AS CIE Cell Membranes

D1: The Cell Membranes
Plan
1 Introduction:
1.1 Where are membranes found in the cell ?
1.2 Roles of the Cell Membranes
1.3 The fluid mosaic model
2 Membrane lipids:
2.1 Types of membrane lipids
2.2 Roles of membrane lipids
2.3 Examples
2
3
4
1
5
3 Membrane proteins:
3.1 Types of membrane proteins
3.2 Roles of membrane proteins
3.3 Examples
4 Membrane carbohydrates:
4.1 Types of membrane carbohydrates
4.2 Roles of membrane carbohydrates
4.3 Examples
5 Conclusions and Overviews:
Xavier DANIEL, Ph.D.
AS

• Cell wall
• Plasma membrane
• Cytoplasm
• Free DNA
• Ribosomes
A Typical Prokaryotic Cell
0.1 – 10mm
Depending on the species
outside
inside
Where are membranes found in the cell ?
1
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Rough ER Smooth ER
Centrosome
CYTOSKELETON
Microfilaments
Microtubules
Microvilli
Peroxisome
Lysosome
Golgi apparatus
Ribosomes
In animal cells but not plant cells:
Lysosomes
Centrioles
Flagella (in some plant sperm)
Nucleolus
Chromatin
NUCLEUS
Flagellum
Intermediate filaments
ENDOPLASMIC RETICULUM (ER)
Mitochondrion
Nuclear envelope
Plasma membrane
A Typical Animal Cell
1
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4
4
Endomembrane System
1
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In plant cells but not animal cells:
Chloroplasts
Central vacuole and tonoplast
Cell wall
Plasmodesmata
CYTOSKELETON
Ribosomes (small brown dots)
Central vacuole
Microfilaments
Intermediate
filaments
Microtubules
Rough
endoplasmic
reticulum Smooth
endoplasmic
reticulum
Chromatin
NUCLEUS
Nuclear envelope
Nucleolus
Chloroplast
Plasmodesmata
Wall of adjacent cell
Cell wall
Golgi apparatus
Peroxisome
Tonoplast
Centrosome
Plasma membrane
Mitochondrion
A Typical Plant Cell
1
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Cell (plasma) membrane
1. Separates the components of a cell from its
environment
Surrounds the cell
The cell membrane is the boundary
IN
food
- sugars
- proteins
- fats
salts
O2
H2O
OUT
waste
- ammonia
- salts
- CO2
- H2O
products
- proteins
Cell needs materials in & products or waste out
Roles of the cell membranes
1
Controls what goes in
and what goes out of
the cell
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1
Cell (plasma) membrane
1. Separates the components of a cell from its environment
Controls what goes in
and what goes out of
the cell
2. Keeps a stable environment inside the cell
Homeostasis of the cell
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Organelle’s membranes
1’. Separate the components of an organelle from its environment
=
Cytoplasm
Surrounds the organelle
Organelle’s membrane is the boundary
1
What goes in ? What goes out ?
Depends on the organelle
Controls what goes in and what
goes out of each organelle
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1’. Separate the components of an organelle from its environment
1
Controls what goes in and what
goes out of each organelle
2. Keeps a stable environment inside each organelle
Homeostasis of all organelles
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2. Compartimentalize the molecules
What is needed in the nucleus may not be needed elsewhere
Genomic DNA
What is needed in the chloroplast may not be needed elsewhere
Chlorophyll
What is needed in the cytoplasm may not be needed elsewhere
Cytoskeleton
What is needed in the vacuole may not be needed elsewhere
Water and waste products
etc...
1
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3. Compartimentalize the chemical reactions
Each organelle is specialized for some reactions only
More efficient ! No mutual inhibition !
Lysosome
Hydrolysis of molecules
Chloroplast
Photosynthesis
Mitochondrion
Last steps of Cellular respiration
Golgi
Modification of proteins
etc...
1
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1
4. Different cellular compartments
can have different conditions
2. Compartimentalize the molecules
3. Compartimentalize the chemical reactions
+
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pH 4.8
Contains digestive
enzymes, optimum pH
4.5 - 4.8
pH 7.2
lysosome
cytosol
Membrane = tonoplast
acts as
a barrier
1
4. Different cellular compartments
can have different conditions
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Phospholipids are
the major structural
component of
membranes
Phospholipid
The fluid mosaic model
1
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aqueous solution
Hydrophilic head
Hydrophobic tail
Phospholipids form
micelles when submerged
in water
air
One-layer membrane ? Impossible !
1
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Hydrophilic
head
Hydrophobic
tail
WATER
WATER
Phospholipid bilayer
1
Mixes with water
Mixes with water
Does not mix
with water
Does not mix
with water
Mix together
Two-layer membrane ? It works !
