The Cell Membrane
*Very thin (7.5-10 nm) *Elastic *Semipermeable
* Dynamic *lipid bilayer *Made of phospholipids,
proteins, CHO& otherlipids
*amphipathic
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Phospholipids
Flexibility & selective permeability Phosphate
 Fatty acid tails

hydrophobic
 Phosphate group head

hydrophilic
 Arranged as a bilayer
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Fatty acid
Phospholipid bilayer
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
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 Protein:
*Hydrophilic & hydrophobic
*Integral & peripheral
 Cholesterol: permeability & toughness
 CHO: recognition sites & attach cells
Glycoprotein
Glycolipid
together
Cholesterol
Peripheral
protein
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Functions of membrane proteins
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1- structural ptns
2- passive channels: ungated & gated(voltage / ligand)
3- carriers for facilitated diffusion
4- carriers for active transport (uniport /symport/ antiport)
5- receptors: number & sensitivity change
6- enzymes
7- identity ptns
8-intercellular connections: a. binding j: tight & desmosomes
b. gap j
9- cell adhesion molecules
10- fixation of cytoskeleton
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Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
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Transporter
Enzyme
activity
Cell surface
receptor
Cell surface
identity marker
Cell adhesion
Attachment to the
cytoskeleton
Membrane carbohydrates
 Play a key role in cell-cell recognition

ability of a cell to distinguish one cell
from another
 antigens
important in organ &
tissue development
 basis for rejection of
foreign cells by
immune system

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Movement across the
Cell Membrane
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2007-2008
Diffusion
 All molecules are in constant motion
 Diffusion

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movement from high  low concentration
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Facilitated Diffusion
 Diffusion through protein channels
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channels move specific molecules across
cell membrane
facilitated = with help
no energy needed
open channel = fast transport
high
low
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Facilitated Diffusion
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Active Transport
 Against concentration gradient
 Needs carrier protein
 Energy is needed
low
conformational change
ATP
high
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“The Doorman”
Active transport
 Many models & mechanisms
ATP
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ATP
antiport
symport
Active transport
 1ry active:
*eg Na+/K+ pump
*α &β subunits
*α subunit contains 2 binding sites for K+ on the outside
& 3 binding sites for Na+ on the inside & an ATP
binding site
*β subunit has ATPase activity.
 2ry active:
*eg Glucose transport 2ry to active transport of Na
1st Na pumped out ….creates concentration gradient…
Na & glucose bind a carrier…transports them to inside
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Getting through cell membrane
Passive Transport

Simple diffusion
 diffusion of nonpolar, hydrophobic molecules
 lipids
 high  low concentration gradient

Facilitated transport
 diffusion of polar, hydrophilic molecules
 through a protein channel
 high  low concentration gradient
 Active transport

diffusion against concentration gradient
 low  high


uses a protein pump
requires ATP
 Vesicular transport
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ATP
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
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ATP
How about large molecules?
 Moving large molecules into & out of cell
through vesicles & vacuoles
 endocytosis

 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”

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exocytosis
exocytosis
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
molecular
signal
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Osmosis is diffusion of water
 Diffusion of water from
high concentration of water to
low concentration of water

across a
semi-permeable
membrane

The pressure necessary to stop solvent mol movements=
osmotic pressure
The numbers of particles per unit volume of fluid
Measured in mmHg
Osmole osmolarity osmolality
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The osmolarity of ICF=that ECF=300 mosmol
280 mosmol is due to Na, Cl & HCO3
20 mosmol is due to protein
Tonicity: is the osmolality of a solution relative to
the plasma
 Plasma proteins of blood is called oncotic
pressure. It is important for capillary circulation
&regulation of ECF
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Concentration of water
 Direction of osmosis is determined by
comparing total solute concentrations

Hypertonic - more solute, less water
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Hypotonic - less solute, more water
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Isotonic - equal solute, equal water
water
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hypotonic
hypertonic
net movement of water
Donnan effect
 The protein anions inside the cells are
non diffusible hinder the diffusion of
diffusible cations
 More osmotically active particles inside
the cell
 The cell tends to swell
 But the Na+/ K+ pump prevents cell
rupture
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Any Questions??
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