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Cell Membrane structure and
transport
Cell membrane of cheek cells,
400X magnification
Cell membrane (http://www.jdaross.mcmail.com/cell2.htm)
Cell membrane at 240,000 X magnification
(http://www.jdaross.mcmail.com/cell2.htm)
Cell Membrane
(plasma membrane)

Separates living cell
from nonliving
surroundings
 Selectively permeable
(semi-permeable)
Membrane Structure

Phospholipid bilayer
 Proteins
– Integral protein
– Peripheral protein

Cholesterol
 Carbohydrates
Phospholipid bilayer

Phosphate group (PO4-3)
– Polar, Hydrophilic

Lipid tail
– Non-polar, hydrophobic


Unsaturated fat: kink,
don’t pack tight
Saturated fat: lie straight,
pack tight
Cholesterol



Breaks up phospholipid
tails
More of them in coldweather cells, fewer in
tropical cells
Makes the cell membrane
– More fluid in colder
temperatures
– More rigid in warmer
temperatures
Proteins

Two main kinds
– Integral
 Lie within the membrane
 Transmembrane proteins
span the entire core
– Peripheral
 Attach to external surface
by weak bonds

Two main functions
– Transporters
– Receptors
Carbohydrates

Short sugar chains =
oligosaccharides.
covalently bonded to
lipids or proteins
– Glycolipid
– Glycoprotein

Function
– Cell to Cell recognition
The Cell Membrane
The parts are dynamic…they move around
and change positions (the membrane is
FLUID)
 The cell membrane is NOT a single, solid
structure—it is made up of many smaller
parts (it is a MOSAIC)

hence, the name…
FLUID MOSAIC MODEL
OF THE CELL MEMBRANE
Transport

Diffusion
– Osmosis
– Facilitated Diffusion

Active Transport
 Endocytosis
– Phagocytosis
– Pinocytosis

Exocytosis
Diffusion



Movement of molecules
from areas of high
concentration to areas of
low concentration.
Passive, requires no extra
energy
Small, non polar
molecules
– O2
– CO2
Osmosis

Diffusion of water
through a membrane
from an area of high
concentration to an
area of low
concentration
– Isotonic
– Hypertonic
– Hypotonic

No extra energy
required
iso- = equal
hyper- = above/over
hypo- = below/under
Plasmolysis and Turgor

Plasmolysis
– The shrinking of a plant cell
when placed in a hypertonic
environment. Cell
membrane pulls away from
the cell wall.

Turgor pressure
– When a cell is full of water.
The water vacuole exerts
pressure against the walls

Helps hold the plant up.
Facilitated Diffusion


Molecules moving from
high to low concentration
with help.
Use Channel or Carrier
protein
– Aquaporins


Requires no energy
Large, polar or ionic
molecules.
– Na+
– Ca+2
– Glucose
Active Transport


Movement of molecules
against a concentration
gradient.
Requires energy
– Na+/K+ pump
– Ca+2 pump in muscle cells
– H+ pump (plants,
fungi,bacteria)
– Builds electrochemical
gradient
CoTransport

A substance that has been
actively pumped to one
side, builds a gradient.
This gradient, potential
energy, can do work as it
leaks back across.
Escorts, in this case,
glucose
– Na+ with glucose

Why should you drink
gatorade or powerade
when dehydrated or
pedialyte when you have
the flu?
A Represents?
B. Represents?
C. Represents?
Endocytosis

Cell takes in
macromolecules or
large particles by
forming new vesicles
from the plasma
membrane.
– Three kinds



Phagocytosis
Pinocytosis
Receptor-mediated
endocytosis
Endocytosis

Phagocytosis
– Cell engulfs a particle
by wrapping
pseudopodia around it
and packaging it within
a membrane enclosed
sac.


Amoeba
White Blood Cell
– Macrophage
Endocytosis

Pinocytosis
– “Cell drinking”
– Unspecific as it takes
in any solute along
with the solvent
Endocytosis

Receptor mediated
endocytosis
– Specific substances
“ligands” bind to
receptors on cells.
– Receptors are in areas
called “pits”.
Exocytosis
The opposite of endocytosis.
Cell gets rid of substances by forming
a membrane around the substance.
The bubble of material moves to the
cell membrane and is released.