Download 16-17 Chapter 7 cell transport

Chapter 7 Membrane structure and
function
Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
Transmembrane
proteins
Cytoplasm
Filaments of
cytoskeleton
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
• 2nd Law of Thermodynamics
governs 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 GRADIENT
from [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 movement
Grab molecule, change shape, flip to other side
Moves from [HIGH] → [LOW]
facilitated = with help
open channel = fast transport
HIGH
LOW
“The Bouncer”
FACILITATED DIFFUSION with ION
CHANNELS
• transmembrane proteins form “tunnels” across
membrane
Moves from [HIGH] → [LOW]
• Moves charged ions (Na+ , K+, Ca++ Cl-) past
hydrophobic tails in center
• Can be “gated” or not
Gates 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 semipermeable 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
water
hypotonic
hypertonic
net movement of water
1
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
KABOOM!
• example: Paramecium
– ex: water continually enters
Paramecium cell
• solution: contractile vacuole
ATP
– pumps water out of cell
– ATP
– plant cells
No problem,
here
• turgid = full
• cell wall protects from bursting
freshwater
2
Managing water balance
• Hypertonic Surrounding
– a cell in salt water (high Solute)_
– low concentration of water
around cell
I’m shrinking,
I’m shrinking!
• problem: cell loses water & can die
• example: shellfish
• solution: take up water or pump
out salt
– plant cells
• plasmolysis = wilt
• can recover
I will
survive!
saltwater
3
Managing water balance
• Isotonic With Surrounding
That’s
perfect!
– 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
I could
be better…
– slightly salty IV solution in hospital
balanced
Practice (not in notes) - Do you
understand Osmosis…
.05 M
.03 M
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
Managing water balance
• Cell survival depends on balancing water
uptake & loss
freshwater
balanced
saltwater
If there is a concentration difference on opposite sides
of a membrane and solute can’t move, water will
Hypotonic Environment
Isotonic
Environment
Hypertonic
Environment
[solute] outside ‹ inside
[solute] outside = inside
[solute] outside > inside
Net movement of water
into cell
Net movement of water
is equal
Net movement of water
out
Animal cells: swell &
burst = CYTOLYSIS
No change in size
Animal cells: shrink =
CRENATION
Plant cells increase
TURGOR PRESSURE
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
Active transport
• Many models & mechanisms
• Requires energy
ATP
ATP
antiport
symport
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 work
EX: COTRANSPORT (see below)
ATP PRODUCTION
during cellular
respiration/photosynthesis
SODIUM-POTASSIUM (Na+-K+) PUMP
• Moves 3 Na+ ions in and 2 K+ ions out
Main electro-genic pump in ANIMALS
• EX: Na+-K+ pump sets up membrane potential
Nerve signal results when Na+ and K+
exchange places
Then pump resets membrane for next signal
CO - TRANSPORT
Na+-K+/ Proton pumps use ATP to create
concentration gradient
Movement 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 TRANSPORTrequires energy
ENDOCYTOSIS
• cell membrane engulfs substance
brings 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 carrier
requires 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 release
substances outside cell
• Ex; Golgi export