Download The Cell Membrane

The Cell Membrane
AP Biology
2007-2008
Overview
 The cell membrane separates a living cell from it’s
nonliving surroundings. It’s a thin barrier ~8nm thick.
 (A nanometer = 1 billionth of a meter)
 It’s function is to control traffic in & out of the cell.


We call this selective permeability.
The cell membrane allows some substances to cross
in or out of the cell more easily than others.
 It is made of a phospholipid bilayer, proteins &
other macromolecules.
Phospholipids
Phosphate
 2 Fatty acid tails

hydrophobic
 Phosphate head

hydrophilic
 Arranged as a bilayer
Fatty acid
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Phospholipid bilayer
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
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A membrane is a collage of proteins & other molecules
embedded in the matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Transmembrane
proteins
Peripheral
protein
Cytoplasm
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Filaments of
cytoskeleton
Membrane Proteins
 Proteins determine membrane’s specific functions

cell membrane & organelle membranes each have
unique collections of proteins
 Membrane proteins:

peripheral proteins
 loosely bound to surface of membrane
 cell surface identity marker (antigens)

integral proteins
 penetrate lipid bilayer, usually across whole membrane
 transmembrane protein
 transport proteins
 channels, permeases (pumps)
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Why do we refer to the
cell membrane as a fluid mosaic?
AP Biology
2007-2008
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
AP Biology
2007-2008
Diffusion
 2nd Law of Thermodynamics
governs biological systems

universe tends towards disorder (entropy)
 Diffusion

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movement from high  low concentration
Diffusion
 Move from HIGH to LOW concentration
“passive transport”
 no energy needed

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diffusion
movement of water
osmosis
Diffusion across cell membrane
 Cell membrane is the boundary between
inside & outside…

separates cell from its environment
Can it be an impenetrable boundary?
NO!
OUT
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
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OUT
IN
waste
ammonia
salts
CO2
H2O
products
cell needs materials in & products or waste out
Facilitated Diffusion
 Diffusion through protein channels


channels move specific molecules across
cell membrane
facilitated = with help
no energy needed
open channel = fast transport
high
low
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“The Bouncer”
Channels through cell membrane
 Membrane becomes semi-permeable
with protein channels

specific channels allow specific material
across cell membrane
inside cell
NH
AP
Biology
3
salt
H 2O
aa
sugar
outside cell
Active Transport
 Cells may need to move molecules against
concentration gradient



shape change transports solute from
one side of membrane to other
protein “pump”
conformational change
“costs” energy = ATP
low
ATP
high
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“The Doorman”
Active transport
 Many models & mechanisms
ATP
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ATP
antiport
symport
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


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uses a protein pump
requires ATP
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
2 Types of Endocytosis
 Pinocytosis

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Pockets form along the cell membrane, fill
with liquid, and pinch off to form vacuoles
within the cell
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
molecular
signal
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The Special Case of Water
Movement of water across
the cell membrane
AP Biology
2007-2008
Osmosis is diffusion of water
 Water is very important to life,
so we talk about water separately
 Diffusion of water from
high concentration of water to
low concentration of water

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across a
semi-permeable
membrane
Concentration of water
 Direction of osmosis is determined by
comparing total solute concentrations

Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water
water
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hypotonic
hypertonic
net movement of water
Managing water balance
 Isotonic

animal cell immersed in
mild salt solution
 example:
blood cells in blood plasma
 problem: none
 no net movement of water

flows across membrane
equally, in both directions
 volume of cell is stable
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balanced
Managing water balance
 Hypotonic

a cell in fresh water
 example: Paramecium
 problem: gains water,
swells & can burst
 water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
ATP
 ATP

plant cells
 turgid
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freshwater
Water regulation
 Contractile vacuole in Paramecium
ATP
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Managing water balance
 Hypertonic

a cell in salt water
 example: shellfish
 problem: lose water & die
 solution: take up water or
pump out salt

plant cells
 plasmolysis = wilt
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saltwater
1991 | 2003
Aquaporins
 Water moves rapidly into & out of cells

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evidence that there were water channels
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Osmosis…
.05 M
.03 M
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell
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Any Questions??
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Key Vocabulary
 Solute

Substance that is dissolved in a solvent
to make a solution
 Solvent

Substance in which a solute is
dissolved to form a solution
 Solution

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Mixture of two or more substances in
which the molecules of the substances
are evenly distributed
Concentration
 The mass of a solute in a given
volume of solution

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Ex. If you dissolved 12 grams of salt
in 3 liters of water, the concentration
would be 12g/3L or 4g/L. If you
dissolved 12 grams of salt in 6 liters
of water, the concentration would be
12g/6L or 2g/L. The first solution is
twice as concentrated at the second
solution.
Hypertonic
 Describes the
side of the
membrane
with the
higher
concentration
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Hypotonic
 Describes the
side of the
membrane
with the lower
concentration
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Isotonic
 When both sides
of the membrane
are in equal
concentrations
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Comparison:
Hypertonic/Hypotonic
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Equilibrium
 When the concentration of a solute is
equal throughout the solution
 Once equilibrium has been reached,
particles continue to move across the
membrane in both directions
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Diffusion
 The process where molecules in a
solution randomly move from an area of
higher concentration to an area of lower
concentration
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Comparison: Diffusion,
Facilitated Diffusion, Active
Transport
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