MEMBRANE STRUCTURE
AND FUNCTION
CHAPTER 7
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



Phospholipid bilayer
Stable boundary between 2
aqueous compartments


Prior knowledge composed of
lipids and proteins
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 Examples
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


B) Encourages reaction





Selective or changes shape
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
Diffusion of Two Solutes
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)

Recognizing Tonicity
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
Practice Problem
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?
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+
3
pump in animals cells
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


Pinocytosis


Cellular drinking (engulfing liquid stuff)
Receptor mediated


Cellular eating (engulfing solid
foodstuffs)
Specific binding must occur first before
engulfing
Vessicles fuse or form from plasma
membrane