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polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
1
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All membranes are phospholipid bilayers
with embedded proteins
1
Transmembrane (intrinsic) proteins span the membrane
Integral (extrinsic) proteins are on either side of the membrane
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All membranes are phospholipid bilayers
with embedded proteins
1
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Oligosaccharides
Cytoplasm
Membrane Proteins
Cholesterol
Outer
Surface
Phospholipid
Bilayer
1
Mostly in animal membranes
Attached to proteins: Glycoproteins
Attached to lipids: Glycolipids (not shown here)
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Membranes are not static
They have a fluid consistency
Most membrane lipids and proteins can drift about
laterally in the plane of the membrane
Cholesterol enhances membrane fluidity
(animal membranes mostly)
1
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The membranes’ “fluid” is
the double layer of phospholipids
1
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The membranes’ “mosaic” is
a collection of various proteins
1
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Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Extracellular fluid
Cholesterol
Cytoplasm
Glycolipid
Transmembrane
proteins
Filaments of
cytoskeleton
Peripheral
protein
Glycoprotein
Phospholipids
1
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The Fluidity of Membranes
Phospholipids in the membranes
Can move within the bilayer
Lateral movement
(~107 times per second) Flip-flop
(~ once per month)
(a) Movement of phospholipids
1
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Proteins in the membranes
Can drift within the bilayer
EXPERIMENT
Researchers labeled the plasma membrane proteins of a mouse
cell and a human cell with two different markers and fused the cells.
Using a microscope, they observed the markers on the hybrid cell.
Membrane proteins
Mouse cell
Human cell
Hybrid cell
Mixed
proteins
after
1 hour
RESULTS
CONCLUSION
The mixing of the mouse and human membrane proteins
indicates that at least some membrane proteins move sideways within the plane
of the plasma membrane.
+
1
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Overview
Cell membranes separate living cell from non-living
surroundings and separate different organelles
 thin barrier = 8nm thick
Controls traffic in & out of the cell/organelle
Made of phospholipids, proteins & other macromolecules
Is a fluid mosaic
1
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Overview
1
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Types of membrane lipids
2
Two types of lipids in the membranes
Phospholipids Cholesterol
Mostly in animal cels
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Phospholipids
Triglycerides that contain a Phosphate Group
One of the 3 fatty acids is replaced by a phosphate group
Variable
2
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Phospholipids
Hydrophobic tail
Hydrophilic head
AMPHIPATIC
2
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Phospholipids
2
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2
Most common Phospholipids in membranes
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2
Outer side
of membranes
Phosphatidylcholine = Lecithin
Proeminent phospholipid in membranes
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2
Inner side
of membranes
Phosphatidylserine
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2
Inner side
of membranes
Phosphatidylethanolamine = Cephalin
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2
Inner and Outer sides
of membranes
Phosphatidylinositol
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2
Phosphatidyl
inositol
Phosphatidyl
ethanolamine
Phosphatidyl
serine
Phosphatidyl
choline
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2
The lipid bilayer is asymmetric
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Roles of Phospholipids
2
The kind of phospholipids influences
the movement of molecules in the membrane
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Fatty Acids
Saturated Unsaturated
Chain of
Carbon + Hydrogen
End in
COOH
Contain double bonds
2
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The type of hydrocarbon tails in phospholipids
affects the fluidity and viscosity of the membranes
Fluid Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
Carbon tails
2
1. Fluid membrane
Not tight
Molecules can drift
1. Viscous membrane
Tight
Molecules cannot drift
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1. Fluid membrane
Not tight
Molecules can drift
2
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1. Viscous membrane
Tight
Molecules cannot drift
2
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2
Aqueous
Aqueous
“Organic”
Can water go through ?
?
Phospholipids regulate the passage of molecules through membranes
YES
Can hydrophilic molecules
go through ?
?
NO
Membranes are selectively permeable
Hydrophilic molecules
do not mix with lipids
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2
Phospholipids regulate the passage of molecules through membranes
Membranes are selectively permeable
Phospholipids restrict the passage of
hydrophilic molecules
through membranes
Phospholipids make membranes become
Barriers between
1. Cells and their environment
2. Organelles and the cytoplasm
Aqueous
Aqueous
“Organic”
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2
Two types of lipids in the membranes
Phospholipids Cholesterol
Only in animal cels
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Lipids
Steroids
Basic form = CHOLESTEROL
In animal membranes
Mostly in the plasma membrane
2
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2
Cholesterol is amphipathic
Hydrophobic
Hydrophilic
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2
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2
Roles of cholesterol
1. Cholesterol maintains the integrity of the Cell Membrane
Part of the steroid ring
(the four hydrocarbon rings in between the hydroxyl group and the hydrocarbon "tail")
is closely attracted to part of the fatty acid chain on the nearest phospholipid
1. This helps slightly immobilize the surface of the membrane
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2
2. Cholesterol regulates the fluidity of the membranes
1. This helps slightly immobilize the surface of the membrane
More cholesterol
Membrane is more ………
Less cholesterol
Membrane is more …….
