A Tour of the Cell

1. A Tour of the Cell Chapter 7

2. How Do We Study Cells?

3. Light Microscopes

4. Electron Microscopes

5. Cell Fractionation

6. All cells have:

7. Prokaryotic Cells

8. Eukaryotic Cells

9. Limits on Cell Sizes

10. Compartmentalization • Internal membranes allow eukaryotic cells to battle the surface area-to-volume ratio problem that limits cell size. • Eukaryotic cells can form “compartments” which segregate certain reactions that interfere with each other (local environments) • These internal membranes participate directly in the cells metabolism (enzymes) • Each membrane has a unique composistion enzymes for cellular respiration - mitochondria

11. Nucleus: Cell’s Genetic Library • contains most of the genes in a eukaryotic cell (some genes are found in mitochondria and chloroplast) • nucleus is enclosed by the nuclear envelope • separates nuclear content from the cytoplasm • envelope is a double membrane, each is a lipid bilayer with associated proteins • envelope is perforated by nuclear pores, lined by a pore complex which regulates transport • nuclear lamina - maintains nuclear shape

12. Nuclear Organization • DNA • Histones • Chromatin • Chromosomes • Homologous Chromosomes • Genome • Nucleolus

13. Ribosomes build a cell’s proteins • sites of protein synthesis = translation • cells with high rates of protein synthesis have …large numbers of ribosomes • free ribosomes - suspended in the cytosol; make proteins that work in the cytosol • bound ribosomes - attached to the outside of the membranes of the nuclear envelope and ER; make proteins that are (1) included in the membrane; (2) packaging within an organelle; (3) exported from the cell

14. Endomembrane System • The membranes of the cell that are either directly related through physical contact or by the transfer of membrane segments by tiny vesicles • the various membranes are not identical in S+F • thickness, molecular composition, and metabolic behavior can change • includes: nuclear envelope, ER, Golgi, lysosomes, various vacuoles, and the plasma membrane

15. Endoplasmic Reticulum • extensive membranes which may account for over half of the cell’s total membrane • network of sacs and tubules called cisternae • cisternal space is continuous with the space between the two membranes of the nuclear envelope • ER membrane separates these spaces from the cytosol

16. Rough ER - ribosomes on surface • bound ribosomes on the rough ER make secretory proteins • cells that are specialized for secretion often have tremendous amounts of rough ER • as ribosome makes protein it is threaded into the cisternal space where it assumes its native conformation • most are glycoproteins; have oligo-saccharides covalently bonded to the protein • rough ER - also the membrane factory of the cell

17. Smooth ER - lack ribosomes • different functions in different cells • phospholipid, steroid, and sex hormone synthesis • metabolism of carbohydrates • detoxification of drugs and poisons • (increased smooth ER production in liver cells is the basis for drug tolerances) • storage and release of calcium ions during muscle contraction (sarcoplasmic reticulum)

18. Golgi Apparatus • vesicles from the ER join at the cis face • individual stack of membrane = cisterna • products are modified as they move from one cisterna to the next • products are sorted into vesicles, pinch off from the trans face • surface molecules on the vesicle direct its movement through the cell

19. Lysosomes • contains hydrolytic enzymes for digestion of macromolecules • lysosomes fuse with food vacuoles that are formed during phagocytosis • some cells recycle their own cell parts • apoptosis - programmed cell death - sometimes uses lysosomes for destruction of the cells • storage diseases

20. Vacuoles • food vacuoles • contractile vacuoles • central vacuole

21. Mitochondria • site of cell respiration, catabolic processing of fuels to produce ATP • two membranes with a narrow intermembrane space between them • inner membrane - highly folded - cristae • encloses the mitochondrial matrix • respiratory enzymes are found throughout the mitochondria

22. Chloroplasts • one of a group of organelles called plastids • amyloplasts • chromoplasts • chloroplasts • two membranes - inner membranes make up the thylakoids • space between outer membrane and thylakoids is the stroma

23. Peroxisomes

24. Cytoskeleton • mechanical support • maintain or change cell shape • anchor and direct organelle movement • control movement of cellular appendages • muscle contraction

25. Microtubules • hollow rods constructed of globular proteins called tubulin • major supporting framework of the cell • serve as tracks along which organelles move with the aid of motor molecules • radiate out from the centrosome • animal cells have a pair of centrioles • cilia, flagella, basal body, dynein

26. Intermediate Filaments • Filaments that are intermediate in diameter between microtubules and microfilaments • made from keratin subunits • more permanent than other filaments • Function: • bear tension • reinforce cell shape • compose the nuclear lamina

27. Microfilaments • composed of two actin chains wound into a helix; built from G-actin subunits • Function: • provide cellular support • participate in muscle contraction • cause localized contraction of cells • cytoplasmic streaming

