Plasma Membrane and Transport of molecules

1. Plasma Membrane and Transport of molecules How do things get in and out of the cell?

2. I. The Plasma Membrane A. The fluid mosaic model describes the structure of the plasma membrane. Different kinds of cell membrane models have been proposed, and one of the most useful is the Fluid-mosaic model. B. In this model the membrane is seen as a bilayer of phospholipids.

3. Figure 5.13 -- Phospholipids spontaneously form Lipid Bilayers Micellesare commonlyformed byFatty Acids(soaps) andDetergents; these have a single hydrophobic tail giving them a moreconicalshape Phospholipidshave twohydrophobic tails and a morecylindricalshape thatfits better in abilayer.

4. Figure 8.3 Rotation Unsaturated hydrocarbon tails have kinks that inhibit molecules from packing together enhancing membrane fluidity Cholesterol reduces membrane fluidity at moderate temperatures and inhibits solidification at low temperatures

5. C. Structure of the plasma membrane 1. The plasma membrane is a liquid (fat molecules) bi-layer structure. Each layer is consisted of lipid molecules with protein molecules embedded in the bi-layers.

6. 2. Membrane lipids are phospholipids with polar, water-soluble heads and long, nonpolar, insoluble tails. • Phospholipids have two fatty acids attached to glycerol instead of three. Because of their polarity, these molecules are attached to H2O molecules. • Phospholipids align with the H2O soluble phosphate ends toward the outside of each layer and the nonpolar tails inside the bilayer.

7. Figure 5.12 -- Phospholipids contain 2 fatty acids, glycerol, phosphate, and an alcohol linked by ester bonds

8. II. Cellular Transport A. Diffusion1. Brownian motion is the random motion of molecules. 2. Most materials in and around any cell are in H2O solution. Diffusion is the movement of particles from areas of high concentration to areas of low concentration. It is the result of Brownian motion.

10. 3. Brownian Motion is continuous motion. When materials are evenly distributed in H2O and no further changes in concentration occur, dynamic equilibrium exists. • Dynamic Equilibrium= random movement continues but there is no change in concentration. (dynamic = change; equilibrium = balance) Dynamic equilibrium is a characteristic of homeostasis in the cell.

12. 4. Diffusion depends on concentration gradients. • Concentration Gradients = difference in concentration between two areas (ex: Inside of a cell and outside of a cell). • Ions and molecules automatically diffuse (move through a membrane) from an area of high concentration to an area of low concentration. This means they move with the gradient. • Diffusion across a membrane continues until there is no concentration gradient. Dynamic equilibrium then exists because the concentration is the same on both sides of the membrane.

13. Figure 8.9  The diffusion of solutes across membranes

14. 5. Selectivity of membrane- only H2O, oxygen, nitrogen, carbon dioxide molecules, and a few other non-polar molecules can diffuse directly across the plasma membrane. • Charged ions of polar molecules cannot automatically diffuse across the plasma membrane.

15. B. Osmosis- Diffusion of waterOsmosis=diffusion of water only through a selectively permeable membrane from an area of high concentration to an area of lower concentration. 1. No osmosis in an isotonic solution because the concentration of H2O is the same on either side of the plasma membrane(dynamic equilibrium). However, movement continues (Brownian motion).

16. Figure 8.10  Osmosis

18. Osmosis in a hypotonic solution- particulate concentration is lower outside the cell so H2O concentration is higher outside the cell, therefore, H2O diffuses into the cell. • Turgor Pressure is the internal pressure of a plant cell. It aids in maintaining rigidity (fluid presses against the cell wall). • Plasmolysis is loss of water from plant cells causes plant to wilt b/c of the loss of turgor pressure. • Most animal cells burst if too much H2O enters the cell. Protists have contractile vacuoles to remove excess H2O.

20. Osmosis in a hypertonic solution- particulate concentration is higher outside the cell so H2O concentrations higher inside the cell, therefore, H2O diffuses out of the cell. • This causes animal cells to shrivel up. In plant cells, it results in plasmolysis which is water loss from the central vacuole leading to loss of turgor pressure (wilting).

