Cells: The Living Unit

1. Cells: The Living Unit

2. Overview of the Cellular Basis of life – • Cells are the structural unit of all living things • 50 – 60 trillion cells in an adult human body • Approximately 200 different types (with different sizes, shapes, and functions) • Cells range from 2µm (1/12,000 inch) to over 3 feet • Cells have the same basic parts (generalized/composite cell) • Nucleus—controls cell activity, centrally located • Cytoplasm—area containing necessary organelles • Plasma membrane—outer cell boundary

3. The Plasma Membrane: Structure – • The Fluid Mosaic Model: • Double-layer (bilayer) of phospholipids with interspersed proteins that are constantly changing • polar, phosphate-containing head contacts HOH • non-polar, fatty acid tail avoids HOH • bilayer has tail in contact with heads outward • allows membrane to self-assemble into a closed structure when forming/repairing • 10% of externally-facing phospholipids have sugar groups attached (glycolipids)

4. Fluid Mosaic Model Continued • Branching sugar groups are on some external proteins that stick far out in the extracellular space (glycocalyx) and function as highly specific biological markers • cholesterol is present for stability • integral (transmembrane) proteins span the entire width of the membrane and help in transporting materials thru the membrane (either as channels or carriers) • peripheral proteins are not embedded in the membrane but attached to the ends of the integral proteins (usually enzymes or perform other mechanical functions)

5. Specialization of the Plasma Membrane: • Microvilli– • Minute, finger-like extensions of the plasma membrane extending from an exposed cell surface • increase the surface area of the membrane • often found on absorption cells (intestinal cells)

7. Specialization of the Plasma Membrane: • Membrane junctions – • With the exception of blood cells (red and white) and reproductive cells (sperm and egg) most other cells are bound closely together • 2factors help in binding: • adhesive glycoprotein in the glycocalyx • plasma membranes of adjacent cells fit together in tongue-and-groove fashion

8. The Plasma Membrane: Functions • Membrane Transport: • Extracellular (interstitial) fluid is composed of HOH, aa, sugars, fatty acids, vitamins, hormones, neurotransmitters, salts, and waste. • Membrane is selectively (differentially) permeable (allows only some things to pass thru-keeps beneficial things inside & harmful things out)

9. Plasma Membrane: Functions • Passive processes • Does not require cellular energy • Move from [high] to [low] (along/down their [gradient]) • Smaller molecules move faster • Warmer temperatures make faster reactions • Eventually the molecules will be evenly distributed thru the whole environment and diffusion stops (equilibrium)

10. Plasma Membrane: Functions • Diffusion: • Diffusion thru a plasma membrane will only occur if the molecule is lipid • soluble and small enough to go thru the membrane pores by itself (simple diffusion) • nonpolar, lipid soluble molecules only • O2, CO2, fats and alcohol • small polar and charged particles can pass through HOH-filled channels made by channel proteins

11. Transport • Diffusion continued: • Molecules too big to go through on their own can get help from carrier molecules (facilitated diffusion) • not exactly sure how this happens • very selective (a carrier will only bind with a specific substrate) • limited by the number of carriers present in the membrane

12. Transport • Osmosis • The movement of HOH is osmosis • cannot move thru lipids bilayer but they can move thru pores • if the [solute] is different on both sides, the [HOH] will be different as well • (more [solute] – less [HOH]) • (less [solute] – more [HOH]) • (both move toward equilibrium)

13. Osmosis Continued: • The [total] of all solutes in a solution is the solution’s osmolarity • osmotic imbalances cause cells to shrink (loss of HOH) or swell (gain of HOH) until [solute] reaches equilibrium or the cells breaks • ability of a cell to alter internal HOH volume is tonicity

14. Tonicity • isotonic solutions have equal [solute] as cells – no net movement of HOH • hypertonic solutions have higher [solute] than cells – HOH moves out of the cell causing shrinkage (crenation) • hypertonic solutions have lower [solute] than cells – HOH moves into the cell causing swelling • distilled HOH is the ultimate hypotonic solution

15. Filtration • HOH and solutes are forced through a membrane or capillary wall by fluid (hydrostatic) pressure • Depends on a pressure gradient that pushes solute-containing fluid (filtrate) from high pressure to low pressure • Not selective – only molecules too large to go thru are held back • Occurs in kidney – first step of urine formation

16. Activity • Create a comic strip for the steps of kidney filtration. Put the steps in your words using vocabulary terms.