28. Plant cell walls • composed of cellulose • primary cell wall - thin, flexible walls that are joined to adjacent cells by the middle lamella made of pectins • secondary cell wall may be added after the cell stops growing; added between the primary wall and the plasma membrane

29. Animal cells have an extracellular matrix (ECM) • meshwork of macromolecules outside the membrane of animal cells • the ECM is: • secreted locally by cells • composed mostly of glycoproteins like collagen • attached directly to cells or by another group of glycoproteins-fibronectins • has fibronectins that attach to transmembrane binding sites called integrins

30. Intercellular Junctions • Allow neighboring cells to adhere and interact through direct physical contact • Plants have: • plasmodesma/plasmodesmata = channels that perforate the cell walls • Animals have: • tight junctions: prevent transport through the intercellular space; desmosomes: allow transport; gap junctions: cytoplasmic exchange

31. Chapter 8 Review • Test - 9/17/03

32. Plasma Membrane • Fluid Mosaic Model • Phospholipids • Bilayers • Membrane Proteins • Cholesterol

33. Phospholipids: • are amphipathic • contain a polar and nonpolar region • head • phosphate • polar, hydrophilic, interacts with water • tails (2) • fatty acids (saturated or unsaturated) • nonpolar, hydrophobic, repelled by water

34. Bilayers: • form when phospholipids are put in water • are arranged from two layers of phospholipids; arranged tail-to-tail • are the basic structure for all membranes • have a hydrophobic region (lipid tails) that acts as a selectively permeable barrier • give membranes their semi-permeability

35. Membrane Proteins: • are either inserted through the membrane = integral proteins • transport proteins - channels and carriers • enzymes • receptor proteins • or are associated w/ the membrane surface = peripheral proteins • act as sites for attachment to the cytoskeleton or extracellular matrix

36. Cholesterol: • is the major derivative of steroid molecules • acts to stabilize the structure of membranes • provides stability by increasing the hydrophobic interactions of the membrane • affects the “fluidity” of the membrane

37. Membrane Fluidity • is affected by membrane composition • ratio of saturated to unsaturated fatty acid tails • amount of cholesterol (animal cells) • unsaturated fatty acids are more oily than saturated ones = increase mem fluidity • cholesterol stabilizes membranes at extreme temperatures • keeps membrane together at very high temps • keeps membranes fluid at low temps

38. Selective Permeability • many molecules are present in the cell’s internal and external environment, the cell must have some way to sort out what can enter and leave the cell = membranes must be selectively permeable • what crosses easily vs. what doesn’t

39. Two ways substances move: • Bulk Flow • Constant, Random Motion (Diffusion)

40. Passive Transport • diffusion • osmosis • hypertonic • hypotonic • isotonic • osmotic pressure (see lab and discussion of water potential) • facilitated diffusion

41. Water Balance • w/out walls • hypertonic-lose water and shrivel (crenation) • hypotonic-gain water, swell, and possibly burst (lyse) • must live in isotonic environments or have adaptations for osmoregulation

42. Water Balance - cont’d. • with walls • hypotonic-water moves into the cell and presses against the wall, causing a turgid cell • isotonic solutions-flaccid cells • hypertonic-plasmolysis - membrane pulls away from the cell wall as it shrivels

43. Examples of Transport • Facilitated diffusion: ion channels • voltage-gated ion channels are important in excitable cells (muscles and neurons) • Active transport: pumps • Na+-K+ pump: important in establishing an electrochemical gradient • proton pumps: • Cotransport: using an ATP driven pump to establish an ion gradient for symport.

44. Membrane Potential • voltage can be created across a membrane using ion pumps • 2 forces involved in the passive transport of ions: • concentration gradient • effect of charge on the ion • Electrochemical gradients: diffusion gradient resulting from combined forces • electrogenic pumps: any transport pump that generates voltage across a membrane

45. Descriptive Terms (these are often clue words on exams!) • Is the substance moving across a selectively permeable membrane? • Only lets certain molecules pass…like the plasma membrane • Is the substance of interest water (solvent), or is it a substance that is dissolved in water (solute)? • In which direction is the substance moving through the gradient? • From higher to lower concentrations (down the gradient) or reverse (up/against the gradient)

46. Cont’d. • How does the concentration of solutes vary from one region to another? • Is the solute hyper-, hypo- or iso- • Is energy required to move the substance? • Energy is used to move against a gradient

47. Transport Terms to Know • Diffusion, Osmosis, Dialysis • Plasmolysis, turgid, flaccid, crenation, lysis • Facilitated Diffusion • Active transport • Exocytosis • Endocytosis, Phagocytosis, Pinocytosis, Receptor-mediated • Gradient