The Cell Membrane & Movement of Materials In & Out of Cells PACKET #11

Transcription

1 1 February 26, The Cell Membrane & Movement of Materials In & Out of Cells PACKET #11

2 Introduction I 2 Biological membranes are phospholipid bilayers with associated proteins. Current data support a fluid mosaic model of the cell membrane. In 1935, Davson and Daniella stated that phospholipids form a membrane two molecules thick. Singer and Nicholson developed the fluid mosaic model in Furthermore, the membrane is only 10nm thick.

3 Introduction II 3 Biological membranes fuse and form closed vesicles. Endocytosis and exocytosis are products of membrane fusion. More later.

4 Cell Membrane 4

5 Properties of Phospholipids 5

6 Phospholipids are Amphipathic 6 Molecules with both hydrophilic and hydrophobic properties are termed amphipathic Other examples Sterols Cholesterol Glycolipids Hydrophilic (sugar) head The aqueous environment inside and outside the cell prevent membrane lipids from escaping the bilayer

7 Fluidity of the Membrane 7 Depends on Two Main Features Saturated vs. Unsaturated Fatty Acid tails (phopsholipids) Unsaturated more fluid Cholesterol Kinks prevent molecules from packing together Absent in plants, yeast and bacteria Fill the holes produced by kinks Stiffens bilayer and makes it less fluid and permeable.

8 Fluidity of the Cell Membrane II 8

9 Proteins of the Cell Membrane 9

10 Functions of Membrane Proteins 10 Cell Membrane Proteins Six different functions Functions of Proteins Transport Enzyme Activity Signal Transduction Cell to cell Recognition Intercellular Joining Attachment to Cytoskeleton

11 Integral vs. Peripheral Proteins 11 Integral Proteins A protein that is firmly anchored in the plasma membrane via interactions between its hydrophobic domains and the membrane phospholipids Directly attached to the membrane

12 Integral vs. Peripheral Proteins 12 Peripheral Proteins Not embedded in the lipid bilayer Can be released from the membrane by relatively gentle extraction procedures Possible key player in cell communication.

13 Transmembrane Protein 13 Protein that spans the entire membrane Have both hydrophobic and hydrophilic regions Alpha helical secondary structure is normally the hydrophobic regions of the protein

14 14 Diffusion

15 Introduction 15 Atoms and molecules, above absolute zero, exhibit motion. This random motion allows particles to move from an area of higher concentration to an area of lower concentration in an attempt to reach equilibrium.

16 Introduction 16 There are 5 ways of transporting materials across the cell membrane Diffusion Regular & Facilitated Passive Transport Active transport Osmosis Phagocytosis Pinocytosis

17 Categories of Diffusion I 17 Regular Diffusion Movement of molecules down the concentration gradient High to low Facilitated Diffusion Movement of molecules down the concentration gradient via a channel Passive Transport Active Transport Regular Diffusion Facilitated Diffusion In cells, these channels are found in proteins Categories of Diffusion Osmosis Special type of diffusion More to come later Phagocytosis Active transport Movement of molecules against the concentration gradient via channels and with the use of energy. Pinocytosis Low to high

18 Diffusion 18

19 Diffusion 19 The movement of a substance from an area of high concentration to an area of low concentration The difference in concentration between the two regions is known as the concentration gradient

20 Rate of Diffusion 20 The rate of diffusion depends on The difference in concentration The greater the concentration gradient, the faster the process The distance between the two regions Smaller distance means faster process The area If the total area is increased, the faster the process The size of the molecules Small and fat-soluble molecules will diffuse faster

21 Passive Transport Regular vs. Facilitated Diffusion 21 Regular Diffusion Movement of molecules is from high concentration to low concentration No proteins are used No energy (ATP) is required Facilitated Diffusion Movement of molecules is from high concentration to low concentration Proteins are used No energy (ATP) is required

22 Active Transport 22 Materials are moved against the concentration gradient Molecules move from an area of low concentration to an area of high concentration Proteins are used to move materials across the membrane Energy is also used.

23 Active Transport II 23 Cells carry our active transport in three ways ATP driven pumps Active Transport Couple uphill transport with hydrolysis of ATP Coupled transport (co-transport)* ATP Pumps Co- Transport Light Driven Pumps Light driven pumps Found mainly in bacterial cells Bacteria Cells Input of energy from light Bacteriohodopsin

24 Active Transport ATP DRIVEN PUMPS 24

25 Active Transport ATP Driven Pumps 25 Energy is used Because energy is used, cells carrying out active transport have A high respiratory rate Many mitochondria A high concentration/reserve of ATP Any factor which reduces or stops cell respiration will stop active transport Cyanide

26 Co-Transport INVOLVES ACTIVE TRANSPORT 26

27 Co-Transport I The Active Transport of H + ions 27 Hydrogen gradients are used to drive membrane transport in plants, fungi and bacteria These are not sodium-potassium pumps

28 Co-Transport II The Active Transport of H + ions II 28 Hydrogen pumps, found in the plasma membrane, pump H + out of the cell This can also be described as primary active transport Setting up an electrochemical gradient