17. Active Processes • Requires ATP to transport substances • Molecules may be too large, unable to dissolve in bilayer, or moving against a [gradient]

18. Active Processes • solute pumping: • moves aa and ions (Na+, K+, Ca++) against a [gradient] • carrier protein uses ATP to change shape to carry solutes • Vesicular (bulk) transport: • moving large particles/macromolecules thru plasma membranes

19. Exocytosis • moving substances out of the cell • hormone secretion, mucus secretion, neurotransmitter release, waste ejection • cell product is enclosed in a membranous sac (vesicle) that migrates to the cell membrane, fuses with it, and ruptures into the interstitial fluid

20. Endocytosis • moving substances into a cell • plasma membranes infolds, a vesicle forms, pinches off the cell membrane, moves into the cytoplasm, and contents are used by the cell • 3 types • Phagocytosis (“eating cell”) – intake of solid material that fuses with a lysosome for digestion • Pinocytosis (“drinking cell”) – intake of liquid material (important in nutrient absorption) • Receptor – mediated endocytosis is very selective – plasma membrane proteins (receptors) bind only with certain substances

21. Cytoplasm • Cellular material between plasma membrane and nucleus major functional area • Most cellular activities take place here • 3 parts: • Cytosol is the viscous, semitransparent fluid where the other elements are found including a variety of solutes (proteins, salts, sugars, etc.) • Organelles are the metabolic machinery that have specific functions • Inclusions are chemical substances present in some cells (stored nutrients, glycogen granules, lipid droplets, melanin, etc.)

22. Cytoplasm Organelles • Specialized cellular compartments with specific functions • Each organelle is surrounded by its own plasma membrane

23. Mitochondria • Sausage-shaped • Movable and change shape continuously • Provide ATP for the cell • Abundant in liver and muscle cells • Contains DNA and RNA for self-replication by fission (very similar to bacteria)

24. Peroxisomes • Contain oxidase that uses O2 to detoxify toxic substances (alcohol and formaldehyde) • Destroy free radicals (highly reactive chemicals with unpaired electrons that can destroy • protein, lipid, and nucleic acid structures) • self-replicating by fission

25. Ribosomes • Small • Made of proteins and ribosomal RNA • 2 globular subunits that fit together • Site of protein synthesis • Some float in the cytoplasm • free ribosomes • make soluble proteins • Some are attached to the rough endoplasmic reticulum • membrane-bound ribosomes • makes proteins for export or use in plasma membrane • Can switch between synthesis locations

26. Endoplasmic Reticulum • Extensive system of interconnected tubes and parallel membranes that make fluid-filled cavities (cisternae) • ER membranes are continuous with the nuclear membrane • 2 types: Rough and smooth

27. Rough ER • Surface is studded with ribosomes • Proteins made are transported thru the cisternae to be secreted • Abundant in the following cells: • secretorycells • antibody-producing cells • liver cells • Considered to be the cell’s “membrane factory” • makes integral proteins • phospholipids • cholesterol

28. Smooth ER • Continuous of the rough ER • Tubules in a branching network • No role in protein synthesis • Responsible for reactions involving lipid metabolism and synthesis • cholesterol • lipoproteins • steroid-based sex hormones • fat absorption/transport • Helps in the detoxification of drugs • Important in storing/releasing Ca++ during muscle contractions • (called the sarcoplasmic reticulum)

29. Golgi Apparatus/body • Flattened, stacked, membranous sacs • Associated with numerous vesicles • Transport vesicles budded off from the rough ER fuse with the Golgi • Modifies, concentrates, and packages the proteins and membranes made in the rough ER • Some proteins are “tagged” for specific delivery, sorted, packaged, and shipped

30. Lysosomes • Lysosomes – • Spherical membranous vesicles • Contains digestive enzymes • Abundant in phagocytes • Digest a wide variety of biological molecules • Works best in acidic conditions (pH 5)

