Chapter 7 Membrane structure and function

Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model

• 2nd Law of Thermodynamicsgoverns biological systems– universe tends towards disorder (entropy)

Diffusion

movement from HIGH LOW concentration

Passive Transport: DIFFUSION

• PASSIVE – Requires NO ENERGY

• Automatic due to kinetic energy of molecules

• Moves DOWN CONCENTRATION GRADIENTfrom [HIGH] → [LOW] until reaches

equilibrium

• Ex: Oxygen/CO2 cross capillary cell membranes

FACILITATED DIFFUSION with CARRIER PROTEINS• PASSIVE- Requires NO ENERGY

Trans-membrane proteins assist in movementGrab molecule, change shape, flip to other sideMoves from [HIGH] → [LOW]

“The Bouncer”

open channel = fast transport

facilitated = with help

HIGH

LOW

FACILITATED DIFFUSION with ION CHANNELS

• transmembrane proteins form “tunnels” across membraneMoves from [HIGH] → [LOW]

• Moves charged ions (Na+ , K+, Ca++ Cl-) past hydrophobic tails in center

• Can be “gated” or notGates can open/close in response to

electrical/chemical signals

The Special Case of Water

Movement of water across the cell membrane

FACILITATED DIFFUSION with AQUAPORINS

• OSMOSIS= Diffusion of water across a semi-permeable membrane AQUAPORIN proteins move POLAR WATER molecules past phobic tails

[HIGH] → [Low]

Concentration of water

• Direction of osmosis is determined by comparing total solute concentrations

– Hypertonic - more solute, less water

– Hypotonic - less solute, more water

– Isotonic - equal solute, equal water

hypotonic hypertonic

water

net movement of water

Managing water balance• Hypotonic Surrounding

– a cell in fresh water (low solute)

– high concentration of water around cell

• problem: cell gains water,

swells & can burst

• example: Paramecium

– ex: water continually enters

Paramecium cell

• solution: contractile vacuole

– pumps water out of cell

– ATP

– plant cells• turgid = full

• cell wall protects from burstingfreshwater

ATP

1

No problem,here

KABOOM!

Managing water balance

• Hypertonic Surrounding– a cell in salt water (high Solute)_

– low concentration of water around cell

• problem: cell loses water & can die

• example: shellfish

• solution: take up water or pump out salt

– plant cells• plasmolysis = wilt

• can recover

saltwater

2

I willsurvive!

I’m shrinking,I’m shrinking!

Managing water balance• Isotonic With Surrounding

– animal cell immersed in mild salt solution (equal salt solution)

– no difference in concentration of water between cell & environment• problem: none

– no net movement of water

» flows across membrane equally, in both directions

– cell in equilibrium

– volume of cell is stable

• example:blood cells in blood plasma

– slightly salty IV solution in hospitalbalanced

3

I couldbe better…

That’sperfect!

Cell (compared to beaker) hypertonic or hypotonic

Beaker (compared to cell) hypertonic or hypotonic

Which way does the water flow? in or out of cell

.05 M .03 M

Practice (not in notes) - Do you understand Osmosis…

freshwater balanced saltwater

Managing water balance

• Cell survival depends on balancing water uptake & loss

If there is a concentration difference on opposite sides of a membrane and solute can’t move, water will

Hypotonic Environment IsotonicEnvironment

HypertonicEnvironment

[solute] outside ‹ inside

Net movement of water into cell

Animal cells: swell & burst = CYTOLYSIS

Plant cells increase TURGOR PRESSURE

[solute] outside = inside

Net movement of water is equal

No change in size

[solute] outside > inside

Net movement of water out

Animal cells: shrink = CRENATION

Plant cells: can’t shrink due to cell wall -PLASMOLYSIS

Link

Osmosis is just diffusion of water

• Water is very important to life, so we talk about water separately

• Diffusion of water from HIGH concentration of water to LOW concentration of water

– across a semi-permeable membrane

symportantiport

Active transport• Many models & mechanisms

• Requires energy

ATP ATP

PUMPS

• Can move against concentration gradient

[low] → [high]

•

Used to create electrochemical gradients across cell membranes

PROTON PUMP

• Main electrogenic pump in PLANTS

• ATP provides energy to pump H+ ions across a membrane

• Stored H+ = potential energy to do workEX: COTRANSPORT (see below)ATP PRODUCTION

during cellular

respiration/photosynthesis

SODIUM-POTASSIUM (Na+-K+) PUMP

• Moves 3 Na+ ions in and 2 K+ ions outMain electro-genic pump in ANIMALS

• EX: Na+-K+ pump sets up membrane potential Nerve signal results when Na+ and K+

exchange placesThen pump resets membrane for next signal

CO - TRANSPORT

Na+-K+/ Proton pumps use ATP to create concentration gradientMovement of substance is linked to return of Na+/H+ as it flows back down its concentration gradient

EX: sucrose is linked to H+

transport

TYPES OF ACTIVE TRANSPORT-requires energy

ENDOCYTOSIS

• cell membrane engulfs substancebrings it into cell in a VESICLE

• PHAGOCYTOSIS- “phage” = cell eating

• large molecules/ whole cells

• PINOCYTOSIS- “pino” = cell drinking

• small molecules, fluids

• Ex: White blood cell eating bacteria

RECEPTOR MEDIATED ENDOCYTOSIS

• Substances (=LILGANDS) bind to specific RECEPTORS in membrane

• Vesicle forms from area with receptors

• Often clustered in coated pits

• EX: uptake of LDL-cholesterol carrierrequires receptor on cell surface

Endocytosis - Summary

phagocytosis

pinocytosis

receptor-mediated

endocytosis

fuse with

lysosome for

digestion

non-specific

process

triggered by

molecular signal

EXOCYTOSIS

• VESICLES fuse with cell membrane and releasesubstances outside cell

• Ex; Golgi export