29 Co-Transport III 29 Pump creates an acidic ph in the medium surrounding the cell H + re-enters the cell via a cotransporter Usually transports a substance in addition to the H + The uptake of sugars and amino acids, into bacterial cells for example, are driven by the presence H + pumps

30 Active Transport Light Driven Pumps H + Pumps in Bacteria 30

31 Active Transport Light Driven Pumps H + Pumps in Bacteria 31 In some photosynthetic bacteria, the H + gradient is created by the activity of light driven H + pumps such as bacteriorhodopsin. In plants and fungi and many other bacteria, the gradient is set up by ATPases in their plasma membrane

32 32 Types of Ports

33 Types of Ports 33

34 Review so far 34 Movement of Materials in/out of Cells Passive Transport Active Transport Regular Diffusion Facilitated Diffusion

35 Sodium Potassium Pump 35

36 Electrogenic Pump 36 These pumps are used to move electrically charged molecules Small organic or inorganic ions MOST cell membranes have a voltage across them and results in a difference in electric potential on each side i.e. the membrane potential.

37 Electrogenic Pump 37 The membrane potential exerts a force on any molecule that carries an electrical charge Cytoplasmic side is USUALLY at a negative potential relative to the outside This tends to pull positively charged solutes into the cell and drive negative charged ones outside the cell Net driving force = electrochemical gradient

38 The Sodium Potassium Pump 38 For some, ions, voltage and concentration gradients work in the same direction Sodium Potassium Pump

39 Movement of Glucose PUTTING IT ALL TOGETHER 39

40 The Movement of Glucose 40 The Na + gradient generated by the sodium-potassium pump can be used to drive active transport of a 2 nd molecule. The downhill movement of the first solute down provides the energy to drive the uphill transport of the second.

41 41 Osmosis SPECIAL CASE OF DIFFUSION

42 Osmosis 42 Concise Definition The diffusion of water (liquid solvent) across a selectively permeable membrane Detailed Definition Transfer of a liquid solvent through a semi permeable membrane, that does not allow dissolved solids (solutes) to pass from an area of high concentration to an area of low concentration

43 43 Osmotic Pressure, Osmotic Potential & Solute Potential

44 Osmotic Pressure 44 Osmotic Pressure Is a measure of the tendency of water to move INTO a solution. The driving force for the water and is the difference in water pressure on both sides of the membrane. The differences in pressure provides a net pressure that is exerted by the flow of water as it moves through the semi-permeable membrane. Class Illustration

45 Osmotic Potential = Osmotic Pressure 45 Osmotic Potential Difference in osmotic pressure that draws water from an area of less osmotic pressure to an area of greater osmotic pressure. The potential of a solution to pull in water Value is always negative The more concentrated the solution, the more negative its osmotic potential

46 Osmotic Potential = Osmotic Pressure = Solute Potential 46 The presence of solutes, in the solutions, impact the direction of the movement of water. The ability of a solution to pull in water depends on the number of solute particles present. The higher the amount of solutes in the solution, the lower the solute potential. The solution is more concentrated. Remember, from previous slide, the value is always suppose to be negative.

47 Osmotic Potential = Osmotic Pressure = Solute Potential 47 All three terms represent a measure of the ability of a solution to pull in water. The value is always negative. The more solutes present, the more negative the value. Represented by s

48 Osmotic Potential = Osmotic Pressure = Solute Potential 48 When two solutions have the same osmotic potential, they are said to be isotonic. Where one solution has a greater osmotic potential compared to the other, it is described as being hypertonic. i.e. It is more concentrated. The solution with the lower osmotic potential is described as being hypotonic. Less concentrated.

49 Pressure Potential 49 Solutions/Water are also under the influence of external pressures. These external pressures are measure as pressure potential. This force (pressure) is not the same as the one caused by the movement of the liquid solvent (water). Represented by p Negative or positive depending on conditions.

50 Water Potential 50 Measure of the tendency of water to leave a solution. Combination of the sum of osmotic potential/solute potential and pressure potential. = s + p

51 Water Potential II 51 When measuring the water potential of two solutions, the solution with the lower water potential receives water from the solution with higher water potential Osmosis!

52 52 Cells and Osmosis

53 Pressure Potential in Plant Cells 53 In plant cells, the cell contents press the plasma membrane against the cell wall producing an external force called turgor pressure. Results in a turgid plant cell Pressure potential is positive

54 Pressure Potential in Plant Cells II 54 Special plant cells that make up xylem, tissue that conducts water in plants, undergoes transpiration. This transpiration results in a negative pressure potential.

55 Cells & Osmosis 55

56 Cells & Osmosis 56

57 Phagocytosis, Pinocytosis, Endocytosis and Exocytosis 57

58 Phagocytosis 58 The take up of large particles by cells via vesicles formed in the plasma membrane The cell invaginates to form a depression in which particles are contained This then pinches off to form a vacuole White blood cells Neutrophils Monocytes

59 Pinocytosis 59 The take up of liquids rather than solids Vacuoles are smaller than those used during phagocytosis

60 Endocytosis vs. Exocytosis 60 Both phagocytosis and pinocytosis involve the taking of materials into the cell in bulk. These are examples of endocytosis The removal of materials from the cell in bulk is called exocytosis.

61 61 Review