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 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 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

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

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

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