The Cell Membrane
Regents Biology
Semi-permeable membrane
 Cell membrane controls what gets in or out

semi-permeable
 only allows some material to get in or go out of
the cell
 GOAL: to maintain HOMEOSTASIS
So what needs to get across the membrane?
sugar lipids
Regents Biology
aa
O2
H 2O
salt
waste
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
Phospholipids
 Phosphate head

“attracted to water”
hydrophilic
 Fatty acid tails

Phosphate
hydrophobic
 Arranged as a bilayer
Fatty acid
“repelled by water”
Aaaah,
one of those
structure–function
examples
Regents Biology
Arranged as a Phospholipid bilayer
 Serves as a cellular barrier / border
sugar
H 2O
salt
lipids
polar
hydrophilic
heads
lipids
nonpolar
hydrophobic
tails
Impermeable to polar molecules
Permeable to nonpolar molecules
(lipid soluble)
lipids
polar
hydrophilic
heads
waste
Regents Biology
lipids
Cell Membrane Functions
 Cell membrane separates living cell from
aqueous environment
 Controls traffic in & out of the cell

allows some substances to cross more
easily than others
 hydrophobic (nonpolar) vs. hydrophilic (polar)
Regents Biology
Permeability to polar molecules?
 Membrane becomes semi-permeable via
protein channels

specific channels allow specific material
across cell membrane
inside cell
salt
NH3 Biology
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H 2O
aa
sugar
outside cell
Cell membrane is more than lipids…
 Transmembrane proteins embedded in
phospholipid bilayer

create semi-permeabe channels
lipid bilayer
membrane
Regents Biology
protein channels
in lipid bilyer membrane
Why are
proteins the perfect
molecule to build structures
in the cell membrane?
Polar areas
of protein
Nonpolar areas of protein
Regents Biology
2007-
Membrane Proteins
 Proteins determine membrane’s specific functions

cell membrane & organelle membranes each have
unique collections of proteins
 Classes of membrane proteins:

peripheral proteins
 loosely bound to surface of membrane
 ex: cell surface identity marker (antigens)

integral proteins
 penetrate lipid bilayer, usually across whole membrane
 transmembrane protein
 ex: transport proteins
 channels, pumps
Regents Biology
Many Functions of Membrane Proteins
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
receptor
Cell adhesion
Attachment to the
cytoskeleton
“Antigen”
Cell surface
Regents Biologyidentity marker
Membrane carbohydrates
 Play a key role in cell-cell recognition

ability of a cell to distinguish one cell
from another
 antigens
important in organ &
tissue development
 basis for rejection of
foreign cells by
immune system

Regents Biology
Cell membrane must be more than lipids…
 In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
It’s like a fluid…
It’s like a mosaic…
It’s the
Fluid Mosaic Model!
Regents Biology
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
Regents Biology
Transmembrane
proteins
Cytoplasm
Filaments of
cytoskeleton
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
Cell (plasma) membrane
 http://www.youtube.com/watch?v=Qqsf
_UJcfBc&feature=related
Regents Biology
Any Questions??
Regents Biology
CHECKPOINT
 Draw and explain what semi-permeable
means, and how this applies to the cell
membrane.
Regents Biology
Movement across the
Cell Membrane
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Movement through a membrane
 Why do molecules move through a
membrane?
HIGH
?
To maintain
HOMEOSTASIS
(equilibrium or
steady state)
LOW
?
Regents Biology
Transport through a membrane
PASSIVE TRANSPORT
No cellular energy is used to move
substances into or out of a cell
 Ex:
H 2O
CO2 O2
 Three types of passive transport

 SIMPLE DIFFUSION
 FACILITATED DIFFUSION
 OSMOSIS
Regents Biology
Molecules move from high to low
 Diffusion

move from HIGH to LOW concentration
 Concentration Gradient: difference between
high and low concentrations
Molecules evenly distributed
Diffusion
Regents
BiologyONLY
occurs if there is a CONCENTRATION GRADIENT
Simple Diffusion Example
Regents Biology
Simple Diffusion Example
 Move from HIGH to LOW
O2
inside cell
LOW
O2
O2
O2
O2
O2
Which way
will oxygen
move?
HIGH
outside cell
O2
Regents Biology
O2
O2
O2
O2
O2
O2
O2
Simple Diffusion Explained
 http://highered.mcgrawhill.com/sites/0072495855/student_view
0/chapter2/animation__how_diffusion_
works.html
Regents Biology
 Simple diffusion is defined as the movement
of…
A.
B.
C.
D.
Molecules from areas of higher
concentration to areas of lower
concentration.
Molecules from areas of lower concentration
to areas of higher concentration.
Water molecules across a membrane
Gas or water molecules across a membrane
Regents Biology
 When sugar is mixed with water ,
equilibrium is reached when…
A.
B.
C.
D.
Molecules of sugar stop moving
Water and sugar molecules are
moving at the same speed
The dissolved sugar molecules are
evenly distributed throughout the
solution
There are more sugar molecules than
water molecules
Regents Biology
 Diffusion is one of the processes
whereby materials are exchanged
between a cell and its environment.
A.
B.
True
False
Regents Biology
CHECKPOINT
 Explain how molecules move through

the cell membrane through the process
of diffusion.
Be specific
Regents Biology
Then what are cell membrane
channels for?
 Need to make “doors” through membrane
 Each “door” is specific to the substance it
carries
inside cell
salt
wasteBiology
Regents
H 2O
aa
sugar
outside cell
Facilitated Diffusion
 Move from HIGH to LOW concentration
 Passive Transport
 no energy needed

