Cell Membranes
&
Movement Across Them
Regents Biology
2006-2007
Cell (plasma) membrane
 Cells need an inside & an outside…

separate cell from its environment

cell membrane is the boundary
IN
food
sugars
proteins
fats
salts
O2
H2 O
Regents Biology
OUT
waste
ammonia
salts
CO2
H2 O
products
cell needs materials in & products or waste out
Building a membrane
 How do you build a barrier that keeps
the watery contents of the cell separate
from the watery environment?
What substance
do you know that
doesn’t mix with
water?
Regents Biology
 FATS 
 LIPIDS 
oil & water
don’t mix!!
Lipids of cell membrane
 Membrane is made of phospholipids

phospholipid bilayer
“attracted to water”
inside cell
phosphate
lipid
“repelled by water”
outside cell
Regents Biology
Semi-permeable membrane
 Need to allow passage through the
membrane
 But need to control what gets in or out

membrane needs to be semi-permeable
So what needs to get across the membrane?
sugar
Regents Biology
aa
lipid
H 2O
salt
NH3
So how do you build a
semi-permeable membrane?
Phospholipid bilayer
 What molecules can get through directly?
inside cell
waste
outside cell
Regents Biology
lipid
salt
sugar aa
H 2O
fats & other lipids
can slip directly
through the
phospholipid cell
membrane, but…
what about other
stuff?
Permeable cell membrane
 Need to allow more material through

membrane needs to be permeable to…
 all materials a cell needs to bring in
 all waste a cell needs excrete out
 all products a cell needs to export out
“holes”, or
channels, in
cell membrane
allow material
in & out
Regents Biology
inside cell
outside cell
Haa
sugar
2O
waste
salt
lipid
Semi-permeable cell membrane
 But the cell still needs control

membrane needs to be semi-permeable
 specific channels allow
specific material in & out
inside cell
waste
Regents Biology
outside
cell
H 2O
salt
aa
sugar
How do you build a semi-permeable
cell membrane?
 channels are made of proteins

proteins both “like” water & “like” lipids
bi-lipid
membrane
Regents Biology
protein channels
in bi-lipid membrane
Protein channels
 Proteins act as doors in the membrane

channels to move specific molecules
through cell membrane
Regents Biology
Movement through the channel
 Why do molecules move through
membrane if you give them a channel?
?
?
Regents Biology
Molecules move from high to low
 Diffusion

move from HIGH to LOW concentration
Regents Biology
Diffusion
 Move from HIGH to LOW concentration
“passive transport”
 no energy needed

Regents Biology
Simple Diffusion
 Move from HIGH to LOW
fat
inside cell
fat
fat
fat
fat
fat
Which way
will fat move?
low

high
outside cell
fat
fat
fat
fat
Regents Biology
fat
fat
fat
fat
Diffusion through a channel
 Move from HIGH to LOW
sugar sugar
sugar
sugar
inside cell sugar
sugar
low

Which way will
sugar move?
high
outside cell
sugar
Regents Biologysugar
sugar
sugar
sugar
sugar
sugar
Diffusion
 Move from HIGH to LOW concentration

through membrane
 simple diffusion
 no energy needed

through a protein channel
 facilitated diffusion (with help)
 no energy needed
Regents Biology
high
low
Active transport
 Cells may need molecules to move
against concentration difference

need to pump “uphill”
 from LOW to HIGH using ATP
protein pump
 requires energy
 ATP

ATP
Regents Biology
Transport summary
diffusion
facilitated
diffusion
active
transport
Regents Biology
ATP
Osmosis
Movement of Water Across
Cell Membrane
Regents Biology
2006-2007
Osmosis
 Water is very important, so we talk about
water separately
 Osmosis

diffusion of water from HIGH concentration
of water to LOW concentration of water
 across a semi-permeable membrane
Regents Biology
Keeping water balance
 Cell survival depends on balancing
water uptake & water loss
freshwater
Regents Biology
balanced
saltwater
Managing water balance
 Balanced conditions

no difference between cell
& environment
 example: blood
 problem: none
 water flows across
membrane equally,
in both directions
 volume of cell doesn’t
change
Regents Biology
balanced
Managing water balance
 Freshwater

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
Regents Biology
freshwater
Controlling water
 Contractile vacuole in Paramecium
Regents Biology
Managing water balance
 Saltwater
a cell in salt water
 example: shellfish
 problem: lose water

 plasmolysis in plants
 shrinking cell

solution: take up water
Regents Biology
saltwater