Download Transport of Substances Across a Cell Membrane

Transport of
Substances Across a
Cell Membrane
Diffusion
 2nd Law of Thermodynamics
governs biological systems

universe tends towards disorder (entropy)
 Diffusion

movement from high  low concentration
Diffusion
 Move from HIGH to LOW concentration
“passive transport”
 no energy needed

diffusion
movement of water
osmosis
Diffusion across cell membrane
 Cell membrane is the boundary between
inside & outside…

separates cell from its environment
Can it be an impenetrable boundary?
OUT
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
NO!
OUT
IN
cell needs materials in & products or waste out
waste
ammonia
salts
CO2
H2O
products
Diffusion through phospholipid bilayer
 What molecules can get through directly?

fats & other lipids
 What molecules can
lipid
inside cell
salt
NH3
NOT get through
directly?

polar molecules
 H 2O

ions
 salts, ammonia
outside cell
sugar aa
H2O

large molecules
 starches, proteins

Small nonpolar molecules such as fats, O2
and CO2
 Diffuse easily across the phospholipid bilayer of a
membrane (nonpolar molecules)
Osmosis is 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
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
Managing water balance
 Cell survival depends on balancing
water uptake & loss
freshwater
balanced
saltwater
Managing water balance
 Isotonic

animal cell immersed in
mild salt solution
 example:
blood cells in blood plasma
 problem: none
 no net movement of water

flows across membrane
equally, in both directions
 volume of cell is stable
balanced
Managing water balance
 Hypotonic

a cell in fresh water
 example: Paramecium
 problem: gains water,
swells & can burst
 water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
ATP
 ATP

plant cells
 turgid
freshwater
Water regulation
 Contractile vacuole in Paramecium
ATP
Managing water balance
 Hypertonic

a cell in salt water
 example: shellfish
 problem: lose water & die
 solution: take up water or
pump out salt

plant cells
 plasmolysis = wilt
saltwater
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
Transport proteins may facilitate diffusion
across membranes
Many kinds of molecules do not diffuse freely
across membranes (size, charge, polarity)
 For these molecules, transport proteins
 Provide passage across membranes through
a process called facilitated diffusion

Solute
molecule
Figure 5.15
Transport
protein
Channels through cell membrane
 Membrane becomes semi-permeable
with protein channels

specific channels allow specific material
across cell membrane
inside cell
NH3
H2O
salt
aa
sugar
outside cell
Facilitated Diffusion
 Diffusion through protein channels


channels move specific molecules across
cell membrane
facilitated = with help
no energy needed
open channel = fast transport
high
low
“The Bouncer”
Facilitated Diffusion
Ion Channels
 allow specific ions to pass through the

protein channel.
regulated by the cell and are either open
or closed to control the passage of
substances into the cell
Carrier Proteins
 bind to specific molecules, change
shape and then deposit the
molecules across the membrane.
 Once the transaction is complete the
proteins return to their original
position.
Active Transport
 Cells may need to move molecules against
concentration gradient



shape change transports solute from
one side of membrane to other
protein “pump”
conformational change
“costs” energy = ATP
low
ATP
high
“The Doorman”
Active transport
 Many models & mechanisms
ATP
ATP
antiport
symport
Sodium Potassium Pump
Cells expend energy for active transport

Transport proteins can move solutes against
a concentration gradient
 Through active transport, which requires ATP
Transport
protein
Solute
1 Solute binding
Figure 5.18
ATP
P
ADP
2 Phosphorylation
P
Protein
changes shape
Phosphate
detaches
3 Transport
4Protein reversion
P
Getting through cell membrane
 Passive Transport

Simple diffusion
 diffusion of nonpolar, hydrophobic molecules
 lipids
 high  low concentration gradient

Facilitated transport
 diffusion of polar, hydrophilic molecules
 through a protein channel
 high  low concentration gradient
 Active transport

diffusion against concentration gradient
 low  high


uses a protein pump
requires ATP
ATP
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
ATP
How about large molecules?
 Moving large molecules into & out of cell
through vesicles & vacuoles
 endocytosis

 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”

exocytosis
exocytosis
Exocytosis and endocytosis transport large
molecules

To move large molecules or particles
through a membrane
 A vesicle may fuse with the membrane and expel
its contents (exocytosis)
Fluid outside cell
Insulin,
Crying
Vesicle
Protein
Figure 5.19A
Cytoplasm
Membranes may fold inward
 Enclosing material from the outside (endocytosis)
Vesicle forming
Figure 5.19B
Endocytosis
fuse with
lysosome for
phagocytosis
digestion
non-specific
pinocytosis
process
triggered by
receptor-mediated
endocytosis
molecular signal