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KEY CONCEPT The cell membrane is a
barrier that separates a cell from the
external environment. Acts like a fence.
Cell membranes are composed of
two phospholipid layers
• Phospholipids have 3 parts:
– Charged phosphate group
– Glycerol
– Two fatty acid chains
• Label the head
• Label the tail
• Which part is attracted to water?
• Cell membranes are composed of two
phospholipid layers.
– The cell membrane has two major functions.
• forms a boundary between inside and outside of the
cell
• controls passage of materials in and out of cell
(border patrol)
cell membrane
inside cell
outside cell
• Cell membranes are composed of two phospholipid
layers.
– The cell membrane is made of a phospholipid bilayer.
– There are other molecules embedded in the membrane.
Functions
cell membrane
Cholesterol:
Strengthens
carbohydrate
chain
Proteins: Gates,
cytoskeleton, differ
for each cell
membrane
Carbohydrates: ID
tags
cholesterol
protein
protein channel
protein
Cell membranes are composed of
two phospholipid layers.
– The fluid mosaic model describes the
membrane.
• Fluid: Cell membrane moves, not rigid
– Does not flip, though, outside stays out, inside stays in
• Mosaic: variety of molecules similar to tiles with
different textures and patterns
• Cell membranes are composed of two
phospholipid layers.
– The cell membrane is selectively permeable.
What does selective
mean?
What does permeable
mean?
So, what does
selectively permeable
mean?
Some molecules can cross the membrane while
others cannot. Which do you think will cross?
• Cell membranes are composed of two
phospholipid layers.
– The cell membrane is selectively permeable.
What is an everyday
example of something
that is semipermeable
or selectively
permeable?
Why is selective
permeability important?
Some molecules can cross the membrane
while others cannot.
• Chemical signals are transmitted across the
cell membrane.
– Receptors bind with ligands and change shape.
– There are two types of receptors.
• intracellular receptor
Examples: Aldosterone
in kidneys
• Chemical signals are transmitted across the
cell membrane.
– Receptors bind with ligands and change shape.
– There are two types of receptors.
• intracellular receptor
• membrane receptor
Examples: red blood cells
carrying carbon dioxide
3.4 Diffusion and Osmosis
KEY CONCEPT Materials move across membranes
because of concentration differences.
3.4 Diffusion and Osmosis
Passive transport does not require energy input from a
cell.
• Molecules can move across the cell membrane
through passive transport.
• There are two types of
passive transport.
• diffusion
• osmosis
3.4 Diffusion and Osmosis
Diffusion and osmosis are types of passive
transport.
• Molecules diffuse down a
concentration gradient.
Where will the
ball roll?
Does it take
energy?
3.4 Diffusion and Osmosis
Diffusion and osmosis are types of passive
transport.
• Osmosis is the diffusion of water molecules across a
semipermeable membrane.
3.4 Diffusion and Osmosis
• There are three types of solutions.
• Isotonic: Iso means equal; outside and inside of cell
have same concentration
• Hypertonic: Hyper = above, more dissolved outside of
cell, or more water inside cell; water Exits the cell
• Hypotonic: Hypo = below, less dissolved outside of
cell, less water in cell; water goes into cell
3.4 Diffusion and Osmosis
Some molecules can only diffuse through transport
proteins.
• Some molecules cannot easily diffuse across the cell
membrane.
• Facilitated diffusion is
diffusion through transport
proteins.
• Tunnels to get through the
mountain
• Still move with
concentration gradient!
• No energy required
3.5 Active Transport, Endocytosis,
and Exocytosis
KEY CONCEPT Cells use energy to transport materials
that cannot diffuse across a membrane. Like a chair lift
moving you up the mountain using energy!
What is different from this picture than the picture in the previous
slides?
3.5 Active Transport, Endocytosis, and Exocytosis
Active transport requires energy input from a cell
and enables a cell to move a substance against its
concentration gradient.
• Passive transport requires no energy from the cell.
• Active transport is
powered by chemical
energy (ATP).
• Active transport occurs
through transport protein
pumps.
• Cells use active transport
to maintain homeostasis.
3.5 Active Transport, Endocytosis, and Exocytosis
A cell can import and export large materials or large
amounts of material in vesicles during the
processes of endocytosis and exocytosis.
• Cells use energy to transport material in vesicles.
• Endocytosis is the process of taking material into the cell.
• Phagocytosis is a type of endocytosis; engulfs Large
particles
Vesicle
3.5 Active Transport, Endocytosis, and
Exocytosis
A cell can import and export large materials or large
amounts of material in vesicles during the
processes of endocytosis and exocytosis.
• Cells use energy to transport material in vesicles.
• Exocytosis is the process of expelling
material from the cell.
Vesicle
Think!
1. What might happen if vesicles in your
neurons (nerve cells) are unable to fuse
with the cell membrane?
2. How is endocytosis and exocytosis
different from diffusion?
3. How are protein pumps different from
protein channels?
4. Cholesterol is located between the fatty
acid chains of the phospholipid layer. Do
you think cholesterol is polar or nonpolar?
Explain.