membrane structure and function chapter 7. life on the edge plasma membranes serve as the boundary...
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Life on the Edge
Plasma membranes serve as the boundary between the living cell and the non-living environment surrounding it
The plasma membrane controls the traffic of substances into and out of the cell
Plasma membranes are selectively permeable, that is it allows some substances to cross while restricting others
Amphipathic molecules which mean they have hydrophilic and hydrophobic regions
Membrane Protein
Example of a transport protein
4 identical subunits bind to form a channel
What level of structure is demonstrated?
Which end faces out? In?
Developing the Membrane Model
Gorter and Grendel. 1925 Prior knowledge composed
of lipids and proteins Phospholipid bilayer Stable boundary between
2 aqueous compartments Hydropphobic and –philic is
respective locations Protein location unknown
Adhesion connection Pure phospholipid not as
strong
Sandwich Model
Danielli and Davson, 1935
Protein formed two continuous layers outside of the phospholipid layers
Same for internal membranes
Problems Different functions
have different membranes
Proteins amphipathic too Hydrophobic
toward outside aqueous
Protein layer
Fluid Mosaic Model
Singer and Nicolson, 1972
Proteins embedded irregularly throughout the membrane Proteins with
hydrophobic and –philic regions
Supporting the Fluid Mosaic Model
Freeze-fracture splits membrane along bilayer Prepares for electron microscopy
Hydrophobic interactions hold together Lipids and proteins drift laterally
Phospholipids move fast laterally, but rarely change orientation
Some proteins more slowly and directionally others not at all
Fluidity Influences
Temperature Decrease temps move from liquid to solid state Pack tighter, e.g cold bacon grease to lard
Components High in unsaturated fatty acid tails, liquid at lower
temp. Hydrocarbon tails with kinks prevent tight packing
Cholesterol Prevents changes from temperature fluctuations,
temp. buffer Warm = maintain phospholipids; reduce fluidity by
restraining phospholipid movement Cool = prevent tight packing; maintains fluidity
Membrane Proteins Differ
Determinants of membrane function Specific to each type
of plasma membrane 2 major protein types
Peripheral attach outside the cell
Integral span entire or partially through bilayer
Functions vary according to area 6 major areas
Protein Functions
A) Channel and carrier proteins
Selective or changes shape
B) Encourages reaction May facilitate sequential
reaction steps C) Shape for a chemical
messenger D) Glycoproteins to
serve as recognition tags
E) Form junctions to hold together
F) Maintains cell shape and stabilizes
Cellular Movement
Plasma membrane is selectively permeable Nutrients in and waste products out Characteristics of cells determine movement
Size Charge Polarity Types of proteins available
Movement based on concentration gradients [solutes] vary on membrane sides Continue till equilibrium reached
No net movement
Transport Types
Passive transport is movement that doesn’t require energy Moves down [gradient]
Substance independent Simple is movement from high to low
concentration Osmosis is solvent movement (water) from high
concentration to low concentration Facilitated utilizes the same process, but has
channels or carriers to transport Active transport is movement that uses ATP
Tonicity
Ability of a solution to make a cell gain or lose water Depends on [solutes] impermeable to
membrane relative to inside of cell Hypertonic solution Hypotonic solution Isotonic solution
Effects vary in animal and plant cells Be able to diagram effects of all 3 solutions
in both cell types Know associated terms
Cellular Effects
Isotonic solutions have SAME [solute] No net movement across membrane
Animal = stable Plant = flaccid (limp)
Hypertonic solutions have MORE [solute] than the cell Water moves out to environment
Animal = crenation or shriveling Plant = plasmolysis, so lose water and PM shrinks
Hypotonic solutions have LESS [solute] than the cell Water moves in
Animal = lyse or burst Plant = turgor pressure (central vacuole)
Osmoregulation
Control of water balance
Paramecium use contractile vacuole to regulate water balance Excess water out so
cell doesn’t lyse Plants that reside in
drier climates Central vacuole
Cell plasma membrane is selectively permeable to water and simple sugars (which ones?)
What direction does each solute move?
What type of solution is this ‘cell’ in?
Practice Problem
Active Transport
Pumps molecules against their [gradients] Carrier proteins responsible for this movement Costs ATP to supply energy
Transfers a phosphate group to protein and forms ADP
Na+/K+ pump in animals cells 3 Na + out and 2 K + in maintains resting state of the
cell Proton pump in plant, fungi, and bacteria cells
Hydrogen ions pumped out of cell Separates charges, (+) out and (-) in
Charge separation stores energy Cotransport moves solutes indirectly
Uses left over ATP from above
Bulk Movement
Exocytosis Move substances out of the
cell from golgi apparatus Endocytosis
Move substances into the cell by forming new vesicles
Phagocytosis Cellular eating (engulfing solid
foodstuffs) Pinocytosis
Cellular drinking (engulfing liquid stuff)
Receptor mediated Specific binding must occur
first before engulfing Vessicles fuse or form from
plasma membrane