Cell Biology. The Plasma Membrane

Transcription

1 Cell Biology The Plasma Membrane recall Fluid Mosiac Model S.J. Singer Semipermeable membrane fluid portion is double layer of phospholipids (=phospholipid bilayer) mosaic portion is the proteins and carbohydrates embedded in the membrane 1

2 recall Role of the Cell Membrane Isolate the cytoplasm and organelles from external environment Regulate the exchange of substances Cell communication Cell identification Membrane Composition: 1. Phospholipids Primary structural building block of the plasma membrane 2

3 Phospholipids contain a hydrophyllic head and non-polar hydrophobic tail Hydrogen bonds form between the heads and the watery environment inside and outside the cell Hydrophobic interactions force tails to face inward Phospholipids are not bonded together which make the double layer fluid 2. Cholesterol (eg. Lipoprotein) An important structural element Provides strength and stability to the membrane Makes membrane less fluid 3. Glycolipid Image: Cholesterol within the animal cell membrane Phospholipid with a CHO attached Provide energy Cellular recognition markers for specific elements 3

4 4. Glycoprotein Protein with a CHO attached Important for cell-cell interactions Important for white blood cell recognition Mucin (secreted in respiratory and digestive tracts) protective coating, water retention capacity 5. Proteins Serve many functions depending on structure and location May be integral (embedded) or peripheral (on the inside surface of membrane) Extracellular Environment Cell Cytoplasm 4

5 Embedded Proteins Channel Proteins Form small openings for molecules to diffuse through Carrier Proteins Binding site on protein surface selects certain molecules and allows them to pass through Receptor Proteins Molecular triggers that set off cell responses (release of hormones) Cell Recognition Proteins ID tags to identify certain cells to the body s immune system Enzymatic Proteins Carry out metabolic reactions 5

6 Transport Across a Cell Membrane What controls how substances Move thru the membrane? Size of the molecule (selective permeability) Charge of the molecule What moves easily thru the membrane? Water Carbon dioxide Oxygen What does NOT move easily thru the membrane? Large molecules such as glucose and proteins molecules or atoms with charges, such as ions (sodium, potassium, calcium) PASSIVE TRANSPORT Does NOT require chemical energy from the cell Depends on permeability of the cell Consider characteristics of the membrane (lipids, proteins, etc) Three main types: 1. Diffusion 2. Facilitated transport 3. Osmosis 6

7 1. Simple Diffusion Movement of molecules from an area of HIGH concentration (more solute) to LOW concentration (less solute) These molecules follow their concentration gradient Cells are able to dispose of O 2, CO 2, and H 2 O without energy n student_view0/chapter2/ animation how_diffusion_works.html 7

8 2. Facilitated Diffusion/Transport Movement of a molecule from an area of [HIGH] to [lower] with with aid of proteins (carrier or channel proteins) works as a facilitating pathway for larger molecules or charged ions (amino acids, glucose, Na + and K+) Cell membranes have specific proteins for each molecule Several factors determine rate of diffusion: Number of molecules in cell (mol) Permeability of a particular molecule Width of cell membrane Concentration of diffusing molecule (mole/cm 3 ) Temperature (rate s as temp s) n student_view0/chapter2/ animation how_facilitated_diffusion_works.html 8

9 3. Osmosis Diffusion of WATER through a semi-permeable membrane Water moves from high H 2 O potential and low [solute] to low H 2 O potential and hi [solute] Given: A - 100% water B - 90% water Question: What direction does the water flow? 9

10 Given: A - 100% water B - 90% water Question: What direction does the water flow? Water flows from A --> B Given: A - 1% glucose B - 20% glucose Question: What direction does the water flow? What direction does the glucose flow? 10

11 Given: A - 1% glucose B - 20% glucose Question: What direction does the water flow? Water flows from A --> B What direction does the glucose flow? Glucose flows from B --> A 11

12 Types of Solutions: 1. Hypertonic Hyper = more ; tonic = solute Thus, a cell in a solute rich environment Water flows outward from region of high [water] to low [water] (or low to high [solute]) Cell undergoes plasmolysis (fluid loss) and crenate (shrivel) 2. Hypotonic Hypo = less ; tonic = solute Thus, a cell in a solute poor environment Water flows inward from region of high [water] to low [water] Cell undergoes cytolysis (cell breaks) and bursts 12

13 3. Isotonic Iso = equal ; tonic = solute Thus, a [solute] both equal within and outside the cell Water flows in and out of cell with net zero movement Equilibrium n student_view0/chapter2/ animation how_osmosis_works.html 13

14 ACTIVE TRANSPORT Requires chemical energy (ATP) Involves the movement of molecules uphill against a concentration gradient Proteins involved are called pumps because they are used to pump substances across the concentration gradients Eg. Sodium (Na) Pump Types: 1. Endocytosis (entering the cell) Phagocytosis Pinocytosis 2. Exocytosis (exiting the cell) BULK TRANSPORT: Endocytosis and Exocytosis A form of active transport (uses ATP) When particles are too big or you are moving in bulk ENDOCYTOSIS (endo=inside; cyte=cell) 14

15 Endocytosis: Phagocytosis Engulf large particles Endocytosis: Pinocytosis engulf very small particles or liquids sipping 15

16 EXOCYTOSIS (exo=outside; cyte=cell) Export of a secretory vesicle or vacuole that will migrate to membrane s inner surface and fuse with membrane Contents are forced out of the cell Opposite of endocytosis Examples: peptide hormones, steroid hormones, wastes 16

17 Test Yourself 1. Describe in your own words, the fluid mosaic model of a plasma membrane. 2. How is active transport different from facilitated transport? 3. Distinguish between phagocytosis and pinocytosis 17