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2
3. Cholesterol reduces membranes’ permeability
to hydrophilic molecules
2. Makes the membrane less soluble to very small water-soluble molecules
that could otherwise pass through more easily
Very small hydrophilic molecules
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2
Membranes contain many important proteins
Sometimes the “normal” phospholipid bilayer is not suitable
“islands” of Cholesterol + Sphingolipids make up Lipid Rafts
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2
Sphingolipid
=
Shingosine + fatty acid (+ phosphate)
Shingosine
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2
Sphingolipid
=
Shingosine + fatty acid + X
Shingosine
Fatty acid
can vary
X
Varies: carbohydrate, phosphate...
If X is a carbohydrate
Sphingolipid = Glycosphingolipid
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2
Sphingolipid
=
Shingosine + fatty acid + R
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2
Sphingolipid
=
Shingosine + fatty acid + X
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2
Lipid rafts
=
Cholesterol + Sphingolipids
a) Shingolipid
b) Cholesterol
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2
Lipid rafts
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Types of membrane proteins
Two types of proteins in the membranes
Integral
3
Span the membrane:
transmembrane
On either side
of the membrane
Peripheral
Bound to protein Bound to lipids
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 Within membrane
 nonpolar amino acids
 hydrophobic
 anchors protein
into membrane
 On outer surfaces of
membrane
 polar amino acids
 hydrophilic
 extend into extracellular
fluid & into cytosol
Polar areas
of protein
Nonpolar areas of protein
Types of membrane proteins
3
Transmembrane proteins
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Functions of membrane proteins
Six main functions for proteins in the membranes
3
Transportation Enzymes
Receptor Cell adhesion Attachment
to cytoskeleton
/ ECM
Recognition
outside outside outside inside / outside
inside
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Channel proteins have pores that allow passage of
ions and small water-soluble molecules
Passive transport
Carrier proteins bind to molecules and change
shape for delivery across membrane
Active transport
3
1. Transport proteins
Regulate movement of ions and small hydrophilic molecules across the membranes
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3
2. Receptor proteins = Signal transduction proteins
Usually after the cell received a signal from outside the cell
Signal = hormone and other substances
Pass on a signal from outside to inside the cell
The cell will respond to the signal that was passed on
Messenger molecule
Activated
molecule
Receptor
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3
3. Enzymes = Effector proteins
Catalyze reactions inside the cell
One or more than one different enzymes grouped together
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3
3. Signal transduction
Receptor + Regulator + Enzyme
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3
4. Recognition proteins
“Identification tags” “Cell signature”
Very important in Immunity
Usually glycoproteins
To differentiate between “self” and “non-self”
Glyco-protein
Cell A
Cell B
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3
5. Cell adhesion proteins
“intercellular compatibility”
Important in Gap junctions and Tight junctions
Cell A
Cell B
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3
6. Attachment to Cytoskeleton / ECM
ECM
Microfilaments
Maintain cell shape
and
Stabilizes the location of
some membrane proteins
coordinate extracellular and
intracellular changes
Integrins
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Outside
Plasma
membrane
Inside
Transporter Cell surface
receptor
Enzyme
activity
Cell surface
identity marker
Attachment to the
cytoskeleton
Cell adhesion
Outside
Plasma
membrane
Inside
3
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Usually branched molecules of 15 or less sugar units
Some are bonded to lipids: Glycolipids
Most are bonded to proteins: Glycoproteins
Glycolipids + Glycoproteins = Glycocalyx
Function: Cell-cell recognition
Membrane Carbohydrates
4
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Membrane proteins and lipids
Are synthesized in the ER and Golgi apparatus
ER
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
apparatus
Vesicle
Transmembrane
glycoprotein
Membrane glycolipid
Plasma membrane:
Cytoplasmic face
Extracellular face
Secreted
protein
4
1
2
3
Conclusion and Overviews
5
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Summary
 The unit membrane consists of a phospholipid bilayer
 Phospholipids consist of a polar, hydrophilic phosphate head and a non-polar,
hydrophobic tail consisting of fatty acid chains.
 Proteins also occur in the membrane and float freely throughout it.
 The model for membrane structure is known as the fluid mosaic model.
 Peripheral proteins occur on the inner or outer face of the membrane and integral
proteins extend through both lipid layers.
 Membrane bound enzymes occur allowing structured metabolic pathways.
5
Conclusion and overviews
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Summary
 Glycoproteins form the glycocalyx and allow cell to cell recognition.
 Receptor proteins can act as binding sites for hormones and other substances and
can transmit the information to the interior of the cell.
 A variety of carrier proteins allow for the controlled movement of substance
through the membrane using both passive diffusion or active transport.
 Non-polar, lipid soluble molecules diffuse through the phospholipid bilayer.
 Ionic, polar molecules require carrier proteins to enable them to pass through the
membrane.
 Membrane structure loses integrity with high temperature or presence of organic
solvents such as alcohol, thereby increasing permeability.
5
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Macromolecules found in Membranes
Lipids
Cholesterol Sphingolipids
Proteins
Transport Receptor
Enzymes Recognition
Cell adhesion Attachment to Cytoskelton / ECM
Carbohydrates
 Oligosaccharides on glycoproteins
 Oligosaccharides on glycolipids
5
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AS CIE Cell Membranes

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