in embryos, and white blood cells show ameboidmovement. • Cytoplasmic streaming through the action of actin microfilaments extends a pseudopodium outward. • Some have specialized pseudopodia with microtubules that are assembled & disassembled to allow movement. http://www.youtube.com/watch?v=zTFsn9xt7b0&feature=player_embedded

26. Junctions • Tight junctions form when cell membranes ofadjacentcells fuse. • Function as seals. • Adhesion junctions occur under tight junctions. Transmembrane proteins link across a small space and connect to microfilaments.

27. Junctions • Desmosomes act as spot welds and increase the strength of the tissue. • Hemidesmosomes are found at the base of cells and anchor them to connective tissue. • Gap junctions are canals between cells that provide intercellular communication.

28. Microvilli • Microvilli are small fingerlike projections that have bundles of actin microfilaments. • They serve to increase the surface area of the tissue as in the intestine.

29. Membrane Function • Membranes surround the outside of the cell and the organelles inside it. • The plasma membrane acts as a selective gatekeeper. • A substance may cross the membrane: • By diffusion • By a mediated transport system • By endocytosis

30. Diffusion & Osmosis • Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. This tends to equalize the concentration. • Down the concentration gradient. • Solutes are molecules (e.g. salt) that are found in a solution.

31. Diffusion & Osmosis • Cell membranes are selectively permeable – water can pass through, but not most solutes. • Gases (oxygen & carbon dioxide), urea, lipid soluble solutes can cross the membrane. http://www.youtube.com/watch?v=sdiJtDRJQEc&feature=player_embedded#!

32. Diffusion & Osmosis • Osmosis - if there is a membrane between two solutions with unequal concentration of solutes that can not cross the membrane, water will flow toward the side with less water / more solute until the two sides have equal concentrations.

33. Diffusion & Osmosis

34. Diffusion & Osmosis • Animals utilize osmosis to control internal fluid and solute levels. • The blood of marine fishes has 1/3 the salt content of the water. They are hypoosmotic to seawater. • Freshwater fishes have blood that is saltier than the water. They are hyperosmotic to the water. • If the solute concentrations were the same, the two solutions would be isoosmotic.

35. Diffusion Through Channels • Charged substances, like water and dissolved ions, can’t simply diffuse across the cell membrane. • They pass through channels created by transmembrane proteins. • Some channels always open. • Some are gated channels. http://www.youtube.com/watch?v=PYP4JYL-KQk&list=FL9N_Px072WuVorSwDfqf-9w&index=87&feature=plpp_video

36. Diffusion Through Channels • Gated channels require a signal to open or close them. • Chemically-gated channels open or close when a signaling molecule binds to a binding site on the transmembrane protein. • Voltage-gated channels open or close when the ionic charge across the membrane changes.

37. Carrier Mediated Transport • Sugars & amino acids must be able to enter cells and waste products must be able to leave. • These molecules cross the membrane with the help of transporter proteins. • Transporter proteins are specific. • Facilitated diffusion • Active transport

38. Facilitated Diffusion • In facilitated diffusion, the transporter protein binds to the substrate molecule on one side of the plasma membrane then changes shape to release it on the other side. • Takes place in the direction of the concentration gradient.

39. Active Transport • Active transport requires energy (ATP) to transport molecules in the direction opposite the concentration gradient. http://www.youtube.com/watch?v=STzOiRqzzL4

40. Endocytosis • Endocytosis is the ingestion of material by cells. • Phagocytosis – cell eating – method of feeding by single-celled organisms. • Pinocytosis – small molecules or ions are enclosed in vesicles called caveolae. • Receptor-mediated endocytosis – method of bringing large molecules into a cell with the help of the protein clathrin.

41. Endocytosis and Exocytosis http://www.youtube.com/watch?v=W6rnhiMxtKU&feature=player_embedded#! http://www.youtube.com/watch?v=U9pvm_4-bHg&feature=player_embedded

42. Exocytosis • Exocytosis - membranes of a vesicle inside the cell can fuse with the plasma membrane to discharge the contents of the vesicle outside the cell. • Transcytosis – a substance may be picked up on one side of the cell, transported completely across the cell and discharged on the other side.

43. Mitosis and Cell Division • Mitosis is the process of nuclear cell division in nonreproductive, or somatic, cells. • A fertilized egg, or zygote, divides by mitosis to produce a multicellular organism. • Damaged cells are replaced by mitosis.

44. Chromosomes • In cells that are not dividing, the DNA is loosely organized so that individual chromosomes can’t be distinguished – it is now referred to as chromatin. • Before division, chromatin becomes more compact and chromosomes can be recognized.

45. Chromosomes • All nonreproductive cells in a species have the same number of chromosomes. • 46 in humans • Half of these chromosomes come from each parent. • Result is two sets of chromosomes. • Diploid • Chromosome 1 from Mom and chromosome 1 from Dad are called homologous chromosomes.

46. The Cell Cycle • Cells come from preexisting cells through the process of cell division. • Cell division – mitosis and cytokinesis – occupy a very small portion of the cell cycle.

47. The Cell Cycle • Interphase includes: • G1 – growth phase where RNA and functional proteins are synthesized. • S – DNA replication. • G2 – growth phase where structural proteins are made. • Mitosis • Cytokinesis

48. Chromosome Structure • During S phase, each of the 2 homologues replicates, resulting in identical copies called sister chromatids. • Chromatids remain connected at a linkage site called the centromere.

49. Cell Division • There are two phases of cell division: • Mitosis – nuclear cell division • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis – division of the cytoplasm • Multiple nuclear divisions not accompanied by cytokinesis result in a multinucleate cell.