Channel protein:
 corridor for specific ion or molecule can pass

Carrier protein:
 undergo a subtle change in shape that
translocates the solute – binding site
HIGH
Regents Biology
LOW
Facilitated Diffusion
 Move from HIGH to LOW through a channel
 Ex: Glucose into cellssugar
of your body
sugar
sugar
inside cell sugar
sugar
sugar
LOW
Which way will
sugar move?
HIGH
outside cell
sugar
Regents Biologysugar
sugar
sugar
sugar
sugar
sugar
Facilitated Diffusion Explained
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animatio
n__how_facilitated_diffusion_works.html
Regents Biology
 Facilitated diffusion requires…
A.
B.
C.
D.
Enzymes
Carrier proteins
Lipid carriers
Carbohydrate carriers
Regents Biology
 Facilitated diffusion occurs…
A.
B.
C.
D.
Into the cell only.
Out of the cell only.
In either direction depending on the
temperature
In either direction depending on the
concentration gradient of the molecule
Regents Biology
 Unlike simple diffusion, facilitated
diffusion required energy expenditure
by the cell.
A.
B.
True
False
Regents Biology
 Facilitated diffusion requires a specific
transporter for a specific molecule.
A.
B.
True
False
Regents Biology
CHECKPOINT
 COMPARE and CONTRAST simple
diffusion and facilitated diffusion.
 Provide an example for each to support
your answer
Regents Biology
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
 Is affected by the concentration gradient of the
dissolved substances called the solution’s
tonicity
 The ability of solution to gain or lose water
 Great impact on cells without walls
Regents Biology
Vocabulary
 A water solution is a mixture of water molecules and
molecules of dissolved substances.
 Solute: material that is dissolved in a liquid (sugar)
 Solvent: the liquid the solute is dissolved in (water)
 Solution: combination of the solute and solvent
(sugar and water)
Regents Biology
How Osmosis Works
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__h
ow_osmosis_works.html
Regents Biology
Vocabulary
 Concentration: amount of water present in a
given volume
 *highest concentration of water is in pure water (highest
water potential)
 The more particles dissolved in water the fewer the
water molecules present
Where is the
highest
concentration
of water
(highest water
potential)?
Regents Biology
Water
Sugar
+
Water
Which way
will the water
diffuse?
Keeping water balance
 Cell survival depends on balancing
water uptake & water loss
Hypotonic
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Isotonic
Hypertonic
Solution
ISOTONIC
HYPOTONIC
HYPERTONIC
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Outside of cell…
Direction of
water
Affect on cell
Water Balance in Cells Without
Walls
Animal cell. An animal cell fares best in an isotonic
environment unless it has special adaptations to offset
the osmotic uptake or loss of water.
Regents Biology
45
Water Balance in Cells with Walls
 Cell Walls

Help maintain water balance
 Turgor pressure

Is the pressure of water inside a plant cell pushing
outward against the cell membrane
 If a plant cell is turgid


It is in a hypotonic environment
It is very firm, a healthy state in most plants
 If a plant cell is flaccid

It is in an isotonic or hypertonic environment
Regents Biology
Water Balance in Cells with Walls
Plant cell. Plant cells are turgid (firm) and generally
healthiest in a hypotonic environment, where the uptake of
water is eventually balanced by the elastic wall pushing back
on the cell.
Regents Biology
47
•
A
Regents Biology
B
Paramecium
(protist) removing
excess water
video
C
A
Regents Biology
B
C
CHECKPOINT
 As solute concentration increases in a
water solution, the concentration of
water decreases.
TRUE
Regents Biology
FALSE
DO NOW
 Draw an arrow to show which way the
water would move by osmosis.
Regents Biology
DO NOW
 Draw an arrow to show which way the
water would move by osmosis.
Regents Biology
CHECKPOINT
 When you go to the movies and eat a
large, salty bucket of popcorn, you
become thirsty and your mouth starts
to crave water.
 Based on your knowledge of osmosis,
explain why this occurs.
Regents Biology
ACTIVE TRANSPORT
 Cells may need molecules to move
against concentration “hill”

need to pump “uphill”
 from LOW to HIGH using energy
protein pump
 requires energy
 ATP

ATP
Regents Biology
Types of Active Transport
1. Protein Pumps:
•Example: Sodium /
Potassium Pumps
are important in nerve
responses.
Protein changes
shape to move
molecules: this
requires energy!
Regents Biology
Sodium
Potassium
Pumps
Types of Active
Transport
 2.
Endocytosis: taking
material into a cell
•
Cell membrane forms food
vacuole “cell eating”
•
This is how white blood cells
eat bacteria!
•
Pinocytosis - liquids
Phagocytosis - solids
•
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Types of Active Transport
3. Exocytosis: Forces
material out of cell
•
Vesicle fuses with cell
membrane
• EX: Hormones or wastes
released from cell
Regents Biology
Endocytosis &
Exocytosis
animations
Comparison of Passive & Active
Transport
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
ATP
Diffusion. Hydrophobic
Facilitated diffusion. Many hydrophilic
molecules and (at a slow
substances diffuse through membranes with the
rate) very small uncharged
assistance of transport proteins,
polar molecules can diffuse through the lipid
either channel or carrier proteins.
bilayer.
Regents Biology
ATP
58