Chapter 7: Membrane
Structure & Function
Plasma membrane
Composition:
primarily lipids
(phospholipids) & proteins with some
carbohydrates (glycolipids or
glycoproteins for cell recognition)
Arranged in a fluid mosaic
Lipid
bi-layer with embedded proteins
Discovery of plasma membrane
structure
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1915- Red blood cell membranes analyzed; lipid
& protein composition discovered
1925- Gorter & Grendel suggest membrane is
phospholipid bi-layer
1935- Davson & Danielli suggest proteins
sandwich phospholipids (FALSE)
1950s- Electron Microscopes used to study
membrane structure
1972- Singer & Nicolson suggest proteins are
dispersed (“float”) within the lipid bi-layer
(further shown by freeze-fracture electron
microscopy
Fluidity of membranes
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Membrane held together by weak
hydrophobic interactions; most lipids &
some proteins can drift within their layer
of the membrane
Protein movement/non-movement may be
dependant on the proteins connection/lack
of connection to the cytoskeleton
Temperature affects level of fluidity
Fluidity affects permeability
**cholesterol & unsaturated fats increase
fluidity (added to membrane to prep for
cooler temps)
Concept Check
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What would happen to the fluidity of the
membrane in the following scenarios?
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Increase in unsaturated phospholipids?
Increase in saturated phospholipids?
A decrease in temperature?
An increase in cholesterol levels?
Membrane proteins
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Determine most of the membrane’s
specific functions
Types:
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Integral proteins: penetrate through the
hydrophobic core of the lipid bi-layer
(transmembrane proteins)
Peripheral proteins: not embedded in bi-layer;
attached to the surface of the membrane
Functions of membrane proteins
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Transport
Enzyme activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton &
extracellular matrix (ECM)
Carbohydrates & the membrane
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Carbohydrates in the membrane < 15
sugar units
Types:
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Glycolipids
Glycoproteins
Function: cell-cell recognition
Synthesis of membranes
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See text book figure 7.10
1. synthesis of membrane proteins & lipids in the
ER; Carbohydrate added to make
glycoproteins
2. Inside Golgi apparatus glycolipids are made
and glycoproteins are modified
3. Transmembrane proteins, glycolipids, &
secretory proteins are transported in
vessicles
4. Vessicles fuse with the membrane releasing
secretory proteins & placing glycoproteins &
glycolipids on the outside of the membrane
**Outside of plasma membrane is made
from the inside of the ER, & Golgi vessicle
membranes
(When vessicles formed in ER & Golgi fuse
with membrane to release material they
become part of the membrane)
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Concept Check
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On which side of the membrane are
carbohydrates found? How is this location
useful to the carbohydrate function in the
membrane?
Selective permeability
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Fluid mosaic model explains how
membrane can regulate passage of
materials
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Hydrophobic (non-polar) molecules can
diffuse through lipid bi-layer easily
Polar molecules & ions which are impeded by
the lipid bi-layer pass through specific
transport proteins
Passive Transport
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No energy required
Diffusion
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Molecules will move from high to low
concentration
Diffusion of molecules is unaffected by the
concentration of other substances
Rate is determined by membrane permeability
to the molecule
Passive Transport (cont’d.)
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Osmosis
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Diffusion of water across a selectively
permeable membrane
Tonicity=ability of a solution to cause a cell to
gain or lose water
isotonic: no net movement of water
 Hypertonic: net loss of water
 Hypotonic: net gain of water
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Passive Transport (cont’d.)
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Facilitated diffusion
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Movement of molecules down their
concentration gradient with the assistance of
specific transport proteins in the membrane
Types of transport proteins:
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Channel proteins: “corridors” for passage of
specific ion or molecule
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Aquaporins (water channel proteins)
Ion channels/gated channels (electrical or chemical
signal causes opening or closing
Carrier proteins: change shape to translocate
substances across the membrane
Concept Check
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What would happen to a Paramecium that
swam from a hypotonic environment to an
isotonic one?
Why do water molecules need aquaporins
to cross the membrane? Why don’t
substances like oxygen and carbon dioxide
require transport proteins?
Active Transport
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Molecules move against the concentration
gradient (low to high)
Energy required
Uses carrier transport proteins
Active transport: sodium-potassium
pump
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Na+ in cell binds to protein
ATP binds to protein
Protein changes shape
Na+ moves out of cell
K+ outside cell binds to protein
P from ATP is removed
(dephosphorylation)
Original protein shape is restored
Active transport: electrogenic pump
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H+ pumped out through protein with the
help of ATP
Outside cell becomes +, inside –
Charge difference across the membrane is
used to do work
Active transport: Cotransport
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Same as electrogenic pump but…
when H+ moves back into cell by diffusion
it carries another molecule with it (i.e.
sucrose)
Passive vs. Active Transport
Concept Check
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Why is the sodium-potassium pump not considered a
cotransporter?
Which solute(s) will exhibit net diffusion into the cell?
Which solute(s) will exhibit net diffusion into the cell?
Which solution “cell” or environment is hypertonic?
In which direction will there be a net osmotic movement
of water?
After the cell was placed in the beaker did it become for
flaccid, or more turgid?
Bulk Transport
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Exocytosis: cell secretes macromolecules
through the fusion of vesicles with the plasma
membrane
Endocytosis: cell takes in macromolecules by
forming new vesicles from the plasma
membrane
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Phagocytosis (“cellular eating”)
Pinocytosis (non-specific “cellular drinking”)
Receptor-mediated endocytosis (specific uptake)
Concept Check
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As a cell grows, its plasma membrane expands.
Is this a result of exocytosis or endocytosis?
Explain.
After a neuron has been stimulated by
neurotransmitters from a neighboring neuron,
the neuron takes in the neurotransmitters by
endocytosis. Is it by pinocytosis or receptormediated endocytosis? Explain.