22. Figure 8.11  The water balance of living cells

23. C. Passive Transport (Diffusion and Osmosis) • Passive transport is the movement of particles across the plasma membrane by diffusion requiring no expenditure of energy by the cell. • Facilitated diffusion- transport proteins embedded in the plasma membrane transport ions and molecules (that can’t get thru the membrane on their own) into and out of the cell as needed.

24. Figure 8.13  One model for facilitated diffusion

25. D. Active Transport Active transport- diffusion goes against the concentration gradient meaning movement from an area of low concentration to an area of high concentration. The cell expends energy doing this. 1. How active transport occurs- the cell uses chemical energy to change the shape of transport proteins so that the particle to be moved is released on the other side of the membrane. The protein’s original shape in then restored.

26. Figure 8.15  Review: passive and active transport compared

27. Figure 8.16  An electrogenic pump

28. Cotransport: Glucose-Na+ Pump • Here is an example of two ways that cells can use ATP and manipulate concentration gradients to transport molecules inside.

29. Cotransport: Lactose-H+ Pump • Here, lactose enters the cell along hydrogen’s concentration gradient. • Hydrogen is then pumped out using ATP, and the gradient is maintained.

30. Figure 8.14  The sodium-potassium pump: a specific case of active transport

31. Figure 8.17  Cotransport

32. E. Transport of large particles Endocytosis- a cell surrounds material and takes it in from its environment by enclosing it in a newly formed vacuole.

33. Exocytosis- vacuole containing what the cell needs to dump, merges with the plasma membrane releasing the material outside the cell.

34. Pinocytosis and Phagocytosis

35. Figure 8.18  The three types of endocytosis in animal cells

36. Figure 8.18a Phagocytosis

37. Figure 8.18c Receptor-mediated endocytosis

38. III. Endomembrane System Q: How do things get around inside of the cell? A: The endomembrane system is a collection of membranous structures involved in transport within the cell.

39. III. Endomembrane System Figure 7.16 The organelles of the Endomembrane System are are connected by transport vesicles.

40. Figure 7.9 The Nucleus contains the cellular DNA and enzymes that copy the DNA. It is surrounded by a double membrane called the Nuclear Envelope, that is continuous with the Endoplasmic Reticulum. Large particles can pass between the cytosol and the nuclear lumen through Nuclear Pore Complexes.

41. Figure 7.11 Endoplasmic Reticulum is the site of synthesis of proteins and other molecules destined for other components of the Endomembrane System and for secretion to the extracellular space. The ER membrane is continuous with the Nuclear Membrane and has two components: (1) Rough ER contains many bound ribosomes that synthesize proteins(2) Smooth ER has no bound ribosomes

42. Figure 7.12 Golgi apparatusreceives proteins, lipids, other molecules from the ER via transport vesicles and modifies these molecules before passing them onto other organelles or the plasma membrane via transport vesicles

43. Figure 7.14: Formation and Function of Lysosomes

44. ***Non-Endomembrane Organelles:The mitochondria and chloroplasts are surrounded by double membranes; The intermembrane space and the matrix have different environments due to the two membranes, and we will later see how this allows certain processes to occur in these organelles

45. IV. Cell Connections and Communication • Desmosomes attach cells together. • Tight junctions leakproof the cell. • Gap junctions and plasmodesmata allow communications between cells.

46. Figure 7.30 Intercellular Junctions are protein assemblies that connect neighboring cells together. Gap Junctions allow neighboring cells to communicate by diffusion of small molecules from one cell cytosol to the other.

47. V. Diseases Associated with Difficulties in Transport across membranes. Diseases resulting from lack of functional channels/pumps • Motor neuron problems -Na+ channel • Cystic fibrosis - Cl- channel • Bipolar disorder -Na+, K+, ATPase • Heart problems -Na+, K+, ATPase, Na+ channels • Resistance to chemotherapy - peptide transporter, p-Glycoprotein, (Multi-Drug Resistance) • Color Blindness, H+ gradient as pump (rhodopsin) • Some Food Poisoning - Ca+ channel