Transcript The Cell Membrane

The Cell Membrane
AP Biology
Overview
 The cell membrane separates a living cell
from its nonliving surroundings

thin barrier = 8nm thick (n=nano=10-9)
 Controls traffic in & out of the cell


selectively permeable
allows some substances to cross more easily
than others- “choosy”
 Made of phospholipids, proteins , cholesterol,
and carbohydrates.
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Phospholipids
Phosphate
 Fatty acid tails
Hydrophobic
 “Water fearing”
Phosphate group head
 Hydrophilic
 “Water loving”
Arranged as a bilayer



Fatty acid
Inside cell
Why do the phospholipids
arrange themselves like
this?
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Outside cell
More than lipids…
 In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
 The composition of the cell membrane is
called the fluid mosaic model because
the phospholipid bilayer and the
embedded proteins can move around
like a “fluid” to let compounds into and
out of the cell.
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Cholesterol also makes up the cell membrane
structure. It is between the tails of the
phospholipids.
Fluid outside the cell
Phospholipids
Cholesterol
Cytoplasm
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Membrane Proteins
 There are 2 types of membrane proteins:

peripheral proteins
 loosely bound to the surface of the
membrane

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integral proteins
 Pass through the lipid bilayer
Why are
proteins the perfect
molecule to build structures
in the cell membrane?
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Protein Structure
 Proteins are molecules that are
composed of amino acids linked
together by peptide bonds.

20 different amino acids exist
 Proteins have complex shapes that
include various folds, loops, and
curves.
 Chemical bonding between portions of
the protein chain aid in holding the
protein together and giving it its 3dimensional shape.
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Some amino acids are hydrophobic
and some are hydrophilic
nonpolar & hydrophobic
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polar & hydrophilic
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
 basis for rejection of
foreign cells by
immune system

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Sketch the structure of the cell membranelabel the phospholipid bilayer, cholesterol,
integral protein, peripheral protein, and
carbohydrate
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Movement across the
Cell Membrane
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2007-2008
 What is the natural movement of
molecules into and out of the
cell?
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Simple Diffusion
 Movement from HIGH to LOW
concentration
“passive transport”
 no energy needed

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diffusion
Diffusion across cell membrane
 The cell membrane is the boundary
between the inside & outside
cell needs food and materials in & products or waste out
IN
food
sugars
proteins
lipids
salts
O2
H2O
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OUT
OUT
IN
waste
salts
CO2
H2O
products
Simple diffusion through phospholipid bilayer
 What molecules can get through directly?

fats & other lipids
inside cell
NH3
 What molecules can
lipid
salt
NOT get through
directly?



outside cell
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sugar aa
H 2O
H2O
Salts
Sugars
How do the non-fat compounds
get in/out?
 Membrane becomes selectivelypermeable with protein channels
formed by integral proteins.

This is called facilitated diffusion
inside cell
NH
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3
salt
H 2O
aa
sugar
outside cell
Facilitated Diffusion
no energy needed
 still moving molecules from high to
low concentration

high
low
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facilitate = to help
“The Bodyguard”
Osmosis
 Water is very important to cell function
 Diffusion of water from
high concentration of water to
low concentration of water is osmosis

across a
selectively-permeable
membrane
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Concentration of water
 The direction of osmosis is determined by
comparing solute concentrations on each
side of the membrane. What is a solute?

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
For each cell, label the solutions as
hypertonic, hypotonic or isotonic and draw
an arrow to show the direction of water
movement.
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Managing water balance
 Cell survival depends on balancing
water uptake & loss
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freshwater
balanced
saltwater
Managing water balance

A cell in fresh water
 example: Paramecium
 What type of environment?
 problem: gains water,
swells & can burst
ATP
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 solution: contractile vacuole
 pumps water out of cell which
requires ATP (energy)
freshwater
Water regulation
 Contractile vacuole in Paramecium
ATP
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Managing water balance
 Another example:

Plant cells
 When in a hypotonic
(freshwater) environment
they are constantly taking up
water.
 Do they burst?
 No!
 Plant cells have a cell wall that
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prevents bursting.
 They build up turgor pressure
which makes the plant stand
tall.
 Plants lose water out of their
leaves= transpiration
Managing water balance
 Another example:

Plant cells
 How do they deal with hypertonic
environment?
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Getting through cell membrane
 Passive Transport

diffusion with the concentration gradient
 high  low

Simple diffusion
 diffusion of hydrophobic molecules
 Lipids (fats)

Facilitated transport
 diffusion of hydrophilic molecules
 through a protein channel
 high  low concentration gradient
 Active transport

diffusion against the 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
Active Transport
 Why is active transport necessary?

The Na+/K+ pump is the way that our
nervous system works. By pumping ions
up their concentration gradients, electricity
is generated.
conformational change
ATP
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Na+/K+ Pump

The steps of the pump are as follows:
1. 3 Na+ ions inside the cell bind to the carrier
2.
3.
4.
5.
6.
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protein (pump).
A phosphate from ATP attaches to the pump
and the pump changes shape.
The 3 Na+ ions are dumped outside the cell.
2 K+ ions outside the cell bind to the pump.
The phosphate detaches from the pump and
the pump goes back to its original shape.
The 2 K+ ions are dumped inside the cell.
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How about large molecules and fluids?

Endocytosis- Active Transport
 The taking in of large molecules (too big to pass
through the membrane) or a large amount of a fluid by
“engulfing”. To engulf means that the cell membrane
pinches in and surrounds what is being ingested. The
molecules are then digested in a pouch called a
vesicle.
 phagocytosis = Ingesting large molecules
 pinocytosis = Ingesting large amounts of a fluid
 Draw endocytosis:
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
Exocytosis- Active Transport
 The opposite of endocytosis. The vesicle fuses
with the cell membrane and it then opens up to
release the contents.
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