Cell
Membranes
What’s wrong with this
picture?
http://www.studyblue.com/notes/note/n/osmolality-and-osmo-gap/deck/1598495
The Plasma Membrane
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Membranes
Cells separate “inside and outside” with lipid
barriers called membranes.
Organelle membranes separate too.
Limits passage of polar substances.
Protein channels allow specific passage.
BILL: What is the difference between polar and
charged?
What passes freely?
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Small, uncharged
polar molecules
Small nonpolar
molecules like N2
Model simple
diffusion
http://www.studyblue.com/notes/note/n/biology-172-lecture-7-flashcards/deck/124266
Cell Walls
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Are outside the
membrane
Structural
Plant cell walls are
made of cellulose
Prokaryotes and fungi
also have cell walls.
http://www.phschool.com/science/biology_place/biocoach/plants/walls.html
Phospholipid Bilayer
• This represents a phospholipidPolar head-Hydrophilic-label
Nonpolar tail-Hydrophobiclabel
• The tails are fatty acid, the head,
phosphorylated alcohol
• These form a sheet two molecules
thick.
http://en.wikipedia.org/wiki/Phospholipid
Why the embedded cholesterol?
What might a membrane in
an arctic dweller look like?
Increase or decrease fluidity depending on
temperature.(Decreases fluidity when warm, increases fluidity when
cold…keeps membranes fluid at very cold temperatures)
Membrane Proteins
Function in
• Transport• Enzyme
• Surface receptors
• ID Markers
• Cell-cell connection
• Attachment
http://www.pc.maricopa.edu/Biology/pfinkenstadt/BIO201/201LessonBuilder/UnitOne/Membrane/in
Embedded Proteins
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Can be hydrophilic with
charges and polar side
groups or…
Hydrophobic, with
nonpolar
Place your proteins in the
membrane.
http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect08.htm
Anchoring in the membrane
What could keep proteins in the membrane?
Transmembrane Proteins
•Carriers- change shape
–Active and passive transport
–Sodium potassium pump
•Channels–are tunnels through the hydrophobic core
•Receptors
–Transmit information from the outside of the cell
–Hormone receptors, neurotransmitters.
What needs a
channel?
Hydrophilic substance like large polar
molecules and ions
http://www.cipsm.de/en/publications/researchAreaF/2007/index.html
Carrier proteins
•Holds ion or molecule
•Changes shape
Have you seen shape
changes before?
•To move something across the membrane
Green ball is one K+ in
a potassium pump
Campbelll p 125 8th edition.
Aquaporins
•Channel Protein
•Each aquaporin allows
3 billion water
molecules per second
to pass into the cell
single file.
What might the positively
charged region do?
http://www.bio.miami.edu/~cmallery/150/me
mb/water.channels.htm
Receptor Proteins
•Example:
–G protein linked receptor
–neurotransmitter
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Cell Surface Markers
Glycoproteins- a carbohydrate combined with a
protein. Add a carbohydrate chain to a protein
embedded in the membrane. Add and label
Important in the self recognition.
Recognized by the immune system.
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Glycolipid- a carbohydrate combined with a lipid .
Add a carbohydrate to a lipid. Add and label.
Important in tissue recognition.
Example is blood group marker.
Cell surface markers
Glycocalyx- “Sugar coating”
Me
You
•Glycoproteins- “self” recognition
The protein/carbohydrate chain shape is different person
to person. For example, the major histocompatibility complex
proteins are recognized by the immune system.
•Glycolipids-tissue recognition
The lipid/carbohydrate chain
shape is specific for a certain
Transport Modes
Through the cell membrane
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Passive - Down the concentration gradient-primary
role in importing resources and exporting waste
1.
Diffusion
2.
Facilitated Diffusion- membrane proteins help charged and polar
molecules pass.
3.
Osmosis
Active- Against the concentration gradient. Energy
requiring. Requires membrane proteins.
1.
Endocytosis/Exocytosis
2.
Na+/K+ Pump
3.
Proton Pump
Diffusion
•Often by Ion Channels
•Direction of movement determined by
–Relative concentration
–Voltage
•Each channel is specific for one or a few ions
•Nervous system
Facilitated Diffusion
•Carrier Proteins
•Specific also
•Bind/release
•Moves things down the concentration gradient
•Passive transport
•Can become saturated
Active Transport
•Uses Energy
•Moves things against the gradient
•Na/K Pump
•Coupled Transport
–Gradients created by one process can power
another
Sodium Potassium Pump
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Cytoplasmic Na+ binds (high affinity in this shape).
Na+ binding stimulates phosphorylation by ATP
Phosphorylation causes shape change, lower Na+
affinity, now high K+ affinity.
The K+ binding causes phosphate to be released
Phosphate release causes shape to return. Now low
K+ affinity,
Endocytosis and
Exocytosis
• Exocytosis-internal vesicles fuse with
the plasma membrane to release large
macromolecules out of the cell.
• Endocytosis-cell takes in
macromolecules and particulate matter
by forming new vesicles from the
plasma membrane.
Bulk Transport
•Endocytosis
–Phagocytosis-particulate
–Pinocytosis-liquid
–Receptor-Mediated endocytosis
–Clathrin coated pits bind to specific molecules.
Bulk Transport
•Exocytosis
–Neurotransmitter discharge
–Hormone secretion
–Digestion enzymes
http://www.kscience.co.uk/as/mo
dule1/pictures/endoexo.jpg
Explain the
diagram.
Eukaryotic cells are compartmentalized
By Membranes
These special areas let things happen by…
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Minimizing competing interactions
Increasing surface area where reactions can occur
Compartmentalizing metabolic processes and enzymatic
reactions.
Examples: Endoplasmic Reticulum, mitochondria,
chloroplast, Golgi, nuclear envelope
Archaea and Bacteria generally lack internal membranes
and organelles.
Protein Pieces