The Plasma Membrane -
Gateway to the Cell
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Aim
• Explain how the exchange of
materials across the cell membrane is
related to its structure
• Objectives
• 1. Test understanding of osmosis,
diffusion, active transport
• Examine the structure of the cell
membrane
• See what aspects of the structure
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relate to how molecules travel
Photograph of a Cell
Membrane
A very powerful
microscope
called an
electron
microscope is
needed to see
this structure
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Cell Membrane
The cell
membrane is
flexible and
allows a
unicellular
organism to
move
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Homeostasis
• Balanced internal condition of
cells
• Also called equilibrium
• Maintained by plasma membrane
controlling what enters & leaves
the cell
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Functions of Plasma
Membrane
 Protective barrier
 Regulate transport in & out of cell
(selectively permeable)
 Allow cell recognition
 Provide anchoring sites for filaments
of cytoskeleton
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Functions of Plasma
Membrane
 Provide a binding site for enzymes
 Interlocking surfaces bind cells
together (junctions)
Contains the cytoplasm (fluid in cell)
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Structure of the Cell
Membrane
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Membrane Components
Phospholipids
Cholesterol
Proteins
(peripheral and integral)
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Carbohydrates (glucose)
Task
Follow the colour code:
Carbohydrate chain (glycocalyx) : yellow.
Function cell recognion
Intrinsic protein :green
The phospho and lipid parts of the membrane in
two colours
Draw in three cholesterol molecules, label and
choose a colour. The cholesterol aids in the
fluidity of the mermbrane
Label an extrinsic (peripheral) protein and choose
a colour for it.
The functions of the extrinsic proteins are
highlighted in blue.
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FLUID MOSAIC MODEL
FLUID- because individual phospholipids and
proteins can move side-to-side within the
layer, like it’s a liquid.
MOSAIC- because of the pattern produced by
the scattered protein molecules when the
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membrane is viewed
from
above.
Phospholipids
Make up the cell
membrane
Contains 2 fatty
acid chains that
are nonpolar
Head is polar &
contains a –PO4
group &
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glycerol
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Cell Membrane
Polar heads are hydrophilic “water loving”
Nonpolar tails are hydrophobic “water fearing”
Makes membrane “Selective”
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Cell Membrane
The cell membrane is
Hydrophobic
made of 2 layers of
molecules pass
phospholipids called the
easily;
hydrophilic
lipid bilayer
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DO NOT
Colour the phospholipid molecule.
Fill in the matching pairs table to get the
definitions
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Solubility
• Materials that
are soluble in
lipids can pass
through the
cell membrane
easily
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Question
• Insulin is a
• ___________
hormone made of
__ _ _______
a protein
____ ____ _
_______
___
Which do you think gets •into_the
cell?
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Semipermeable Membrane
Small molecules molecules move through
easily.
e.g. O2, CO2,
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Semipermeable Membrane
Ions and large molecules such as proteins do
not move through the membrane on their
own. Hydrophobic molecules like fat can
dissolve and pass through
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Aquaporins
• Water Channels
• Protein pores used during
OSMOSIS
Label one
of your
intrinsic
proteins
and show
water
passing
through it
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WATER
MOLECULES
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Question
How does this explain osmosis?
Draw and label an aquaporin on your diagram
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Types of Transport
Across Cell
Membranes
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Simple Diffusion
• Requires NO
energy
• Molecules
move from
area of HIGH
to LOW
concentration
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DIFFUSION
Diffusion is a
PASSIVE process
which means no
energy is used to
make the
molecules move,
they have a
natural KINETIC
ENERGY
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Diffusion of Liquids
Your diagram shows water molecules , a coloured
crystal B, which is full of crystal molecules C.
Colour and explain
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Osmosis
• Diffusion of water
across a
membrane
• Moves from HIGH
water potential
(low solute) to
LOW water
potential (high
solute)
Diffusion across a membrane
Semipermeable
membrane
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Diffusion of H2O Across A
Membrane
High H2O potential
Low H2O potential
Low solute concentration
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High solute concentration
Cell in Isotonic Solution
10% NaCL
90% H2O
ENVIRONMENT
CELL
10% NaCL
90% H2O
NO NET
MOVEMENT
What is the direction of water movement?
equilibrium
The cell is at _______________.
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Cell in Hypotonic Solution
10% NaCL
90% H2O
CELL
20% NaCL
80% H2O
What is the direction of water movement?
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Cell in Hypertonic Solution
15% NaCL
85% H2O
ENVIRONMENT
CELL
5% NaCL
95% H2O
What is the direction of water movement?
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What Happens to Blood Cells?
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Your diagram
Shows what happens to red blood cells
Colour and explain briefly what has happened in each
case
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Three Forms of Transport Across the Membrane
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Passive Transport
Simple Diffusion
 Doesn’t require energy
 Moves high to low
concentration
 Example: Oxygen
diffusing into a cell and
carbon dioxide diffusing
out.
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Passive Transport
Facilitated diffusion
Doesn’t require energy
Uses transport
proteins to move high to
low concentration
Examples: Glucose
moving from blood into a
cell.
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Proteins Are Critical to
Membrane Function
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Types of Transport Proteins
• Channel proteins are embedded
in the cell membrane & have a
pore for materials to cross
• Carrier proteins can change
shape to move material from
one side of the membrane to
the other
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Facilitated Diffusion
Molecules will randomly move through
the pores in Channel Proteins eg
water
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Carrier Proteins
• Other carrier
proteins
change shape
to move
materials (eg
glucose)
across the cell
membrane
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Facilitated diffusion Glucose
• “I” shows the
concentration
gradient. Glucose
is carried high to
low via carrier
proteins (J) in
the cell
membrane (K)
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Active Transport
Requires energy or ATP
Moves materials from
LOW to HIGH
concentration
AGAINST concentration
gradient
Label an intrinsic
protein as channel for
active transport
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Active transport
Examples: Pumping
Na+ (sodium ions)
out and K+
(potassium ions) in
against strong
concentration
gradients.
Called Na+-K+ Pump
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Diagram
• This shows the transport of an amino
acid (A)
• For active transport to happen, energy
is supplied in the form of ATP.
• This is broken down into ADP plus P
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Moving the “Big Stuff”
Larger molecules are moved into the cell by
endocytosis.
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Pinocytosis
Most common form of endocytosis.
Takes in dissolved
molecules
as a vesicle
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Pinocytosis
• Cell forms an
invagination
• Materials
dissolve in
water to be
brought into cell
• Called “Cell
Drinking”
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Example of Pinocytosis
pinocytic vesicles forming
mature transport vesicle
Transport across a capillary cell (blue).
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Endocytosis – Phagocytosis
Used to engulf large particles such as
food, bacteria, etc. into vesicles
Called “Cell Eating”
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Phagocytosis About to Occur
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Phagocytosis
- Capture
of a Yeast
Cell (yellow)
by
Membrane
Extensions
of an
Immune
System Cell
(blue)
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Your diagram
Large particles (H) suspended in extracellular
fluid (I). The inside of the cell is labelled J
The membrane folds inwards, forming a vesicle
(K)
The vesicle will fuse with a lysosome and the
contents digested
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Answer the questions on your sheet
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