31. Lysosomes • Digesting particles ingested by endocytosis (especially bacteria, viruses, toxins, etc.) • Degrading worn-out or nonfunctional organelles • Breakdown of stored glycogen • Breakdown of non-useful tissue • embryonic digit web • uterine lining during menstruation • bone tissue to release Ca++ • Autolysis happens when the lysosomal membrane breaks down and digests the cell (low O2, injury, high vitamin A)

32. Cytoskeleton • Series of rods going thru the cytoplasm • Supports cellular structures • Helps generate cell movement • Not covered by a membrane • 3 types:

33. Cytoskeleton • Microtubules • hollow tubes • all originate near the nucleus in an area called the centrosome • determines the overall shape of the cell and distribution of organelles • (some organelles are attached to microtubules and are pulled thru the cytoplasm) • constantly grows out from the cell center, breaking up, & reassembling

34. Cytoskeleton • Microfilaments • thin strands of contractile proteins • dense cross-linked network in a unique arrangement for each cell • responsible for cell motility or changes in cell shape

35. Cytoskeleton • Intermediate filaments • tough, insoluble protein fibers that vary with cell type • diameter intermediate between microtubules and microfilament • most stable and permanent of cytoskeletal elements • high tensile strength to resist pulling forces on the cell • helps form chromosomes

36. Cilia and Flagella • Whiplike extensions from the exposed surface of certain cells • Cilia moves substances in one direction across cells • Flagella are usually singular and used for movement

37. Nucleus • Control center of the cell • Most cells have only one(except skeletal muscle cells, some liver cells, and bone destruction cells which are multinucleate)(and red blood cells that are anucleate) • cannot reproduce and only live 3-4 months • Largest organelle in the cell • 3 distinct regions

38. Nuclear Envelope • Double layer membrane surrounding nucleus • Outer membrane is continuous with rough ER • At various points, the two membrane layers fuse and forms a nuclear pore • to allow for the import of proteins and export of messenger and ribosomal RNA

39. Nucleoli • Spherical bodies within the nucleus • Not membrane-bound • Assembly site for ribosomal subunits (very large and numerous in cells making tissue proteins)

40. Chromatin • System of bumpy threads in the nucleoplasm • Made of DNA and histone (a globular protein) • During non-dividing times, the chromatin is extended and not easily seen • During cell division, the chromatin condenses to form visible Chromosomes

41. Cell Cycle • Cell Growth and Reproduction – The Cell Life Cycle: • Series of changes a cell goes thru from the time it is formed until it reproduces • 2 major periods – interphase and mitotic phase

42. Interphase • Period from cell formation to cell division • Cell is carrying out all its routine activities • Also prepares for cell division • Divided into 3 subphases: G1, S, G2

43. G1 • G1 (growth 1) • Cells are metabolically active • Time length is variable • Cells that will not divide are in the G0 phase • Centriolesbegin replication

44. S • S (synthetic/synthesis) • DNA replicates itself • Must occur before cell can divide • Replication must be exact to avoid mutations • Trigger is unknown • Once replication starts it continues until it’s finished

45. G2 • G2 (growth 2) • Cell makes more organelles and membrane to allow for division

46. Mitotic Phase • Some cells divide almost continuously (skin and intestinal lining) • Some cells divide more slowly (liver cells) • Some cells do not divide at all (nervous, skeletal, & cardiac muscle) • The amount of nutrients a cell requires is directly related to its volume • Surface area does not increase proportionally with volume and a growing cell will eventually “outgrow” its surface area

47. Mitotic Phase • Cell division creates daughter cells that have a favorable surface area: volume ration • Two events: • Mitosis (nuclear division) • Cytokinesis (cytoplasmic division)

48. Mitosis • 4 phases • P • M • A • T

49. Phrophase • Prophase – first and longest phase of mitosis • chromatin coils and condenses to form chromosomes (each chromosome is two identical chromatin threads held together by a centromere) • microtubules from the centriole lengthen and push the 2 centrioles apart (toward the poles) • nuclear membrane disappears when centrioles are at the poles, some microtubules anchor the centriolesto the plasma membrane and other microtubules attach to the centromere of each chromosome

50. Metaphase • Metaphase – chromosomes cluster at the equator • Chromosomes align along the equator or center of the cell. • Spindle Fibers are attached to centromeres for each pair of sister chromatids.