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Cell Membrane Structure
Plasma membrane: outer membrane: defines the cell

Fluid mosaic model
 Phospholipid bilayer
 Hydrophilic (water loving) polar heads of phospholipid molecule
 Hydrophobic (water fearing) non-polar tails
 Proteins
 Integral proteins: go all the way through the membrane
 Membrane gates and channels, receptors
 Peripheral proteins: attached to only 1 side of membrane
 May be found in layer facing cytoplasm
 May be found in layer
facing outside of cell
 Enzymes, Receptors
 Some proteins and phospholipids
have carbohydrate parts attached
 Glycoproteins
 Glycolipids
Passive Transport vs. Active Transport


Passive = without adding energy
 Watching TV is passive (but it is NOT
transport)
Active = energy must be added
 Playing soccer is active (but it is also NOT
transport)
Passive Transport

Does not require energy at the point of
transport


But some force is at work
Moves substances from where they are in
high concentration to where they are in low
concentration

Move down the concentration gradient
Types of Passive Transport
Diffusion

Random movement of molecules

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Does not REQUIRE movement through a
membrane, but it CAN BE through a
membrane
Moves substances from where they
are in high concentration to where
they are in low concentration until
equilibrium is reached
Gasses, liquids and solids can all
diffuse, but solids take a LONG
time!
Some materials can diffuse through
some membranes
Lipids can diffuse through cell
membranes
Types of Passive Transport
Facilitated diffusion


Diffusion through a
membrane, but where a
“door” is needed
When ions, proteins and
other non-lipids move
through membranes, they
usually need to go
through a gate or channel
 Gate or channel is also
called a carrier protein
molecule
Types of Passive Transport
Osmosis

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Movement of WATER across a selectively
permeable membrane, from where water
is in high concentration to where water is
in low concentration

Water can move through but the
dissolved substances can’t
It may be easier to think of the other
substances’ concentration

Water moves toward the higher
concentration of dissolved
substances
Each dissolved particle exerts about the
same amount of osmotic pressure

How many osmotic particles per
molecule?
(hint: ionic or covalent bonds?)

Glucose, NaCl, CaCl2, a protein
Osmosis , cont.
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When one side of the membrane has a higher
concentration of dissolved substances, that side is
said to be hypertonic to the other side
When one side of the membrane has a lower
concentration of dissolved substances, that side is
said to be hypotonic to the other side
When both sides of the membrane have the same
amount of dissolved substances, they are said to be
isotonic to each other
The substances on either side DO NOT have to be
the same substances ( for example- salt on one side
of membrane and sugar on the other side)
Osmosis , cont.
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Physiologic condition (PC) = the normal
osmotic pressure of body fluids
 PC is isotonic to 5.0% glucose solution
 PC is isotonic to 0.9% NaCl solution
 But body fluids do NOT have 5%
glucose and/or 0.9% NaCl!
(a) A red blood cell in an isotonic solution
looks normal, because the same amount
of water moves into the cell as moves out
(b) A red blood cell in a hypertonic solution more water moves out of the cell than moves
in, so it shrinks (crenates)
(c) A red blood cell in a hypotonic solution more water moves into the cell than moves
out, so it swells & may burst (hemolysis)
Osmosis in Plants

Turgor pressure results when plant cells are placed in
hypotonic solution
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Central water vacuole gains water
Cytoplasm expands
Plasma membrane pushes against cell wall (cell does NOT burst)
Maintains plant’s upright posture
Plasmolysis results when plant cells are placed in hypertonic
solution

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Central water vacuole
loses water
Plasma membrane pulls
away from cell wall
Active Transport

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Requires energy in the form of ATP at
the point of transport
Moves substances across a membrane
from where they are in low
concentration to where they are already
in high concentration

Move against the concentration gradient
Types of Active Transport
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Active Membrane Transport

Movement of materials across a
cell membrane against the
concentration gradient, using a
carrier protein that bonds to ATP

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Bonding causes the protein to change
shape
May move one substance in one
direction (uniport), or two
substances in the same direction
(symport) or two substances in
opposite directions (antiport)
 The Na+/K+ pumps are an
example (they move the ions in
opposite directions = antiport)
Types of Active Transport
The “–cytosis family”
 Endocytosis
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Moves materials into the cell by having
the membrane engulf them and form a
vesicle around them (selective
membrane fusion)
 Pinocytosis = “cell drinking”
 Phagocytosis = “cell eating”
Exocytosis

Moves materials out of the cell by
surrounding them in a membrane
(vesicle formation) and merging the
vesicle to the plasma membrane
(selective membrane fusion)
 Secretion is a type of exocytosis
Types of Active Transport
Special application of
endocytosis:

Receptor mediated
endocytosis

Essentially just
endocytosis, but with
special receptor proteins
that trigger the
formation of the vesicle