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3.3 Cell Membrane
KEY CONCEPT The cell membrane is a barrier that
separates a cell from the external environment.
3.3 Cell Membrane
The cell membrane has two major functions
– forms a boundary between inside and outside of the
cell
cell membrane
inside cell
outside cell
3.3 Cell Membrane
The cell membrane has two major functions
– forms a boundary between inside and outside of the
cell
– controls passage of materials
cell membrane
inside cell
outside cell
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• Polar Head is charged PO4
• Nonpolar tail of two fatty acid chains
3.3 Cell Membrane
The fluid mosaic model describes the membrane
• The arrangement of molecules making the membrane
• Flexible like fluid with a variety (mosaic) of molecules
cell membrane
carbohydrate
chain
cholesterol
protein
protein channel
protein
3.3 Cell Membrane
There are other molecules embedded in the membrane
• Cholesterol – strengthens
• Proteins – movement and signaling
• Carbohydrates – identify the cell
cell membrane
carbohydrate
chain
cholesterol
protein
protein channel
protein
3.3 Cell Membrane
The cell membrane is semipermeable
Some molecules can cross the membrane while others
cannot.
3.3 Cell Membrane
Selective Permeability
-some things can cross while others cannot
-semipermeable
-maintains homeostasis
-molecules can cross in a variety of ways
3.3 Cell Membrane
Chemical signals are transmitted across the cell
membrane.
• Receptors bind with ligands and change shape.
• There are two types of receptors.
3.3 Cell Membrane
Receptors: communicate between cells by detecting a
signal molecule to perform an action
• There are two types of receptors.
– intracellular receptor: bind to molecules to cross the
membrane
3.3 Cell Membrane
•
Membrane Receptor: binds to molecules that cannot cross
the membrane; change shape and transmit the message
3.3 Cell Membrane
Person A
Person B
What is the function
of the cell membrane?
What is a
phospholipid? Why is
it called that?
Describe the fluid
mosaic model.
What is selective
permeability?
How does the fluid mosaic model and
phospholipids make the cell membrane
semipermeable?
3.3 Cell Membrane
Page 80 Formative Assessment Questions
3.3 Cell Membrane
3.3 Cell Membrane
KEY CONCEPT Materials move across membranes
because of concentration differences.
3.4
and Osmosis
3.3Diffusion
Cell Membrane
Passive transport does not require energy input from a
cell.
• Takes NO energy
• Substances move DOWN their
concentration gradient
3.4
and Osmosis
3.3Diffusion
Cell Membrane
Diffusion is a type of passive transport.
• Molecules diffuse down a
concentration gradient.
• -small
• -nonpolar molecules
• -for example:
• O and CO2
3.4
and Osmosis
3.3Diffusion
Cell Membrane
Osmosis is a type of passive transport.
• Osmosis is the diffusion of water molecules across a
semipermeable membrane.
3.4
and Osmosis
3.3Diffusion
Cell Membrane
Diffusion and osmosis are types of passive transport.
• There are three types of solutions.
• isotonic
•
hypertonic
•
hypotonic
3.4
and Osmosis
3.3Diffusion
Cell Membrane
Facilitated Diffusion = some molecules can only
diffuse through transport proteins.
• Some molecules cannot easily diffuse across the cell
membrane.
• Large, Polar molecules
• Facilitated diffusion is
diffusion through transport
proteins.
• Does NOT require energy
3.3 Cell Membrane
Person A
Person B
What is diffusion?
What types of
substances move
through diffusion?
What is osmosis?
What types of
substances move
through osmosis?
Make a Venn diagram to compare/contrast
osmosis and diffusion
3.3 Cell Membrane
Person A
Person B
Describe what
happens to a cell in a
hypertonic solution.
Describe what
happens to a cell in a
hypotonic solution
The hospital gives patients’ a saline
(slightly salty) solution to keep them
hydrated. In terms of hypo- hyper- and
isotonic solutions explain why a pure
water solution isn’t given instead?
3.3 Cell Membrane
Page 83 Formative Assessment Questions